Bias vs. heatsink temp on Bryston 4BSST

[jimangie1973]

I recently bought a used 4bsst and noticed immediately that one heatsink channel was much warmer than the other. I checked the bias using the procedure here:

http://www.bryston.com/BrystonSite05...STRUCTIONS.pdf

I tweaked both channels to get them within 1 mV of the 25 mV value when fully warmed up. Even so, one heatsink is still warmer than the other, not dramatically, but enough to feel the difference.

Any ideas on a reason for this? Does it even matter?

Thanks,
Jim

[AndrewT]

Hi,
can you check the temperature of the individual transistors mounted on the heatsinks?

The cooler heatsink may have 1 or more transistors running excessively hot.

[jimangie1973]

Not sure I can get to them. I'll take a look. Are you thinking there could be 1 or more badly matched pairs?

I did some THD tests over the weekend and both channels look good for a 20-20KHz sweep (THD < 0.002%). I didn't do high power tests though, only a few watts.

[AndrewT]

no,
I am wondering if the thermal conductance from output device to heatsink has deteriorated with age.

A bad thermal joint will make the heatsink appear cooler and cause severe overheating of the badly connected devices.

[jimangie1973]

Ah ok. I suppose that's possible but the amp is only 6 years old.

[nigelwright7557]

Originally Posted by jimangie1973

I tweaked both channels to get them within 1 mV of the 25 mV value when fully warmed up. Even so, one heatsink is still warmer than the other, not dramatically, but enough to feel the difference.

Any ideas on a reason for this? Does it even matter?

Thanks,
Jim

Have you checked DC offset on the channels ?
A high Dc offset on one channel will put dc through the speaker and so use more power.

[jimangie1973]

Yea, checked it. It's only a few mV.

I'm wondering if I should just set the bias based upon minimizing distortion, rather than setting to a predetermined bias current.

[nigelwright7557]

I design my own amps and i use a signal generator and a scope to check for crossover distortion. I give the amps just enough bias to get rid of cross over distortion.

[jimangie1973]

The schematic of the Bryston output stage is here:

Bryston Limited - Music For A Generation

I'm not sure where the test points are taken from on the schematic, but regardless, if one channel has matched BJTs with a higher beta value, wouldn't that channel have a higher bias current with the same bias voltage, thus produce more heat? If so, it's probably more accurate to match heat sink temps than go by voltage. Just an uneducated guess.

[AndrewT]

no.
The bias current is setup and measured with the amplifier input shorted and open circuit load on the output.

Now look at a typical push/pull output stage.
The top half sends a bias current down.
The bottom half sinks a bias current down.
In between one usuaully has a feedback resistor and a Zobel.
The sum of the currents passing these two components must be tiny, <<10mA
From this, the upper and lower half currents must match fairly closely. As output offset approaches zero mVdc, the two half currents become exactly equal.