Question for the One and Only. (Mr. Pass)

K-amps · 3rd February 2004, 01:35 AM

Mr. Pass I have just come across a Forte Model 6a. I think it is 200wpc and uses IGBT's.

Can I reduce the rails by half (by rewiring the Toroids in series) and bias it to 50 watts class-A?

I have done it in the Forte Model 3 with good results but that uses BJT's, I am worried playing with these IGBT's.

If so, what idle bias do you recommend going to with rails of approx +/-38vdc?

Thanks much in advance,

K-amps.

K-amps · 3rd February 2004, 11:49 PM

Anyone with ideas?

pinkmouse · 3rd February 2004, 11:51 PM

I'll move this to the Pass forum, you stand a better chance of getting a reply there.

K-amps · 3rd February 2004, 11:53 PM

Thanks!!

Petter · 4th February 2004, 01:02 AM

Rule of thumb:

To stay in the same thermal envelope: When halving the PSU voltage, you can double the current.

The other alternative is to aim for a temperature which should not be more than 70 degC.

You might also want to do some reading up on IGBT's to evaluate their thermal stability. If I know NP, I suspect that the emitter resistors are large enough to offset any thermal runaway should the devices be so inclined.

Petter

K-amps · 4th February 2004, 01:08 AM

Thanks Petter,

Additionally I'd like more insights especially from NP. I suspect he knows something about this that I should know.

I just dont want to blow the IGBT's and have a worthless amp on my hands.

I agree with you on the value of NP's emitter resisters, probably a minimum of 0.47 ohms... right?

Nelson Pass · 4th February 2004, 05:10 PM

Quote:

Originally posted by K-amps
Mr. Pass I have just come across a Forte Model 6a. I think it is 200wpc and uses IGBT's.
Can I reduce the rails by half (by rewiring the Toroids in series) and bias it to 50 watts class-A?
I have done it in the Forte Model 3 with good results but that uses BJT's, I am worried playing with these IGBT's.
If so, what idle bias do you recommend going to with rails of approx +/-38vdc?K-amps.

Should work just fine, although I don't have a schematic on
it, as it was after my time. I would just crank up the bias until
the sinks reach a continuous 50 deg C (touchable for 5 seconds).

K-amps · 4th February 2004, 06:35 PM

Thanks all, especially Mr. Pass!

3rd February 2004, 01:35 AM	#1
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Question for the One and Only. (Mr. Pass) Mr. Pass I have just come across a Forte Model 6a. I think it is 200wpc and uses IGBT's. Can I reduce the rails by half (by rewiring the Toroids in series) and bias it to 50 watts class-A? I have done it in the Forte Model 3 with good results but that uses BJT's, I am worried playing with these IGBT's. If so, what idle bias do you recommend going to with rails of approx +/-38vdc? Thanks much in advance, K-amps.

3rd February 2004, 11:49 PM	#2
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	C'mon anyone? Anyone with ideas?

3rd February 2004, 11:51 PM	#3
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	I'll move this to the Pass forum, you stand a better chance of getting a reply there. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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3rd February 2004, 11:53 PM	#4
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Thanks!!

4th February 2004, 01:02 AM	#5
Petter diyAudio Member Join Date: Jan 2001 Location: Scandinavia	Rule of thumb: To stay in the same thermal envelope: When halving the PSU voltage, you can double the current. The other alternative is to aim for a temperature which should not be more than 70 degC. You might also want to do some reading up on IGBT's to evaluate their thermal stability. If I know NP, I suspect that the emitter resistors are large enough to offset any thermal runaway should the devices be so inclined. Petter

4th February 2004, 01:08 AM	#6
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Thanks Petter, Additionally I'd like more insights especially from NP. I suspect he knows something about this that I should know. I just dont want to blow the IGBT's and have a worthless amp on my hands. I agree with you on the value of NP's emitter resisters, probably a minimum of 0.47 ohms... right?

4th February 2004, 06:35 PM	#8
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Thanks all, especially Mr. Pass!

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