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MM Phono Pre with balanced input
MM Phono Pre with balanced input
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Old 25th November 2021, 10:31 AM   #1
ZackPlonk is offline ZackPlonk  Germany
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Default MM Phono Pre with balanced input

Hello,

As a learning project to push myself into starting to design my own stuff rather I have been trying to come up with (yet another) phono stage.

I wanted something with a balanced input b/c I think the superior CMMR is worth it in my particular case. I do have quite a bit of EMI where I'm listening to records. So my design goals:
- Optimized for MM. (For MC carts, a head-amp can be added later)
- Balanced input to cancel out CM interference
- Keep capacitors out of the signal path as much as possible
- passive RIAA
- Optional sub-sonic/rumble HPF

So using what I've learned so far about op-amps and looking at what many others did, I've put something tohether in LTSpice.

I'm using an offset compensation through another op-amp in order to get rid of the DC offset introduced in the initial two stages.

The rumble filter is a 3rd order S-K Q=.707 filter with a 20Hz cutoff. The idea is to have switch that selects the output jacks to either connect to U3 directly (out1) or to alternatively to the HPF output (out2)

Resistors in the RIAA filter should be .1% types and the caps there should be 3% types.

I think I'm pretty ignorant about all the things I don't know yet, so if someone can spare a few minutes to look at this thing and can share some feedback on what to do differently that would be awesome...

Some points I'm wondering about in particular:
- Better choices for op-amps?
- Is it worth considering a discrete approach for the first stage instead of op-amp
- is a a good idea to place the HPF as the last piece of the chain? Would there be anything advantegeous about putting it between the RIAA and the second gain stage?
- is the offset compensation a worthwhile approach or should a DC blocking cap in the output be considered. It might just be a matter of taste, but I'd love to read opinions on this...
- Is there a better way to wire up the input to have best flexibility with balanced and single-ended sources? Also modifications to the input loading resistance and capacitance (R1/R2, C1) are interesting to me. How can that be put into practice? Some RCA jacks in parallel where one could fit caps and resistors seems to be one way to go about that but the fact that the resistive load is split across R1/R2 makes this more complicated. Is there a way to have a single load resistor and still have a balanced input?

Here's the schematic and a bode plot w/ and w/o HPF
Screenshot 2021-11-25 at 11.00.13.png

Cheers,
Lars
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Old 25th November 2021, 02:21 PM   #2
jackinnj is offline jackinnj  United States
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MM Phono Pre with balanced input
Opamps and passive RIAA equalization not optimal due to the likelihood of over-loading at the high frequency end of the audio band. You can tame this by lowering the values of R1/R2 to take into consideration the inductance of the cartridge. This puts the 75uS compensation before the first stage of amplification.

Bob Cordell demonstrated in his "VinylTrak" phono preamp in Linear Audio Volume 5.

Of course, the values of the rest of the compensation network change.
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Old 25th November 2021, 02:36 PM   #3
billshurv is offline billshurv  United Kingdom
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MM Phono Pre with balanced input
vinyl track puts a pole at 8kHz with the loading. Too much noise penalty otherwise.


Hans Polak did design a balanced transimpedance MM amplifier but it's only for the brave.
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Old 25th November 2021, 04:02 PM   #4
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by ZackPlonk View Post
Here's the schematic
The 10k values of R4 and R5 are high and contribute a lot of noise, about 18nV/√Hz according to my calcs. That's much more noise than the OPA1641 devices you've selected.

perhaps R4=R5= 470 ohms and R3=R6= 22k would be better, reducing the Johnson noise contribution to 3.9nV/√Hz.
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Old 25th November 2021, 04:09 PM   #5
ZackPlonk is offline ZackPlonk  Germany
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Quote:
Originally Posted by Mark Tillotson View Post
The 10k values of R4 and R5 are high and contribute a lot of noise, about 18nV/√Hz according to my calcs. That's much more noise than the OPA1641 devices you've selected.

perhaps R4=R5= 470 ohms and R3=R6= 22k would be better, reducing the Johnson noise contribution to 3.9nV/√Hz.
This is very useful advice. Much appreciated, Mark!

LTSpice noise sim with original 10k/470k
Screenshot 2021-11-25 at 17.14.57.png

and with 470R/22k as suggested...
Screenshot 2021-11-25 at 17.14.27.png

Last edited by ZackPlonk; 25th November 2021 at 04:17 PM.
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Old 25th November 2021, 08:40 PM   #6
ZackPlonk is offline ZackPlonk  Germany
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Quote:
Originally Posted by jackinnj View Post
Opamps and passive RIAA equalization not optimal due to the likelihood of over-loading at the high frequency end of the audio band. You can tame this by lowering the values of R1/R2 to take into consideration the inductance of the cartridge. This puts the 75uS compensation before the first stage of amplification.

Bob Cordell demonstrated in his "VinylTrak" phono preamp in Linear Audio Volume 5.

Of course, the values of the rest of the compensation network change.
Putting that pole into the input/loading makes it dependent on the exact cartridge model, doesnít it?. I like to switch around cartridges so thatís not very practical.

Can you talk a bit more to what sort of overloading is potentially happening with opamps and passive RIAA? I donít really know what Iím doing so this is one more thing I have no clue about. Hopefully I can learnÖ
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Old Yesterday, 01:53 PM   #7
jackinnj is offline jackinnj  United States
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MM Phono Pre with balanced input
Quote:
Can you talk a bit more to what sort of overloading is potentially happening with opamps and passive RIAA? I don’t really know what I’m doing so this is one more thing I have no clue about. Hopefully I can learn…
It's been discussed on DIYAUDIO and elsewhere -- while there isn't a lot of audio information above 10kHz, the recording physics/mechanics dictate that the output at 20kHz will be 10x the output at 1kHz.

The surface noise, clicks and pops, can overwhelm the first gain stage, causing it to distort.

Search the web for articles on "phono stage headroom"
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