Jfet BOZ

I have fresh batteries...12 AA's, about 19 volts.

I changed the the source side resistors to 5 ohms, and the V+ input drain resistor from 2.2K to 1K. Thats giving me about 10V across the Jfets. No more crazy distortion.

It's making music now. It sounds well...good I think.

Also, The voltage across D-S of the Jfet is 10.1V on one channel and 9.5V on the other. Does this indicae a bad match on the jfets?

It's got lots of noise If I crank up the volume without a signal. Maybe I need to sort out the wiring a bit. But it's working.

What voltage should I be aiming for across the Jfets? They are BL grade. I am starting to understand the relationship between the drain and source resistor to the distortion and voltage across the Jfets.

Also, I have a Pass f5 that I jacked up the gain by using 200ohm gain resistors instead of the stock 100 ohm. So, I have more feedback and 6db more gain than stock in the amp. maybe I should go back to the 100ohm.
 
What is the smallest value cap I could use on the output? I have some Ampohm 2.2uf's would they be useable?

I have seen some people suggest a 5.6ish UF cap, i am using an 8.2UF and have no DC on output.

I finally got mine sounding good by playing with the source and drain resistor values. I'd pick up an assortment of resistor values to play with for the source and drain if you don't have a lot on hand already.

Also, I had a lot of hum and noise when turned up. It was my unshielded interconnects. Never noticed it when using a passive preamp setup.
 
Hi all, thanks for an interesting thread. I have started dabbling with solid state audio recently starting with a DCB1 and the ACA, both originating from Mr Pass (Thanks Nelson). Compared to my valve power amp (a 6L6 SE with EF86 drivers) I have to crank up the LS volume on my DCB1 to get sufficient SPL from my 90dB speakers.

Now I'm planning to build a jBOZ preamp and B1 buffer in the same case. As the two designs have have a potentiometer at their inputs I would like ask where is the best place to have the volume control: at the jBOZ input or at the B1? I am thinking of putting a 25K trimmer at the input of the other design.

Is this plan OK, and will it work? I don't want to have two volume controls on the front facia, one to control the JBOZ and another for the B1.
 
its difficult to understand what you are saying, or trying

OK let me have another go: I intend to connect my sources to a jBOZ (for some gain) which is in turn connected to a B1 (housed in the same case) feeding an ACA amplifier.

If I interpret your reply (for which I thank you) correctly, the jBOZ will, as you said, be the input and the B1 the output.

The Bride of Son of Zen article has quite something on volume control placement iirc, might wanna check the PassDIY site.

Thanks, I will look it up for enlightenment.
 
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In my opinion the only drawback by the JFET BOZ is the relative high distortion for higher output voltage (Post 58). So I just wondered if any of you have tried the modification as shown in the attachment. By tying the lower end of the source resistor to a negative voltage, the distortion should be lower. This solution is very similar to raising the gate to a positive voltage and using a coupling capacitor.
 

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Hello, today i received 2 f5 boards and jfets from the store, those are the last parts needed for my F5, i actually usa a AmpCamp and a B1 and i'm really satisfied.
I'd like to build a JfetBOZ for my f5 but i have some doubts, can someone help me?
-i plan to use a 18v smps (from the b1)
-To lower the gain i need to increase the R on the Source pin, i'm planning to use 20R, is that ok?
- i have a 20k stereo stepped attenuator can i use it in place of the trimmer at the input, or i need the pot and the trimpot?

Thank you everyone
 
I have just received my HP339A last week, and this gave me a perfect oppertunity to play with it....I measured the original schematic as postec by Nelson on post#2 with a few JFETS.. 1Vrms out, 1KHz, here goes:

Idss=4mA, optimum Vsupply = 7.7Volt, THD = 2.5% mostly H3, some H2

Idss=8mA, optimum Vsupply = 16.3volt, THD=0.43% , pure H3, gain=26dB

Idss=14mA, optimum Vsupply = 24.5volt, THD=0.17% pure H3, gain =28dB

Idss = 16mA, optimum Vsupply= 28.9Volt, THD=0.115% pure H3, gain =28dB

JFETs with simmilar Idss measured the same THD, gain and optimum Vsupply...

!

I loaded the data into excel and set it to a 3rd order polynomial for a good estimator. I also made another graph that is fairly easy to read.
You can also punch the equation into a wolframalpha and get an estimate for your specific idss.

y = 0.0152x3 - 0.4738x2 + 6.1317x - 10.22

Below is my estimate for idss = 9.3 (just replace the "9.3" with your idss)
y = 0.0152(9.3)^3 - 0.4738(9.3)^2 + 6.1317(9.3) - 10.22 - Wolfram|Alpha

https://drive.google.com/file/d/0BzO309CR8CcpWU1sYS1tZTlNeHM/edit?usp=sharing


Thanks to all for the data and hope this helps another DIY'er! and as always critique welcome!
 
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