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LeforaGuest

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Mar 17 12 11:05 AM

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Hello all,

We just finished a major over-haul of our A&DR 700 comp and 760 compex units. This was not just a tune-up and repair, we went through them thoroughly and found some very interesting things that I thought might be valuable to others.

So far, we've seen 4 different schemes for the compexes, 2 for the F700s. Evolution of the designs includes things like extra trim pots for the compression curve, attempts to tame FET gain differences and on-board v regulation.

Here are the high spots, but feel free to ask questions.

- Current draw from pot wipers causes difficulty in adjustments and eventual
failure of the trim pot. Generally a current draw of no more than 20
microamperes DC should be drawn from a potentiometer wiper to avoid this
although pot manufacturer specifications vary on this point. This shows up
in several pots: compression curve, OS bias (F700), meter range, expander
curve. There are more suspect places but if an old pot has a 'dead spot'
then, wiper current draw is a prime suspect. This was not really well known
when these compressors were designed, but it is a problem that we know
about now.

- Drift in zener diode voltages causes some matching issues. Main power
supply might want to use a bandgap reference instead of a zener diode. Also,
I would stay away from Diodes, Inc. zeners and use Vishay or maybe NXP. Not
all zeners are created equally. We changed them all.

- 'inverter' stage setting is very twitchy and probably should not be. The
pot probably should be split up into three resistors to make the pot less
sensitive - the actual trim range should not be that high. We made this mod
with one of our units with good results.

- The control voltage pot of the 760 could be replaced with a shaping
network like found on the later F700 PC board to help linearize the FET gain
law. The risk here is that the F700 configuration would draw significant
wiper current. Adding buffers (just a 2N4401 or something like that) after
each pot might help. Considering this one for our "frankenstein" unit.

- On the F700 the output stage bias current setting is a bit problematic as
when the wiper goes open circuit the stage blows up. Four series diodes work
much better in this position, or perhaps a fixed resistor. We prefer the
diodes. 4 of our vintage units had previous damage here.

- For gain curving adjustments, the compressor should be placed in 1:1 mode
(no detector activity) and with an external CV applied to trim the FET bias and
compression curve for light compression matching, then the parallel compression
curve stages for further gain reduction in stages. Our feeling is that three breakpoints
should be sufficient. Once the compression curve is set with the external CV, the
internal detector gains should be tight enough and only the detector output
should need adjustment. The units would all be stereo matched(mix-and-match)
at this point.

- 1% resistors, please.

- no electrolytics in the compression time constant circuit. These get leaky
and cause the compressor to behave very strangely.

- plan on replacing the input and output pots. ALL of ours were shot or very dicey.

- There's no good way to linearize the threshold control POT on the 760 expander. We
used a type 01 12 way Elma switch in there. Works great and gives a quick match in stereo.

- We managed a .5 db match for pairs in the 6 Helios desk units. This includes CV links
for the 760 XN Expanders. NO attempt to pre-match FETs.

- If there was such a thing as a very compact PC mount multi-turn trimmer, I'd use them
in the curve trims. TOUCHY, even with new pots.

- Contrary to popular belief, there IS a trim for the limiter threshold, it's internal. Tempting
to bring it to the panel, but it's already very crowded.

The sound of the compressor is very nice, I'd put it up there alongside any
of the 'great compressors'. Once adjusted properly, they sound great. It
seems, though, that many problems are attributed to current draw from the
trimpot wipers and twitchy adjustments, as well as the sometimes too compact
layout of the F760's used on the Helios console.





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LeforaGuest

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#3 [url]

Mar 19 12 5:28 PM

Excellent information, David... should be a sticky!

-zmix


Thanks, Chuck. More coming on this subject soon.

Question: if you WERE going to try and develop a method for matching FETs, how would you do it? Also, do you think the beta changes much with age?


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LeforaGuest

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Mar 20 12 12:28 AM

Thanks, Chuck. More coming on this subject soon. Question: if you WERE going to try and develop a method for matching FETs, how would you do it? Also, do you think the beta changes much with age?

-phantom309

Chuck, my edit time-out came and went to quickly....what I MEANT to say is: do you think FET beta changes more with age (and temperature) more so than BJ transistors?

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zmix

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Mar 20 12 9:53 PM

Thanks, Chuck. More coming on this subject soon. Question: if you WERE going to try and develop a method for matching FETs, how would you do it? Also, do you think the beta changes much with age?

-phantom309


If you really wanted to match FETs "properly", I'd hire Larry Janus to develop a 'solid-state' variant of his tube testing rig... !

I have no data on FET aging, but they do tend to be thermally sensitive, and current flow is actually reduced as the temperature rises (unlike a BJT).

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LeforaGuest

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Mar 21 12 9:57 AM

If you really wanted to match FETs "properly", I'd hire Larry Janus to develop a 'solid-state' variant of his tube testing rig... !I have no data on FET aging, but they do tend to be thermally sensitive, and current flow is actually reduced as the temperature rises (unlike a BJT).

-zmix


A FET, data logging, curve tracer. Larry?

So, "thermally sensitive" makes this a constantly moving target. In use (so far) and installed in the console, our calibrations seem to be holding well. Maybe the FET gain delta isn't as important as we feared.


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studiochap

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Nov 28 12 3:49 PM

I've done a miserable thing - swung round in a control-room chair and the chair arm totalled the meter on my friend's Compex limiter which was sat in a rack behind me...doh. Not just the glass, the needle and all.

I emailed Sifam with the meter serial # and spec  ( FSD 1ma) , who asked which country I was from, so they could pass my query to the relevant people, but have since heard nothing...

Anyone got an idea of the specific Sifam type number, or a replacement which could take the existing calibration plate? Or got a spare meter they could sell?

Any help here greatly appreciated!

Thanks,

Gwyn

Gwyn Mathias Mixer Madness in one form or another since 1972... Happy Sequoia user!

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LeforaGuest

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Nov 28 12 11:52 PM

I probably have one, Gwyn. I'll look for you tomorrow. Sifam is practically out of business in the UK. The company was sold to and East Indian firm and now only the BAD versions of the Director series are still made. Ask anyone that has ordered them lately and you won't be enthused.

Anyway, I'll see what we have. They might have all gotten used for our desk rebuild.

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LeforaGuest

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Nov 29 12 12:39 AM

I have one Gwyn, BUT it's gain reduction reading to the right and it's a white face. I haven't tested it yet. Would that work for him?

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silvertone

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Nov 29 12 6:39 AM

Tony Arnold is doing the remakes of the 760.  I have one here right now for beta testing.  It has the black faced meter.  Try getting a hold of him if that's the style your looking for.

I also have a preliminary manual for the 760 as they are a little tricky to set up if anyone is interested I could mail a pdf.





Click here to view the attachment

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johnr

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Posts: 1,561 Member Since:25/01/2011

#14 [url]

Nov 29 12 1:02 PM

Andy, you might not realise it but putting your email address in a non-obscured form on a publically accessible web forum will result in a lot more spam. It's better to make it unreadable by spammers' email harvesting bots but still understandable to humans, eg. by changing "@" to "at" or "." to "dot".

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compasspnt

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Nov 29 12 3:35 PM

And because of that, here is Andy's message converted:

Hi Silvertone,

   I have a couple of original 760 from a Helios console that are in a very sorry state. I would be very very grateful to get hold of the manual if that is ok?

  I am


strangeandbouncy at mac dot com

   kindest regards,

     ANdyP

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studiochap

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Posts: 254 Member Since:05/02/2011

#17 [url]

Nov 30 12 6:51 AM

Thanks guys for your knowledge and suggestions re a replacement meter! 

David, I'll PM you.

Thanks again,

Gwyn

Gwyn Mathias Mixer Madness in one form or another since 1972... Happy Sequoia user!

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ssltech

Aqua Marine

Posts: 4,034 Member Since:22/01/2011

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Nov 30 12 9:44 AM

Gwyn, Be aware that the newer 'directors' are the ones like the one shown in Larry's photo. -They are LED-illuminated, have a very slender "stick" pointer and no 'mullion' frame around the glass. The mounting screw holes are "east and west".

The original one has a broader "sharpened-tip" pointer, is unilluminated and has a strong 'frame' around the lens... (though not strong enough to protect against control room chair backs!) and the mounting holes are "Northwest and Southeast" ...if you're picking up what I'm laying down...

They are left-reading, though it may be possible to convert a right-reading meter by rotating the rest position to make it sit to the right, and reversing the terminals to reverse the direction of motion.

There are some out there to be found, but they're not common. The Orange County limiters also used them (I think they were a poor copy of a Compex) and i do have a couple of them myself, but I'm saving them for a Compex replica build ("one day...")

Here's the meter next to an A&D meter (and a 1mA RIGHT-READING Director atop the ADR case for comparison):

The motor on the back of the original is MUCH larger and better damped than the newer version:

(Two original meters on the right hand of this 'identity parade', newer Director is second from the left, GE meter on the left).

If I find another one, I'll let you know Gwyn, but from memory, I think they're 1mA, left-reading GR scale Director 14's, which I'm pretty certain which you won't find new any more...

-Keith Andrews -If I can't fix it, I can fix it so [i]NOBODY[/i] can fix it!

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