Aksa Lender P-MOS Hybrid Aleph (ALPHA) Amplifier

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#1,643
AKSA said:
The VAS is driven by voltage at the base and current at the emitter, with a very tight connection with the LTP. This makes it very stable and extremely fast.
Click to expand...

Thanks Hugh, for all the explanations!
I think I'll soon be running out of things to investigate and question about.
Instead I will start building it 🙂

So far we had:
4R version, changing gain (lower), adjustable damping factor, balancing CCS and tuning fb compensation cap.

All are possible, the Alpha amplifier is a very flexible design!

Regards,
Danny
 
#1,644
AKSA said:
ED,

I think that is a tube headphone amp?
H2 at -40dB H3 -60dB H4 -60dB is typically a tube profile. With SS I try to aim for -65dB to -72dB, just on the level of audibility, the Woo product would be very tubey and perhaps to many it would be a bit too much tubey!

Hugh
Click to expand...

Yes, you're right. It's a tube headphone amplifier.
I have seen a lot of reviews about SET with similar profile. All of them quoted with very good sound quality due low order harmonic and no use of feedback.
So, I want to know if possible "to clone" that behavior in SS. After my results in some simulatios, I think yes ! 😀
 
#1,646
There is also Hugh’s own headphone amplifier design, the Silicon Harmony. A 5w into 8ohm SE Class A amp with zero global feedback. It has a harmonic profile similar to SET tube amp but less THD so as not to go overboard with the “Tubey” sound.

il_fullxfull.1271976441_b796.jpg


It’s the best headphone amp I have heard - although not a public DIY design. Achieves about -66dB for H2 and -90dB for H3 and nothing else. Can drive any headphone load.
 
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#1,648
X
I received the ALPHA 20 boards today. My goal is to have two 1 unit rack mount amps to match some of my other gear. A quick look for low rise fans gave a number of options. I'm sure to have many questions during the build. These amps will be paired with a WAW consisting of a Fane 15 inch fullrange and LF drivers. Expecting they will make beautiful music together.
 
#1,649
Danny, Hugh:
Can you please summarize the changes needed for a 4ohm capable ALPHA 20? I think we need to increase bias but wasn't clear if it was 1.56amps or 1.9 amps and which output devices and what Aleph feedback resistor value?
Thanks.

This list is still for 8R ALPHA 20?:
R131 | R132 | R128 |
source | sensor | ccs-fb | bias
-------|---- | ----- |---------
0.33 | 0.12 | 680 | 1.95A version
0.44 | 0.12 | 510 | 1.46A version
Click to expand...
 
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#1,652
Good timing !
I was just busy with making a summary of all the possible options 🙂
Feel free to adjust or add, I left the BB specs for you X 😉


Summary:

Code: 
        | R131   | R132   | R128   |        |               
        | source | sensor | ccs-fb | bias   | speakers
--------|--------|--------|--------|--------|--------------
Alpha20 | 0.33   | 0.12   | 680    | 1.95A  | 8R  5-6R dips
Alpha20 | 0.44   | 0.12   | 510    | 1.46A  | 8R  6-7R dips
Alpha4R | 0.27   | 0.12   | 860    | 2.38A  | 4R  3R dips
Additional changes for the Alpha4R: shunt fb 2k2+56uf, compensation cap 22pf

The minimum ohm load(speakers) depends also how much power(voltage) the speakers need,
less voltage=more stable at lower ohm, high voltage=only stable at higher ohm
Alpha current rule nr1: the more current it delivers the more it is stable for low ohm speakers.
For the Alpha4R use TO264 Ixys devices like IXTK88N30P and IXTK90P20P

Gain settings: max. Vin for 17 Vout (max power):
Alpha20 with shunt fb 820r: 0.69 Vin
Alpha4R with shunt fb 1k8+68uf: 1.45 Vin
Alpha4R with shunt fb 2k2+56uf: 1.74 Vin (recommended, ratio H1-15/H1-4 = 99.74)
Alpha4R with shunt fb 2k7+47uf: 2.09 Vin
The FFT profile of the Alpha4R 2k2 looks a lot like the Alpha20 8R.

Damping factor:
depends where feedback is taken: before, after(Vout) or between the sensor resistor.
For between sensor resistor: put two resistors in series (100 ohm total), parallel to the sensor resistor.
FB taken before sensor, DF: 57(8R) 33(4R)
FB taken at amp output, DF: 300 (<100Hz)
FB taken between, DF: 57-300(8R) 33-300(4R)

FB compensation cap:
Alpha20: 18pf
Alpha4R: 22pf
Tune by listening, find the value with the best spatial imaging.
When compensation cap is too high the sound is subjectively compressed and dull,
when it is too low it is subjectively expanded and tizzy but when you find the sweet spot you will find the amp takes on a 'magical' sound relative to the other positions.


For the power hungry/low sensitivity speakers there's the BB.

I took the liberty to copy from some posts, I hope there are no copyrights 🙂

Regards,
Danny
 
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#1,655
It's part of the fb high-pass filter.
47uf is what Hugh recommended for the 2k7.
On the ac analysis I see it's flat to 10Hz,
lower, it starts to roll off.
1000uf seems too much, there's no roll off, FR is flat down to 1Hz.
If it is detrimental, that's a question for Hugh 🙂
 
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#1,658
danny_66 said:
It's part of the fb high-pass filter.
47uf is what Hugh recommended for the 2k7.
On the ac analysis I see it's flat to 10Hz,
lower, it starts to roll off.
1000uf seems too much, there's no roll off, FR is flat down to 1Hz.
If it is detrimental, that's a question for Hugh 🙂
Click to expand...

Thanks, I recall Hugh saying a 220uF was plenty. Also, the voltage is not that high as there is the divider, so for 10x divider it sees no more than 5v or so. This allows us to use super low ESR caps like 4x100uF 16v oscons for improved tight bass response.
 
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