Is a Trident Fleximix pre a class A circuit?

[Gregg Sartiano]

A potential buyer is asking me. Anybody know?

[ulysses]

The fact that your buyer is asking demonstrates that (s)he probably doesn't know what "class A" actually means. The correct answer is that it doesn't matter.

[primalsteve]

Actually the correct answer is hell no!. It's not even discrete if I remember correctly. I seem to remember IC's all over the place in that thing. Kind of on the level of an 80b module. Rumor is they were designed while Queen was signed with them. It was supposedly designed to be their live desk. They sound kind of cool. As far as I know the only "Class A" item ever made by Trident was the "A-Range" console.

I'm no technician but a technician once explained to me that one of the first signs of whether something is Class-A is whether there is a bipolar supply. If you see a PSU supply that has an audio voltage that has separate + and a - voltages there is a good chance it is not Class A. Something about "push and pull".


Steve Mabee

[Jim Williams]

Those pre's are opamp based with a pair of TL071's in front of a transformer. I decribed the circuit and a fix for the loading problems of it in previous posts. These could be pushed into Class A if a current source is used to "turn off" one of the opamp's output transistors giving a single ended class A output up to around +6 dbu. A simple 4.7 to 7.5 k resistor from the - rail to the opamp output pin is all that's needed to do this.

Jim Williams
Audio Upgrades

[wwittman]

The B Range desks were also discrete Class A.

[Jim Williams]

I'm not too sure about that. I remember the opamp designs used a pair of complimentry output transistors in class AB although the input and driver transistors were class A.

It's been a few years since I've worked on one.

Jim Williams
Audio Upgrades

[Wavebourn]

Originally Posted by Jim Williams

I'm not too sure about that. I remember the opamp designs used a pair of complimentry output transistors in class AB although the input and driver transistors were class A.

It's been a few years since I've worked on one.

Jim Williams
Audio Upgrades

It is simple to convert them to class A soldering one resistor per opamp. However, as the result it will draw more current.

PS: recent British patent converts power amps to A class using similar approach.

["DB"]

Originally Posted by primalsteve

I'm no technician but a technician once explained to me that one of the first signs of whether something is Class-A is whether there is a bipolar supply. If you see a PSU supply that has an audio voltage that has separate + and a - voltages there is a good chance it is not Class A. Something about "push and pull".
Steve Mabee

Well, I agree with the part about not being a technician, and probably the guy who told you that as well.

From the Wikipedia:

"Amplifier classes

Amplifier circuits are classified as A, B, AB and C for analog designs, and class D and E for switching designs. For the analog classes, each class defines what proportion of the input signal cycle (called the angle of flow) is used to actually switch on the amplifying device:
Class A
100% of the input signal is used (conduction angle a = 360° or 2π)
Class AB
more than 50% but less than 100% is used. (181° to 359°, π < a < 2π)"

Has nothing to do with power supplies, a class "A" amp can be designed with bi-polar rails or just one power rail, same is true for class "AB".
The class "A" amp, being "on" all the time tends to run hotter.

OBTW, the Fleximix was an IC console, class "AB", and sounded (IMAO) great.

[ulysses]

Originally Posted by primalsteve

I'm no technician but a technician once explained to me that one of the first signs of whether something is Class-A is whether there is a bipolar supply. If you see a PSU supply that has an audio voltage that has separate + and a - voltages there is a good chance it is not Class A. Something about "push and pull".

It's so easy to take a tiny little fact and extrapolate it into something utterly false. The tiny little fact is that if an amplifier is NOT push-pull - that is, if it's single-ended - then it had better be class A if you want it to produce audio that's not horribly distorted (as in beyond intelligibiity). That's the end of the tiny little fact. This fact doesn't say that push-pull amps can't be class A (they can) or that single-ended amps have to have a single-ended power supply (they don't) or that push-pull amps have to have a bipolar power supply (they don't). And the big gaping hole in this fact is the point of my previous post: As DB's Wiki quote should make abundantly clear, the class of amplifier operation has NOTHING to do with sound quality.
So please let's not confuse amplifier classes with the utterly unrelated Stereophile Magazine hi-fi gear review ratings, or grades of eggs, or grades on report cards, or degrees of burns or homocide, or any other qualitative rating system. Amp classes serve one purpose and that is to denote operating conditions of the output devices.

["DB"]

Very well said ulysses.

[Wavebourn]

Originally Posted by "DB"

Very well said ulysses.

...except one little fact: other than A class amps were invented in order to save big part of electricity loosing small part of sound quality.

Otherwise, his speech is well done.

[ulysses]

Oh, yes. Class A amps are the least efficient, that's true.
Your suggestion that sound quality is inherently lost is pure bullshit though.

[Tim Farrant]

And I'll add to ulysses comments that Class AB amps are in fact also Class A to the point where the AC (audio current) demanded by the load exceeds 1/2 the AB stage bias current.

Thank you
Tim.

[Wavebourn]

Originally Posted by ulysses

Oh, yes. Class A amps are the least efficient, that's true.
Your suggestion that sound quality is inherently lost is pure bullshit though.

Your suggestion that my suggestion is pure bullshit is the poorest bullshit.

[monkeyt]

I'm so sick of hearing people talk about Class A.

My freaking toilet is Class A, i still take a dump in it!

[JohnRoberts]

Originally Posted by monkeyt

I'm so sick of hearing people talk about Class A.

My freaking toilet is Class A, i still take a dump in it!

I'm inclined to agree with that sentiment and am glad my toilet isn't push-pull.

There is IMO way too much halo effect surrounding "Class A". Sound quality isn't defined by the topology but the quality of the execution of whatever appropriate topology is used. Lets not forget that lots of cheap battery powered consumer gear was Class A.

Regarding the rather old trick of forcing an opamp's output stage into Class A operation, it is worth note that this primarily affects the output driver but could also introduce an small input error to account for AC zero now being 1/2 FS at the output. In a well designed opamp this asymmetric load could imbalance internal thermal dissipation and perhaps cause input tracking errors in critical designs. This may have offered some small benefit to a very slow early technology opamp, with modern parts if anything it will likely be neutral or negative for performance, drive capability, power supply, thermal, etc.

JR

[ulysses]

The myth that Class A amps offer some sonic advantage comes from a few particular sources, and acknowledging those sources might go far to illustrate that there is no real substance behind the myth:

1. Stereophile magazine offers a rating system alongside its equipment reviews that uses the same "class" name designation, with of course Class A being the best. This rating system is completely unrelated to the system of classifying operation of amplifiers, and is an absolute red herring.

2. Push-pull amplifiers have an inherent cancellation of 2nd-order harmonic distortion. Since 2nd harmonic is generally pleasing to the ear, and single-ended audio amps are necessarily operated class A, there is a mental correlation that actually has nothing to do with class of operation (since a push-pull amp can be operated class A and still lack the pleasant 2nd-order distortion).

3. Because power supply requirements are often the limiting factor in amplifier design, a class A amplifier will run out of headroom more quickly than a class AB amp run off the same power supply. This often translates into higher distortion. Many users have their first exposure to different Classes of Operation in the world of tube guitar amps, where high distortion is generally greeted warmly. The particular types of distortion associated with tube amp overdrive (soft saturation of both triode voltage stages and output transformers; power supply sag; etc) has been incorporated into the popular musical vernacular, and so terrible amplifier performance is not only tolerated, but embraced and even sought after.

4. Many marginally-informed enthusiasts are under the mistaken impression that Crossover Distortion (a much less pleasant-sounding type of distortion) cannot be avoided in an amplifier without running it Class A. Judgements about entire classes of operation are based on a few experiences with amps that are malfunctioning or poorly designed. This is misguided, as there are literally billions of amplifiers in use in the world right now operating in classes B and AB that do not produce audible (or even measurable) crossover distortion. Proper design, correct bias, and negative feedback work very well to eliminate this and many other sources of distortion in amps regardless of their class of operation. The fact that popular music is not based on this type of distortion should be adequate evidence that the classic Class AB amplifiers (used in the creation of what we've come to know as musical-sounding) do not typically exhibit this type of distortion - at least not when they're functioning correctly.

[Paul Gold]

I propose making "doped silicon wafer" cool. Kids like dope.

[wwittman]

Here's what's not a myth

some mic pres sound a lot better than others.

some of the best sounding ones are Class A

that may not be the reason, but it may be.

[ulysses]

Originally Posted by wwittman

Here's what's not a myth
some mic pres sound a lot better than others.
some of the best sounding ones are Class A
that may not be the reason, but it may be.

To put it another way:

some mic pres sound a lot better than others.
some of the best sounding ones are not Class A
that may not be the reason, but it may be.

or

some mic pres sound a lot better than others.
some of the worst sounding ones are Class A
that may not be the reason, but it may be.

Is it starting to sink in? Or do we need a bigger hammer?

Try looking at some other variables. Many of the classic preamps have single-ended output amplifiers, output transformers, and DC current for the amplifier running through the transformer primary. Each of these things is often, but not universally, associated with Class A operation. And they each have far more impact on the sonics than CoA does.

[JohnRoberts]

I think it's good that consumers try to learn about the inner workings of their gear but it often reduces to oversimplifications and trying to attach significance to things like brand of capacitor used, or general circuit topology. The unfortunate reality is there is a great deal of importance in the small details of how any given approach is executed. To wit a cheap electrolytic applied well will outperform an expensive one used poorly, etc.

I realize this sounds like the classic trust me, you just don't understand rant, so I won't say that. I will say only that it isn't that simple. All class A circuits do not sound the same. While they will have some common attributes, these alone do not completely define how a circuit will perform or sound.

In response to the correlation between a number of well respected products all using class A topology, if I were designing a high end product to merchandise to consumers who are predisposed to expect class A to sound good, why not play into that prejudice if it doesn't cause any major compromises otherwise.

A number of products have been within a fraction of a dB of theoretical performance for decades now. In such cases merchandisers need to find other ways to stand out and differentiate themselves. If a product sounds different than the pack, as often as not that deviation is probably an error of some sort. Ideally this error will be a euphonious coloration and perceived as superior (especially if the product is expensive enough).

Good luck and caveat emptor.

JR

[Wavebourn]

Originally Posted by JohnRoberts

Regarding the rather old trick of forcing an opamp's output stage into Class A operation, it is worth note that this primarily affects the output driver but could also introduce an small input error to account for AC zero now being 1/2 FS at the output.

Also, it increases current in VAS, as the result it is faster. Pitty, no pin is connected to diffpair 's tail to bypass an odd transistor and feed NFB directly to the emitter.

[Wavebourn]

Originally Posted by ulysses

2. Push-pull amplifiers have an inherent cancellation of 2nd-order harmonic distortion. Since 2nd harmonic is generally pleasing to the ear, and single-ended audio amps are necessarily operated class A, there is a mental correlation that actually has nothing to do with class of operation (since a push-pull amp can be operated class A and still lack the pleasant 2nd-order distortion).

"Push-pull in class a" sounds like "saddle for a cow".

["DB"]

A good example of why the "class 'A" is better than class 'AB' statements" are so meaningless can be found in the Distressor (which has both in it). Here's what Dave Derr has to say about it:

"Dave Derr
15th October 2005, 06:26 PM
Im still waking here on a Saturday afternoon and figured Id put my usual, irritating two cents in.

Since the Distressor has been mentioned in the post a few times, Id like to point out something ironic. We use IC opamps all over it. Before we calibrate the unit, we jump out the VCA and disortion generator. We then measure the noise and distortion and make sure the circuit is really really clean and colorless. Typically the distortion measures around .006% and is all harmonic in nature, 2nd or third. The -3dB point of the frequency response is like 2Hz to 200kHz. Soo the opamps are designed to be really clean and linear.

THEN.... we jump in the VCA and distortion generator. Everything changes. These circuits are what add all the "sound" of the Distressor. And Interestingly, the VCA is a class A discrete circuit, biased on a little bit all the time.

If you haven't gotten the irony yet, folks have mentioned that we use opamps all over, and yet the sound of the Distressor without compression, is predominantly the result of a discrete class A circuit! Class A circuits rarely act as colorlessly as a well designed IC opamp circuit, and so, those that say Class A are more linear and accurate, may have been mislead. In fact, in the Distressor case, we use the Class A circuit to do all the coloring!

One other thing, I have been using IC opamps for almost 20 yrs, and have never seen any sign of crossover distortion with analyzers. On a daily basis, I often look at the distortion residual from my Audio Precision. This is what's left over after the original signal has been removed, and again, I have never seen a half way modern IC Opamp create crossover distortion with its accompanying sharp edges and non-harmonic distortion. So I'm confused with the concept that IC opamps somehow have crossover distortion when after all these years, Ive never seen a single sign of it in a semi-modern, correct functioning op amp IC. Even textbook writers mention it, but if a lowly TL072 or NE5532 create crossover distortion when driving a decent load, I HAVE NEVER SEEN IT! I swear to ya."


The only place I ("DB") have ever heard crossover distortion (except for stuff clearly in serious need of repair) was in real cheap CB radios that used a class "B" amplifier, and most people know that really nasty "CB radio" sound; if API, Neve, Trident, etc. class "AB" gear sounded like that I doubt anyone would use it.

Mic pre's, and other simple circuits like limiters, or a guitar amp are fairly easy to design and build class "A", and to build with discrete components. Many circuits are easier, and cheaper to build class "AB" with chips. I suppose it would be theoretically possible to build a discrete, class "A" DAC, but who would want to try? It would take hundreds of transistors and would probably double as a great space heater.

Most of the people who have posted replies are probably listening through power amps that are class "AB","D","E"', "F", or "G", so the old saying about "the weakest link determining the sound of the whole chain" would seem to apply. And if they are happy with their chain with big hunks of non-class "A" in it, then it really is ALL about the design and the implementation, not the class.

Kind of like: what's the difference between an oboe and a bassoon?
Not much, but the bassoon burns longer in the fireplace.

It is still comforting to know that thirty years later I still only agree with WW about half of the time.

[JohnRoberts]

Originally Posted by Wavebourn

Also, it increases current in VAS, as the result it is faster. Pitty, no pin is connected to diffpair 's tail to bypass an odd transistor and feed NFB directly to the emitter.

I'm unfamiliar with VAS terminology (perhaps voltage amplifier stage?). Biasing the output will indeed change the operating point of that stage too. Of course you need to be sure to apply your pull-up or pull down output resistor to the correct supply or you could reduce the nominal current in that stage.

I stand by my original assessment that this might be of limited benefit to very early opamp technology but if you're going to muck around inside, and have the power supply capacity to burn just swap out the opamps for something from the last couple of decades, for even better performance.

JR

PS: If you think class A is the bee's knees, disregard my suggestion. Saying your product is now class A sounds sexy, in an Austin Powers way.

[Matt Syson]

In a vein similar to the last few posts.
Mr Neve designed an EQ (in a black box) for AMEK a long time back, which had a 'warmth' control. It used a VCA to deliberately add distortion. Early versions suffered some complaints because the effect was too subtle and it was assumed that the circuit was not working. Later units were modified to be able to provide up to around 2% from the original 1% range. Forgive me if the actual values are incorrect but it was certainly in the whole numbers not the 0.004% which seems to upset so many people.
Matt S

[wwittman]

Care to site a few examples of the WORST sounding mic pres that were also Class A?

[JohnRoberts]

Originally Posted by wwittman

Care to site a few examples of the WORST sounding mic pres that were also Class A?

Um, the one in your cell phone... dictation machines, cheap cassette recorders, etc.

JR

[ulysses]

Worst-sounding class A mike preamps? How about the ones in those little battery-powered Shure mixers? There are probably worse, but those ones are pretty bad. Similarly, the Shure Level Loc would be a good example of a bad-sounding compressor (same kind of circuitry, same crappy transformers). Once again, the CoA has absolutely nothing to do with why they sound bad. But you asked.

I could also point you to piles of $30 "class A discreet" phono preamps that are noisy, have low headroom and high distortion, and sound like total ass too. The even cheaper opamp based units wipe the floor with them.

But the point (once again) is not that one CoA or topology sounds better than another. The point is that these are not the determining factors for sound quality.