A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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

Post by billshurv » Wed May 02, 2018 4:14 pm

mediatechnology wrote:
Tue May 01, 2018 1:37 pm

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.
Yeah. Initially direct into MiniDSP HD but longer term am leaving space in the preamp for the TI EVM that you are using, so if it works with that I'm happy.

I'll ask around, I have a selection of duff MCs but not that one.

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

Post by billshurv » Wed May 02, 2018 4:18 pm

mediatechnology wrote:
Tue May 01, 2018 5:21 pm
I went back and revisited AES48-2005.


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.

I need to go away and think about that. My assumption was that, once inside the case simple twisted pair was assumed. Let me check around and see if I can find anything in the whitlock presentations.

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

Post by mediatechnology » Wed May 02, 2018 4:41 pm

The low-level MM input leads cry out for the electrostatic shielding of shielded twisted pair from other stuff inside the box including tail biting itself.

MC leads, inside the box, would seem to benefit from a nice controlled tight twist with good balance more than shielding.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by billshurv » Wed May 02, 2018 5:54 pm

Agree on the twisting of the MC leads. My MM solution is somewhat different so doesn't suffer from some of the normal problems, but has it's own set of issues :).

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

Post by mediatechnology » Fri May 04, 2018 8:15 am

I remembered that one of my MM preamp customers shared his MC experiences with the network in series with pin 1.

Image

He uses the JT-346-AX in his MC setup and reported in his situation it was always quieter with the network in rather than hard-grounded.

I could put that on the board at the G connection but it really should be at the XLR with short leads to the chassis as in AES48-2005.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by paschulke2 » Sat May 05, 2018 3:27 am

mediatechnology wrote:
Mon Apr 30, 2018 9:11 pm
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.
The idea with the nut is already a good one.

What you want to achieve is that the dies of both transistors have the same (not necessarily constant) temperature. A good thermal contact of the transistor cases alone does not provide this – instead you only make sure that the temperature is the same where the two cases touch. With an outside thermal gradient in the right direction …

Code: Select all

          T1       T2
       +-------+-------+
       |       |       |
warm   |  die  |  die  | cold
       |       |       |
       +-------+-------+
… the temperature of the two dies will still be different. Therefore you need to make sure that there is no thermal gradient on the outside of the transistor pair. The nut is already quite good for this purpose but the best solution is a small can of copper or aluminium (silver for the high end version …) that also extents a bit over the wires of the transistors. I'd probably take a round piece of aluminium (~8mm long, ~8mm diameter) and drill a ~6mm diameter blind hole.

Thomas

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

Post by mediatechnology » Sat May 05, 2018 6:17 am

Thomas thank you for posting.

Tubing is exactly what I need. I'm looking for some form that's already cut.
When I'm out this weekend I was going to look for some eyelets used in leather craft.

A spacer or sleeve would serve as a tube but one with a hole larger than a #10 is bard to find.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX851

Post by Speedskater » Sat May 05, 2018 8:04 am

mediatechnology wrote:
Fri May 04, 2018 8:15 am
I remembered that one of my MM preamp customers shared his MC experiences with the network in series with pin 1.
.............................................
The thing about that resistor, capacitor, switch network is that it lets the camel's nose into the tent. It would be better to hardwire pin 1 to the chassis and put the network in the cable shell.
Kevin

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

Post by billshurv » Sun May 06, 2018 6:53 pm

Been looking more into this Pin 1 issue, specifically the cases where you would want to bring the ground into the case (or compromises force it), but wondering if it's unwanted noise in this thread and perhaps warrants splitting off into another discussion as it can get contentious very quickly...

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

Post by mediatechnology » Sun May 06, 2018 9:04 pm

billshurv wrote:
Sun May 06, 2018 6:53 pm
Been looking more into this Pin 1 issue, specifically the cases where you would want to bring the ground into the case (or compromises force it), but wondering if it's unwanted noise in this thread and perhaps warrants splitting off into another discussion as it can get contentious very quickly...
I agree. We can figure out what works once we get boards.

I've run some more noise tests that I'll post tomorrow. I used a 5532 for the input op amp.

I'm getting a NF of 1.6 dB for a 30Ω R source (approx. a DL-103's 40Ω||100Ω).
With 10Ω Rsource a NF of 4 dB is measured. Ein -138.68 dBu.

With a 1Ω source, in which the preamp's contribution dominates, the NF is a poor 12dB. Ein -140.65.
But -140.65 represents a noise density of 0.51nV√Hz which is pretty good for only two ZTX851.
If a 1Ω Rgain were used that figure would improve a little bit.

I think the ZTX851 do about as good at 5.5mA as they did at 8 mA.
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