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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

AKSA's Lender Preamp with 40Vpp Output
AKSA's Lender Preamp with 40Vpp Output
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Old 14th November 2017, 08:55 PM   #81
AKSA is offline AKSA  Australia
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Bogdan,

It was my opinion that I passed to X, and he saw no reason to disagree.
As has he mentioned, mosfets do not make good VAS in common source.
I don't believe this is incorrect, and is based from my personal experience, in particular, in the FetZilla, where a bipolar VAS is far superior to a ZVN, specifically in imaging. No reference to using common source output devices, although I don't that role much either.

Somewhat aggressive point you make. Why criticise X when he is on a roll, doing so much good work for the forum?

HD
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Last edited by AKSA; 14th November 2017 at 08:58 PM.
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Old 14th November 2017, 09:32 PM   #82
JPS64 is offline JPS64  Germany
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Mofo PCB arrives tomorrow and I need preamplifier (B1 KORG work on progress).

Schematics attached. I know, xrk971 would prefer SMPS + BOOSTER + CRCRC.
My idea, 3x SMPS + CLC: 2x 24VDC@>4A for Mofo left/right and 1x 48VDC for AKSA/LENDER preamplifier.

Not ordered yet, comments welcome.

Final design (EAGLE binaries, Gerber and all manufacturing datas) will be available for DIY.

JP
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Old 14th November 2017, 09:41 PM   #83
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
Quote:
Originally Posted by maty tinman View Post
Yesterday I heard your fourteen amplifiers and I loved the F5 Juma sound.

Virtual Audition of Very Simple Quasi MOSFET Amp

Maybe the 15th would be the Cubie3
That's a nice amp, I agree. Perhaps it's liberated from the confines of a 2d flat PCB that makes it sing? Certainly, Juma designs some very nice amps.

Click the image to open in full size.
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Old 14th November 2017, 09:52 PM   #84
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
Quote:
Originally Posted by JPS64 View Post
Mofo PCB arrives tomorrow and I need preamplifier (B1 KORG work on progress).

Schematics attached. I know, xrk971 would prefer SMPS + BOOSTER + CRCRC.
My idea, 3x SMPS + CLC: 2x 24VDC@>4A for Mofo left/right and 1x 48VDC for AKSA/LENDER preamplifier.

Not ordered yet, comments welcome.

Final design (EAGLE binaries, Gerber and all manufacturing datas) will be available for DIY.

JP
Looks very good JP, it's the MoFo arrival that drove the rapid layout of this preamp. Couple of comments. On CLC, I have found adding a moderate value parallel resistor like 51ohms helpful to prevent that LC from oscillating. The output bypass caps should be film, it looks like you may have MLCC and that's not good as it has high distortion. I have not simulated with DN2540 yet so not sure what the values will be.
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Old 14th November 2017, 10:32 PM   #85
bogdan_borko is offline bogdan_borko  Serbia
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I wasn't criticising him, he was generalizing. I'm listening to a amplifier with mosfet vas, i tried bjt, but mosfet sounds better. In some sense it' simmilar to fetzilla... it's up to concrete aplication and a amplifier as a whole.Why didn't you use output mosfets in old aksa amps and use output bjts in new naksa amps? It is generalisation,period.
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Old 14th November 2017, 11:25 PM   #86
bogdan_borko is offline bogdan_borko  Serbia
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Quote:
Originally Posted by xrk971 View Post
My mistake, I have built Juma's F5 which did not use ZTX BJT and went straight from JFET to MOSFET gate. If original F5 uses the BJT for VAS then that is good and agrees with what I said about not using a MOSFET as a VAS. But I am no F5 expert so leave it at that.
Yes, original F5 uses bjt for vas, Juma ruined it with his mosfets
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Old 14th November 2017, 11:34 PM   #87
JPS64 is offline JPS64  Germany
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@xrk971 Thanks for comments: output bypass is VISHAY/ROEDERSTEIN MKC1860 (very cheap at pollin.de).

JP
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Old 14th November 2017, 11:44 PM   #88
Max Headroom is offline Max Headroom  Australia
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Quote:
Originally Posted by AKSA View Post
......mosfets do not make good VAS in common source.
I don't believe this is incorrect, and is based from my personal experience, in particular, in the FetZilla, where a bipolar VAS is far superior to a ZVN, specifically in imaging.
HD
Any ideas of why this is...the THD is not particularly different ?.

Dan.
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Old 15th November 2017, 02:17 AM   #89
Bigun is offline Bigun  Canada
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AKSA's Lender Preamp with 40Vpp Output
In my very limited experience with Class AB amps the sound is especially sensitive to the VAS and what happens there (typically compensation but the VAS is often at a key juncture between the Class A input stage/error amp and the non-linear impedance of the output stage). It's waaay more than the THD can explain. The detailed nature of non-linear capacitances and operating curves are all important.

Low OLG amps can also be sensitive to the design of the error amplifier - LTPs work best with high OLG if you want to avoid large differential signals sweeping the LTP gain curve into non-linear regions.

Of course, for comparable OLG you need a high current through a FET VAS if you want to match the transconductance of a bjt... and a bjt wrapped up in Cdom is already quite linear.
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Last edited by Bigun; 15th November 2017 at 02:23 AM.
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Old 15th November 2017, 02:35 AM   #90
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by Max Headroom View Post
Any ideas of why this is...the THD is not particularly different ?.
I'm also curious about this. My first guess is that it might have something to do with MOSFETs having much higher 1/f noise than bipolars. So low frequency details get lost, ISTM low freqs are very important to getting good soundstage depth.
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