Hey all, I am checking up on an amp that is getting warmer on one channel than the other. This amplifier doesn’t make it super easy to check his current. I took out the positive busbar and installed leads so that I could measure the current.
For the warm channel I’m measuring about 30 mA per device and the cooler just a little under 10-1/2 mA per device.
For amplifiers using lateral MOSFETs in TO3 cases it seems the general rule is to shoot for 100-110 mA per device. At least for the Hafler stuff. For my B&K amp that uses TO3p it’s about 50 mA per device.
What would be suggested for these smaller TO220s?
Dan
For the warm channel I’m measuring about 30 mA per device and the cooler just a little under 10-1/2 mA per device.
For amplifiers using lateral MOSFETs in TO3 cases it seems the general rule is to shoot for 100-110 mA per device. At least for the Hafler stuff. For my B&K amp that uses TO3p it’s about 50 mA per device.
What would be suggested for these smaller TO220s?
Dan
Looks like the individual transistor Source resistors would be the place to measure, giving you numbers for each device.
So measure across the large 2 W resistor? What number should I be looking for? Does 30 mA seem appropriate, I say that because that’s what one channel was sent to. Or should it be lower?Or higher?
Dan
Just bumping this thread, see if someone might have a suggestion as to an appropriate bias current for each output?
Thank you,
Dan
Thank you,
Dan
Times 8 would be .24 amps. That's a healthy start, but we haven't seen the rest of the circuit or the supply rail voltages.
Good point! For the outputs it’s a +/- 75v. Hopefully the circuit is clear enough.
Dan
Sounds like 30 mA might be the way to go. The reason I want to make sure is because the designs I have that use TO3p are set for 50 mA and are quite a bit larger in size. I just want to make sure I don’t set it too high.
Dan
Dan
Looks like the 100R is the source resistor. Since it is a common drain or source follower, a biasing scheme to ground or voltage divider with variable resistor cannot be used. The bias scheme appears to use some combination of BJT/lm358. If it were me I would check component values of source resistors and the autobias(?) circuit and figure out how that circuit works. I have some ideas, but really no clue on that one...
Thank you, I appreciate the response. I’m actually replacing every IC on this board. That’s what my friend wants.
There is only one trimmer for each channel, what would you suspect this would be for if not bias? I really wish they were better service information for this amp.
Dan
I missed that one and at first glance, it looked like the OPS is biased via drain feedback config. and not the gate. The trimmer appears to affect bias of the drivers in front of the mosfets, which in turn would bias the mosfets through the gate. Some testing of how that trimmer affects bias would sort it.
What would cause a channel to run hotter even though they’re biased equally?
I have both channels at roughly 30 mV across one of the 1 ohm resistors that is coming off of the source.
I’m sorry, I’m fairly new to this, biasing MOSFETs that is. I’ve gone across the rail fuse measuring current, but not this method. Am I doing this right?
Here is the issue, this is after about 10 minutes. And by the way, the channel that was getting warmer before because it was biased higher is the channel that’s not getting as warm. So that was cooler before now the warmer. So yeah after 10 minutes.
About 104 degrees
85.5 degrees
Both channels are hovering right around 30 mil volts across the resistor. They do fluctuate a little bit going down to as low as 28 1/2 or so but really don’t go much higher than 30 mV. if they are biased equally, why would one be getting warmer than the other?
Both channels are about equal on measurements of the outputs. They both have right around 6 mV of DC offset and have about 2 mV of AC voltage on them.
I’m not entirely sure how I would go about diagnosing this which is why I’m asking, but would it be wise to go across every single 1 ohm resistor to make sure that they are all biased equally. Or am I measuring on the wrong resistor altogether?
Thanks,
Dan
OK, measuring across those 1 ohm resistors is probably not the best way to set the bias. I went across every resistor on one channel and the measurements were anywhere from 18 mV to 53 mV most were in the 30 to 40 mV range, but several were outside that.
So clearly, these are not like emitter resistors, or they are and some of these 1 ohm resistors are no longer 1 ohm. Does this mean that each mosfet is not conducting equally? Unfortunately there is no way to break the rail so that I can measure the current of the entire channel without desoldering and removing the large buss bar. Suggestions as to where to go from here would be great.
I have to get my kids to sleep now, but in the morning I’ll measure across each 1 ohm (2 watt) resistor to see what the other channel looks like.
Dan
So clearly, these are not like emitter resistors, or they are and some of these 1 ohm resistors are no longer 1 ohm. Does this mean that each mosfet is not conducting equally? Unfortunately there is no way to break the rail so that I can measure the current of the entire channel without desoldering and removing the large buss bar. Suggestions as to where to go from here would be great.
I have to get my kids to sleep now, but in the morning I’ll measure across each 1 ohm (2 watt) resistor to see what the other channel looks like.
Dan
I got it real quick chance to check the other channel and I can see why it’s running warmer so the resistor I was measuring on was still sitting right about about 30 mV but I’m majority of them are between 50 and 80 mV. The lowest being about 24. But two thirds of them were between 50 and 83 mV. So clearly, I am measuring the wrong spot. There is one other resistor that comes off of the source which is the small 100 ohm resistor. Adjust for 3 volts across that resistor?
Dan
Dan
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see document how to set the bias
thanks to Jason Gibson from https://freemanscarstereo.com/ for the info
thanks to Jason Gibson from https://freemanscarstereo.com/ for the info