Gyrator loaded Son of ZV9/F3

The current depends on Idss of bf862 so your Q3 in one channel is very different from the JFET in other channel.
You should either use parts with same/similar Idss or increase the value of R3 to lower the current to 10mA.
Using similar JFETs is a better solution.
 
The only active part you need is this:
Mikro Princ | BS170 | FET | Tranzistori | Poluprovodnici | on-line prodavnica
Kelco - Proizvodi - Rezultat pretraživanja
and it costs 10 cents a piece. The rest is trivial...

Hi Juma,
I bought the BS170 mosfet and I will build a proposed preamp. I think I will try several versions :)
But, starting with the simplest, if you were to use just a resistor instead of CCS, and you had 60 v supply, but you can afford to drop 40 v across the resistor...should it work? I saw that you draw several variations, but none of them was with just a resistor..I guess that its value should be around 4.7 k, then I could place a 22 k between the Drain and Gate, and a 10k pot between gate and Source, and maybe 4.7 k at the front of the gate. I want to try the "micro-zen" preamp approach. If unhappy, it's easy to make it more complicated ;)
 
Hi Juma,
I re-read the whole thread. Right now I am trying various preamps and trying to figure out what it is in the preamps sound that I like and vice versa. Also trying not to get lost among myriads of preamp variations :)
So, the story goes: I have two amplifiers: Zen V9 and a Mosfet follower, something like MoFo, but with a lightbulb instead of an inductor and with around 45 v suppply. So I was playing with various preamps trying to find which one I like the most. I had a BoZ which I then modified (added Schade feedback) and made it look something like Aleph L. Then I quickly bulit a Jfet Boz, according to your sugestion (with J310 jfet, 220 source degeneration resistor and a 680 ohm drain resistor, 24 v supply). I was pleasantly surprised to find out that I like its sound more than the original BoZ and also the modified Boz. The difference would be described as more natural sound, more open and relaxed, more transparent and easy flowing . (The amp was ZV9)
Wondering why, was it becasue of the Jfet vs. Mosfet (irf610-with a largish input capacitance), lack of feedback, or sometjing else - I don't know. But Jfet boZ with J310 sound more like tube, (I like it) and a Mosfet one with irf610 sounds more like Solid state (I don't like it).

So, as I am trying to make an improvement over Jfet Boz with J310, i am thinking that maybe it is better that I don't mess with BS170 mosfet as a gain device (although I may just quicky try it-I am curious), but instead focus on building this amp, as it is logical step up from a Jfet Boz, with a Jfet at its heart. Even though that means I will have to get the BF862.
To get to the point, I was thinking about the Gyrator circuit. Nothing against it, but have you tried to use a real inductor here instead?

I was thinking, If you happen to have a neon lamp (220 v 18w) inductor, is it possible to use that one instead of a gyrator?

Also, regarding the gain, If I use it with Zen V9, maybe 6 db of gain will suffice, but If I use a follower, I would like to have the option to increase the gain up to 15-18 db...so mabe I should use schematic from post nr.1 as a basis...
 
Seems like you are having a lot of fun.. :D
Real chokes are (in my experience) inferior sounding solution, but many like them, so give it a try, you might like it.
Circuit from post #1 is nice, just take care about the parts you use in your build, some had problems with noisy resistors and similar stuff...
And yes, stay clear from BS170, if more people find out how good it really is, it will soon become much more expensive... :)
 
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Hi Juma,

I am still playing with simple jfet and mosfet preamps. I have builthe one with BS 170, in fact I did the simplest version, I used the 4.4 K drain resistor (22k, 5 pcs in parallel) on a 60 V supply, 39 K from drain to gate, I also added a small gate stoppeer resistor, then a pot from the gate to ground to set the proper bias (around 20v at the drain, 40 v across the 4.4 k resistor). I can vary the input resistor from about 5.6 to about 12.4 k to vary the gain. (think of it as 5.6 k+ 6.8 K resistors in series, but the 6.8 resistor can be bypassed with a small switch for higher gain and vice versa). Since the input impedance thus varies from 5.6k to 12.4 k, I put 20uF cap at the input. To make sure it has low output impedance, I also added an output buffer (maybe it is not necessary). Currently it is driving F3, but could be needed to drive a Mosfet follower as well (Sewa- like Mofo, but with huge lighbulbs instead of an inductor and 40v supply). That's why I wanted to have a variable gain.
I did some listening to this preamp on F3. The sound is clear, neither cold nor warm. When not compared to anything, seems about right (on F3 amp).
But, then I reversed to the J310 Boz, this one:
http://www.diyaudio.com/forums/pass-labs/103050-jfet-boz-75.html#post1832806

And...found that I like the J310 jfet Boz more. It is slighly less analytical and more warm sounding. I find it more enjoyable and less tiring than the one with BS170 mosfet.
But, it has just enough gain for F3, and certainly not enought for a Mosfet follower.
Now, my next step is to build the circuit as you presented on post nr.24. I guess that it should sound better than the variants I have built up to now.
But, since I don't have the BF 862 mosfet, I wanted to try that circuit with J310. Yep, I know it needs some modifications- that's what I am kindly asking you here :)
Also, instead of a gyrator with BS 170, could it also have a Jfet 310 CCS on top? Should be a tad simpler. Power suply is not a problem, in this case I can vary it from 20-40v :) (single rail)
If you are busy and don't have time to draw a schematic, maybe you could give me a few hints on what values to change? (source resistor etc)...

Thanks and regards,
Vix
 
... I don't have the BF 862 mosfet, I wanted to try that circuit with J310. ...
... instead of a gyrator with BS 170, could it also have a Jfet 310 CCS on top?..
You want J310 with occasional BJT to deliver clean and high voltage swing to drive the unity gain power amp - it's a no go here; J310 can not easily replace neither BF862 nor BS170 in this circuit, it's too different.

It would have to be a whole new circuit and I would have to build a prototype to test it. I could do it on commission but it would cost you much more than few pennies for appropriate parts. :D
Actually, for your needs, this (see attachment) is safest bet, stick to it ; #24 is too specific, no fooling around with it.
In attached circuit, you can use J310 instead BF862, but R14 should be 220R or so (for Id of 10mA or so), R20 should be 22k or so, and R27 should change to 56k. It will sound different, try it to find out if you like it
 

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Well I finish

Hello Juma
Finally I found them. Two jfet one 15,93 mA and the next 16.2 mA. The preamp sound very good. Round with B'52 rude with Ramones and sweet with the radio. My son love it
Thanks Juma
 

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Juma,maybe i am offtopic but some years ago i found this schematic of yours on the net but i have no info about it and i cant remember where i found it.Is there a dedicated thread with info on sounding impressions,improvements (if any)and comments etc.?
 

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It's a small MOSFET, common source connected, loaded with a CCS (JFET). Gain is defined through local, Schade feedback (R4/R5 ratio). R3 sets the Drain of Q2 at half of the power supply voltage. Any usual CCS (at about 10mA) can be used instead of JFET.
It sounds OK - I like the circuit from post #1 of this thread better, but it's easy and cheap to try both and decide for yourself... :cheers:
 
It's a small MOSFET, common source connected, loaded with a CCS (JFET). Gain is defined through local, Schade feedback (R4/R5 ratio). R3 sets the Drain of Q2 at half of the power supply voltage. Any usual CCS (at about 10mA) can be used instead of JFET.
It sounds OK - I like the circuit from post #1 of this thread better, but it's easy and cheap to try both and decide for yourself... :cheers:
Thank you.I made a layout(not tested for errors yet) for this small MOSFET version to test.I read all posts from this thread and i saw on post #37 that you made an other version of schematic from post #1.Between those 2 schematics (post#1 and post#37) which one you like most in terms of sound?
 

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