How about this 12au7 pre?

sith · 13th April 2004, 07:27 PM

Hi guys,
I found this one over the internet , so, is it good , bad? Something like foreplay, or Bruce Rosenbilts pre, or Berman, or 'something completely different'

analog_sa · 13th April 2004, 08:02 PM

Depends on your sound preferences. To me it's a horrendous, badly designed high-feedback circuit. But some like the way it sounds.

richwalters · 13th April 2004, 08:27 PM

Hi there.........the way to the <bootstrap> concept is a good one, a little modification and one has a real bootstrap (far better than using two single R/C coupled stages) the dummy nicely raising the input impedance...and can take a good overload.
I currently use 2 xEF86's (1st in triode config and second pent). This circuit config has excellent overload capacity with high inp Z and high feedback of about 35dB. THD is very low throughout audio pass band as is noise <75dB below 1volt. Sim results with ECC83's except noise is higher. As gain/perfomance /BW is vastly improved there can be the risk of HF positive feedback if gain is too high or from crummy layout . Best results, use screened pentodes instead of naked triodes as HF response is much higher.

The 20KHz perfmance with a good layout should expect av <0.1% THD at the 10V rms output level, then into cathode follower. At lower levels the THD drops proportionally.

keep at it

rich

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13th April 2004, 08:02 PM	#2
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	Depends on your sound preferences. To me it's a horrendous, badly designed high-feedback circuit. But some like the way it sounds.

13th April 2004, 08:27 PM	#3
richwalters diyAudio Member Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru.	Hi there.........the way to the <bootstrap> concept is a good one, a little modification and one has a real bootstrap (far better than using two single R/C coupled stages) the dummy nicely raising the input impedance...and can take a good overload. I currently use 2 xEF86's (1st in triode config and second pent). This circuit config has excellent overload capacity with high inp Z and high feedback of about 35dB. THD is very low throughout audio pass band as is noise <75dB below 1volt. Sim results with ECC83's except noise is higher. As gain/perfomance /BW is vastly improved there can be the risk of HF positive feedback if gain is too high or from crummy layout . Best results, use screened pentodes instead of naked triodes as HF response is much higher. The 20KHz perfmance with a good layout should expect av <0.1% THD at the 10V rms output level, then into cathode follower. At lower levels the THD drops proportionally. keep at it rich