A Flat Moving Coil Preamp Using Paralleled Amplifiers

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mediatechnology
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by mediatechnology »

I'm using the Lundahl 1678. 1:16 for the Denon 103. I have a 3k83Ω termination resistor in parallel with the Shure SE22 47kΩ input termination resistor. The SE22 data sheet says there is 54dB of gain. I'm using at least 50dB gain in this setup to get 7cm/sec lateral to +4dBu.
Thanks for the data.
Gain of 74 dB (1 kHz) overall or about 5000. (Av=5011)
With the 250 uV Dennon DL-103 that works out to be 1.25 Vrms.
Probably a TAD less due to insertion loss of the L-1678.

The active input gain would then need 60-66 dB gain.
With 66 dB in the flat front-end, plus about 2 dB gain in the post-RIAA EQ stage, plus the final 6 dB gain in the balanced out it looks like I easily get to +74 dB gain and +4 dBu EQ'd output.
For a 100 uV cart there needs to be 8 dB in the post-RIAA stage to get to +4 dBu.

I doubt it will be as quiet as the Lundahl and probably not as quiet as a super-duper single-ended active input.
The Lundahl has the advantage of noise-free gain, high intrinsic common mode rejection and almost-infinite common mode impedance.
I have an extra set of 1678's currently wired 1:32 for the Ortofon SPU. I'd be happy to send them to you to check out if you think it would be useful.
Wow, thanks Paul.
What I need more than anything is a MC cart.
Amazon had the DL-103 for $169 and I noticed that they dropped in the last couple of days to $165.
What I might do is have you do that comparison on your end.

One possible combination I just thought of is the Lundahl ahead of the flat MM preamp.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by Gold »

I only have one DL103 which I'm using. I could send you the SPU I have with or without the transformer. It's pricey so you'd have to be careful but I'm happy to do it. It would be a good test since the output is so low. I destroyed the stylus on a DL103 a while ago. I can see if I still have it.I could send that if I have it and an electronically working but physically broken cart would help.

I recently discovered that a Shure V-15 III has the option of a conical stylus. I plan on picking one up eventually and trying that. I am a big fan of conical styli. I always think they sound better despite worse specs. The DL103 tracks heavy. It's the one thing I don't like about it for checking lacquers.

Edit: Since I am getting an enormous benefit from this I'll spot you a Denon 103. If you order one I'll reimburse you.
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mediatechnology
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by mediatechnology »

Since I am getting an enormous benefit from this I'll spot you a Denon 103. If you order one I'll reimburse you.
Wow, thank you Paul that's very generous.
The DL-103 body from the damaged one would help.
Have you had any luck getting those re-cantilevered?

I just did some noise measurements on the MM preamp that I'll be posting in that thread.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by Gold »

mediatechnology wrote: Have you had any luck getting those re-cantilevered?
I haven't tried. It looked like it was almost as expensive as getting a new one. When mine broke I needed an immediate replacement. I know Soundsmith hot rods them.

I'll look for the broken one but I probably tossed it.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by mediatechnology »

I looked up the Shure SE22 and immediately remembered the UREI 1122.
I think UREI made that for Shure because they are identical except for the silkscreen.
There were a ton of UREI 1122s in broadcast.

EDIT: Just found this:

Image
Richard Lee, Moving Coil Preamp, Wireless World July 1981.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by Gold »

I have an Shure SE20 and SE22. The SE20 is discrete and the SE22 uses an IC opamp. I think the SE20 was the same as a UREI 1122. I'll look at the boards in the SE22 and see if they say UREI on them. The SE20 boards say UREI on them.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by ricardo »

mediatechnology wrote:Image
Richard Lee, Moving Coil Preamp, Wireless World July 1981.
Well that certainly brings back memories.

Those versed in the art will recognise it as a simplified version of what Leach published in Audio. The story of my version is a friend built Leach and was disappointed so I said I'd make him a better one. My efforts are a LOT quieter & have MUCH less distortion than Leach or any of the commercial copies.

At one stage, I was going to give up the day job & go into manufacture but was persuaded to join Calrec by the chance to design the Mk4 Soundfield Mike.

I believe one version running 3mA with Hitachi 2SA1084 / 2SC2546 is STILL the lowest noise such device in the known universe.

I compared the 1981 version with everything I could lay my hands on and found it at least 10dB quieter than anything else (depending on how you measured noise) often much quieter. One surprise was that it was quieter in real life than a large & VERY expensive Denon transformer matched to their cartridges. My guess is that though the Denon transformer provided free voltage gain, the Rs presented to the typical MM RIAA stage was still sub-optimal

The 15.7kHz bandwidth is in fact an early attempt to do sensible noise measurements (-3dB 6dB/8ve 10kHz filter)

The recommended devices are now all Unobtainium though I believe JR has a secret stash. :(

Wayne, I'll send you my updated thoughts cos the Denon is less difficult to match than eg the Lo R Ortofons. eg 2 pairs of 2sc2240/2sa970 should be good for DL103

This little circuit has many subtleties and I've spent a LOT of time in my previous life attempting something better without success.
_________________________
I recently discovered that a Shure V-15 III has the option of a conical stylus. I plan on picking one up eventually and trying that. I am a big fan of conical styli. I always think they sound better despite worse specs. The DL103 tracks heavy. It's the one thing I don't like about it for checking lacquers.
I don't think you need to worry about DL103 tracking at 2gm with its 0.5mil conical stylus.

Though the indentation eqn isn't exactly proportional to pressure this is 'equivalent' to (0.2/0.5)^2 x 2gm = 0.32gm with a 0.7x0.2 elliptical

Besides there have been several studies which show its tip mass which is the real killer ... even with soft lacquers.

But for real cossetting of vinyl, you need to raid the Library of Congress. When SHURE announced the end of V15-V MR production, the LoC bought up the entire supply in the known universe.
Last edited by ricardo on Fri Aug 14, 2015 10:32 pm, edited 1 time in total.
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mediatechnology
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by mediatechnology »

Thanks Richard.

One thing that's not obvious in that circuit is the collector load.
For our readers it's the grounded 1K at the output.
I also spotted a non-floating supply version having separate collector loads to split supplies.

The common base topology looks ideal.

I tried a THAT1512 in common base with In+ and In- grounded.
It's similar topology to the mic preamp-based summing amp circuits you see in the SSM applications.

Balanced input is fed to Rg+ and Rg-.
The cartridge and termination, in parallel, serve as Rg and set gain.

It works, but the internal resistor values of the 1512 are not suitable and it doesn't lend itself to paralleled devices.
There are also active current sinks in the NPN emitters vs. resistors to improve linearity in mic preamp applications.
A single 1512 in this configuration isn't quiet enough.
There's also no control of gain - it's determined by the cartridge and it's termination.

There just aren't many common-base audio circuits for moving coil preamps published.
I've been asking myself "Why aren't there any common-base opamp-assisted fully-differential instrumentation input high gain moving coil preamps?"

I found this gem which was apparently published by Jean Hiraga in "L'Audiophile."

Image
Common Base Moving Coil Phono Preamp, Jean Hiraga, L'Audiophile.

It's not as simple or elegant as Leach and Lee but it is floating input.
It looks very similar in a way to Leach's "Current Mirror" common base moving coil preamp only with all PNP and has split balanced inputs.
I don't think the original Hiraga used 2SB737s.

Seems like Hiraga could be used with helper op amps to have more gain than a head amp.

What's cool about the Hiraga is that it is a common base differential pair....
You just don't see those often.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by JR. »

I recall the Leach version from back in the day (9v? ) and IIRC I asked Fitchen (of the well know Motchenbacher and Fitchen text on low noise design) what he thought about the topology. He was a professor at university of Bridgeport (CT) literally a few miles from my house back then. It was surprisingly easy to look him up in the phone book, it was even a local call.

Not only did he have nothing to offer regarding the topology, he asked me to write a chapter for his next book. :lol: Seriously, I told him I wasn't up to the task of writing a text book and thanked him for his time.

Regarding unobtanium I have a small stash of 2sb737 a very low Rbb pnp but I don't have any of the npn complement (2sd786)

I have long since lost my enthusiasm for vinyl and MC carts were out of my price range back then let alone now... The last batch of Shure MM carts I got I traded Shure a couple of my delay line kits for (when Shure was thinking about entering surround sound market).

JR
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mediatechnology
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Common Base Moving Coil Preamp with Feedback to Base

Post by mediatechnology »

Leach, Lee, Hiraga all used common base topologies for "head" amps.
Virtually all the rest I can find are common emitter inputs.

Common base isn't used often in audio small-signal design probably due to it's low input impedance.
Fortunately, for moving coil inputs, the tens of Ohms input impedance of the common base stage is not a problem.

If you turn the "Demrow" (or in audio "Cohen') instrumentation amp input transistors on their side and feed input to the emitter it becomes a common base (CB) input.
You can either ground the bases or float them by one Vbe (e.g. Hiraga) to have the emitters at 0V.
My experiment with a single THAT1512 is this topology.
With the input transistors turned on their side to form a CB input stage feedback, so-called "current feedback" is returned to the emitter.
There are commercially-available "current input" MC input preamps using this topology.
With the THAT1512 having its' Ra and Rb internally tied to the emitters there are no other options to return feedback. (The THAT1570/1571/1580/1583 do not have this limitation.)

Having feedback returned to the emitter, which is also the input, makes the cartridge output impedance part of the gain equation.
Maybe that's good - maybe it's not. Lower cart impedance generally equals more needed gain.
Cable capacitance being part of the feedback node is certainly not good.

My question is: "Why not return feedback, both DC and AC, to the base of the input transistor and use all three ports for signal?"
I searched the web high and low for various combinations of "audio moving coil phono preamp common base feedback to base" etc., etc.
Most all roads lead to Leach, a few to Hiraga and then not much else.

Paul Kremble's website however is the treasure-trove that led me to ricardo's MC head amp: http://www.angelfire.com/sd/paulkemble/sound4.html

My search for common base MC phono preamp topologies with feedback to the base did not directly lead me to Paul's website but after staring at it for awhile I stumbled upon this:

The Technics SL-10 internal common base moving coil preamp.

Image
Technics SL-10 Moving Coil Preamp, common base topology with AC and DC voltage feedback to the input transistor base.

Bingo!

Q604/606 are the common base input transistors operating in parallel.
Q602 is the collector load.
Q608 and Q610 are a complimentary cascade buffering Q604/606.
DC feedback to the base of the input is via R610 and R614.
The collector load of Q610 is the AC feedback network and is very low impedance.
The shunt value is 1R8 Ohms.
AC feedback from the shunt network of R624 and R626, 56R and 1R8, are fed to the input base by C612.

I finally have an example of a common base moving coil input stage with feedback taken to the base of the input.
There's only one pic of this I can find on the web.
Now there are two and an index for the search terms.
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