Jfet BOZ

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#1,452
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.
 
#1,453
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.
 
#1,454
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.
 
#1,455
Rodeodave said:
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.
Click to expand...

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.
 
#1,458
c3474 said:
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!
Click to expand...

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