# Audio amplifier power supply question

## Main Question or Discussion Point

So I had an old home receiver which broke down so i took out the large transformer 6800uf caps and rectifier. This receiver put out 60 watts out of each chip so from my many hours of being on google in the past, the chips should require +-35v, and because of the increase in voltage after rectification and decoupling, this transformer should put out 24vac. 12-0-12

when i tested the transformer is was putting out 30-0-30!
after the rectifier it was putting out above 70 and the 6800uf caps are only rated for 50v!
And i looked inside the receiver before taking these parts out and found it to be just the simple split power supply, nothing else.

for my project i need around 30-40v split supply. I cant think of a real way to drop the voltage down half of what it is but i did have an idea of rectifying between both the 30s and 0 giving two 30v outputs, then i would put them in parallel. but it continued to give me 70 for some reason.

anyone have an idea for getting what i need out of this transformer?

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If this was the original power supply, and the transformer was not damaged, replace the filter capacitors.
Is this a tube type? or a solid state? What is the rectifier?
Advice is to find the original schematic and do a restoration on the parts. Electrolytic capacitors as well as paper capacitors go bad over time.

Averagesupernova
Gold Member
Your math doesn't make sense to me. Driving an 8 ohm load with 21.9 VAC will get you your 60 watts average. The peak value of 21.9 VAC is about 31 volts so when rectified and filtered will be about 31 volts DC assuming zero ripple. So you need a 31 volt dual supply. Naturally the supply will need to be higher than this since there WILL be ripple and the valley of the ripple voltage cannot be lower than the peak of the largest signal. Are you sure the rectifiers and capacitors are configured in the circuit the way you think they are? If you measured 30-0-30 off the transformer then rectifying each half will get you about a 42.5 volt dual supply. Low enough to not harm the filter caps and high enough so the ripple voltage will not affect the output.

Ok wait a minute I think I might be looking at split shpply wrong...
Puting the 2 main lines through the rectifier gave me like 70v...is this because it is posotive 35 and negative 35? And did the 2x35v caps not pop because they were only seeing 35v each sence they were grounded?

I have always tgought that for say a 12v split supply you need 12v dc the the 2 caps between negative and posotive and then that gives -+12v

But is it actually the difference between the 2 that you read I am reading?

In other words if I have +12 and -12 then will I read 24v?

Because if so then I am a dumb butt

Averagesupernova
Gold Member
Well I wouldn't call you a dumb butt just yet. LOL. You are correct in that the +12 and -12 volt supplies will make a voltmeter read 24 volts. Just think of it as two 12 volt batteries in series with the node between the batteries the common. Put one lead of the voltmeter on this node and move the other one around. One configuration will get you a +12 reading and the other configuration will get you a -12 reading. Measure across both batteries and you will get 24 volts.

So... I am soposed to read 70v!

I have actually built amps for people in which I now see I was powering with half the voltage...darn! Thanks guys!

Hey if you have 2 amplifiers in parralel will your total harmonic distortion double?

AlephZero
Homework Helper
Hey if you have 2 amplifiers in parralel will your total harmonic distortion double?
What is "1% of 100W plus 1% of 100W", as a percentage of 200W?

Oooooooooooooh

You are genius. Thank you so much.

BAAAAAD idea to run amps in parallel!

Regards

Malc

Edit... Unless you know what you're doing!

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AlephZero
Homework Helper
BAAAAAD idea to run amps in parallel!
No, it can be a very good idea when done right. E.g. these speakers (retail price about $6,000 - that's$6,000 each, not for a pair)
Go to http://www.genelec.com/products/previous-models/1038a/ [Broken], click on "documentation" then "data sheet"

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Averagesupernova
Gold Member
No, it can be a very good idea when done right. E.g. these speakers (retail price about $6,000 - that's$6,000 each, not for a pair)
Go to http://www.genelec.com/products/previous-models/1038a/ [Broken], click on "documentation" then "data sheet"
So what? I don't see any power amplifiers in parallel.

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So what? I don't see any power amplifiers in parallel.
Have to agree with that. No worries guys I know what I am doing, there are certain things you can do withing the circuitry to make it safe.

Bit of a misunderstanding here. I assumed you were connecting the outputs of your amps together i.e. +ve to +ve and -ve to -ve (or hot to hot and ground to ground)

Of course, using separate amps for different frequency bands using active crossovers is common practice, particularly in large audio systems.

Then there's bridging amps for a bigger voltage swing into higher impedance loads but that's a different topic.

Malc

Averagesupernova
Gold Member

Bit of a misunderstanding here. I assumed you were connecting the outputs of your amps together i.e. +ve to +ve and -ve to -ve (or hot to hot and ground to ground)

Of course, using separate amps for different frequency bands using active crossovers is common practice, particularly in large audio systems.

Then there's bridging amps for a bigger voltage swing into higher impedance loads but that's a different topic.

Malc
The separate amps in the case of the link provided by AlephZero is simply one amp per speaker except it looks like the subwoofer uses two amps in bridge mode.
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Anyone here familiar with strapped amps? I have never tried it.

Hi AverageSN...

I've never *deliberately* paralleled (strapped?) amp outputs - that's how I know it can be a bad idea - mega instability and other nasties!!!

Bridging amps, as you probably know, can either be dead easy, not quite so easy, or somewhat tricky.

I'm assuming that we're talking about stereo (two channel) amps in the same enclosure, as are most professional power amplifiers. Just in case anybody's unfamiliar with bridging output stages, here's a brief explanation.

Most pro amps have a bridge mode switch. Press it and it inverts the signal phase on one channel. This turns the amp into a sort of "push/pull" configuration. When the output from one amp goes +ve the other one goes -ve. This has the effect of doubling the voltage swing into the load, which is connected between the +ve outputs of each amp. Some amps which have Neutrik Speakon output connectors have a separate connector wired for bridge mode. C Audio amps among others have this output configuration.

Slightly more tricky are amps which don't have a bridge switch but have balanced inputs, usually on XLR connectors, if they have link through input connectors, connect channel 1 to channel 2 with a link cable wired pin 1 to 1, 2 to 3 and 3 to 2, i.e. a phase reverse cable. You will need to turn both amp gains to the same level, preferably to max for this to work properly. Output is connected as before between the +ve outputs.

If your amp has unbalanced inputs, you'll need to find a way to apply a phase inverted signal to one of the inputs. If the equipment feeding the amp has a balanced output on XLR or 3 pole jack, you can make a cable that connects XLR pin 2 or jack tip to one input and XLR pin 3 or jack ring to the other one. XLR pin 1 and jack sleeve goes to both input grounds.

If the equipment feeding the amp has unbalanced outputs, you'll need to find a way to make the signal balanced. A D.I. box with a balanced output might work, or a balanced line driver or centre tapped transformer. You'll need to make connections as above. You can make a balanced line driver from a dual OP-amp like an LF353 or equivalent. There are plenty of circuits for this on the web.

Wow! This turned out a bit longer than I thought it would. My apologies if I've gone over familiar ground.

Cheers

Malc

Averagesupernova