Altec 1592A - Direct Outs Mod

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smilan

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Joined
May 29, 2017
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463
Hi, I have an altec 1592A mixer that I would like to add to it direct outs from each pre amp.
I'm thinking to mod the music / speech switch, remove the 1uf cap and use this switch to select if the signal from pin 1 of the 1588B will go to the direct out jack or go to the master channel.
Here you can see what I've done with one of the channels:

2mrsit5.jpg


It seems to work well but there's one problem that I don't know how to solve: when the modded switch passing the signal to the direct out jack there's a leakage of the signal going to the master channel at the same time...
I was thinking that is coming from the channel pot but it acts the same way also when I disconnectedpin 1 of the 1588B from the 750 ohm pot, so it seems like there is a connection from the 1588 socket to the master channel that I cant find.
Please advice, what else should I check ?
Here's the schematics of the 1592A  and the 1588B
2uffz12.jpg


[
altec-1588b-microphone-transformer-schematic-modification.png

 
I think that the GROUNDING of the new output jack is part of the problem. How much "leakage" do you have? Does it change when the level control in fully CCW? You may want to look @ DPDT switch where you ground the output signal when not used.
Duke.
 
The 820r goes with the bus, not the pot.
 

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Arguably, the un-used 820r bus resistor should be grounded (perhaps through another ~~100r to preserve mix balance).

If you also need to switch the Direct Out on/off, this needs a 2-pole switch, so may not be possible on the stock part.
 

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Thanks Duke and PRR,

Audio1Man said:
I think that the GROUNDING of the new output jack is part of the problem. How much "leakage" do you have?

When I applied 1 khz at 3.2 v peek to peek (measured at pin 2 of the xlr input jack), while pin 1 of the 1588 was disconnected from the 750 ohm pot I got 1.2V on the output.

Audio1Man said:
Does it change when the level control in fully CCW?

The channel level control doesn't effect on the leakage while the master level control effecting the leakage in a minor way.

Audio1Man said:
You may want to look @ DPDT switch where you ground the output signal when not used.

I'm lucky because Altec already put a dpdt switch for the music / speech switch (and used only 1/2 of it).
I've tried to connect it as PRR suggested (on his second reply) and still I got this leakage...
index.php

So I disconnected the direct out jack, the the connection between pin 1 of the 1588 to the 750 pot and the connection between the 750 pot to the bus and still, signal from the microphone goes to the master out.
What else can I check?

 
> What else can I check?

C26.

But how much is "leakage"? There will always be SOME leakage. -60dB? -10dB?
 
PRR said:
But how much is "leakage"? There will always be SOME leakage. -60dB? -10dB?
I hope I'm measuring it right:
When measuring on pin 1 of the 1588 1 khz at 0 dbu (0.775 v) I'm getting on the main output -23 db (when the master pot is turned 100% CCW) and -15 db (when the master pot is turned 100% CW).
 
Dubious design. EVERYTHING runs from C26 with NO local decoupling.

I guess when C26 was new and 2,000uFd, it sucked-up signals trying to sneak along the power rails.

And now, some years later, when C26 is feeling flabby, it doesn't control sneakage so well.

Lucky guess. Glad it helped you.
 
PRR said:
Dubious design. EVERYTHING runs from C26 with NO local decoupling.

Interesting that there is not even a decoupling cap inside the preamp module.  Possibly due to space limitations.  Maybe 100uf across pins 5 + 6 would be a good idea?
 
mjrippe said:
Interesting that there is not even a decoupling cap inside the preamp module.  Possibly due to space limitations.  Maybe 100uf across pins 5 + 6 would be a good idea?
I'll try that and see how it works
 
One more thing, I really like the EQ on the master channel and I was wondering if I can duplicate the EQ circuit (C23, P8, R42, R43, P7, C21, C22, R41) and add it to the direct out without any additional active amplification?

2uffz12.jpg
 
smilan said:
I was wondering if I can duplicate the EQ circuit (C23, P8, R42, R43, P7, C21, C22, R41) and add it to the direct out without any additional active amplification?

Unfortunately no, to have the same result you'll also need the section around Q4.
(Ignoring the intermediate buffering & amplification from Q1,2,3 for now)

There might be re-design alternatives (passive, or IC-opamp), but these circuits are not that large, could consider duplicating the original approach. 

Bye
 
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