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Old 22nd November 2012, 06:42 AM   #3011
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Just look at post 2992 from Nelson.
He says how to.
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Old 22nd November 2012, 12:16 PM   #3012
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Quote:
Originally Posted by bobodioulasso View Post
Just look at post 2992 from Nelson.
He says how to.
A stand alone ACA has an open loop gain of 23 dB. Thus, it falls short of the specified 39 dB in F6; per Mr. Pass. IRFP240 may have higher distortion, lower Gm, and an added source degeneration required relative to R100A. We look forward to his simple solution and those of other DIYers' to attain an open loop voltage gain of 80. This race for a simple solution is on.

I hope that Variac can keep the shelves of diyAudio Store stocked with the existing ACA PCBs and components. My proposed schematic may be tweaked to become the basis of a high power modified ACA; instead of a modified F6.
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Old 22nd November 2012, 12:29 PM   #3013
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Based on shematic in F6 Amplifier

Can I assume that P3+P4 will be normal 1/4-1/2 watt pots? Not certain at all being in line with Source degeneration resistors.

Also - do we still want to see a pot across the upper tranny pins 5 and 7?

I am stuck on this issue on my current layout.
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Old 22nd November 2012, 01:16 PM   #3014
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Can I assume that P3+P4 will be normal 1/4-1/2 watt pots? Not certain at all being in line with Source degeneration resistors.

>>yup... Even assuming 10'amps thru rs, that means max 1 volt or so across rs ... Let's call it 2 to give us extra margin - gives us max dissipation in the pots of 4/10 = 0.4w so 0.5 w should be plenty. Guess 0.25w would be fine too...

Also - do we still want to see a pot across the upper tranny pins 5 and 7?
>> can't imagine why you'd need or want them since you've got the zen love pots across Rs already ...

I am stuck on this issue on my current layout.[/QUOTE]
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Old 22nd November 2012, 01:24 PM   #3015
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Ilquam noted that the pots across the secondaries did not do as much for him, but he had balanced the windings outputs with a cap on input side I believe. In doing this, I believe he pushed the adjustabillity to the output.
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Old 22nd November 2012, 04:16 PM   #3016
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I didn't see a lot of effect from the pot loading of the secondary windings and went for the pots across source resistors instead. There doesn't appear to be any significant ill effect from using small source resistors (like 0R1).

The cap on the input side was to reduce the high frequency rolloff at 20kHz. Without the cap, 20kHz is 0.5 dB down.


Quote:
Originally Posted by buzzforb View Post
Ilquam noted that the pots across the secondaries did not do as much for him, but he had balanced the windings outputs with a cap on input side I believe. In doing this, I believe he pushed the adjustabillity to the output.
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Old 22nd November 2012, 04:17 PM   #3017
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To determine the power dissipation of the Zen pots, use the following formula:

Zen_watts = (Rs*Ipeak)^2/R_Zen,

where Rs is the value of the source resistor, Ipeak is the peak current through Rs (usually 2*Ibias) add R_Zen is the reistance of the Zen pot.

Example: Zen_watts = (0R1*3A)^2/10R0 = .09 watts.


Quote:
Originally Posted by Tea-Bag View Post
Based on shematic in F6 Amplifier

Can I assume that P3+P4 will be normal 1/4-1/2 watt pots? Not certain at all being in line with Source degeneration resistors.

Also - do we still want to see a pot across the upper tranny pins 5 and 7?

I am stuck on this issue on my current layout.
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Old 22nd November 2012, 04:24 PM   #3018
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Here is an LTSpice simulation of the F6 using IRFP240 fets, and "tuning the Zen pot to keep the ratio H2/H3 approximately constant. I estimate the transconductance of the IRFP at 1.4A bias to be 3.5S.

This looks pretty good. I need to build it and see how it measures and sounds.
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File Type: jpg irfp240-rswip-p03-sweep.jpg (92.1 KB, 433 views)
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Old 22nd November 2012, 06:23 PM   #3019
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I will suggest that you degenerate the IRFP240's with .47 ohms on the
DC bias portion and then connect the capacitor to the Source pin so that
it is not degenerated AC. I expect the distortion figure to be a little higher,
but no big deal.

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Old 22nd November 2012, 06:26 PM   #3020
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Rs=0R47 sounds about right for thermal stability. Any reason not to use one or more "Zen pots"?

Quote:
Originally Posted by Nelson Pass View Post
I will suggest that you degenerate the IRFP240's with .47 ohms on the
DC bias portion and then connect the capacitor to the Source pin so that
it is not degenerated AC. I expect the distortion figure to be a little higher,
but no big deal.

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