bias a trace elliot ta100r

[lowell]

Just got finished doing a repair on a trace elliot ta100r and am wondering if anyone has any information on biasing these amps.

Thanks!

[lowell]

FYI this amp has 2 power mosfets driven by BJT's. I'm wondering what should be the DC voltage across the bias pot, which is b/t the GATES of the mosfets. Here's a schematic.

[lowell]

Enzo, JM, Steve Conner, J pbass, any of you guys able to give me basic instruction on biasing power mosfets? I'm under the assumption that they're similar to tubes in that the if Vgs is 0v then it's ON. Therefore the gate needs negative bias to limit quiescent current. ?

[Jazz P Bass]

Never had one of these on the bench.
But, it is a push pull output stage.
Therefore, I would monitor the mains currect that the amp is consuming at idle.
I would think 50 watts should do.
I never heard of 'negative bias' on a mosfet gate.

[mozwell]

Its a standard mosfet power amp stage.
Set the trimpot to MINIMUM resistance.
Monitor the power supply current with an ammeter (multimeter set to amps, use 10A range). You will need to disconnect either the positive or negative lead & put the ammeter in series.
Adjust the bias pot for approx 20 to 50mA on the ammeter

Alternatively, add a 0.33 ohm 5W resistor in series to each mosfet's "source" terminal.
You can then measure the voltage across this resistor to set the bias, 0.33 ohm 50mA = 16.5mV. Anywhere from 5mV to 16mV should be fine
This source resistor also will compensate slightly as mosfet temperature increases, so bias current will remain more stable with temperature.

Im not a big fan of power amps without emitter or source resistors, i just dont think they are as reliable. I am willing to sacrifice a few watts output, to get a more reliable output stage. Any paralled output devices, i think should always have emitter or source resistors, for current sharing & to minimise bias current shifts with temperature.

[lowell]

Mozwell so are you saying the the mains current reading will only be reading the mosfet current? Won't it also be measuring current for all other devices that are drawing current? Or is it so inconsequential that it shouldn't matter? Finally why 50ma?

[Steve Conner]

Enzo, JM, Steve Conner, J pbass, any of you guys able to give me basic instruction on biasing power mosfets? I'm under the assumption that they're similar to tubes in that the if Vgs is 0v then it's ON. Therefore the gate needs negative bias to limit quiescent current. ?

No, all power MOSFETs are enhancement mode. A Vgs of 0V is off, and positive voltage turns it on. (For N-channel devices that is. The P-channel ones are off for Vgs = 0, and turned on by negative gate voltage.)

With the BUZ901/906, a reasonable voltage across the bias pot would be about 1V. These are lateral MOSFETs with a low threshold voltage, not that different to the 0.6V of a BJT.

Mozwell's recommendation makes the most sense so far. At least, if you assume he meant to splice the meter into the DC supply to the output stage. Not the AC line. I'd also disconnect the speaker so that DC offset wouldn't skew the result.

[J M Fahey]

Agree: Mozwell's suggestion is the way to go.
And add, since you seem to be confused: nobody mentioned *mains* ; what he means is you put your current meter in series with one of the MosFets, so you are reading its current only, not the rest of the circuit.

[lowell]

"Monitor the power supply current with an ammeter" sounds an awful lot like putting the meter in series with the mains and the power supply. It's all good though. I get it now. Thanks yall. The thing is up and running with no crossover distortion.

[Jazz P Bass]

So how did you bias the output stage.
I would be curious to know what the mains draw was when you were finished.

[lowell]

haha you busted me. I actually put it as cold as it would go and then scoped the output w/ no speaker connected. I warmed the bias until crossover distortion was gone.

[Jazz P Bass]

Sounds like a good game plan to me.
By the book.
Thanks.

[lowell]

So this thing is back. It has a low volume fizziness on the notes. Very subtle. I monitored the current per Mozwell's suggestion and it's at 24ma. However, turning the bias pot does nothing. I have measured the resistance across the pot and it is indeed changing when it's turned. Not sure what's up here. All those diodes checkout with the meter.

TR4 C: -375mv
TR6 C: -1v

This seems odd to me. I'd expect to see these symmetrical.

[MarkusBass]

I monitored the current per Mozwell's suggestion and it's at 24ma. However, turning the bias pot does nothing.

I'm not sure what this means - previously you wrote that:

I warmed the bias until crossover distortion was gone.

Both statements cannot be true.
I'm not sure which Mozwell's suggestion you followed; he had two suggestions: to measure the power supply current, or to add a ballast resistor and measure voltage drop on the resistor. But he also suggested (maybe not directly) to verify that you can change the current flowing through the mosfets. Did you verify this?
There are several important notes regarding amp that use lateral MOSFETs (like 2SK135 and 2SJ50 in this amp):
- 2SK135 and 2SJ50 are very difficult to be purchased and you should be very careful in order not to short them.
- P1 trimmer changes the voltage between the gates of output transistors (and changes the bias). If this does not happen in your amp, it still needs to be fixed.
- the method of measuring the power supply current of the whole amp is highly inaccurate because you also measure the current of the Zener diodes (at IC1) and TR4 and TR6. Especially if you measure such low current like 24 mA. You have to measure the current of the MOSFETs only.
- the other method (with 0R33 ballast resistors) is also not optimal. It's because lateral MOSFETs have high resistance and usually such amps do not use ballast resistors. Remember that lateral MOSFETs have very special characteristics - check it on Wikipedia.
- the best method is to measure the current of one MOSFET only. But this may require cutting a track or desoldering the transistor - you choose.
- if you have problem finding what is wrong with the amp, you may desolder the MOSFETs and solder in their place two 1k5/5W resistors (from drain to source). Then you can work with the amp witout destroing the MOSFETs (and add them at the very end).
- I'm not sure whether the voltages on TR4 and TR6 should be symmetrical - this may depend of Vgs(off) voltages of both MOSFETs but I would expect at least one to be positive and the other negative. If you measure the (total) current of the amp, it is still possible that the MOSFETs are not properly biased.
- does the voltage on TR4 and TR6 collectors change when you turn the pot?
- do you have 0V on the speaker output?
- did you measure the amp with a dummy load (and full power)?
Sometimes simillar problems are caused by failed Zener diodes but first I would clarify the problem with bias.

Mark

[lowell]

Markus,
Thanks for the reply. Yes I DID warm the bias until the crossover notch was gone. Now, however, there seems to be no effect on my scope, or when reading current draw when the bias pot is turned.

I measured the current of 1 mosfet, not the entire PA.

- does the voltage on TR4 and TR6 collectors change when you turn the pot?
I will check this

- did you measure the amp with a dummy load (and full power)?
Not sure what you mean here. What am I measuring?