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
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
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.
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.
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.
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.
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?
audiobomber said:
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.
audiobomber said:
There is also the original Leach common base headamp. A variant which is used in the MC-stage in the RIAA from LC-Audio (danish link, as I did not find the RIAA on LC-Audio's english pages), which i use to good results my self.
Best regars
KJ
audiobomber said:
It depends.
I believe (i.e. I am not sure and would appreciate a cartridge designer stepping in here) that there are cartridges that have little mechanical damping and thus rely on (external) electrical damping and that there are cartridges with significant mechanical damping and no need or desire for much electrical damping.
The former would like a current input, the latter a voltage input.
BTW, not every transimpedance input really has a zero impedance. The Aqvox with its discrete input stage is about 20 Ohms.
Opamp-based inputs can be zero Ohms, but I can image that quite a few have a small-value resistor in series with the input so as to offer something that works well enough with both styles of MC cartridge.
--
About 'automagic' impedance adjustment: often claimed by manufacturers. I investigated this a couple of years ago and it simply doesn't exist, not even in extreme Japanese DIY circles.
It needs a low-noise supply voltage that is isolated from signal ground, which may make the implementation a bit tricky. LC-Audio uses a bulb/lamp and three photo-diodes for the supply voltage. The design is also sensitive to specs for base-spread-resistance/bulk-resistance (?) - r(BB) and Base-Emitter-resistance - r(BE) for the transistors, which has significant implications for the noise performance of the design. r(BE) is of course strongly dependent on quiescent current. I have not seen many transistors with spec for r(BB), though I believe the Base/Emitter-matrix transistors form Zetex may be suitable. LC-Audio uses SB747 and SD786, which are out of production and may be difficult to get hold on.audiobomber said:
By the way, I have for a very long time been tinkering with an idea of a differential input stage based on/inspired by the original Leach design. The idea has since evolved to a completely differential phono-/RIAA-preamp specifically for low impedance MC cartridges. Hopefully the ideas will materialise in to the real world later this year … though it will be a rather complicated build so don’t hold your breath. In theory (that’s by SPICE-simulations) it may have very impressive performance, noise-figure with reference to a 6 ohm MC-cartridge about 2-3 dB, distortion below noise floor ( at about -100 dBr for low frequencies, mostly 3rd harmonic) and band with in excess of 100 KHz (-3 dB).
A potential disadvantage with current input designs for MC-cartridges is that the source-inductance of the cartridge may become significant with respect to frequency response in the treble area.
Best regards
KJ
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