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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 10 responses by atmasphere

atmasphere's avatar
atmasphere
11,577 posts
 
The idea that power supplies somehow compare to engines falls apart fairly easily. What if you have large capacitances *and* tight regulation for example?

In addition to excellent power supplies, a good RIAA section will be low noise with as few stages of gain as possible, to minimize distortion. In addition, the best of them will be tube, and passive EQ. Passive EQ allows one to avoid negative feedback, which puts an unnatural 'sheen' on top. Expect first-rate components throughout- handpicked to meet the RIAA curve and for low noise, etc. If on a PCB the material will be something other than FR4 to minimize dielectric effects of the board. Layout will include star grounding for low noise, and attention to RF suppression and improving stability by providing grid stop resistances and local power supply bypass.

I am also of the opinion that since the LP is recorded in balanced mode, and since the cartridge is balanced, that the phono section ought to be balanced too and to that effect we created the first balanced phono preamps in 1989.

The line stage ought to have similar characteristics- simple signal path, zero feedback, quality components, etc. A little understood issue is that the line section is supposed to control the interconnect cable to the point that the cable drops out of the system equation- IOW the cable between the amp and preamp will make little difference. If this is not the case for you your line section lacks this ability.

The volume control will be manual multi-position switch, or if it is remote, a motor drive operating a multi-position switch which is the only way to build up a decent volume control.

Power supplies will be outboard to minimize noise, and locally bypassed for the same reason. I can go on and on...
atmasphere's avatar
atmasphere
11,577 posts
 
Hi Thom, having been in the business of balanced phono production longer than anyone else (since 1989- all the prior balanced phono circuit art were not *production* circuits) I can safely tell you that there is a lot of misconception brought out in the links you provided. I wish I had seen them when they were current!

The advantage of a balanced differential phono section is not that the cartridge will somehow act differently, it is the fact that the cable and the electronics act differently! To take advantage of the improved behaviors, we need to hook up the cartridge itself in balanced mode. This is easy as cartridges take quite naturally to this.

The result is lower noise throughout the phono system. In our case this allowed us to eliminate a stage of gain. That made the preamp more transparent, as it now makes less noise and distortion with wider bandwidth. IOW the signal path is actually simpler, not more complex, quite the opposite of the usual drone of balanced circuits being more complex!
atmasphere's avatar
atmasphere
11,577 posts
 
Ok- I have to clear up some misconceptions:

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.

The reality is, the pins of the cartridge are the inverting and non-inverting outputs of the cartridge. In a differential balanced system, there is no ground signal at all- ground exists only for shielding and it is possible to operate a balanced line without shielding (IOW with only 2 wires...). In this case, the ground wire is the shield and is also the tone arm itself which is shielding the wires.

>>RIAA eq deviation no more than 0.05db<<

Why?

Because if there is an error, it won't be *our* error. You'd be surprised how good the pre-emphasis on older cutting machines actually is. My Westerex system employs hand-picked components to insure accuracy against the serial number of my cutter head.

If I recall correctly, when you "balance" a phono cartridge (using a center tapped transformer, for example), you don't get the normal 6dB common mode rejection, because you are "halving" the signal in order to balance it.

The balanced source which is the cartridge arrives at the input of the balanced phono section without any transformer. If you refer to my comments above, the inverted ('-' pin) goes to the inverting input (pin 3 of the XLR), the non-inverted output ('+' pin) goes to pin 2 of the XLR and the tone arm ground to pin 1 of the XLR. Very easy, very simple, and nothing 'halved'. The cartridge is normally a balanced source and you get common mode noise rejection like crazy.
atmasphere's avatar
atmasphere
11,577 posts
 
Hi Thom, I thought I ought to point somethings out. A phono section needs to be really accurate. Many seem that way, but are not as they add something in trying to be 'detailed'.

I have found that high overload is a good thing. So our MC section overloads at about 250mV so that even a high output moving magnet can't overload it; at overload the output is making over 120V peak to peak.

What happens is that less than perfect recordings can thus still be enjoyed. IOW bad sounding LPs should be utterly playable on the best of systems without a lot of fatigue while the best sounding LPs transport you to the music.

In practice this works and yet our RIAA accuracy is within 0.05%. We extended the curve all the way to 100KHz and the low frequency cuts off at about 1.5Hz, so tone arm resonance defines the lower cutoff. In order to pull this off the power supply has to be very very stiff, and I think I mentioned before that we created a proprietary regulation setup just for this purpose (our regulators are quieter than the 'Super Reg' for example).

Differential amplification comes into play here as differential amplifiers have a 'cross mode rejection ratio' which is an ability to reject noise in the power supply. This is further enhanced by using two-stage constant current sources that are also designed to reject power supply noise.

Overall this gets you is a phono section that is unperturbed by poor recordings. It does seem if a phono section has issues, that often bad recordings will reveal that more than good ones!
atmasphere's avatar
atmasphere
11,577 posts
 
Dr. Stanley Lipshitz also published a set of simple formulas for exacting RIAA reproduction. His articles on the RIAA curve can be regarded as reference material.
atmasphere's avatar
atmasphere
11,577 posts
 
Hi Thom, what is it about my explanation that you do not understand?
atmasphere's avatar
atmasphere
11,577 posts
 
Two of the main points I can't get around are:

(1) Whether or not a cartridge is a balanced or a floating single ended device. Floating single ended makes more sense to me.

and

(2) Since everyone more or less agrees that there isn't a 6dB noise advantage to running a cartridge in "balanced" mode, what might the advantages of running a cartridge in balanced mode be.

Hi Thom, since a cartridge is an inductive device, it has something in common with other inductive devices like audio transformers. Any output of an audio transformer can be used in the balanced mode, even if it is driven single-ended. All that is required is that neither side be at ground. Ground can be the chassis the transformer is on; in the case of a cartridge ground can be the tonearm/turntable.

BTW a 'floating single-ended' device will hum like the dickens. Single ended devices are always grounded.

I have a tube mixer I built that is single-ended for my keyboards and drives a single-ended output transformer that is designed for 600 ohms. The output of the transformer is tied to pin 2 and pin3 of the XLR; pin 1 is ground. It drives 600 ohm balanced lines effortlessly.

Cartridges work the same way. It may not have been the intention of a cartridge manufacturer to make a balanced source out of it, but that is in fact how they behave since neither side of the cartridge is 'grounded', i.e. tied to its metal body. In fact many cartridges don't have a metal body! So really the question is more like: how in the hell can this thing be single-ended?" When looked at that way, you suddenly see why there have to be special grounding considerations (ex.: the third grounding wire) that you would not normally expect to see on your typical single-ended output (like from a tuner).

One obvious advantage of operating a cartridge balanced is the interconnect cable itself, which is less likely to have the usual interconnect cable colorations and less propensity for hum/noise pickup. Although the *output* of the cartridge is going to be the same regardless of balanced or single-ended, 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.

IOW 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.
atmasphere's avatar
atmasphere
11,577 posts
 
Years ago the Library of Congress did a study; they concluded that any laminated media was not/is not servicable as an archival media. IOW any laminated media would fail within years or 2 decades at the most.

Non-laminated (amorphous) media is preferred as it cannot delaminate over time. So far the only examples of this sort of media are LPs and 78s, and the stampers that made them. LPs have a projected storage life that goes into centuries if properly stored!
atmasphere's avatar
atmasphere
11,577 posts
 
It is really helpful to work with live recording sessions and master tapes, especially if they find their way to LP!

I've done a lot of recording and after a while you get used to the way your mics 'hear' and place them where they will get the effect you are after- not always where you would put your ears.

Hagtech is quite correct that the device does not have to be inductive; IOW a balanced source is certainly not limited to inductive devices (like microphones, transformers and cartridges). You can even get ceramic devices to be balanced, but who would want to- like frying an egg on the sidewalk :)

IMO, careful design of differential balanced circuits includes the use of properly designed constant current sources and you will want to have a properly regulated power supply too. A proper CCS will dramatically increase the differential amplifier's ability to reject power supply noise, and again if done right seems to reduce the overall noise floor of the differential amplifier as well.

I my case where this took me was less stages of gain overall. Figuring out what the tubes needed to be quiet was the key.
atmasphere's avatar
atmasphere
11,577 posts
 
That is why stiff regulation and effective Constant Current Sources are a must. IMO, an effective CCS is one that will create immunity to the supply- independent of the fact that the amplification is fully differential.
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