[ACMP investiupgradifications] All things PREAMP

[Steve Hogan]

OT about the faraday shield.. just wondering about the process. Do you wind the primary, then wrap the shield, then wind the secondary on top?

That is exactly how we do it, although one could put the secondaries on first and then the shield and then the primaries. If the transformer were a step-up instead of step-down, I might put the primary on the outside.  In the case of toroids, the coupling between windings is really good whether the winding is on the top or the bottom.

Steve

 

[mitsos]

Thanks for the clarification... Would you say that is generally a good thing to do if one is winding toroids? Or is it generally preferred, not necessarily needed?

From time to time I have to have trafos wound and I wonder if I should have the shop add a shield. Guess it can't hurt?  Any spec for material? I need to give these guys pretty specific instructions...

 

[Steve Hogan]

Thanks for the clarification... Would you say that is generally a good thing to do if one is winding toroids? Or is it generally preferred, not necessarily needed?

From time to time I have to have trafos wound and I wonder if I should have the shop add a shield. Guess it can't hurt?  Any spec for material? I need to give these guys pretty specific instructions...

Toroidal power transformers have their windings spread out over the surface of the core, with primaries and secondaries on top of one another. This construction makes for low leakage inductance (passes high frequencies well) and high capacitance (passes high frequencies well) between windings. If you are interested in rejecting high frequencies from the power line, then the Faraday (copper foil) shield is necessary between windings to prevent capacitive coupling of HF garbage from the power line (RF, dimmer hash, etc.) from primary to secondary and vice versa.  Adding the shield makes your power transformer an isolation transformer at the same time, in a similar way to how "balanced power" tranformers work. I believe it is an excellent thing to have in all audio equipment power transformers, but it is seldom done because it is an additional expense to add to the cost of the power transformer and most manufacturers of audio gear really don't know how important it is and they are also under competitive cost pressures to cheap out.

There are some construction techniques and tricks that we use to get maximum performance from our Faraday shielded power transformers, but we are now beginning to enter into trade secret territory, so I'm afraid that I don't have more any construction details that I am prepared to explain.

If you are interested in a quote on your next power transformer for audio design, contact me off list.

 

[Steve Hogan]

I personally appreciate the time your giving to this preamp fiasco. I hoping for the best.
Although something tells me this is not a typical DIY project. Its more of a engineers
puzzle than a simple circuit fix. But educating, nonetheless.

Thanks for the kind words. Since I upgrade and modify pro audio gear for a living, and end up fixing many problems that were originally designed into the gear, my approach is hopefully very scientific.  The transistor swap problem, for example is one that I want to see for myself.  It is true that substituting different transistors may have stopped the apparent oscillation, but I want to see the PCB layout, and figure out what is REALLY going on.  The power dissipation in these Q4, Q5 transistors isn't very high (unless it is oscillating), so a 2 Amp rated transistor isn't really necessary here. Q5 is just a current sink load for Q4 and it is set to 8.5 mA.  Q4 should only dissipate about 125 mW when it's working right.  All that to say that the large, metal can BC441 and BC461 are perhaps overkill to resolve this oscillation problem.  I have spent considerable time researching beefy transistors that have similar capacitance, gain-bandwith product and Hfe as the BC441/BC461, but will fit easily into the existing PCB.  I am not inclined to go for the TO-126 package as a solution (mechanical nightmare) when there are many appropriate TO-92 and TO-92L (tall package for audio driver) transistors that may work perfectly, and be a very close mimic to the Neve BC441/BC461 parts, which are hard to get and unnecessarily expensive.  I am seeking the very best, if you will, engineered solution.

Further investigation of the original Neve 338 plug-in amplifier as detailed in the AMS Neve '81 owners manual available as a download shows the reason for Neve using BC461-6 and BC441-6 TO-39 packaged transistors with heatsinks.
Pin 4 of the Neve BA338 Plug-In amplifier is labeled "boost (39 Ohm for 300 Ohm O/P)."  Strapping a 39 Ohm resistor betwen this pin and Ground increases the current sink Q5 collector current from 8.25 mA in the "non-boosted" position to 40 mA in the "boost" configuration.  This will increase Q4 dissipation from about 100mW to 480 mW which will be quite toasty without a heatsink and a big part.  The -6 suffix was the highest gain selection available on the BC441/BC461 transistors.  The TNC '81 clone does not use the "boost" position, so a hefty TO-92 transistor should work just fine.
I have now selected what I believe to be an optimal replacement solution to the Q4/Q5 transistor oscillation problem.
The replacements I have selected have a TO-92 package with CBE pinout-- same as the Chinese supplied BC557/BC547's. They are, however. rated at 800 mA current instead of 100 mA.  They are cheap and readily available.  The junction capacitances are more in line with the BC441/461 parts, and they are available in a selected gain range that should mimic the performance of the BC441/461 transistors.  Unfortunately, I must order them to try them out, so I won't have answers until next week.
Until then I am not going to recommend them by part number until I confirm that they work well. because it may just cause more confusion than already exists as to the best transistors to use as replacements.

I will confirm that a PN2907A looks like an excellent replacement for the BC461-6.  Gain is similar, they have 800mA rated collector current, and they will fit perfectly in the existing PCB with only one change.  Since they are EBC and the original factory transistors are CBE pinout, one must install a PN2907A with the Flat of the transistor body rotated 180 degrees from the original transistors.

While investigating the transistor problem, I discovered another mistake in the same circuit that came about from just stealing the Neve schematic to make a clone and not stealing it very carefully. Unfortunately those to whom the word R&D means "Rob and Duplicate" often have no clue how any of the circuitry works or what it does.  I haven't decided  what to do about this yet, but my uncomfortable tingles are beginning to go off.

Fellow DIY group members,  Let me detour for a moment to share some thoughts with you all regarding my efforts to help the owners of these TNC preamps to get their units working and usable, and at the same time provide myself with additional income to pay my bills.  I have been contacted by various engineering and manufacturing professionals who make superb quality (but not inexpensive) products.  They have warned me to be careful about the investigative work that I have been doing that has included detailed explanations and solutions that you DIY folks can learn from and make improvements to your gear (which is good).  The truth is, that there are folks out there, both Chinese and domestic, who are making money selling clone preamps that have been "developed" by simply copying the Neve circuitry.  Most of these folks have absolutely no clue what is really going on in the circuits.  They substitute parts that are hard to find or unusual because they are hard to get and they have no understanding of why Neve chose that part in the first place.  The power transformers, audio transformers and inductors in most of these clones are sub-par to be charitible, and the manufacturers of the the magnetic components don't even know how to fix their own parts, because they don't understand the circuit in which they are used, and don't understand how to make their own product better.
It has been said by others that the TNC folks have expended thus far only a minimal effort to address the problems with these units themselves, but have just watched others in the various Forums like this one tackle the various problems and attempt to solve them without any real effort on their part.  The present group buy preamps were apparently sold as-is.  TNC has made it clear to me that the work I am doing to the preamps is not under their authority but is strictly between me and owners of the preamps.  I have worked out a system for bringing in the preamps to modify them and add the new power transformer and other upgrades, and also making available a kit of upgrade parts, for the DIY types who are capable and wanting to do the upgrade and transformer installation work themselves. I think that plan will work well for both me and the preamp owners.

I started this project based on the idea that there were these preamps out there that had hum and buzz problems that could be fixed if I provided a good power transformer for them.   As I have learned more about these ACMP preamps, I have come to realize that there are a lot more design problems in these preamps than just the power transformer.  I can fix every one of the problems.  I have already designed a new power tranformer.  I have already fixed the Power supply PCB and the wrong capacitor values and voltages that were chosen.  I have now figured out what I believe to be the best replacement transistors, and located another problem in the EQ boards that will require changing another part. I can design new 5% tolerance inductors that are consistent and free from high Q resonance, and I can design meter driver circuitry and clip lights that work correctly with real audio waveforms.

The moral and economic dilemma I find myself in is that I design and engineer first-class audio gear to keep a roof over my head. I certainly do not need to make a lot of money on fixing the problems in these preamps, but if I cannot make it pay something, then I cannot continue to provide really good engineering services.  The difficulty I am facing right now is that at the same time I help you, the purchasers of the preamps, to make your gear work correctly, I am simultaneously giving away free engineering services to the manufacturer(s) who should rightly be paying for that engineering work.  If I solve the problems for you in a manner that "shows all my cards", the manufacturers get all their boxes re-designed and the problems fixed without them having to pay for it.  I have been warned that I have been engineering fixes for all the problems of the TNC preamps for free.  The truth is that most of the manufacturers of the really cheap Neve clone preamps truly don't even know that their circuitry has problems, let alone how to fix them.
As I write up detailed fixes for my DIY friends, I am at the same time writing it up for these manufacturers who have already demonstrated by their product line that they have no qualms at all about stealing intellectual property from others to make a buck for themselves. It is because there is some rumblings of another "Group Buy" in the works that I am especially concerned about fixing all the problems in the future boxes for free.

The truth is that, if the "Rob and Duplicate" folks get all their engineering for free, it really makes it hard for the legitimate audio equipment manufacturers to stay in business.  Their products cost more, sound better, and many of them have invested heavily in test equipment and engineering costs to make their products the best they can be.

I'm searching for a balance between helping the individual owners, which I really want to do, and inadvertantly assisting the unscrupulous, which is neither in my best interest nor in the best interest of legitimate manufacturers of the really good audio gear.  In reality, it isn't even in the best interest of every one of us who wants to have great audio gear available, because the folks who care enough to really design and build the best audio equipment will go out of business.

I apologize for such a long rambling post, but I wanted to share my concerns with you all, and invite you to comment.



 

 

[0dbfs]

I'm searching for a balance between helping the individual owners which I want to do, and assisting the unscrupulous which is neither in my best interest or in the best interest of legitimate manufacturers of the really good audio gear..

You could always send an invoice to the mfg.

BTW Mr. Robin Hood,

Thanks for all the knowledge and information you have provided up to now. I'm looking forward to see where you go with all of this.

Cheers,
jonathan

 

[okgb]

TNC  [ or Alan Hyatt ] should pay you and then get the jump on another run before
, as will happen ,  the chinese push whatever " improvements "
they get into the next round  , and there is no doubt that will happen
, even if it was tnc saying we need better transformers , they make for
grp A , Improve for grp B , take those and sell to Grp C , of course
getting away with as little as possible .

You can amortize the numbers and wait till you have a enough
to sell the upgrade packages like a grp buy at once , and then
piece meal with overstock [ or keeping some stock after ]
as it's adaptability to other design develops
[ I would hope this doesn't turn out more expensive  ]
based on percentage of numbers out there .

Of course too , the people doing the whispering have some interest for themselves as well ,
but for years now these chinese sales have been a run of X amount , then move on to the next item ,
no support , no looking back .
Some of your things may be too expensive for them and could be a low stocking item ,
but i think using their model consider it a one off run , that may have some ongoing life after ,
certainly if all goes well and people are happy with the value / service it will enhance  ,
further establish your good reputation and may be allot of good promo

As a separate product , you could make whole , external power supplies ,
or look for other other products to adapt  [ & retro fit ]
the things you are working now for greater numbers .
[ i don't think the chinese care about old things , it's like the new car
model , where they gotta keep pumping out new product for new profit
The world needs decently priced api 500 supplies .

Another point is , that as some are not in this as a business [ recording ]
they wouldn't likely buy the better product as it would not pay itself off
and therefore would go to the cheaper item as the only option , maybe
hoping to improve performance themselves but this isn't business lost
for the high end market .
On another hand as others end replacing too many components , they may
get the idea buying cheap is not worth it , or not to expect [ accept  ] much .

Supporting local business and not buying Chinese where possible ,
couldn't hurt our own economies , sometimes cheap is not worth it ,
as long as you can afford it .

But clearly , someone is stirring things up again , mostly we avoid that here ,
but as it is a grp , let us know how we can help .

 

[crazydoc]

Steve

Those are indeed some difficult thoughts and questions as to how to resolve this dilemma: fix the buyers defective preamps, but at the expense of providing free engineering to those who have stolen the designs in the first place.  I myself am no businessman, lawyer, electronics or audio professional, so my postings will certainly be naive and ignorant.

It seems we have a number of players in this game, each with his own (and many times conflicting) interests at heart: the manufacturer (Chinese), the distributor (TNC), the purchaser (including us DIYers), the re-engineering/repair people (at this time Steve), and the designers and manufacturers of high-end audio equipment (including Steve) who's product designs are being stolen and who are subsequently losing income from their own work.

First, I would have to say that those of us who bought these knew (or should have known) that it was a crap shoot, and should have had no real expectations that the product would work correctly or be warranted. There is also the ethical/moral dilemma of buying goods built from another's hard work (at no profit to that person), but I guess that by buying we all showed where we stood on that. So there should be no expectation that these will be fixed at anyone's expense but our own.

The fly in the ointment here is that TNC, the distributor, through some ignorant mistake, I'm sure, has printed a one year warranty on the users manual. I don't know what this implies legally. I am reasonably sure, having participated in and followed these group buys over several years, that the organizer of these is an honest, but business-unwise naive man who wants to do the right thing. He has done this at little or no financial profit (and possibly a loss) to himself, and I don't think there is any possibility of his honoring the warranty, for financial reasons. In this latest group buy he has picked up a partner, well known in the recording community. I have no knowledge of his interest or capabilities in all this. Anyway, I don't think there will be any help or responsibility from this sector.

I would divide the fixes for these into two categories - those that merely call for the replacement of parts with existing substitutes (transistors, capacitors, etc.) or minor circuit corrections, and those that require the manufacture of more expensive components, such as inductors.

Obviously the cheap modifications could be picked up on by manufacturers at little expense, and incorporated into future production runs should they be interested. Manufacturing the more expensive inductors might be cost prohibitive for them, but you can't be sure. They are some pretty wily people. Any commercial mods that are made, whether unpublished, potted or otherwise hidden, will probably get into their hands.

So to me the answer boils down to two possibilities:

1) Do the fixes, unpublished or not, with the real possibility that you will shortly see cheap Chinese equipment incorporating the fixes, to the detriment of the domestic producers. This would probably include even just supplying the new power transformer (though I have no real idea if it would be possible or cost-effective for the Chinese to reverse engineer and produce these, nor how much of a fix this alone is going to be.)

2) Let this project drop, and leave those of us who played this game to reap the rewards (another of life's learning experiences.)


I'm sure there are alternative answers I've not thought of. Have at it. 

 

[okgb]

A long & tangled web for sure with many unknown truths
It would be better however to keep this thread technical
as is the " Lab "
And we don't need this turning into H.R. or PSW

could start another thread in the Brewery to discuss
& debate mutual assistence . [ and hopefully determine
how to keep this positive for everyone ]

also a few of the more clever here still have not recievced
their pres yet  [ Clint ?  ] which will also feed more
knowledge into this problem [ & solution ]

 

[Steve Hogan]

Steve

Those are indeed some difficult thoughts and questions as to how to resolve this dilemma: fix the buyers defective preamps, but at the expense of providing free engineering to those who have stolen the designs in the first place.
. . .
So to me the answer boils down to two possibilities:

1) Do the fixes, unpublished or not, with the real possibility that you will shortly see cheap Chinese equipment incorporating the fixes, to the detriment of the domestic producers. This would probably include even just supplying the new power transformer (though I have no real idea if it would be possible or cost-effective for the Chinese to reverse engineer and produce these, nor how much of a fix this alone is going to be.)

2) Let this project drop, and leave those of us who played this game to reap the rewards (another of life's learning experiences.)

Although I am new to this forum, I am delighted with the mutual respect that is displayed to every participant here. I love that one doesn't have to plow thru tons of inanity to get to the answers that everyone so freely shares here.
In that spirit, since I have brought up this off topic subject (for this venue, at least), I will do my best to clarify my intentions and close off the discussion in this place.  Having gotten myself involved in what I thought were strictly technical discussions in other forums, and then coming to realize that there was way too much personality issues that cluttered up the ability to simply get to the truth, I confess that I really like the signal to noise ratio here and I don't want to have a part in creating static.

It was not my intention to stir up any kind of hornet's nest or to even make accusations. The world we live in is what it is.  I do not believe that the TNC partners had anything but the best intentions to bring decent technology to many people who simply are not in a position to spend thousands on mic preamps. I believe that they (naively, perhaps) had no idea of the magnitude of the problems that the preamps would manifest, and they appear to have held back essentially no funds with which to effect warranty repairs.

Crazydoc has, as always, spoken with a great deal of clarity in his post. I indeed agee with him that the buyers of these preamps are on their own to spend more money to really make them perform, if not spectacularly, at least decently.
It is my desire to make my services and expertise available to those who want to fix/mod/upgrade their preamps accordingly.

I have tried to think of ways to provide the more subtle fixes privately,  to just those whose preamps I actually mod/upgrade or to those to whom I sell the upgrade kit of parts with directions of what to do.  The reality is, that in this day of internet communication, once you publish even a set of directions with an explicit copyright notice and all rights reserved legal clause, I suspect that I would be able to Google the preamp mod directions about 30 seconds after it popped out of my printer :>).  It is my opinion that trying to make the upgrade changes a trade secret is doomed from the get-go, so I have resigned myself to publishing the entire list of mods that I will be making available on the preamps so there will be no surprises.  The one thing I have going for me is that I am the sole supplier (for now) of the upgraded power transformers, so every one who wants one will get one from me.  The fast, soft recovery rectifiers that I am retrofitting to the power supply boards for those who want the cleanest power supply are no longer being made and I have pretty much the only remaining stock.

I have gone way too far already to drop the project, and I still feel that I can make such a substantial improvement in any owner's preamp that it will be well worth the upgrade cost.  There are enough of these units out there right now that if even 10 or 20 percent of the owners choose to upgrade, I could still make enough money to cover my initial development costs and make a little on each preamp that I fix.

I love to teach, as you can probably tell from my long-winded, but hopefully instructive posts.  I spend a lot of time writing my posts because I strive to be technically accurate, and still be able to make the concepts clear to those who aren't uber technical.  I have always felt that if one really understands his subject well, then he should be able to simplify its explanation and still maintain technical accuracy. "The Art of Electronics" by Horowitz and Hill (my favorite electronics book) is an excellent example of that abilty to simplify and clarify concepts.  There are many who read and participate in the DIY board who are eager to learn more about audio electronics.  There is so much that goes into good sound that isn't published or taught anywhere that I know of, and I am willing to share much of what I have learned over the years with the DIY group.  I trust that my continued participation here will be helpful to everyone.

So I will procede full speed ahead to finalize the list of upgrades to all three ACMP preamps.  I will post again when I have some new measurements or insights and I will keep it technical. The next thing for me to test is the effect that the new power tranformer has on the hum levels.  Those who are interested in the fixes/mods/upgrades that I will be offering can contact me off list to make arrangements.

 

[rodabod]

Steve, I agree with everything you've said. You have already given away a lot of free work already, and if this is your line of work then you should think about yourself first.

As you suggested, instructions posted by yourself with a mod kit would spread like wildfire on the internet. Perhaps you could offer a send-and-return mod solution for those who are unable to study the circuits themselves.

You can guarantee that your work will be incorporated into future Chinese designs. Whether or not this is a good thing is up for debate I suppose.

I'm still waiting to get my racks and will investigate when they arrive, but it'll be great if Peter (Clintrubber) can get his hands on his as he seems to have a fairly solid understanding of sandstate devices.

Roddy

 

[tarnationsauce]

Perhaps we could do a Group Buy for Steve's engineering? If even 1/4 or 1/2 of us get together and help it should be a fair sum of money. Although I have no idea what kind of money a job like this is worth.
am most definitely willing to participate in something like that. With enough people it will be pretty cheap for individuals.

The only caveat being that any aspect of his engineering that's released will most likely become someone else's "R&D".

Plus there will be money made for the specific parts, kits, and/or labor. Everyone wins!

 

[okgb]

Right Steve can have accountable " numbers " and maybe a better
idea of what to expect , how much to order .
BUT once it's out there  , it's out there  ,
so the finacial model has to be based on the initial run ,
gravey after that .

Although grp buys do have ecomnomy of scale , that's up to steve

I have sent my 81 inductors to Cinemag and expecting replacements
to be under the 20.00 mark in small quantities [ or 1's & 2's]
once they're set up anyone will be able to order at anytime

And , no they weren't bought  " AS IS " that wouldv'e hurt sales
i didn't expect a neve preamp  but i did expect it to work
but the changing story is typical of the vague details from the start
again  , no problem to open a thread in the brewery , but might as well
move forward and fix these enough to be usable

 

[Steve Hogan]

So I will procede full speed ahead to finalize the list of upgrades to all three ACMP preamps.  I will post again when I have some new measurements or insights and I will keep it technical. The next thing for me to test is the effect that the new power tranformer has on the hum levels.  Those who are interested in the fixes/mods/upgrades that I will be offering can contact me off list to make arrangements.

I spent yesterday finalizing the capacitors for the power supplies, and the phantom mods, and ordering same.
For you technical types: I will be using three different series of premium quality, low impedance, 105C electrolytic capacitors, depending on value, voltage, case size and function in the circuit.  I will be using Panasonic FM series for the smaller caps, Panasonic FC series for a couple of the 63 Volt caps since the FM series stops at 50V.  The big filter caps are Nichicon HE series which has in addition to the high temperature rating and low impedance, exceptionally long life.  I have ordered caps for all three types of preamp.  The new input filter caps have conservative voltage ratings and will be loafing instead of panting as they do their job.  I will be reducing the 24 Volt main filter cap values from 4400uF to 2000uF total for the 73 and 84 power supplies, and from 4700uF to 2200uF on the 81.  As I explained in an earlier post, that reduces the peak charging currents in the caps, which reduces magnetically induced noise from the power transformer.

In my first modified power supply, an 84, I removed all 12 of the 1N4004 rectifiers, and replaced them with fast soft recovery rectifiers.  The standard recovery 1N4004 rectifiers generate little bursts of RF when they turn off 60 times per second.  The good rectifiers eliminate that problem completely.  I plan to add an RC snubbing network to the PCB that will damp the tranformer ringing when the rectifiers turn off as well.

I used 9 of the original 1N4004 rectifiers that I removed from the power supply board as protection diodes for the regulators. They were easily tacked on in all the right places on the solder side of the PCB.

The phantom supply from the factory uses a 51 Volt zener diode as a reference.  This means that if the Zener is right on the money, the phantom regulator has 50.4 Volts output.  Diode tolerance is 5%, so it can be as low as 47.9V and as high as 53 Volts.  My modified phantom circuit works best with 52 Volts, so my plan is to replace the single zener with 2 series zeners, a 20V and a 33V.  I have hundreds of each, so I can select a pair that yields 52.6 Volts, which will give 52 Volts from the regulator.  The kits will have the selected pair of Zeners.

It is really important that the 6K81 phantom feed resistors be well matched.  When I match them by hand I usually select them to be within a couple of Ohms of each other. Almost everybody I know uses 1/4 watt or RN55 sized 6K81 resistors for Phantom.  I did the power calculation for the resistors and it turns out that the worst case power dissipation (shorted) is  0.397 watts!  With the most power-hungry microphone I know, (Scheops 12V), the power that must be dissipated by the resistors is just over 1/4 watt.  It is never a good idea to let metal film resistors run that toasty, so I will be using 1/2 watt sized resistors when I replace the phantom feed resistors in the preamps.
I have two choices here, buy a bunch of cheap 1% resistors and select them myself into matched pairs, or spend more on the parts and buy 0.1% tolerance resistors that I don't have to select. Because of the extra labor for selecting the pairs, the matched pair of resistors will end up being $1.50 in the upgrade kit of parts.  For the same money, I can purchase PRP GP1/2 25PPM 6.81K 0.1% resistors and not bother with all the matching of lesser parts.  These PRP resistors have copper leads, non-ferrous end caps and all the things that make a resistor sound good.  Their close cousins the PR9372 Audio metal film resistors are probably the best sounding metal film resistors available, better than Roederstein and many others.  The phantom resistors are in series with each other and provide a parallel load to the microphone across the input tranformer primary. Its effect as a load is about 20 dB less than the much lower input impedance of the transformer. It may seem really silly to put such a high quality resistor in such a low cost preamp, but I think in this case it ends up giving higher quality for about the same money as a selected matched pair of cheap resistors.  At this point, I am about 80% convinced that using the PRP resistors is the better way.

I have been offering the fast soft recovery rectifiers as an extra cost option ($1 per rectifier. $12 for a 73/84 and $16 for the 81) to those who have been sending in their preamps. (The Diotech-USA rectifiers that I use have been discontinued, and are unobtainium except for my remaining stock).  I  have to supply 9 or 13 1N4005 regulator protection rectifiers anyway at about $0.25 per rectifier, so it may make sense for almost everyone to get the good ones and use the ones they take out to do the regulator protection.

Thoughts on these two choices of components for the Preamp mods?

 

[clintrubber]

Thoughts on these two choices of components for the Preamp mods?

Not specifically on those, but I must admit something else came to mind: reading all this good & valuable info
(thanks !) I started to realize (or at least I got the impression) that this goes well beyond tackling 'the Chinese flaws'.

I guess that most people want to go all the way once upgrading these beasts, but as a what-does-what it might be
interesting/good to roughly sort the upgrades on a scale ranging from say
'absolutely essential' to
'soundwise lifting it somewhat towards the original' to
(entering the realm of diminishing returns here) 'nice to have for technical reasons' to
'maximum overengineering but hey why not while at it ?' 

Please note this is not meant as criticism, but as a discussion item on how far to go with the technically refreshing on a 'character-preamp'. In all respect I somewhat felt that some of them might be on the path of making these circuits fully blameless (in D.Self-speak). Since these are 'character preamps' I figure we probably shouldn't make them too technically perfect, I assume at least part of the charm comes from ingredients that are technically probably pretty bad but still sound good. I mean, for a totally transparent preamp I know quicker ways to buy or solder one 

Best regards (& again, please don't feel offended or anything since I'm not after criticising your good work in any way,
it's much respected),

  Peter

 

[tommypiper]

Steve, many thanks, as others have said. 

Why not do something like we did for the Poor Man's 660, which was adopted by Analag and apparently is a real success.  He was concerned about his design becoming a Chinese product.  So I/we suggested in the thread that he withhold the schematic and include it only with the paid PCBs when shipped.  This has worked great.  No one has spilled the beans or published the schematic.  DIY and troubleshooting discussions haven't revealed enough for a clone maker to rob and duplicate.  On the other hand, no one can ever safely keep things under a lid when selling anything these days.  But at least this method keeps the design off the web. 

In your case, you could offer something like three services:
1) send in the box to you for repair, with perhaps various repair options.
2) purchase a full bag of PSU and parts for a full repair, DIY, with full documentation.
3) purchase a partial bag of parts for those who want to DIY or source some parts themselves, or use external PSU, but still with full documentation for all the fixes.
4) full documentation only, for a nominal small fee, or free, to members only.

You get paid.  You get to share all your fixes.  Keeps your fixes off the web.  You offer a cheap way to share all your fixes for those who want to tread lightly on purchasing parts (option 3) and consider option 4 (or as described below, perhaps free for members).

Finally, to complete the full spirit of DIY, you could also allow full documentation for members here via email, who can't afford or don't want to buy parts.  This would be important because it would reduce/eliminate the need for troubleshooting questions like, "what's the fix Steve said for x?" Therefore, troubleshooting discussion would be limited to implementation, and not design or drawings.  This keeps the noise down and makes it hard for interlopers to understand the whole picture.

Just my 2 cents.  It's a way to charge some coin for your time and engineering and your parts, while still also sharing in the DIY spirit.  And keeping it all somewhat discreet. 

EDIT: On the other hand, I can imagine Chance getting a copy of your fixes, and rightly so, since he owns some of these, and then including that list next time he goes to China for a new group order.  Once it's released to the Chinese factory, it's gone.  So, maybe my idea is silly.  Maybe it's silly to try and control this information...  :-[

I personally would like to get my hands on your soft recover rectifier diodes...  The PRP resistors seem fine.  I really appreciate the attention to the PSU, it's often overlooked.

 

[rodabod]

I wouldn't trust anyone outside of this forum to keep any documented fixes to themselves to be honest. The 660 is a great example of how that has worked here though. One look at some of the other forums discussing the topic of these preamps, and I'd feel pretty doubtful.

I reckon like you suggest, Steve could get a decent bit of work from servicing these units himself. It would be quite sad to see his efforts stolen for future designs at someone else's benefit.

I reckon a lot of people with next-to-no understanding of electronics might be annoyed at the suggestion of not giving instructions for free, but they have to understand the effort involved and the years of work required to be able to do this; something which someone has worked for, and now provides their main income.

 

[Steve Hogan]

Not specifically on those, but I must admit something else came to mind: reading all this good & valuable info
(thanks !) I started to realize (or at least I got the impression) that this goes well beyond tackling 'the Chinese flaws'.

I guess that most people want to go all the way once upgrading these beasts, but as a what-does-what it might be
interesting/good to roughly sort the upgrades on a scale ranging from say
'absolutely essential' to
'soundwise lifting it somewhat towards the original' to
(entering the realm of diminishing returns here) 'nice to have for technical reasons' to
'maximum overengineering but hey why not while at it ?' 

Please note this is not meant as criticism, but as a discussion item on how far to go with the technically refreshing on a 'character-preamp'. In all respect I somewhat felt that some of them might be on the path of making these circuits fully blameless (in D.Self-speak). Since these are 'character preamps' I figure we probably shouldn't make them too technically perfect, I assume at least part of the charm comes from ingredients that are technically probably pretty bad but still sound good. I mean, for a totally transparent preamp I know quicker ways to buy or solder one 

You bring up a very important point. I understand completely that it is not desirable to re-engineer or modify these units to "fix" all their circuitry shortcomings.  To do so would eliminate the very coloration that they were purchased to impart to their owner's recordings. 

Let me comment on the mods that I plan for these "character" preamps. The fixes that I have thus far proposed make little or no change to the desirable colorations, but should comprehensively fix the problems that are just problematic and compromise the intended sound quality.

1. Neve modules don't come with power supplies.  Since most of the actual audio circuitry in these units is copied directly from the Neve schematics, there aren't too many mistakes there.  Replacing the power supply tranformer lowers the stray magnetic field that was inducing unacceptable hum levels into the EQ circuits (fix).

2. The caps in the power supply had wrong voltages and or values and no protection diodes. These should have been there from the start, and the over voltage on the filter caps was likely to cause early failure of the power supply (fix).

3. Phantom supply -- Chinese added 1K resistor in series with the two 6K8 feed resistors.  Neve has only the 6.8K resistors.
Lack of balance in the phantom resistors can cause low-frequency THD and saturation of the microphone output tranformers. The 1K resistor can cause the mic to starve for power due to excessive voltage drop across the resistor.
Grounding the off position for the phantom switch discharges the phantom power, otherwise the cap stays charged and discharges when you plug in the next microphone.  Off isn't really off on these preamps. (mostly fix -- otherwise preventing the preamp from degrading the microphone's performance)

3. Gain switch -- speaks for itself -- not a problem in the Neve, but they didn't copy that part correctly.

4. New transistors in the 81: the need is for transistors that more closely match the characteristics of the original. The transistors that I have on order are very close in electrical characteristics to the Neve parts, but they fit in the Chinese circuit board correctly. (fix to eliminate oscillation and to more closely match Neve)

5. This is a new one,  The Neve circuit uses an AA144 Germanium diode as a Baker clamp for Q4 in the BA338 amplifiers.
This is the same Q4 that must be replaced in the ACMP preamps.  The Chinese, having no clue what an AA144 diode is, nor what it does in the circuit, substituted a 1N4148 silicon diode, which cannot not clamp Q4 because its forward voltage drop is about 0.7 Volts instead of about 0.2 Volts on the Germanium diode. The Baker clamp prevents Q4 from saturating when the amplifier overloads (clips) in the positive direction. If Q4 saturates, it "sticks" to the rail and takes a long time to recover from clipping (sounds terrible).  The Baker clamp helps the amplifier to overload gracefully and come out of clipping immediately. The Chinese circuit won't work like the Neve.  I plan to replace that diode with a Schottky diode that has a similar low forward voltage drop to the original AA144 which is pretty much unobtanium.  The diode only works when you clip the amp, but now it will act the same as the original circuit. (fix)  BTW  the substitution of that diode may be partly responsible for the oscillation problem -- I don't know for sure yet though.

6. Polarity checking and correction (fix).

7. Premium rectifiers in the power supply are probably overkill, but they just make the power supply quieter. I can't see how it hurts.

8.  When the time comes to shield/rework the inductors, my goal would be to measure some original Neve parts and duplicate them in terms of Value and DCR (which affects the Q of the Equalizer circuitry).  The shielding, if it proves necessary after the power supply mods, just gets rid of hum -- can't see that as a change in the "character".

From your point of discussion -- absolutely no offense taken.  I am in complete agreement with your caution not to "fix" out the color.  In many of the mods I do to vintage gear, the goal is to fix the nasty, noisy stuff, without changing the character that made that piece of gear prized in the first place.

 

[recordinghacks]

I don't want to reopen the business discussion here, but I wanted to propose a solution to this scenario,
which seems to sit at the heart of many folks' fear about publishing details of circuit fixes for the ACMPs:

...I can imagine Chance getting a copy of your fixes, and rightly so, since he owns some of these, and then
including that list next time he goes to China for a new group order.

The simple answer to that is to impose a licensing fee for TnC. I don't believe the TnC guys would forward
this group's solutions to the factory if they understood that they hadn't been given permission to do so. That said,
if Steve and TnC could negotiate a licensing fee, so much the better -- the product is improved, and Steve gets
compensated for his considerable time and expertise.

That said, even if the factory does get an illicit copy of Steve's circuit fixes, can they source all the right parts?
Can they reproduce the high-end power transformer? Not as far as we've seen. Case in point: having Neve schematics
in hand sure didn't generate a functional Neve preamp.

I'm if favor keeping the details off the public websites, as was suggested above, but it seems to me that the best
protection is not secrecy, but rather a negotiated license.

 

[clintrubber]

From your point of discussion -- absolutely no offense taken.  I am in complete agreement with your caution not to "fix" out the color.  In many of the mods I do to vintage gear, the goal is to fix the nasty, noisy stuff, without changing the character that made that piece of gear prized in the first place.

Hello Steve,

Thanks for taking my words as they were intended and thanks as well for the added info, indeed all fixes without touching the character one would say.

The three cap types you mentioned remain as the thing I'm wondering about, unless these were meant for the supply only, and not for the AC-coupling ones, for which one might or not might want to use those (often dreaded) tants, at least already 'for authenticity'.

Thanks & regards,

  Peter

 

[clintrubber]

I reckon a lot of people with next-to-no understanding of electronics might be annoyed at the suggestion of not giving instructions for free, but they have to understand the effort involved and the years of work required to be able to do this; something which someone has worked for, and now provides their main income.

Hi,

I'm not sure this would still work. I mean, I don't want to give ill-meaning people an idea, but isn't the relevant info already out there ? (= here in this thread)

Various people* already contributed and then Steve joined and gave it an additional & completer 'scientific touch' and the result is a collection of experimental & common sense & 'experienced' info that would someone who's at least a bit skilled in the art enable to transform his/her preamps into 'cured' versions.

The only thing that I still could see working is to close this thread in some way & arrange access for 'legitimate people' (...) only. Not sure if we would & could go that path....

Bye,

  Peter 

*: the lucky b&st*rds that got their preamps already