[ACMP investiupgradifications] All things PREAMP

[clintrubber]

Thanks for those added schematics :thumb: Good to see it's close to the original AND this way of drawing is actually more convenient than the original documentation at the same time.

Bye,

Peter

 

[clintrubber]

[quote author="ChuckD"]They have copied the B338 correctly it seems however they are using BC557 and BC547 for the constant current emitter follower output stage. Also they are using 1N4148 diodes instead of the correct ones BAX13 and the AA144 [/quote]
For the current sources I guess it'd be hard to discern between 'original' and 'generic NPN & PNP' here, but it's an easy mod if one wants to 'correct'.

I haven't looked more detailed, but that AA144 should be in reverse during normal operation so can't imagine it'll affect 'sound'.

Bye,

Peter

 

[clintrubber]

FWIW: ... I saw the gain-stage for the treble section does have BAX13 diodes though, but likely these will be forgotten diode-properties when converting the original schematics to the ones they were actually going to make...

... and there's of course no guarantee that the schematics & semiconductor-types in the actual units will actually exactly match, but that's true for more brands...

 

[clintrubber]

What's the deal with the dual +24V supply section ? Obviously spreading the load, but the schematic doesn't specify which supply goes where. No problem though.

W.r.t. how alike it is to the original (as can be with the different output-stage), obviously this one doesn't use tant-caps but 'normal electrolytics'.
If it really matters I dunno, but it might be another mod-candidate.

 

[CHANCE]

I'm getting the detailed schems for thr 73 and 84 too if anyone wants them.

 

[clintrubber]

[quote author="CHANCE"]I'm getting the detailed schems for thr 73 and 84 too if anyone wants them.[/quote]
Sure, that'd be nice :thumb:

And without wanting to overask, but while at it, if they have the various condenser-mic-schematics available then that wouldn't hurt either :wink:

Thanks

 

[mfdu]

so theres been a noise issue with the '81. any problems reported on the '73's?

chris.mfdu

 

[clintrubber]

[quote author="mfdu"]so theres been a noise issue with the '81.[/quote]
No!

Let's call it a hum issue.

Or is that just me ?

noise = hiss,
hum = mains-nasties in the audio

nitpicking, sure :wink: but it keeps the conversation clear.

Bye,

 

[CHANCE]

A friend of mine who worked with Paul at Manley Labs said that they once had a simular problem and it was paint insulating the cover to the chassis. These ACMP's have a heavy coat of black wrinkle paint on the cover. I doubt that this is the problem, but you never know. I'll check later.

 

[Svart]

Don't be so sure. I just figured out a huge hum/hiss issue and it turned out to be poor grounding in a computer due to powercoating and lack of good contact between sections..

 

[floxe]

even more strange:

i just figured out some hiss coming from my canon printer! though i use professional balanced cables there's lot of hiss when the cables are somewhere near the printer.

best,
flox

 

[Mylithra]

Most printers are just a plastic shell.. Theres no shielding in them. I ended up moving mine away from the studio and just print to a network printer.

 

[CHANCE]

With regards to the buzz problem we are having, this is from another board:


"Ok everybody, I'l speak...

First of all the modules sound fantastic. Granted 99.999% of the time I am mixing, so I haven't checked out the preamp portion, but the line amps / EQs are great.

It's interesting that all of the modules use the single ended class A output stage based on the BA283 card ( same as what's inside the N£V£ 1064, 1066, 1073 1272, etc) even the "81".
In the N£V£ 1081 the output is a class A/B push pull design.

The "81" uses a A/B amplifier in the EQ buffer amp cards, so it has a bit more of the 1081 sound.

In all cases the units produce a nice amount of 2nd and 3rd harmonic and the top end sounds sweet.

I ran some minimally miced drums through these and A/B'ed an identical setting on a popular plugin N£V£ emulation.
The sound was quite similar, but the long term impression was that when listening through the TNC..
the room that the drummer was in seemed like it was 20˚ hotter and the drummer sounded like he was smiling.... I know.. Rolling Eyes Ok?

I HIGHLY recommend that you run some pretty hot signals through these when you first get them and exercise every switch and pot. I had one or two that didn't pass signal until it got LOUD. This is a symptom of new switches with anticorrosive coatings on the contacts which are intended to self clean through use, not a problem.

Now to more serious matters. I have the same buzzing in my 81s that a few of you have reported. I disassembled the 81 and ran some tests.

Here are my findings so far:

It does not appear to be caused by power transformer induced EMI.

I ran the unit off an external supply and had the same problem.

The "73" and "84" have their inductors equally close to the power supply as the "81" and have none of these problems.

Re-routing the the audio cables (which go to the output pot and phase switch) away from the PSU made the noise floor higher.

Moving my hand in the proximity of the EQ cards made the noise floor higher.

There is a transistor on each of the EQ amp cards that is running very hot (Q4).

I suspect that there may be ultrasonic oscillations in the EQ amp cards. Every symptom I found and every other reported symptom lead me to this hypothesis.

If this is indeed the case, the solution will be much simpler than if it was power supply related. It will require a few very very small capacitors be tack soldered across a few components. I'll run more tests when I am able. Unfortunately, I don't have a working Oscilloscope here and am in the middle of producing one record and mixing two... plus having major dental work performed this morning.. ugh."

 

[zmix]

[quote author="CHANCE"]With regards to the buzz problem we are having, this is from another board:
[/quote]
[quote author="ZMIX"]
"Ok everybody, I'l speak...

First of all the modules sound fantastic. Granted 99.999% of the time I am mixing, so I haven't checked out the preamp portion, but the line amps / EQs are great.

It's interesting that all of the modules use the single ended class A output stage based on the BA283 card ( same as what's inside the N£V£ 1064, 1066, 1073 1272, etc) even the "81".
In the N£V£ 1081 the output is a class A/B push pull design.

The "81" uses a A/B amplifier in the EQ buffer amp cards, so it has a bit more of the 1081 sound.

In all cases the units produce a nice amount of 2nd and 3rd harmonic and the top end sounds sweet.

I ran some minimally miced drums through these and A/B'ed an identical setting on a popular plugin N£V£ emulation.
The sound was quite similar, but the long term impression was that when listening through the TNC..
the room that the drummer was in seemed like it was 20˚ hotter and the drummer sounded like he was smiling.... I know.. Rolling Eyes Ok?

I HIGHLY recommend that you run some pretty hot signals through these when you first get them and exercise every switch and pot. I had one or two that didn't pass signal until it got LOUD. This is a symptom of new switches with anticorrosive coatings on the contacts which are intended to self clean through use, not a problem.

Now to more serious matters. I have the same buzzing in my 81s that a few of you have reported. I disassembled the 81 and ran some tests.

Here are my findings so far:

It does not appear to be caused by power transformer induced EMI.

I ran the unit off an external supply and had the same problem.

The "73" and "84" have their inductors equally close to the power supply as the "81" and have none of these problems.

Re-routing the the audio cables (which go to the output pot and phase switch) away from the PSU made the noise floor higher.

Moving my hand in the proximity of the EQ cards made the noise floor higher.

There is a transistor on each of the EQ amp cards that is running very hot (Q4).

I suspect that there may be ultrasonic oscillations in the EQ amp cards. Every symptom I found and every other reported symptom lead me to this hypothesis.

If this is indeed the case, the solution will be much simpler than if it was power supply related. It will require a few very very small capacitors be tack soldered across a few components. I'll run more tests when I am able. Unfortunately, I don't have a working Oscilloscope here and am in the middle of producing one record and mixing two... plus having major dental work performed this morning.. ugh."[/quote]


That was my post over at the Pro Sound Web... I've done some more investigating.

The major difference between the 1081 and the TNC81 is that the EQ line amps in the 1081 use a complementary BC461 / BC441 pair and the TNC use a complementary BC547 / BC557 pair .
The BC441 and BC 461 are rated at 2A Ic and the BC547 / BC 557 are rated at 200ma Ic and they run quite hot.

I've got some BC441 and BC 461s on the way and I will know exactly what's what when I swap them in to the TNC81.


-CZ

 

[clintrubber]

[quote author="zmix"]The BC441 and BC 461 are rated at 2A Ic and the BC547 / BC 557 are rated at 200ma Ic and they run quite hot.[/quote]

Silly us that we didn't spot that when the schematics popped up... nice you spotted that ! :thumb:
Those BC441 and BC 461 in the Neve-DOA's even use heatsinking for them, won't be without reason... and then we/they expect that a simple plastic TO-92 would do as well... :?

Should check the pics, but what's your idea so far, is there room on the PCB to do an elegant replacement ? (including at least some small clip-on heatsinks for those TO-39's)

Regards,

Peter

 

[zmix]

Here is the board layout for the amp:

http://recforums.prosoundweb.com/index.php/fa/10261/5734

And the schematic for the amp:

http://recforums.prosoundweb.com/index.php/fa/10262/5734

Why aren't the images appearing in the post??

 

[clintrubber]

Thanks. That should easily fit. If I'm not mistaken the pinouts are identical as well (EBC).


Apart from that & this hot-hot-issue & the hum, it'd be interesting to keep one preamp with only the necessary mods and compare it with one that tries to come as close as possible to a real one (without swapping the likely dominating audio-TXs): replacing the ceramic caps with polystyrene, using tants etc...

 

[wmtunate]

[quote author="zmix"]
Why aren't the images appearing in the post??[/quote]

The URL has to end in ".jpg" or another image format for the IMG tag to recognize it.

You can add .jpg to the end of those urls, like so:

 

[zmix]

[quote author="wmtunate"][quote author="zmix"]
Why aren't the images appearing in the post??[/quote]

The URL has to end in ".jpg" or another image format for the IMG tag to recognize it.

You can add .jpg to the end of those urls, like so:

[/quote]

Not seeing pictures..!?!

 

[wmtunate]

I don't know. I see them fine.