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Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier
Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier
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Old 29th January 2018, 08:05 AM   #11
AKSA is offline AKSA  Australia
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A few words about the ALPHA 20W from the design POV.

There are two significant issues with audio amps. Crossover artefacts, usually H5 and higher odd order harmonics; and harmonic profile, which influences how the ear perceives the sound, that is, natural, or unnatural.

Crossover distortion happens when there are transconductance changes as one device hands over to another, back and fro, across the 0V point on the waveform. It's particularly evident with Class AB amps, because the inactive device is actually switched off, while the active device is going hard at it. Turning on and off semiconductors is difficult because the ear perceives the subliminal high order harmonics which are sprayed into the signal output as 'distortion'. Class A amps fix this problem because neither of the output devices turn on or off; they may current cycle up and down, but they do NOT turn off, and so the death throes 'artefacts' are not created so the sound is more 'natural', whatever that is........ The ALPHA output devices do not switch, although they do current/voltage rise and fall with the load.

Lastly, the harmonic profile has been thrown into prominence in the last ten years, but in fact has been pivotal for good sound and Jean Hiraga demonstrated this in Le Revue du Son in Paris in the mid-sixties. He said that if you can arrange the first five or six harmonics off the fundamental along a straight, descending line, with at least 10dB reduction with successive higher harmonic, the sound will be perceived as 'natural'. H2 should start no higher than 68dB down from the fundamental. This was a controversial statement at the time, and even now, because it pays little regard for THD, which is a rms summation of the harmonic percentages of the fundamental, with no information about each actual harmonic. I have been designing amps using this philosophy for twenty years, and if you give regard to the monotonic decrease of the harmonics and a moderate THD (no more than 0.03%) you can create a good sounding amplifier. There are other issues, of course, but it certainly explains why many amps with very low THD still do not sound good. The fact that this is a subjective assessment, and humans enjoy squabbling about the subjectives has only clouded high end. During the eighties the Japanese opened up the 'THD Wars', to very little benefit. Still a lot of people like single ended triode amplifiers, and they deliver outrageous THD figures.

I do not go for Class A amplifier as a rule. They are brute force machines, with less efficiency than a gasoline engine, usually less than 20%. Even on this 20W amp with 24V rails, the output dissipates around 96W, and for only 19W this is 19.8% efficient with a lot of heat. The real skill is in designing good sounding AB amps, and all my commercial amps are AB with very high efficiency and perceptible natural sound with deep stereo sound field. That said, the ALPHA uses Nelson Pass sliding CCS in the output stage and used here with little change; it's a brilliant, creative solution to increase the efficiency of a simple Class A in SE, where the output currents cycle up and down but nothing turns on or off.

In sound field for this amp, I'd suggest building it, but we assured that all my commercial amplifiers are just a little better and much more powerful. But you'd have to pay for them - the ALPHA is cheap as chips, excepting of course the heatsinking......

My sincere thanks to XRK, a fantastic guy to work with. He should take great care his fingers never melt, he works so fast.......

HD
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Last edited by AKSA; 29th January 2018 at 08:18 AM.
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Old 29th January 2018, 08:40 AM   #12
schillg11 is offline schillg11  Germany
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I only can find the FQA36P15 on the ON homepage. Is the P20 a follower of this one?
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Old 29th January 2018, 08:59 AM   #13
AKSA is offline AKSA  Australia
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The 15 indicates it can withstand 150V, the 36 is the current maximum.
So, being only 48V rail to rail, the FQA36P15 is fine, and I must apologise it might have been a typo!

HD
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Old 29th January 2018, 10:27 AM   #14
dadod is offline dadod  Croatia
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Quote:
Originally Posted by AKSA View Post
My sincere thanks to XRK, a fantastic guy to work with. He should take great care his fingers never melt, he works so fast.......

HD
Yes XRK is fantastic guy, fast but afraid of more complex circuit.
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Old 29th January 2018, 12:58 PM   #15
xrk971 is online now xrk971  United States
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Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier
Hi Dadod,
Ah, the 29 transistor amp! You can see I have no problem making 5 or 6 transistor jobs . The hurdle on your GW3 amp is requirement to match a bunch of BC550/560’s. I don’t want to build it without the matching. So the extra steps get in the way sometimes. I want to do another headphone comparison thread though and would like to include your amp so hope to build it in next month.
Cheers,
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Old 29th January 2018, 01:21 PM   #16
marconi118 is offline marconi118  Germany
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can I use BC560C instead of KSA992

and BC550C for the NPN one?
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Old 29th January 2018, 01:35 PM   #17
xrk971 is online now xrk971  United States
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Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier
Hugh will confirm but I think BC550/560 will work fine. On Aksa Lender Preamp I used SMT version of 2N5401 and it was good. The KSC1845 is special though - you need that one. But when PCBs come out, beware of pinout of BC560 vs KSA992.
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Old 29th January 2018, 04:23 PM   #18
ZoltanChivay is offline ZoltanChivay
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Hi xrk971

Thanx for another nice project!

Can you posted the .asc files?
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Old 29th January 2018, 05:08 PM   #19
xrk971 is online now xrk971  United States
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Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier
Just wanted to show you the prototype without the heatsink blocking the view of the MOSFETs. As you can see, it is a very simple amp but with extraordinary performance.

Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier-alpha-20w-prototype-bare-jpg
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Old 29th January 2018, 06:06 PM   #20
xrk971 is online now xrk971  United States
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Aksa Lender P-mos Hybrid Aleph (ALPHA) Amplifier
Just an update that the Master layout artist JPS64 is on the case. He will make two layouts: the 20W board and the 55W board. I think the 20W made to allow compatibility with the DIYA Store’s UMS specification might be a good idea? Whilst the 55W is strictly CPU cooler territory. I don’t think anyone wants a passive heatsink that can handle 300w per channel.

So outputs will be three per side closely spaced to all fit on copper block of the Dell surplus workstation heatsinks good for 130w to maybe 150w per side. The Dell FD841 heatsink designed for Xeon 5500 cpu’s Is supposed to be good for 130w with heatsink at 41C. We can tolerate 55C heatsink temps so perhaps can push 150w or slower fans speeds.

Just search eBay for Dell FD841 T7400 heatsink. About $17ea.

I am not choosing new COTS CPU coolers because modern CPUs are much smaller and there isn’t enough area to fit 3 TO-247 packages side by side.

Look how big the copper pad on the FD841 is:
Click the image to open in full size.

If someone can find a modern heatsink this big with heatpipes and a fan on Amazon, please let us know.
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Last edited by xrk971; 29th January 2018 at 06:20 PM.
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