A Flat Moving Coil Preamp Using Paralleled Amplifiers

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JR.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by JR. »

ricardo wrote:
mediatechnology wrote:

____________________


If I was doing more serious work on this, I would want to provide the 'floating' PSU without introducing noise into the 0.28nV input.

Paul Kemble has a link to http://www.lcaudio.com/index.php?page=8 which is essentially my circuit powered by some photocells

Anyone have experience with these photocells? There doesn't seem to be much data on them at all :(
I looked at that link and ASSume that the schematic is wrong... A 2sb747 is a to-220 power transitor, me thinks he meant the 2sb737.

I recall messing with the Leach circuit when he published it, but not that much since the powering scheme is impractical,.

Using photocells to provide floating power is interesting but I suspect there is a better way to skin that cat.

====

Looking at the Kemble schematic the 1k base resistors in combination with the relatively high beta 737/786 could probably drain a floating battery in short order. Using the optical power sources they will be power limited so the circuit will stabilize at some modest voltage, I gather from some comments the typical battery for these is AA so 1.5V (or maybe 2x in series?).

Thinking out loud, instead of battery, or light power, how about driving the top and the bottom collectors with positive and negative current sources. The familiar 5 resistor and one op amp synthesized current sources should be reasonably low noise and that noise would be referenced to the output not input. There should not be a problem from running the current sources from conventional +/-15v rails. Further a servo circuit could be included detect for DC errors and balance the current sources to deliver 0V nominal output.

or not....

JR
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ricardo
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by ricardo »

JR. wrote:I recall messing with the Leach circuit when he published it, but not that much since the powering scheme is impractical,.

Using photocells to provide floating power is interesting but I suspect there is a better way to skin that cat.

====

Looking at the Kemble schematic the 1k base resistors in combination with the relatively high beta 737/786 could probably drain a floating battery in short order. Using the optical power sources they will be power limited so the circuit will stabilize at some modest voltage, I gather from some comments the typical battery for these is AA so 1.5V (or maybe 2x in series?).

Thinking out loud, instead of battery, or light power, how about driving the top and the bottom collectors with positive and negative current sources. The familiar 5 resistor and one op amp synthesized current sources should be reasonably low noise and that noise would be referenced to the output not input. There should not be a problem from running the current sources from conventional +/-15v rails. Further a servo circuit could be included detect for DC errors and balance the current sources to deliver 0V nominal output.
I've spent a long time trying to feed my little circuit from sensible power rails without success. One problem is you still need passive decoupling as the 'output' is the 'input' of an RIAA stage with <5mV sens. for full output. Wayne might be better served as he can decide what level he wants to run the output of the MC/MM pre-preamp.

In my case, the size of the Electrolytics required and/or the complexity of the PSU soon exceeded the real estate I was prepared to devote ... and didn't provide better results than the single C cell. (Yes. 1v5 is sufficient. I suggested a single AA for Wayne cos a C cell won't fit in the traditional Altoid tin :D )

The one mains powered commercial version stuck with 0.5mA for this reason ... and also cos there was still nothing near it in terms of noise circa 1990.

Paul, which SPU is yours? From the Ortofon website, I have the Meister at 1R5 to the Royal at 6R. I'd like to dream up a version for Wayne to set a Noise Benchmark at least.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by ricardo »

For something with balanced In/Out, here's something I dreamt up for 50R nominal ribbon mikes & P48V.

A couple of people with these rare 50R unicorns have built it and reported good results. (There's practically no modern gear which gives low noise with 50R ribbons).

But its certainly not ready for prime time as the only way to twiddle input/output offsets is to hand match the devices in situ. I can't dream up a sensible way to do this so I just offer this as an idea which may inspire true gurus like Wayne.

It's Common Emitter so THD is higher than my MC amp and its noisier cos the input devices are in series. Common Emitter means the LN bucket is wider. This helps with Denon DL103 at 40R but not with SPU at 5R or less.
______________________

I think SPU at 5R or less is a serious problem for any feedback scheme unless you change the gain structure of the whole system to allow zillion dB gain in that first stage. That's why I favour the push pull Common Base stage without feedback.

Something which may help is TI INA163, their DIP14 version of THAT 1510 etc. The important internal feedback resistors are accessible and TI tell me the stages that drive them can easily drive 500R or less at low THD
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Active P48V circuit for 50R nominal ribbons
Active P48V circuit for 50R nominal ribbons
Ribbon2.gif (19.34 KiB) Viewed 24116 times
Gold
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by Gold »

ricardo wrote:
JR. wrote: Paul, which SPU is yours? From the Ortofon website, I have the Meister at 1R5 to the Royal at 6R. I'd like to dream up a version for Wayne to set a Noise Benchmark at least.
It's the SPU Classic N. It is the most vintage like one they make but without the head shell. I've been told that the head shell 'contributes to the sound'. It sounds plenty good as is. It says the internal impedance is 6Ω.
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mediatechnology
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by mediatechnology »

Thanks everyone for your replies and comments.

I'm not seeing on the US futurlec site any of the LN transistors. The 2SC3329 comes up no stock; the rest do not return search results.

The goal is a flat MC preamp that in one stage (or two cascaded ones) would bring levels up to the 100-300 mV that are required for the RIAA EQ/Monitor.
A "pre-preamp" is not the most elegant solution given that the RIAA stage is line level input and the desire also includes line-level flat output.

I revisited Self's figure 8-5 pre-preamp which uses 3X 2N4403.
It's right out of Motchenbacher/Fitchen which also show a CB CFB and a CE with a combination of emitter and base feedback.
When Self's figure 8-5 is re-drawn to be fully-differential it looks like a conventional transistor/op-amp assisted Demrow/Cohen or differential Motchenbacher/Fitchen with paralleled transistors.

Self's numbers for 3X 2N4403 are not that far from what I got with 4X 1512 around -138 dBu unweighted 20 kHz BW.
(With 2SB737s his was a good 3 dB better but what's the point if they aren't available.)
Cascaded into a traditional MM input for the rest of the needed gain the final results are certainly going to be noisier.

My measurements were at 60 dB gain with a 10R source, 157 kHz BW with 20 kHz bandwidth estimated to be around -137 to -138. (AudioTester shows slightly less OP noise around -139 dBu with a 15K7 filter.)
Direct comparison to Self's SE design is not possible because his gain was unspecified but only 46 dB max, his input R was 3R3, and he doesn't specify the measurement BW.
(Assuming it might be the 22 kHz equivalent noise bandwidth of the AP he uses.)

The 4X 1512 has a built-in 3 dB or so penalty from being fully-differential.
What it's being compared to is SE.
I'll gladly accept the real-world advantages of a balanced input at the penalty of some thermal noise.

The 4X 1512 and their DC-coupled fully-balanced input are certainly in the league and they have the advantage of not requiring input coupling caps.
With the 1512 there's no significant base voltage - it's pinned to ground by the Rbias (47R). (658 uV max in common mode.)
The differential Ios - the cartridge current - of a 1512 is +/- 1.4 uA max which is far less than the leakage current of a 1000-2200 uF coupling cap. (Maybe 3-10 uA?)

Neither Self or the 4X stacked 1512s are as good as the Altoids tin "pre-preamp."
The Altoids and Self don't have the 60 dB gain.

4X 1512 may not be the most economical approach compared to 6X 2N4403 in a op-amp assisted discrete design but it sure is simpler.
I may try Self's design in a fully-differential topology but I doubt it will do much better noise-wise. It certainly will have a lower BOM but be more complex.

Image
Common Base and Common Emitter Feedback Circuits, Motchenbacher, Fitchen, "Low Noise Electronic Design," figure 7-3.

Image
Common Base WIth Feedback to Base and Emitter, Motchenbacher, Fitchen, "Low Noise Electronic Design," figure 7-4.
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JR.
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by JR. »

Not to quibble with the great M&F but I am a little uncomfortable with the suggestion two different techniques for measuring the input impedance return different answers and "both" are correct. That is making my head hurt. :lol:

But yes a really good book for back in the day..

JR
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ricardo
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by ricardo »

Gold wrote:
ricardo wrote:Paul, which SPU is yours? From the Ortofon website, I have the Meister at 1R5 to the Royal at 6R. I'd like to dream up a version for Wayne to set a Noise Benchmark at least.
It's the SPU Classic N. It is the most vintage like one they make but without the head shell. I've been told that the head shell 'contributes to the sound'. It sounds plenty good as is. It says the internal impedance is 6Ω.
Paul,

I have from the Ortofon website SPU Classics at
http://ortofon.com/hifi/products/histor ... pu-classic
http://ortofon.com/hifi/products/cartri ... /classic-n
&
http://ortofon.com/hifi/products/cartri ... -gm-e-mkii

They are all 2R with 0.2mV @ 5cm/s. There's a SPU Royal at 6R 0.2mV

I enclose details of 2 Altoid [*] specials, one for 6R 0.2mV and the other for 2R 0.2mV

I'll dream up something to work with Wayne's MM special in a bit but I've got some microphone goodies to play with :D

[*] I don't really recommend Altoids cos you can only get a couple of AA cells in. Something which would house a couple of C cells would be better.
I'm not seeing on the US futurlec site any of the LN transistors. The 2SC3329 comes up no stock; the rest do not return search results.
Damn! In 2012, Futurlec Oz, Futurlec US & IIRC a European Futurlec site had 2sc3329 and also the Hitachi 2sc2547. My recommendation for this project and the Ribbon P48V mike might have cleaned them out.

But 2sb737 is still available from JR's Olde Unobtainium Shoppe.
Attachments
MC pre-preamp for 6R Ortofon SPU
MC pre-preamp for 6R Ortofon SPU
SPU6R.gif (13.06 KiB) Viewed 24088 times
MC pre-preamp for 2R Ortofon SPU
MC pre-preamp for 2R Ortofon SPU
SPU2R.gif (12.97 KiB) Viewed 24088 times
Last edited by ricardo on Thu Nov 19, 2015 7:34 pm, edited 1 time in total.
dirkwright
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by dirkwright »

The "Altoids" circuit is from Marshall Leach, RIP:
http://users.ece.gatech.edu/mleach/headamp/
ricardo
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by ricardo »

dirkwright wrote:The "Altoids" circuit is from Marshall Leach, RIP:
http://users.ece.gatech.edu/mleach/headamp/
Yes. But there's a HUGE difference in noise & THD between Leach and my versions.

"The history of this design is that a friend built http://users.ece.gatech.edu/mleach/headamp/

and was disappointed with the performance so I said I'd make him a better one. This is a good example how understanding a circuit thoroughly can transform its performance from mediocre to "best of breed".

This version has MUCH better noise and distortion compared to the Leach version(s) and its commercial copies; especially with a real MC cartridge."

This is from MCamp.doc in my Yahoo MicBuilders directory. Loadsa other stuff of interest to anyone still wanting to use a MC cartridge. You have to join.
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mediatechnology
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Re: A Flat Moving Coil Preamp Using Paralleled Amplifiers

Post by mediatechnology »

Richard published his design in 1981: viewtopic.php?f=6&t=749&start=14

Leach came 18 years later.
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