A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
billshurv
Posts: 375
Joined: Thu Feb 01, 2018 7:07 am

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by billshurv » Mon Apr 30, 2018 7:03 pm

Put me down for a set when you get around to getting boards made.

Dumb question, on the input side you have a ground connection. As that is pin1 for a balanced connection when would you actually connect something there?

User avatar
mediatechnology
Posts: 3911
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Mon Apr 30, 2018 9:11 pm

Thanks Bill!

I need to figure out a good way to thermally-couple the ZTX851 before I do layout.
The M7 nut filled with epoxy works very well but doesn't look particularly elegant.
I'm going to look at using a short spacer or tube.
I'll post back when I start layout.

To answer your question the G connection at the input is for the input XLR pin 1 and the cart cable shield.
"G" doesn't carry signal current.
The common mode rejection stage does forward-reference to it.

Is your current cart wiring already balanced and floating?
https://ka-electronics.com

Free Hong Kong.
"We’ve Seen This Movie Before–How Communist China Is Following Nazi Germany’s Playbook Against America" https://www.breitbart.com/politics/2019 ... t-america/
"China Warns Brands to Accept Communism, Freedom of Speech ‘Ridiculous’" https://www.breitbart.com/asia/2019/10/ ... idiculous/
"There's A War On For Your Mind" Banned.Video https://banned.video/

billshurv
Posts: 375
Joined: Thu Feb 01, 2018 7:07 am

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by billshurv » Tue May 01, 2018 7:02 am

The main tonearm is already floating. The cable, although RCA terminated is STP and the screen comes out separately to a Tag to mount on the chassis from each side, so perversely there are 3 connections to be made to the ground stud, one from each cable screen and one from the cartridge itself. Done more to impress people paying full list for the tonearm than anything else I suspect :). Other arm is being modded, but is for MM use.

As the cart itself floats I couldn't see a need to reference the screen so assumed could wire as per AES48.

User avatar
mediatechnology
Posts: 3911
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Tue May 01, 2018 7:30 am

It sounds like you're already there.
What cart are you using and what are its impedance, level and loading requirements?

I have to wonder "how many modern RCA phono cables are actually STP with only RCA terminations?"
I don't dare cut mine to find out.
I don't use mine and someday they'll belong to someone else.
I made my own home brew STP cart cable.

I have to use two different screen connections in my setup as well.
One obviously for the STP cart wiring.

It took me awhile to find the second which is the drain wire/ground connection for the platter itself on the SP15.
Finding that grounding post was a breakthrough.
It had been years since I had looked underneath it.
I hate pulling the platter if I don't have to.
Always risky.
https://ka-electronics.com

Free Hong Kong.
"We’ve Seen This Movie Before–How Communist China Is Following Nazi Germany’s Playbook Against America" https://www.breitbart.com/politics/2019 ... t-america/
"China Warns Brands to Accept Communism, Freedom of Speech ‘Ridiculous’" https://www.breitbart.com/asia/2019/10/ ... idiculous/
"There's A War On For Your Mind" Banned.Video https://banned.video/

billshurv
Posts: 375
Joined: Thu Feb 01, 2018 7:07 am

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by billshurv » Tue May 01, 2018 11:31 am

My daily driver is a Benz, which is 0.4mV 12R coil and 1k loading
I have a plan to get an ortofon retipped which is .25mV 6R and 100R loading

I also have a very sad Denon DL304 that may or may not get a new cantilever. That is an edge case at 0.18mV but 20R as an air coil. But that is off in dream world at the moment. So 0.25 to 0.5mV and 6-12R covers my sweet spot.

User avatar
mediatechnology
Posts: 3911
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Tue May 01, 2018 1:37 pm

billshurv wrote:
Tue May 01, 2018 11:31 am
My daily driver is a Benz, which is 0.4mV 12R coil and 1k loading
I have a plan to get an ortofon retipped which is .25mV 6R and 100R loading

I also have a very sad Denon DL304 that may or may not get a new cantilever. That is an edge case at 0.18mV but 20R as an air coil. But that is off in dream world at the moment. So 0.25 to 0.5mV and 6-12R covers my sweet spot.
Thanks for the data.
I'm trying to remember - were you going to convert flat and do RIAA in DSP?
If so, what is the overload point of your A/D?
Mine are +8 and +15 dBu.

Trying to figure out the best gain ranges.
My rule-of-thumb for flat is 23 dB less gain than what is needed for EQ'd playback.

I'm looking for a damaged DL-103 - to use for test coils - if you know anyone that has one.
https://ka-electronics.com

Free Hong Kong.
"We’ve Seen This Movie Before–How Communist China Is Following Nazi Germany’s Playbook Against America" https://www.breitbart.com/politics/2019 ... t-america/
"China Warns Brands to Accept Communism, Freedom of Speech ‘Ridiculous’" https://www.breitbart.com/asia/2019/10/ ... idiculous/
"There's A War On For Your Mind" Banned.Video https://banned.video/

User avatar
mediatechnology
Posts: 3911
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Tue May 01, 2018 5:21 pm

I went back and revisited AES48-2005.

Image

I agree AES48 is the way to go.
It doesn't show how to treat internal shields and doesn't even show having them perhaps assuming all connectors in 2005 were board-mounted.
You would think they would have shown STP between the XLR and PC board.

For internal shields in a phono preamp what I would do is use the G terminal on the preamp input to telescope the internal shielded input cable to the input XLR.
Then drop that shield at the XLR so there's no connection of the internal shield to pin 1. (Pin 1 then follows AES48 as shown.)
Phantom complicates that for Mic preamps.
For MC and MM pickups is my recommended shield treatment technically correct?

For the 9 pin D connector on my flat MM preamp I used a slightly different approach telescoping the shield out all the way to the arm base.
https://ka-electronics.com

Free Hong Kong.
"We’ve Seen This Movie Before–How Communist China Is Following Nazi Germany’s Playbook Against America" https://www.breitbart.com/politics/2019 ... t-america/
"China Warns Brands to Accept Communism, Freedom of Speech ‘Ridiculous’" https://www.breitbart.com/asia/2019/10/ ... idiculous/
"There's A War On For Your Mind" Banned.Video https://banned.video/

Speedskater
Posts: 19
Joined: Mon Jun 02, 2014 8:14 am
Location: Cleveland, Ohio, USA
Contact:

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by Speedskater » Wed May 02, 2018 8:06 am

It would be better if:
a] the AC line voltage Safety Ground attached to the chassis near where the AC cord enters the chassis.
b] the audio circuit common is attached to the chassis at a single point near the audio input connectors.
c] the DC supply common is attached to the chassis at the same single point as the audio circuit common.
Kevin

User avatar
mediatechnology
Posts: 3911
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Wed May 02, 2018 9:55 am

Speedskater wrote:
Wed May 02, 2018 8:06 am
It would be better if:
a] the AC line voltage Safety Ground attached to the chassis near where the AC cord enters the chassis.
b] the audio circuit common is attached to the chassis at a single point near the audio input connectors.
c] the DC supply common is attached to the chassis at the same single point as the audio circuit common.
Thanks for the insight.
I've never liked seeing anything else tied to the safety ground either.
I wondered about that from a regulatory/safety perspective when I saw it.

WRT a] Isn't there also a UL requirement that the safety ground attachment to the chassis "shall not be used for any other purpose?" I've been told in the past that excluded using the same screw and nut for instance to mount the ear of a transformer. The "shall not be used for any other purpose" requirement seems to apply to electrical connections as well.

It seems like when "a, b and c" share the same connection as in AES48 "a" isn't being used exclusively for safety ground.

The other thing I noticed on most of the switchers I've tested is that the primary-side input filter, and often the output filter, tie to the mounting spacers in order to bond to chassis. viewtopic.php?f=7&t=943
https://ka-electronics.com

Free Hong Kong.
"We’ve Seen This Movie Before–How Communist China Is Following Nazi Germany’s Playbook Against America" https://www.breitbart.com/politics/2019 ... t-america/
"China Warns Brands to Accept Communism, Freedom of Speech ‘Ridiculous’" https://www.breitbart.com/asia/2019/10/ ... idiculous/
"There's A War On For Your Mind" Banned.Video https://banned.video/

User avatar
JR.
Posts: 2942
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by JR. » Wed May 02, 2018 3:27 pm

Don't quote me on this but IIRC the safety ground indeed needs to have it's own nut and be intimately attached to the chassis. There may be some allowance for other wires with their own nuts on top of the first.

Another tidbit I learned the hard way. If any audio terminations are labelled "ground" they must pass the ground bonding test... something like 50A with only single digit voltage rise...

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Post Reply