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Old 29th June 2020, 07:37 PM   #331
TungstenAudio is offline TungstenAudio  United States
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Probably the main reason that 10uF electrolytics were used in the ACA is that the particular one used (Elna Silmic II) were small and of known good sound quality. This made it easy to put together a simple kit that would be relatively inexpensive, but still produce an amp that was easy to assemble and sounded great for new DIY enthusiasts.
The smaller cap positions can certainly be tweaked to one's inclination and content. I might suggest a 2.2uF, 50V Nichicon ES series bipolar cap in the C3 position, though I have continued to use the 10uF Elna Silmic II that was part of the original kit. Film caps tend to be physically large, and are not always a good substitution for that reason. If you can find a 1.0uF, 63V polypropylene or PET cap that fits reasonably well, then that may be worth trying. The C4 position is a good place to try something different, especially if you have managed to add R16. I like larger value (100uF) organic polymer caps that have a small diameter case and close lead spacing for C4.
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Old 29th June 2020, 10:20 PM   #332
JOIMONF is online now JOIMONF
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Hi Den, I placed SJEP120R100 only in Q1. May have marginal benefit also on Q2, but if any, not deserves the extra expense there.
Concerning the capacitor you add to R14 (0,33uF)
I was hesitant from the beguinning to try. Adding components to the smart N.Pass circuitery.
You know me, I was happy actually further reducing components to their desing. But your suggestion bypass R14 with an small cap seem easy to try (can be done from the speakers connections). I have done today is St. Peter day, not working in my town.
I tried different small capacitor values, all russian teflon, but in the end the much preferred in my case was the smaller at only 10nF.
That not meaning has an small benefit, no no, it is very important find, giving the sound more 'loudness' but at the time more airy and treble extension.
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Old 30th June 2020, 04:54 AM   #333
Variac is offline Variac  Costa Rica
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ACA amp with premium parts
The stock kits are back in the store!

Amp Camp Amp – diyAudio Store

Cool new labeled rear panel and wiring to make it easy to select 2ch stereo or
Monoblock balanced, bridged or balanced.
"The geek shall inherit the earth"
2019 Burning Amp is Nov. 9 & 10 in S.F. Thread here on diyAudio:
Burning Amp 2019!!!
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Old 30th June 2020, 02:35 PM   #334
TungstenAudio is offline TungstenAudio  United States
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These kits are great to work with. Buy two
One of the best ways to 'upgrade' the ACA is still to bridge the two channels inside one chassis, then use a pair as monoblocks. The type of bridging that works best will depend on your speakers, and it is easy to try the different types.
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Old 2nd July 2020, 10:44 AM   #335
wrath is offline wrath  Poland
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What do you think about using IXFB82N60P

(V-MOS, N 600V/ 82A/ 1250W/ 0,75R) to ACA?
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Old 2nd July 2020, 01:28 PM   #336
dfoulk is online now dfoulk
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Default IXFB82N60P

Hi there Mr. Wrath,

I don't know that much about it, but those are kinda pricey, around $20 a piece.
I had asked Tungsten what he thought about the IRFP440, a beefier selection than the IRFP044 that I am currently using for Q1, with 29 volt rail voltage, and he suggested the FQP44N10, as he was using them and was pleased. I like the IRFP044, but I think I will try the IRFP440 just to see. They are around $3 a piece, and are 500 volt rating. If your pockets are deep, there is the Semisouth SJEP120R100,
at $100 a piece, supposed to be very nice, used by Mr. Pass in some of his highly regarded amps. I have also heard from Jordi that they really sound good in his ACA for use as Q1. Of course they are not made anymore, so the high prices, and beware of fakes. When I try the IRFP440 for Q1 and the current used IRFP140 for Q2, I will post what I hear.
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Old 2nd July 2020, 03:42 PM   #337
TungstenAudio is offline TungstenAudio  United States
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The IXFB82N60P is one of a new series of parts that have been designed for switching applications, rather than linear. It has a different set of design tradeoffs from the IRFP440, which is also a newer part. Where the IRFP440 has a moderate input capacitance, low gate charge and low transconductance, the IXFB part has very high input capacitance (23 nF) and high gate charge in order to obtain its high transconductance. Both parts would be rated at about 140V for continuous Vds, rather than 500V or 600V for pulsed operation.

The IRFP440 might be Ok for linear operation, but its low transconductance doesn't offer any advantage, at least on paper, over the IRFP240 or IRFP140. The IXFB82N60P, with its extremely high input capacitance, is probably not suitable for the ACA. Certainly not for the Q1 position, though it might work for Q2, but with different sonic character. Remember that Q2 in the ACA functions as part of an interactive current source, so its dynamic behavior is important.

These are examples of some of the different tradeoffs that are inherent in the fabrication of Mosfet transistors. High Vds capability and high current both tend to result in high gate capacitance and high gate charge. High transconductance and low Rds-On are similar. Optimizing for those parameters extracts a toll in other areas of performance that we find desirable for audio.

There are other amplifier designs that make good use of the newer physically large Mosfets. Probably the best example is the Pass Labs XA25, which has enjoyed great critical and commercial success. Though its exact design remains a topic of much speculation, we can safely say it uses IXFN140N30P and IXTN40P50P transistors. Another design is the BA2015 50Watt Shade amp which uses a pair of IXFN140N20P Mosfets. These are examples of big "hockey puck" transistors that require very different designs to drive them properly. The front end of the XA25 is a power amplifier in its own right. The BA2015 hockey puck amp uses a transformer to drive the lower IXFN140N20P, and in turn requires an input stage (or preamp) of very low output impedance. The simple design of the ACA cannot support such large transistors.
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Old 2nd July 2020, 04:34 PM   #338
dfoulk is online now dfoulk
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Default mosfets

I must say that I am just curious about the sonic character of the IRFP440, and it
is an inexpensive part. I know from the past that every transistor does have it's own sonic character, and mixing them can sometimes give you something different that you would not be able to obtain otherwise. Kind of like the taste of the right amount of lemon in your sweet iced tea Also, I am experimenting with the specific sound of my own home brew open baffle speakers, so what I hear may not be applicable to everyone. There are parts that just won't work in the ACA circuit, but I sure enjoy the exchange of ideas and information from knowledgeable experimenters!
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Old 2nd July 2020, 05:01 PM   #339
twocents is online now twocents  South Africa
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ACA amp with premium parts
Default R12 - feedback?

If I understand correctly R12 is the feedback resistor here. I find another member, xrk971ís use of carbon film resistors for the feedback very interesting and wonder if someone here has considered or tried it for the ACA yet? See the comment from another thread:
Originally Posted by AKSA View Post
I was always fascinated with X's measurements using carbon resistors v. metal film resistors in the feedback networks of the Alpha. He demonstrated conclusively that metal films bring up the third harmonic over the second; the carbons do the opposite.
Best sounding amps have a monotonic decrease of successive harmonics, and the more linear, the better. This is from Jean Hiraga in the 60s - nothing is new and demonstrates how the marketing speak drives the equation rather than the truth.


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Old 2nd July 2020, 06:35 PM   #340
dfoulk is online now dfoulk
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Default carbon resistor

Maybe this phenomenon is why Tungsten has found that a small capacitor added in parallel to R12 improved the sound of his ACA when driven hard. Perhaps the capacitor has reduced the odd harmonics in a similar way to the carbon resistor?

Just a guess, I do not have the equipment to test the idea
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