HOT 92/90

Posted by: Erik (DK) on 29 October 2001

I have the 92 / 90-3 combo, and at the moment im only using the poweramp as a powersuply for the 92, not for driving speakers. For some (this?) reason the amps are getting really hot from time to time. I have tried to reconnect the poweramp to my speakers, but that hasn't solved the problem. Must the poweramp always be connected to the speakers, while turned on?

I hope somebody's
willing to give me
some advise

Erik Vindbjerg

Posted on: 29 October 2001 by bam

Strange, there has been a recent spate of overheating threads on the forum? Curious. Although timely with the cold weather setting in.

I agree with Omer. If the amp gets hot enough to concern you then there is a fault somewhere for sure. The power consumption of the pre should not be enough to cause this. I assume you are not feeding any signals into the amp.

Posted on: 29 October 2001 by Martin Payne

You can damage a power amp by running it with a signal but no speaker load.

Don't know if this applies to NAPs.

cheers, Martin

Posted on: 30 October 2001 by bam

I am not aware of any amps that can be damaged by not having a speaker connected...which are you refering to? In my experience, feeding signals in with no speaker load should not make any difference under normal circumstances. It might if there is some electrical fault in the amp. That's the only reason I asked.
In general, amps are happier with no external load at all (in fact, internally, the NAPs are loaded with an 8ohm resistor in series with a cap so that the amp sees a load at high frequencies; this helps keep it stable). Very low resistance and high capacitance tends to piss them off.

Posted on: 30 October 2001 by Martin Payne

quote:
Originally posted by bam:
in fact, internally, the NAPs are loaded with an 8ohm resistor in series with a cap so that the amp sees a load at high frequencies

If the preamp is set for a considerable volume, then the power amp is driving a large voltages.

Would large currents be sent through the resistor if there is no other load on the amp? This would cause it to heat up considerably.

cheers, Martin

Posted on: 30 October 2001 by bam

Martin,
Good question.
The series resistor has a value of about 8-ohms. The series capacitor has a value of 0.22uF. The capacitors reactance (which is a word for out of phase resistance) decreases as frequency increases. At low frequencies the capacitor reduces the current flow through the pair and at dc the pair have near infinite resistance. As the frequency increases the capacitors reactance drops until at infinite f the capacitor looks like a short circuit. So the power dissipated by the resistor will increase with frequency.

The maths for the average power dissipated by the series resistor for a sinewave signal is:

P = V^2 / { R + 1/[(2 x pi x f x C)^2 x R]}

X^2 means X squared.
V is the rms voltage at the output of the amp
f is the frequency in Hz
R is the series resistor value in ohms
C is the series capacitor value in Farads

For example, the output of a NAP250 at full wack is about 25Vrms (I think). The resistors power dissipation for a sinewave at this voltage would be as follows:

1KHz 0.01W
20KHz 3.6W
50kHz 18W
1MHz 77W

With filtering in the preamp and the amp's own roll-off you would not get 25Vrms much above 20kHz, probably less than half at 50kHz (giving 4.5W). The power dissipated by the series resistor is independent of whether a speaker is connected. Since the amps are class AB they are less than 70% efficient - so 2W in the resistor would mean about 1W in the amplifier circuit giving a total of 3W per channel.

I think that with a full volume, stereo music signal into a NAP250 with no speakers you will probably be generating under 5W of heat within the case. This isn't a lot: it should keep the frost off but certainly won't get "hot" by any means.

BAM
I hope my calcs are right! Ready to be shot down in flames if not

Posted on: 01 November 2001 by Erik (DK)

Well I guess I was feeding the poweramp with signal, as the cable for powersupply (from the poweramp to the preamp) also carries a signal (from the preamp to the poweramp).
I ran the signal this way in order to use another poweramp.
I guess I should have pressed "mute"?