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Old 21st June 2012, 04:43 PM   #1
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Default Bryston 3BST operating conditions

Does anyone here have some experience with Bryston 3BST power amps ?

A friend has one which failed one day. We found a 2.2uF tantalum cap had shorted which came after a 1 watt 10 volt zener connected to the regulated 28 V supply.So it took out the zener also and shorted out the 28 V which came from a LM317 regulator. We replaced the zener and the tantalum with another tantalum.

Now after about a year another tantalum doing the same job in another section of the circuit shorted out.Replaced the parts and its working fine. The tantalum seems to be just a small value filter after the zener which just drops the 28 V to 18 V for the opamps that work off +/- 18 V ( TL074).

Since things appear hot inside I measured the temperatures after one hour. The ambient here is 30 deg C !

Tab on LM 317 was at 74 deg C
Heat sink fin was at 68 deg C

Tab on LM 337 was at 58 deg C
Heat sink fin was at 52 deg C

Metal cans on 4 transistors on the input board ( part of the diff input amp etc.) were at 81 deg C !!

Are these normal operating temperatures ?
The heat sinks of the power transistors were at 35 deg C !

By the way I replaced all tantalum caps with Samwah 10uF/63V caps. Leakage might be more but that isn't an issue in the place where they are used. Can't keep opening the amp every time the tantalum's fail. I replaced 5 of them.The amp is 16 years old and so the tant's might just keep dropping dead !

What is the bias current through the output devices in the discrete opamp on the input board and how to fix it if it's wrong. Remember the output devices are currently running at 81 deg C ! There are no heat sinks on them and no space to fit one if desired !

Any information , suggestions or opinions welcome.
Thanks.
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Last edited by ashok; 21st June 2012 at 04:47 PM.
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Old 21st June 2012, 08:58 PM   #2
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And I thought these items ware guaranteed for life - why bother fixing them, go dump it on the MDs desk.
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Old 21st June 2012, 09:33 PM   #3
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I hope that you have a schematic to help understand the unhealthy temperatures inside the amp.
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Old 22nd June 2012, 04:47 AM   #4
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The owner suggested emailing Bryston directly. I did and had an immediate reply. They do not use those tantalum's anymore and told me to use alternative caps. The caps are not in the signal path.

Their designs apparently do run warm but I'm going to cool off those transistors a bit according to their suggestions . They said they have never had any issues with their standard operating conditions. We do not have sub 20 deg C ambient anytime during the year. Most times it is between 30 and 33 deg C !

Anyway, Bryston responded immediately as I was told they would do !
We can't ship this amp half way around the globe to fix a few inexpensive caps ! The amp sounds very nice and really has great control over the bass. Deep AND tight. My amp comes close but isn't there yet ! It sounds better than the Behringer nu3000 DSP class-D power amp !

However I have heard better ( sweeter !) mids and highs elsewhere.

Nice solidly built amp. Everything seems to be aluminum and so the weight seems to come mainly from the two large transformers.

With the very prompt technical support , I'd be inclined to pick up a used one myself ! Can't afford a new one !
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Last edited by ashok; 22nd June 2012 at 04:49 AM.
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