DIY ACA mini

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#1,461
audiosteve said:
Kevin's questions is very important.
'Sounds' but no music is exactly what the amp will sound like when music is being played (loudly) but the bias supply is OFF. Double check your bias and offset voltages and that you have close to 12V DC at the gates of Q1/Q2.
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Thanks for the info. You are correct. It is my mistake. I have 11.4v on both but I misread the VB. 30.3mv instead of correct 300.0mv.
 
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#1,466

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#1,468
KevinHeem said:
Good call Steve…from experience maybe?

I have experience swapping inputs and outputs (at the board of a FW amp)…
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Yeah, I use mine in the office and it's on a shelf behind my monitor so I can't tell if it's on or not when sitting.
Therefore I cannot count how many times I have turned the music up w/o first turning the amp on,
The sound bojo75 described is VERY familiar to me and its cause well known.
BTW, when I use the system properly I play audio with SubSonic going via USB to a Grace Design SDAC and then to the ACA-Mini feeding MarkAudio CHR-70 sealed speakers. It's a really great office rig. Really great. I dig this amp more and more every day. It always sounds top notch and never fatigues.
 
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#1,470
I got a number of requests for the FQP transistors, but I only had three sets to offer. If you contacted me through PM and I didn't respond its because they are gone and I had too many PMs to answer. However, there is another member here who has a stash (in fact that's where I acquired mine). I think he'll be offering sets either here or in the Swap Meet.
 
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#1,476
Answering my own question, or sort of LOL...

Note in the same series the 1F supercap is available at Mouser's and not restricted, perhaps a solution?

It is with 1F way higher than needed for the cut off frequency, and same sizes apart from non relevant height, and has even a lower ESR (not that it should really matter here given the figures but well...)

Claude
 
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#1,478
KevinHeem said:
I had two hours of free time tonight giving me chance to sit down and listen to both ACA Mini amps I have. Both amps use the “Almost Clone” boards… artwork done by Mark Johnson using the original circuit designed by Nelson Pass. The first amp, the red “prototype” amp has matched Fairchild FQP3N30/FQP3P20 installed in it, while the green “REV A” amp has the matched Harris 520/9520. Both amps sound amazing, but I’m hearing a clear difference between the two. I made some notes while listening so I can share my findings.

I’m going to kill the punch line here… I prefer the amp with the Fairchilds by a wide margin, with or without the jumper in place. The reason I prefer this amp is that I find the midrange more clear and detailed, making instruments come into focus. This makes it easy to the hear the delicate sound of a finger pressing against the string of a guitar. Instruments sound more full and dimensional as well. With this amp though, the highs are a bit rolled off (which I don’t mind at all). The bass may be a bit rolled off as well not going quite as deep.

Here's the funny thing…with the “Fairchild” amp I prefer the jumper in, but with the “Harris” amp I prefer the jumper out. It gets even better…with the Fairchild amp I prefer the absolute phase non-inverted, while the Harris amp I prefer the sound with the phase inverted meaning + and – switched at the speaker terminals. Go figure.

I’ve sent sets of the Fairchild FQP’s to three people and I believe @rhthatcher is going to try them as well. I'm more than curios to know their findings.
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What bias level do you use for the FQP-ACAmini, Kevin?
 
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