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Old 23rd December 2011, 04:20 PM   #41
Bensen is offline Bensen  Belgium
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I've made two prototype boards:
- board 1: diff input pair +ccs + vas
- board 2: drivers (2SK2013/2SJ313/ + vbe multiplier + OPS (on old copper clad)

What have I done:
- Made board 1 and choosen the right resistor value for the biasing of the diff input pair. All voltages and currents looked good.
- Made board two with 1output pair soldered directly to the board.
- powered everything on with 10Ohm resistors instead of the fuses
- all measured good and set the bias current
- hooked up a speaker and an input signal
- and we had music,

BUT then:
- I've used a second output pair
- used wires with a lenght of 20mm between board and output FET's.
- Gate stoppers of 220Ohm are mounted on the board.
- Powered it on and measuring continous the bias current.
==> I saw that bias current keeps rising, I pulled the plug and recheckt everything, no error found.
- Then, turnen bias current to minimum
- Powered it on and increasing bias current , once I got to +-100mA the bias current keeps rising!?
- I desoldered everything and soldered the FET's directly to the board (see picture below)
- Now I can set the bias current to a fixed value, but I noticed that the bias current between the four FET's are varying a lot (from 100mA - 400mA)?

Question:
Could it be that due to the wires of 20mm between board and output FET's (2SK1530 / 2SJ20), the FET's destroid them selves due to oscillation?
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Old 23rd December 2011, 05:13 PM   #42
AndrewT is offline AndrewT  Scotland
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Does the right hand pic show a 5th transistor trapped behind the clamp bar?
Is that the heatsink located Vbe multiplier? Is the Vbe transistor actually in thermal contact with the heatsink?

Is a two bolt fixing for the clamp plate, sufficient to ensure all four output devices are in flat contact with the heatsink? I recommend a five bolt fixing for four output devices.
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Old 23rd December 2011, 05:26 PM   #43
Bensen is offline Bensen  Belgium
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Hi Andrew,

Yes, behind the bar is a2SK792 FET for the bias setting, it is pressed to the heatsink with the same bar.

The board as you see it (blue/green material) the heatsink and mounting of the output fet's is only temporary for testing purposes. Once everything is finetuned, new PCB will be made and each FET will have its own bolt for connecting to the heatsink.

My guess is that it's not he VBE multiplier which is defect, because the bias current rises +- 5mA/second.

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Old 23rd December 2011, 05:29 PM   #44
AndrewT is offline AndrewT  Scotland
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Have you tried moving the Vbe multiplier transistor?
With the two bolts located as in the pic, the tighter you make the bolts the less clamping force on the Vbe multiplier.
It could be quite loose or not in thermal contact. Is the Vbe the same thickness as the output devices?
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Old 23rd December 2011, 05:34 PM   #45
Bensen is offline Bensen  Belgium
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The VBE Multiplier FET is thinner than the output devices. This is wy I put extra silicon insulation pads between this FET and the metal bar.
I tried moving it yes, even with a screwdriver, I'm not succeeding to move it.
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Old 23rd December 2011, 05:40 PM   #46
AndrewT is offline AndrewT  Scotland
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Then I suspect a wiring error.
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Old 23rd December 2011, 05:47 PM   #47
Bensen is offline Bensen  Belgium
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But how can one than explain that before this happened, the four ouput FET's measured the same bias current. The situation now is that, 1 FET measures 80mA, the second 400mA, the 3th 200mA and the 4th 120mA?

I have four new Fet's laying I'll try these tommorow, but then directly soldered to the pcb.
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Old 23rd December 2011, 06:01 PM   #48
AndrewT is offline AndrewT  Scotland
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Remove the output mosFETs and re-wire the NFB back into circuit.
Then test the LOW POWER amplifier into a 10k load.
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Old 23rd December 2011, 07:45 PM   #49
Bensen is offline Bensen  Belgium
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Yes, I'll do that first thing tommorow. I did exactly the same by building it up.
I've putted a 1K resistor between the Gate and Source connection where normally the output FET's will come. The NFB is than guaranteed.


Greetz
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Old 24th December 2011, 03:57 PM   #50
Bensen is offline Bensen  Belgium
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Default I'ts running again but unstable bias

I removed the output FETS and placed 2 1K resistors instead, this to keep the NFB network.
I measured all voltages/current, everything looked fine.

I mounted 2 new output FET's, but this time close the PCB and the gatestopper right at the legs of the FET's.
I powered it on, and yes, all measured fine.



Than I started following the bias current throught the OPS.
Cold amp: Set bias current at 180mA
Warm Amp (40C): Bias current dropped to 140mA
Than increased bias with trimpot back to 180mA
Amp cooled down again: Bias measured 215mA

The VGS multiplier is overcompensated. I'm using a 2SK792 FET instead of the irf9410 in the schematic. The resistor values in the schematic are the same as in practice.

What should I do to improve it?
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