Determining power supply needs for custom-built preamp

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jdurango

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Joined
Sep 22, 2014
Messages
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If I have a custom built preamp that's missing it's power supply, without any schematics or labelling on the PCB, is there any way to determine what type of power supply voltage rail(s) it needs? It's all discrete, transformer balanced.....would any of the components or anything else give away what type of power it needs? Thanks fellas!
 
It might help to inventory the active devices. 300B or 12AX7 vacuum tubes? 30V transistors? 150V transistors? TL072 chips?

Heatsinking? If all naked TO-92 transistors it is unlikely to be 0.3W each, and likely to be 0.01W for 90% of them. Figure resistors might take as much power again. A Volts-guess and a Power-guess leads to a Current-guess.

We usually set the supply Voltage to design spec and let the current be what it is (what was designed). A variable-voltage power supply of apparently excess capacity is a help.
 
Well it's all discrete, so no IC chips. I'll have to look around for voltage regulators, but don't recall seeing any.....they were probably in the power supply.

If I could just determine the voltage(s)  needed, I can get a beefy power supply that'll handle the current. Figuring out the voltage(s) is the hardest part though.

Perhaps use a variable DC supply, start at a low voltage and ramp up until the circuit "engages"? I'm not even sure how this would work.....feel like there is a high risk of creating a magic smoke generator.
 
You may need to sketch out some of the topology. Many discrete designs use asymmetrical PSU rails, or a single ended PSU.
Start with the input section of the circuit, probably a differential pair of some sort. That can usually help in working out what the rails are intended to be.
 
gswan said:
You may need to sketch out some of the topology. Many discrete designs use asymmetrical PSU rails, or a single ended PSU.
Start with the input section of the circuit, probably a differential pair of some sort. That can usually help in working out what the rails are intended to be.

Okay, I'll try building a little block diagram and share it here. I'm far too amateur to figure this stuff out on my own, even if I knew all the proper equations, it's just beyond me.....and paying a local repair shop is beyond my budget.....and i like learning! Thanks very much for the help!
 
Also worth posting a photo of it, both topside and underside of any PCB too.
Many people here could work out a lot with just a few photos.
 
gswan said:
Also worth posting a photo of it, both topside and underside of any PCB too.
Many people here could work out a lot with just a few photos.

Okay, definitely will do! Thanks a ton for the help!
 
So this is actually a mixing console I just purchased. I had the seller take same better pictures. It appears the PS rail caps are actually INSIDE the mixer, and it's intended for the transformer(s) to be external. AC comes into the mixer, and immediately goes OUT/down  the external PSU jack. I'm assuming the AC feeds external xformers, which then feed the caps/rectifier circuits inside the console. Pics attached. Watcha think?

When I recieve the mixer I'll make a little block diagram with the values of all these components. Thanks fellas!
 
Yeah I'm not sure what the problem is there. I previewed the post and it said the pics were attached. Reported it to the moderators, we'll see what they come up with. Will try later. Thanks for checking anyhow!
 
Okay, this whole uploading one pic per post with 90 second wait in between and still getting a 404 isn't working for me. Here's a public Google Drive folder with a buncha pics. Thanks again everyone for your help! Will have component values soon.

https://drive.google.com/drive/folders/0BxiPOzSDUqGKSGk1V3haS0ROX1k?usp=sharing
 
See it now. It looks like an unregulated supply. I can see bridge rectifier and some caps only at the main PSU section.
It will probably be bipolar, since there are appear to be a couple of caps in a couple of places.

In a couple of photos there is what looks like a simple regulator board at the end, feeding a single PSU rail along one strip of modules (red and black wiring). It has a trimmer on it, probably to adjust the rail voltage. It appears to be responsible for another regulated rail. The wiring to the input modules appears to come from somewhere else.

However the channel circuitry appears to use those old (and noisy) tin-can 741 op-amps. These usually need a bipolar supply of around +/-12 to +/-15V. Since it appears unregulated, it is likely to be +/-12V, since line surges could destroy the op amps if they are run too close to their maximum ratings. Tracing out the wiring to the power pins of the op amps would identify what the power source is for these.



 
Whoops said:
isn't the Power Supply inside?

theres a power cable going into the mixer, it can be seen in the photos and the mains fuse also

There is no mains transformer inside in the photos. The OP mentioned that the mains goes back out through the multipole connection, which sounds like a dangerous idea. Given the vintage of the circuitry I'd suggest it went to an old E core transformer, which may have cause hum in the circuitry due to pickup. It is probably practical to mount a toroid inside now, since the PSU requirements don't look particularly large, a 50VA should do it.
 
Yeah I'm starting to have doubts about how this was thought out. AC goes into the console, then out to an external transformer. It uses LM741 op amps (not discrete) and unshielded Peerless / Altec 15356 line transformers, which are not very ideal for mic pres.

Watches think? Is this thing even worth messing with? Am I better off with a Sound Workshop 1280B or PM1000 or something?
 

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