Choke Loads for Zen/Aleph Amps - diyAudio
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Old 24th December 2003, 02:30 AM   #1
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Default Choke Loads for Zen/Aleph Amps

Choke loads have been informally discussed before:

Inductors, Lightbulbs As Current Sources

Time to look a little closer at the driving factors of why we build stuff the way we do.

Be nice to the tube guy, now. Looking at the Zen design (and the Aleph design), a current source is used to force an output when the amplification MOSFET modulates. In the tube world, we call this "parafeed".

In parafeed amps, a choke is used as the current source. The DC bias current shoots through the choke and into the tube. When an AC signal appears at the tube, the choke resists the AC signal, and forces the signal to go elsewhere - the output.

If the AC signal is low enough in frequency, it appears like slowly changing DC to the choke. The choke doesn't try to fight it, so nothing appears at the output.

This theory can be applied to Zen amps (Figure below). The Pass design uses a current source that burns 50% of the voltage as heat in Figure (a). In Figure (b), the choke doesn't burn any voltage, except a little due to DCR.

Now, tube amps usually use about 100mA, and chokes are relatively easy to make for lower currents. The Zen amps use an order of magnitude more current, which means the chokes have to be that much larger. Pass Labs probably selected not to use chokes because it may be financially and commercially infeasible.

However, for the DIYer, chokes may be acceptable. The tradeoff is as follows: higher construction costs of the very large choke will result in much less power dissipation, less heat sinks, and cheaper operating costs. Those who are thinking about water-cooling their amps may find relief. Those who are concerned about ecology may also find solace, not wasting as much energy as heat.

Is there sonic differences? A tube guy would think not. However, those die-hard may find Figure (c) desirable: not only does the active current source exist, but a passive source also exists. This does not save any heat/energy dissipation problems, but it may increase the fidelity of the design.
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Old 24th December 2003, 02:49 AM   #2
tiroth is offline tiroth  United States
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Well, the impedance of an active current source is generally so high as to swamp any effect of the impedance of the choke. I'm not sure I see the point...

Since all of the current is passing through either way it seems to me that you either bite the bullet and use huge iron or just go active.
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Old 24th December 2003, 03:07 AM   #3
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Let me make a correction: Figure (c) may be considered a natural progression from (a) and (b). If each of the things is good independently, would they be even better combined? This was more of a rhetorical question to spawn discussion.

What is more germane to the discussion is why DIY hobbyists have chosen (a) more often than (b).

If chokes may not be financially feasible for commercial products (such as Pass Labs), are chokes feasible for DIY projects? I believe that chokes are within the scope of the average DIY hobbyist, at least on this forum.

Pass has tried chokes before (refer to previous thread) and had good results. Why do the Pass Labs amps not all have chokes? In general, some things may be more feasible for commercial operations – such as mass produced circuit boards or anodized aluminum faceplates. However, other things may not be feasible for commercial operations. High-current chokes may be one of those things. That may be the primary value of Pass’s patents: giving an amplifier choke-like performance without the cost of the choke.

As a DIY hobbyist, high-current chokes may be feasible. Do the performance advantages warrant exploring the option more often?

This may be implying that Pass’s invention is a “band-aid” to get around using a choke. I don’t endorse that theory. I endorse the theory that Pass’s invention was a way to bring his vision of high-quality audio to commercialization. Otherwise, it may have not been possible.
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Old 24th December 2003, 03:20 AM   #4
Magura is offline Magura  Denmark
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Forgive my lack of knowledge, but wouldnt those chokes have to be size x-large to be useful for bias in a SOZ or is there something ive gotten wrong?

I made a set of 4 pcs 2mH chokes for the psu for my SOZ project (air gap) and they are simply huge (600W a piece)

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Old 24th December 2003, 06:29 AM   #5
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This subject has been addressed before (in this forum, by me,
don't ask where) and I repeat:

Chokes (especially air core) are great loads for Alephs, and
I have built such with a ~ 1 H load using a full roll of MWS
magnet wire. The bias is a little tricky (left for you to either
work out or wait for an article) but the performance is excellent,
with an symptotic approach to 50% efficiency and just about
the specs you get from an Aleph current sourced Zen.

The roll of 1000 ft of MWS wire ( I forget the gauge - probably
16 or 18) doesn't cost much <$100 but it does take up a little
space on top of the box.

The bias is a little more sensitive, but nothing you can't trim -
I recommend a meter on the front panel with a knob.



Oh, I forgot to mention, when you do this, you tend to do it
at 1/2 supply rail with twice the current.
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Old 24th December 2003, 10:49 AM   #6
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Default If you can't hide it, make a feature of it.

http://www.diyaudio.com/forums/showt...5181#post65181
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Old 25th December 2003, 02:54 PM   #7
Fuling is offline Fuling  Sweden
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How ´bout some choke loaded IT-coupled balanced SE tube/mosfet hybrid action???

I have all the iron except the power transformer, which should be easy to find used or surplus. Shall I???
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Old 25th December 2003, 03:27 PM   #8
Magura is offline Magura  Denmark
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I find your idea interesting.

I think i once saw an inductor calculator online...in that case it should be no problem to make the 50mH inductors....though they are going to become huge.

Are you sure 50mH is enough?

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Old 25th December 2003, 04:45 PM   #9
Fuling is offline Fuling  Sweden
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I *think* 50mH will be enough, at 20Hz they have a reactance of 6,28 ohm and since the circuit is bridged each half should "see" a 4 ohm load, correct me if I´m wrong. I guess the higher inductance the better, but this should work.

Experience shows that 100mH is enough for an unbalanced SE circuit loaded with 8 ohms.

The chokes I´m planning to use are 25mH @ 3A, double C-cores.
Have a look at page 11 in the thread Circlotron linked to if you wanna see a pic of my earlier attempt to use them in an amp.

Just a thought: In a balanced circuit it should be possible to use one centertapped choke with just a small airgap instead of two separate chokes gapped for 3A.
Though, I guess that would make it a PP amp which could be biased in class A, AB or B. Not funny anymore...
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Old 25th December 2003, 05:05 PM   #10
Magura is offline Magura  Denmark
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There it was...

http://www.colomar.com/Shavano/inductor_info.html

The inductor calculator that is.

Looking at a 50mH air gap you can achieve 8.81-1.13 ohms, but wasnt the point to bias the mosfets without the low efficiency??

If we see the same resistance as with the resistors, but have added some monster inductors....whats the benefit then??


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Old 25th December 2003, 05:15 PM   #11
Fuling is offline Fuling  Sweden
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Uhm, you might want to use some iron cores for the chokes, otherwise they will be huge and the DC resistance will be way too high.
Again, have a look in the thread Circlotron posted, there he shows how to make really high inductance chokes out of old microwave oven transformers.
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Old 25th December 2003, 07:45 PM   #12
Magura is offline Magura  Denmark
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True...no problem to make some 50mH inductors of low resistance.
If needed it would maybe be even easier to just make the E and I cores yourself, in order to obtain a bunch of matched inductors.


All of a sudden a SOZ could be quite efficient

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Old 25th December 2003, 07:50 PM   #13
Fuling is offline Fuling  Sweden
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I don´t think DIY core laminates will be easy, but has anyone ever tried actually?

Yes, the maximum theoretical efficiensy of a class A SE choke loaded amp is 50%, not that bad. I reached about 40% which isn´t bad either I think.
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Old 25th December 2003, 07:53 PM   #14
Magura is offline Magura  Denmark
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Quote:
Originally posted by Fuling
I don´t think DIY core laminates will be easy, but has anyone ever tried actually?


Yes, the maximum theoretical efficiensy of a class A SE choke loaded amp is 50%, not that bad. I reached about 40% which isn´t bad either I think.
Ive machined cores for trafos before, no big deal.....epoxy does wonders




Sounds rather appealing...especially if you think of the original SOZ power efficiency.

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Old 25th December 2003, 07:57 PM   #15
Fuling is offline Fuling  Sweden
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SOZ has like 4% efficiency or something, right?
If i had the necessary heatsinks lying around I´d love to build one...
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