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Old 10th March 2015, 02:15 PM   #1
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Default Help with Vertical Mosfet power amp improvements

Hi
I have this power amp that is on a board complete with power supply. Looks Italian.
It works, but I have substantial ''crossover sounding'' distortion if a play a 200 to 450 HZ sine wave into it.
Bias is set to 40mA monitored with amp meter in the positive rail.
On the scope I can just see the distortion. The same amount I would leave for any other amp, but for some reason on this amp I can still hear it in the previous mentioned freq range.
If I then increase bias until I hear no more distortion, amp meter shows quiescent current of 150mA. It heats up a bit more in idle.
I then let the unit cool down, and when switched back on, bang, the Mosfets are taken out along with the mains fuse. Obviously it doesn't like the higher bias.
I have attached a schematic, and would like to improve on the design.
Any ideas would be great.
Also note that on the schematic I have written that the biasing transistor is not mounted on the heatsink.
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Old 10th March 2015, 02:41 PM   #2
juma is offline juma  Germany
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Biasing transistor has to be mounted on the main heatsink (in order to perform its duty) and the IRFP240/9240 need to have Source resistors (0R22 at least, each). Iq 150-250 mA is OK but you need an appropriate heatsink.

BTW, 2 pairs of HEXFETs per channel is a bit too ambitious for +/-80V rails, 4-6 pairs is more realistic...
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Old 10th March 2015, 02:58 PM   #3
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If that schematic is drawn correctly, the problem is that the base drive to VAS transistor MJE15034 is taken from the wrong place, it should be the collectors junction of that leg of the LTP.

Also, the bias transistor must be on the heatsink, however that is not the issue you are having at this stage.

And you also need source resistors 0.22 - 0.47 ohm. Again that is not the issue at hand though.

Your 10k feedback resistor could do with being 1 watt.
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Last edited by richie00boy; 10th March 2015 at 03:00 PM.
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Old 10th March 2015, 04:21 PM   #4
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You are correct. I made a mistake in my drawing.
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Old 10th March 2015, 07:40 PM   #5
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Here is the corrected schematic. My bad.
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Old 10th March 2015, 11:00 PM   #6
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You still don't have 0r22 source resistors on the output mosfets.
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Old 14th March 2015, 04:22 PM   #7
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OK, so I have taken advice from all of you. Thank you all very much!!
I'm in the proses of adding 0,47ohm Source resistors, and want to attach the two Vbe multipliers to the heatsink. I say two, because this is a bi-amp system. My question is, where do I mount them? I was thinking of using TO-126 packages for ease of mounting.
Do they have to be mounted close to their relevant FET's? ie. the Vbe multiplier for the tweeter, close to the tweeter Fets, and the other close to the woofer Fets? ;or doesn't it matter?
Also, can i mount them on top of one of the output Fets using the same mounting bolt?
I will however need to link these transistors to the preamp board with 5cm wires. Anything I need to lookout for when doing this.
I have included a photo of the output/heatsink board.
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Old 16th March 2015, 08:26 AM   #8
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It is essential that the bias transistor senses the output devices of its own circuit!

TO-126 package sharing a main transistor mounting bolt is a very good way to achieve good sensing.
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Old 16th March 2015, 09:05 AM   #9
AndrewT is offline AndrewT  Scotland
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the Source traces of the output devices are quite wide.
There is room to add source resistors to these traces.
Drill two holes in the pcb through the centre of each of the traces, to match the pin pitch of a radial resistor. One resistor to every source lead. Scape off the coating around each hole and tin while the shiny copper is still fresh.
Cut the trace between the new resistor pads.
Ensure the feed back comes off the speaker trace and not off the output trace where all the sources feed into.

I would suggest you change the feedback.
keep the 220uF and increase the 330r to around 750r.
select a new value (around 22k to 25k) for the 10k to suit the gain that the speaker driver requires. it is +29.9dB now. Stay fairly close to this otherwise you will need to revisit the compensation. This reduces the dissipation in the resistor. A series pair of 12k & 11k gives double the dissipation capability and gives +30dB gain.

The input can now be altered to match the new NFB. Change the 10k to match the new NFB value.
Select the input DC blocking cap to give a F-3dB ~2Hz (24k & 4uF plastic or 10uF+6u8F electrolytic in series)
The low pass could be reduced slightly change the 680pF to 1nF, or even as high as 2n2F
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Last edited by AndrewT; 16th March 2015 at 09:09 AM.
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Old Yesterday, 02:19 PM   #10
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The unit now works great!!!
No distortion, and I can see Vbe multiplier is working great. I mounted them directly on top of the output devices.
I have one more question. The Fets for the tweeter aren't paralleled, as it only uses one irfp9240, and one irfp240 for the tweeter amp. Do they also need ballast resistors?
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