Phono Transfer System Construction Information

Construction information for DIY projects, including the MS Mid Side Matrix, Elliptic Equalizer, Mastering Console, Phono Transfer System, Insert Switcher and the Dual Class-A Amplifier. You can post your baby pictures here.
Sergio
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Re: Phono Transfer System Construction Information

Post by Sergio »

Thank you very much!
Gold
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MM Preamp R4 & R5

Post by Gold »

Wayne,
I went through the threads but couldn't find or missed an explanation of the function of R4 and R5 in the flat MM Phono Preamp. It says the value can be 0Ω -499Ω. Could you explain what the function of these resistors is and how to pick a value? I think I knew this at one time but forgot.
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mediatechnology
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Re: Phono Transfer System Construction Information

Post by mediatechnology »

Paul -

R4 and R5 serve a couple of purposes. I build the boards with them at 100Ω.
In high RF environments R4 and R5, along with along with C3+J3, form a differential LP filter around 8 MHz but C3+J3 are mostly for added termination C.
What I found however is that with R4 and R5 in circuit and J3 open measured CMRR was improved.
I've never found a concrete explanation for that since C3 is differentially-connected.

Jensen, Rod Elliot and others have various explanations as to what these series resistors can do.
Jensen writes about degeneration; Elliot reduced rectification.
An author by the name of John Dunn writes that R4/R5 also "de-Q" the inputs to prevent them from resonating out-of-band.
When the Thevenin equivalent of the feedback network and input resistor are nearly equal value CM distortion is reduced.

Totally optional however.
They can be made 0Ω because at 100Ω they may have a small noise penalty but it's not much relative to the cart's 500Ω to 1400Ω DCR.
I run my MM with R4 and R5 in circuit and C3+J3 open.
Gold
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Re: Phono Transfer System Construction Information

Post by Gold »

Thanks Wayne!
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mediatechnology
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Re: Phono Transfer System Construction Information

Post by mediatechnology »

I edited it quite a bit you might want to refresh one more time.
You're welcome!
Gold
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Re: Phono Transfer System Construction Information

Post by Gold »

I guess if I use the MM preamp as an MC preamp with a transformer in front of it I should use 0 ohm so it doesn’t contribute to the termination resistance. I think that’s what I did last time for the DL103 MC pre. I should open that up and take another look.
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mediatechnology
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Re: Phono Transfer System Construction Information

Post by mediatechnology »

R4 and R5 should probably be 0Ω if you use a transformer for MC.
They don't terminate it though because R4 and R5 are in series with a high impedance op amp input.
Gold
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Re: Phono Transfer System Construction Information

Post by Gold »

mediatechnology wrote: Fri Dec 08, 2023 7:37 pm They don't terminate it though because R4 and R5 are in series with a high impedance op amp input.
i guess termination is the wrong word. Wouldn’t it alter the source impedance the opamp sees? Best to avoid that.
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mediatechnology
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Re: Phono Transfer System Construction Information

Post by mediatechnology »

It raises the source impedance seen by about 200Ω which is relatively low to the cart's 1350-ish Ohms.

Rnv for 1350Ω = 0.66µV
Rnv for 1550Ω = 0.70µV or about 1/2 dB more.
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mediatechnology
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Re: Phono Transfer System Construction Information

Post by mediatechnology »

I just made some dummy cart measurements and found that the source impedance of a 539 mH/1342Ω cart can be modeled, over a 20 kHz BW as a 25KΩ resistor. viewtopic.php?p=17535#p17535

The 100Ω/leg resistors for rectification reduction (R4 and R5) in series with the op amp non-inverting inputs aren't going to have much effect on noise.

If a MC step-up transformer is ahead of the preamp then they can be made 0Ω. But if the source is a cart I'd leave them in.
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