Transformer Load Regulation Problem..

I have a 24-0-24 5A transformer for powering up a 25+25 Watts RMS Stereo Amplifier that drives two 8-Ohm speakers. The amplifier requires around 22.5 Volts to produce 20Volts output across each of its speaker loads.

But the transformer provides only about 20Volts at the secondary for current of 1.5 Amperes.. That essentially means, my amplifier simply cannot produce the required voltage at the speakers and generate the rated power output.

Should I get hold of another transformer or do wat? Spmebody please assist me on this ..

Regards,
Prasanna
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Hi,
something wrong with your measurements.
24Vac should read around 26Vac when no load and mains at nominal supply voltage.

26Vac should then produce a peak voltage of around 36.7Vdc and after the rectifier the smoothing caps should be charged to around 36Vdc.
When supplying quiescent current to the chipamp this supply voltage will probably fall by between 0.5V and 1V to around 35Vdc.

Where could your PSU have gone wrong if you are seeing only 20Vdc?
 
xitronics said:
I have a 24-0-24 5A transformer for powering up a 25+25 Watts RMS Stereo Amplifier that drives two 8-Ohm speakers. The amplifier requires around 22.5 Volts to produce 20Volts output across each of its speaker loads.

But the transformer provides only about 20Volts at the secondary for current of 1.5 Amperes.. That essentially means, my amplifier simply cannot produce the required voltage at the speakers and generate the rated power output.

Should I get hold of another transformer or do wat? Spmebody please assist me on this ..

Regards,
Prasanna
_______________

Hi Prasanna,

Is this a newly-purchased transformer? Did you acquire the '24-0-24' rating by measurement, or from a label on the transformer? Or where?

What was the setup when you measured 20V at the secondary? Firstly, what type of measurement was that? i.e. Was it AC Volts RMS, DC Volts, AC Volts Peak, or what? And was it actually measured across the secondary winding, or across power supply filter capacitors, or where?

What setup did you use to get a 1.5A current, and how did you measure that?
 
Ya, its a newly purchased transformer.. Label shows 24-0-24 as I explicitly told him to give me a 24-0-24 5A transformer..

Now some info about the measurements I made on it:

Under no load condition my DMM shows 24.1 Vac approx on both sides of the centre-tap..

Rectifier output is around 33.6 Volts under no load..

Speaker's DC Impedance is 8 Ohms exact. I get around 19.78 Volts across its terminals(and 2.4A current) while it is fed rectifier's output -- stupid I knw as it cud hve burnt the coil windings but I had it in contact with the PSU for only abt a second. Not a skillful work though.. :-

Also, mine is not a chipamp, its using the 2N3055-2955 pair.. Works great but not its best..

_________
Prasanna
 
xitronics said:
........ Label shows 24-0-24 as I explicitly told him to give me a 24-0-24 5A transformer..

Now some info about the measurements I made on it:

Under no load condition my DMM shows 24.1 Vac approx on both sides of the centre-tap..a
what is the rated input voltage?
What was the mains input voltage.
Without these two bits of info your measurements are almost worthless.
 
xitronics said:
Ya, its a newly purchased transformer.. Label shows 24-0-24 as I explicitly told him to give me a 24-0-24 5A transformer..

Now some info about the measurements I made on it:

Under no load condition my DMM shows 24.1 Vac approx on both sides of the centre-tap..

Rectifier output is around 33.6 Volts under no load..

Speaker's DC Impedance is 8 Ohms exact. I get around 19.78 Volts across its terminals(and 2.4A current) while it is fed rectifier's output -- stupid I knw as it cud hve burnt the coil windings but I had it in contact with the PSU for only abt a second. Not a skillful work though.. :-

Also, mine is not a chipamp, its using the 2N3055-2955 pair.. Works great but not its best..

_________
Prasanna

Hi Prasanna,

Wow. I wouldn't do that to your speaker, any more!!!

What I am wondering is why you didn't, instead, measure the rectifier output voltage with some worst-case 'amplifier & speaker' load. If it doesn't sag below what you expect, then you shouldn't have any problem.

And I am not sure that measuring DC applied directly from the rectifier output to the speaker terminals for one second or less will give any meaningful indications about your transformer.

And I guess I don't really know what your 2.4A measurement means. If your speaker stayed at 8 Ohms, for that, then that would imply that your power supply gave it 8 x (2.4)^2 = 46.08 Watts.

But, if the 19.78 volts and 2.4A are both measured values, then that would imply that your speaker's resistance was actually 8.24 Ohms. Otherwise the current would have been 2.4725A. Using the 8.24 Ohms and 2.4A figures, power given by (I^2)xR was 47.46 Watts, which is about the same as V^2/R gives.

So, that was for one channel only? :)

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If you REALLY need to know what your maximum output power is, and assuming that you have no other test equipment, I have thought of a way that you could create a mains-frequency signal for the input of your amp, so that your RMS meter could be used to somewhat-accurately measure the RMS voltage and current of a mains-frequency sine signal being applied to the speaker by the amp, so that you could then calculate the measured output power into your speaker as Vrms x Irms, with the volume control set at max and the input voltage at the maximum safe value. But it's probably too risky for your equipment, and you, and might not be a very accurate method anyway, and would probably just be 'another whole can of worms', not worth trying.