[ACMP investiupgradifications] All things PREAMP

[okgb]

not quite so lucky peter
but hope you get your's soon

 

[crazydoc]

Re: gain switch. Since so far there doesn't seem to be an optimal solution that keeps all the gain settings, I've decided to mount a NC SPST microswitch, pushbutton or toggle, next to the gain knob on the face of the pre (hopefully I can find the space there.) This will connect the two sides of the trace I've cut at position 6 of the gain switch, leaving this gain setting intact. When rotating the switch through it's settings, I'll toggle this switch to break the connection, thus preventing the oscillation. Of course, it only matters when the switch passes from position 6 to 7, but I'll just have to get into the habit to doing every time I change gain. Not a good solution functionally, but I'll get that setting back.

It would be nice to mechanically couple this action, but I can't come up with a simple, cheap way to do it.

Does anyone know if, taking the switch apart, there's a way to modify it to make that position break-before-make? I don't feel like going to all that trouble to find out.

For reference, see my post # 440   http://www.groupdiy.com/index.php?topic=27791.440

 

[tarnationsauce]

Re: gain switch. Since so far there doesn't seem to be an optimal solution that keeps all the gain settings, I've decided to mount a NC SPST microswitch, pushbutton or toggle, next to the gain knob on the face of the pre (hopefully I can find the space there.) This will connect the two sides of the trace I've cut at position 6 of the gain switch, leaving this gain setting intact. When rotating the switch through it's settings, I'll toggle this switch to break the connection, thus preventing the oscillation. Of course, it only matters when the switch passes from position 6 to 7, but I'll just have to get into the habit to doing every time I change gain. Not a good solution functionally, but I'll get that setting back.

You can simply hit the mic/line button. The line input does not use the 2nd gain stage.

It would be nice to mechanically couple this action, but I can't come up with a simple, cheap way to do it.

Does anyone know if, taking the switch apart, there's a way to modify it to make that position break-before-make? I don't feel like going to all that trouble to find out.

For reference, see my post # 440   http://www.groupdiy.com/index.php?topic=27791.440

Firstly, I don't think you would want that witch be break-before-make. I am pretty sure there is DC running through the switch and if you used break-before-make you would get a pop at every position.
I took apart my switch. There is a possibility of modifying it to add an extra position. You would have to grind a bit of that wheel off.

 

[Steve Hogan]

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'.

All but one of the caps in the Bill of Materials for the "kit'o'parts" are used to re-populate the power supply PCB with different caps. As a matter of fact, every electrolytic on the power supply board gets replaced for one reason or another.  Collectively, along with the replacement low-noise power transformer, they provide a robust, reliable power supply with lower ripple, noise, and significantly more "reserve" due to bigger output caps on the regulators than the original supply.

  The remaining Panasonic FC 220uF/63V electrolytic replaces the 100uF/63V electrolytic in the Phantom power local filter on the mic preamp card.  In addition to reducing the 1K to 200 Ohms, and making the 6K81 feed resistors matched, this completes the beefing up of the phantom mic powering circuitry.  This cap is a DC filter that is in common mode with the microphone signal, so it really has close to Zero effect on the sound quality except for final filtering of ripple and noise from the microphone's power.

  I understand that the original Neve preamps used Tantalum caps for signal coupling from stage to stage. Tantalum caps are a mixed bag.  When used with DC bias as in the Neve circuitry which runs on a single 24V rail, they work OK.  My understanding is that there is a built-in diode in a tantalum cap that causes excess distortion if it is not biased on.  Tantalum coupling caps in audio circuits that run on Bipolar supplies which have little or no DC bias  aren't that pretty sounding.  I have no plans to offer tantalum replacements at the present time for the audio coupling caps.

I am thinking at the present time of handling parts kit orders for DIY types and/or people who live too far away to ship me their preamps for a reasonable cost, in the following way. I am thinking of charging some amount for the instruction booklet, which must be purchased with every order.  If you have multiple preamps and you are ordering multiple kits, you only have to buy one instruction/documentation booklet --  (sorry, please no group buys for multiple owners.) The price of the booklet will cover both the publication costs and some compensation for my IP.

 

[Steve Hogan]

Re: gain switch. Since so far there doesn't seem to be an optimal solution that keeps all the gain settings . . .

Does anyone know if, taking the switch apart, there's a way to modify it to make that position break-before-make?

Whether or not a switch is "shorting" or "non-shorting" in rotary switch lingo -- aka "make-before-break" or "break- before-make" is determined by the width of the rotor that moves through the contacts that are at fixed intervals of rotation.  A fat tab on the rotor stays connected to the previous contact position as it passes on to the next. (shorting) and a skinny tab on the rotor leaves the previous contact completely before making contact to the next (non-shorting).
To make a rotary switch non-shorting on only one position would require that the contacts, and not the rotor be different widths.  It's possible but I don't know of a switch manufacturer who has ever designed such a switch.
That's why the Neve has the "OFF" position in the middle of the range.

I think the best solution on the present run of ACMP preamps is to make one position an "OFF" position, just like the Neve.  You end up with one less gain click, but one can spread out the remaining resistor values in a manner, yet to be determined, to prevent a big gain gap and distribute the loss that way.  I hope to figure this out very soon.  For my mods, that will have to do, because the labor involved in adding a switch would be astronomical if you are not DIY.

I have taken the plunge -- the upgrade parts kits will contain two PRP GP1/2 6.81k 25PPM 0.1% Phantom feed resistors. I will be using the same resistors in the next revision of my Jensen Twin Servo preamps. They will arrive next week, along with my remaining capacitors.
The Q4, Q5 resistors arrived today, so we will soon know if they are a good fix to the '81 EQ oscillation problem.

More Later . . .

 

[maxwall]

Man, o man

This is some of the best reading I have come across next to Kevins ( KHStudio ) pultec inductor thread. Just great reading and knowledge here. You cant get this in any book I have ever read.

Steve,
I have read thru everyones postings and I have never seen a more sensible solution. As long as it remains a option to DIY fix and at reasonable cost , I fully support this. The information leak is a complex one and I have no easy answers. But something tells me that you already know how to deal with this and it should work out just fine.

I have several 81's and I looked at them with contempt every day before you came here and started helping out. Its nice to know some one actually cares about audio like myself and many other members around here. My thanks goes out .

Wish I had more Engineering chops , but that comes with experience and not just reading books on electronic circuit behavior which happens to be my sole resource next to this forum.

I have built Neve preamps already with second hand components and my own etched pcb boards more successfully and I cannot figure out how something like this can so easily have been screwd up. just annoyes me how this stuff can be produced with no oversight , accountability or care.

why build a worldclass circuit to have it sound like crap = pointless.

 

[crazydoc]

Thanks for your replies regarding the gain switch.

Tarnation - I know you can bypass the audible noise from the switch by using the line position. However, the first preamp stage will still oscillate and produce the signal when the switch goes through this position - it's just not being passed through to subsequent stages. I doubt if this is doing damage to anything, but it's still there and I have to remember to be in line mode when switching through it.

It's like, if a tree falls in a forest and there's nobody there to hear it, does it still make a sound? In my world it does.

Steve - I know the gain switch problem is relatively unimportant compared to most of the others, but something in my mind desires order. My kids are somewhat OCD and I think I've inherited this from them. Unless the entire resistor ladder is reworked, there will not be equal gain steps between switch positions if one position is off (not too hard to do, I know, and not really a big deal anyway.  ) But I would like to keep position 6 particularly (which I lose when I cut the trace) because this passes the cleanest signal, directly from the mic transformer to the input of preamp stage 2. No 1st stage preamp noise, and no resistors in the path (which probably don't produce any audible signal degradation anyway.)

A switch between the cut traces is still the best solution for me. The problem lies in remembering to open and close this switch at the appropriate time. I'm now thinking of a simple SPST toggle switch whose toggle lies against the gain switch knob when it is closed (to activate position 6), making it difficult to rotate the knob in this position, or at least jogging the memory to throw it in the other direction (opening the trace) before changing the gain.

This is just for my neurotic DIY purposes - I'm not suggesting it as an option for any commercial kit or mod.

 

[clintrubber]

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'.

All but one of the caps in the Bill of Materials for the "kit'o'parts" are used to re-populate the power supply PCB with different caps. As a matter of fact, every electrolytic on the power supply board gets replaced for one reason or another.  Collectively, along with the replacement low-noise power transformer, they provide a robust, reliable power supply with lower ripple, noise, and significantly more "reserve" due to bigger output caps on the regulators than the original supply.

  The remaining Panasonic FC 220uF/63V electrolytic replaces the 100uF/63V electrolytic in the Phantom power local filter on the mic preamp card.  In addition to reducing the 1K to 200 Ohms, and making the 6K81 feed resistors matched, this completes the beefing up of the phantom mic powering circuitry.  This cap is a DC filter that is in common mode with the microphone signal, so it really has close to Zero effect on the sound quality except for final filtering of ripple and noise from the microphone's power.

...

I have no plans to offer tantalum replacements at the present time for the audio coupling caps.

Hello Steve,

Thanks again, it confirms my assumption of cap-replacements only 'outside' the signal path (yeah, uhh, just like DC-servo caps... well, you'll understand how it's meant  )
I'm with you, after the replacement of supply-caps the tants might be a later step, better first use & enjoy it as is & then later trying signal-path alternatives... or simply continue making music with a 'now fixed' preamp.

Have a good weekend,

  Peter 

 

[mitsos]

Hi Steve,

Thanks for all the effort you've put in so far, and for keeping this more or less open source.

I have two burning questions..
1. Have you tried any of the PSU mods (trafo and/or cap/diode changes) in the circuit yet? Any observations?

2. Can I use MUR860 diodes (supposedly fast/soft recovery) instead of the unobtainable ones you are using (have a small stash of them and no immediate use)..

thanks again!

 

[Steve Hogan]

Tarnation - I know you can bypass the audible noise from the switch by using the line position. However, the first preamp stage will still oscillate and produce the signal when the switch goes through this position - it's just not being passed through to subsequent stages. I doubt if this is doing damage to anything, but it's still there and I have to remember to be in line mode when switching through it....

Unless the entire resistor ladder is reworked, there will not be equal gain steps between switch positions if one position is off (not too hard to do, I know, and not really a big deal anyway.  ) But I would like to keep position 6 particularly (which I lose when I cut the trace) because this passes the cleanest signal, directly from the mic transformer to the input of preamp stage 2. No 1st stage preamp noise, and no resistors in the path (which probably don't produce any audible signal degradation anyway.) ...

This is just for my neurotic DIY purposes - I'm not suggesting it as an option for any commercial kit or mod.

I haven't tackled the gain switch yet, but two points come to mind --

1. If having the mic/line switch doesn't fix the connection between input and output, then some parts are going to eventually get damaged. IMHO this oscillation is very harmful and should be completely eliminated under all circumstances.  I hope to eliminate any possibility of a speaker and/or ear destroying pop from this preamp no matter how I fiddle with the switches and knobs.
I always go for goof-proof in my designs.

2. Perhaps it would be possible to have postition 7 be the position with the straight thru connection, and then spread out the other steps with resistor changes.  (Using good resistors here won't hurt if you have to change them anyway).

Sorry to take so long to get answers, but I at the same time I am working on these mods, I must be working on client's gear.

 

[Steve Hogan]

I have two burning questions..
1. Have you tried any of the PSU mods (trafo and/or cap/diode changes) in the circuit yet? Any observations?

2. Can I use MUR860 diodes (supposedly fast/soft recovery) instead of the unobtainable ones you are using (have a small stash of them and no immediate use)..

Answer to 1:
I have my first 84 Power Supply PCB completely modded now, except that I may still remove the 0.1uF ceramic caps, especially the ones on the phantom supply.  The only marks on them are "104" which stands for 0.1uF, but the voltage rating and the ceramic formulation is a mystery, but they are really unlikely to be anything better than the cheapest. Their presence in the power supply is of dubious value. Some regulators really don't want too low an esr on their Input and Output terminals.  The Low Z electrolytics I am using really negate the need for them.  If they are 50 volt parts, then they are not a good thing for the phantom supply. I am also still considering a couple of minor tweaks to the Phantom Supply regarding startup current limiting to protect the pass transistor which has to work harder to charge up the bigger output cap.

I also have an unmodified 73 PCB as well.

To measure the difference between the original circuit and the modified phantom supply circuit I simply capacitively coupled the output of the supply into my AA5001 THD analyzer reading Level dBm with true RMS rectifier, unweighted, unfiltered.  The original circuit measured about -58dBu with distinct 120 Hz ripple and really low frequency bouncing as the power line amplitude changed.  Always jumping, jumping.  With the bigger filters, and the current regulator diode in place of the 2K zener feed resistor, the output had NO discernable ripple and measured -89dB.  Interestingly the low frequency "jumping" perturbations were completely gone as well. Needless to say, I was pretty happy with the results.  That's a 30 dB reduction in hum and noise for about $5 worth of parts.
I hope to publish scope photos soon.

You might ask, why work so hard on the Phantom power supply?  The problem as I see it with ANY discernable ripple and noise on phantom is that most microphones have really crappy regulation – some none at all.  Their circuitry is single ended and they usually have crappy CMRR.  This low frequency bounce looks like it may be audible or at least intermodulate with audio if it gets into the microphone circuitry.  I may be wrong about all this, but I figure if the phantom is really clean, it cannot hurt..

Still no overall report yet,  I am still documenting the improved regulator performance before I reassemble the supply into the preamp and see where we are at.  I have wired up the power transformer to the voltage selector switch and Molexed the secondaries.  It fits in the same location as the original using different hardware 1/4-20 x 1.5" Button-head capscrew with a low-profile Nylock nut. It barely fits heightwise, but it fits.  I need to come up with an insulator on the lid. Maybe just some tough tape, so that if you push down on the lid and make contact with the screw that holds the transformer, you don't end up with a shorted turn (bad). The lid clears the screw, but not by much, and in keeping with "goof-proof" design philosophy, I would rather add an ounce of prevention to prevent a pound of problems that need a cure.  It's easy to press down on the lid inadvertantly if you place something, even your elbow, on top of the preamp.

Answer to 2:  The MUR860 rectifiers are fine.  The only down side is mechanical, in that it is very difficult to cleanly replace an axial leaded rectifier with a TO-220. The metal tab is "hot" and connected to the cathode, so don't let the tabs touch each other.

More Later

 

[tommypiper]

...I need to come up with an insulator on the lid. Maybe just some tough tape, so that if you push down on the lid and make contact with the screw that holds the transformer, you don't end up with a shorted turn (bad). ....

Uh, like, just use a nylon bolt for the power transformer.  Done.

 

[Steve Hogan]

...I need to come up with an insulator on the lid. Maybe just some tough tape, so that if you push down on the lid and make contact with the screw that holds the transformer, you don't end up with a shorted turn (bad). ....

Uh, like, just use a nylon bolt for the power transformer.  Done.

This is where the mechanical engineer side of me kicks in. Because nylon is non-conductive, it would indeed solve the accidental shorted-turn-by-the-lid-touching-the-threaded-end-of-the-screw problem.  I would not, however, trust mounting a 2 pound chunk of steel and copper to the very modest tensile strength of a single nylon screw. By the time you get the screw tight enough to compress the two rubber washers enough to securely captivate the transformer, the bolt is under considerable tensile strain.  The nut would have to be at least standard thickness, if not heavy instead of the thin nylock that I propose.  As long as the SS screw is blunt and has no sharp edges where the threads were rolled that could potentially cut through it, one or two pieces of tape on the cover will do the job nicely.

 

[crazydoc]

I always go for goof-proof in my designs.

Aye, there's the rub. But I have an idea (always dangerous, those ideas.) I'll see if I get a chance to try it out this weekend.

 

[mitsos]

Steve, thanks for the reply on testing the new PSU... looks promising! Can't wait for the complete write up!

 

[crazydoc]

Aye, there's the rub. But I have an idea (always dangerous, those ideas.) I'll see if I get a chance to try it out this weekend.

Well, here's the idea. And it worked. It switches through positions 5, 6, and 7 without a noise.

I mounted a small SPST switch http://www.radioshack.com/product/index.jsp?productId=3190382 on the gain switch, drilled and tapped a hole in the shaft, and mounted an arm on it (made from a plastic Lysol bottle in my recycle bin) that closes the microswitch when it gets to position 6. The microswitch connects the cut traces of position 6, after the rotary switch breaks contact with either adjacent position.

Here's the switch assembly wired up, and in the preamp.

Not very elegant - just a prototype. The final version should have a mechanism for adjusting the distance from the shaft axis to the microswitch lever, as this is a critical distance for timing the closing and opening of the switch. (And put the protective cover back on the switch.) It's a lot of trouble to go to for very little gain (pun intended), but it was an interesting exercise.

 

[Steve Hogan]

Well, here's the idea. And it worked. It switches through positions 5, 6, and 7 without a noise.

I mounted a small SPST switch  on the gain switch, drilled and tapped a hole in the shaft, and mounted an arm on it (made from a plastic Lysol bottle in my recycle bin) that closes the microswitch when it gets to position 6. The microswitch connects the cut traces of position 6, after the rotary switch breaks contact with either adjacent position. ...
Not very elegant - just a prototype. The final version should have a mechanism for adjusting the distance from the shaft axis to the microswitch lever, as this is a critical distance for timing the closing and opening of the switch. (And put the protective cover back on the switch.) It's a lot of trouble to go to for very little gain (pun intended), but it was an interesting exercise.

That's a really nifty solution.  You have a lot of mechanical ingenuity, which is really evident in this clever solution.

I began to investigate the gain switch mods today by realizing that the recently published schematic of the ACMP 73 doesn't match reality -- many of the resistor values are wrong.
I typically use a spreadsheet to work out resistor values when tightening up voltage dividers for meters and things like the gain switch.  I have already tidied up a  set of optimum values for the 3 dB steps in the Line in  attenuatuator. The values in the gain switch, however, made no sense at all.  Their calculated impedance and loss did not match the  measured mic gain steps of 6 dB +/- about .5 dB.  It appears that there is a complex loading effect of the 2nd stage amplifier's input impedance on the gain switch resistor values.

Not really being familiar with how the Neve input circuitry was realized, I printed out and studied up on the Neve circuitry.  The adaptation of the 11 position switch with 6 dB steps from the 22 position switch with 5 dB steps and 2 off positions was perhaps not done as cleverly as one could have.  My initial concept of a fix would be to create an off position between steps when in mic and perhaps spread the whole microphone gain range of 60 dB into nine 6.67 dB steps instead of ten 6 dB steps. If that proves unworkable, then I am pretty sure that I can at least spread out the high gain steps.  The weird loading on the attenuator network makes the calculation of these values difficult, and in addition, it is important to not change the total load seen by the transformers, or the sound will change as well as the gain.  A lot of stuff has to be figured out simultaneously to do these networks correctly.  I did notice in the particular '81 unit that I am working on, that some resistors had their coating damaged (wrinkled up) by whatever solvent they used to clean the PCB.  These resistors had endcaps and body showing through and MUST be replaced.  Any metal film resistor that is open to moisture will be ruined almost immediately due to the thin, deposited nichrome film corroding away.  Looks like only certain values were affected -- must have bought the coating for that lot of resistors from Uncle Chan, who got it half-price from Mr Lee, who got it . . .

More Later

 

[crazydoc]

I did notice in the particular '81 unit that I am working on, that some resistors had their coating damaged (wrinkled up) by whatever solvent they used to clean the PCB.  These resistors had endcaps and body showing through and MUST be replaced.  Any metal film resistor that is open to moisture will be ruined almost immediately due to the thin, deposited nichrome film corroding away.  Looks like only certain values were affected -- must have bought the coating for that lot of resistors from Uncle Chan, who got it half-price from Mr Lee, who got it . . .

I noticed the same thing - a few resistors have their coating almost dissolved off - and the adjacent ones seem to be fine. Would a glop of clear nail polish protect them without replacing the resistors?

And thanks for all your input in this.

 

[Paul G]

These resistors had endcaps and body showing through and MUST be replaced.  Any metal film resistor that is open to moisture will be ruined almost immediately due to the thin, deposited nichrome film corroding away.  Looks like only certain values were affected -- must have bought the coating for that lot of resistors from Uncle Chan, who got it half-price from Mr Lee, who got it . . .

More Later

And the plot thickens.....

Thanks for all the hard work, Steve and everyone else.

Paul ;D

 

[Steve Hogan]

I noticed the same thing - a few resistors have their coating almost dissolved off - and the adjacent ones seem to be fine. Would a glop of clear nail polish protect them without replacing the resistors?
And thanks for all your input in this.

I recommend replacement in the strongest possible terms. Metal film resistors are dependent on their coating to keep moisture from attacking the deposited metal film which is very, very thin.  The slightest bit of moisture with a voltage across the resistor will corrode the film and cause it to open up/change value/be ruined.  For more information see http://www.irctt.com/tanfilm/index.aspx

There are two reasons that I have no use for the old black Holco resistors. First of all, they were longer than every other brand and didn't fit in the circuit board worth a darn.  Secondly, the black laquer coating they used formed a huge miniscus to the lead and if you bent the lead too close to the body (like trying to fit it into a PCB) the coating would crack, breaking the moisture barrier, and leading to failure.  One of the nice things about tantalum nitride resistors, is that they have a natural oxide coating that passivates the resistance material and prevents moisture from harming it.  Like the bridges in Michigan that are made out of a special steel that forms a rusty coating that seals out moisture and prevents the distructive type of rust without periodic (perpetual) need for paint.  Metal film resistors are extraordinarily reliable when their coating is intact, but if not, all bets are off.  The film will be attacked in minutes.  The coating on some of these particular resistors looks like paint that's been hit by paint remover and is wrinkled and ready to be scraped off.  As far as nail polish is concerned, the ship has already sailed.  Metal films are cheap.  Make a list of the bad ones and let's get them replaced.  Heck, it may be just the excuse to tidy up the tolerances and values.