Power rating of an IGC

[Dr.H]

Janneman:

Maybe it was unclear from my post, but I am using a 300VA, 22VAC transformer per channel. Thus I have about + and -31V DC per channel. This would suggest that I am able to swing up to 31 V on the output, which is not what I measure.

Any views on waht could be causing this issue?

[GregGC]

Originally posted by Dr.H
Janneman:

Maybe it was unclear from my post, but I am using a 300VA, 22VAC transformer per channel. Thus I have about + and -31V DC per channel. This would suggest that I am able to swing up to 31 V on the output, which is not what I measure.

Any views on waht could be causing this issue?

So what is the rail voltage when you start seeing the distortions and what is the peak amplitude?

Thanks.
Greg

[macboy]

I suggest that if you have a dual trace scope, connect the other channel to monitor one of the power supply rails. This way you can literally see whether your power supply is up to the task.

You transformer is more than big enough, in fact it is overkill. But with only 1000 uF of cap on the power supply, you are asking for problems. A 40 W RMS amp (for example) will draw an average current from each supply rail of over 1 ampere (P=I^2*R, and remember each rail supplies power for 1/2 the time). Drawing 1 ampere from a 1000 uF cap will discharge it in much less than the 8.3 ms that it must hold its charge (assuming a full-wave rectifier, the power supply crests are 1/120 second = 8.3 ms apart). If you want more power, you need more capacitance.

[PRR]

> the maximum output voltage should correspond to the rail voltage?

In an ideal amplifier of this type, yes. The peak output voltage is equal-to or less-than the rail voltage (and in real life, always less-than).

Assuming you have +/-31 volts and that does not change with load, and assuming the amplifier had zero loss, you should clip at 31V peak or about 58 watts in 8Ω.

If you only had say +/-15V, perfect regulation, perfect amplifier, 15V peak is 10.5V RMS or 13.7 watts. Power goes by square of the rail voltage, so modest changes in voltage make significant changes in power numbers. (Not so much difference in Loudness: loudness is more like voltage than power, and double-voltage is not-quite double-loud.)

Real amplifiers have real losses, often 5 volts or more per side at high current. Still assuming a perfect power supply, but 5V loss in the amplifier, 31-5=26V, 26V peak is 18V RMS is 40 watts RMS.

You have an oversized transformer but undersized capacitors. Some say 1,000µFd is a magic value for "sound"; in which case don't question "authority" or ask about power (after all, who cares about power, it is sound we want to hear).

If you must do the numbers, I will guess that under load your +/-31V sags to +/-25V DC or lower at the ripple-dip. 25V-5V=20V peak, which is 14V RMS, about what you measured. 31V-25V droop is quite reasonable for an under-capped power supply.

On purely mechanical grounds, I want to see over 2,000µFd across the rails of an 8Ω amplifier eating 60Hz power. That would be two 4,000µFd caps for split supply. And at modern cap prices, I would use 2 to 5 times that much. The rails will be stable, and there won't be much buzz-breakthrough in clipping.

Other people with better ears feel that a too-stable supply is un-musical, and use 1,000µFd caps max. I see the argument for guitar amplifiers, where clipping and power-sag is part of the tone. I am unconvinced about "minimal" rail capacitance for general audio; I do know many happy designs with startlingly small power caps. I do know that in the days when I tinkered, I often liked the sound better with huge caps (and they certainly look better on the scope).

If you want big power numbers, put in big caps.

If you want "sound", try the classic under-capped design and compare with some fat caps added upstream of the chip (close to the rectifier DC output).

[Dr.H]

Thanks guys, very useful info. I will have to measure the supply voltage when the clipping starts and will report back.

[carlosfm]

Originally posted by macboy
If you want more power, you need more capacitance.