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Old 18th April 2002, 08:39 PM   #1
nar is offline nar
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Cool Aleph choice

I have built an Aleph 4.It sounds very nicely!!!
Your question is a bit strange.Power is not a rule for sound quality.
I have not heard an Aleph with 3 stages.Might be interesting if your precious load has very very low impedance,such as electrostatics,etc.
If your speaker is kind an Aleph 4 will bring you satisfaction,for me the 100 W over my horns and bass 97 dB/Watt is large enough!!!!

Have a good search...:
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Old 18th April 2002, 10:10 PM   #2
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If you want to hear an Aleph with 3 stages,
try the 0 or the 0s.

Both had considerably better bass control and
lower distortion than the later 2 stage designs,
and the 0 gave the same distortion curve into
2 ohms as 8 ohms.

Most people still preferred the 2 stage overall. I think
that part of it was that the 3 stage design sounded
more like the other amps on the market, and the 2
stage did not.
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Old 19th April 2002, 11:42 AM   #3
Bakmeel is offline Bakmeel  Netherlands
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Mr Pass,

If the 2 stage design sounds so much more differently than the 3 stage design, then how would you describe the difference between the two designs?

In a Dutch Forum, www.hifi.nl, I am involved in a discussion with someone who claims that the Aleph series can't deliver their power specified. Translated, he claims this:
Quote:
(...)

The Aleph 1.2 operates in Single ended class-A. If I correctly interpreted the schematic, it runs at about 330 mA idle current per output transistor. This times 12 gives a total idle curret of 4A. Totally there is exists 120V across the amp. That means it dissipates 480 Watts. Such a design can never deliver more than 4A peak at its load. At 4 Ohms, that would be 32W max, and at 8 Ohms 64W. But, at these powers it will distort a lot.

I don't believe that this Aleph will remain stable unconditionally. It is an "Overall feedback" design. And there seems to be quite a lot "local" feedback. There is no "Boucherot" or "Zobel" network. That can give problems when driving capacitive loads. If that generates a problem you can always place such a network behind the amp.

(...)

Yours, Peter
It disturbs me, because I can't refute his calculations. But I also cannot believe that the Aleph manuals are a bunch of nonsense. Can you comment on this?

Yours,
Bouke van der Weerdt
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Old 19th April 2002, 12:15 PM   #4
Geoff is offline Geoff  United Kingdom
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Bouke

It seems that your correspondent, Peter, has overlooked a basic fundamental of the Aleph series, the active current source. His calculations are correct for a single-ended output stage with a constant current source, but the Aleph has an active current source which enables the peak output current to be twice the quiescent current. If the quiescent current is 4A, the peak output current will approach 8A and this gives around 120Wrms into 4ohm. The 8ohm power will be limited by the supply rail voltage.

Geoff
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Old 19th April 2002, 03:37 PM   #5
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Geoff is correct. The Aleph current source varies. It is driven by a signal derived from the resistor network at the output. Yes, this can also be contrued as local feedback. He would prefer more global feedback, perhaps? The patent would be good light reading for your acquaintence.
Regarding feedback. I've got two pairs of Aleph 2s and a Mini-A (actually at the moment, it's a Mini-XA, but that's another topic entirely...) on hand to play with. In particular, I have tortured the Mini-A in ways that I wouldn't treat a 'normal' amp; since the parts cost is so low, if I blow something I won't shed any tears. Among other things I've tried: running the amp with the feedback loop disconnected entirely (open loop--something I normally only try with tube circuits) and various other ratios of closed loop feedback. Nary a problem. In fact the only thing that has given me a moment's trouble is the placement of the negative power lead from the power supply. For whatever reason, this can cause the circuit to get twitchy. I hooked a 1uF cap across the output and the amp didn't even notice. How much more capacitance the fellow might want to put on there, I don't know, but even the Mini-A can handle some capacitance. Note that this is something I wouldn't care to try with a lot of other solid state amps.
Granted, I usually think in terms of tubes, but I seem to recall seeing a number of schematics without Zobel networks. If the circuit can be contrived to be stable, there's no reason to put one in. It certainly isn't graven in stone that every amp shall have an output network. They interfere with the sound and are to be avoided if possible.

Grey
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Old 19th April 2002, 03:51 PM   #6
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Thumbs up Get twitchy

Grey did you try a 1000 pF compensation cap (see C104 in Aleph 3 schematic). Works wonders.... I used a Panasonic PP to replace the ceramic cap in my stock Aleph 3. Sounds better. A 1000pF polystyrene works great here also. Shall I send you a few.....?

H.H.

P.S. Mr Pass explains the Aleph dynamic current source in some detail in the latest Zen article on www.passdiy.com
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Old 19th April 2002, 04:23 PM   #7
jam is offline jam  United States
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H.H.

How did the compensation capacitor help? Is the circuit not stable with this capacitor removed?
Just curious.

Jam
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Old 19th April 2002, 04:36 PM   #8
nar is offline nar
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Talking Aleph can reply

A lot of people doesn't understand the way Nelson's active current source works.But the main idea is that bias is set DC,but the active current source can see the AC bias go up the initial DC bias,and let the active stage without stress while the current source takes the work,and so to the loudspeaker seems to have higher impedance that it really has,or so it is the way the amp "sees"the thing.


This idea is bright!!!!!

I can't sleep thinking about it...
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Old 19th April 2002, 07:15 PM   #9
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Just a comment on the notion that such a circuit would
require a Zobel (or RC to ground) network at the output
for stability.

None of the two gain stage Alephs ever required such
a network, and there has been no instance of instability
that I have heard about.

The issue is one of feedback. Designs which use a lot of
feedback require something to stabilize the load at high
frequency so that it has a known phase margin at the
unity gain point.

The 2 stage Alephs simply did not have enough feedback
to become unstable with any load.
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Old 19th April 2002, 07:51 PM   #10
jam is offline jam  United States
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Mr.Pass,

I had a question about C104 that Harry mentioned, which is used in the Aleph3. Is it really necessary? I notice that it is not used in the Aleph 60.

Jam
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