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Max safe operating temperature for transistors inside modified factory amplifier
Max safe operating temperature for transistors inside modified factory amplifier
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Old 30th September 2008, 09:19 AM   #1
azzajonesy is offline azzajonesy  New Zealand
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Default Max safe operating temperature for transistors inside modified factory amplifier

Hi all i have a Dussun DS-99 amp which i always suspected was biased quite low into class a then switching to b.

i finally opened it up the other day and found it was running quite low, measured between 13-17 mv across the .33 emitter resistors for a estimated 45 ma. With the lid off i and powered on for 10 minutes measured 39 degrees C at the top of the heatsink at about 25 C ambient. DC offset was less than 2mv. power consumption with no signal was 8-12 watts (average 9) measured at the power socket.

i bumped the quiescent up to around 48mv for estimated 148ma, checked dc offset (still under +/- 2mv), checked temp with the lid off after stabilizing for 10 minutes and saw 53 C. power consumption was 20-24 mostly 23 watts. put the lid back on, put back in the rack and played some music. utterly stunning at all volume levels instead of just really quiet. jammed the temp probe between the lid and the heatsink and watched it climb up to around 63C before finishing for the night (around 1.5 hours of listening).

thought the next day that 63c might be too high to be seeing in there and dropped the qc to an estimated 125ma, dc still fine but tweaked it to +/- 1mv anyway, temp measured 55c, i assumed this to be due to warmer ambient though didn't measure it at the time assuming that because i was running the current lower that it would be cooler overall, i also assumed that the power consumption would be less as well so didn't measure either. put the lid back on put it back in the rack and listened this time it was pretty much the same as 150 but it seemed to me like it was running out of power at higher levels, it wasn't sounding harsh but it was sounding muddy on extreme kick drums ext. i was monitoring the temperature and noticed it creeping above 60 then to 65 then to 70. i switched off at 73 and pulled the lid off and measured again, this time i noticed that qc was rising with the temperature as well as the power consumption up to around 28-30 watts.

So i reset back to 100ma estimated and the power consumption is down to 20 watts, the temp with the lid on goes no higher than about 53 and although the class a to b point is higher than before i really would like to see it higher.

My questions are.
Why did it go into thermal 'overload' at 125 but not 150 ma?
Is the power supply section likely to be put on any additional stress by running it harder into class a rather than the same sort of power demand but in class b mode (i intend to replace the psu caps to high temp versions possibly with a bit more capacity)?
Shouldn't running signal through the amp take some of the heat away from the heatsink and convert it into output?
The amp has a quite nicely designed convection cooling system with the transistor heatsink contacting the thick front alloy panel and transfering some of the heat to this, would anyone consider fan cooling (fans would need to be mounted on top sucking from the bottom due to space considerations) or would i throw away the heatsink and use a better chassis at whcih point i would probably be better off going full diy??
sorry for the long post but i think it best to supply as much info as i can?

thanks for reading and thanks in advance for your ideas.

i really do like the sound of this amp at 150 ma and i liked it before but after hearing it there i dont see any need for upgrades any time soon. The speakers im driving are a set of passive linn Keilidhs with the ninka tweeters and the ku stone bases which i also quite like, they are a 4 ohm load.
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Old 30th September 2008, 09:35 AM   #2
destroyer X is offline destroyer X  Brazil
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Default Keep them bellow 52 degrées celsius

Measure into the heatsink and near the transistors.

If the transistor has black case, the temperature there must be lower than the heatsink...if not, your transistor is trying to dissipate more than it can.... so... check the insulators and the heatsink dimensions.


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Old 30th September 2008, 01:46 PM   #3
AMV8 is offline AMV8  United Kingdom
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If you are trying to set the bias by checking heatsink temperature then when the power amp is heated up you should be able to keep your finger on the heatsink just by the output transistor for 5 or 10 seconds. This will be a heatsink temperature of around 60 degrees just by the heatsink. The output device will have a higher temperature.

Having said all of that; the amplifier designer probably pre set the bias for the best amplifier performance. Unless you have a circuit for the amplifier giving details of the voltage and current amplification stages and of the power supply capability I would stick to the original setting.

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Old 30th September 2008, 01:54 PM   #4
ferencz is offline ferencz  Hungary
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Some design note - my

I use to allow max. ~66% of the Tj-max., assuming the worst case (= max. dissipation).
Then for example for a Tj-max=150°C rated transistor the actual max. junction should never exceed 100°C.
Having the datasheet of a transistor (and the datasheet of the insulating device) you can easly calculate the max. allowed heatsink temperature (you will need junction to case thermal resistance for the calculations - Rjc).

Anyway I'd keep the bias current as it is (as trimmed in the factory) - and I'd search for an other modification to improve the sound of the amplifier (recap, etc.). I never preferred such "Turn up your OP bias and your amp will sound better!" mods.
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Old 30th September 2008, 03:12 PM   #5
Jan Dupont is offline Jan Dupont  Denmark
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I may have missed something reading your description on how you have adjusted the bias, but here is my advise:

Adjust the bias to the desired value and monitor it for at least 30 min. The bias WILL change due to temperature raise/fall, so keep adjusting it to the desired value during this period.

When bias has been stable for more than 10 min., then you can put on the lid.

Also keep monitoring the temperature during this adjustment to be sure that you haven't set bias to high for size of heatsink.
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Old 2nd October 2008, 03:23 AM   #6
azzajonesy is offline azzajonesy  New Zealand
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Thank you all for your wise advice.

ACD yes the last time i adjusted the bias i did leave it to stabilise both in temp and bias as well as power consumption about 30 minutes, as pulling it out of the rack and dis/reconnecting everything is a pain

ferencz and AMV8 i thought mass market amplifiers were set to perform trouble free and well within spec for the common person (this is china grey market version) for the common conditions of the market they are produced for. it will never hit 40 ambient inside my house and i have a temperature monitor, the amp sits in an open rack plus cooling available if needed. most common people wouldn't monitor or even check the heat. it really does sound much better without switching. thanks for your temperature measuring tips however

destroyer x i thought the temperature will always be hottest at the device itself, it is a couple of degrees warmer on the ceramic/plastic part than the heatsink 2mm away from the contact point. Perhaps i can disassemble and redo with some arctic silver thermal paste at some stage.

Please i have 1 more question about bias then i promise i will leave you all alone and go and build one of these DIY amps

i have attached a blown up pic of the internals (i cannot find a schematic), i was measuring at the emittor resistors and when i check the standard settings noticed that at the resistors there was an imbalance 13 : 17 between left and right. I noticed a couple of raised pins on the board (of the type that look like they are there for attaching alligator clips to for some reason) but didn't measure there until last night. I find that when i have balanced the current at the resistors that if i measure at those 2 points they read out of balance. I assume this is the correct point to measure the bias from, i have included a blurry pic with the points circled, i can take a better pic next time i have it open however. the points appear to be on the same piece of trace as the matching resistors on each channel.

the only other amp i've biased was an a400x pioneer and that from memory just had a single resistor per channel.

if you can point me in the right direction i promise i will set it back close to factory and not bother you guys again (until i start on the DIY project).

thanks again, it's good to know you guys are here.
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Old 2nd October 2008, 08:26 AM   #7
Patrik Floding is offline Patrik Floding  United Kingdom
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You better measure the voltage across all emitter resistors to make sure that one transistor is not carrying a lot more current than the others. Then adjust based on the transistor that gives the highest current reading, and check the others after you are done (when it's all stable) so they are not running low. If you do have a large imbalance then it's time to try to figure out why..
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Old 8th October 2008, 09:50 AM   #8
d3imlay is offline d3imlay  United States
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At my day job I work with UPS up to 100KW. Any device temperature below 80C is cool. this is taken on the hot spot of the device with a thermocouple.
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Old 8th October 2008, 12:00 PM   #9
EchoWars is offline EchoWars  United States
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Thermal compensation in that amp may be poor, but you DO need to monitor the bias for a minimum of 30 minutes (an hour or more is better) before you put it all back together...just like ACD said.

I'm not sure what you feel you're accomplishing by cranking up the bias, but I'd surmise that any sonic gains are likely in your head. If you want a Class A amp, build or buy one.
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