Zen vs. Douglas Self Trimodal Power Amplifier? - diyAudio
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Old 4th March 2001, 05:11 PM   #1
Thunau is offline Thunau  United States
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Hello everybody! My first post on this forum. Fairly technical mood around here but I see a lot of common sense posts also that I can undestand. I see a lot of Nelson Pass fans here. I was looking through available PCB's and came across this little wonder.

http://www.dself.demon.co.uk/trimo.htm
and
http://www.signaltransfer.freeuk.com/trimodal.htm

It lets you select mode of operation between A, B and A/B. I guess for critical quiet listening class A would be prefferable, but when you want some power you can switch over to class B and give up a little quality. Anybody heard/built this amp? Any thoughts? I think Monarchy Audio sells something along those lines and it seems to be respected in certain cicles. But then again some swear by Bose!
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Old 4th March 2001, 07:19 PM   #2
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I havent built the trimodal but i have the book from Self and have read about it. It is a good concept but it is too complicated to be good quality. It al depends on what you want. The Zen amp is not good for most cases. Like I mean if you want very high sound levels or to use it with with unsensitive speakers. It is really a good amp for low levels or for active speakers.
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Old 8th March 2001, 11:36 PM   #3
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Have you read all Self articles on their original form (EW&WW magazine) with all the follow ups? And then the book?

I do agree that the Trimodal is far too complicated and that simpler can be better (not always).

What I intend to do is modify a Pioneer receiver's amp section, which resembles Self's design a lot, though lacking his "tricks" to improve distortion figures of course. Only a few pcb tracks have to be cut to add the improvements.

Self does not approve on what I want to do, though probably because he may be afraid that I may sue...

In any case, the circuit I will be using is his "simpler" one. The output will be regular emitter-followers (EF) arrangement instead of complementary feedback pair (CFP), mostly because it would be tricky to modify that on the Pionner's pcb.

All parts will be improved too, and the first stages will get a regulator.

What do you think?
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Old 9th March 2001, 03:15 AM   #4
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Something that Douglas Self never quite makes clear (nor does G. Randy Slone) is whether they have capacitance downstream of the regulators. I gather they do not. Any circuit I've ever heard which directly dumps the output of the regulator into the active circuitry runs out of steam when asked to produce any sizeable dynamics. Some equipment (and I'm thinking in particular of Jeff Rowland's stuff, here, as I believe he uses that strategy) sounds really, really sweet. Your first impression is that this must be the finest sounding stuff in the universe, then someone does something rude, like hitting a drum, and...nothing happens. It's as though there's a limiter unit inside his gear.
If that is, indeed, what Self and Slone think regulation is all about, I can understand their disdain for regulated rails. Only...it's not that difficult or expensive to put a couple of caps after the regulator.
The short answer...go for it...but be careful. I've never had much luck modifying existing PCBs as the traces never seem to be anything remotely like what I want, and it would take an entire spool of wire to do the little point-to-point patches necessary to bring things to a conclusion.

Grey
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Old 9th March 2001, 07:38 AM   #5
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Nobody would say that a regulated power supply is a bad thing. The reason that no one uses one in an amp (maybe only in very expensive, complex units) is because a regulated power supply is as hard to make as the amp.

Say that the amp is rated max 3 amps output with 4 transistors in the output/channel. To make a regulated PSU for 3 amps that is capable of going up to high frequencies around 20-100 KHz is not easy. Itīs not like making a power supply for general use. You need at least 4 transistors and double the heatsinks again for the power supply in the worsed case. I say you would need more so the PSU is fast and doesnt reach itīs max capabilities making it hang over. The design has to be very good or else the improvement isnīt a lot better. Maybe you could have the same improvement in sound with just putting double capacity after the rectifiers. We are talking of a cost here of plus 60-80 %.
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Old 9th March 2001, 07:28 PM   #6
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I, myself, haven't found things to be as bleak as promitheus does, particularly as regards to price. The voltage regulator doesn't have to drop as much power/heat as the output because, although clearly the current must be accounted for, the voltage drop isn't nearly as bad. A decent regulator generally has a dropout of, say, 1 to 3V, so in theory you could get by with, let's say, 5V of headroom. In the real world, I'd provide more (on the order of 10%, work backwards from a reasonable voltage sag in the mains line), but certainly not the 20-30-40% that Self/Slone seem to think is necessary. Since we're not dropping as much voltage, the power dissipation is correspondingly less.
Self & Slone seem to be *very* stingy with their capacitors, acting as though it's profligate waste to put anything more than 10,000 uF caps into the main bank. No wonder they feel they need such headroom on their rails! What they regard as normal, acceptable ripple makes me cringe. In their view, it's all taken care of by the CMRR and NFB, but fer cryin' out loud, guys, we're DIY--what's a few more dollars on bulk caps? We're not responsible to anyone but ourselves for the amount of money we put into our projects. Even when I'm running unregulated rails--hell, *especially* when I'm running unregulated rails, I go with large filter caps in the main bank.
(Actually, I suspect that a lot of their intransigence on the audibility of things stems from their attitudes towards their power supplies, and the fact that they don't seem to want to bother matching their gain devices. [Yes, they will--grudgingly--match the devices for the input differential.])
Regarding the frequency response of the regulator...I quite agree with promitheus that a wide-bandwidth regulator is a good thing, but here again I feel that a bank of good caps after the regulator (I'm prone to using lots of film caps at this point in the circuit) helps take care of the high frequencies. Caps here also take care of any residual noise generated by the regulator itself.
In any event, as I understood carlmart's post, he's only planning to regulate the front end, which is a considerably less daunting thing, current-wise. Perhaps I misunderstood.
In regulators, as in other places in audio circuitry, I find that less is more.

Grey
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Old 9th March 2001, 08:33 PM   #7
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Talking regulate yes, regulate no

First of all: can someone explain to me what those funny message icon globes mean? I must be making a face very much alike those when I look at them, trying to guess which should I pick. Today I went for the toothed smile...

GRollins is right when he says that I am intending to regulate just the low current side of the amp, that is all but the output transistors. Full regulation is something I am still not too sure about.

I certainly do not agree with Self (or Slone if he says the same) over the regulation thing. The regulator should be good though, as well as the parts you use. First stages voltage, after regulation, should also be higher than the output stage unregulated voltage. And that is not easy to implement. Many people, like N.Pass simply go for a voltage doubler and then regulate that, so as to be able to use the same transformer.

Some weeks ago there was a 3-part piece on <http://www.tnt-audio.com/> on "Solid State Power Amplifier Supply", by Dejan Veselinovic, that I think should be read by many people. Myself always learns something new on this question.

What type this regulator should be, totally discrete or using 3-pin regulators, may not be the issue. On my case I have had very good luck using 3X7 types. Those are the ones I intend to use on the Pioneer's regulator, even if left "floating", that is unprotected from an eventual short circuit. That is because those types have a maximum working voltage of around 40v: within that they are fully protected. But you can use them on higher voltages as long as the difference between input-output is never greater than 35v or so. No protection though.

There are new regulator types, like the LT1085 and similar, that supposedly improve on the 3X7, but the latter are the minimum you should go for. Much better than 78XX or 79XX families.

To go for a full regulation, that I may try some time to see how it sounds, you need plenty of space and current to spare. In this case I wonder if you don't limit dynamics somehow.

But larger (and $$$) models from Krell or Mark Levinson are wholy regulated, if I am not wrong. And they get good reviews

When I make up my mind and go for a try I would like to start by using James Boak's and Kit Ryan's designs published in The Audio Amateur some years ago. Builders reported great results from using them.

This year there was un upgrade article on Audio Electronics (former TAA and present Audioxpress), by Walt Jung, on his top grade supplies published some years ago. Pity these supplies are designed for lower voltages, up to +/-24v.

In any case, over this regulators question, a friend of mine says they are not the right way to go because regulators use feedback to lower impedance and that feedback gets imprinted on the sound. A valid question that I can't really say much about, except that I have never listened to unregulated (amps or preamps) that sounded better than regulated (somewhere in the circuit) ones.

Until proven different, I think regulation IS the way to go. For me, an audio circuit using a regulated supply sounds better than an unregulated one.

Going for a high bandwidth on the regulator should be good too, but I don't know how to work on them for being stable and have a large BW at the same time.

This supply question deserves a quite thorough discussion, that hopefully we will carry on. Though maybe it exceeds the original issue of "ZEN VS DOUGLAS SELF TRIMODAL POWER AMPLIFIER" that started it.


Carlos


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Old 1st April 2001, 08:47 AM   #8
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One big issue about full power supply regulation is
that the full supply voltage has to be dropped across
the regulator, the >transformer< being a weak link here.
Because of line and load regulation variances, one has
to drop ten or fifteen, maybe more, volts per rail to
stay within regulation from no load to full load.

While I've had past experience working with switch mode
regulation, I don't feel confident to attempt designing
a regulator based on this topology, nor on a resonant
mode converter, which would have lower noise.

Best one can usually do for a practical design is to
put plenty of iron into the transformer and plenty of
capacitance in the power supply. Decouple generously
on the driver board and sub-regulate there.

According to various sources, a diff amp with a current
source tends to have good power supply rejection anyway.

It's a bit of a complex subject since power supply design should also include issues like rectifier noise and RFI
intrusion from the AC line.

While toroidal power supply transformers are popular, one
weakness is that they do have good HF coupling between
the primary and secondary, unless electrostatic shielding
is incorporated. This is why I prefer split-bobbin transformers for preamp and such power supplies. But try
and get that with big iron!

There are various tradeoffs you have to live with in the
engineering world; those are some of the few I know about
and had encounters with when I had a real job as an electronics technician ten years ago. I got to look over the
engineer's shoulder as the new supplies were designed, debug
'em and fix 'em in production. I really miss having that
kind of a job.
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Old 2nd April 2001, 01:09 AM   #9
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Damon,
For me, the key to bulk power supply design was clearly expressed when you wrote "plenty of iron [in] the transformer and plenty of capacitance in the power supply." Self/Slone, for instance, believe in puny little power supplies, counting on CMRR to take out the nasties. Then they stick a class B output section in. This is a good recipe for collapsing rails, as at high power a class B output section will present a widely varying load to the power supply. If the power supply isn't stiff enough to handle it...who cares? Or so they (and many others) seem to say.
With a stout power supply, you can minimize the variation of the rail voltage, simply by having enough electrons flowing in from the transformer to keep the output happy without having to draw down the 'savings account' in the bulk supply caps.
Of course, one could always go for a class A output, which doesn't vary at all...
Yes, heavy iron costs money, but I used to have this little thing I'd say when I was faced with a customer who couldn't decide between two different pieces of equipment because of price. First, I'd warn them up front that I was about to play with their heads. (Honest Abe, that's me.) Then I'd say--Okay, I'm a magician. I have a magic wand. When I wave my magic wand the calendar will advance one year. (I'd wave my imaginary wand.) I'd then say--You're listening to your stereo. Now, are you saying to yourself I'm glad I bought XXX, or I *wish* I'd bought XXX? Then I'd remind them that pain (even financial) is temporary. A year later the amount they spent would only be a memory, but the stereo would still be with them. The moral of the story being: Save up, beg, borrow, or steal (okay, maybe not steal) enough money and build the bigger power supply. If you need to, rationalize it by telling yourself that you can reuse the transformer and caps (if they're big enough) in your *next* project. Think of the money you've saved!

Grey
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Old 2nd April 2001, 04:02 AM   #10
blmn is offline blmn  Brazil
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Carlmart and Damon,

Actually, the big problem with regulated PSUs for the output stages is the cost and, as Promitheus said, the design itself must be done carefully. Since the design allows good responses to transient current demands and no problems with HF instability, no problem except the price and additional heat generation.

The standard textbooks use three factors to define how good is the power supply: input regulation factor, output resistance and temperature factor.

To be good the first one needs good mains connection, the second needs good transformers, wiring and high quality components and the last one needs stable components under many temperature conditions. All these things will change if you consider a regulated PSU, but the main idea is the same.

A good transformer is not always a bigger one. Good transformers are always done with good iron (low losses, good flux density), wire (good insulation and the correct area) and construction (as Damon said in his statement about toroids). If you want a really good transformer, forget about the cheaper ones (little or big), or design your own.

In regulated power supplies, the amount of voltage under the series regulator is (not only) a function of the current drawning by the load. So, in many cases you really need much more than a few volts to maintain a good regulation factor. I tested it myself, in some cases (40 Amps peak capacities from car audio SMPS amps, I needed 60% of over voltage).

Self just says the following about reservoir capacitances is his book (page 220):

"For amplifiers of moderate power the total reservoir capacitance usually ranges from 4,700uF to 20,000uF, though some designs have much more. Ripple current ratings must be taken seriously. It is often claimed that large amounts of reservoir capacitance give "firmer" bass; this is untrue for all normal amplifier designs below clipping."

He says very interesting things about decoupling caps considering all the amp stages. He didn't recommend underated transformers, cheap components or even less capacitance than necessary.

The calculation of reservoir capacitor is not a mysterious or a not determined thing, you can find it in many textbooks. Considering the ripple voltage needed, the peak current and using good quality caps, wiring and diodes these calculations work very well. As far as I know, the rule of thumb proposed by Slone is his book (1000uF per 10 watts), works well too and didn't disagree with the equations about the matter. Of course if you use larger amounts of reservoir caps you will have less ripple and a better capacity of response on high current demands (there is a practical limit for it too), but you must be aware of surge currents and your bank account. A good transformer is much more important, in this case.

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