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FH9HVX - Budget Conscious 100w Class AB for Lean Times

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Francois G

Francois G

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I noticed that in the second schematic in post #1, V131 (FQA40N25) the Fairchild N-channel QFET is specified as a 250V device, while V132 (FQA36P15) the P-channel is a 150V device. These are the Power MOSFETS listed in Vunce’s Mouser BOMs, which I had just ordered today.

Out of curiosity, could someone explain the reason for specifying a 250V N-Channel, while the complementary P-channel is 150V?

Mouser has in stock an N-Channel QFET (FQA46N15) specified for 150V. Would that be a better complementary device for the 150V P-Channel?

https://www.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/FQA46N15?qs=7j7f/9GINBGCx53nOtb/Dw==
 
xrk971

xrk971

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Hi Francois,
Great question and I think the FQA46N15 might work even better. The one we spec’d was based on what Hugh had recommended for the Alpha 20 long ago. In reviewing the specs it seems the 46N15 has lower input capacitance and higher trans conductance and only 30w less dissipation power. It was the 280w vs 250w that was probably the deciding choice as we were looking for something very thermally robust. Both Vunce and myself have even used IRFP9240 here and it works fine although pushing the safe operating limits for the IRFP to only use a single for a 100w amp.

I’ll make a note on Post 1 that the 46N15 is also a good (perhaps better) choice. I’ll wait until Hugh chime in on this too before we say it is definitively better.

Thanks.
 
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wtnh

wtnh

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Joined 2015
Quick question - the feedback resistors, R141 and R108 in the original schematic from post #1, are specified as carbon. It is my understanding that in general, carbon resistors will exhibit more noise in the audio band than thin-film metal, so I am wondering about the rationale?

I was looking at this article from EE Times:

https://www.eetimes.com/selecting-r...ifier-and-other-high-end-audio-applications/#

Thanks
 
xrk971

xrk971

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You want carbon film and noise is not the factor here but the generated higher second order vs third order harmonic distortion of the carbon film vs metal thin film. Metal thin film or metal foil or bulk laser trimmed metal are lower overall THD. But carbon film will sound more natural with just a tad higher THD. You need to hand match them though as they come 5% from factory.

But metal thin film will work and sound just fine. Use good quality ones here.
 
wtnh

wtnh

Member
Joined 2015
xrk971 said:
You want carbon film and noise is not the factor here but the generated higher second order vs third order harmonic distortion of the carbon film vs metal thin film. Metal thin film or metal foil or bulk laser trimmed metal are lower overall THD. But carbon film will sound more natural with just a tad higher THD. You need to hand match them though as they come 5% from factory.

But metal thin film will work and sound just fine. Use good quality ones here.
Click to expand...

Good to know - I was pretty unaware of the carbon impact on harmonic signature. I guess Erno Borbely was hinting at something similar when he stated in this article that he preferred Caddock MK132s (since replaced by MK232 series) for feedback and input use in FET amps. These are thick (as opposed to thin) film deposited metal resistors which, according to the EE Times article I mentioned before, are more similar to carbon in terms of noise. They are also serious money, > $5.00 each, but are 1% tolerance.
 
wtnh

wtnh

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Joined 2015
The LEDs in the BOM are indeed 12 Volts and contain an integrated current limiting resistor - looks like they are intended for automotive applications. The schematic already shows a resistor for the LED so you would not want to use the one in the BOM - just a regular LED.
 
xrk971

xrk971

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Joined 2012
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Hi guys, which LED are you talking about? If this is the indicator LED for the PSU, a regular one with circa 2v drop is fine. I don’t recall any that have a built in resistor and meant for 12v. If so, that’s an error - a standard LED should be used.

On the FH9HVX amp board, there is a red LED that is an indicator on the input stage CCS. That one is less than 2v drop.

@Meanie, regarding +/-56v - I think it can work and if caps are 63v rated it will be fine. I might have to look at if some setpoints need adjustment for optimum performance though.
 
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