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Old 31st May 2001, 01:15 PM   #11
jam is offline jam  United States
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Alex,

Could you e-mail me a copy of Ben Duncans article on the ADEQ or is there a web site that I could go to.

I plan to use cascoded J-fet (I can vision Grey turning in his grave) differential with switchable gain (for moving coils). This would be followed by a passive eq. stage and then by a second differential stage which would be buffered with followers (Grey is moving again). There will be no loop feedback anywhere in the circuit, and the circuit would be totally solid state.
This would be similar in concept to Allen Wright's RTP phono section (link above). Another thought would be to replace the second differential with a variation of Nelson Pass's X circuit.
But all this is just an idea I don't know when I will have the time to persue this,as I am working on an amplifier now.

Jam
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Old 31st May 2001, 03:57 PM   #12
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Jam,
Looked at the phono stage on the link you provided. Once I saw it, I remembered looking at it at some point in the past. Looks fairly reasonable. My only reservation about using a cascode as a first stage is that cascodes have comparatively high output impedence. Another point is that he's using his RIAA across the whole differential; you can't back up and use half the circuit easily. More on this in a second.
Alex,
Yeah, I had this blinding revelation about cartridges being ideal as balanced sources about two or three years ago. The differential nature of the circuit would cancel noise, distortion, etc., so why wasn't everybody doing it? Seemed to me to be such an obvious thing. I asked and asked and asked. Heard of a few circuits out there, but not many. Finally one guy ventured that some cartridges have the negative lead tied to the body of the cartridge as a grounding/shielding scheme, and that you could run into trouble *if* you owned such a cartridge. I said *harrumph* and started planning a balanced phono stage.
(N.B.: Nelson (okay, actually Wayne), who does *everything* balanced, did a single-ended first stage in the Aleph Ono [and presumably the Xono, as it supposedly is very similar], then created a balanced output from that...)
Now, back to what I was saying to Jam about symmetrical RIAA. If you have such a cartridge on hand (or a friend brings one over) it would be nice to run the phono signal through *half* the balanced stage (thus allowing the cartridge to ground in the manner it expects), which would have to have its own, independent RIAA eq. So my circuit uses two identical RIAAs, one for each half. The only thing to keep in mind is that the 47k input has to be adjusted for balanced or single-ended operation, and you'll need some switching or jumpers to reconfigure +- to +gnd. I intend to hardwire the thing as balanced, leaving the unbalanced option as a desperation option requiring a soldering iron.
Okay, onwards: Alex, I considered, and can still arrange, a JFET first stage for noise purposes, as those little SK critters are awfully quiet. But I don't have any on hand, and wanted to try all-tube first, anyway. (Who's got the cheapest prices on those JFETs these days? I need to get some.)
Jam again,
Turning in my grave? YIKES! I hope I didn't die and not realize it...I've got *way* too much to do.
I'm in agreement on the no FB aspect. My phono has no FB as it stands now. I could easily add local, I suppose, but for the time being I'm going open loop.

Grey
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Old 31st May 2001, 04:13 PM   #13
Alex M is offline Alex M  United Kingdom
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Grey wrote:

"Yeah, I had this blinding revelation about cartridges
being ideal as balanced sources about two or three years
ago. The differential nature of the circuit would cancel
noise, distortion, etc., so why wasn't everybody doing it?
Seemed to me to be such an obvious thing. I asked and asked
and asked. Heard of a few circuits out there, but not many.
Finally one guy ventured that some cartridges have the
negative lead tied to the body of the cartridge as a
grounding/shielding scheme, and that you could run into
trouble *if* you owned such a cartridge. I said *harrumph*
and started planning a balanced phono stage."

This is why I like the logical grounding scheme SME (among
others) use in their tonearms. Their scheme is to have a
separate wire connected to the tonearm, which you can (or
maybe not!) screw into the earthing post on your preamp.
If there is a conductive path from armbase to headshell,
the cartridge body is then grounded. Mind you, if the
cartridge body itself is internally connected to the signal
return, you could have a nice hum loop. Rega, by contrast,
ground the signal return on one of the channels of their
tonearms - one of the reasons for the popularity of
aftermarket rewiring of the RB250 and RB300.

I guess the overriding reason for not doing these things
properly in the first place is cost.

Anyway, Ben Duncan claims that when a cartridge is connected
in balanced mode the hum disappears in a very dramatic
fashion.

Alex
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Old 31st May 2001, 07:21 PM   #14
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Alex,

I take it that you have a Rega tonearm. I would highly recomend that you re-wire it as a soon as possible. I have heard the differences on a friend's tonearm and was skeptical of the magnitude of the differences till I heard it myself. It takes the Rega into the same league as the mighty SME V (no kidding!). In my book this makes the Rega the best bargain in audio. It is a pity the manufacturer of the Rega does not see it this way.
Ben Duncan is absolutely right, once you hear the lack of noise or hum you can't go back. The differences can be positively eerie in a high resolution system.

By the way, most cartridges that have their body tied to one of the signal leads usually can be fixed for balanced operation. Look fof a metal tab between the one of the cartridge pins and the body on the back of the cartridge, all you have to do is cut this tab and begin to enjoy the music.

Jam



[Edited by jam on 05-31-2001 at 03:21 PM]
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Old 31st May 2001, 08:05 PM   #15
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Grey,

The beauty of Allen Wright's approach is that you can go into the circuit single-ended or balanced and come out single-ended or balanced and more importantly you have only one set of eq. components, which means the outputs will be totally complementary to each other and have a high Common Mode Rejection Ratio. (CMRR)
In your circuit (if I can visualise it correctly), you have to match two sets of eq. components to some great tolerence or run the risk non-linear addition and subtraction of the circuit halves and possibly end up with a non-flat RIAA equalized response.

I believe a good option in my idea is the use of an X-amp in the second stage of phono amplifier after the eq. I have to go on record and say that the X-amplifier, in my opinion, is the single most important advancement in analog amplifier technology in the past two decades. Zen-Master Pass Rules!

All this brings up an interesting question about your x-over in your quad amped. system. Are you converting the signal to single-ended before the x-oxer and then back to balanced after the x-over? If not I can see a whole host of problems (like the RIAA stage) and would like to know your solutions.

Jam

P.S. All this from a guy that don't even own a turntable.
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Old 31st May 2001, 08:38 PM   #16
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Red face BEWARE



You are entering the land of the long posts!





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Old 1st June 2001, 12:21 AM   #17
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Jam,
Let's see if I can keep all this straight in my head long enough to get it written down.
As for the CJ ART and other paralleled tube circuits...it's an ideal situation for a current source. As the tube(s) age, they are held to the same current, regardless. I have no idea whether CJ uses resistors or current sources in the ART. Sonically, I would vote for a resistor (somehow, you knew that...right?), but this might be one of those situations that would justify a current source.
Mis-match in my RIAA: Keep in mind that differentials tend to balance out (cancel) unequal signals, so in fact having two RIAA sections allows me to average errors, hence greater accuracy. Add to that the fact that I'm using as tight a tolerance parts as I can get my little sweaty paws on, and it should work out pretty well. (Just tossing parts in without trimming or matching capacitors got me +-.2dB accuracy to the RIAA curve, which ain't so bad. With a little fiddle factor, I can probably nudge that even closer.)
Currently my entire system is running single-ended, so the crossover is single-ended. As I go through this convulsion of revamping things, I'm making it all capable of balanced input, but not yet actually running balanced. Since I mostly listen to phono, there's not much point in balanced operation until I get the phono stage ready. Otherwise, I have to create a balanced signal out of thin air. Okay, I'm already doing so in the first stage of my tube amp, but it's semi-ready to go balanced.

Grey
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Old 1st June 2001, 01:34 AM   #18
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Grey,

I believe the CJ uses resistors and the BAT is a current sourced differential. The idea of using multiple devices is to lower the output impedence and hence improve drive capability. Don't get me wrong the CJ sounds rich and lush but in my opinion is slightly fuzzy and lacks the focus of a circuit that uses one or two tubes. The problem is that when one tube starts to go bad the other tubes mask it to some degree a question a CJ representive doged when I posed it to him. The problem is keeping good tubes in pre-amp at all times, I certainly don't want to be testing tubes every time I want to listen to my system, I bet you that there are people out there using pre-amps that are not performing to their optimum.

The BAT is a far superior design and is designed to drive impedences as low as 10k which is great in most
applications,but let's take the case of an an amplifier that has unequal impedences on it's balanced input, as low as 1k on one leg and 25k on the other. This is why I like followers as long as they are done right, the exception being the case,as you mentioned, controlling the impedence of the load. I like designs that have some degree of immunity to the load presented to them (makes comparisons easier).

I am impressed with the accuracy of your RIAA curve, I was not trying to be critical but play the devil's advocate, that being said I usually prefer the simpler more direct approach (something following Nelson's designs has taught me).

Jam
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Old 1st June 2001, 02:31 AM   #19
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Jam,
I haven't heard the ART, nor have I heard any BAT stuff, so I don't have an opinion on either one. (No excuse on the BAT, since there's a dealer here in town. I'm just trying to get so many things done that I haven't taken time.)
We'll see what happens with line stages and crossovers and such soon. Right now, I've got to get the phono up and running, and that in turn is waiting on me finishing the new cabinetry for mid/tweeters.
And as if that's not enough, I'm starting to think that I might want to use the Aleph 1.2 project as the excuse to begin experimentation on melding the X front onto the Aleph rear. Somebody better save me from myself...

Grey
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Old 1st June 2001, 12:42 PM   #20
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Grey,

Now you are talking! I hope the Aleph X (or is it the X Aleph) is now on the drawing board. Please keep us informed on your progress.
I had a thought about a line stage using a 6SN7 in a current sourced differential configuration (I suspect this is one of your tubes of choice), followers optional. Every one else seems to be using 6DJ8's, but that was some time ago.
Another tempting project is Allen Wrights zero feedback amplifier.

http://www.vacuumstate.com/images/PP-1C_a.gif

Both projects are not practical for me due to nature of my system. You on the other hand.......

Jam
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