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Gyrator loaded Son of ZV9/F3
Gyrator loaded Son of ZV9/F3
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Old 18th June 2015, 09:48 PM   #11
bogdan_borko is offline bogdan_borko  Serbia
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Quote:
Originally Posted by juma View Post
When I saw ZV9 for the first time it looked to me like a beautiful preamp topology made to an power amp thanks to existence of LU1014.
Few months ago I started to entertain this single ended preamp idea again and not only that the sound was exceptional (I like it so much that I removed BAF 2013 preamp from the case to move this one in although I thought that I'll keep the BAF2013 for a very long time), it showed other great qualities too: great drive/swing capability (Zout less than 50R and ability to drive power buffers like F4), great versatility in terms of gain and power supply used, low and steady consumption (10-12mA per channel), it's stable into reactive loads and the freq./phase response is linear up to 100kHz.

The story about this preamp actually started here but I think it deserves a dedicated thread. The main change from the original ZV9 is that I use gyrator instead of aleph CCS - it sounds better to me but it's easy to test both.

As a power supply I use 32V SMPS taken from dead HP inkjet printer, followed by a capacitance multiplier. The PCB is 74 x 25mm, copper side view. R9 is mounted on the output terminal and C3 (WIMA-MKP10) is beside the PCB due to its size. C2 is 2-5pF, made from two twisted isolated wires cut to length for best square wave response (I did this just because I don't have small value caps in my parts repository at the moment). As shown, the gain is 13dB (4.5V/V) but can be set as ratio of R1/R3 in a wide range (I tested 6-26dB). The clean swing at the output with 30V rail voltage is 24V peak-to peak (8.5V_RMS) into 10k load.

EDIT:
One thing I forgot to mention - due to RC constants present in the preamp and cap. multiplier, 10-15 seconds are needed for capacitors to charge and the sound starts to come out. You might call it: very slow start
Juma, nice and simple as always! Couple questions:
1. Whay is R6 referenced to the output?
2. Referencing C8 to the source of jfet (not ground) always sounded better to me, have you tried it?

Cheers, Borko.
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Old 19th June 2015, 07:20 AM   #12
juma is offline juma  Germany
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Hi Borko,
Quote:
Originally Posted by bogdan_borko View Post
1. Whay is R6 referenced to the output?
That's the trick with "modulated cascode" as we now it from ZV9/F3 - practically it's the same idea that is more often used in source follower circuits where the botom end of cascode is modulated from the JFET's source which is the output node. It audibly linearises the operation - I noticed that with it this circuit can be used without the local feedback and it still sounds very good (although the gain is higher) which is not exactly the case with standard cascode.


Quote:
Originally Posted by bogdan_borko View Post
2. Referencing C8 to the source of jfet (not ground) always sounded better to me, have you tried it?
Connecting the C8 to GND leaves some small "room" (in terms of voltage) for JFET to "breathe" (the Vds is not strictly fixed) and that solution sounds better to me. So, as always, it's matter of personal taste, although in this case the swing on the source of the JFET is very small - it swings only the amount of current that's drawn by the load times 4R7...

Best regards !
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Old 23rd June 2015, 05:42 PM   #13
Dennis Hui is online now Dennis Hui  Canada
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Gyrator loaded Son of ZV9/F3
Hi Juma,

I'm wondering how high impedance do you think one can use for the input pot?

Thanks,
Dennis
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Old 23rd June 2015, 06:05 PM   #14
juma is offline juma  Germany
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Hi Dennis,
10k is an optimal value. If you happen to have an extra impotent source component that can not drive a 10k pot, use 25k pot, but in that case the R3 should be changed to 10k and R1 to 47k. If 25k is still too low for the puny source component, add a buffer between it and the pot.
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Old 23rd June 2015, 07:16 PM   #15
Dennis Hui is online now Dennis Hui  Canada
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Thanks Yuma. It's probably best I get a new pot.

Cheers,
Dennis
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Old 8th July 2015, 01:37 PM   #16
juma is offline juma  Germany
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A fellow member asked me if this preamp can be made with simple BJTs only so I gave it a try and it works lovely.
He is OK with max. 6V_RMS at the output so the rail voltage is down to 24V.
Since the use of a BJT at the input and single rail PSU imply the use of an input cap I introduced local Schade feedback (R10, R3) to control the gain (about 13dB). Z_out is a bit higher (150-200 R) , Z_in too (47k), some elements' values are changed, but it still sounds very good...
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Old 8th July 2015, 02:07 PM   #17
Dennis Hui is online now Dennis Hui  Canada
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Gyrator loaded Son of ZV9/F3
I ordered some BF862; hope I can actually see them.
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Old 9th July 2015, 07:04 AM   #18
juma is offline juma  Germany
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Tiny little bastards, aren't they ?
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Old 10th July 2015, 03:39 AM   #19
Dennis Hui is online now Dennis Hui  Canada
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Quote:
Originally Posted by juma View Post
Tiny little bastards, aren't they ?
Got them today. Dang, they're small...
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Old 11th July 2015, 01:48 AM   #20
kp93300 is offline kp93300  Malaysia
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JUma,
Is this the BF 862 used in the schematic?
It has two numbers ..
NXP BF862,215 RF FET, N CHANNEL, 20V, 25MA, 3-SOT-23
BF862,215 - NXP - RF FET, N CHANNEL, 20V, 25MA, 3-SOT-23 | element14 Malaysia

thanks
kp93300
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