simple preamp from half 12ax7

[erez1012]

Your circuit shows about 1.5mA idling current - are you sure that's enough?

no, what is your recommendation?

nice video about Gate Loop Ringing at 07:25

 

[erez1012]

update

 

[Chris Hornbeck]

Approximately half of your B+ supply voltage will appear across the ? resistor, so it will also dissipate about the same power as the MOSFET. It's value is simple Ohms Law: R = Volts (about 150 for your case) divided by Current desired.


Only you can know what current is desired because you're keeping that a secret.


YOS,
Chris

 

[erez1012]

if i understand you correct, from post 21.
20mA is a good start. so 15Kohm,6W

 

[Chris Hornbeck]

Well, 20mA would mean 7K5, but yeah.


YOS,
Chris

 

[Bigun]

Perhaps get a couple of 15k, use one for 10mA which will likely be plenty and both resistor and FET will run a bit cooler - generally a good thing in my view.
Put a second 15k in parallel with the first for 20mA, which will generate more heat, see if it sounds any better to your ears

 

[erez1012]

hi again.
All the parts arrived, I started building the pre amplifier.
It works well, makes a bit of noise, probably power or GND.
12au7 works great for gain.
The spectrum picture is not so good. no 2nd harmonics

 

[nigelwright7557]

The 12ax7 has a lot of gain so any noise around will be amplified too.
Try extra decoupling on B+.

 

[kodabmx]

12au7 works great for gain.

The 12ax7 has a lot of gain so any noise around will be amplified too...

Which is it? 12AU7 or 12AX7? If it's 12AU7, those plate and cathode values are way off, and therein lies the problem most likely.

 

[erez1012]

Which is it? 12AU7 or 12AX7?

i try with 12ax7 and 12at7, but the volume is jump to high level about 9 o'clock. so i put 12au7.

those plate and cathode values are way off,

what is you're recommendation?

Try extra decoupling on B+

OK

 

[kodabmx]

If you have the parts, you can try 47k plate resistor and 220R - 330R on the cathode for a 12AU7. With a 300k plate load, the 12AU7 is so current starved it's virtually worthless 🙁

Otherwise, use 12AX7, and a voltage divider on the input to reduce the gain (or install negative feedback).

 

[erez1012]

thanks.
i try the 47k and 220 with 12au7

 

[erez1012]

If you have the parts, you can try 47k plate resistor and 220R - 330R on the cathode for a 12AU7. With a 300k plate load, the 12AU7 is so current starved it's virtually worthless

Otherwise, use 12AX7, and a voltage divider on the input to reduce the gain (or install negative feedback).

work good. thanks

Try extra decoupling on B+.

yes it help

Summary:
pcm56p DAC 1ma output to 100ohm
12au7 with 47k plate resistor and 220R - 330R on the cathode

 

[kodabmx]

Glad it helped.