preamps & amplifiers with no negative feedback

[tubesound]

Are these the only Conrad Johnson amps and preamps that use no negative feedback?

Amplifiers:
Premier 350
Evolution 2000
CA200

Preamps:
Premier Seven
Premier Ten
Premier Fourteen
Premier 18LS
Evolution 20
Evolution 20SE

[AnotherJohnson]

Regarding the preamps, there is no mention of negative feedback for either the GAT or ET7 series.

Frankly, I don’t think negative feedback would be needed in these small signal single ended triode (hence Class A) preamp circuits.

As for the amps, which are often push pull designs, some negative feedback is necessary for stability as gain factors rise. Both ARC and CJ use small amounts.

Feedback is always used to control distortion figures. There is a summer/comparator, and the feedback maintains linearity between the shape of the input wave and the output.

Both ARC and CJ seem well aware of the transient and phase shift issues that feedback can introduce… so both companies do their level best to find the right compromise. You’ve got to have some for bandwidth and stability.

Design features are only interesting when they manifest themselves in the form of better sound. CJ and ARC have this figured out, at least when my ears are the judge.

[tubesound]

Thanks!
How about the CAV 45 circuit? Does it use negative feedback?

[AnotherJohnson]

Maybe Roberto or RJ can answer definitively, but a pair of EL34s together in push pull would need some negative feedback, so I expect some is used.

FWIW, I ran across a feature in some Manley preamps where they let the listener adjust preamp negative feedback.

[roberto]

Hola tubesound,

AJ is right, the negative feedback is needed, but the amount is nothing. Just to have a stable circuit design and prevent to have an oscillation that without the feedback, it might occur. Yes, there are some models that are zero feedback, but the circuit topology used is different than the push-pull. The new circuit topology is to design and build the shortest signal path with the best electronic components that the standard electronic industry produces.

The result is an outstanding sound with incredible signal to noise ratio (over -100dB) with a richness sound fulled of pure harmonic texture of the musical instruments and voices. The CAV 45 S2 is fantastic. I had it for some time, and really has the signature of Conrad Johnson quality sound. Here is an explanation of it:

...The use of carefully selected, high-quality parts enables the CAV45-S2 to properly exploit the dramatic potential of a control amplifier to enhance the immediacy of musical experience afforded by fine recordings. All resistors in the audio circuits and related power supplies are precision metal film types. With the exception of the large power supply reservoir capacitors (which are specially chosen electrolytic capacitors bypassed by polypropylenes), all capacitors in these circuits use polystyrene and polypropylene dielectrics...

You can't go wrong with it...have a careful audition and let your ears decide. Your ears are the final judges, not specs.

Happy listening!

[AnotherJohnson]

I do think that this is another red herring issue, like slew rate. And I’m not referring to any specific people… I’ve even been guilty myself on similar follies.

People latch onto the spec as a descriptor, and assume based on spec sheets that one amp is better than another without listening. Listening was really the driving force behind the establishment of ARC, CJ, Stereophile and TAS. The fact that Stereo Review and their contemporary counterparts, and the Japanese designers who shaped the mass market amps, seemed to have tin ears, drove the former to success.

I mention slew rate as another red herring. It shows up as a nebulous spec on many spec sheets. Higher is automatically “known” to be better. But your ears can tell you a different story if you listen long enough.

There is an anecdote about this in ARC’s 50th Anniversary book. They’ve had legendarily amps that had very modest slew rates.

If amps are judged based on listening to music, negative feedback and slew rate aren’t linearly, or even monotonically, related to performance. Some of the lousiest sounding amps have some of the “best” specs.

If it sounds good, it is good. If it measures good but sounds bad, it is bad.

[Big Dog RJ]

Yes, correct there AJ! Good points.

The CAV45 does use a bit of negative feedback, as is the case with all of CJ's latest offerings, from their LP series onwards. It is definitely required to keep things stable and distortion free in such top grade circuits.

Speaking of specs... there was this chap who harped on specs & nothing but specs! So just to pull his leg, another member posted specs of one particular amplifier, and this guy was very highly taken up by it, that he proclaimed thus is the best amp so far that he's ever come across... then the other guy asked would you like to try it? And he jumped up & down in excitement wanting to know what that brand of amp was... it turned out to be a Kenwood car amp!!! Go figure

Correct on the Manley Labs gear. With the Ref350's that I had, the monoblocks had adjustable feedback on 3 separate settings. It was fun to begin with but later proved to be useless as you'd have to constantly make changes each time you loaded different kinds of music.

Cheers, RJ

[AnotherJohnson]

There is another “aside” related to the 70’s when Bill Johnson, Bill Conrad, Lew Johnson and others were swimming upstream against the SS mania.

McIntosh was abandoning tubes. One of my McIntosh heroes, Gordon Gow, maintained to the end that SS was better. Of course eventually McIntosh came back to capitalize on the market success of the tube resurrectors. They even built a commemorative model of the Mc 275 in honor of Mr. Gow.

Bill Johnson, on the other hand, believed that for the same level of design and development effort, tubes will always beat SS.

And obviously Jeff has abandoned SS at CJ, focusing on tube based products (although both ARC and CJ seem to sprinkle FETs and other bits within their mixes).

But my point is that as many critical listeners were choosing tubes, McIntosh was focusing on SS and arguing that it was better. As good as McIntosh is, I don’t think that listening (except for noise floor and possibly rms power) has been their strongest suit.

[tubesound]

Thanks everyone. The CV 45 is ab amplifier I really look forward to hearing!
Out of curiosity, when the CAV 45 is setup in the triode configuration, does the circuit still use a bit of negative feedback?

[AnotherJohnson]

There is a user who goes by Joenid who has not posted for a while. You might pm him.

There is no factory mod to turn a CAV 45 to triode mode.

There is a triode first stage to the amp, but the output is by a push pull EL34 configuration.

Joenid used to post things that make me think he would be the best one to address your specific interests.

[tubesound]

Thanks AnotherJohnson, I didn't realize that the CAV 45 can't be configured at the factory to run in triode! I will look into it.

[AnotherJohnson]

Sometimes JF will authorize modifications that are not on the official list, like in cases where a phono preamp is to be used with a specific load other than one from the normal list.

So you could inquire of CJ whether this could be done. Most likely power would drop from its already modest level. And I expect it would not be a low cost mod, just because of round trip shipping, the required new packaging to satisfy FedEx insurance rules, and the bench fee, plus whatever other changes would be necessary in JF’s view to make it first rate.

Have you heard one that was modified?

Most people rave about the performance of the stock s2 unit. It might be an expensive lesson … but expensive is definitely subjective.

[AnotherJohnson]

Someone else might do such a mod. I’ve had guitar amps that could be switched to triode operation. I’ve never found it worthwhile if pentode operation sounded good to begin with. But this is a matter of personal taste.

Once you go down the road of an unauthorized modification, it can be hard to sell the unit on the used market. Once such a mod was done, I doubt that CJ would help undo it.

[AnotherJohnson]

Another aside: Introduction of negative feedback is done by comparing the difference between the input and the output, aside from gain. The difference is fed back out of phase as a correction, with the goal of reducing the difference in real time.

It was used extensively to get distortion specs down to minuscule levels for rms power specs when driving resistive loads. However, it impacts transients and phase information, and is considered a major factor in amps that measure great but sound mediocre.

A good design respects negative feedback as producing both goid and bad artifacts. Designs that sound good typically use small amounts of negative feedback to achieve the good, without introducing much that is not good.

[AnotherJohnson]

And yet another diversion:

I recently picked up a Prima Luna Prologue 4, and when you started asking about feedback, something in my memory seemed to call up PrimaLuna as a company that had claimed improvements without feedback.

I just realized that this is the very claim they make for the Prologue 4 I’ve just finished evaluating.

From their web site-

*********
The ProLogue Four starts with the same basic topology as the top-rated ProLogue One integrated amplifier. But further improvements are realized from new driver and phase splitter circuitry. The result is no need for any negative feedback. This means even more natural tonality and better overall musicality.
*********

Well …. My own impression of it did not agree with this assessment. I would not have described the amp as having more natural tonality and better musicality. But maybe it does exhibit these characteristics when compared to an earlier model PrimaLuna that I have not heard.

The specs of the Prologue 4 look pretty darn good, but you don’t listen to specs. To my ears, it’s performance is commensurate with its price point, ie entry level.

[tubesound]

I haven't heard a CAV 45 that was modified, but generally I prefer the sound of SET designs to push pull. I should chat with JF about the possibility of getting a new CAV 45 with some factory modifications.

[AnotherJohnson]

I know guitar amps and audiophile amps are not the same.

But … some principles are the same. To quote Gerald Webber, “All single ended amps are class A, but not all class A amps are single ended.” And the idle current is key. You’d like it set exactly halfway from cutoff to saturation if you could get it there.

The sticky wicket with a push pull design, as you probably know, is the hand-off from one tube to the other. If this is done well, all but some gifted .1% would not notice switching distortion.

If you’re really tuned in, you may want to do some extended listening to candidates. Even consider travel to a good dealer.

One I could recommend would be Paragon Sight and Sound in Ann Arbor, MI. If you spend $10k, they’ll cover some of the costs of your travel. You can see the policy on their web site.
You could call them to discuss the audition in advance.

The one fly in the ointment is that they are not a CJ dealer, so you’d have to shift your interest in order to get a good audition.

It is REALLY hard to get a CJ audition in the US.

Lyric Sound was good until they dropped the line. Audio Arts was good, but seems to have dropped the line. Spearit Sound was good until they closed. CJ may be able to suggest a competent stocking dealer … maybe Steve Javaherian in South Florida.

[tubesound]

From conradjohnson.com:
"The MV55 was offered in both ultra-linear and triode configurations. Triode operation of the output stage achieves an exceptionally rich and musical sound, but at reduced power levels."

Can the MV45, MV50, and MV52 also be run in triode mode?

And in triode mode, could these, including the MV55, be run without any negative feedback?

[AnotherJohnson]

If you’re willing to solder and desolder, you can do just about anything with Classic tube amp circuits.

From The Gear Page (a guitar gear website):

“It's an inexpensive way for manufacturers to add a feature to an amp and to make a somewhat different tone available.

Your output tubes are typically pentodes, meaning that they have five elements inside (plate, cathode, control grid, screen grid, suppressor grid). Pentodes (and beam power tetrodes, such as the 6L6/6V6 family) are very efficient... as tubes go. The screen grid eliminates the capacitance between the plate and the control grid, which makes the tube more linear and controls electron flow more accurately, for more power.

Connecting the screen grid directly to the plate supply (same voltage as the plate) makes the tube function more like a triode, hence the name. It almost halves the maximum power, so people sometimes call it a half-power switch. The tube wastes power in triode mode.

At anything less than full volume, the difference between full and half power is not that noticeable. It definitely does not halve the loudness of the amp.

Triode mode on a pentode sounds thicker, mushier, less articulate. Some people think it sounds more bluesy; I think it just sounds sloppy, like the difference between sober and drunk.”

Translating this to audio amplifiers, it boils down to the idea that at low power levels triode and tetrode or pentode mode sound similar to most ears, but as you approach and surpass the limits of the triode configuration power, the tetrode or pentode configuration will continue to track the input signal accurately.

If you want CJ to do these mods, you’ll have to talk with Mr Fischel. I don’t know if he’d do it or not.

If you want an aftermarket modifier or DIY approach, you can probably do more.

It would not be that hard to jump pins to create a triode out of a pentode. With the old amps you propose, you wouldn’t be risking much.

As for zero feedback mods, I think success would depend on the proposed load. The feedback is there for a reason. Someone thought it was necessary and desirable when the amps were designed. Maybe you would disagree.

Call CJ, or the best local tube gear tech in your location. You’ll get a better sense of what could be done if there were no goal of preserving the original integrity of the amp.

[roberto]

Wow, what a great explanation AJ, your analogy with being sober or drunk made my teeth to clack. And I am with you 1000% with it. Any power amplifier by Conrad Johnson is a serious design. If you want to have a triode power amp, then get another model that was designed as it. There are too many things that was taken in consideration with the design. Same thing for the pentode design.