A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

[mediatechnology]

Updated the schematic to remove a couple of Cs and change the TL431 shunt current.



Large schematic: https://proaudiodesignforum.com/images/ ... ematic.png

I do have one assembled unit for sale on the KA Store: https://ka-electronics.com/shop/index.p ... uct_id=104

[billshurv]

Before I drive myself mad playing spot the difference, does the new schematic reflect the BOM or do I need to make a couple of tweaks?

[mediatechnology]

The Mouser BOM has been updated.

Removed 4 Cs and changed two R values.

C1, C16 not installed.
C6, C21 not installed.
R34, R74 were 100Ω now 150Ω.

[mediatechnology]

A an FFT/THD measurement of the second PC board.

Signal flow: DAC > THAT1646 > 60dB Pad > Preamp > A/D

Left channel (blue): Preamp Out
Right channel (red): THAT1646 Out Generator Monitor

I used 0 dBu output since at higher levels the THD differences between the test chain and the DUT become very small.


ZTX851 Moving Coil Preamp, THD, Gain 62 dB Blue Out Red Gen Monitor

It appears that most of the added distortion, about 0.0002%, is third.

I'm using a flat preamp gain of 52 dB for the DL-103 with an additional +6 in the EQ/Monitor switcher for RAW recordings.
Seems low, but its not for an A/D with a 4V RMS (+15) overload point.

[Hans]

Wayne,

In the course of this thread I probably missed it, but why have you chosen for a 62dB MC amp.
For a head amp to be connected to an MM input it's way too much gain, and for a full version including Riaa you will still need another 20dB.
Or was this amp intended to be connected to an A/D converter, doing the remaining part digitally ?

Hans

[mediatechnology]

Hans -

This is in no way designed to be used as a "head amp." Where did you get that idea?

It's part of the Phono Transfer System which is designed for direct A/D conversion or "line-level" analog RIAA EQ. https://ka-electronics.com/shop/index.p ... duct_id=79 (This is the assembled MM combo. I have bare MM and EQ boards as well.)
Schematics here: viewtopic.php?f=7&t=753

In a flat, "RAW," capture 1 kHz 5 cm/s should be at a maximum of about -20 dBFS if not -20 to -25 dBFS.
The A/D overload point, sometimes 2 or 4 V RMS, usually determines the preamp gain.
The RIAA EQ gain is separately adjustable with a trim.

Precise gain adjustment with a trim below 5-10Ω is a bit problematic so for the higher gains jumpers are best.
The exact best setting is a little hit and miss unless one installs their own Rg.

For the DL-103 I'm using an Rg of 3.2Ω. (jumpered 2.2 + 1Ω).
The total gain of the flat preamp front-end is 52 dB.

Since I'm routing the signal through the PTS there is another 6 dB gain built into the THAT1646 for a total flat, RAW gain of 58 dB.
So just under 60 dB flat for the DL-103.

The 62 dB gain maximum (+68 with a PTS) is for lower-output carts.

IIRC my 1 kHz "0" is around -28 dBFS which isn't as much headroom, in an unequalized workflow, as you might think.
I routinely see peaks of -6 to -3 dBFS on really hot bright cuts.
Clicks are another story.

[mediatechnology]

I haven't built this head amp (for which I have no need) but with ZTX 851/951s it ought to perform very well.

"Moving Coil Phono Head Amp," Zetex AN-23, March 1996. viewtopic.php?f=12&t=1166

[billshurv]

There is some discussion on the horrendously amatuerish approach of a phantom powered MC headamp feeding the mic input on a USB sound card. A lot of people have these and it's a cheap way of setting up a vinyl ripping setup. Although a few things on the user interface of my Focusrite annoy the hell out of me. But worthy of experimentaion. But that's possibly where Hans confusion came in.

[mediatechnology]

There is some discussion on the horrendously amatuerish approach of a phantom powered MC headamp feeding the mic input on a USB sound card. A lot of people have these and it's a cheap way of setting up a vinyl ripping setup. Although a few things on the user interface of my Focusrite annoy the hell out of me. But worthy of experimentaion. But that's possibly where Hans confusion came in.

Well that may explain it.
We didn't hear back from Hans.

Seems like that if you're going to go to all that trouble of building a head amp for a mic preamp input that it would be better to have more front end gain and just use the USB sound card's Line input.

Most of the head amp circuits I see are designed to feed a 47KΩ load rather than the 2-10KΩ impedance of a mic preamp so how well they'd work and how quiet the USB's mic preamp is would seem to make any solution highly variable.

I had a client once who just connected an MC cart directly to the mic input of his USB I/O and got what he claimed were good results.
I don't remember the details of the equipment but he was streaming/ripping his vinyl with JRiver.

[billshurv]

well my personal interest started from explorations into MM carts with the conclusion that almost none of the commercial solutions are fit for purpose. Holman was onto something back in the 70s. For MM you can make a very cheap phantom powered headamp that you can put inside the turntable. Worth knocking up for a bit of fun*. Plus the look of horror on audiophiles faces when you mention phantom power and Vinyl in the same sentence .

*Other's definition of fun may vary. Not sure JR can have fun unless an angle grinder is involved these days