u-turn Original Pluto MM RIAA Pre-Amp - Look Inside

[Pete B]

I found it odd when most phono preamps went OP amp based since they're all push
pull class AB. Even boutique brands such as Levinson and others. Then audiophiles
promote the use of single ended class A power amps that are hugely inefficient at a
fairly high power level. Why don't they demand single ended class A in their preamps
where the inefficiency has little impact given the very low power level?

On the other hand, OP amps have gotten so good that they're hard if not impossible to
beat. Still, it feels like cheating using OP amps.

I'm of the school of thinking that if the noise and distortion are low enough, the frequency
response matches to a tight tolerance and you keep them out of overload, then amplifiers
sound the same. I've done the A/B experiment many times. In the last few years even
with phono amps. It is as if the A/B switch is not connected even when it is.

Thanks for the reference to the MM-5/6 I'll take a look at them.

 

[DavidTT]

I suspect that many of us would have a hard time distinguishing between competently designed phono preamps in blind A/B testing. As a too-often gear churner, I've been eyeballing phono preamp upgrades recently but it's a never-ending thing that I wish to avoid going nuts with over time. Go back to tubes in phono pre? Class A like Darlington? Go big with Sutherland? When does it end, rhetorically asked...

 

[krichard2496]

I suspect that many of us would have a hard time distinguishing between competently designed phono preamps in blind A/B testing. As a too-often gear churner, I've been eyeballing phono preamp upgrades recently but it's a never-ending thing that I wish to avoid going nuts with over time. Go back to tubes in phono pre? Class A like Darlington? Go big with Sutherland? When does it end, rhetorically asked...

We would say that the type of input device is equally important, if not more-so, to the operating topology of that device. It is important to provide distinction to the construction of an individual gain block (most op-amps use Class-A input stages as well, but combine those with a Class AB output). While there are rare examples of Class-A output op-amps, the uA702 and uA739 being notable examples, you have to travel back to 1963 and 1977 respectfully and those never gained market traction for fairly obvious technical reasons.

We have tested BJT conventional transistors in class-A (at up to 80V+) for the input stages of our devices and found them severely wanting, from a sonic quality standpoint. In addition to the fact that BJTs have their own noise issues when dealing with MM-style cartridges (due to the inductive nature of the source).

A J-FET-input op-amp is nearly always too noisy in the real world, due to process limitations (the input J-FETs are run at a very low current inside the op-amp). To our ear, however, if you can ignore the high noise floor in a prototype situation, carefully deployed J-FET input op-amps, in some ways, outperform many competitors all-discrete BJT input phono amps.

Our discrete high-voltage J-FET design, running class-A, is working well for us - sonic accuracy as well as low-noise floor, high overload margin (etc.) at a price point which makes high quality sound accessible to a wider group of audiophiles than previously expected.

We don't claim to do any better than the very-best-implementation of vacuum tubes. But due to the inherently limited PSRR (power supply rejection ratio) of a single-ended Class-A design (used by the vast majority of tube-type phono preamps), power supplies become very critical. A simple (high-impedance) power supply, used by most, tends to lead to limited dynamics, perceived soft or overly-warm bass, a soft treble, and other faults. The fact that many tube designs use a single-loop RIAA feedback may cause other issues like mid-treble glare.

Additionally, most tube designs use a 12AX7 (1/2 of a 6AV6) with high plate load impedance, and therefore have a slightly higher-than-desirable noise floor, and require physical Step-Up Transformers for use with Low-Output MC cartridges.

The result is that tubes - "properly implemented" - in our view are limited to the $2K to $5K+ region, with a separately tube-regulated power supply, whereas we can produce surprisingly musical and accurate sound for much less, using our extensively-regulated 56V-rail internal architecture with our High-Voltage, High-Linearity J-FETs and surrounding components, and our separate-box Active LOMC step-ups using multiple parallel-arrays of high-transconductance, low-noise J-FETs.

 

[WizardWG4]

@krichard2496 Very informative and thanks for the the info.

The fact that many tube designs use a single-loop RIAA feedback may cause other issues like mid-treble glare.

Curious about the above. Are you referring to 12AX7 active feedback with the RIAA network from 2nd stage plate to 2nd stage grid? Would feedback from 2nd stage plate to 1st stage cathode be the better option?

 

[krichard2496]

@krichard2496 Very informative and thanks for the the info.


Curious about the above. Are you referring to 12AX7 active feedback with the RIAA network from 2nd stage plate to 2nd stage grid? Would feedback from 2nd stage plate to 1st stage cathode be the better option?

Active tube EQ around the second stage tube is quite rare, one revision of the Sherwood S-5000-II integrated amp being an example (Amplifier Handbook, Richard F. Shea, 1966, p. 17-82) and the H/K Citation IV. In our review of hundreds of existing units, your second scenario appears to be far more common in practice.

Because of the predominance of the single-loop (second scenario) our main point was that this all-in-one loop feedback [from 2nd stage plate (usually via a cathode follower) to the 1st stage cathode] tends to produce audible glare in the upper midrange due to the phase shift of F3 rolloff at 2122Hz. While it's true that the F3 rolloff is actually the second pole (F1 being the first at 50Hz) our belief is that the ear's greater sensitivity in the upper midrange causes F3 to be more sonically problematic.

From a strictly technical point of view it "should" make little difference assuming that the loop isn't running out of current in the upper midrange and treble due to lowered impedance in the RIAA network (see Jung 1977 TIM articles in The Audio Amateur).

However, clients who have compared our gear to, for example, the EAR 834 (Marantz 7 variant) and ARC SP-8E (SP-3 and SP-6 are similar) have noted that this reduction of glare, while retaining openness, clarity and transparency via our implementation of high-headroom passive EQ. This "absence of glare" is consistently one of the first things they've noticed compared to their previous high-quality gear which featured single-loop tube RIAA feedback EQ.

 

[WizardWG4]

Thanks I appreciate that clarification.

This gives me another reason to try out a passive phono stage. I've only listened to active so far.

 

[Boltman92124]

I found it odd when most phono preamps went OP amp based since they're all push
pull class AB. Even boutique brands such as Levinson and others. Then audiophiles
promote the use of single ended class A power amps that are hugely inefficient at a
fairly high power level. Why don't they demand single ended class A in their preamps
where the inefficiency has little impact given the very low power level.

On the other hand, OP amps have gotten so good that they're hard if not impossible to
beat. Still, it feels like cheating using OP amps.

I'm of the school of thinking that if the noise and distortion are low enough, the frequency
response matches to a tight tolerance and you keep them out of overload, then amplifiers
sound the same. I've done the A/B experiment many times. In the last few years even
with phono amps. It is as if the A/B switch is not connected even when it is.

Thanks for the reference to the MM-5/6 I'll take a look at them.

Comparing my new Darlington MM-6 to the Emotiva XPS-1 is no contest to my ears. A big win for the Darlington.

 

[Pete B]

Comparing my new Darlington MM-6 to the Emotiva XPS-1 is no contest to my ears. A big win for the Darlington.

I'd like to hear a bit about how they are different, if you don't mind.

 

[Boltman92124]

I'd like to hear a bit about how they are different, if you don't mind.

No problem! Keeping it brief>>>
1.) The Emotiva is noisy in comparison. Audible noise floor even at moderate volumes. The DL MM-6 is black backgrounds. No noise audible even with volume up to 12 o'clock on my line stage.
2.) The Emotiva just sounds harder/harsher. The MM-6 is one smooth operator! You want to turn up the volume with it. Just a different league of natural tonality and lack of strain to it's sound. Vocals more lifelike.
3.) Subtle detail perks up my ears with the DL. Almost like a better mastering/pressing of the same LP. Makes all my LP's sound better.

You can start with the DL MM-5 like I did and I heard some of the above right away. But I sent it back for a new MM-6 config and a little boost in the gain to 42db. When it was gone, the Emotiva was a tough listen in the meantime(didn't listen much). Not even the equal of the MM-5, which you can get for $179. And the DL MP-7 owners are even more giddy. Little giant killer phono preamps.

 

[Pete B]

@Boltman92124 Were you using an MC or MM cart with the DL MM-6 and Emotiva in the past post?

 

[Boltman92124]

@Boltman92124 Were you using an MC or MM cart with the DL MM-6 and Emotiva in the past post?

Sorry. Need to visit AK more often. The MM-6 doesn't do LOMC. So compared them with various MM carts including vm540, 95ml, HOMC SAE 1000e etc.. Don't get me wrong, I enjoyed the Emotiva for years. And the Emotiva did sound good with a Denon 103 LOMC, but it had some hum. Not a deal breaker though. Gave it to a buddy recently.