Amp Camp Amp - ACA

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Variac

Variac

diyAudio Editor
Joined 2001
BurningAmp.org
Paid Member
Also remember that the amp was designed for a laptop power supply, so there were some concerns about just how "Clean" the power would be.
Ah I get it , that was the point! You were considering another supply and wanted to know if C2 was just a filter.

Thanks for the response ZM, good to know..
 
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Macfixer

Macfixer

Member
Joined 2011
mymacfixer.com
ID just like to say thanks for the AHA!! Ive had them in my system now for the last year and i had blown the large caps on one amp while building and had ordered some cheap replacements off ebay not the same make or model Id changed out on two channels and had been living with them and another one bi amping B&W 603 II very happy last week I got some Panasonic caps and replaced the cheap ones....What a difference night and day...just sounds cleaner more open etc etc, just better to our ears. This morning I took the plunge and did the 2.21K resistor solder on all four. Im astounded a major move forward more dynamics clarity audibly less distortion lovely tone etc etc. I suspect some of its down to the extra watts driving the reasonable inefficient speakers better.

Anyhow just wanted to say thanks to everyone for the amp that keeps on giving.

What next to tweek? Any suggestions below is the current set up

macbook pro os x10.9.4
flac files
audirvana
toslink cheapo!
firestone spitfire II DAC with upgraded op amp
twisted pair cat5 interconnect
Dact 10k volume knob ebay! 23 stepped in process
twisted pair cat5 interconnect
ACA bi amp
cat 5 cable white - coloured + speaker cables
B & W 603 series II
 
S

Samuel Jayaraj

Member
Joined 2001
My single channel version of the ACA is as follows:-

Supply Voltage - 29 Volts DC;
Voltage for 2SK170 - 23 Volts (Dropped by 2K2//2K7 resistors);
Q1 Drain Voltage - 11.8 Volts;
R7 - 15K
CRC Filter - 15KuF - 0.27E 8watts - 15KuF;
R1, R2 - 0.18E 5Watt
R3, R4 - 0.36E 5Watt
C1 - 4700uF 50V (Elna)
Input Cap - 10uF (Plastic Film)
Drain of Q4, Q2 - 10uF Decoupling (Plastic Film)
Measured Iq - 1.60 Amps

Absolutely dead silent on 94db Fostex 168Sigma speakers, paralleled for 4E impedance. The sound is spot on in terms of Timbre, PRAT, Musicality. It is LUSH. I have compared the ACA to Juma's BAF2013 Lateral Mosfet, Zero FB Amp. They are near identical in their sonic signature, except that the ACA is a little more fleshed out and organic in the mid frequencies, whereas Juma's Amp is a little more defined and slightly more authoritative in the bass region.

Do you all think I should lower the supply voltage for Mosfets' life? Can I expect about 15 watts into 4 ohms in the given setup?

Thanks.
 
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Loudthud

Loudthud

Member
Joined 2007
Samuel Jayaraj said:
Do you all think I should lower the supply voltage for Mosfets' life? Can I expect about 15 watts into 4 ohms in the given setup?
Click to expand...

I tried to duplicate your amp as close as I could. For R1, R2 I used 0.15 Ohm and for R3, R4 I used 0.3 Ohm. Idle current was 1.80 Amps. This setup reached 15W into 4 Ohms at 6% THD at 1KHz. Open loop gain measured 26.7dB which is pretty close to where it needs to be. Tweeking the quiescent voltage for minimum THD I got 12.8V.

Mosfet life will depend on the effeciency of your heatsinks. Mine ran about 31C with a fan, way too hot without it.
 
S

Samuel Jayaraj

Member
Joined 2001
Loudthud said:
I tried to duplicate your amp as close as I could. For R1, R2 I used 0.15 Ohm and for R3, R4 I used 0.3 Ohm. Idle current was 1.80 Amps. This setup reached 15W into 4 Ohms at 6% THD at 1KHz. Open loop gain measured 26.7dB which is pretty close to where it needs to be. Tweeking the quiescent voltage for minimum THD I got 12.8V.

Mosfet life will depend on the effeciency of your heatsinks. Mine ran about 31C with a fan, way too hot without it.
Click to expand...

Loudthud, thank you so much for your interest and for earlier pursuing me to try experimenting with the ACA with your own variations.

I found that the ACA as I had set up with just a pair of multimeters and ears because I have no other test equipment, went pretty loud. But then the tonal quality had a certain brightness, not too objectionable, but can't live with for long.

Ofcourse, it might sound better with a different pair of speakers than the Fostex I had tried on, as Fostex have a reputation of being a bit bright and shouty.

But then I immediately compared the ACA with my other recent build of Juma's BAF2013 Lateral Mosfet 25Watt Class A amp. The latter sounds just as detailed or a little more detailed than the ACA, has no sound of its own (almost) and is the most 'acoustic' type of sound I have ever heard from an amp; everything else has an 'electronic' sound to it, in comparison.

At the end of a rather exciting and learning journey, I am going to revert my ACA to the Master's original configuration and run it off a laptop supply. I would say that Nelson, for the genius he is, configured the ACA with a sound which you can live with for a long time - as a great hi-end 'primary amp' for beginners and as an equally great 'secondary amp' for any veteran.

Thanks, once again Loudthud and Mr.Pass.
 
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