Low noise Balanced MC Pre

[syn08]

Not sure why you guys don't try using a floating power supply, as originally Hawksford did in the LFD phonostage. That would guarantee a zero current through the cartridge under all conditions.

 

[Bonsai]

Sounds like a good idea but you have to generate it - either another trafo or one of those little dc-dc converters.

Is it not just easier to servo the offset out?

 

[syn08]

Or one of those little solar cells and some white LEDs .

As you can see, a servo may not cancel the transient currents through the cartridge (it that's a concern, of course). Otherwise, not even a servo is required.

In fact, I believe a servo, while certainly cancelling the offset, may make the transient currents even worse.

P.S. https://linearaudio.nl/sites/linearaudio.net/files/Didden-Hawksford_Part2.pdf page 28

 

[Bonsai]

I'm looking at fig. 13 of your link - very interesting!

 

[syn08]

That's the Hawksford LFD phonoamp core. I don't think a more detailed schematic was ever published by himself (I hope I'm wrong).

All kind of implementations can be found on the net, many showing a lack of understanding of his topology advantages.

 

[as_audio]

Complexity is moved to floating supplies.

 

[Bonsai]

Look at jackinnj proposal - two 8 pin packages

 

[as_audio]

Three for one channel according to fig. 13

 

[jackinnj]

I believe this is the ZXTN2018F is the equivalent of ZTX851

 

[Hans Polak]

That's the Hawksford LFD phonoamp core. I don't think a more detailed schematic was ever published by himself (I hope I'm wrong).

All kind of implementations can be found on the net, many showing a lack of understanding of his topology advantages.

At switch off, supply caps may partly discharge through the Cart.

Hans

 

[mkc]

That's the Hawksford LFD phonoamp core. I don't think a more detailed schematic was ever published by himself (I hope I'm wrong).

It can be found in the mpp thread. I believe Joachim posted it.

 

[syn08]

At switch off, supply caps may partly discharge through the Cart.

No, take another look.

 

[Hans Polak]

I measured it

Hans

 

[mkc]

Not easy to search for. But, found it.
https://www.diyaudio.com/forums/analogue-source/154210-mpp-145.html#post2215423

 

[as_audio]

Schematic added

 

[mkc]

Thanks, as_audio

 

[syn08]

Schematic added

Some additions to the original schematic (the servo, the balancing pot) are triggering (transient) cartridge DC currents. Not a big deal, but one of the original schematic features is lost.

 

[syn08]

I measured it

Please post the schematic. As I said, there are many variants flying around that are losing this feature in the process of "improving" it. One good example above.

 

[as_audio]

The example in post 115 has the Hawksford drawing style.

 

[as_audio]

Concerning cartridge dc concerns

Not sure why you guys don't try using a floating power supply ..snip..
That would guarantee a zero current through the cartridge under all conditions.

On the other hand :

With or without input cap, with or without a floating power supply, with bipolars or jfets,
I believe a power on/off current pulse through the cartridge is unavoidable. ..

It is frequently claimed that pre-amps with floating supply can
by definition not inject dc into the cartridge coil.

I would love to see a formal explanation for this.