[ACMP investiupgradifications] All things PREAMP

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This post describes the latest updates in developing the modifications/fixes for the ACMP preamps.

1. Gain Switch mods:
I modeled the input stage used in the 73 and the 84 in LT Spice to be able to determine the input impedance of the first and second stage.  It is not very high, and it changes depending on the gain.  That means that the second stage, which has +28dB of fixed gain in Mic mode and +18 dB of fixed gain in Line mode has a different input Z depending on whether it is in mic or line mode.  I added much more complete formulas to determine the proper resistors for the gain switch incorporating the input Z.  The schematic for the ACMP 81 has another mistake.  The load on the line transformer is shown as 6K8 but it is actually 2K2 as in the other schematics and the Neve.

2. Phantom Mods:
The TNC directions instruct you to turn off the phantom power berfore patching a mic.  Good advice, but without the 48V switch grounding the filter cap thru the feed resistor, the phantom stays up until externally discharged. It turns out that the ground is not carried through from the phantom power supply to the preamp card.  They use a 2 conductor patch cord, but only the +52 is wired.  The 48V ground ends at the preamp card.  The original 100uF/63V phantom filter cap (soon to be 220uF/63V) is grounded to the chassis by the adjacent screw/pad. I am considering changing that connection and using the 2-conductor cable to return the bottom of the filter. I'm not crazy about using the chassis as a ground wire.

3. New power supply transformer/power supply mods:
I determined the required RC network to properly damp the resonance of the new power transformer.  The fast, soft recovery rectifiers that I have added to the power supply do not generate bursts of RF oscillation when they turn off. However, because the transformer is loaded (draws current) only at the peaks of the AC waveform and is unloaded during the transition from positive peak to negative peak, when another rectifier turns on and current is drawn from the transformer winding again. The current from the transformer is drawn in short bursts, and then abruptly stopped.
The stored energy in the transformer causes the ringing.  If you add a series RC network across one of the secondaries, the ringing can be almost eliminated, removing one more source of unwanted spurious garbage from the power supply and simultaneously preventing it from negatively affecting the audio.  The scope photos are instructive:

48Voltwinding10kHzundamped.jpg

The above photo is the 48 Volt winding of the new transformer when excited by a 10 kHz square wave into the primary. In normal operation the winding is passing sine waves, not square waves, but when the rectifiers turn off the same ringing is excited due to the sudden discontinuation of the current flow.

48Voltwinding10KHzdamped.jpg


After determining the proper RC network and wiring it across the secondary, the result is pretty dramatic.

48Voltwinding1kHzundamped.jpg


It's a lot harder to see the HF ringing at lower frequencies -- This is 1KHz undamped.

48Voltwinding1kHzdamped.jpg


You must look at lower frequencies to check your network, however.  Sometimes things look great at 10kHz but lower frequencies have a big peak that was not seen at the higher frequencies. This network works well at all frequencies and is the one that I will add to the Power supply PCB.  Interestingly, you really don't have to damp each of the windings individually.  By damping the 48 Volt winding, in this case, all the other windings behave as well.  The transformer just has to have some way to dump the stored energy and one winding is sufficient to do the job.  It makes it a lot less hassle, since the 73 and 84 have three windings and the 81 has four.  Damping the 48V winding does it for all of them.  I found a nice place for the RC network on the PCB to make it a clean mod. BTW all power transformers need this network. Some power supply manufacturers add capacitors across the diodes and/or capacitors across the secondary with no series resistor.  Although this kills the HF ringing, it causes a very large lower frequency resonance to occur.  On a commercial power supply I recently modded, there was a 0.33uF cap across the secondary and the result was a 20dB peak at 15kHz.  Needless to say, I prefer adding the series resistor.  The caps across the rectifiers, by the way are in series with each other and put a similar capacitive load on the transformer secondary, which makes for an unnecessary peak.    That's why I use the good rectifiers which require no snubbing,  and then I properly damp the power transformer, and all the spurious stuff goes away.

More Later. . .



 
Steve,

In previous posts you make mention of the AA144 Ge Diode and its relative scarcity, and  been doing some followup on that diode part and sadly that part is either very hard to get or very expensive to get for unknown reasons other than they are well out of production and contain gold.


The AA144 / Sub is BAT86 Schottky Diode






 
Hi,

Good question you bring up. I guess the clamp-function is not too depending on
the super-specific properties of that original type, while at it we obviously want to insert a proper one.

Note 1N4148, 1N914 are Si-types

I saw FX-builders-sites have decent prices, 1N277 & 1N34A for EU0,30 each ( www.musikding.de )
while www.banzaieffects.com has a large range of possibly suited types I won't be repeating here.

I'm with you on not wanting to pay those prices for a simple Ge-diode, but since each pre doesn't need too many
and while the lid is off then better make it a replace-&-forget thing, despite it can in total be done a few $ cheaper.
$4.75USD is a sick price though...

Bye,

  Peter

I'm lazy, how many does each ACMP81 preamp need ?


Bye,

  Peter

 
Hi,

Those markups indeed silly & sad.

[OT & just kidding]But hey, I bet those most expensive discrete semiconductor devices live in the nifty boxes of the brand that shall remain nameless but here's a thread about it: http://www.groupdiy.com/index.php?topic=6481.0... I bet the Ge-types on that box come out at several hundreds a piece  ;) [/OT & just kidding]

And wasn't someone selling BC184-somethings recently on eBay, at ehh, $9.99 a piece ?!?

but on the other hand ,  I was made a sucker to buy the ACMP 81.
Hmm, sorry it feels like that, seriously. We knew free lunches are rare & we knew some DIY-ing would most likely
be in order. It now sure looks more 'repair' than most people (incl. me) would have expected, but ain't it still fun ?
What do others think ?

Bye,

  Peter
 
electrochronic said:
Is there another Germaniuim diode suitable for sub-ing the AA144 like a 1N100A, 1N270, 1N276 1N277 , and maybe  these  1N34, 1N55  All DO-7 packages. Silicon diodes as indicated have a higher foward voltage drop unlike germanium diodes which is the desired behavior on this diode.

I have a friend that has repaired literally thousands of Neve modules and the first thing he checks on a broken one is that diode, which is often cracked and broken.  From the information that I have gathered so far, it is a weak sister in the design, that breaks often.  Since we have agreed to "open source" on these mods -- (please buy the parts kits when they come out), I will offer that I would not use a Germanium diode at all since these are fragile.  I plan to use a BAT86 small signal Schottky diode in that location.  It's forward voltage drop is not quite as low as germanium, but it is about half that of a regular silicon diode and most importantly, lower than the Vbe of the transistor that needs the Baker clamp.  That Schottky diode is readily available, relatively rugged, and has a couple of features that make it work really well in this application.  It is very fast, has no reverse recovery issues like regular silicon diodes, and its junction capacitance is more linear than regular diodes, so it contributes less distortion.  This is a clone, so I don't feel obligated to use exactly the same parts, as long as they function in the same way and don't mess with the "character".  The substitution of a Schottky diode for the Germanium in this application should work just fine.

The clamp is used on the Q4 in the 81 that needs replaced, so one needs 4 pcs, one for each BA338 amplifier clone.

While we are on the subject of "sticker shock" I will warn you that the current limiting diode that gives the more than 30 dB reduction in ripple and noise in the phantom power supply regulator costs about $3.00. -- about $0.10 per dB noise reduction.  Each preamp power supply only requires one. I am not aware of any other solution that works so well, with such a simple change, in that application.
 
electrochronic said:
Wow , did'nt realize that problem with the rack space. When does the bad news end with these.

Hopefully once Steve is done with these :)
We all really appreciate all the work you've done on this.

Yeah.. they dont quite fit if you leave no space. As they are now, I dont see how you could use them stacked on top of anything else. The inductors pick up power supplies from other pieces of gear 3 sometimes 4 spaces away. My rack is otherwise quiet. 
 
if no better alternative , a 3.00 diode that you only have to buy once
doesn't seem too bad [ and not if balenced with other reasonable parts ]
i have 4 pres so 12.00 i can handle ,getting to 4 x psu xfmrs is sticker shock
in my economics world i have to ponder whether i should do them a couple at a time

btw good grp citizenship of you contributing to other threads ,thanks
 
Yeah, when does the bad news on these Chance pres end?  Indeed.

I'm even wondering if I should sell mine.  I expected to replace the PSU automatically, and maybe some caps and transistors -- way back when I ordered them, just from looking at the photo.  But now, not only is it the switch and the inductor, and who knows what's next, maybe more resistors and rewiring this and that... I'd rather start from scratch with quality parts.

The comments about seeing the effects of a toxic cleanser on the PCB hitting the resistors and partially stripping them of insulation has me thinking as well.  These are the cheapest, crappiest Chinese products we've seen.  And who knows WHAT kind of solvents they used in there?  Are we endangering our health just by handling and inhaling fumes off these PCBs? 

I was willing to DIY some fixes, now I'm wondering if these belong in a hazardous waste bin.
 
It's true.

Chinese products as a whole really suck. These pres bring that suckiness to a whole new level in terms of QC.
 
tommypiper said:
Yeah, when does the bad news on these Chance pres end?  Indeed.

I'm even wondering if I should sell mine.  I expected to replace the PSU automatically, and maybe some caps and transistors -- way back when I ordered them, just from looking at the photo.  But now, not only is it the switch and the inductor, and who knows what's next, maybe more resistors and rewiring this and that... I'd rather start from scratch with quality parts.

The comments about seeing the effects of a toxic cleanser on the PCB hitting the resistors and partially stripping them of insulation has me thinking as well.  These are the cheapest, crappiest Chinese products we've seen.  And who knows WHAT kind of solvents they used in there?  Are we endangering our health just by handling and inhaling fumes off these PCBs? 

I was willing to DIY some fixes, now I'm wondering if these belong in a hazardous waste bin.
What do you have, Tommy?  I may know of a buyer or two.  Thanks.

Paul ;D
 
hi guys!

I've been reading this long, long, long story since the begining and hoping there's a new GB for that kind of stuff!

but now!!! dude...these things... :-X
I'm really curious about what you paid fot these.

I'd rather start from scratch with quality parts

more expensive but painless, I think.
 
tommypiper said:
The comments about seeing the effects of a toxic cleanser on the PCB hitting the resistors and partially stripping them of insulation has me thinking as well.  These are the cheapest, crappiest Chinese products we've seen.  And who knows WHAT kind of solvents they used in there?  Are we endangering our health just by handling and inhaling fumes off these PCBs? 

I was willing to DIY some fixes, now I'm wondering if these belong in a hazardous waste bin.

Actually, I believe that the problem with the affected resistors is with the resistors, not the cleaning process.  On the same board with the wrinkly finish resistors are all the rest of the resistors that are just fine. I think the coating on only some of the resistors was defective. Whatever residue is left of any solvents they used to clean the board is long gone, and I seriously doubt that any health hazard exists.
 
Maybe those solvents are what caused the rust-colored spots on the chassis under the PS PCB in one of my 81s.  I also have a splattering of some sort of brownish residue on the inside of the top lid in that one.  Haven't opened the other one yet.
 
Hi Steve,

Thanks for your good work. I have only received my 73 a few days ago, and read my way through this. I would expect that there may be many like me, who live on the other side of the world, and would welcome a parts repair kit.

All the best,
aomahana.
 
Steve Hogan said:
I think the coating on only some of the resistors was defective. Whatever residue is left of any solvents they used to clean the board is long gone, and I seriously doubt that any health hazard exists.

Thanks for the reassuring words.  However, could it be that the solvent wasn't carefully applied and hit some resistors and not others?  Doesn't that fit the pattern of the quick and most uncareful production that we see before us in every other aspect?

I keep hearing about the toxic cleaning solvents companies use on their PCBs and in fact it is a health concern, it doesn't go away from the PCB.  The computer industry has been under pressure on these for some time and yet have not changed from chemical wash to water wash PCBs.  Last I heard.  There's also flame retardants, which are very bad.

It makes you wonder in this case because the production seems to have been so totally out of control. 
 
wmtunate said:
Maybe those solvents are what caused the rust-colored spots on the chassis under the PS PCB in one of my 81s.  I also have a splattering of some sort of brownish residue on the inside of the top lid in that one.  Haven't opened the other one yet.
It is much more likely that the brown spots came from the plater who plated the steel prior to painting.  The board would have been cleaned prior to assembly in the chassis.  I noticed that only certain values of resistor are affected, with perfectly intact resistors all around.
 
aomahana said:
Hi Steve,

Thanks for your good work. I have only received my 73 a few days ago, and read my way through this. I would expect that there may be many like me, who live on the other side of the world, and would welcome a parts repair kit.

All the best,
aomahana.

I am planning to make kits of parts available with the new power transformer with mounting hardware, the fast soft-recovery rectifiers, replacement power supply and Phantom caps, a matched set of zeners to trim the Phantom to 52 Volts, a current limiting diode to improve the Phantom supply, new resistors for the 317 regulators, matched 0.1% PRP Phantom feed resistors,  Mounting hardware for the transformer, etc etc. New transistors and schottky diodes for the '81 and whatever else I find out is needed. I have many of the parts already, but it is slow going getting to the bottom of some of the problems and confidently solving them in reliable way.

 
Steve Hogan said:
aomahana said:
Hi Steve,

Thanks for your good work. I have only received my 73 a few days ago, and read my way through this. I would expect that there may be many like me, who live on the other side of the world, and would welcome a parts repair kit.

All the best,
aomahana.

I am planning to make kits of parts available with the new power transformer with mounting hardware, the fast soft-recovery rectifiers, replacement power supply and Phantom caps, a matched set of zeners to trim the Phantom to 52 Volts, a current limiting diode to improve the Phantom supply, new resistors for the 317 regulators, matched 0.1% PRP Phantom feed resistors,  Mounting hardware for the transformer, etc etc. New transistors and schottky diodes for the '81 and whatever else I find out is needed. I have many of the parts already, but it is slow going getting to the bottom of some of the problems and confidently solving them in reliable way.

Steve, thank you for all the hardwork you've put into this.  I will be first in line when the kit is released.  You should be compensated for your solutions.

I have no faith in TnC standing behind these things at all.
 
Here are some pics of the two offending resistors on my 81's preamp board. The other boards and resistors seem to be in pretty good shape, though some of the resistors have a dull rather than shiny surface. It looks like the coating was partially dissolved (and maybe dried with pressurized air?), as some of it is stuck to an adjacent resistor.

Steve - are these the same damaged resistors in your boards? (They have different values - 39 and 120k ohms.)

ACMP-resistor-crud.jpg


ACMP-resistor-crud-detail.jpg
 

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