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Old 16th September 2007, 11:29 AM   #1
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Default Jfet Cascoded BOZ

Hi all!
I have received some 2sk170BL and built this preamp circuit using Nelsons articles about Cascode operation/ZV9 and of course BOZ...the preamp did actually work after some hours trimmings...
it shows good square waves and linearity figures on the scope, and sound is clear, and bass sound impressive...
this lead me to think that I could now show it to you as I would really like to have your advices concerning any trimmings that could be done in order to have it work in the good spot for the Jfet...I , for sure have made several beginner mistakes , as it is one of my first trial as building something from scratch, but I would be glad if someone could help me point them out
here is the schematic:
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Old 16th September 2007, 11:42 AM   #2
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By the way, the nodes voltages that are on the schematic were from 2 diffrent trimmings, the ones in the triangles were from a first bias setting, then the others are the actual ones that are now set...
here are some picts of the work bench
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Old 16th September 2007, 11:47 AM   #3
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a close-up showing the regulated PSU, it delivers 71V, but as there are diffrent switches acccross the Zeners, it can be ajusted to 60v/71V/82V and even 94V, so it's versatile enough for many preamps projects...(it is a copycat of Nelsons BOZ PSU except that the TIP 29 is replaced by a TIP 41C...
Vince
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Old 16th September 2007, 12:47 PM   #4
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OOPS! I made a mistake in the schematic drawing, the resistor between Gnd and the source pin of the Jfet is a 450Ohm, and not 270Ohm...
Vince
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Old 16th September 2007, 02:53 PM   #5
flg is offline flg  United States
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bobsinclar, Nice I would like to give advice but I'm no expert either. However, Let me stimulate some thinking in a few areas... Don't you think that 50K "volume ctrl" is a problem for source impeadance of the following stage? I would evaluate using a pot at the input instead...
And, what's the 330k and the 4.75k at the input for? And, how about this? I believe that 10u at the input could be a higher quality cap of lower value ($$$) with the 100k input R...
What do you think? Thanks
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Old 16th September 2007, 04:07 PM   #6
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Hi Flg!
actually you pointed the 50K and it is right that I could use a lower pot, but I have no 5K...so I used it...but I will try to find a 25K I may have one lying around...

I put it at the end of the preamp, because Nelson did it on BOZ and I thought it was just right keeping that...but I will try it in front, and we'll see what happens, do you think I can keep it as 50K if I put it in front?

actually the 330K is a key part of the schematic, remove it, and the amp doesn't work, the sine waves are cut flat on the upper side, I don't know exactly why, but it happened that when I put it in place the sine wave become perfect, and the IRF 610 actually become conducting the way it should to make a voltage drop in the CCS resistor(6K), I guess this resistor helps the Gate of the Jfet to be referenced at an other voltage than a virtual ground...the truth: I don't know

as for the 4.75k at the input, you are right I wasn's sure either that it was necessary to use it...mayve 200Ohm to avoyd parasitic oscillations would be enough...

about the 10uf cap at the input, do you mean that I could have used a 1uf? and then that would have cost less? actually the whole schematic is built with trash components, and these are electrolytics...I for sure have good audio caps of nearly every values...I will try them, but I would like to set right all the concept before...
Thanks for your help Flg, I will try to modify the pot right now!
Vince
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Old 16th September 2007, 04:31 PM   #7
flg is offline flg  United States
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Well, NASCAR is getting ready to start and I'm trying to get my L Loaded ZV9 proto set up to watch racing and play with it at the same time. What a combination ha?
Yes, I think a 50k pot would be good at the input. It will serve as the input load risistance and the hi Z of the JFET probably won't mind at all.
330k? I'm still thinking about it,,, but, I think you can remove it. You probably need a little tweak somewhere else???
I have used JFETs many times without any input Rs or oscilations...
The 10u... Well, I think without the 330k and the 4.75k, you don't need it at all! But it's a good idea sometimes, depending on what your source is. Having said that, it needs to be sized based on the resistance after it for low frequency rolloff. If you only have that 100k you could probably go down to 2.2u or so. Then you could spend the same money on an expensive "audiophile approved" type. I've used Mundorf "M-Cap Supreme" in places like that but they are $$$.
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Old 16th September 2007, 04:33 PM   #8
Magura is offline Magura  Denmark
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Quote:
Originally posted by flg
Well, NASCAR is getting ready to start and I'm trying to get my L Loaded ZV9 proto set up to watch racing and play with it at the same time.

How far are you with your LZV9 ?

Magura
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Old 16th September 2007, 04:39 PM   #9
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Well I'd be ok to remove the 330k if only I manage to make it work without! but looks like a key part...
I already changed the pot value to 4.7k, but I 'll let it at the output for a time, looks like nothing has been changes, except a little less gain(what is actually desirable as it had too much!)
actually my question, would be concerning the IRF 610 VGS, if you look at the voltages on my schematic, it seems that it is only 9.5V-6.8V so 2.7V or so...this means that the fet isn't running correctly?? any idea?
Vince
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Old 16th September 2007, 04:45 PM   #10
flg is offline flg  United States
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Quote:
Originally posted by Magura



How far are you with your LZV9 ?

Magura

Hmmm. Well, Ive had it running with the sig gen and the scope mostly with unmatched parts. I've listened to it on some cheapy spk'rs a little. but it had to low of an output cap for descent low freqs. Let me show you a pic of what it's set-up like for todays playing around...
The 120mH is a Hammond 195T5 5A 100mH Choke (measures 120mH @ 3.5A)
I'm going to look at the Open Loop Gain and Freq Response today and then add the R1 & R2 and see what I can get for Closed Loop response and gain...
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