Was experiencing some problems with soundstage so checked the bias on each channel, the readings were widely differing. The problem I now have is I have no idea what the bias settings should be, the amp is supposed to be heavily biased into Class A. I have read quiescent current could be anywhere from 75mA up to 200mA, depending on various forum threads.
I have settled on a value of 150mA which seems to give a very good sound, 75-100mA sounded a little bright and insipid. Obviously I don't want to blow the output transistors and the heatsink at 150mA is very warm to the touch, I can hold my fingers on it for about 4 seconds after about an hour at low volume levels. Anyone have any idea as to what the quiescent voltage should be for this amp?
I have settled on a value of 150mA which seems to give a very good sound, 75-100mA sounded a little bright and insipid. Obviously I don't want to blow the output transistors and the heatsink at 150mA is very warm to the touch, I can hold my fingers on it for about 4 seconds after about an hour at low volume levels. Anyone have any idea as to what the quiescent voltage should be for this amp?
Have dismantled the fridge and have icicles forming
Finally settled on about 124mA, nice and toasty but not silly. Once amp is warmed up am within 0.1mV across the 0.22Ohm output resistors, not too bad at all considering they are one turn pots on the bias.
DC offset is 0V on one channel and 0.1mV on the other, seems all my servicing has turned out well. The upgraded OPA 627 are an absolute joy and well worth upgrading from the LM 318's.
Have finally settled on just short of 180mV and it sounds smooth, detailed and powerful. A few of the words said in jest set me thinking and now have 120mm PC fan blowing on heatsink, it doesn't even get warm to the touch now. I now have to build up an enclosure for the fan, got to keep the furry ones paws away from the blades.
The amp is now a different sounding beast and have never heard it sound so musical before. I would much preferred the bias to sound better at lower values but the higher I go the better it sounds
Have finally settled on just short of 180mV and it sounds smooth, detailed and powerful. A few of the words said in jest set me thinking and now have 120mm PC fan blowing on heatsink, it doesn't even get warm to the touch now. I now have to build up an enclosure for the fan, got to keep the furry ones paws away from the blades.
The amp is now a different sounding beast and have never heard it sound so musical before. I would much preferred the bias to sound better at lower values but the higher I go the better it sounds
And if you get bias to 1A will sound even better believe me!
You will rediscover class A
and will have a heater in the cold winter days (or nights maybe)And if you get bias to 1A will sound even better believe me!
You will rediscover class A
It is my understanding that the P170 was full class A for the first 18W. I never use that living in an apartment with people next to and below me.
1A of (total) output bias current gives 2Apk of maximum ClassA output current from a Push-Pull amplifier.
2Apk into 8r0 is 2*2*8/2 = 16W of ClassA output power.
Above that, the 1A bias amplifier will transition automatically over to ClassAB.
Whereas 450mA of bias, gives 0.9^2*8/2 = 3W of ClassA before transitioning over.
2Apk into 8r0 is 2*2*8/2 = 16W of ClassA output power.
Above that, the 1A bias amplifier will transition automatically over to ClassAB.
Whereas 450mA of bias, gives 0.9^2*8/2 = 3W of ClassA before transitioning over.
Ok! I confess this is all new to me and I haven't touched on these subjects since taking my physics O level 30 years ago.
If I am doing all the math correctly I have a current of 0.8mA (0.18mVx0.22Ohm) per transistor per channel. So that gives me 0.8x2x2x8/2=12.8W, more than enough for my needs. Please correct me if I have this wrong.
Thank you Andrew, I think I now understand, it's nice to be made to think rather than be given the answer So if I have it correct I have 1.6Ax2x2x8/2 which gives me a nice 25.6W of class A, it also confirms my memory of about 18W for the original bias setting approx 0.11mV/.22Ohm =500mA(x2) x2x2x8/2=15W.
So if I have it correct I have 1.6Ax2x2x8/2 which gives me a nice 25.6W of class A, it also confirms my memory of about 18W for the original bias setting approx 0.11mV/.22Ohm =500mA(x2) x2x2x8/2=15W.124mA? How?!
Measuring the factory set bias on one of my P170s, I get approximately 20mA per channel. (Heatsink temperature of approx 50 degC, electrolytic case temperature of approx 40 degC). Despite the almost traditional criticism (and often deserved) criticism of 80s-era MF build quality, a '30 degC above ambient' temperature rise is good, solid, safe (consumer equipment) engineering.
Upping the bias by 50% (to 30mA per channel) gives a heatsink temperature of approx 60 degC, electrolytic case temperature of approx 50 degC).
Though the bias increase has transformed the amplifier (smoother and more refined, with wonderful, luscious treble) I'm not comfortable going any higher.
According to previous posters, 75mA - 100mA is pretty much a minimum!
I've checked my meter and it is spot on accurate. Any explanations for this discrepancy? Some '"I biased my P170 to the sky!" type hyperbole? A genuine preparedness to run amps as hot as a frying pan?
Any ideas?
I can only guess that you are measuring it wrong......?
About 100mA is about normal.
You should be measuring 22mV across one of the 0R22 resistors per channel.
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