Group Buy Interest?

[crazydoc]

My units have their spikes at 60, 120, 180, 240, 300 etc Hz, so I doubt that transistor oscillations would coincidentally occur at these frequencies. Brad Whatshisface over at Gearslutz, though not forthcoming about his methods, has pretty obviously got ahold of some Mumetal to shield the power transformer, and considerably improved, though not fixed, the problem.

Although ground loops could certainly be part of the problem, I'm still going for induced EMF from the power supply as the major contributor. The problem is likely to be multifactorial, in any case.

If I get a chance this weekend, I will look through all my shit still boxed up from a move several years ago, and find my scope and other tools to see if I can check mine out further. I did find my soldering iron and some solder last week!

 

[antichef]

The problem is likely to be multifactorial, in any case.

That's it -- multiple different causes, varying at a higher degree from model to model, and at a lower degree from unit to unit. I put mumetal foil around the inductors and the toroidal power transformer in my 81s, and it didn't seem to make any difference (but I did *cut the crap out my fingers* a couple of times - careful with that stuff!). I think Brad had good results on a 73. He also made clear that he had to try several different types or grades of the material - it may be that the stuff I'm using isn't right.

I also had good, but temporary hum-reduction results from rotating the toroidal transformer in one, but not both, of the 81s. After I jiggled the unit externally (by putting it back in the rack) the hum came back and then would not respond to moving (twisting, flipping, repositioning, hanging-outside, etc.) the toroidal transformer at all. This is the one I just swapped the transistors in, and it's pretty quiet now.

 

[dgatwood]

Given the size and value, I'd guess that's somewhere in the power supply, perhaps between filter caps or something. I would replace that with the same rating, or perhaps higher seeing what happened to it . . . 1W at least . . .

That burning down the studio scenario is starting to look more likely

If it's an 81, that resistor is specified as 47ohms, 1/2W in the audio output amplifier. If it's not an 81, I don't have schematics, so I can't help you. Be sure the resistor you replace it with is at least a half watt resistor. I assume they wouldn't specify the more expensive 1/2W if a 1/4W would do just as well.

D'oh. I missed half a page of discussion and thought this wasn't covered yet. Never mind.

 

[BudgetMC]

Spent a bit more time tonight playing with the ACM-1200 microphones. I swapped in a NOS GE 12Ay7 (6072) in one of the mics. It had a nice clear upper mid/high sound to it, but, interestingly, didn't seem to have the bottom end that came from the Mullard 12At7/4024. The Mullard seems to take some of the "edge" off the high end of this mic.



More significantly, I think, the 12Ay7 didn't seem to provide the clear transients in the "key dangle" test that the 12At7 was able to produce. In fact, in the key test, the 12Ay7 sounded every bit as bad as the stock China 12Ax7.

 

[antichef]

Once you replace the resistor (or before), measure the voltage across it. If that's much more than 4V, then the resistor's rating is exceeded (4V^2/47=0.34W). 7W1 probably would need to be adjusted until that is a safe reading, but I'd wait for more expert confirmation before you go screwing with it . . .

My trimpot (on the one I'm working on) has glue on it, too.

I measured 2 volts across that resistor (7R8 on the ACMP-81), except for at the poppy place, then it spiked to 14 volts and settled back to 9. I could hear the unit clicking even with no output cable attached!

 

[crazydoc]

...then it spiked to 14 volts and settled back to 9.

Do you mean it settled at 9 volts if you held the switch "at the poppy place ", or if it continued on through its rotation?

 

[antichef]

If I held the switch in the place of popfulness, the voltage settled at 9 after about a half second, and stayed that way for 2 seconds (as long as I could bring myself to leave it there). If I moved off that popsome place in either direction, it went back down to 2.

 

[antichef]

OK - there are 4 EQ boards in the 81 that have the same transistor configuration that zmix was talking about, but he only specified the 2 with inductors as needing the BC461/BC441 mod. On my first 81, I found that to be true, but went ahead and made the other two match for good measure.

On my second 81, however (the one with rust on the inside top and a missing brass screw in one of the inductors), things were different. I first put the new transistors in each of the inductor boards - after each, the hum was still present. I then worked on the rightmost board (the one with the phase switch), and the hum was still there -- I was bummed out. I then swapped the transistors in the fourth from the right - immediately to the left of the two inductor boards -- hum gone!

As a bonus (and not an insignificant one), the EQ seems to "work" on both units now - it seemed flaky before, where I couldn't use the full range of the controls or it would cut out - now it seems good. It's going to take me quite some time to learn how to use those EQs - they're complicated

Now I've got two working 81s

There's still one thing I have to chase down -- before, I tried passing a mix through the two 81s using the stereo line outs, and feeding them back in. When I posted my results, everyone immediately noticed that the mix sounded thin and emaciated when it went through the 81s. Then I tried hitting the phase switch on one, and it got better -- could the phase be reversed on one of them? -- admittedly, using outboard effects is not something I do often, and I'm less than 100% confident it wasn't another problem in my system, but I'm about 98% confident -- it was a fairly simple setup.

 

[thajeremy]

I know you guys have a lot going on with trying to fix these pre's and all, but could someone post details or post where I can find the details on how to test my 73's and 84's for the hum issues? I see all these frequency graphs and things you guys are posting and I have no clue of how to do this. I have a friend who can handle doing the work on them, good with electronics and such....but I need to know how to test them to show him what I want fixed. Iv had mine for over a week now and have not even hooked up the pres, just kinda sitting back and waiting till someone finds a fix for them...

 

[mshilarious]

My trimpot (on the one I'm working on) has glue on it, too.

I measured 2 volts across that resistor (7R8 on the ACMP-81), except for at the poppy place, then it spiked to 14 volts and settled back to 9. I could hear the unit clicking even with no output cable attached!

9V yields 1.7W. I would go with a 2W resistor (note the 14V spike is actually 4W, but since that's very transient, 2W is probably OK). I'd also make sure not to hold the gain knob in that position . . .

 

[gemsbok]

OK - there are 4 EQ boards in the 81 that have the same transistor configuration that zmix was talking about, but he only specified the 2 with inductors as needing the BC461/BC441 mod. On my first 81, I found that to be true, but went ahead and made the other two match for good measure.

On my second 81, however (the one with rust on the inside top and a missing brass screw in one of the inductors), things were different. I first put the new transistors in each of the inductor boards - after each, the hum was still present. I then worked on the rightmost board (the one with the phase switch), and the hum was still there -- I was bummed out. I then swapped the transistors in the fourth from the right - immediately to the left of the two inductor boards -- hum gone!

As a bonus (and not an insignificant one), the EQ seems to "work" on both units now - it seemed flaky before, where I couldn't use the full range of the controls or it would cut out - now it seems good. It's going to take me quite some time to learn how to use those EQs - they're complicated

Now I've got two working 81s

There's still one thing I have to chase down -- before, I tried passing a mix through the two 81s using the stereo line outs, and feeding them back in. When I posted my results, everyone immediately noticed that the mix sounded thin and emaciated when it went through the 81s. Then I tried hitting the phase switch on one, and it got better -- could the phase be reversed on one of them? -- admittedly, using outboard effects is not something I do often, and I'm less than 100% confident it wasn't another problem in my system, but I'm about 98% confident -- it was a fairly simple setup.

my 2 81-s have the single brass screw missing. i wonder if its supposed to be this way or if its a problem. also anyone have a link to purchase the trasistors or any extras for sale. let me know.

 

[antichef]

my 2 81-s have the single brass screw missing. i wonder if its supposed to be this way or if its a problem. also anyone have a link to purchase the trasistors or any extras for sale. let me know.

As far as I could tell, the screw did not play an electrical part, but was there to secure it to the board (it also seems to be glued to the board) - unless someone who knows better than me (and there are plenty of folks in that category) says something, I'm not going to worry about it.

I got the transistors at: http://www.newark.com - just search for the part numbers - looks like they're running low on BC441. If I wind up with some spares, I'll post.

 

[cnix]

I know you guys have a lot going on with trying to fix these pre's and all, but could someone post details or post where I can find the details on how to test my 73's and 84's for the hum issues? I see all these frequency graphs and things you guys are posting and I have no clue of how to do this. I have a friend who can handle doing the work on them, good with electronics and such....but I need to know how to test them to show him what I want fixed. Iv had mine for over a week now and have not even hooked up the pres, just kinda sitting back and waiting till someone finds a fix for them...

Well, you have to fire them up to see what you have, who knows, you may have a diamond in the rough. Both my ACMP-73 and 84 pre's are very nice on mic's that do not require a lot of gain, no unwanted noise. They both exhibit some noise when I max out the gain, but as of yet I have not needed that much gain. I will do the fix once it is determined the best way to go about it. The EQ on my 73 does induce just a little noise, but the EQ on my 84 does not, which is great because I like my 84 EQ options better for my vocal and acoustic guitar.

Zmix also stated that there is a coating on the knobs that burns off with use, and that coating induces noise, so at least you can eliminate that noise from your pre's with some playing around.

So I say, fire them up and have some fun!!

Charlie

 

[pohaku]

As far as I could tell, the screw did not play an electrical part, but was there to secure it to the board (it also seems to be glued to the board) - unless someone who knows better than me (and there are plenty of folks in that category) says something, I'm not going to worry about it.

I got the transistors at: http://www.newark.com - just search for the part numbers - looks like they're running low on BC441. If I wind up with some spares, I'll post.

I've got 2 x 81s and I just bought the last 7 BC441s (ordered 8 so I expect they'll back order them). Hopefully they will stock more soon. Bought 2 watt resistors as well. I figure, as long as I'm in there, I may as well replace those as well. The resistors are cheap so I bought enough for my 4 x 84s as well, although I might wait before I start dinking around with those. I haven't had time to test any of them yet, aside from seeing whether they simply power up and that all the LEDs appear to work. I'm in no rush so I may also wait to see what Chance wants to do about getting any of these units fixed before I start messing with them in earnest. I'm underwater at work and then off to Hawaii for a week, so it might be awhile before I can get to them anyway.

 

[crazydoc]

I know you guys have a lot going on with trying to fix these pre's and all, but could someone post details or post where I can find the details on how to test my 73's and 84's for the hum issues? I see all these frequency graphs and things you guys are posting and I have no clue of how to do this. I have a friend who can handle doing the work on them, good with electronics and such....but I need to know how to test them to show him what I want fixed. Iv had mine for over a week now and have not even hooked up the pres, just kinda sitting back and waiting till someone finds a fix for them...

Well, you can just fire them up and hear the hum/buzz and tell him that's what you want fixed.

Or you can look in your recording software and see if it has a FFT function (fast fourier transform.) This will plot the amplitude versus frequency of a sampled track. You record the output of the preamp with nothing hooked up to it (in theory with 100-200ohms of impedance across pins 2 and 3 of the mic input - I just shorted them together). Then use the FFT filter to plot the graph. I'm sure you can get software just to do this alone too.

I have a free recording program http://audacity.sourceforge.net/ that has a FFT filter (called Analyze->Plot Spectrum) that I used for this.

 

[crazydoc]

And the result:

 

[crazydoc]

An interesting post from Group DIY:
http://www.prodigy-pro.com/diy/index.php?topic=27791.msg380327#msg380327

After replacing the transistors in one of my 81's with the BC's I still had hum/buzz... Rotating the transformer changed the quality of the buzz but did not eliminate the fault.

I made up a 10 conductor test extension cable to remote the power xformer and that has solved all of the buzz issues. Nice and quiet with EQ engaged.

I tested the remote xformer with an 81 that has the original trannies and still have some oscillation.

My conclusion is that for the best and most reliable results the power transformer needs to be outboard and the transistors need to be swapped.

 

[antichef]

My conclusion is that for the best and most reliable results the power transformer needs to be outboard and the transistors need to be swapped.

-- prolly true. I'm not hearing any hum right now, but I'll try and do some testing. I can't immediately find a FFT "audio unit" plugin, but I'll keep looking.

So to terminate the mic input, I could put a 150 ohm resistor across pins 2 and 3 in an XLR plug, but do I need to ground pin 1?

One thing I noticed when I had a mic plugged in -- if I moved the mic within about a foot of the power transformer, even with the case fully assembled, I got a lot of hum through the mic -- that thing must be like a miniature EM sun.

 

[gemsbok]

-- prolly true.

One thing I noticed when I had a mic plugged in -- if I moved the mic within about a foot of the power transformer, even with the case fully assembled, I got a lot of hum through the mic -- that thing must be like a miniature EM sun.

i noticed if i moved a mic near the output xlr cable going to the mixer i got more hum. i guess you need to seperate any cables. this is getting crazy.

 

[jasonbmerrill]

I know you guys have a lot going on with trying to fix these pre's and all, but could someone post details or post where I can find the details on how to test my 73's and 84's for the hum issues? I see all these frequency graphs and things you guys are posting and I have no clue of how to do this. I have a friend who can handle doing the work on them, good with electronics and such....but I need to know how to test them to show him what I want fixed. Iv had mine for over a week now and have not even hooked up the pres, just kinda sitting back and waiting till someone finds a fix for them...

http://www.stillwellaudio.com/?page_id=26