Quote from: Jazz P Bass on November 21, 2024, 06:11:07 PMTo try and prove whether or not the power switch is the issue, turn the power switch to On & then plug the power cord in to the outlet.

Did you do this ? What were the results ?

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMQ: Should I be concerned with the millivolts across the .62R resistors while biasing the amp?? Does this matter?? What's considered to be too high of a voltage (if there is?)

This is a long thread and I don't want to contradict anything from members who might have more experience with this amp. I have none. This amp is a little unusual in that one pair of output transistors have the 0.62R emitter resistors and the other pair has 0.27R resistors. The first pair act like driver transistors to the second pair that act more like a Class B output stage.

Second point, going by the schematic in post #7, there is no info regarding biasing from the original designer. We don't know where the bias was set when the amp left the factory. If we did, I would go with that. Without guidance from Randall, I would start with 100 millivolts across the 0.62R to see how that looks with a dummy load. A crossover notch might not be exactly at the zero crossing but at a slightly higher Voltage where the second pair of output transistors turn on at around +/- 3V.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMOr am I only concerned with the notch in the sine wave?? Which there is very little of at min bias adjustment.

The problem with bipolar transistors is that if the bias Voltage on the Base it a fixed Voltage, the transistor will draw more and more current as the transistor heats up. In a power amp, the output transistors are going to heat up, they can get so hot that even with a large heat sink, they will get too hot and fail. This is called thermal runaway. Special circuits have been devised to keep the current under control as the transistors heat up. These circuits include the thermister and the diodes near the bias adjustment.  Keeping the bias as low as possible while still eliminating the crossover notch is the goal.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMAlso, I used to get -.6v on the bottom lug of the trim pot which I don't have anymore. It's now a positive voltage. Starting at .876V at the minimum bias setting. It only increases with the adjustment of the bias pot.

Without knowing exactly how the amp is wired, I can't comment on that and I'm not looking back on a 13 page thread to find that info.

Quote from: RG100ESROX on November 26, 2024, 02:22:31 AMI also have a bad hum that gets worse as the bias is increased.

Where is the hum coming from ? Do any of the Volume or Gain controls affect the amount of hum ? If you ground the signal at the Effects Return jack, does the hum go away ? If it does, suspect C45. If it doesn't, suspect C38 and C39 or possibly C31. If that doesn't fix it, suspect any mechanical ground connection. Tighten any screws or nuts where things are grounded to the chassis. Cables that go between PCBs might need to be jiggled or give them a shot of contact cleaner.

Bias current is usually adjusted with no signal, then look at the output with a scope and a small signal to confirm there is little to no notch where the sine wave crosses zero Volts. Sometimes a dummy load will disturb the DC readings across the emitter resistors in the power amp if there is DC Voltage on the output. In that case, disconnect the dummy load before adjusting the bias. Then reconnect the load to check for crossover notch. In some cases you might adjust the bias with a small signal to observe the crossover notch and set the bias to the lowest value that eliminates the notch. If you are really diligent, you might check the bias when the amp is hot after being played for some time to see if the temperature compensation is working as it should. You should see no notch, but bias current may have drifted up or down with the change in temperature. That's OK, don't adjust the bias unless you see a gross notch.

Clipping is when the input signal is too big for the power amp to reproduce without distortion on the peaks. The peaks of the sine wave are "clipped" off so the wave starts to look more like a square wave. This insures that the transistors in the power amp can withstand the Voltages that they might see in normal use, not just the Voltages with no signal.

The next test is with a dummy load. This test makes sure the transistors can withstand the current required for the amp to produce full power. If the power amp can pass this test, it us usually good to go.

Myself, I would look at the output with a scope and drive it to clipping with no load. If the amp survives, try it with a dummy load resistor. If that's OK, hook up a speaker and Rock ON !

Those Voltages in the preamp are close enough for Rock and Roll. About as good as you can get with JFETs and fixed (not pots) resistors.

The one with the smaller cap is providing a treble boost. The one with the larger cap sets the low frequency response.

You can't really expect anyone to reply without knowing exactly what model amp you have. A quick ooogle search shows at-least a dozen models. With only a 12V supply, my guess is it's probably a Class D design. If Fishman won't fix it, trash it and buy another.

Edit Oooge shows no "Loudmini". There is a Loudbox mini, but it has an IEC connector for a line cord. No input for a 12V supply

Edit 2 Ok, found it. Its a Loudbox-Mini-Charge. Uses a 12V 5A power supply. Same advise applies. If Fishman won't fix it, trash it and buy another.

What a poor design. No gain from Q1. Q1 is a switching FET not intended for Audio. High Impedance tone stack more appropriate for a tube amp. Low input impedance inverting opamp stage loads the tone stack too much. Power amp is OK but 100K input is higher than recommended for that chip.

Looks like it was hacked together by some internet hack without a clue.

What does it use for a power supply ? What did you build it on if you didn't have a PCB ?

What is the DC at the output with no signal ?

Because this thing has a pretty high output impedance, the signal amplitude at the output as well as the DC offset could change considerably when you connect a dummy load.

That signal is about 3KHz. Is it coming from a generator, or is the preamp creating it ?

Were the 2N5484s put into the board without crossing the legs or twisting the part around ? It seems like the TIS98s would have the Base as the middle leg, but the 2N5484s would have the Gate on one end.

The 2N5484 is rated at 25V BV(GSS) so not likely to survive in the power amp circuit.

I think it's safe to assume the amp is working correctly. The vast majority of the Class D chips will protect themselves against shorts on one or both outputs.

Quote from: Miyagi_83 on November 08, 2024, 05:41:49 AMNext, I plugged it into the amp module, turned it on and tried testing it following your suggestion. A sine wave appeared on the screen when the volume knob was at minimum, but nothing happened to its shape when I turned the volume knob up. It just started floating up and down. The same was true for both outputs.

See on the screen where it says the frequency is 284.09 kHz ? That is the class D modulation being filtered by the output filters on the class D amp. The floating up and down, that is the signal from the generator. Set the frequency of the generator to 500 Hz and set the scope to 10mS (Ten millisecond) per division. Set the Volts per division to 5 to 10 Volts per division, whatever it takes so the trace doesn't go off the screen vertically. The scope trace will look really thick or out of focus because the 284 kHz will still be there.

Use the scope to see if there is signal on each side to ground.