Adding a 3rd gain stage to an MM phono stage, to make it suitable for LOMC? - diyAudio
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Old 15th August 2012, 08:39 AM   #1
andyr is offline andyr  Australia
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Default Adding a 3rd gain stage to an MM phono stage, to make it suitable for LOMC?

I'm hoping that someone can confirm my thinking is correct.

I have a DIY, battery-powered, JFET-based MM phono stage (40dB) ... which is similar (but a bit different! ) to several designs shown on these pages.

It consists of 2 gain stages separated by a passive composite RIAA network - and works very well! . I would now like to add a 3rd gain stage (x20) to act as a head amp for my Benz LOMC.

To avoid the use of switches, I would like to have 2 sets of RCA input sockets:
* the 1st pair feeds the "MM" part directly (for my Grado).
* the 2nd pair - for my Benz - goes into a x20 gain stage which then feeds the MM section.

The phono stage will only be used with 1 TT at a time - but I want the option of the cart being either a LOMC or a standard MM.

Am I correct in assuming that, if I use a series cap following the new front-end gain stage (to stop DC offset from the new "headamp stage" being fed into the 1st gain stage of the MM phono stage), the "head-amp gain stage" will not exert any load on the 1st gain stage of the MM phono stage? IE. having it there won't affect the signal from an MM which is plugged into the "MM RCA sockets"?

Thanks,

Andy
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Old 16th August 2012, 01:24 AM   #2
kevinkr is offline kevinkr  United States
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Nope, the output stage of the MC head amp will still appear in parallel for audio signal with the cartridge connected to the MM input. It is unlikely that your MM cartridge will like that too much..

Also based on considerable experience I would recommend trying a step transformer. Lundahl makes some reasonable priced good ones...
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Old 16th August 2012, 01:38 AM   #3
andyr is offline andyr  Australia
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Quote:
Originally Posted by kevinkr View Post

Nope, the output stage of the MC head amp will still appear in parallel for audio signal with the cartridge connected to the MM input. It is unlikely that your MM cartridge will like that too much..
But, Kevin - there will be no output from the MC head amp stage ... because there will not be a cartridge plugged into those input socktes - the MM cartridge will be plugged directly into the input sockets connected to the MM phono stage.

Regards,

Andy
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Old 16th August 2012, 02:20 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

The output stage of the MC step up is low impedance,
nothing to do with it having any input signal or not.

The MC stage will load the MM if powered up.

A switch and one pair of inputs sockets is a lot more sensible.

My amplifier has two pairs of sockets and a switch so you
can have a two turntables connected, one MC and one MM.

rgds, sreten.
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Old 16th August 2012, 02:29 AM   #5
andyr is offline andyr  Australia
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Quote:
Originally Posted by sreten View Post
Hi,

The output stage of the MC step up is low impedance,
nothing to do with it having any input signal or not.

The MC stage will load the MM if powered up.

A switch and one pair of inputs sockets is a lot more sensible.

My amplifier has two pairs of sockets and a switch so you
can have a two turntables connected, one MC and one MM.

rgds, sreten.
Thanks for your input, sreten.

Sure, a switch and one pair of inputs sockets is a lot more sensible ... but I want to understand the reason why I need this arrangement.

Your answer "The MC stage will load the MM if powered up" would be the reason ... but can you explain why the 1st gain stage (the "MC head amp stage") will load the subsequent gain stage - which is the 1st gain stage of the actual MM phono stage - if there is a (series) coupling cap after the "MC head amp stage"?

Thanks,

Andy
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Old 16th August 2012, 03:06 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

DC coupling arrangements have no effect at AC frequencies.

The MC stages low output impedance will load the MM cartridge.

rgds, sreten.
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Last edited by sreten; 16th August 2012 at 03:10 AM.
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Old 16th August 2012, 03:19 AM   #7
kevinkr is offline kevinkr  United States
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And the end result will be rather low output from the MM cartridge, scewed frequency response, and probably a significant amount of distortion.

With this configuration the cartridge will try to drive the low output impedance of your MC amplifier as well as the intended input to the MM amplifier. It's not designed to drive low impedances, and won't do it well. Additionally the MC output is not necessarily going to be linear when back driven and may or may not reward you with distortion.
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Old 16th August 2012, 03:24 AM   #8
andyr is offline andyr  Australia
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Quote:
Originally Posted by sreten View Post
Hi,

DC coupling arrangements have no effect at AC frequencies.

rgds, sreten.
Aah, thanks sreten. Yes, that makes sense.

Regards,

Andy
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Old 16th August 2012, 03:25 AM   #9
andyr is offline andyr  Australia
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Quote:
Originally Posted by kevinkr View Post

With this configuration the cartridge will try to drive the low output impedance of your MC amplifier as well as the intended input to the MM amplifier.
Yes, I now understand.

Thanks for your input, Kevin.
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Old 16th August 2012, 03:31 AM   #10
kevinkr is offline kevinkr  United States
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Use a really good switch with contacts designed for switching extremely low voltage ac signals or even better a mercury wetted reed relay if you can find one.

I just use a single input and plug in what ever I am going to use at a given moment, this I feel results in less degradation than a switch at these very low signal levels. (I have had problems with both relays and switches at low signal levels, if the contact oxidizes slightly the signal cannot overcome the breakdown potential of the barrier and the result is no sound, or in a marginal case lots of distortion.) Gold plated contacts can help a lot here.

I have a bunch of MC cartridges and one solitary MM which I almost never use, if you have a good MC you may find you are not using the MM much.
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