Need advise with NFB loop

[brucetassin]

Hello All and Happy New Year.
I'm posting my latest creation for some advice.
I'm playing this amp now and it sounds very good but I would like more bass.
I dont have a pre-amp with controls just feeding it with "line out" from my CD player. I have read some treads that suggest that the 1000 ohm resistor in the NFB may cause oscillation and should be changed. So far I have not seen or heard any oscillation. Which way should I go with the resistor/capacitor values to get a little more bass?
I could build a passive tone control to put at the input. Any Ideas about this?

 

[Cassiel]

Hi,

I would do the following:

Install a 470 Ohm resistor to bias the 12ax7 (in parallel with the 50uF cap)
Change the 1000 Ohms resistor for a 3K resistor or a 5K trimmer to be able to adjust the feedback. Once you find the sweet spot remove the trimmer for a fixed resistor. Bypassed it with a 270 pf cap. Small mod that may or may not solve your bass problems but worth a try. Think it would improve it.

 

[DF96]

Increasing the 1K resistor will reduce the feedback, and so increase the gain and output impedance. This may cause a bass peak at the loudspeaker bass resonance - sometimes known as "one note bass".

Changing the 1K resisor will also change the bias for the 12AX7, as in this design the resistor has two functions: feedback and cathode bias. Hence the need for a separate bias resistor if the 1K is increased.

 

[Cassiel]

DF96- Yes, decreasing feedback affects the bass response but no "one note bass" here. IMO, it sounds better. Depends on a lot of things though: OT's, speakers, etc.

Cheap mod -worth a try.

 

[Cassiel]

Forgot to say that I'd also change the power supply to silicon diodes and at least 100uF caps but you'll have to adjust the B+ and I don't see how (don't like the idea of increasing the resistance). Maybe a regulated supply? As you can imagine this mod isn't as cheap and easy as the first one.