Power supply build question from a first time Amp buldier

[richard.C.]

Hi all,
First time building a chip amp, well the amp and power supply are prebuilt, and for reference the links below are what i purchased off ebay to work with.

Amplifier Power Supply Assembled Kit for DIY NEW | eBay

150W LM3886 X 3 mono channel Audio Amplifier Board HiFi | eBay

The amplifier circuit asks for a 30-35VDC power supply, the PSU caps handle 50V, easy. Now the transformer i pulled from an old blown amp i have, the secondary side measures 0-34-68VAC or 34-0-34. Now immedietly i belived this transformer is a perfect match for the amplifier circuit voltage demands. I have hooked the 34-0-34 to the ~~-GRD-~~ to the AC side of the rectifier/PSU when i measure the DC side of the rectifier/PSU (Luckly i did) DC+ DC- gives me 92.4VDC, DC+ and (ground) gives me 48.2VDC.
So 92V is deadly for this amp, why is it so high, is it an anomaly in my measuring or is it expected. Do i need a lower V transformer from here?
Also i may add, i wired up the 0-34 (no ground) to the AC side of the rectifier/PSU ~~-~~, and i got from the DC+ DC- side, 48.2V, still too much. Is this all expected for someone more schooled in the subject. If i was to get lower V output transistor, what would it need be? 25-0-25?

thanks,
rick.

[Mooly]

No anomaly

The AC voltage is an RMS value. Lets take the 34 volts AC rms. If you viewed that on a scope it would measure 34 * 1.414 which is 48 volts peak.

1.414 is square root of 2

The capacitors in the PSU charge to this peak value.
You need to do some reading...

http://pyramid.spd.louisville.edu/~k.../labs/lab2.pdf

[Mooly]

And this,
Unregulated Power Supply Design

[richard.C.]

Ok, there is a little math involved here. But a long story short, i need another transformer with a lower AC voltage output to work from, i may have another wreck to scrap. Oh thanks for the link, i don't mind a maths challenge, see if i can work it out.

[Mooly]

Yes, take the DC voltage required say 30-0-30 volts DC and work back.

So that's 30/1.414 which is 21 volts. So you need a 21-0-21 volts AC tranny.

The theory and maths is cast in stone... in practice be aware of the following.

The transformer will be quoted as a given voltage when supplying its rated current. In other words at full load.
Off load... which is what your amp will see when just sat idling or at low or "normal" listening levels and the voltage will be higher. Maybe by as much as 10% or so. That figure is the "regulation" percentage of the transformer. As a general rule bigger heavier transformers have much better regulation than small one.

Your mains voltage... can that vary by much ? If its 230 volts nominal then what does it go to worst case... here ours can be as high as 253 so again that difference reflects in the output.

Always better to play safe and the difference in audio output (subjectively) is minimal given a slighly lower DC voltage... and the amp runs cooler and is more reliable.

A 20-0-20 vac tranny will give -/+28volts DC... maybe a bit higher as mentioned off load so I would go for something like that.

A 25-0-25 which would give -/+35 volts DC, but again the voltage at low loading will be a little higher.

[richard.C.]

Thanks again for the links Mooly.

[Mooly]

You are welcome... hope the amp goes well.

[richard.C.]

300VA, about 20-0-20 toroidal might be the way to be then!? though the problem might be finding one of those specs, commonly i'm seeing 18-0-18 and 25-0-25 on the net, not 20V or 21V ones

[digits]

I'd go for the whole 35V, the supply will likely sag under load anyway.
Common voltages for transformers for the the national chipamp range is about 24 to 26VAC i.e. 34VDC to 36.8VDC. Yah OK now you wanna know what 25VAC is...lol, just over 35 and a quarter VDC.

Of coarse with single chips you'd pick a much lower voltage if you wanted to drive 4 ohms.

[Mooly]

Providing you have sufficient heatsinking on the chips the higher 25-0-25 should be OK as having checked the data sheets for the LM3886 it's well within limits.
Worst case you could approaching perhaps -/+40 volts DC with the amp idling. Again che
ck the tranny specs... a 230 volts tranny on 240 volts (or a bit more) will give a correspondingly higher output.

Don't underestimate heatsinking though... remember that watts (power dissipated in the silicon) doesn't rise linearly with increase in supply voltage.

To make this easy to understand imagine thinking of this as a DC problem. If an output transistor (in the IC) is supplying 16 volts across the load (8 ohm) then the current is 2 amps.

Case 1. Amp running on -/+28vdc. The IC is "dropping" 28-16 volts which is 12 volts. 2 amps is flowing. That gives a dissipation of 2*12 which is 24 watts (in the IC)

Case 2. Amp running on -/+35 vdc. The IC is "dropping" 35-16 volts which is 19 volts. 2 amps is flowing. That gives a dissipation of 2*19 which is 38 watts (in the IC)

So don't underestimate things...