My measurement of the heat on Q624/5/6/7 are not very scientific - just me touching those transistors, and also the bigger one its bolted on to, and also the surrounding larger transistors which do not have these passengers on them.
Another set of very hot transistors are Q613/4/5/6 ; they are atleast as hot as the regulator transistors.
Ref test points - there are two, but you are right that's just two of 8 of the .22ohm resistors.
I measured the voltages across all Eight 0.22 Ohm resistors, and they all fall between +/- 1mv to 5mV
Ref oscillation: I have acquired and old scope. I am happy to check but any pointers on what, would be appreciated. But purely going by the audio output, I dont get any odd sounds or hisses with no input, just a little hum at max volume.
In any case, I am brave enough now to hook it up now to my main speakers and give it a critical listen for distortion or some thing missing.
Another set of very hot transistors are Q613/4/5/6 ; they are atleast as hot as the regulator transistors.
Ref test points - there are two, but you are right that's just two of 8 of the .22ohm resistors.
I measured the voltages across all Eight 0.22 Ohm resistors, and they all fall between +/- 1mv to 5mV
Ref oscillation: I have acquired and old scope. I am happy to check but any pointers on what, would be appreciated. But purely going by the audio output, I dont get any odd sounds or hisses with no input, just a little hum at max volume.
In any case, I am brave enough now to hook it up now to my main speakers and give it a critical listen for distortion or some thing missing.
When you set the bias to a given voltage across the test points then you should see that same voltage across all the 0.22 ohm's in that channel.
A range of 1 to 5mv is a big variation in current. 1mv across 0.22 ohm is just 4.5 milliamps while 5 millivolts is 22 milliamps. The difference in heat per transistor at that variation is 0.22 watt vs 1.13 watt per device. A big difference when you have eight of them.
Use the scope to make sure there is no visible high frequency oscillation at the output. Use a high sensitivity such as 5mv/DIV and once you have a stable trace open the timebase out to a faster speed to see what is there.
Although it is a power amp it is just the same as this for checking.
Swapping Op-Amps... you have checked to see it's stable haven't you ? - diyAudio
A range of 1 to 5mv is a big variation in current. 1mv across 0.22 ohm is just 4.5 milliamps while 5 millivolts is 22 milliamps. The difference in heat per transistor at that variation is 0.22 watt vs 1.13 watt per device. A big difference when you have eight of them.
Use the scope to make sure there is no visible high frequency oscillation at the output. Use a high sensitivity such as 5mv/DIV and once you have a stable trace open the timebase out to a faster speed to see what is there.
Although it is a power amp it is just the same as this for checking.
Swapping Op-Amps... you have checked to see it's stable haven't you ? - diyAudio
I realised i made my volts measurements with the inputs and speakers in place. I took them off and re verified, and you are right. One channel shows 2.5mV (+/-) for all 4 resistors, and the other shows 0.3mV for all 4 there.
Ref the oscilloscope - did you mean i need to check the +/- of the speaker outputs? Apologies, if it is a silly question, but I am trying to learn!
Ref the oscilloscope - did you mean i need to check the +/- of the speaker outputs? Apologies, if it is a silly question, but I am trying to learn!
The channel with 2.5mv has 11 milliamps per pair set as a bias current, the one with 0.3 millivolts has just 1 milliamp (so virtually nothing) and that channel should run cold.
The scope can be connected across the speaker terminals.
Be sure to connect the scope probe ground lead only to the negative speaker terminal... if the scope and amp are mains grounded then connecting the probe ground to any point other than ground in the amp (which the speaker terminal is) would result in a short via the leads.
The scope can be connected across the speaker terminals.
Be sure to connect the scope probe ground lead only to the negative speaker terminal... if the scope and amp are mains grounded then connecting the probe ground to any point other than ground in the amp (which the speaker terminal is) would result in a short via the leads.
Hello Mooly and Co
Sorry for no more updates, but given work commitments, I have decided I like the sound as is with the bias at about 5mV, and it runs a bit warm but not scorching.
I will park it for now and let it run. If it breaks for any reason, will roll up my sleeves again 🙂
Thanks for your time and inputs.
Sorry for no more updates, but given work commitments, I have decided I like the sound as is with the bias at about 5mV, and it runs a bit warm but not scorching.
I will park it for now and let it run. If it breaks for any reason, will roll up my sleeves again 🙂
Thanks for your time and inputs.