Peavey mark IV input amplifier

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Chrisfromthepast

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I have an old mixer collecting dust, and I can’t let it go, the frame is a road case so its easy to store.
The mixer/road case functioned as a table and backup for the various live consoles we owned over the years at a local dive.
The console isn’t much to look at and has no $$ value, but I remember loving the preamps.
Intent on redrawing the circuit, Im curious and doubting if I can do this at all with an off the shelf potentiometer.
The pot that controls gain in the feedback loop of the op amp seems to have a different type of deck from the attenuator at the input, but this is a single knob.
Its neat, and Id love to try it out in a little modular package, but do I need to tear some pots apart to make this happen?
 

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  • peavey mark iv 16, 24 ch mixer service schematics.pdf
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Whoops

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Hi,
I think it's nice if you want to restore it.

I got a bit confused with your post, in a sense I didn't understand well what were your intentions or what do you need.
I'm pretty sure we could help, but could you rephrase and explain what you really need or what help we could give?
 

Chrisfromthepast

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Mar 5, 2018
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37
Sorry for the vagueness.
If that was too much, please skip ahead.
My lab reports never got high grades in college
I doubt I’ll use the desk because it is not quiet enough for recording work, and it lacks sweepable mids so I can’t throw it in a dive bar even as a backup.
Even if I lowered the noise floor, this mixer still has cheapo 60mm faders. I can’t see getting past that. I have dreamed of restoring it in various ways, but will probably wait to throw it in a practice space.
Im creeping toward the build of one of those 500 series homemade consoles that is so fashionable to dream about these days.
For my drawing board console, I want to use the dimensions of API 200 series faceplate to control the input gain and selection.
So, I’ve been looking at all sorts of designs. This all beside the point...
Anyway, during one of these investigations, I took a good look at the first op amp stage in this peavey mixer.
I can harvest the transformers from the old mixer or just use the circuit as a line amp. I can harvest the input pots too for that matter...
———————-
But, if I wanted to redraw this circuit and make it with new potentiometers, I would need a specialty pot right?
In the schematic posted, I see that R3a is rev log 50k pot
And R3b is log 50k pot.
I can’t find a part like this.
Has anyone had luck taking pots apart and mixing and matching the tracks?
Is there a super cheap stepped attenuator solution?
The design goal is a quick and dirty 5532 based input amplifier thats accepting a wide range of signals, and cheap to build.
 
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Chrisfromthepast

Active member
Joined
Mar 5, 2018
Messages
37
Thanks for your interest!
So, in this peavey circuit, the gain knob is a potentiometer that is similar to a stereo pot, in that it has 2 resistive tracks.
However, the designer at peavey needed a rev log track that worked backwards from a log track, so that the gain would be smooth as you turn the knob up.
Rev log for deck 1 and log for deck 2
I feel like i’ve seen someone swapping resistive tracks on some of the cheaper alpha pots, but can’t find the link.
Is this a thing? Old school analog guys have any tricks?
This is achievable with custom stepped attenuators, but thats not really fitting... spend $150 for input attenuator on your custom bespoke peavey preamp?
That sort of pot is not an inventory item, so I was wondering if there are any creative solutions to classic circuits with this topology:
50k rev log track and 50k log track on the same potentiometer.
 

Chrisfromthepast

Active member
Joined
Mar 5, 2018
Messages
37
Perhaps JR will chime in here.

While crossing my fingers that JR will chime in, I can get a little farther on my own.

In LT spice with a generic op amp model and no input transformer, Im surprised by the lack of gain with the knob maxed out. its more or less unity gain.
With linear stereo pot the 12 o clock position is roughly down 6db, and with the pot almost all the way down, the output signal is down 33db
I've attached some graphs.
This input circuit seems to be less of a gain stage, and more of a buffer to drive the insert send that comes next in this console.
Are there more classic examples of 1 stereo knob acting as attenuator and feedback resistor at the same time?
Id love to compare them.
thanks for your time!
Chris
 

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  • pot centered.jpg
    pot centered.jpg
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  • pot maxed.jpg
    pot maxed.jpg
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  • turned down.jpg
    turned down.jpg
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