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Flat Moving Magnet Preamp Input Capacitance

Posted: Tue Feb 12, 2019 3:17 pm
by mediatechnology
Just measured the Balanced Flat Moving Magnet Preamplifier stray PC board capacitance without any added termination.

Differential: 4.6 pF plus op amp typically 2-5 pF.
Common Mode: 9 pF per leg both within 0.2 pF of each other plus op amp typically 2 pF.

With the cart connected to the input using a balanced/floating configuration the load capacitance, without added termination or cable C, would be about 10 pF plus the op amp.
Jumpers can then add 50, 100, 150 and 200 pf.

Re: Phono Transfer System Construction Information

Posted: Mon Feb 18, 2019 5:56 pm
by mediatechnology
Update February 18, 2019. Updated RIAA EQ Monitor Switcher assembly instructions with test procedures and schematics.: https://proaudiodesignforum.com/images/ ... 021819.pdf

Image

Full size schematics are in the pdf.

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 4:25 am
by billshurv
I'm reconsidering packaging today. Following some detailed measurements on some of the cartridges that I have the generator side by Hans some interesting things have come up, not least that, to get the best generator linearity 47K is often not the best and in many cases very low capacitance is preferable, of the order of 40pF. Without hardwiring into the base of the turntable this could be on the edge of what is possible but certainly means putting the preamp very very close to the turntable.

For now I am ignoring the fact that generator linearity is only half the problem and we still have the damned motor to deal with, but if you at least know how one half is behaving you have a fighting chance :).

I am also ignoring the fact that getting a flat response above 40kHz on a phono cartridge is a daft goal!

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 10:43 am
by Gold
I think the main reason to locate the preamp at the turntable is noise. One of the reasons I’m sticking with a step up transformer into the MM preamp for a DL103 is for the transformer CMRR and galvanic isolation.

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 11:07 am
by billshurv
For MM noise is not an issue (interference of course still can be). I won't argue with anyone who likes transformers. Not for me but that is just a preference rather than any engineering reason (although keeping kirchoff happy is always a good thing if you can :) ).

Domestic harmony is my challenge, but I do have a LOT of stacked up brownie points so could afford to lose some on this implementation...

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 11:21 am
by JR.
billshurv wrote:
Fri May 17, 2019 4:25 am
I'm reconsidering packaging today. Following some detailed measurements on some of the cartridges that I have the generator side by Hans some interesting things have come up, not least that, to get the best generator linearity 47K is often not the best and in many cases very low capacitance is preferable, of the order of 40pF. Without hardwiring into the base of the turntable this could be on the edge of what is possible but certainly means putting the preamp very very close to the turntable.

For now I am ignoring the fact that generator linearity is only half the problem and we still have the damned motor to deal with, but if you at least know how one half is behaving you have a fighting chance :).

I am also ignoring the fact that getting a flat response above 40kHz on a phono cartridge is a daft goal!
IIRC there was an early 4 ch(?) record playback system that put a HF carrier on top of the audio (40-50 kHz?)... That ended up in the dustbin of history with several others.

JR

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 11:49 am
by Gold
billshurv wrote:
Fri May 17, 2019 11:07 am
For MM noise is not an issue (interference of course still can be).
I've found that in NYC at least even a couple of feet of unbalanced cable on an MM cartridge picks up quite bit of crud. A balanced front end goes a long way. A DL103 with a transformer front end into the MM preamp was quieter by about 6dB than a V15 directly into the MM preamp at matched output level IIRC. The noise spectrum of the DL103 with transformer was much less objectionable also. The noise was mostly gaussian with the transformer. Without the transformer I got noticeable edgy RF/EMI type noise.

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 3:38 pm
by mediatechnology
IIRC there was an early 4 ch(?) record playback system that put a HF carrier on top of the audio (40-50 kHz?)... That ended up in the dustbin of history with several others.
JVC's "CD-4" system.

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 3:56 pm
by billshurv
Gold wrote:
Fri May 17, 2019 11:49 am

I've found that in NYC at least even a couple of feet of unbalanced cable on an MM cartridge picks up quite bit of crud.
Ah, in my quiet little village I think I am the main source of crud :).

Ref CD4 there are a few brave souls who still collect those and play them back. I am fully supportive of their obsession, but do not wish to be sucked into it myself!

Re: Phono Transfer System Construction Information

Posted: Fri May 17, 2019 4:21 pm
by mediatechnology
billshurv wrote:
Fri May 17, 2019 4:25 am
I'm reconsidering packaging today. Following some detailed measurements on some of the cartridges that I have the generator side by Hans some interesting things have come up, not least that, to get the best generator linearity 47K is often not the best and in many cases very low capacitance is preferable, of the order of 40pF. Without hardwiring into the base of the turntable this could be on the edge of what is possible but certainly means putting the preamp very very close to the turntable.

For now I am ignoring the fact that generator linearity is only half the problem and we still have the damned motor to deal with, but if you at least know how one half is behaving you have a fighting chance :).

I am also ignoring the fact that getting a flat response above 40kHz on a phono cartridge is a daft goal!
I found that when my S680 was lightly-loaded with capacitance that cymbal crashes turned into trash can lids.
It became very harsh and sibilant.