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Old 31st July 2007, 08:14 PM   #1
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Default Trans Impedance Interface?

My Meitner preamp has separate interchangeable modules for MM and MC. The MM module has adjustability for capacitance and impedance. The MC module allows no adjustment, because somehow the circuit presents the cartridge with the proper impedance. Does anyone know how this is accomplished?

Here's the website and the applicable excerpt:

Phono Input Modules: Trans Impedance Interface (MC) and Constant Impedance Interface (MM).

http://www.museatex.com/pa6i.htm
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Old 31st July 2007, 10:28 PM   #2
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As it says it uses a transimpedance amplifier, which is basically an amplifying I to V converter.

The usual way to accomplish this is to use the inverting input of a feedback amp with no input resistor, so the current into the node establishes the current through the feedback resistor.

Since the inverting input is a virtual earth and the output is at Vout this means that Vout / Rfb = Iin so the transfer function of the amplifier is equal to Rfb, eg it is has the units of an impedance rather than a numerical gain.
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Old 1st August 2007, 10:26 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

In other words the load on the cartridge is a virtual short, all voltage is
dropped across the internal resistance and cartridge output is a current.

The voltage gain of the MC stage is inversely proportional to the
cartridge resistance, so higher output types with higher resistance
windings have lower gain than lower output types with low resistance
windings which get high gain.

/sreten.
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Old 1st August 2007, 11:02 AM   #4
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So does that seem like a good way to implement a phono stage? Any idea of the advantages/disadvantages? Do any other manufacturers use similar circuits in their phono stage?

The impedance of the load adds damping to the cartridge. What impedance would the cartridge see in this circuit?
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Old 1st August 2007, 02:13 PM   #5
oshifis is offline oshifis  Hungary
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I have a Dynavector MC/MM/MI preamplifier unit. It uses the same principle at MC. Unfortunately no circuit diagram :-) There are pot transformers and grind-top opamps inside, it seems quite complex...
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Old 1st August 2007, 03:10 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by audiobomber

So does that seem like a good way to implement a phono stage?

Any idea of the advantages/disadvantages?

Do any other manufacturers use similar circuits in their phono stage?

The impedance of the load adds damping to the cartridge.
What impedance would the cartridge see in this circuit?

Hi,

Nothing wrong with it (MC's only though).
+ Changes gain to suite cartridge "automatically".
- Experimenting with loading is not easy to not possible.
Yes.
Zero.

/sreten.
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Old 1st August 2007, 04:39 PM   #7
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Does the circuit present the same impedance to every cartridge then, or would a different cartridge see a different load?
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Old 1st August 2007, 05:18 PM   #8
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The usual recommendation for a gain stage is to have a load impedance equal to several times the cartridge's internal impedance. With a transformer, the load should be equal to or slightly higher than the cartridge impedance. Would the cartridge see its own impedance as the equivalent load into a trans-impedance amplifier?
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Old 2nd August 2007, 11:49 AM   #9
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Quote:
Originally posted by audiobomber
Would the cartridge see its own impedance as the equivalent load into a trans-impedance amplifier?
No Dan, the cartridge sees zero impedance.

Well what does that do to the damping?

I have no idea, but it sounds damn fine. Say, did you know that the Phonoclone everyone around here is discussing is also a transimpedance amplifier?

Why no, I hadn't heard that

It is, and so is the 47 Labs phono stage the clone is based on.

Thanks for the info.

You're welcome anytime.
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Old 2nd August 2007, 11:53 AM   #10
SY is offline SY  United States
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Historical note: the first use I'm aware of for using an I-to-V as a phono amplifier was from Bob Fulton back in the 1970s. His amp was specifically tailored to the Shure V15 (surprisingly).
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