Threshold S200 Bias - again

Hello Gents
Sorry to bother you again with yet a thread on biasing the S200 - but despite the guide found here on this site (ZenMod provided plenty of times) I'm still in doubt...

I've measure the temperature on the top of the heatsinks - around 58 Celcius on right channel, and 62 celcius on the left - thinks this is what classifies as "¤!#* hot" in the Pass Nomenclature?

I've measures voltage over the big 1 ohm emitter resistors - on startup (cold amp) the right channels stabelizes after about 10-20 seconds on appx 230mV, the left (hottest) on around 280mV.

Now - the document states that the initial voltage at startup should be 180mV when starting the adjustment - so am I correct that in order to bias the amp correctly I should:

1. Power on the cold amp, and carefully adjust the two potmeters so the voltage over the 1 ohm resistors reads appx 180mV

2. Put the lid back on

3. Wait xx minutes - check temperature - if less than 49 celcius, raise bias, if more, lower it?

I states that the amp will "lock" into biasvalue after a while and will become non-responsive to changes on the potentimeter - so - how long should I wait before adjusting?

BTW - this amp sounds great!!!

Best regards
Hans
 
Official Court Jester
Joined 2003
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values for cold amp are , for my logic , more of informative character, confirming expected behavior ....... being of secondary importance

in other words, Iq in temp.. equilibrium is of primary importance

besides that , first thing I can think of - if there are NTCs , check cold resistance of each , desoldering one leg from pcb

if there is 10% difference to written value , replace
 
Easy now, working on it.... :)

It's a little tricky - first 20-30 seconds after power on, the amp needs to stabilize the bias, then I find you only have few minutes to adjust - after that, the bias pot kind of loses it's function, the voltagedrop over the resister does not really change much when you turn the pot after this. So - quick adjust, then power off, then reassemble amp, let it cook for 1-2 hours, then measure with my IR temp probe.

I'm down to 54 degrees on one side now, and 53 on the other, so getting close to the target 49 degrees - but I can only make one run on it each day (busy man, little kids, animals and wife to feed etc. :) ). I'm measuring in the top bolt hole on the back - inside one of the "feet" which is mounted on the back...
 
Looks like I nailed it now - 49,2 degrees on one side and 48,7 on the other measured with IR thermometer in upper bolt hole on the back of the amp :)

Offset is stable 16 and 34 mV :)

Ended up with about 170mV and 160 mV as initial voltage drop over resistors - a little less than recommended.

Plays nice music :)

But now I have to turn on the electric heater in the listening room again :(

Thanks for your assistance! :)

Regards
Hans
 
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