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Old 9th March 2006, 02:23 PM   #1
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Default Possibility query: MyRef + Tube?

Hey all,

This is just a quick shot quesiton because I don't want to ressurect an old thread. I was looking at the MyRef approach and was wondering if the configuration can be adjusted to use any configuration of tubes as a gain stage instead of an OPAMP. Admitedly I don't know enough about the LM3875 to know if it can be used as a buffer.

Thanks,
Tim
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Old 9th March 2006, 03:16 PM   #2
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I'm afraid you're wishing too much....

My_Ref relies on a close integration of the devices used, and the techniques employed to achieve its permformance are closely realted to the internal structure of the components, so closely that Mauro even specifies the brand (National) of the LM318 (because of different compensation masks used by other manufacturers).

Moreover it is NOT a buffer but gives voltage gain and drives (quite hardly) the LM3886....

If you want to use tubes with this amp I've heard that someone used Joe Rasmussen's basic buffer in front of a my-ref with good results.

Cheers

Andrea
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Old 10th March 2006, 06:17 AM   #3
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Ah, thanks for setting me straight

I'm just curious though. I know the LM318 is just there for voltage gain. However I rwad in the MyRef documents that he chose the LM318 mostly because it's a nice compramise between price and sound out of many choices for OPAMP. Also it looked like the LM3886 is acting more like a current source that has its currents varied by the voltage, ie it "senses" the dynamic load of the speakers and attempts to compensate. Is this the case? If so that was why I thought a tube would work in the first place. What I don't know is if the LM3886 is implemented in a way that HAS to take advantage of an OPAMP's characteristics.
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Old 10th March 2006, 07:46 AM   #4
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Hi Tim

I think your idea is viable but difficult. Assuming the tube works as common cathode and NFB is applied to the cathode you will obviously need some readjustment of dc levels in order to directly couple the anode to the 3886 current pump. More importantly, the dc stability of the output will now be dependent upon the tube dc stabilty which doesn't seem like a good idea.
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Old 16th May 2007, 07:10 PM   #5
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Red face Re: Possibility query: MyRef + Tube?

Quote:
Originally posted by eVITAERC
I was looking at the MyRef approach and was wondering if the configuration can be adjusted to use any configuration of tubes as a gain stage instead of an OPAMP.
Tim
indeed this is an idea I was wondering about since quite some time now. Sadly, I never had the time to develop it.

BUT, It's not goin' to be a trivial thing! This is not like sticking a tube stage in front of a gainclone, here you have to really carefully redesign the whole thing knowing what you're doing.

BTW: to refer to such a thing as to a "MyRef + Tube" would be rather improper... one should rather say "Hybrid MyRef -like amplifier" or something like that.


Quote:
Originally posted by analog_sa
I think your idea is viable but difficult. Assuming the tube works as common cathode and NFB is applied to the cathode you will obviously need some readjustment of dc levels in order to directly couple the anode to the 3886 current pump. More importantly, the dc stability of the output will now be dependent upon the tube dc stabilty which doesn't seem like a good idea.
I guess the easy way would be to AC couple the tube "VAS" to the current pump and care about 3886 offset locally (either statically and/or with a DC servo).

Otherwise (as you said) one may take care of DC levels (likely using a bipolar supply) to keep the DC coupling and use a servo (as in the Mauro's my_Ref "Evolution") to keep DC out of the way. Of course care must be taken to keep the tubes operating within proper range...

At least for a first try, I would likely go for the AC coupling.

I would also lower the overall NFB to get less stability problems, of course at the expense of a reduced nominal DF. But IMHO that may only improve rather than worsen the results soundwise, at least as far as DF is kept no less than a few units.

Mmmhh... this last sentence may sound heretical, isn't it?

Well, let' see. Perhaps it deserves some explanation.

IMHO, a good audio amplifier should be able to put out so much current as to be able to "drive it's own output impedance". That is, the overall (closed loop) output impedance of the amplifier should be such that it can keep working "normally" (producing a linear output current, without clipping or going wild) even with a 0 ohm (short circuit) load.

(this is to make sure it will never misbehave under any condition with a real load, leading to possible overload/clipping, slew rate limiting, etc).

For a single 3886, we have ~ max 10A typical. Assuming supply rails at about +-35V, that would imply that we would have a Zmin = Vsupply/Imax = 35/10 = 3.5 ohm (!).

Given that of course we don't care to drive an actual short circuit at the output and assuming perhaps ~ 1 ohm of "stray" DC resistance (voice coil, xover, cables, etc.) to be always present in the output circuit, we would have to have a minimum Zout of about 2.5 ohm, for a "nominal" DF of ~ 3 on 8 ohm and ~ 1.5 on 4 ohm (I said "nominal" 'cause effective DF must take into account the "stray" resistance too, thus would be ~ 2 and ~ 1, respectively).

Worried about this low DF figure?

Well, that's about the maximum "real" DF you can get out of a single LM3886 anyway!

Well, at least @ +-35V; from the above, higher supply rails would make things even worse, while reducing them would allow for a smaller Zout -> higher DF... e.g. at a mere +-12V supply you could go as low as 0.2 ohm for a nominal DF of 40 on 8 ohm.

(BTW: maybe that's why some Class D with "virtually unlimited" output current capability have an higher "perceived DF" than just about any other amp out there?! )

Notice that no matter how small is the amplifier Zout, the actual DF would be limited by the "stray" resistance in the output circuit and thus would be no more than a few units anyway (unless you go to negative Zout, you can't get a DF higher than that).

This is to say that it make no sense to hunt for ridiculously low Zout... while a not-so-small, "properly chosen" Zout will provide just about the same "real" DF while assuring that the amplifier will always behave properly and correctly under any (dynamic) load condition.
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Old 16th May 2007, 08:26 PM   #6
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You have allready been advised that the myref is not a simple 2 stage amp, but a current pump...

I was the person to use the rasmusen buffer, it is very warm sounding and too mellow to say its a good allround solution...certainly nice for some jazz, and olde worlde stuff...

I eventually matched the myref to a pedja rogic buffer... which I think is the best thing after flavoured milk.
You can search the solidstate forum, for my stripboard layouts etc... Come to think of it , I should upload the new PCB files for it...
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Old 17th May 2007, 10:13 AM   #7
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Quote:
Originally posted by Nordic
You have allready been advised that the myref is not a simple 2 stage amp, but a current pump...
Ahem... first of all, it's NOT a current pump in itself as a whole, it does USE a current pump. That's not the same...

Though the "my_Ref" proper integrates a number of different ideas and techniques to achieve certain design goals, the very basic idea behind it is that to wrap a transconductance amplifier (a voltage controlled current source, the LM3886 op-amp based "current pump" in this case) within a voltage NFB loop.

(this offers a number of advantages with respect to a more "classic" design which typically use a voltage amplifier within the loop and opens up a wide range of opportunities to control the intimate operation of the whole. But this is a long story, which have been already explained many time elsewhere and is not the case to repeat here).

Back to the "my_Ref" design, it's perfectly correct to see it as two stages, a voltage amplifier (let' call it "VAS" for short), which is based on an LM318 followed by a transconductance amplifier which is based on an LM3886 configured as "current pump". Those two stages are wrapped within an overall voltage NFB loop. That's it (what's tricky about the my_ref proper is how many details have been carefully thought out, but that's another story).

Once you have understood how it works and you know what to do, nothing prevents you to design an amplifier based on (some of) the same concepts and ideas using different technologies.

You may replace the LM3886 current pump with a different transconductance amplifier, such as e.g. one using discrete MOSFETs configured in common source... (that way basically you'd be cloning the Tim De Paravicini's Musical Fidelity A370 design).

Or you may keep the 3886 current pump and replace the LM318 "VAS" with a different gain stage using discrete components or even -why not?- tubes, as we (at least analog_sa and myself) where talking about here.

Of course you could even change both stages to end up with an "all discrete" design. The SKA GB150 may be an example of something like this (or is it using ICs somewhere? I heard a lot 'bout it, but unfortunately didn't had the chance to see its schematic diagram).

Eventually, you may even make up an all tube design... and likely you will end up with nothing but a typical OTL tube amplifier!

Yes, if you think a bit about it, the De Paravicini / Penasa idea may be seen as nothing else than a typical tube OTL amplifier "transposed" to sand state!

(N.B.: this is NOT intended to take any merit off Mauro or Tim, but just to note how breaking new ideas are often old concepts and ideas put together to new life in a somewhat different context).
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Old 17th May 2007, 01:44 PM   #8
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Quote:
Originally posted by UnixMan


Ahem... first of all, it's NOT a current pump in itself as a whole, it does USE a current pump. That's not the same...

(N.B.: this is NOT intended to take any merit off Mauro or Tim, but just to note how breaking new ideas are often old concepts and ideas put together to new life in a somewhat different context).

This is very true, you covered some things I wanted to point out.

Since Mauro showed this design to us I have tried various permutations of the Transconductance Amp + Voltage Amp design, including discrete fully symmetrical versions (with super symmetrical voltage stage, balanced input, bridged output). This has proved very successful, but the amp much more complex then the two op amp single ended version Mauro presented. Definitely not a beginners amp. A beginner can easily build a MyRef.

The wonderful thing about what Mauro did, is that he took a proven concept, and made it very simple and elegant. This is one reason why it is so special.

It is relatively simple (if you understand the amp) to duplicate (or even improve) the results we get from Mauro's great design using other parts. But you will have to "design" it. Mauro already did that work using easy to get parts. Mauro put a lot of research into what he showed us, and got a lot of us thinking. For that I at least owe him a great debt of gratitude because it really started me down the path of understanding many electronics concepts I had not really understood before. Like "what is transconductance anyway?"

So lets just understand the terminology:

A simple transconductance amp in the feedback loop of a voltage amp is not necessarily a MyRef... though it may be "MyRef like." Only if it follows Mauro's compensation schemes (any of the three major versions, even including feed forward) and uses the LM3886 and the LM318 is it a MyRef. Also I would go so far as to say that the power supply (shunt reg for V stage) is also a part of the MyRef signature, some would say for the better, some would say for worse, but all would agree its a fine amp altogether, and very cheap to build.

Anyway, I hope, if Mauro sees this, he is proud of all the good he did. Thanks Mauro!

Cheers!
Russ
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Old 17th May 2007, 02:34 PM   #9
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Quote:
Originally posted by Russ White

Since Mauro showed this design to us I have tried various permutations of the Transconductance Amp + Voltage Amp design, including discrete fully symmetrical versions (with super symmetrical voltage stage, balanced input, bridged output). This has proved very successful, but the amp much more complex then the two op amp single ended version Mauro presented.
that's quite interesting...

have you presented these nice works here or elsewhere?


Quote:
A simple transconductance amp in the feedback loop of a voltage amp is not necessarily a MyRef... though it may be "MyRef like." Only if it follows Mauro's compensation schemes (any of the three major versions, even [...]
of course, only a "my_Ref" is a "my_Ref"!

If you change anything it's no longer gonna be Mauro's design anymore, but something different. It may be better, worse, just plain different or even apparently undistinguishable from the original, yet it will NOT be it and may not be called "my_Ref". For quite obvious reasons only Mauro has the right to use that name for his own designs and developments.

That's why I previously cared to hint about proper naming...

And of course it's not just a matter of name/"trademark": the name here represents the designer and the many concepts, ideas and goals (and taste and skill ...) that he have put into his own design(s). Different designers will always have different tastes, ideas and point of view, and in the end different goals too (or at least different priorities).


Quote:
Anyway, I hope, if Mauro sees this, he is proud of all the good he did. Thanks Mauro!
100% agreed...
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Old 17th May 2007, 03:00 PM   #10
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Quote:
Originally posted by UnixMan


that's quite interesting...

have you presented these nice works here or elsewhere?


I have posted bits and pieces here and there. But I am holding off on the details until I have really proven the concept. I also need time to finalize the project (many other things competing for time).

Here is the general idea.

The "master" voltage amp has been tested as both a discrete super symmetric amp (ala XBOSOZ), and a THS4131(which is also super symmetric).

One note, my best results have been with no R/C filter at the inputs of the Gm stages.
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