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Old 14th June 2015, 06:36 PM   #1
juma is offline juma  Germany
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Default Gyrator loaded Son of ZV9/F3

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
Attached Images
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File Type: gif pcb-layout.gif (34.8 KB, 722 views)
File Type: jpg pcb-made.jpg (65.4 KB, 664 views)
File Type: jpg modul.jpg (159.8 KB, 660 views)
File Type: jpg built.jpg (195.3 KB, 346 views)

Last edited by juma; 15th June 2015 at 06:06 PM.
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Old 14th June 2015, 10:55 PM   #2
wdecho is offline wdecho  United States
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I like the fact that you use readily available cheap transistors. It is a preamplifier I would like to hear.
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Old 15th June 2015, 09:27 AM   #3
GuidoR is offline GuidoR  Italy
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Hi Juma,
being me a Juma-follower (no, it's not a new topology ) I have already collected all the components for this new pre-amp (and drawn the PCB before you published this one...).
A question about the JFETs: I do have many BF862, but they are all in the range >16mA. If I'm well understanding, it's all matters of current dissipated in the BC and in the BS, isn't it? (I've built some copper TO-92 heatsinks)
And (you always replied to a similar question in the other thead), would a 2SK170V be used instead? (with Idss in 12-15mA range).

Many thanks again
Guido
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Old 15th June 2015, 10:50 AM   #4
juma is offline juma  Germany
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Quote:
Originally Posted by GuidoR View Post
... it's all matters of current dissipated in the BC and in the BS, isn't it? (I've built some copper TO-92 heatsinks)
Hi Guido, it's not about current only but power (P = V x I ) but I guess that's what you meant and you got it right. If you keep the power dissipation on these two to less than 200mW the heatsink is not needed. You have to know what is the max. voltage swing you want at the output and that will determine the optimal power supply voltage (which influences the power disipation in BJT and MOSFET) . If you don't need more than 19V_peak-to-peak (6.7V_RMS) at the output, the power supply voltage can be 24V and then you can use 20mA JFET with no heatsinks on BJT and MOSFET (in that case the R22 should be 51k to ensure symmetrical clipping).

Quote:
Originally Posted by GuidoR View Post
... would a 2SK170V be used instead? (with Idss in 12-15mA range)
Sure, but I think it will sound a bit different - compare it with your higher Idss BF862 since you alredy have TO-92 heatsinks to find out what you like more. JFET won't mind higher current since it operates at Vds of about 4V only.
In this preamp I use BF862 with Idss of 14.1 mA and got the Id of 11mA with 4R7 degeneration (R2). If you use, for example, BF862 with Idss of 18 mA I supose you'll get the Id of about 14 mA which is still OK. Do some testing and you'll see...

Last edited by juma; 15th June 2015 at 11:11 AM.
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Old 15th June 2015, 03:12 PM   #5
GuidoR is offline GuidoR  Italy
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Thanks you Juma, it's time for some test...
Again, many thanks for your clarifications (and your patience).

Guido
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Old 15th June 2015, 05:07 PM   #6
zkri is offline zkri  Croatia
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Hi Juma,

I would like to try this preamp, butÖ I canít find BF862 locally. I have some 2SK170BL (10-11mA Idss) and J310. Can I use any of these, and what is needed to change in this case? I donít need more than 6V RMS at the output. Thanks.

Zdravko
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Old 15th June 2015, 05:57 PM   #7
juma is offline juma  Germany
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Hi Zdravko,
you can buy BF862 from Farnell.de and Mouser.de online shops but if you are lazy like me, try 11mA k170bl with 24V power supply - only thing to change is R22 = 51k (because of lower PS voltage, in order to ensure symmetrical clipping). Also, you can try different values for R19 (it sets JFET's Vds, between 2-5V you'll find the value that soundwise suites the k170bl the best). k170 has higher parasitic capacitance so you might lower the value of R3 to avoid making the bandwidth narrower i.e. nasty phase shift in the upper range. Of course, that means adjusting the R1 to suit the chosen gain ratio with the new value of R3. C2 might need minor adjustment too. Of course, none of these changes are mandatory, the preamp will work OK as it is, with just k170bl instead of bf862, but I think it won't sound its best...
So, as usual, being lazy means more work

Last edited by juma; 15th June 2015 at 06:00 PM.
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Old 15th June 2015, 06:13 PM   #8
gadut is offline gadut  Indonesia
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Superb

it's great that power supply is reduced now, however is there any changes for voltage doubler that you previously posted? I already have 2x15V transformer to be used.

this is a request : can I buy from you all transistors used for this project? I'm sure someone else will ask the same
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Old 15th June 2015, 06:45 PM   #9
juma is offline juma  Germany
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Quote:
Originally Posted by gadut View Post
...it's great that power supply is reduced now...
It's reduced because I don't need to drive the F4 to full power which is what you asked for.

Quote:
Originally Posted by gadut View Post
... any changes for voltage doubler that you previously posted? I already have 2x15V transformer to be used.
You don't need voltage doubler for this version - 30V AC rectified is about 40V DC so drop some voltage with RC filter or use the regulator of your choice.
If you mean to use that transformer with high voltage version from the other thread it's OK, just drop some more voltage on the resistor in the RC filter.


Sorry but I don't have enough parts to sell them. Anyway, they are all current production transistor that can be bought all over the world.
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Old 15th June 2015, 09:05 PM   #10
juma is offline juma  Germany
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Quote:
Originally Posted by zkri View Post
.... and J310....
That one might be interesting. To use it here its Id should be set to about 12mA (R2 in 120-180 R range).
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