Formula to calculate output power

[martinbls]

Hi everybody!

I have a small solid state power amp, handmade about 25 years ago (not by myself), and I don't know any of the specifications for this amp. However, it delivers a lovely sound and I'm thinking about upgrading the toroid transformer to a larger one. Currently there is a 50VA transformer with 2x15Vac secondaries, delivering (measured) +-21,5Vdc for both channels.

Is there a simple formula to calculate the maximum constant output power at 4 and 8 ohms, only by the given power supply voltage?

Thank you!
Martin

[AndrewT]

Yes.

[martinbls]

;-)
Thanks Andrew, very enlightening!

By the way, here are a few pics.

[AndrewT]

Joke over.
Yes, the PSU supply determines in part the maximum output voltage from the amplifier.
One must subtract the PSU voltage sag when trying to deliver a short term or longer term output current.
Then one must subtract the voltage lost through the amplifier and any resistances before you reach the output terminals.

For a +-21.5Vdc supply expect ~43/3V into high impedance loads, i.e. low current transients
For lower impedance loads requiring maximum current capability of the PSU and amplifier expect Vpk ~ Vpsu-5V. i.e. ~16Vpk into 8ohm and maybe ~14Vinto 4ohm.

Maximum average output power is given by applying the Formulae:

P = I^2 * R
P = V^2 / R
P = V * I
strictly these only apply to DC operation.

For AC one must add in ac for both Voltage and current. i.e Vac or Vrms and Iac or Irms must be used for AC calculation.
One can also use the peak voltage and peak current (obtained from an oscilloscope as an example) by modifying the formulae slightly.

P = Ipk^2 * R / 2
P = Vpk^2 /R / 2
P = Vpk * Ipk / 2

[Nico Ras]

Nicely made. So why do you want to upgrade the transformer?

[Nico Ras]

I see 2A written on the transformer so probably 60VA. If it uses transistors in the output stage I would say that the output would be around 24 watts sine wave into an 8 ohm load per channel. To audibly detect a reasonable difference you will need to double the power, is it worth the effort?

[sakis]

alps blue line ...relay selector neat and nice construction dontr know who made it and when ...but trust me the man knew exactly what he was doing ...

hei Nico good to see you !!! i hope you are doing just fine

[martinbls]

Hey Andrew, thanks a lot!!

Hi Nico,

well, I always thought a 50VA transformer for a stereo amp might be a little bit on thin ice when it comes to short peaks in current delivery from the ps. The "2A" you read on the transformer is just the suggested secondary fuse. It is a Monacor International RKTM 5015, so it's definitely a 50VA type.

I heaven't calculated the output power by Andrews formula yet, but if you are even close with your estimation of around 24W sine wave per channel at 8 ohms, then a 50VA tranformer is DEFINITELY too small, given the average effeciency of around 60% for transistors!

Regards!
Martin

[AndrewT]

I would estimate 16W into 8r0 and 25W into 4r0.
Since this is a 2channel amplifier, then when 8ohm speakers are used the total output power is ~32W. A transformer rated from 32VA to 64VA will work properly. A transformer outside that range will still allow the amplifier to work.

If you use 4ohm speakers then expect a 50VA to 100VA to work properly and again a transformer outside this range will allow the amplifier to work.

I see nothing wrong with 50VA for a 25+25W domestic use amplifier.

You would have to check the temperature de-rated SOAR for the load impedance of your speakers for that single pair output stage, before considering modifying your amplifier.
It maybe that the designer uses the low VA of the transformer and the low smoothing capacitance to let the output and driver devices survive abuse without needing to use current limiting nor fuses in the secondary side.

[martinbls]

Hi Andrew,

again: Thanks a lot! I did a few calculations and a bit of research and my estimate of about 17W into 8r0 and 27W into 4r0 comes close to your values.

Some time ago I had the opportunity to talk to the constructor and the reson why the transformer is that small is mainly because he wanted to put all the goods of this small stereo amp with integrated passive pre and source selection into this enclosure of just 44mm hight (37,5mm inside).

I was planning to rebuild the amp as a power amp without the volume pot and the source selection, because I want to build a remote controlled passive pre. This is why I thought about using a larger enclosure (same type, but 88mm hight) and use the empty space for a larger transformer of 120VA, just to add some more headroom for peak power. Smoothing capacitance is now 2x4700uF and could be increased to 2x10.000uF (I already have some of these lying around, brand new ones.)

Thank you all for your help and your comments, much appreciated!!

Regards!
Martin