# Dual-differential input stages

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#### Jon T

I'm wondering if anyone can explain to me what the advantages (and disadvantages) of the dual-differential input stage are? I've seen this design used in a handful of freely available schematics (Pass A75 & Leach Amp) and bragged about in some commercial amps' marketing hype, but no one seems to have any information about why one would want to use this configuration versus an ordinary differential pair.

Thanks!
Jon

#### vdi_nenna

Paid Member
I think it has to do with Common Mode Rejection Ratios. Cancelation of differences or similarites...I'm not positive of which, but read the Pass/ Thagard A75 amp article at Passlabs.com. I know it's explained in there. You can learned alot from that article.

#### Anagod

Differential Input

Balanced or differential input ignores any signal that is of the same polarity and is present on both inputs at the same time. This is called common mode rejection, and the more balancd or symmetrical the input is, the higher the ratio will be(CMRR). This topology does not produce second order harmonic distortion, only third. I have built both tube and transistor devices with that topology and find it to exhibit a clean clear sounding midrange. Try it.

#### Jon T

I'm familiar with the advantages of differential input stages. Definitely the way to go for solid state, though most tube designs I've seen shy away from it, for some reason.

I'm curious, though, about using dual differentials in the input stage. In this configuration, you have two differential pairs, one of NPN devices and one of PNP devices. The balanced input signal drives both of these diff pairs (or one side of each with the other sides grounded for single-ended inputs). The amps that use this configuration seem to use balanced circuitry throughout (i.e. dual voltage amp and driver stages, all implemented with complementary devices). Of course, the output stage is push/pull using complementary emitter followers.

The two public domain examples that come to mind are the Pass A75 and the Leach Amp, neither of which really explain what the advantage of this fully-balanced approach is. If you look at these schematics, it's almost as if there are two amps on top of each other: an NPN one for the positive signals and a PNP one for the negative ones.

Ideally, I can see how such a balanced approach will result in great CMRR for noise and cancelation of distortion. However, there are no "ideal" devices, and I can't help but think that since NPN and PNP devices are never exactly complementary, you'll introduce asymmetries in the output, which would essentially be second harmonic distortion. I guess enough feedback will clean this up, but I'm of the ounce-of-prevention-beats-a-pound-of-feedback school of thought.

I'd be curious to hear what a fully balanced design that uses only one type of device sounds like. Basically the idea would be to use one or more diff pairs as voltage amplifiers and then a totem-pole style output stage. I bet you could get the balance between the two halves very close, but of course, then you'll have to deal with the asymmetric impedance of the output stage. I saw a push/pull 2A3 amp that used this design, but of course it had a transformer to handle the output.

BTW, anagod, are you using balanced interconnects, too? Do you find this makes a big difference?

Thanks!

-Jon

#### promitheus

Using a differential input stage helps you combine easily two signals in an amp. The input signal and the negative feedback signal from the output. It helps the cmrr to be high. The dual differential input has 2 of these circuits one with a pair of npn transistors and one with a pair of pnp transistors. They both are joined at the bases of the transistors. You can see that in a schematic better then I can describe. The Elektor magazine amps that are the best projects around for non class A use them all the time. The use of the double differential input makes the whole circuit fully symmetrical from input to output making this easy to have low distortion specially from even harmonics.
So even if the whole thing sound more complicated to use it makes the amp design easier.

#### piertr

Dual differential input stages have nothing to do with common mode rejection ratios and something (not everything) to do with even order distortion cancellation. There is a big difference between a differential or "balanced" input and an emitter-coupled differential pair input stage. In the case of the latter, having two of them (complementary, as in the Leach amp), the benefit is a slight decrease in even order distortion. The downside is that it may be more difficult to compensate (two amplifier halves need compensation, not just one), and the amp becomes more expensive due to more PCB area, more parts, etc. Leach talks about this on his web pages. Whether this amplifier 'architecture' sounds better or not is questionable. After all, likely the reason so many people follow the "simpler is better" philosophy religiously is that there's usually more second order distortion, which has been proven to be euphonic. Of course, then you're listening to your amplifier and not your source material as it was recorded. It's all personal preference.

#### thespeakerguy

One advantage, though not significant in audio power amps, is the first order cancellation of the input bias current. The input impedance is increased too. Again, not a significant gain, just an observation for those who may want to know.

#### Michel

thespeakerguy
incorrect. input impedance is halved as the dual differential input stages are in parallel at a.c....however this is a moot point, as input impedance is determined ultimately by value of input resistor to ground. cmrr is a non-issue in a compentently designed amp. (see self's book)....really no discernable benefits to dual differential pair in practice.

#### P.Lacombe

Michel,

Because of the cancellation of the input current, the resistor connected across input terminals can be high, without disturbing the input transistors bias.

Thus, the input impedance can be higher... but this is not very important.

Regards, P.Lacombe

#### Xavier

Guys, are u talking about a differential input stage or differential input with mirror-image.

Differential Input Stage:
- To provide feedback, so that only the error signal is amplified.
- To avoid DC offsets at the output if its DC Coupled.

Mirror-Image Differential Input Stage:
Besides the advantages mentioned above, this provides excellent PSSR and Slew rate.

PS: If I have misunderstood ur question, pardon me.

-XL.

#### P.Lacombe

Xavier

I've understood we are talking about *double* differential input stage, with one npn differential pair, and one pnp differential pair, base connected together....

Regards, P.Lacombe.

#### promitheus

Yeah I think that is correct. One differential towards the plus power supply and current source towards negative power supply and the opposite.

#### Xavier

Then I'm right u'll are talking about Mirror-image differential Input stage( One differential stage complementary to other and their bases tied together).As I have said before this provides excellent PSSR and slew rate.

PSSR:
An amplifier with single differential input stage can be made hum free at ideal( without input signal) but its difficult to know whether it will remain hum free at all loads, dual/mirror-image differential stage deals with it seemlessly.

Slew-rate:
In an amplifier slew-rate is defined by how Cdom sources and sinks current while input stage sinks and sources. Sourcing and sinking of currents through Cdom( Capacitor at dominant pole) does not follow the same positive and negative levels because the current passes through two different routes. This leads to asymmetry in the output. Since dual/mirror-image differential stage is complementry to each other the output is exactly symmetrical.

Complementry actions cancels transistor parameter variations.

#### promitheus

For me this is the best kind of input circuit for an amplifier. It has excellent performance and it solves a lot of problems.

#### Michel

john t
tube designers do not shy away from dual diff. stages....they have nochoice as there are no complementary tubes.

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