Lowering gain in 12AX7 SRPP preamp

[Asen]

You can find more info about my DIY project here:

http://www.diyaudio.com/forums/showthread.php?threadid=250

 

[JBasham]

Have you tried using a 5751 instead of a 12AX7?

It's usually a drop-in substitute and it has less gain.

Also many of my friends and I prefer the sound of the 5751 to the sound of the 12AX7. It ususally sounds less "fuzzy" or "muddy."

 

[Asen]

OK, I took a picture for you...

 

[fdegrove]

Hi,

Looks nice, Asen.

Instead of using the resistive voltage divider at the input you could try some degenerative feedback by removing the decoupling cap across the cathode resistor if Zo is not crucial.

Also, why do you have a 1M and 47K R in // at the output?

Cheers,

 

[ALBQ]

12AX7 Pictures

Asen,
Very Nice - also, good job on the internal wiring. Thank you for for taking time to post the pictures. I will need to get a few pictures off on my latest preamp project as it progresses. Thank you and nice work.
ALBQ

FYI - link to my webpage - need updating with latest projects
http://mywebpages.comcast.net/gillespie147/AlbuquerqueAudio-MainPage.html

 

[Asen]

Thank you all for the comments!

fdegrove,

I tried it without cathode caps but I didn't like the results. Also I don't think the input voltage divider is quite a problem. It is the same thing to have a 250k pot and to turn the volume just a little. The resistor gives me more possibilities to play around with the volume control.

Regards

 

[Asen]

The 47k at the output is just a load resistor I put for simulation purposes. It doesn't exist in reality. There's 1M resistor only.

ALBQ, nice webpage. Your amps look impresive.

 

[petermc]

Found it!

Happy to say that I have found the issue! Just need to work out exactly how I will solve it.

I spent so much time with a multimeter looking over the components and connections, looking for a dry joint. I started to think my amp was fine in that regard. This amp is so simple.

I haven't really gone with a stock layout, so based on what I read elsewhere I thought about the wiring and where things were located. I unscrewed the pot and pulled it out with the wires on it. The amp came to life and sounds great.

The centre wire on the pot is likely the issue. It is picking up interference from the heater wire, or perhaps it is picking up interference from the wire going to the primary of the power transformer.

Just need to experiment and work out where that wire can go.