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Old 31st August 2004, 11:20 AM   #1
polsol is offline polsol  South Africa
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Default Ouput Stage Distortion

Articles that I have read over the years on amp distortion tend to deal with the distortion characteristics in the voltage amplification stage and then just lump on the output stage without any further comments in this respect.
If one considers a class A output (for convenience purposes) what is the distortion added by this stage?

Following on from the recent forum topic of no NFB in an amp because of the possible (probable) effects of speaker "back EMF" to the input stages, would it not be possible to design an amp that only had NFB applied to the voltage gain section and run the output in a non NFB manner. In other words the current output acts as a "buffer" between the input stage and the speakers?
Has this been tried before?

I seem to remember from some time back a power amp that was built from and Op-amp input with complimentray pair output transistors and, from memory, the NFB was only on the op-amp.

I assume one of the problems faced would be the output resistance of the amp?

Once upon a time the amp's "Damping Factor" was all in vogue but this seems to have bitten the dust of late (at least in some of the specs I have seen). I take it that the "damping factor" would also play an important part in reducing the feedback from the speakers to the input stage?

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Old 31st August 2004, 12:33 PM   #2
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Default Re: Ouput Stage Distortion

Quote:
Originally posted by polsol
would it not be possible to design an amp that only had NFB applied to the voltage gain section and run the output in a non NFB manner. In other words the current output acts as a "buffer" between the input stage and the speakers?
Has this been tried before?
Hi Polsol

Yes it is possible and it can be easely done.
The feedback is taken from the colector of the VAS transistors and the output stage only do the current gain.
Many comercial amps use that aproach , come to mind Densen in the DM10 and Onkyo in the Integra 9711 series .
They are advertised as Non Negatif Feedback amps , when they must really be called Overall Feedback Free amps.... a la Nelson Pass...

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Old 31st August 2004, 01:30 PM   #3
AKSA is offline AKSA  Australia
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Tony,

Many issues in your question; it's complex. Pardon me if I cover a little ground before a fuller explanation.

First, the voltage amplifier of any amplifier is operating in common emitter mode. This is the most distortive of all the configurations, known loosely as CE, common collector (emitter follower) and common base.

The distortion introduced is an order of magnitude higher than that of an emitter follower (EF) and compounded by the interelectrode capacitance between base and collector which changes with applied voltage and current. This capacitance, usually called Miller capacitance, is effectively multiplied by the beta of the transistor, and is considerable, leading to HF rolloff and some nasty distortion effects.

In any global feedback loop there is a propagation delay which at some very high frequency turns negative feedback into positive feedback. This abruptly turns the amplifier into an oscillator, so it is immediately clear that something must be done to the voltage amplifier to cut its gain back to below unity so that by this very high frequency the now positive feedback does not bring a destructive paroxysm of oscillation.

Thus, the heart of any amplifier becomes the voltage amplifier, principally because all the compromises lie in this circuit element.

It is certainly possible to drive the output stage in open loop. I've been down this path, and the results are interesting. In brief, taking feedback from the voltage amplifier rather than the output stage makes the amplifier sound very like a tube amplifier, with a lovely, twee sound and not too much bass control. Incidentally, imaging is superior, and you can drive almost any load with ease....... so, you can't have it all, where would you put it?

Obviously, this configuration reduces damping factor, which explains the effect on the bass. But it shrinks the feedback loop, and propagation delay no longer encompasses the output stage, so the pole frequency, the point at which the amplifier would ordinarily go up, is much higher, making stability much easier to ensure. We can use less lag compensation across the VAS to achieve this; this is good for sonics.

It is adviseable to run Class A in any output stage which is running open loop. This is because the crossover disjunction, the so-called 'dead zone', is no longer important because neither of the two output devices is ever switched off. And the high quiescent current of Class A will ensure lower large signal distortion arises in the Emitter Follower(s), and thus helps to improve damping factor. Large signal distortion is caused by higher current flow through collector/emitter as the signal swings higher (and lower); to generate more current flow, the base must be driven harder, and the result is that the step voltage between input and output, the base/emitter voltage, increases. This has the effect of compressing the waveform, adding harmonic overtones, generally H2 to H6. Ordinarily, this distortion mechanism is well nulled in a global feedback loop, but if the output stage is open loop, that is, outside the feedback loop, it can be minimized by biasing both ouput devices at high current as in Class A.

Cheers,

Hugh
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Old 31st August 2004, 01:48 PM   #4
polsol is offline polsol  South Africa
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Dean,

Just a quick reply to say thanks for a very comprehensive reply - much appreciated - which I will study in detail tomorrow (I'm just about to head home from work - now 4pm in Kuwait).
I mentioned Class A as I couldn't see a way around trying to get around the cross-over distortion in class AB.

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Old 31st August 2004, 01:49 PM   #5
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I think you may try feedback at the current driving dstage just before the last stage of power transistor. It seems a good compromise worthy of trying.
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Old 1st September 2004, 01:26 AM   #6
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Quote:
Originally posted by polsol
I mentioned Class A as I couldn't see a way around trying to get around the cross-over distortion in class AB.

Cheers
In order to use a class AB emitter follower current amplifier outside of the NFB loop, as just an emitter follower, you have to bias the two transistors separate of the rest of the circuit that is enclosed in the NFB loop. Using current sources that are equal so that no major DC offset is present, and a bias servo for thermal compensation establishing an overall temperature coefficient as close to 1 as possible to eliminate crossover. If you make one of the current sources slightly adjustable, DC offset can be adjusted and output EF stage can be capacitor coupled to input drivers. Also output impeadence is more or less fixed and lower impeadences may cause problems.

There is one major drawback to not including the output stage in the NFB loop. If you desire to have say 6A peak output, then the transistor must be fairly linear up to 6A or symetrical harmonic distortion will occur. However, if included in the NFB loop, and the amplifier circuit before the emitter folower output stage has been over designed for peak voltage & current, and can operate at high frequencies (much higher than audio), then harmonic distortion will be generated equal and opposite in phase to the distortion created by the saturation of the device, and will mathmatically cancel out leaving a much lower distorted sine wave. This will compensate somewhat for certain nonlinearities inherent to all transistors. Simply stated, transistors are not linear devices, just have a region of operation that is very close to linear. This is where a hybrid SS feedback driver stage combined with a power tube output stage not included in the NFB loop would be more prudent in achieving linearity if this is the route of choice. Tubes are very linear by design. More expensive and usually requires a lot of iron but would be worth the $$$.
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Old 1st September 2004, 01:58 AM   #7
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Quote:
Originally posted by cunningham
This is where a hybrid SS feedback driver stage combined with a power tube output stage not included in the NFB loop would be more prudent in achieving linearity if this is the route of choice. Tubes are very linear by design. More expensive and usually requires a lot of iron but would be worth the $$$.
Hi Cunningham

But a tube output stage without overall feedback will have a much higher output impedance (lower damping factor) then a triple follower...even if we use power triodes...
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Old 1st September 2004, 09:44 AM   #8
upstart is offline upstart  United States
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There seems to be a correlation between output stiffness (low output impedance) and variously described unpleasant sonic characteristics.

Solid state amplifier designers take great pains to keep output impedance low exactly in order to mitigate against feedback from the speaker reactance back to the earlier stages (input or VAS).

Is it possible that today's high efficiency tweeters would prefer to be driven with a higher impedance? I mean, if people are willing to turn amplifiers into room heaters to avoid output distortion, why not put the heat to good use by increasing the output impedance of the amplifier, attempting to approximate the speaker's conjugate impedance?

Any thoughts?
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Old 1st September 2004, 12:53 PM   #9
polsol is offline polsol  South Africa
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Thanks all for your responses.

Are there any DIY (i.e. schematics) available on the web that utilize this type of approach?

Any subjective comments as to the "sound" of these amps when compared with standard NFB types?

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Old 1st September 2004, 01:09 PM   #10
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One of N.P.'s "hits", the famous Stasis amp used this kind of feedback:

http://www.diyaudio.com/forums/attac...&postid=109546

And I think I have seen it on a schematic of a GAS or ML as well.

regards

Charles
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