Melcor opamp and RCA mixer cards

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Deepdark

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Hi

I can't find info on these 2 cards (see photo) the one on the top is the preamp and below is the eq. I thought first it was from a BC100 from RCA but it has 2 transistor on each boards, like the GME-20 (Melcro preamp). So, basically, I would like to know if someone recognise these board and know where they came from :)

Thanks
 

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PRR

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They look the same to me, not minding the replaced caps.

The opamp has pin functions marked. The transistors are a power output buffer or a supply regulator. The board is translucent. Shouldn't take 20 minutes to work out the schematic.
 

Deepdark

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yes they are quiet simple. I guess an input and output transformer in the same order than an API 312 should do the trick, or Melcor has a specific transformer ratio to minimise noise?
 

Deepdark

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Hi there. I just take a look last week at the board and drawn out the schematic, then found over here a document called "The UOAB". I compared both schematic and it's the same. So, What I have is really is that board.

So, it looks to be a kind of multi purpose amp board. What I want to do is an mic preamp, like an API 312. I've just modded the schematic according to. I cut some traces on the schematic and drawn out the pot and transformers. I think it's pretty easy to do but maybe there is simpler way to get it done that I'm not aware of. Let me know :)

The transistors are there to stabilise and set both voltages to +15v and -15v. I would like to know if they tend to fail before I put some juice? Other than that, I'll have to replace all caps, they just dryied out.

Thanks guys :)
 

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Deepdark

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Oh and I would like to know, do I need to cut traces of un-needed parts of the board or it's not necessary?

Other than that, what is the goal of R4 output resistor 4.7R ?

And also, I have the application notes by Melcor Electronics where we find some usefull application of their 1731 opamp. They always put an 10uf coupling capacitor and a resistor of 600 to ground. So, the output is AC coupled through the coupling cap and the resistor charges the cap so the output is at DC ground potential? Any benefit?  Why API don't put these coupling capacitor? For a humble direct coupling to the transformer, I guess we can go straight without any coupling cap and resistor, just like the API?
 

PRR

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> For a humble direct coupling to the transformer, I guess we can go straight without any coupling cap

There is always some DC error at the opamp output.

The old Melcor is not the slickest opamp on the planet, and may have significant offset. (11/9/67... dinosaurs still walked the earth!)

A good OPT has low-low DC resistance.

Some voltage across low-low resistance can be real DC current.

Transformers do NOT like DC current. Spoils the bass.

OTOH, adding R or C in front of the OPT also shaves bass.

OTTH, "some" dumb resistance between opamp and reactive load is often needed to damp parasitics.

Resolving all these trade-offs is hard thinking based on much more information than you have here.
 

Deepdark

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Thanks prr. I think i will try it as à API 312 and judge if it needs some sort of cleaning. I'll try with a cap between opamp and the transformer. Not sure about makinf an RC, since the Hammond has à bandwidth of 50hz to 15khz.

What is the goal of the really liw resistor at the output?

Other than that, should i cut all the un-needed traces or it's just useless?
 

pucho812

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Those cards, are uoab cards.  Uaob = universal operational amplifier board, I handed that document to ethan to post as it was too large to upload.


I have a client who has an old RCA console, same model that was found at RCA nashville in the late 1960's - early 1070's. Depending on when those UOAB boards were made they make have melcor  discrete opamps, RCA branded discrete opamps, or even API 2520. The console itself is a bunch of passive stages with the UOAB as a make up stage.  The console  runs those cards at +/-28VDC  with the transistors as oncard voltage regulation to be +/-15VDC to run the opamps.  The console of which there were only 4 made( so I've been told) is designed  very similarly to an API of the era, controls on the surface with lots of wiring going down to card cages that house all the UOAB's. much like an API with racks of 312 and 325 cards.

The UOAB paperwork is very detailed as to how to hook them up.

In the case of the RCA console for preamps, they have passive attenuators for preamp control  which usually follow the UOAB board.  So there is not really variable gain as far as preamp section, just -10, -20, -30dB attenuation.  There are other places in the console path that have variable attenuation via gotham/neumann 600Ω faders.  but again all passive with make up UOAB make up stage.
 

Deepdark

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Thanks pucho. I see that they handle gain selection via a combination of resistor/capacitor in // with the option to add one other resistor/cap. All of which, wired to a rotary switch or like. I prefer the use of a pit, which is why i opted to go that route and used the free space for an 10k/120pf cap, and a 10k rev log with an 100r resistor, ust like an api. Of course there is probably a pletora of way to handle it, but i only need a mc pre, no rumble attenuator, or all other possibilities it offers. I'll just swap out the output resistor and put an 10uf coupling cap to be sure there is not dc going into the OPT. I plan making an regulated psu with +28/-28v and an 48v rail. With the teansistors handling the +/-15v and all the 50uf caps, i think it will ve a very clean signal. There is a couple of gnd connection, so should be easy to rout everything up. What do you think of the modded schematic i did to use it as an api 312?
 

Deepdark

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pucho812 said:
fwiw you can skip the transistors /regulation and run them +/-15VDC

Thanks. I'll try them at 28v and see if it runs good. In doupt, i'll skip it and run it straight to the opamp

Does the 28v supply was originally regulated in these mixe?
 

pucho812

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The console in question which used these ran the following a bipolar(+/-) 15VDC and bipolar(+/-) 28VDC. The bipolar 15 was  used to power the eq  and compressors, which were both melcor.  the eq a 2 band eq model gme-20, the compressor a melcor cl-20.

The rca designed portion of the desk ran of +/-28VDC but is all UOAB with the  regulation per card.
 

Deepdark

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pucho812 said:
The console in question which used these ran the following a bipolar(+/-) 15VDC and bipolar(+/-) 28VDC. The bipolar 15 was  used to power the eq  and compressors, which were both melcor.  the eq a 2 band eq model gme-20, the compressor a melcor cl-20.

The rca designed portion of the desk ran of +/-28VDC but is all UOAB with the  regulation per card.

Thanks pucho. Plenty of information here. I'm making an regulated psu now to feed both card at +/-28v. Is there any trouvle feeding with already regulated signal to feed the regulated portion of the board?

In other words, I don't know if rca ran the bipolar voltages regulated before the boards, but by my side i will regulate voltages before entering the UOAB cards so i was wondering, what if we feed transistors with already regulated voltages?
 

mjrippe

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Having the voltage already regulated will not make a difference.  You could use unregulated to save a few cents. 

The real question is why RCA chose to use the higher rails, only to lower it at the module.  They could have used an unregulated +/-20v supply and wasted a lot less energy in heat.  It is possible there were some parts running at +/-28v somewhere in the board, perhaps Pucho knows?
 

Deepdark

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Maybe they had some cards running at like 21v or more and 28v was an average, I don't know. For now, i got an 24v power transformer, so about 33v once rectified. 33v-15v made about 17/18v of v difference. An lm317 can handle a max on 40v diff. Between vin and vout. So i could try it out at 28v and if i prefer to run at 15, i could drop it out to 15v i think. I plan using silent art pcb.
 

pucho812

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mjrippe said:
Having the voltage already regulated will not make a difference.  You could use unregulated to save a few cents. 

The real question is why RCA chose to use the higher rails, only to lower it at the module.  They could have used an unregulated +/-20v supply and wasted a lot less energy in heat.  It is possible there were some parts running at +/-28v somewhere in the board, perhaps Pucho knows?

There is nothing in that board running off of +/-28VDC  that I can recall off hand.  There are no relays, all the switches are toggle switches or button switches with real audio on them,. Perhaps it's for the lamps in the switches? but yeah nothing comes to mind.
 
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