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What Makes a Good RIAA or Line Stage?

Hi Doug,

In a currently running thread on a certain RIAA / Line stage beginning with the letter "E", some very provocative comments were made that are of a general nature.

I fear that this conversation will be lost on the many individuals who have soured on the direction which that particular thread has taken. For the purpose of future searches of this archive, those interested in the "E" thread can click this link.

For the rest of us who are interested in some of the meta concepts involved in RIAA and Line Level circuits, I've kicked this thread off - rather than to hijack that other one. In that thread, you (Doug) mused about the differences between your Alap and Dan's Rhea/Calypso:

... the Alaap has the best power supplies I've heard in any tube preamp. This is (in my admittedly unqualified opinion) a major reason why it outplayed Dan's Rhea/Calypso, which sounded starved at dynamic peaks by comparison.

Knowing only a bit more than you, Doug, I too would bet the farm on Nick's p-s design being "better", but know here that "better" is a very open ended term. I'd love to hear Nick's comments (or Jim Hagerman's - who surfs this forum) on this topic, so I'll instigate a bit with some thoughts of my own. Perhaps we can gain some insight.

----

Power supplies are a lot like automobile engines - you have two basic categories:

1. The low revving, high torque variety, characteristic of the American muscle car and espoused by many s-s designers in the world of audio.

2. The high revving, low torque variety characteristic of double overhead cam, 4 valves per cylinder - typically espoused by the single-ended / horn crowd.

Now, just as in autos, each architecture has its own particular advantage, and we truly have a continuum from one extreme to the other..

Large, high-capacitance supplies (category 1) tend to go on forever, but when they run out of gas, it's a sorry sight. Smaller capacitance supplies (category 2) recharge more quickly - being more responsive to musical transients, but will run out of steam during extended, peak demands.

In my humble opinion, your Alap convinced Dan to get out his checkbook in part because of the balance that Nick struck between these two competing goals (an elegant balance), but also because of a design philosophy that actually took music into account.

Too many engineers lose sight of music.

Take this as one man's opinion and nothing more, but when I opened the lid on the dual mono p-s chassis of my friend's Aesthetix Io, my eyes popped out. I could scarcely believe the site of all of those 12AX7 tubes serving as voltage regulators - each one of them having their own 3-pin regulators (e.g. LM317, etc.) to run their filaments.

Please understand that my mention of the Aesthetix is anecdotal, as there are quite a few designs highly regarded designs which embody this approach. It's not my intent to single them out, but is rather a data point in the matrix of my experience.

I was fairly much an electronics design newbie at the time, and I was still piecing my reality together - specifically that design challenges become exponentially more difficult when you introduce too many variables (parts). Another thing I was in the process of learning is that you can over-filter a power supply.

Too much "muscle" in a power supply (as with people), means too little grace, speed, and flexibility.

If I had the skill that Jim Hagerman, Nick Doshi, or John Atwood have, then my design goal would be the athletic equivalent of a Bruce Lee - nimble, lightning quick and unfazed by any musical passage you could throw at it.

In contrast, many of the designs from the big boys remind me of offensive linemen in the National Football League. They do fine with heavy loads, and that's about it.

One has to wonder why someone would complicate matters to such an extent. Surely, they consider the results to be worth it, and many people whom I like and respect consider the results of designs espousing this philosophy of complexity to be an effort that achieves musical goals.

I would be the last person to dictate tastes in hi-fi - other than ask them to focus on the following two considerations:

1. Does this component give me insight into the musical intent of the performer? Does it help me make more "sense" out of things?

2. Will this component help me to enjoy EVERY SINGLE ONE of my recordings, and not just my audiophile recordings?

All other considerations are about sound effects and not music.

Cheers,
Thom @ Galibier
128x128thom_at_galibier_design

Showing 9 responses by hagtech

hagtech
163 posts
 
Other designer's comments? Well, I think you guys got it right here in that power supply design is critical to optimal performance. It is often overlooked and/or not thought of as integral to a linestage of phono design.

I'm not familiar with the Alap, so cannot comment.

For me, I design a power supply as though it is part of the gain circuits. They are tied together and symbiotic in so many ways. Having said that, there is no single topology that works everywhere. A balanced differential stage works best with one type of supply, single-ended another. Opamps prefer something else. It is not about the amount of capacitance or inductance. It's about providing exactly what is needed for each particular case.

You also need to think of the box and system as a whole. That is, a box will have both low voltage and high voltage supplies; input circuits and output circuits. Each has to do their job without disrupting the other, yet play together as needed. And that box also has to integrate with other boxes and not cause them problems. A power amp should not mess up the linestage. So a good power supply thinks outside the box.

Hmmm. I seem to be saying exactly the same thing Raul did above.

The car engine analogy sort of leaves me short. To me, there are so many other parameters involved. It would be nice if it were so simple, but I can see how it helps to advance the discussion. Bruce Lee would indeed make a great power supply, if he could do so quietly without all the howling.

jh
hagtech
163 posts
 
Whoah, Thom! Look what you started here.

I have to disagree with the assertion that a cartridge is balanced. It isn't. It's floating single-ended. To have true balanced, you need 3 signals. A reference level (return), positive polarity, and negative polarity. A cartridge obviously has only 2 pins per channel.

However! You can turn force it into balanced mode by creating the reference (middle point) for it externally. This is most easily done using a step-up transformer that has a center tap on its primary. You ground the center tap and connect the cart across the entire primary. Ralph, I assume this is what you do? I hooked up a Trumpet phono once this way through XLR inputs. Works great.

The other alternative is to use an opamp type differential stage. Also called instrumentation amps. The drawback with this technique is a relatively weak dc impedance on the reference (permitting hum), unless you also drive it actively.

jh
hagtech
163 posts
 
>>RIAA eq deviation no more than 0.05db<<

Why?

Just how many cutting lathes in the 50's and 60's were built to this specification? And let's put this into perspective by comparing to the average speaker response. Why are speakers allowed 100x more error? Does that make sense?

The key here is that the stereo channels be matched. They need to be very tight. The relative matching is what preserves the spatial cues, soundsage, focus, etc. An absolute error of even 1dB can be reasonable as long as the channels are equal.

Hey, I've been promoting accurate RIAA eq for years, but let's not go off the deep end like the 70's chase for lower THD. What's next? +/-0.00005dB equalization? This is looking to me like its all about marketing.

jh
hagtech
163 posts
 
>>3.18uS turnover point ... switchable ... other components altered at the same time<<

You are indeed correct. The 3.18us cannot simply be switched in and out. Adding the corner affects the other component values.

jh
hagtech
163 posts
 
Oops, I made a mistake. The above is true only for the standard Lipshitz method of EQ. If EQ is split across two stages (the way I do it, for example), then the switch is possible.

jh
hagtech
163 posts
 
>>Again, your quality sound reproduction targets/priorities are a little different from ours<<

Indeed, and that is the beauty of freedom and liberty. As designers we can choose different paths. The result is that the marketplace is offered more choice.

I do not disagree with your approach. It is sort of what I used to do. However, chasing good numbers sort of limits potential. I am discovering that the only way to reach the next level is to get caught up in the emotion of the music. It is the unmeasurables that begin to matter. Great technical performance is a good start (I would almost say mandatory), but it can also be a trap. And that was the point I wanted to make. Don't get caught in a marketing game where specifications rule.

So in that sense, I am more in the camp of Thom. I can live with a +/-0.75dB RIAA error if the compromise gets me better connected with the musicians and their message. For me, this emotional connection is more important than absolute accuracy.

jh
hagtech
163 posts
 
>>is frequency response at the phono stage is magnified by the time it reaches the speakers?<<

No it isn't. Deviations add up, but they don't multiply. A 1dB phono error will give you a 1dB shift in frequency response at the speaker.

How much is that? Well, take your listening position. Now move your head about 6 inches in any direction (up, down, front, back, left, right). That change you hear is probably more than 1dB.

>>A deviation of 0.1dB is I think 1% accurate<<

Yep. Here's a plot I made years ago showing what happens when the capacitors are off by +/-5%. You get peak errors of about +/-0.4dB. I'm sure the same sort of thing happens with resistor tolerance.

www.hagtech.com/images/accuracy.gif

>>Johnothan's calcs re the effect of cart resonance<<

He got it right. Resonances can be ultrasonic.

www.hagtech.com/loading.html

More info on RIAA at:

www.hagtech.com/pdf/riaa.pdf
www.hagtech.com/equalization.html
www.kabusa.com.riaa.htm

jh
hagtech
163 posts
 
>>since a cartridge is an inductive device<<

Being inductive should have nothing to do with balance. The transducer could be capacitive (touch sensor) or resistive (thermistor). It's just a two-terminal device.

>>there is in fact a noise advantage to the input amplifier, simply because it is differential and makes less noise than a single-ended input amplifier<<

I don't believe this is true. You get double the gain, but same SNR.

>>there is little advantage insofar as the cartridge itself is concerned, but plenty of advantage from everything that you use with it: the cable and the preamp itself<<

Yes, this is where we agree!

jh
hagtech
163 posts
 
This circuit would produce 3 dB [20log(sqrt(2))] more noise than a single gain cell operating in identical conditions

Except the signal is split between the two inputs! This is not the same as two amplifiers operating in parallel, whereupon you would get the 3dB SNR advantage.

jh
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