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Old 26th August 2016, 04:21 AM   #1
ridikas is offline ridikas  United States
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Default Automatic Input Impedance Load Phono Preamplifier?

Have any of you heard of a phono preamplifier which automatically loads a cartridge with a proper input impedance? No need for resistors. I was wondering if someone could give a clear explanation on how such a circuit works, or maybe even post a schematic?

From the little that I was able to find online, it apparently works with MC cartridges only, low output, and low self impedance.

Some commercial units include Van Den Hul, Aqvox, Clearaudio Smart, Goldmund Mimesis 7PH (the one built into the 7 preamp), WLM Phonata, and I'm sure many others.

Are there any benefits or drawbacks to this type of design? Just very curious to get a discussion going.

Last edited by ridikas; 26th August 2016 at 04:25 AM.
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Old 26th August 2016, 06:23 AM   #2
Calvin is offline Calvin  Germany
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Hi,

its just feeding the pickup signal into the inverting input of an OPAmp.
The gain of such a stage is the quotient of feedback impedance over pickup impedance.
Now the impedance of a MC-Pickup is assumed a resistance plus a very small inductance (iIrc: 40R+17H for the Denon DL103 for example).
The impedance plot beeing almost fully resistive over the audio band.
The lower the resistance -and at the same the inductance and output volume- of a MC the higher will be the gain of the stage -> automatic adjustment.
Feeding into the inverting amp also omits with commonmode distortions -probabely not of relevance at these low levels, but then...
As the inverting node of the OPAmp is a very-low impedance node (virtual ground) the damping of the cartridges HF-resonance is maximal.
You cant simply switch in a different input resistor to tune the HF-response of the cartridge.
I assume to many a Denon 103 would appear too dark tainted, overdamped with such an input stage.
These inputs seem to suit low inpedance cartridges better.
MM-pickups have much higher impedances and the high inductance of the coils make the impedance plot non-constant over the audio band.
Together with the high damping of the virtual gnd input it rendes this input stage unsuitable for MMs.

jauu
Calvin
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Old 26th August 2016, 06:53 AM   #3
ridikas is offline ridikas  United States
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Thank you so much for that explanation. Now I perfectly understand how this sort of circuit operates.
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Old 26th August 2016, 09:57 AM   #4
DF96 is offline DF96  England
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So far from providing an automatically adjusting load, this simply places a virtual short across the cartridge? It automatically adjusts gain, not impedance.
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Old 31st August 2016, 02:01 AM   #5
wobbly is offline wobbly  New Zealand
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Default Aqvox

The Aqvox phono seems to be what you are describing.
This uses the low impedance MC as a current source, as you say into a virtual short, and amplifies the derived voltage from there.
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