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Old 30th December 2012, 06:43 AM   #701
billyk is offline billyk  United States
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Quote:
Originally Posted by bcmbob View Post
General question

I'm curious about how many are using a preamp/gain stage with the ACA as opposed to a pot or other attenuation device for volume control.
ODAC -> B1 buffer in front of mine.
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Old 30th December 2012, 08:22 PM   #702
waam68 is offline waam68  United Kingdom
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Quote:
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General question

I'm curious about how many are using a preamp/gain stage with the ACA as opposed to a pot or other attenuation device for volume control.
I only have a Yaquin MC12B phono/pre so I build all my amps without a pot and use the one on the Yaquin. My ACA drives my big old 15ohm wharfedales pretty loud but then again my listening room is only about 15 by 15ft. The ACA goes noticibly louder than my 3.5w tube SET with better grip on the bass. Really lovin this little amp in my system with vinyl.
Mike
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Old 31st December 2012, 04:55 PM   #703
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Default question on bias - 3x

I've run this amp with double bias for some time now, and my sinks are having no trouble keeping up. I'd now like to triple the bias. For double bias I simply halved the values for R1-R4. For 3x bias, is it simply a matter of taking those values and dividing by 3? And, do I need to adjust the feedback loop (R11-R12) for any reason when upping the bias? I'll be running this to Tony Gee's Solo 103 for near field listening, also building a Pensil 10.2, not sure if this thing will drive the Alpair 10.2 though, will be experimenting.

Any input would be appreciated!!
Thanks......HAPPY NEW YEAR!!!!!!!!!!!!

-John
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Old 31st December 2012, 05:13 PM   #704
bcmbob is offline bcmbob  United States
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Ma - can you report on how that bias change effected the sound/output of the ACA?
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Old 31st December 2012, 06:45 PM   #705
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I would highly recommend increasing bias; as to the sound, all I can say is that it was just better. My little inefficient bookshelf speakers certainly presented a bigger soundstage. Nelson typically advocates maximum bias for best sound, and based on my experience building many of his designs over the years, I tend to agree.
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Old 31st December 2012, 08:02 PM   #706
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MA, I would suggest you keep an eye on the power dissipation of the resistors. It might be better to just parallel several to get the value down.

The open loop gain of the ACA would increase at higher bias currents. Nelson likes to keep feedback at 10dB to control higher order harmonic generation so open loop gain needs to be measured. I'll do some experiments in the next week.
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Old 31st December 2012, 08:26 PM   #707
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Quote:
Originally Posted by Loudthud View Post
parallel several
Threw a few 10W on there. At 2x bias, they're lukewarm.

Concerning the feedback, I'm fairly clueless on this adjustment, so your input would definitely be helpful. I'd like to go no feedback....is it a matter of just putting a jumper at R11, and completely removing R12?
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Old 31st December 2012, 09:34 PM   #708
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Quote:
Originally Posted by MEGA-amp View Post
Threw a few 10W on there. At 2x bias, they're lukewarm.
That should get the job done.

Quote:
Originally Posted by MEGA-amp View Post
Concerning the feedback, I'm fairly clueless on this adjustment, so your input would definitely be helpful. I'd like to go no feedback....is it a matter of just putting a jumper at R11, and completely removing R12?
That will work. I left R11 in place just for a little protection against ESD. THD can be lowered by tweeking the R1-R2/R3-R4 ratio and adding a Source resistor to Q1. (Search for my earlier posts in this thread.) But, in such a simple design, there are unintended consequences. I don't know if it will sound any better. As is it really warms up (pun) the sound of my cheap CD player.
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Old 3rd January 2013, 10:22 AM   #709
Nisbeth is offline Nisbeth  Denmark
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Any ETA on the next round of boards?

/U.
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Old 3rd January 2013, 11:54 AM   #710
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Any ETA on the next round of boards?

/U.
See post #695 from Variac.
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