Single or dual differential?

[Steven]

Even without biasing of the bases of Q14 and Q15, these transistors still act as regulators. They have only limited collector-emitter voltage, though. For no ripple on the supply line the collector will be 0.7V more positive than the emitter (for Q14). That means that the supply may go down approx 0.7V before Q14 will go into saturation, and 0.7V is not much for ripple on the supply of a power amp. But as long as the ripple is less than that, Q14 will regulate and lower the noise/ripple to the pre-stage. If the supply goes down more, the collector base diode of Q14 may go into conduction and C6 will be discharged, and consequently C2 will not be charged anymore and will loose voltage after a while, dependant on the value of C2.
I think I would put protection diodes from the emitter to the collector of Q14 (and Q15); anode to E(Q14) and cathode to C(Q14), for Q15 the other way around. These will protect Q14 from being damaged if the supply goes down quickly and C2 is still charged. The emitter base diode of Q14 will zener, the base collector diode will conduct and C2 is discharged to the supply via Q14. Q14 may not survive, depending on transistor type and value of C2. For Q15 a similar story.

Steven

[lumanauw]

Q14 and Q15 is capacitance multiplier. The basic equation is just simple, how many current you wanted to pass Q14 and Q15, divide it by Hfe, and there you have the C-B resistor. You can put zener to ground, but it will be zener regulator, not cap multiplier.
It acts to multiply the capacitance, so there is capacitance from base to ground.

[lukio]

Hi...Lumanaw

The different between pre and power amplifier basic design are 1. bias current at driver stage to achieve the stability at desired bandwith and
2. most linear region transistor operation at high signal level
3. capability to drive lower impedance load..



Cheers,,,

Lukio

[mikeks]

Originally posted by PRR
> You folks are worrying too much.

Who is worrying? lumanauw has an idea that one topology is better than another. In the hands of a skilled experienced designer, this may be true (though all-else is never equal). But at lumanauw's apparent level of learning, it is important to realize that no topology is "unavoidably" better than another. We can easily design a "dual differential" that is bad (heck, I've done that), and a "single differential" that is better. Many sing-diff inputs work very well. Complementary diff has some "obvious" advantages, but they can be botched with careless detailing. lumanauw's theory may be right. But he needs to understand that the devil is in the details.

That's where Doug Self is useful. He has worried the Classic Topology to the bone, and pointed out many "small" details where most designs could be significantly better. The concept and many of the details can be applied to other topologies. But you have to learn them before you can apply them. Self teaches this well. He gets fabulous performance out of something that looks like a 1972 Fisher. We can criticize that he works mostly by measurements, not by ear. But his details look like things that "should" be better to the ear, and others with golden-ears can judge his work for themselves. There are details where I disagree with his theory, but he is honestly investigating and sharing which is all anybody can do.

> you lose GAIN (6db)

Too true. Though gain in BJT is cheap. And with overall feedback and slow output devices, gain has to droop inside the audio band to keep the loop stable. That -tends- to lead to a design where almost all gain is made in one stage, usually the second stage, so the total gain can be compensated to one-pole response. With the huge potential gain of BJTs, this usually means deliberate reduction of input stage gain with emitter resistors.

FETs is different. Gain is harder to come by. In a sense, they already have the "emitter resistors" built-in that we add to many BJT stages. If you can find semi-matched complements (I sure agree that small mismatching is no big deal) then all-complementary is one preferred topology.

> you INCREASE even harmonic production, because you are not equally driving the second stage transistors.

Less if you detail so the standing current is very-large compared to the signal current (base current in the output stage). This is one place where many BJT designs come up a dollar short.

I'm also, at my current level of understanding, becoming less concerned about THD and more about distortion spectrum. A spectrum with no even-order is un-natural. If the slope of the spectrum is steep and/or all products are far below system noise, that won't matter. And I'm not sure I can equate lumanauw's observation "more focus" with distortion spectrum.

Hi prr...you've become my all-time favovourite commentator on this forum with nuggets such as this.....cheers!

[mikeks]

Originally posted by john curl
Many advanced audio designers use techniques similar to or better than the concepts described in IC designs in previous threads. Often, we invented them first, such as the complementary differential input stage.

What advantages have you located with respect to the fully complementary (visually!) voltage gain stage vis a vis the generic topology in audio band applications??

[PMA]

Hello Michael,

the same positive and negative slew rate, e.g.

Pavel

[mikeks]

Originally posted by PMA
Hello Michael,

the same positive and negative slew rate, e.g.

Pavel

This is not an advantage, as you can have symmetrical slew rates with the conventional design...subject to modifications....eg bootstrap....

[thanh]

This is not an advantage, as you can have symmetrical slew rates with the conventional design...subject to modifications....eg bootstrap....

Mieks! What thing make you can continue to discuss?

[ppl]

I have been working on a DIY Headphone Amp using a similar Folded Cascode topology and I find mine has a lot in common with this




A lot of researching reveals also that this is a popular implementation of Folded Cascode Amplifiers I see similarities in these circuits to early Solid state Luxman Amps, however As expected each designers implementation of this idea while generically similar are unique in individual details. One classic variation is if a Diff amp is included in some designs such as the design I just cited, while others dispense with this additional diif amp in the second stage as is done in this Implementation,

in addition other subtitle variations include to current source actively or passively along with other housekeeping duties. Feedback and compensation are also critical. This brings me to my question Referring Back to the first design the Load Resistors for the vas stage design the 47k to ground from the Collector and emitter of the VBE multiplier. This appears to be the most popular method however I was thinking of returning them to the inverting input sum node along with the Overall loop feedback loop. ( Again I cite this design)



I would like your views on these two methods as I hope to be able to make my design stable without any compensation capacitors and yet retain a comfortable Amount of overall feedback. The second method appears to have the advantage of using the vas second stage load resistors as an additional local feedback loop around just the Voltage gain section and thus not have to deal with the excessive phase shift of the output stage. I understand a simple solution is a cap from the collector of the Vbe multiplier to the inverting input nd in effect rolling off the bandwidth of the vas stage to a point the output stage can deal with. In my version I am using a three stage Darling connected complementary output so stability is an issue with my wish list however I want to add to that list no frequency compensation capacitors be required.

[lumanauw]

Hi, PPI,

I've got (bad) experience with folded cascode. If your output stage is classA or having error correction scheme, you can use folded cascode and have good result.
I made low biased classAB without error correction, and the front end is folded cascode, it's just not working. OL gain is lack too much, the sound is like all treble to the music.
It is right that folded cascode (ie : low OL gain) can dispense stability caps like miller cap, and with folded cascode usually stabilizing is done by 5pf-20pf cap from collector of folded cascode to the inverting input, or even no cap at all, but the main problem is how many OL gain minimum is required to fix all the bad things in the whole amp? Sometimes folded cascode just don't provide enough OL gain to be burned for good CL gain performance.
Other method to linearize OL curve (beside putting R from collector of VAS to ground) is to put 220k in the place of miller cap (base-collector of VAS) and this works good too But my experience, load to ground or 220k in VAS local loop does not affect stability much, the best way to stabilize an amp is still put caps.