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MOSFET Source Follower Headamp
MOSFET Source Follower Headamp
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Old 24th December 2016, 10:39 PM   #11
AKSA is offline AKSA  Australia
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I really like this circuit. It is a traditional tube topology using jfets, delicious, X!!
Thank you for bringing it to us!

HD
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Old 25th December 2016, 12:39 AM   #12
xrk971 is offline xrk971  United States
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That preamp is a another great Juma design. Everything by Juma that I have made works well and sounds great.
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Old 25th December 2016, 06:43 PM   #13
xrk971 is offline xrk971  United States
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After 5 hours of extended listening session with all sorts of music, I can positively say that this amp rocks. Incredible bass and dynamics. Stunning detail without being taxing. Clear smooth mids with wonderful vocal articulation. A joy to listen to. You rediscover songs and hear things you missed with other amps/headphones. My DT880-250's are a perfect match for this amp. The bias is rock-stable and the DC offset is very steady as well (about 20mV on one side 8mV on the other). In short, no other headphone amp has dynamics and bass authority like this one. I love the absolute silence and blackest blacks when music is not playing or there are quiet passages.

Hope everyone has great Xmas and new year. Happy holidays folks.
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Old 25th December 2016, 10:16 PM   #14
AKSA is offline AKSA  Australia
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X,

I'm really not surprised...... the SE topology and the wonderful harmonic profile almost guarantees good sound.

I see these BF862 from NXP are rated at 20V, and at 24V Vcc is might be pushing it.... can you reduce it to 18V and keep the wonderful sound quality?

I notice you use a 1M biasing the gate of the input device. Since this is a bootstrapped input stage you could reduce the 1M to 220K keeping in Zin very high whilst picking up on resolution. This is an old tube trick; this is a SS analog after all.

Of course, more resolution might fatigue, don't know, but an easy change to see if it makes any difference to the sound.

Ciao,

Hugh
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Old 25th December 2016, 11:02 PM   #15
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by AKSA View Post
X,

I'm really not surprised...... the SE topology and the wonderful harmonic profile almost guarantees good sound.

I see these BF862 from NXP are rated at 20V, and at 24V Vcc is might be pushing it.... can you reduce it to 18V and keep the wonderful sound quality?

I notice you use a 1M biasing the gate of the input device. Since this is a bootstrapped input stage you could reduce the 1M to 220K keeping in Zin very high whilst picking up on resolution. This is an old tube trick; this is a SS analog after all.

Of course, more resolution might fatigue, don't know, but an easy change to see if it makes any difference to the sound.

Ciao,

Hugh
Thanks for the tips Hugh. I just double checked and supply is actually 20v after cap Mx. Without the class A load the voltage was 24v. So it seems to sound fine at 20v anyhow. I will reduce that input resistor to 220k and see if it makes a difference. I am looking for a suitable case to put this in. Not too many of my amps make it past plywood board stage. So you know it must be good.

That preamp is insanely simple and just surprises the heck out of me how good 3 FETs can sound and provide gain. We are talking about "AM radio" gain stages as they are called by the manufacturer. BF862's are dirt cheap - one of the best bargains in all of DIY audio.
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Old 26th December 2016, 05:11 AM   #16
jameshillj is offline jameshillj  Australia
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... you could just 'tack-on' a 220kR across the 1MR - a standard resistor with short leads
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Old 26th December 2016, 05:18 AM   #17
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by jameshillj View Post
... you could just 'tack-on' a 220kR across the 1MR - a standard resistor with short leads
Are we talking about changing R4 or R11? (I assume R4?):

Click the image to open in full size.

(Hugh, what does bootstrapped input gate mean in this case? Usually I thought that meant that there is bias provided by a cap fed from a rail? Or is R4 the bootstrap and R11 the input gate impedance setting resistor?) in the sims it showed reducing R4 to 50k reduces distortion.

Last edited by xrk971; 26th December 2016 at 05:22 AM.
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Old 26th December 2016, 05:32 AM   #18
AKSA is offline AKSA  Australia
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X,

R4, the bias resistor which generates the operating point for Q2. Sorry I was not clear.
If you decrease R4 you might increase C1 to 220nF. Aside from that, no changes.

This bias generator, sometimes called 'self-bias' with tubes, carries the bias voltage from the two resistors (10R/270R) to the gate, but the operating point moves up and down and so does the gate voltage with it, so the impedance of R4 is magnified by this bootstrap action. Self talks about bootstrapping on input stages to SS as well; worth a look.

I have been following Juma's work for decades, from back when he lived in Zemun. There are lots of very clever people from this part of the world; Dadod is another. Juma's transposition of tube ideas to jfet is up there with NP in my view.

I do this trick on self-biased cathode followers, and the improvement in resolution is astonishing.

Hugh
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Last edited by AKSA; 26th December 2016 at 05:39 AM.
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Old 26th December 2016, 06:27 AM   #19
xrk971 is offline xrk971  United States
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Thanks for the explanation Hugh. I am about to give this a try. Will lowering R4 change the voltage gain of this preamp? I need the gain in order to drive the unity gain follower power amp.
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Old 26th December 2016, 06:50 AM   #20
AKSA is offline AKSA  Australia
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No, X, it will not change the gain at all.....

Really interested in the results. I have a feeling it will improve the resolution, not that it needs it! Making something wonderful spooky supernatural, huh?

Hugh
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