Ramsa WP-9110 amp runs warm when idle

[cablercdd1]

I have a Ramsa WP-9110 power amp that we're looking to donate to a school. I replaced a driver transistor a couple of years ago and noticed it runs warm just sitting there with no signal being amplified. Does anyone have experience with amps warming up the heatsinks with no signal? The amp runs fine otherwise. Maybe this is normal for this amp but I thought I'd ask.
Paul

[mozwell]

How warm is warm ?
The output section will draw current, even with no signal. When you replaced the driver transistors, did you check the bias ?
The higher the bias current, the warmer the heastink will get at no signal. It the heatsink is slightly warmer the ambient air temp, i woudl not worry.
The bias current is there to keep all transistors "slightly" on, so as to get rid of crossover distortion.

You can check the bias. Near the power transistors, there should be some power resistors, normally 0.1 ohm to 0.47 ohm, somewhere in that range.
Measure the voltage drop across each one. Use V = IR to work out the current in each one.
If we have 0.22 ohm and 11mV across it, we have a bias current of 50mA
There will be a trimpot to adjust the bias, it will normally be close to the power transistors. slowly adjust this, while measuring the voltage across one of the power resistors.
Turning down the bias current will reduce the temperature of the heatsink with no signal, but if you turn it down too much, you may get crossover distortion.

For your amp, if the heatsink only rises by (say) 15 degrees C above ambient, leave it as it is. A school most likely wont drive the amp hard, so even if bias is a little high, it wont matter.

[g1]

Further to what Mozwell said, if you end up checking the bias, see if one channel is biased hotter than the other. If so, chances are it will be the channel you replaced the driver on. If this is the case, adjust it to match the other (cooler) channel.
If both sides turn out to be biased the same, then it is probably normal amount of warmth for this model.

[Enzo]

Did that channel run hotter two years ago? or do you simply note it runs warm, and it was the channel you fixed two years ago?


Circuits age, parts age, components drift. Amps that were properly biased some years ago my be marginal now. I have found any number of amp that were overheating, even shutting down, and all I needed to do was adjust the bias.


SO yes, amps can get pretty warm just sitting there.

[cablercdd1]

Ok, thanks for all the replies. I checked the resistors which are .27 ohm and had .041 volts on ch A and .05 volts on ch B which give me 15 ma and 18 ma. I don't remember which channel I replace the driver because I retouched the solder connections at the time. Those currents seem reasonable so should I leave it as is.
Thanks
Paul