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
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
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
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.
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.
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.
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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That should get the job done.
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.
There's lots of margin in this design. As long as your original voltage is
not really much larger, I suggest you use it, and set the Drain voltage
at approximately 1/2 of the supply.
Many thanks for the info. Will be very useful.
Its a really nice little Amp. Many thanks for sharing your ideas.
Some Pics with the big brothers the F5, J and the B1 as Pre
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An ACA with a 0.22 ohm resistor in place of R1, R2 and a 0.33 ohm resistor in place of R3, R4 was tested for open loop gain at 1kHz, then the closed loop gain was adjusted via a trimpot to be 10dB less. The R12 value listed is just a calculated value. The distortion was measured along with the 2nd harmonic. For the 2X and 3X columns additional resistors were added in parallel with the exsistong 0.22 and 0.33. The 2X via jumper column is the standard ACA with a 1K resistor connected from Base to Emitter on Q3 effectively doubling the current. The open loop gain is higher but distortion numbers are about the same. All measurements were taken with an 8 ohm load and the voltage at the plus side of C1 adjusted to 10.0V.
The biggest surprise was that distortion at 1 Watt decreased at higher currents, but only below about 2kHz. At 10kHz distortion actually increased slightly.
The biggest surprise was that distortion at 1 Watt decreased at higher currents, but only below about 2kHz. At 10kHz distortion actually increased slightly.
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Loudthud and all, so it appears that there is no real benefit in increasing current as compared to Nelson's standard configuration. My considered guess is that in the long run the standard current setting and its distortion profile would be sonically more pleasing.
However, it is possible that Bass might be a little tighter with the higher current setting.
However, it is possible that Bass might be a little tighter with the higher current setting.
WOW! Thank you for the detailed testing results!
Looking at your results, and using them as a rough guide for my own experimentation, it seems I am in agreement with Samuel Jayaraj's sentiments. I enjoyed the sound with no feedback, with feedback, 2x/3x bias, but it seems I've reached the limit....with my inefficient speakers anyway. I picked up a pair of Alpair 10.2 and FE126En, with the hopes of pairing them up with this amp, choosing a design is another matter....Frugal Horn MK-III...?
-john
Mega,
Don't know if you have been watching, but these are a lot of fun and sound surprisingly good. Might be a match for the FE126En.
Foam core Spiral Horn.
Don't know if you have been watching, but these are a lot of fun and sound surprisingly good. Might be a match for the FE126En.
Foam core Spiral Horn.
My new table heater, first side, first own class A experience in mono:
http://www.abload.de/img/acarunningxlqu3.jpg
Sounds very clean and round. Can't wait to listen to it in stereo.
Thank you Mr. Pass
And thank you once again for the layout Rudi!
http://www.abload.de/img/acarunningxlqu3.jpg
Sounds very clean and round. Can't wait to listen to it in stereo.
Thank you Mr. Pass
And thank you once again for the layout Rudi!