Choke Loads for Zen/Aleph Amps

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Choke loads have been informally discussed before:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=7393&highlight=

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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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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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.

:wiz:

Oh, I forgot to mention, when you do this, you tend to do it
at 1/2 supply rail with twice the current.
 
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...
 
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.
 
Fuling said:
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.

Magura
 
how to calculate the efficiency of a SOZ with chokes??

Uhm, I don´t know. I guess those "tail" resistors (from sources to negative rail) dissipates alot of heat, but maybe they can be replaced with a third choke, acting as a constant current source?
I don´t know, I´m just thinking loud here.
BTW with chokes instead of resistors in the SOZ the gain probably goes way up so some kind of negative feedback might be necessary. Not attractive.

Thinking about cores, as I see you live in Denmark maybe you could have a look at www.svebry.se and check out their EI transformer kits. Much easier than ripping old power transformers apart.
 
Fuling said:


Uhm, I don´t know. I guess those "tail" resistors (from sources to negative rail) dissipates alot of heat, but maybe they can be replaced with a third choke, acting as a constant current source?
I don´t know, I´m just thinking loud here.
BTW with chokes instead of resistors in the SOZ the gain probably goes way up so some kind of negative feedback might be necessary. Not attractive.

.


Sure not attractive...so this is most likely the end of chokes for SOZ....or what??

Great link...postage could be mean though :(

Magura
 
I suspect that the easiest way to make a mosfet work good with at choke load is to use it as a source follower and use a preamp for voltage gain.
Applying feedback to a choke loaded Zen-like amp seems to be a ***** unless the rail voltage is well regulated, the DC resistance in the choke is too low to make the feedback loop work DC-wise.

Yeah, postage...:bawling:
Maybe it would be cheaper to pick them up yourself??:D

BTW, you can also get custom wound transformers and chokes from Svebry and they don´t charge much for the work.
 
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