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Power amplifier running too hot?

Timcat

pfm Member
I have two Bryston 4B3 power amps; one a dealer's demonstrator that is 5 years old and a second that is brand new (the demonstrator will be going back this week). The new one runs much hotter than the older one, both at idle (switched on but with no music being played) and under load. The difference is very obvious to the touch. I put a temperature probe on the heat sinks of both amps and the old one measured at 37.5 C and the new one at 45 C, both under the same very light load. The probe was placed in exactly the same place on both amps and both channels of each amp measured the same. The amps were measured in the same location.

This is quite a large difference, anyone know of any possible reason for this? I am concerned that something is wrong with the new one. Maybe something is out of adjustment, or could there have been a minor design change in the 5 years since the older amp was built?

The new amp does sound a little more detailed and refined than the older one, but the difference is very slight.

Anyone have any ideas?
 
It sounds likely that the newer one has the quiescent current set slightly higher than the older one.
In the context of a 300WPC amp with voltage rails high enough to give this power it could be caused by just a very few mA difference in this current.
I wouldn't worry about it.
 
Most of the transistors and resistors can withstand a temperature of 125 C so they are all well within their normal range.
 
It sounds likely that the newer one has the quiescent current set slightly higher than the older one.
In the context of a 300WPC amp with voltage rails high enough to give this power it could be caused by just a very few mA difference in this current.
I wouldn't worry about it.

Thanks Jez!

Thinking about it and bearing in mind that the two channels are running at the same temperature, maybe this is a deliberate change by Bryston, as the amp is a true dual mono design with a separate PSU (including the transformers) for each channel.
 
Heat sink at 45 degrees doesn't mean transistors at the same temp. Transistors at 125 degrees will be fried in no time.
 
Thanks Jez!

Thinking about it and bearing in mind that the two channels are running at the same temperature, maybe this is a deliberate change by Bryston, as the amp is a true dual mono design with a separate PSU (including the transformers) for each channel.

There's a bias setting that is optimum in a class A/B amp and they need to set up within a few % of this setting, with a 300WPC amp that temp difference could well be just the effect of each amp being at different ends of that few % range or they could have maybe changed the output transistors to ones which need a few more mA to be at optimum.
 
There's a bias setting that is optimum in a class A/B amp and they need to set up within a few % of this setting, with a 300WPC amp that temp difference could well be just the effect of each amp being at different ends of that few % range or they could have maybe changed the output transistors to ones which need a few more mA to be at optimum.

Thanks again Jez!
 
if they are both the same then happy days and a bit more bias into class a wont be a bad thing.. I asked Atc the same question as i had an older sia2-150 which ran hotter than a newer and they confirmed that they dropped the bias down a bit as people complained they were running too hot....
 
if they are both the same then happy days and a bit more bias into class a wont be a bad thing.. I asked Atc the same question as i had an older sia2-150 which ran hotter than a newer and they confirmed that they dropped the bias down a bit as people complained they were running too hot....

Both channels are running at the same temperature in the new amp. The same applies to the older one too.
 
Idle power for the 4B3 is <=80W. I.e. up to 40W into each heatsink.

I would expect that to get quite toasty. 45C seems reasonable (20C rise with 40W means the heatsink is 0.5 C/W, which seems about right for the size).

Bryston specify 85C as the max (ouch :eek:). Both channels run at 1/3rd max power is the worst case for dissipation. I doubt that most domestic speakers could cope with anything like 100W average power!
 
Idle power for the 4B3 is <=80W. I.e. up to 40W into each heatsink.

I would expect that to get quite toasty. 45C seems reasonable (20C rise with 40W means the heatsink is 0.5 C/W, which seems about right for the size).

Bryston specify 85C as the max (ouch :eek:). Both channels run at 1/3rd max power is the worst case for dissipation. I doubt that most domestic speakers could cope with anything like 100W average power!
What is the reason for 1/3rd max power being the toughest test for an amp? I apologise in advance if the answer doesn't fit onto the back of a postcard! :D
 
What is the reason for 1/3rd max power being the toughest test for an amp? I apologise in advance if the answer doesn't fit onto the back of a postcard! :D

The version that does fit on the back of a postcard is the for class A/B (and class B really) roughly 1/3rd power is the point where the output devices have to dissipate the maximum heat. Of the power provided by the power supply some goes to drive the speaker and some gets wasted as heat. Efficiency is worst at very low powers and at max power the greatest efficiency is reached as the greatest proportion of the input power is transferred to the speaker. For class b the max theoretical efficiency is 78.5% but in practice for class A/B somewhere around 65% ISH is more typical.
 
I decided I didn't like hot running amps. I had an Emotiva XPA-1 and XPA-2 per side. XPA-1s, when switched into "Class A" (just much higher bias setting, so enriched A/B) got VERY warm.

When I decided I didn't need two mono amps per side, I sold XPA-1s and kept the 2s.
 


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