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Old 8th January 2011, 04:50 PM   #1
diyAudio Member
Join Date: Dec 2005
Location: Loganville, GA
Default Need advise with NFB loop

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?
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Old 8th January 2011, 05:48 PM   #2
Cassiel is offline Cassiel  Libya
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Join Date: Sep 2004
Location: Madrid

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.
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Old 8th January 2011, 06:03 PM   #3
DF96 is offline DF96  England
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Join Date: May 2007
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.
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Old 8th January 2011, 06:15 PM   #4
Cassiel is offline Cassiel  Libya
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Join Date: Sep 2004
Location: Madrid
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.
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Old 8th January 2011, 07:02 PM   #5
Cassiel is offline Cassiel  Libya
Account disabled at member's request
Join Date: Sep 2004
Location: Madrid
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.
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