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Old 23rd June 2020, 01:11 AM   #241
still4given is offline still4given  United States
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Fetzilla Build Documentation
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Hi Terry,

If the cap multiplier is fitted to the FetZilla and the quiescent is only 350mA, then two channels would draw about 720mA at idle and bounce up from there with music signals. I would not expect much heat from the cap multiplier because the total idle dissipation would be 0.72 x 1.5 = 1.08W, almost nothing for a large series pass transistor, maybe an average of about 3W passing heavy music.

Very different to a Class A like the Alpha Nirvana which will be passing and average of 1.7A and up to 4.5V across it. That is 7.65W and you'd really notice that.

The offset issue is a nuisance but based on past experiences with singletons and bipolars, it seems to be caused by the heavy tempco of the jfet input device and mosfet VAS device. Small temp changes there will cause big changes on the offset, and its origin is first and second stages, not the output stage. Understandably this could be improved; but to do it, and improve the depth of image as well, the best way would be replace the mosfet VAS with a bipolar, like a KSA1381. I believe a bipolar is far better for the VAS role; it has much more transconducance and therefore more accuracy than a small mosfet.

Thanks for keeping the thread moving........

Hugh
Hi Hugh,

The offset was weird but I don't think it will cause any grief. I'm just going to forget about it. The speakers won't care. Good to know about the Cap multiplier being sufficient because it is very clean and I think a good option since I cut off the original PSU portion of the boards.

I've added this amp to my A/B stack and am looking forward to comparing it to other amps.
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Old 23rd June 2020, 02:26 AM   #242
AKSA is offline AKSA  Australia
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.....and I will be fascinated to hear from you how it compares, particularly with the Very Simple Quasi, one of my favourites!!

Hugh
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Old 25th June 2020, 08:11 PM   #243
still4given is offline still4given  United States
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Hugh, Hugh, Hugh look what you've done to me. OK, so now I had to hook up the Very Simple Quasi again. I forgot what a nice little amp that is. I listened to it for most of the day before comparing it to another amp. First I compared it to the Ovation NX since I had just recently added it to the amp stack. They were very comparable with the Quasi being a little easier listening on some stuff. This morning I hooked up the Fetzilla and compared it to the Quasi. They are quite different. At first I preferred the Quasi as it has deeper lows and I felt like I needed to push the Fetzilla to get more oomph from it but then I put on some Mel Torme and Ella and the Fetzilla showed it's worth. The vocals sounded like they were singing in my ear. So natural and clear. Beautiful. So I would say the Fetzilla isn't for everyone but it certainly has its place.

Blessings, Terry
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Old 25th June 2020, 10:26 PM   #244
AKSA is offline AKSA  Australia
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Many thanks, Terry,This actually surprised me!
I sheet the great vocals back to the unusual bootstrap/CCS hybrid we used in the FetZilla.

It seems it hits the spot on vocals, which for me - and maybe for you too - is the big spotlight for a good amp. It tells me that I should use that hybrid in all my amps!

Many thanks Terry, you have given me incredibly useful information.........

Hugh
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Old 25th June 2020, 11:22 PM   #245
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