Jfet BOZ

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For one, it's a very common value cap.

Also, you want to make a versatile preamp with good bandwidth into demanding loads. Generally, you'll want to put your -3dB frequency two octaves below the lowest frequency you want to have unaffected by the highpass formed by the coupling cap and the input impedance of the following stage. f=1/(2*pi*R*C) for 10uF and 10k gives f=1.6Hz for -3dB. Two octaves above is like 6Hz, which is reasonably far away from 20Hz. A 5k input impedance gives f_-3dB=3.2Hz. Two octaves above that is almost 13Hz, so the performance at 20Hz is theoretically still unaffected by the output cap. 10uF is thus a reasonable choice for an output coupling cap for a wide range of input impedances.

So, in short, 10uF is a common value and pretty much guarantees that the output coupling cap does not harm performance.
 
Fellas, I’ve read the thread completely from page 1 and now I want to make one.
My JFET is matched 5.87mA Idss. I am planning to use 18V battery power supply. This will drive a class D amplifier with 20k Zin and 20dB voltage gain.

Since I don’t have oscilloscope nor the knowledge to simulate the circuit, please be so kind so share:
- What value for Rdrain? I understand this needs to bias half the voltage at the Drain.
- What value for Rsource? I understand this is the local feedback and sets the gain? I don’t need too much gain, let’s say 1.5-2.5x amplification is okay, anything between a buffer and a standard preamp would suffice.

I want the lowest possible distortion while maintaining the character and the simplicity of the circuit. It has been described as lively-sounding which really piqued my interest. I’m not set on the power voltage, but pretty much stuck with the JFET.

Any suggestion will be much appreciated.
 
if i use standard JFET BOZ, to connect to one of my class D amps (TPA3116), which has lots of gain, the overall gain is too much and you can hear hissing noise with high sensitive speakers.
in my case i need small additional gain with hiraga monster in active speakers to match with the other amps and speakers.
I used 18Volt batteries to fire it.
 
For one, it's a very common value cap.

Also, you want to make a versatile preamp with good bandwidth into demanding loads. Generally, you'll want to put your -3dB frequency two octaves below the lowest frequency you want to have unaffected by the highpass formed by the coupling cap and the input impedance of the following stage. f=1/(2*pi*R*C) for 10uF and 10k gives f=1.6Hz for -3dB. Two octaves above is like 6Hz, which is reasonably far away from 20Hz. A 5k input impedance gives f_-3dB=3.2Hz. Two octaves above that is almost 13Hz, so the performance at 20Hz is theoretically still unaffected by the output cap. 10uF is thus a reasonable choice for an output coupling cap for a wide range of input impedances.

So, in short, 10uF is a common value and pretty much guarantees that the output coupling cap does not harm performance.

Thanks, that was the answer i wished to hear :)

Now i can use my pio 1uF caps and do not need to buy bigger ones.
If forgot to say, that i use them above 200hz for mid/high range amp, so 1uF should be more than enough.
 
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

Idssvsvoltage.jpg - Google Drive


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

With this formula, Idss of 5.88 shows power supply voltage = 12.5V :D
Does that sound about right? :D