12AU7/12BH7 Headphone Amp - Page 5

sorenj07 · 15th June 2007, 11:57 PM

I'll take a look. Is there any chance that the negative feedback could be fed through TWO small capacitors in series? that way the caps could block DC, and the AC signal would be twice-inverted and not make positive feedback..?

Brian Beck · 16th June 2007, 12:09 AM

No, you'd only need one cap in series with the feedback resistor to block DC. Caps don't invert signals, but at very low frequencies series coupling caps do introduce phase shift. This is workable though.

sorenj07 · 16th June 2007, 12:14 AM

If you say so. I don't remember tenth grade physics that well... Anyway what size capacitor are we talking here? I've got a few cheap .22uF caps I could use, but I don't want to solder the bulky pots in there again. is 47K in series with .22uF a good start for feedback? I'm still using 7AU7's.

p.s. the amp sounds pretty good, very solid imaging and very little hum. i'm happy

16th June 2007, 12:09 AM	#42
Brian Beck diyAudio Member Join Date: Dec 2004 Location: South Florida, USA	No, you'd only need one cap in series with the feedback resistor to block DC. Caps don't invert signals, but at very low frequencies series coupling caps do introduce phase shift. This is workable though. __________________ Brian

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15th June 2007, 11:57 PM	#41
sorenj07 diyAudio Member Join Date: May 2006	I'll take a look. Is there any chance that the negative feedback could be fed through TWO small capacitors in series? that way the caps could block DC, and the AC signal would be twice-inverted and not make positive feedback..?

16th June 2007, 12:14 AM	#43
sorenj07 diyAudio Member Join Date: May 2006	If you say so. I don't remember tenth grade physics that well... Anyway what size capacitor are we talking here? I've got a few cheap .22uF caps I could use, but I don't want to solder the bulky pots in there again. is 47K in series with .22uF a good start for feedback? I'm still using 7AU7's. p.s. the amp sounds pretty good, very solid imaging and very little hum. i'm happy