[ACMP investiupgradifications] All things PREAMP

[crazydoc]

Again , anybody using thier's with the 2n2907 & 2n 2222a transistors ?
mine seem to through allot of heat , you can feel it as soon as you take
the cover off , i'm wondering about these in the long term .

I checked mine today (though I can't find my non-contact thermometer.) They do run on the warm side, though I can hold my finger on them for 10 seconds or so without major discomfort.

 

[chris carter]

Anyone hear from Steve Hogan??

 

[Paul G]

Anyone hear from Steve Hogan??

I got an email from him today.  He's been very busy and promises to post soon.

Paul ;D

 

[redddog]

He's been saying that for two months.

 

[audioforge]

i ve receive my second 81 bought from a member here.
i confirm my first report.
i have no Humm from the inductors.
220V 50Hz here.
what i ve done change the 8 bad transistors with 2N5321 and 2N5323.
mod the pop mic input with the "0dbfs" tip.
add a 600R on a switch at the back of the output TX.
remove the input capacitor on the line TX.
eveything work good.
BUT this time i use a square wave to test the Traffos.
I ve done it for the line TX and remove the input capacitor to tame the overshoot.
but oh my god the ouput TX is AWFULL.with overshoot and crosstalk...
remove the 10n :it s worse. solder it again..
then I engage the EQ on (all others EQ switches on OFF) crosstalk nearly disappear. and small gain increase ...
Square wave is good when mesured on the 2n3055 so only the output tx is the culprit.
So I swap the output tx for a 30 Ohm/600 schlumberger traffo i have (bigger than a utc
A10 ,shielded ,pinouts within "glass pearl"..) put only a 600r across the output:
perfect square from 30 hz to 30Khz.
i will try with a carhnill output (but they re mounted on my 1084s clones)asap.
so my advice will be:
if you live in europe swap the inductors don t seems necessary.
try 2n5321 5323 instead of bc441 61 if you don t have some..
remove the input line tx capacitor. throw away the chineese output tx..
or maybe it s only this one i have who s bad....

ps i would like to read rodabod comments on the chineese output tx since i remember he change to carhnill ones..
audiforge

 

[maxwall]

Audioforge

Did you get the buzz in the mid and upper mid bands of EQ ? This is a different noise that is
usually found after the transistor replacement. Then again your on 220V power so you may not
have the same problem as those running on 115V

To confirm your findings, I'll probably need to use a step up power trafo from 115V to 220V and switch my 81 mains power supply switch (red switch on back of 81) from 115V to 220V on back ( swap out fuses as well - 100ma type for 220V mains) and test to see if the buzz disappears.

add a 600R on a switch at the back of the output TX.
remove the input capacitor on the line TX.

can you give reference to the PCB pad locations from above quote ( example r43, 1c21 ) so its easy to follow your modifications. Would like to try them out.

those 2N5320/21 NPN and 2N5322/23 PNP parts are somewhat pricey, but this category of transistor is higher current rated , hence the price. Here is another sub 2N5681/82 NPN and 2N5679/80 PNP both 1amp rated/100-120V/hfe 40-150 (TO-39)
and another  TIP110/112 NPN and TIP115/117 PNP (TO-220 case, observe BCE leg orientation) 2amp rated/60-100V/hfe 500-1000 with high power dissipation around 50W, this will last forever compared to the 10W T0-39 types and can be remotely wired with short leads from the PCB and heatsinked to the case with proper insulator. In fact it shoud'nt need a heatsink at all since it can handle much higher power.

The 2N2907A, 2N2222A ( T0-18 metal case) power dissipation capability is just marginally rated over the BC547,557 (TO-92 plastic case) parts pending replacement in the ACMP 81. This is why they still run hot. I think around 600-800mw as compared to the 10w rating of the BC441/461 and 2n5321/5322 (T0-39 metal case) variety .

 

[stereokillah]

hello electrochronic, audioforge,

i 'm in europe too.

i've changed the transistor and hum lost some db 
changed C11 and C16 (10uF/35V) to 100uF/35V hum lost 1or 2 dbu
put shielded sheet on the tx psu (bought on black market) and  hum lost around 5db
changed the two inductor for carnill shielded with oep case and when i give mid low and mid high i have a very litlle changement aournd 50hz and mid hash

only have some noise when i give some gain on the last  low band 330 to 33.

So i don't have information about the 600r  across the output where i need put it please.

 

[rodabod]

i would like to read rodabod comments on the chineese output tx since i remember he change to carhnill ones..
audiforge

They are actually ok. Primary inductance is good and matches well with the Carnhill equivalent. It does need to be terminated correctly though; you can adjust by measurement or ear. The original Carnhill has some interaction in the low-end which depends on how you terminate it.

Audioforge, PM me your email address.... I will give you some details that could be handy.

Roddy

 

[audioforge]

electrochronic
yes ,inductors hum being relative to the power traffo EMI it s seems that 220v 50hz tame this trouble. it was my conclusion after testing the first unit you sent me (thanks again franck)(and i beleave i was the first to have one in europe at this time).

i do my test for the EQ with sine wavefrom a generator  sent to the line input .choose a frequency eg 220 hz or 500 hz etc at 0db. according to the frequency choosen for  the mid eq  ; and see the sine wave at the output with probes and oscillo. i see very good sine wave form whatever eq gain.
i still wait for a technical procedure to reproduce your test conditions showing hum on the inductors....
franck and sterokillah.
a output TX need to be terminated to do measurement.in real world too.
what i usually do on my diy's with 600r output TX is very simple.
take a two pole switch .solder a 600 or 620r on a pole then a wire lead to one of the output tx pinout. a second wire from the other switch pole to the other  output pinout of the output TX.
that s all..
this way i can let the the output traffo unterminated if i go to 600r input gear or terminate it when i go to other gear without 600r input tx.in this case i can use the switch on or off and let my ears choose what i like the best....
stereokillah :i didn t change any capacitors..i need to try later...
franck :
my local dealer don t have bc461 but 2n5321/23 .around 1.1 euros vat included each.sourcing the original bc transistors in small quantity with the shipping will cost me at least the same and the spec are very similar to the original with less noise and more consistant hfe...
they seems to be happy in the 81 boards ;no oscillation nor overheating....
rodabod.
i was thinking at first glance that the mic and line TX from the chineese units will be quite bad but it appears there quite good (at least the line tx) when testing them with a square wave and removing the input capacitor(line tx; i don t do it this time for the mic tx).my thought for the output tx was :well it s just a output tx that look quite similar to the original one..square wave show me i was wrong.
i will do a test on thursday with the first unit i ve mod (with a  chineese tx to compare the 2 chineeses between them) and will connect one of my carhnill output tx to compare how they hold on with square wave...
btw i sent you a pm
will post my report as tests will be done...
audioforge

 

[audioforge]

new tests report
i ve test my first tnc81 (the one iv e bought from franck) vs the second one.
again with SQUARE wave send to line in.
the output chineese tx ,600 ohm terminated, of the first unit don t show crosstalk
and have less overshoot than the second unit which is very bad.
removing the 2 1000pf capacitors on the output board tame a little the overshoot but not in a significant way.
engage the EQ boost a few the output level and degrade the square wave but it s due to the design.
first conclusion :chineeses output TX quality could be very variable. one just "acceptable" and one frankly bad .
i ve try different r+c combinations but if it show better responses with sine wave ;square wave (which translate the best a build in tx quality) clearly show thoses chineeses tx are not up to the task.
final test : i swap the worse output tx to a 1066 carnhill repro i have in my 84 clone.
with his 10n+R ,as always 600r terminated. GOOD square wave from 100hz to 25kz,
very little overshoot ,no crosstalk. engage the EQ do the same square wave degradation and little boost (i presume due to fact of the circuit resistance and capa added).
So my final? conclusion IS:
if you want an acceptable tnc81 (unless you live in an 110v 60hz area and have inductor hum)  the best is to BUY a 1066 output tx. the carnhill repro are good and quite cheap from Colin. what you have at the 2035 output will be what you have 'except from 20hz to 100 hz not perfect" at the output TX.
audioforge
btw my 2 carhnill output tx "sing". do someone know how to tame this. it drive me nuts.

 

[stereokillah]

hello,
someone should know where i can buy push button to change EQ on off.
this switch 4 pole is broken in one my units

i find it here but it's out of stock.

http://www.rapidonline.com/Electronic-Components/Switches/Latching-PCB-Switches/Illuminated-push-button-switches/70729

thank you

 

[rodabod]

I have noticed one final issue with my '84 preamps, and unfortunately I think it stems from the power transformer.

The problem mainly affects low output ribbons and some dynamics. I believe the issue is a generally noisy earth (hum). When the unit is powered up and set with high gain but with no microphone connected, it is very quiet. However, connecting a microphone suddenly increases hum level.

This hum can be fixed by simply removing the top of the rack case, or even just loosening the screws on one side. This makes me think that the transformer is radiating EMI into the case and forming a circuit (a turn) which is leading to earth currents.

I am considering just swapping the toroid for a good quality one. I did consider breaking the ground connection around the case by using washers, but I though this might be a crude solution. My other idea was to lift the microphone input pin1 away from chassis earth and derive it from the line output pin1 so that it takes its earth from the next (clean) piece of kit. Again, probably not the most elegant solution, especially if the line output / line input following has pin1 lifted at one end!

Roddy

 

[audioforge]

"This hum can be fixed by simply removing the top of the rack case, or even just loosening the screws on one side"
hi rod .have you try out some isolation sheet above the psu tx screw. maybe it touch the top of the rack case....
audioforge

 

[rodabod]

That was the first thing that crossed my mind since it would be very dangerous. But no, it seems to just be the electrical connection between the top and bottom halves of the case; the toroid bolt only touches one side.

 

[crazydoc]

That was the first thing that crossed my mind since it would be very dangerous.

Why would that be dangerous? (The toroid screw touching the top of the case.)

 

[jdbakker]

That was the first thing that crossed my mind since it would be very dangerous.

Why would that be dangerous? (The toroid screw touching the top of the case.)

Dr. Egon Spengler: There's something very important I forgot to tell you.
Dr. Peter Venkman: What?
Dr. Egon Spengler: Don't cross the streams.
Dr. Peter Venkman: Why?
Dr. Egon Spengler: It would be bad.
Dr. Peter Venkman: I'm fuzzy on the whole good/bad thing. What do you mean, "bad"?
Dr. Egon Spengler: Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light.
Dr Ray Stantz: Total protonic reversal.
Dr. Peter Venkman: Right. That's bad. Okay. All right. Important safety tip. Thanks, Egon.

If the bolt through the toroid touches the top of the case, it (plus the bottom, side and top of the case) forms an extra, shorted winding on the transformer. Best case: a blown fuse, worst case: studio fire.

JD 'oh, the embarrassment' B.

 

[crazydoc]

"I see," said the blind man to his deaf dog as he picked up his hammer and saw.

Thanks

 

[okgb]

Btw Burdij's posting on the W.M.  links his site
which has xfmr shields that one could use for
the inductors of these pres .

i never go to H.R. but anyone know what the tnc boys are up to
these days , do they ever say anything ?

 

[chris carter]

Btw Burdij's posting on the W.M.  links his site
which has xfmr shields that one could use for
the inductors of these pres .

i never go to H.R. but anyone know what the tnc boys are up to
these days , do they ever say anything ?

This is the ONLY forum where TNC communicates with the masses:
http://recforums.prosoundweb.com/index.php/m/0/16069/2592/15306/#msg_16069

 

[rodabod]

I have noticed one final issue with my '84 preamps, and unfortunately I think it stems from the power transformer.

The problem mainly affects low output ribbons and some dynamics. I believe the issue is a generally noisy earth (hum). When the unit is powered up and set with high gain but with no microphone connected, it is very quiet. However, connecting a microphone suddenly increases hum level.

This hum can be fixed by simply removing the top of the rack case, or even just loosening the screws on one side. This makes me think that the transformer is radiating EMI into the case and forming a circuit (a turn) which is leading to earth currents.

Right, that's another evening of work on these. I've just swapped out one of the mains transformers for a good quality Czech model which I have used before. Total silence hum-wise with the case top off. Hums badly with the case top back on. Balls.

As I said before, the hum disappears with the mic input unterminated. I don't think this is a case of having a dirty earth in the mic interfering. I found that terminating the primary with a short causes the same response, so I'm guessing the terminated coil is acting as a generator due to EMI. I was surprised the better quality transformer did not improve the issue. I'm also surprised the Carnhill's metal can isn't helping significantly here either, unless it is maybe being picked up by the wiring from the input XLR. I will terminate the trafo locally to test this rather than at the XLR.

More work to be done I reckon. I still have the option of using plastic washers as a fix, but it's a crude fix, and not safe in some respects.

Here's a photo if anyone's interested: