[ACMP investiupgradifications] All things PREAMP

[mitsos]

Thanks for doing all these tests as well as for the toroid info..

 

[Mylithra]

A post I made on another board:

I had been messing with only 1 of my 73s up to this point. I read in another post where a guy was wiggling connectors and got some of the noise to go away so I thought Id look into it.

I took the cover off the one I had been messing with and starting poking around with it on and humming. When I touched the 1000uf 25v Cap in the back of the mid EQ board, I hear a pop. I took the board out of the chassis and found the soldering job on this cap was horrible. I just had to touch my iron to it and the whole solder joint got sucked up on my iron. I cleaned and re-soldered the part. Beware, there may be some poor soldering jobs on some of these.

I put the cover back on and this time decided to try both at the same time. The one I had not been messing did have a very minor hum (almost un-noticable) but was not humming like the other one. I suspect this will be mostly fixed by Brad's shielding. (which Im still in for BTW) Something else I noticed. I had both units, one on top of the other on my lap. The one that didnt hum all of a sudden started humming when I turned on the power of the 2nd unit. This to me sounded like something 2 fold going on here. The inductors are picking up EMI, but it also sounds like power transformers may not be consistant from unit to unit. My next test when I get some time will be to take the EQ board out of the one that doesnt hum and place it in the one that does to see if it inherits the hum or not and vise versa.

/end old post

I still havent gotten around to that test yet but I willng to bet the EQ board from the one that doesnt hum,will start once its in the other case. I think the power transformer in this case is most likely causing my hum.

 

[0dbfs]

More notes on my 81's:

1.
Line input sounds heavily filtered (no low's, exaggerated hi's) and attenuated. Suspect bad line-in transformer.

2.
This is a comparison of a 400hz square wave from line in connection to the secondary of the xformer (maybe you can imagine what that sounds like?):

3.
The same test using the mic in xformer both looks and sounds much more as expected. Like this:

4.
This is the same for all four 81's that I have.

Thoughts?

Cheers,
jonathan

 

[playboss]

arent those xformers supposed to be the same as chameleon labs?

 

[rodabod]

Does the line-in sound very, very tinny? You can't see any sign of the fundamental there, let alone any much other low frequency content. You could try taking the trafo out of circuit by lifting the secondary.

 

[0dbfs]

Not sure if they are the same as CL's or not. There are no markings on the IP trafo's.

The symptoms with the line in turned out to be wiring related.

Unfortunately the analog outs on the motu 24io and 2408 aren't really balanced + and -.

Cheers,
jonathan

 

[0dbfs]

I added an OFF switch-position to the gain knob in the #7 position where the second gain stage first kicks in. Now switching to the next position (#8) does not cause a pop.

I chose to test it out by replacing a single 680R resistor (R43) with a 1.5k, disconnecting one end of that resistor and jumpering it to the position between R42 and R43.

This pretty much replaces the #7 position sensitivity setting with OFF and keeps the original remaining 4 higher gain settings.

You could do this with a different value (replace R43 closer to 620 ohms) and replace R42, 41, and 40 with different values to make the gain steps more uniform up to the original full gain if you want.

Here are some snaps. Sorry for the bad cell phone pics (wife is out of town with the camera).

Cheers,
jonathan

 

[mitsos]

Nice... Thanks once again.. cheaper than a new switch that's for sure!

When your better half gets back home, I'd love to see some better pics.. If you can do macro that would be best.

 

[0dbfs]

Hey Mitsos,

I'll post some better pics after this weekend.

I'm going to go ahead and finish my remaining units and try to even out the gain steps so that what was position 7-11 is now 7-OFF -and- 8-11 with smoother gain steps leading up to full-open-gain.

Just another couple notes:

1.
These units have spacers/washers in the boards between the chassis screws that appear to orient the 48V and polarity switches into proper alignment for the front panel holes. In other words, after dis-assembling and re-assembling them several times the push-button switches will become "stuck" if you don't get the alignment correct on re-assemble... Making the holes slightly larger might be a solution if you have them apart and don't want to worry too much about the washers.

2.
The issue I had before with the i/p trafo was not due to the motu. I was using an unbalanced TS cable to send line level to the 81..  :-[

3.
It appears that there are currently a handful of mods to get these units in better condition. Best to try and address them in one go if possible. Here's my running list:

• Swap out the EQ board transistors
• Check polarity on the line input and change at the board connector (swap red/white) if needed (all four of mine were reversed polarity @ the line input)
• Remote the original toroid to prevent induced hum in the EQ (or replace with non-hum-inducing toroid?)
• Add OFF setting to mic-input sensitivity switch position #7 and smooth out the gain of the remaining four positions.
• Enlarge the front panel holes for the push-button switches slightly to prevent sticking. Maybe that's a stretch but you might wish you did it after the units are in the rack and you don't feel like taking them out of service and opening them up again.

Cheers,
jonathan

 

[druedger]

Added observation as copied from Black Market:

If anyone has received preamps here, or has read about the hum problems, if you have time, then could you contribute any thoughts you have on fixes?

I just put my unit through its paces last night, and the um is bad.  I'm sure others have seen posts elsewhere about this, but the main issues are that with the EQ engaged there is a nasty ground loop buzz when the notch filter selection is set to 360 Hz.  It seems to be across each frequency selection for that switch.  I also noticed that even the hi pass filter has hum problems as well.  As you continue to move the switch towards 300, the hum gets progressively louder.  The worst problem appears to be a nasty pop that occurs when increasing the gain on the gain switch.  Somewhere around 8 o clock on the dial it pops hard.  It's so loud that it hurt my ears with headphones on.  If it had gone to my monitors, there's a good chance it could have damaged the speakers.  It's that bad.  So keep that in mind when you get your units.

Does anyone have a schematic for this?  When I got the unit I pulled the cover and noticed that there is no star grounding scheme for all the card connections internally.  I'm thinking of redoing the internal grounding, but I'd love to see a schematic so I can do it properly.  I'm sure I can suss it out just by tracing wires and such, but a schematic would make it much easier and would save some time.

Chance, is there any way you can make schematics available for these units?  I hear the ACMP-81 and ACMP-84 units have the same hum problem.

BTW, question for schematic already answered.

Perhaps I should have elaborated here.  I need the FULL schematic, not just the signal path of the 73.  The 81 power schematic is significantly different than the 73 board, and in particular I need to know what the 9CZ7 connector is providing.  I think it might be +48 phantom, and I'll be scoping this today to try to suss it out.  Basically I am going to completely replace the power supply section and custom build my own.  I think I can do it with some PS parts I have lying around that will allow me to completely redo the supply and still keep the IEC plug and mains transformer in the box (I prefer this for convenience).  I have a switching power supply that provides + and - 12v, so I just need to add the +24 and +48 outputs, which I can custom design a PCB for pretty easily.  Having the power board schematic with all the connector marking for the 73 would save me some diagnostics and guessing, but for the most part I know what I need to do.

I'll definitely keep everyone posted how the hum is affected after I integrate my new supply.

 

[maxwall]

great to see folks are experiencing the same issues I have in my ACMP81's

now how to figure out the corrective actions ? I have had zero time to work on mine as I have been too busy with my regular line of work, but I'm following this closely.  GOOD WORK odbfs, thanks for your efforts !!!

I really would like to add a Elma 4130 gain switch and wire it to the board to maintain all
the proper gain selections ( resistors on switch, bypass resistors on the board) . Toss the cheap gain switch. This option comes second to the noise in this box. currently I have a 100% failure rate on all my boxes. The noise issue is the primary problem. Good to see that moving the transformer confirmed my findings in most of the noise.

Sure wish we were forewarned about these problems before the purchase. The boxes are unworthy for studio recording. Disclosure disclosure disclosure = integrity, right mates.

 

[0dbfs]

I have tried a couple temporary shielding options in lieu of an external toroid.

I don't have any mu-metal to experiment with but have found that some perforated aluminum works better than steel or solid aluminum as well as nulling the toroid and mounting it at a slight angle so that it's not parallel to the chassis... Still, it's audibly better with the trafo OTB.

If a shielded toroid were available (avel lindberg?) with similar voltage and current ratings then that might well be a best "ITB" solution. Ie; remote box's and connectors start to add up depending on how nice you want it to look (for remote of the toroid) and a shielded trafo might cost similar at the end of the day. The included toroid does not indicate current ratings (at least in english! anyone know a method to compute current ratings on a given trafo?

The inductors are not shielded either. I've got an altec-4722-trafo can which may be mu-metal and would fit nicely around the inductors. Have not yet been able to empty the can to test that yet and i'm not sure whether I want to sacrifice 8x 4722's for that if it does help (in my case).. I was planning on using those for a couple four channel one-bottles possibly re-engineered for 12ax7's

I briefly googled the elma 4130 and it looks to be 103mm deep at first glance... There is about 55 mm behind the current/included switch before the mic and line input trafo's would get in the way. Of course anything is possible. 

Cheers,
jonathan

 

[wmtunate]

I've got the schematic saved here, and it gives the following ratings for each secondary of the transformer:

2 * 15V (CT) / .1A
26V / .2A
26V / .2A
45V / 20mA

 

[maxwall]

no need to try and sheild the inductors. Pot core inductors by design are supposed to inhibit noise fields. Cinemag sells their Pultec inductors without shielding and they too are pot core designs. you can also compare the factory Neve inductors which are not shielded either.

On the other hand, a custom wound shielded power torroid might work well. But not one made in China.

Ever polish a turd ? well thats what this China preamp is , a turd. try to polish it and its still a turd.

 

[okgb]

It's not uncommon for preamps to have seperate power supplies ,
often just to make it easier to sell into other countries , but as well
to  eliminate the noise caused [ radiated ] especialy in cramped
designs .

Although there was disscussion about it being better to feed D.C. from the extrenal supply because of the same a.c. radiation ,
IT is easier and in this case for a retro fit , to
Simply extend the secondaries [ anyone know how many  ? maybe not ] move the power xfmr and make an extension cable

So A.C. or D.C. ?

And on the torriods some kind of low pass filter won't hurt eirther

 

[clintrubber]

So A.C. or D.C. ?

I'd say that AC would be better, meaning the reservoir caps inside the preamp-box. But it ain't no
real big-big poweramp, so...

If there's significant regulation and/or additional cap-stuff inside then it may not matter and
then passing DC from external supply to the preamp-box would even avoid
bringing in big 50 (60) Hz signals. But as I think to have understood it's the TX-strays,
not the AC-sec-wires, that cause the nasties, right ?
Then the AC should be doable.

 

[mitsos]

I think we may be on to something, but it's hard to say without doing an actual test. Does anyone have a Gauss meter? Is this something that could be checked with one? It'd be nice to test this toroid and a known good one for EMF. Maybe check inside a few pieces of equipment. I don't know if there are normal levels, but if I had to guess, I'd say there are limits to the amount of EM garbage a toroid can put out in, say, a medical device used around people with pacemakers..

anyway an easy test could be if someone had a working JLM 5 rail PSU and could bypass the whole PSU+toroid in the box to see if there is a change. I don't think this has been done yet? I hope my 84s come to me perfect(wishful thinking) but I will try this if the PSU gives me problems. If there were a noticeable improvement, I don't think it would be too off the wall to ask Chance to arrange for new toroids.  Not to be a dick, but at the quantities we're looking at, they'd be cheap. Wishful thinking with China I guess, but Chance did offer a warranty and to make things right in case of problems. Has anyone had any contact with him at all lately?

 

[b3groover]

...I don't think it would be too off the wall to ask Chance to arrange for new toroids.  Not to be a dick, but at the quantities we're looking at, they'd be cheap. Wishful thinking with China I guess, but Chance did offer a warranty and to make things right in case of problems.

Yeah right.  Good luck with that.  The warranty Chance is offering is "send it back to China".  He's not standing behind these at all.

 

[0dbfs]

Chance did mention on the PSW thread that he would honor and deal with warranty issues after shipping is complete. I haven't heard anything else yet.

I don't think a toroid with these secondaries is a stock item but maybe China will honor a replacement.

I'm waiting to see how that plays out before spending the $$ on making remote boxes.

I think China should pony up a replacement (shielded or whatever)..

No gauss meter here but maybe a coil connected up to an amp-meter would provide a reference point against some other typical situations?

FWIW, I even threw in the kitchen sink with no difference  :

Cheers,
jonathan

 

[rodabod]

Did you remember to earth the kitchen sink?

Have you swapped out the transistors already?