[ACMP investiupgradifications] All things PREAMP

[clintrubber]

By the way, the BC461,441 ( TO-5, 39 ) cans don't fit very nicely on the board since their bigger than the T0-92 types.
Its a tight sqeeze in some spots and the legs stand higher than I care for just to make room. In fact the legs should be close to the pcb board to prevent possible oscillation or EMI interference as I understand it.

I don't have them handy here to compare them to TO-39, but would using BD13x-16 (TO-126; say BD139-16 & BD140-16) help here maybe ? I expect these will be electrically close - most likely even the very same die, just different package.

These might need drilling the holes a bit because of their thicker leads, but I expect they can be mounted closer to the PCB-surface.

Bye,

  Peter

 

[zmix]

This is certainly not a 'new problem".... :   RTFF..! 

ALL of the EQ amp boards have the wrong transistors and ALL of them will oscillate.

This is exactly what I stated in my initial assessment. I specified that ALL of the output transistors on the 4 EQ boards need replace

ment.

Oscillation contaminates the power supply and will therefor appear in all connected circuits.

Once you have replaced all of the Q4 and Q5 on the 4 EQ filter boards this oscillation will go away.

PS: Look at the parts I put in mine (photo below) certainly no better than using the correct BC461 BC441 parts, yet no oscillation problems!

Just received my ACMP81's

New problem discovered

replaced 4Q4, 4Q5 on the Upper EQ Board which fixed buzzing oscillation issue on this board.
Payed careful attention to pin orientation between the BC547,557 and 441,461 when soldering back
into the PCB. After power up , I noticed a buzz oscillation coming from Lower EQ board. It also has a
output complimentary NPN/PNP pair so I replaced them ( 5Q4,5Q5 ) like the upper eq pcb board but this did not
cure its oscillation problem. So as it stands there is a possible oscillation problem on the lower eq pcb board
that is evident when engaging the rotary switch from 220 -1200 hz followed by its associated 10K
sweep fader. when the rotary switch is dis-engaged to the off position the oscillation is gone - eq amp out of path.

Looks like I'll have to replace the comp transistor pairs for the following boards below, which I have not
done yet. It may be possible that the oscillation is being passed by another eq amp , not sure.

3Q4,3Q5
6Q4,6Q5

Any ideas on this being another transistor issue on the 220 -1200 hz lower eq board ?

I have some test equipment handy but don't know where to start dropping the scope probe ?
maybe I'll start checking transistor emitters for broken waveforms.

The noise I'm getting is not a ground hum or 60 cycle low pitch hum , its sounds very close to the oscillation
that I heard on the upper eq board before I fixed it but a different pitch.

By the way, the BC461,441 ( TO-5, 39 ) cans don't fit very nicely on the board since their bigger than the T0-92 types.
Its a tight sqeeze in some spots and the legs stand higher than I care for just to make room. In fact the legs should be close to the pcb board to prevent possible oscillation or EMI interference as I understand it.

 

[maxwall]

I manually bypassed each EQ section with a jumper and discovered that this distortion was coming from the HP / LP Filter section, and specifically from the first ampllifier stage.
I traced the source of the distortion to Q2 (a BC557) and replaced this defective transistor with one that I had previously removed from the other EQ amps and am very happy to report that the distortion is now gone.

RTFF ? - yeah a lot to F*%^*$ reading already.

Zmix ,
I read your assessment above looks like you had problems in a different area Q2.  Chance mentioned the 4 EQ board amps needing possible replacement , maybe he cc'd this info from you, dont know. The HP/LP Board obviously does not contain any comp 'Q' pairs so an isolated issue with Q2. What method was used to check distortion on Q2 and how ? can you explain ? some other members might learn something here.

Why when I switch the 200-1200HZ rotary to 'off ' position on the lower eq board amp the oscillation disappears ? When I switch to 'off' does'nt this isolate the problem to the audio path of this eq module or does the oscillation travel thru the power supply or ground path from a different eq board and find its way thru to the lower eq board when switched to 'on' position ?

I'm still going to to replace the other eq's complimentary 'Q's pairs regardless. I report back my results.

Also love the modular design and layout , very easy to remove boards and work on.

 

[maxwall]

Just replaced all output transistor comp pairs with BC461,441's on all 4 EQ boards
and the oscillation buzz is still there. A little on the upper mid board EQ and really pronounced on the lower mid EQ board. WTH !!! This ain't right

The buzz is gone when the rotary switch is in the off position. All the other EQ's are fine except the Upper mid EQ. It stil has a little buzz in it when the rotary switch is engaged.

I'm going to get to the bottom of this , for sure !!!

I'm going try swapping in another board from another 81's and see what happens.

 

[zmix]

Chance was quoting my posts on the Pro Sound Web....

I manually bypassed each EQ section with a jumper and discovered that this distortion was coming from the HP / LP Filter section, and specifically from the first ampllifier stage.
I traced the source of the distortion to Q2 (a BC557) and replaced this defective transistor with one that I had previously removed from the other EQ amps and am very happy to report that the distortion is now gone.

RTFF ? - yeah a lot to F*%^*$ reading already.

Zmix ,
I read your assessment above looks like you had problems in a different area Q2.  Chance mentioned the 4 EQ board amps needing possible replacement , maybe he cc'd this info from you, dont know. The HP/LP Board obviously does not contain any comp 'Q' pairs so an isolated issue with Q2. What method was used to check distortion on Q2 and how ? can you explain ? some other members might learn something here.

Why when I switch the 200-1200HZ rotary to 'off ' position on the lower eq board amp the oscillation disappears ? When I switch to 'off' does'nt this isolate the problem to the audio path of this eq module or does the oscillation travel thru the power supply or ground path from a different eq board and find its way thru to the lower eq board when switched to 'on' position ?

I'm still going to to replace the other eq's complimentary 'Q's pairs regardless. I report back my results.

Also love the modular design and layout , very easy to remove boards and work on.

 

[maxwall]

Problem solved !!!

swapped in another eq board to rule out other component failures, ie inductor , cap  etc.
the problem stays with the box . The eq board I pulled out from another 81 , had the same buzzing
noise problem when engaged in the test platform . I even replaced the transistors again like before BC441, 461 and still the buzzing noise remains.

Before Testing the inductor , I removed the toroid power transfomer and moved it slightly away from the eq boards and this instantly romoved the buzzing hum problem from the mid upper and lower eq sections when engaged. (Consequently, these are the only two boards which contain the only two inductors) This is somewhat strange because I'm used to hearing a low 60 cycles hum
but this usually points to a ground loop noise, not a transformer EMI noise like what is evident here.

I found that if I rotated the transformer left or right in its mounting location the noise did not change significantly enough to call it a fix. so again I found that the transformer needs to be relocated pretty far away from the eq boards to get the signal quiet like it should be.

It appears the transformer will have to be moved outboard of the box to get a good clean noise free signal. It may also be that these transformers put off a field that is not found in higher cost toroids, not sure. I don't have any Mu Metal to cover the donut shaped transformer to see if shielding will help. maybe the inductor on the eq boards needs sheilding instead. I read thru the Prosound web forum and this problem has already been encountered, this is what led me to at least trying it. and it worked !!!

Looks like the best solution is to mount the transformer and maybe the power supply in a outboard box.
Its a hassle but I don't think there is any other choice for getting a clean signal out of this ACMP 81.

 

[okgb]

Nice work ,
generally speaking torriods pass allot more hi freq **** than normal pwr xfmrs
curious though only the 81's suffer from this ?

 

[maxwall]

okgb,

81's , definitely.

It seems likely the others would get effected as well, but I have not heard any reports yet.

I might also add that this preamp sounds damn good for the price , really damn good.

That is when the power transformer gets relocated and the transistors get switched out for the BC441, 461's.  it will make for a really decent recording tool.

 

[zmix]

Can you provide measurements? What's the noise floor with the EQ in and EQ out?

okgb,

81's , definitely.

It seems likely the others would get effected as well, but I have not heard any reports yet.

I might also add that this preamp sounds damn good for the price , really damn good.

That is when the power transformer gets relocated and the transistors get switched out for the BC441, 461's.  it will make for a really nice recording tool.


Thanks
Chance

 

[clintrubber]

Look at the parts I put in mine

Thanks for the various info.

Just wondering, how is the PCB-quality ? You've done several mods'n'swaps now,
have the circuit-traces been a bit sturdy so far ?

Bye,

  Peter

 

[zmix]

Look at the parts I put in mine

Thanks for the various info.

Just wondering, how is the PCB-quality ? You've done several mods'n'swaps now,
have the circuit-traces been a bit sturdy so far ?

Bye,

  Peter

Peter,
I had an opportunity to do some more extensive repairs to one of mine after I had accidently swapped two side by side identical wire connectors on the preamp board. This neat trick resulted in 2 popped electrolytic caps and 2 BC184 transistors.  I would rank the quality of the PC boards as being as good as any modern epoxy / fiberglass boards, certainly better than the boards in any of my 1960s and 1970s gear.  As you can see in the photo, I needed to fit those large leads from the NTE parts into the 0.1 holes in the board. I was concerned about lifting the donuts around the holes, but I used my normal care when desoldering and no traces were harmed during any of the repairs. 

 

[maxwall]

Zmix,

I think its safe to say that when the EQ is engaged the noise floor does increase. But to what level ? not sure ,  technically I think a audio analyzer would work nicely for this but I often use my ears because in the end the final test is listening to the results. If everything is quiet and the headroom is
big and harmonics smooth, its good for me.

The niceness in this amp is that most all the caps are Rubycon 105's throughout and transistors are Fairchild , very delighted to see this level of quality. the film caps in the passsive sections seem to be nice too.

I'm going to make some stainless steel covers out of tubing for the inductors and power transformer to see if I can't eliminate the buzzing from the power supply. I would really like to keep the power supply
self contained within the chassis if possible.


Here is a snapshot of the inductors in the Eq section , BC transistors upgraded already.
I noticed they don't get hot to the touch, rather on the warm side. The heat tends to increase on the metal case when the EQ is at different settings. i don't think they will require heatsinks so far.

Out of curiosity, I'm tempted to wind my own Neve inductors using the pultec toroid cores and sub them in for these just to compare....keeping the Neve inductance specs the same of course. Might be interesting.

 

[tommypiper]

Um, I'm having problems with this new forum software.  I don't know where to post these problem comments.  Anyone?

Maxwall, et al, thanks for posting, guys.  I am only surprised that it's taken this long for someone to discover problems coming from the toroids.  When I first saw the photos I suspected they would be causing noise.  I was planning to shield them, or remove them, and possibly shield the inductors, it was the first item on my list of things that would probly need fixing.  Unfortunately, I still don't have a shipping invoice so I can't comment on actual experience and join your fun.  Keep the updates coming, it's very helpful!

 

[zmix]

Um, I'm having problems with this new forum software.  I don't know where to post these problem comments.  Anyone?

Maxwall, et al, thanks for posting, guys.  I am only surprised that it's taken this long for someone to discover problems coming from the toroids.  When I first saw the photos I suspected they would be causing noise.  I was planning to shield them, or remove them, and possibly shield the inductors, it was the first item on my list of things that would probly need fixing.  Unfortunately, I still don't have a shipping invoice so I can't comment on actual experience and join your fun.  Keep the updates coming, it's very helpful!

Well, as I've posted and / or been quoted before the TnC 73 and 84 use the same inductors  and PSU  toroids without issue.

 

[maxwall]

Zmix

The Tnc 73,84 inductor boards are they the same distance and orientation away from the transformer as the 81 ?

Can some one snap a photo so we can see the board layout in a 73,84 ?


Also for those who intend to replace the BC557, 547 transistors with BC 461,441 , there are substitutes
which can be used like the 2N5320 or 21, 2N5322 or 23, but these are a little pricey.

If you want a cheap substitute use TIP31B or C(NPN) and TIP32B or C(PNP) both rated at B=80V C=100V, 3A. These parts will work and are overrated in their specs to carry out their jobs as subs. The BC441,461's are 75V, 2A rated.

IMPORTANT : the Legs on the TIP31B or C, and 32B or C are not pin compatible with the BC 557,547's. The legs will need to be shaped/bent to match the Collector , Base, Emitter pin arrangement exactly. The TIP transistor legs will need to be thinned to fit in the pcb pads holes as well. View available data sheets on web to familiarize yourself with both the original and replacement transistors regarding Emitter , Base and Collector pin differences before attempting installation. Also the gain on these transistors are rated lower than the 2N and BC parts

 

[maxwall]

Has anybody confirmed that the power toroid transformer in this ACMP 81 does not have a electrostatic shield in it.
I don't believe it does, and this may be part of the problem with China transformers.


Also, the suffix 6 or BC 441-6, 461-6 is not required to make these work. You'll have to buy a load of transistors and measure the hfe of each one to get selected parts. Newark sells the BC 441,461 but you won't find suffixes
available when you make the purchase. I suspect suffix 6 parts are really pricey. Kind of like selected vacuum tubes.

The non suffix 6 BC parts wont need a heatsink , they don't get hot in my ACMP 81.

Its just the damn toroid power transformer ruining the party. Its too close. I sheilded the inductors and it still picks up noise from the toroid power transformer, regardless.

 

[rocketattack]

Anyone have any suggestions regarding the "killer pop" when turning up the gain? It seems that the pop exists on at least two of the models. Someone on gearslutz said they have it on their 81s, and it's on both of my 84s.

 

[b3groover]

Ask and ye shall receive:  Inside the ACMP-73

I have not had a problem with hum nor oscillation of the EQ.

 

[maxwall]

Rocket Attack,
Your absolutely 100% correct with the Brutal pop in the 81 gain switch, mine even cuts out for a few seconds then comes back very slowly. Sounds like the preamp goes dead for a second or two. Is this a non-shorting switch ? I'll have to check it to see wether it is or not. I'm hoping not. It should be a shorting type. Yes, ?


b3groover,
The effected area in the BUZZZZ zone is highlighted in yellow below. On a 73 you wont get it because the eq boards are far enough away from the transformer flux field to avoid the EMI.  There only needs to be about  4-5 inches of empty space between the transformer and boards then the buzz is almost non-existent. On the 81 , the same area highlighted below in yellow is occupied by the low mid and low eq boards and no extra space exists to be free from the transformer flux field. The 73 and 81 share the exact same case, but the 81 has more boards.  The 81 owners are stuck with the problem ,  its a simple fix but a pain in the arse. Mount the transformer and power supply outside in its own box. It will probably make it quieter anyhow, as long as the lines are kept fairly short ( 3-4) feet and generous guaged wires are used utilizing safety ground by default.

I just picked up some Cannon multipin connectors to start the process on my 81's for remote placement of the power supply. The power connector on the back of the 81 can be removed and a plate installed in its place for use with the Cannon multipin power connector. Yes , its a pain is the arse...... The only thing I have'nt tried is a Mu-metal barrier
between the transformer and afflicted boards. MU-Metal might be another easier alternative.

Why is it when I lift the PS grounds I always get a AM radio station in a foreign language, why can't it be Michael Savage from the Savage Nation. Sheeple

 

[rocketattack]

Rocket Attack,
Your absolutely 100% correct with the Brutal pop in the 81 gain switch, mine even cuts out for a few seconds then comes back very slowly. Sounds like the preamp goes dead for a second or two. Is this a non-shorting switch ? I'll have to check it to see wether it is or not. I'm hoping not. It should be a shorting type. Yes, ?

I haven't noticed a cut, but that isn't to say it doesn't cut out.

Has anyone noticed it on the 73? Or is it only on the 81 and 84?