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Old 9th November 2019, 12:34 PM   #1451
ralf is offline ralf  Germany
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thanks,

when i calculate on the suggested site, 1uF should be enough.
Why suggests Pass 10uf on the output?
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Old 9th November 2019, 02:02 PM   #1452
Rodeodave is offline Rodeodave  Austria
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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.
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Old 9th November 2019, 11:53 PM   #1453
jwicaksana is online now jwicaksana  Indonesia
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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.
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Old 10th November 2019, 08:19 AM   #1454
ralf is offline ralf  Germany
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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.
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Old 10th November 2019, 08:24 AM   #1455
ralf is offline ralf  Germany
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Quote:
Originally Posted by Rodeodave View Post
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.
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Old 10th November 2019, 05:58 PM   #1456
mjf is offline mjf  Austria
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[QUOTE=jwicaksana;5970448]Fellas, Iíve 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.

what jfet is it? (2sk170gr?).......

do you have a dmultimeter?
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Old 10th November 2019, 10:35 PM   #1457
jwicaksana is online now jwicaksana  Indonesia
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Quote:
Originally Posted by mjf View Post

what jfet is it? (2sk170gr?).......

do you have a dmultimeter?
Hi, it’s 2SK117. Yes I do have a multimeter.
Thank you.
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Last edited by jwicaksana; 10th November 2019 at 10:38 PM.
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Old 11th November 2019, 12:57 AM   #1458
jwicaksana is online now jwicaksana  Indonesia
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Quote:
Originally Posted by c3474 View Post
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
Does that sound about right?
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Old 30th November 2019, 08:14 AM   #1459
ralf is offline ralf  Germany
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Default JFET BOZ gain inceaseable too?

Quote:
Originally Posted by Zen Mod View Post
you'll find your sweet spot
Hi,
am i right, that i can increase gain with using lower values for source and higher values for drain
say 5 Ohm for source and 2k7 for drain
Resaon is, that with F5 overall gain is litte too small, in comparison to Hiraga amps

best
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Old 30th November 2019, 08:23 AM   #1460
Zen Mod is offline Zen Mod  Serbia
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yup

thread lightly ..... and it'll be fun to learn
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