Single or dual differential?

[john curl]

Dimitri, it would seem that this current drive question does belong on a separate thread.
For the record, current drive is not unusual for motor drive applications, and has been used for many decades. However, loudspeakers, under some circumstances, could benefit from current drive, BUT NOT typical speaker systems.
Also, I could care less whether SE is on this website or not. I like this website, because it is fast moving, and many inputs post interesting schematics and other references. I would prefer to keep on subject, if possible, but I'm sure I also have diverted from the original subject on many occasions, over the years. We were having an interesting discourse on differential input stages, but now this is pretty much lost in the noise of other inputs.

[hitsware]

>I would prefer to keep on subject, if possible,

refost 199

If I understand Hugh right, the single pair configuration has more even harmonics not from the pair itself, but from the single ended (as a rule) next stage. Correct? Isn't the pair itself push pull by definition? .............. mike

[lumanauw]

Sorry, my mistake. I will get back into track.
Like one post said, is it true dual differential gives less harmonics, because they cancel each other? What is really the benefit of having less harmonics? Isn't that tubes have warm sound because they have big harmonics?
About the current drive power amp, I make it a new thread, because it is about very different topic in Current Drive Power Amp

[fdegrove]

Hi,

Like one post said, is it true dual differential gives less harmonics, because they cancel each other? What is really the benefit of having less harmonics? Isn't that tubes have warm sound because they have big harmonics?

No, differential would give less distortion with less even harmonics due to distortion cancelation.

Tubes can be made to sound icecold just the same.
The idea of tubes having a warmer ( richer?) sound is just plain BS.

Cheers,

[dimitri]

Jc wrote: The complementary differential has lower distortion, all else being equal.

This is absolutely right. The differential pair alone is symmetrical, but when the signal is passed on to the VAS stage, it loose symmetry in current mirror, or in resistive load, or simply the VAS stage transfer function is not symmetrical.

[hitsware]

>Tubes can be made to sound icecold just the same.

Ya Mon ! I once had to modify a tube preamp to make it sound like a tube preamp.........mike

[AKSA]

Yes, true, the fully complementary input stage should give less distortion because of even order cancellation, particularly at the voltage amplifier, which is then fully complementary as well.

The single diff stage will not, however, have particularly low TOTAL distortion, other things being equal. Its configuration merely gives an inverting node at like potential - the base of the feedback transistor - so that like phase signal from the output can be injected as negative feedback.

Truth is, the diff pair as used in most SS amplifiers is dual input, one inv and the other non-inv, but single-ended output. This is an important distinction.

If it were used as a differential output then its even order distortion would be nulled. Lots of people would be happier....

No, as was suggested, the chief benefit of a dual diff pair input stage is that it permits use of a fully complementary voltage amplifier, which will null its own even order distortion, leaving only very low levels of odd order.

However, and here's the rub, do we want to reduce the even order distortion, while leaving the odd order essentially unaffected? Do we really want to skew the formation of harmonics towards low levels of odd-order, with even-order submerged below the noise floor?

I would suggest we don't. It just ain't musical. Another point which NP made, a bloody good point, is that a single diff stage leads to a single-ended voltage amplifier, which is almost always supplied with current from a current source supported from the rail. This arrangement facilitates marvellous offset control - no small feat.

The feedback node on a SS amplifier is required to undertake two tasks; AC voltage feedback, very important and quite obvious; and DC offset control, holding the speaker output at very close to zero volts. This task is vital; it's the amp equivalent of a regular heartbeat for a mammal. If the output deviates even a couple of volts from zero (and the nature of offset control failures are normally rail excursion catastrophes), then the speaker either overheats, or dies outright, and often takes the output stage with it too.

So, here are two very good arguments for a single differential input stage. Of course, I need not emphasize that this topology halves the component count, and is cheaper, simpler - less to go wrong! (that's the third argument!)

Cheers,

Hugh

[jcarr]

A comment on terminology and how it is used by different people to mean different things:

At least in Japan, "dual differential" specifically means a topology in which an input differential pair feeds its output to another differential pair (in series). A topology with two input different pairs - one N and one P - is called (at least in Japan) "complementary differential".

I raise this issue because both of these terms are commonly used in catalogs and brochures published by the big Japanese electronic conglomerates (and other audio manufacturers), and unless you are clear as to who is using these terms and what what they mean by doing so, you may be left with a totally erroneous impression of what the circuit topology is.

Previously alluded to in this post.

http://www.diyaudio.com/forums/showt...889#post258889

hth, jonathan carr

[lumanauw]

So, we've got 2 confincing answer about single differential from AKSA and Mr.Pass. Is there any designer from pro-audio that used to applicate dual differential? What is the main reason to use dual differential? Is it just for easiness, or symmetrical look? Even though it LOOKS symmetrical, I think npn and pnp are not exact opposite in behavior. So if the behavior is not an exact opposite, would it be that the output like "have a shadow" or "blurry"?

[AKSA]

Thank you JCarr,

I stand corrected. When I mentioned dual diff input stage I meant fully complementary diff input stage. I see it might have meant one diff pair as VAS following the input diff pair.

There is an engineering elegance in the complementary diff input stage, and there is no doubt it can be made to sound very good. As PRR remarked, a lot of the sonics are in the fine detail; components, dimensioning, layout, etc. But we all have a belief structure, and I lean towards the single diff.

Cheers,

Hugh