It took a 3.9k resistor in series with the 1.27K for a total resistance of 5.17 K to bring the voltage down to 1.0 mV. The pot has very little range 1.0 mV full c.c. to 1.2 mV full c.w.. I set the other side to 1.2 mV and it seems to be running okay.
Both channels output and wave form are the same at the speaker terminals. The left channel sounds a little different to me, but the wife and son, both of whom can hear much better than me, could hear no difference, so I don't know.
Should I test it at the maximum load, the amp is 1 ohm stereo stable? I currently have two 2 ohm 100 W wire wound resistors. I am willing to buy whatever resistors I need. How would you set up the load to test it?
It is rated at 75W @ 4 ohms and 125W @ 2 oms.
Both channels output and wave form are the same at the speaker terminals. The left channel sounds a little different to me, but the wife and son, both of whom can hear much better than me, could hear no difference, so I don't know.
Should I test it at the maximum load, the amp is 1 ohm stereo stable? I currently have two 2 ohm 100 W wire wound resistors. I am willing to buy whatever resistors I need. How would you set up the load to test it?
It is rated at 75W @ 4 ohms and 125W @ 2 oms.
The two 2 ohm resistors are good enough to test the thermal stability. The resistors may need to be fan cooled. When testing, you will need to re-check the bias at regular intervals. Does your DC power supply have an amp meter?
Do you have a scope?
Please (and this applies to anyone who needs repair help) use your sig line to list all equipment you have, editing it as equipment changes. Include the model numbers.
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Oscilloscope (yes or no)
Multimeter(s)
Type of signal source (grounded RCA shields preferred).
Soldering iron
Desoldering pump
Power supply
2 ohm current limiting resistor (hollow cylindrical ceramic 100w preferred)
Do you have a scope?
Please (and this applies to anyone who needs repair help) use your sig line to list all equipment you have, editing it as equipment changes. Include the model numbers.
Top of page, menu USER CP >> EDIT SIGNATURE
Oscilloscope (yes or no)
Multimeter(s)
Type of signal source (grounded RCA shields preferred).
Soldering iron
Desoldering pump
Power supply
2 ohm current limiting resistor (hollow cylindrical ceramic 100w preferred)
I do have a scope and my power supply does have an amp meter. I have been using an automotive head lamp as a current limiter.
I also have an adjustable voltage, 3 amp max wall wart type dc supply.
I also have an adjustable voltage, 3 amp max wall wart type dc supply.
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As the amp heats up, regularly reduce the input level to 0 and check to see if the idle current has increased. There shouldn't be any significant change from room temperature to thermal shutdown.
I'd recommend adding heatsink compound between the bias transistors and the heatsink.
You stated that you thought that one channel sounded different. Do you see any problem with your scope?
I'd go through the heating process with music instead of a sine wave.
I'd recommend adding heatsink compound between the bias transistors and the heatsink.
You stated that you thought that one channel sounded different. Do you see any problem with your scope?
I'd go through the heating process with music instead of a sine wave.
So hook up each channel to the dummy load, what is the ideal voltage from P.S.? Do you set the gains to just before clipping at say 1 KHz? How long do you let it run?
I will definitely add the heatsink compound to the bias transistors.
I am still learning about what I am looking at on the scope, but a 1 KHz, at the speaker outputs they both look almost identical, except that with the channel in question it is 52 VPP and on the other amplifier channel it is at 54 VPP.
What I perceived as different was more like a little muddy, not quite as "bright" as the other channel, but I know all of that is very subjective. The dc offset on that channel has increased a little since adding the resistor, from 5 mV to ~ 11 mV.
Thank you.
I will definitely add the heatsink compound to the bias transistors.
I am still learning about what I am looking at on the scope, but a 1 KHz, at the speaker outputs they both look almost identical, except that with the channel in question it is 52 VPP and on the other amplifier channel it is at 54 VPP.
What I perceived as different was more like a little muddy, not quite as "bright" as the other channel, but I know all of that is very subjective. The dc offset on that channel has increased a little since adding the resistor, from 5 mV to ~ 11 mV.
Thank you.
The power supply voltage isn't critical. if it's adjustable, set it between 13 and 13.8.
I would use music instead of a 1k tone. Set the level so that the output clips intermittently on peaks.
Does one channel clip at 52v and the other at 54?
11mv offset is insignificant.
I would use music instead of a 1k tone. Set the level so that the output clips intermittently on peaks.
Does one channel clip at 52v and the other at 54?
11mv offset is insignificant.
If I set the gain using the left channel, just before clipping it is 54 Vpp then moving the probe to the right channel it just is starting to clip.
If I set the gain with the right channel first it is probably ~54 Vpp, then switch to the left channel it is ~52 Vpp.
I will use music. When you are talking about input level are you referring to the amplitude of the signal from the source or the gain on the amplifier?
How long do you run the test, while periodically checking idle current?
If I set the gain with the right channel first it is probably ~54 Vpp, then switch to the left channel it is ~52 Vpp.
I will use music. When you are talking about input level are you referring to the amplitude of the signal from the source or the gain on the amplifier?
How long do you run the test, while periodically checking idle current?
If there is one gain pot, it could be an imbalance between sections. I wouldn't be concerned about that.
I'm referring to whatever gets the amp to intermittently clip. Either/both of those will do it.
When I make a modification to a bias circuit (rare), I run the amp up to thermal shutdown. You can speed up the process by covering the heatsink with a towel.
I'm referring to whatever gets the amp to intermittently clip. Either/both of those will do it.
When I make a modification to a bias circuit (rare), I run the amp up to thermal shutdown. You can speed up the process by covering the heatsink with a towel.
Finally got it to shut down, it ran for over an hour with music clipping about 10% of the time wrapped in a towel except for periodically checking bias current. I ran a 1 khz sine wave to it and it shut down within about 5 mins. the heat sink was over 200 F° (temp. when I switched to sine wave).
The bias voltage started at 1.2 mV cold, while running and heated up stayed fairly stable around 1.8 mV. Once the amp shutdown and allowed to cool then started back up the bias voltage is 0.9 mV and rotating the pot has no effect on voltage. The other channel started at 1.2 mV stayed stable at 1.3 to 1.4 mV through loading and after restart is stable at 1.3 mV.
The bias voltage started at 1.2 mV cold, while running and heated up stayed fairly stable around 1.8 mV. Once the amp shutdown and allowed to cool then started back up the bias voltage is 0.9 mV and rotating the pot has no effect on voltage. The other channel started at 1.2 mV stayed stable at 1.3 to 1.4 mV through loading and after restart is stable at 1.3 mV.
I think it's good enough. What you didn't want to see was the idle current going extremely high when it heated up.
Since the bias had little to no effect at high temp, re-set it when it cools down.
Since the bias had little to no effect at high temp, re-set it when it cools down.