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-   -   Modification of an headphone amp to real preamp (http://www.diyaudio.com/forums/headphone-systems/8251-modification-headphone-amp-real-preamp.html)

FrankDIY 3rd December 2002 12:35 PM

Modification of an headphone amp to real preamp
 
Hello tube experts :)

I am wondering what need to be changed in my headphone amp:
http://headwize2.powerpill.org/proje...bender_prj.htm

to modify it to a preamp. The last week-end I used it as a preamp and was working fine. Do I need to change the input/output impedance??

Thanks

Joel 3rd December 2002 03:22 PM

That's cute! I like the small box...

Anyway, no - I don't see anything you need to change to make it a "preamp". People seem to think a headphone amp is a dedicated unit, that will not work as a valid preamp - but this is nonsense. It's a high impedance input (good), and a low inpedence output (good). The big caps on the output guarantee wide bandwidth.
I presonally would remove the 1 meg resistor on the input tube's grid. The 100k volume pot is sufficent to stabilize the bias voltage.

FrankDIY 3rd December 2002 06:10 PM

that is fine in that case. thanks. any other opinions?

G 3rd December 2002 06:55 PM

If you plan on using it as a preamp only the ouput caps can be as small as 2.5 uf. Of course this depends on the input impedance of your amplifier. The formula for calculating this is:


C = 1,000,000,000/(2 Pi F R) where:

F = the lowest frequency you mant to reproduce. (usually 15 or 20 Hz)

R = the input impedance of your amplifier

I hope this helps.

G

argo 4th December 2002 08:34 AM

Re: Modification of an headphone amp to real preamp
 
Hey François,

How do you find that amp? Is it worth in your opinion to use this amp as high quality preamp?
Could you give some sound impressions or comparisons to other preamps perhaps?
I have planned to build a low output impedance preamp for a long time now. I have a couple of 6N1P-EV tubes and some powertrannies lying in my parts box so should I give it a try?

Argo

FrankDIY 4th December 2002 12:29 PM

thank you very much, I am a good loudspeaker builder but still learning tube amp :)

Argo, I did a setup on my system to switch between this tube preamp and the built-in preamp of my amplified with one button. The difference were notable: a real warm sound compared to the solid state preamp, less bass, detailed mids and smooth highs. A very good sound for the low cost.

If you want to build it, you have to be aware that the cathode-heater voltage of the top half of V2 exceeds the 6N1P's spec of 100V so I have no idea on the long term reliability of the amp.

By the way, I invite you guys to see the picture that I have posted of this amp a few weeks ago: http://www.diyaudio.com/forums/showt...&threadid=7005

argo 4th December 2002 12:55 PM

Quote:

Originally posted by François

If you want to build it, you have to be aware that the cathode-heater voltage of the top half of V2 exceeds the 6N1P's spec of 100V so I have no idea on the long term reliability of the amp.

Thanks for pointing that out.
Quote:

By the way, I invite you guys to see the picture that I have posted of this amp a few weeks ago: http://www.diyaudio.com/forums/showt...&threadid=7005
Very nice work indeed!

Argo


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