Flat Balanced Input Moving Coil Phono Preamp Construction Information

[mediatechnology]

Very nice!

I tried the MC preamp with the TDK switcher and as long as the preamp board wasn't near the primary-side and heat sink bar it was as quiet as the linear bench supply.
With yours at right angles and on the far side of primary it ought to be quiet as well.

I like the RJ-45 bulk-head female-to-female termination. I would never have thought to use a RJ-45 turn-around. That's very clever.

You might want to reverse the jumper shunts - they're upside-down from they way I usually install them and will be easier to pull if inverted on the header with the large end on the top. They're easier to grab with needle-nose.

[grufti]

I can report on two improvements since I finished the MC preamp. I switched to tabbed jumper shunts. Those are so much easier to move around and I had simply run out.

The other item is more interesting. I found a very nice 5W +/-15V switcher at mouser for $20. It's a RECOM RAC05-15DC. The 10W version RAC10-15DK/277 costs only $10. Not sure why that is. I'm very happy with the tiny 5W switcher.

https://www.mouser.com/ProductDetail/RE ... KYOmsuw%3D
https://www.mouser.com/ProductDetail/RE ... vSbg%3D%3D

[mediatechnology]

I put one of those supplies in my cart to check out on the bench.
Thanks for sharing that.

[mediatechnology]

When I first built the MC preamp I "temporarily" used the box I had made for the MM preamp.
That was about a year and a half ago.
The MM preamp was left without an enclosure.
I finally got around to making a box for the MC preamp and was able to remount the MM preamp in its original enclosure.

The box for the MM preamp wasn't really tall enough for the MC board.
I used a Hammond 1590JBK for the MC board.


Flat Balanced Input Moving Coil Phono Preamp Using the ZTX851.

DB-9 connectors are used to allow me to switch between the MM and MC preamps.

[mediatechnology]

Updated January 25, 2021: A new PCB version, assembly instructions and Project Manager link are available. Improvements include a simpler servo, selected components to improve CCIF IMD2 distortion and an output pad for head amp applications.

[mediatechnology]

I've decided to pre-assemble the matched dual ZTX851 transistors to avoid having to epoxy the nut on after assembly.

Here's how the transistors are matched: viewtopic.php?f=6&t=1153

The latest version uses a smaller 10-24 or 10-32 "acorn" cap nut. (An M6 is close.)
Though they are harder to find I use aluminum nuts for greater thermal conductivity.
The hole is drilled out to 15/64" to clear the TO-92 E-line transistors mounted back-to-back.
I set the depth of the hole so that the top of the transistor rests on the ledge formed by the end of the bit.

The transistors are first epoxied together on the flat face using an old Protoboard to hold them while they cure.

In a separate step I fill the acorn nuts with epoxy and let it begin to cure to increase viscosity.
(Do not use plastic glue if you try this at home. Plastic glue melts transistors. It has to be epoxy. I use JB Weld. See: viewtopic.php?f=7&t=1179&p=14337#p14337 )
I then insert the transistors and wait for them to cure.



The flat moving coil preamp doesn't require the thermal hat but I found that the servo activity at infrasonic frequencies is far less due to greater thermal mass and greater device coupling.
If a nut seems like overkill gluing the transistors together is still a good idea.

The nut seems to lower the 1/f-like "noise" thermal gradients cause.
You can't really see this very low frequency noise in the actual output - its high pass filtered by the servo - but you can see it in the servo output.

From an earlier post:

This is the servo response (at the servo output not the audio output) of one channel without the transistors glued in open air:



This is the servo response (at the servo output not the audio output) of both channels with the transistors glued and the thermal "hat" installed in open air:



(Some of the noise in the above images are the Velleman scope which is noisy even with the input grounded.)

[mediatechnology]

I'm frequently asked how to do switchable termination for the Flat Moving Coil Preamp

The simplest termination scheme uses a 6 pin female connector plugged onto the termination jumper header. This option provides four values: 10, 30, 100 and 1KΩ or any three values <1KΩ plus 1KΩ.

The switch wires directly to the header and uses the on-board resistors.
A Mouser BOM for the female housing and pins is here: https://www.mouser.com/ProjectManager/P ... 08ae674507

For more than four values a 2 pole 6 position miniature rotary switch such as the NKK MRA206 can be used with resistors mounted on the switch and located at or near the XLR input connectors.

NKK MRA206 2 pole 6 position switch:
https://www.mouser.com/c/?q=633-mra206
https://www.mouser.com/datasheet/2/295/ ... 663588.pdf

Here is a connection diagram for the Flat Moving Coil Preamp showing switchable input termination.



Large format: https://ka-electronics.com/images/png/M ... 2923-1.png

The NKK MRA206 pinout (front view) looks like this:



Resistors R1-R6 are in parallel with the internal 1KΩ bias resistors and larger values require calculating termination with them in parallel.

The values of R1-R6 can be calculated by:
1/Rdesired - 1/1000 = 1/Ractual

So for a 400Ω termination:
1/400 - 1/1000 = 1/Ractual = 667Ω = 665Ω rounded to an E96 value.

A spreadsheet to calculate Moving Coil Input Termination is here: https://ka-electronics.com/calculators/ ... ulator.xls



Two Input with Independent Termination



Large format: https://ka-electronics.com/images/png/M ... 2923-1.png

For two inputs with only one termination selector move the switch connections to after the 4PDT switch.

Mouser Project Manager link for the NKK 4PDT (gold contacts) and MR206 Rotary Switch: https://www.mouser.com/ProjectManager/P ... e465205d3a

On the BOM above the switch that includes the knob is about $2 less expensive.

A similar NKK rotary switch wired with resistors. This one is a hastily-wired 12 position mic preamp gain switch I use on the bench.

Tip: When wiring the switch be sure to solder the lead to the center common pins before installing the resistors. If you don't you won't be able to get the soldering iron tip into the recess formed by the resistor bodies.



A DIN power pigtail for the Meanwell P25A:

[mediatechnology]

Updated the FMCP assembly instructions, Mouser Project Manager BOM and Schematic here and in the original post.

Assembly Instructions:
https://proaudiodesignforum.com/images/ ... 2325_1.pdf

Mouser Project Manager BOM:
https://www.mouser.com/ProjectManager/P ... e01a329726

Large Schematic:
https://proaudiodesignforum.com/images/ ... amp_19.png

[Tubetec]

The first question I had as I looked down the page was what were the stainless nuts for .
So its about providing a kind of thermal inertia , where the devices heat and cool less over very short periods of time ?

[mediatechnology]

The first question I had as I looked down the page was what were the stainless nuts for .
So its about providing a kind of thermal inertia , where the devices heat and cool less over very short periods of time ?

Yes. I think they do two things.
The nut and glue help to thermally-couple the two devices and the added thermal mass slows the rate-of-change.

You may have missed it in this thread but I ran some experiments in the design thread that shows less servo activity. https://proaudiodesignforum.com/forum/p ... 651#p13651

I eventually found some Aluminum nuts for better thermal conductivity. Most of the hardware store offerings are stainless with lower conductivity. The experiment was using stainless.

An FFT of the infrasonic response with the servo off the thermal "drift" looks a lot like 1/f noise. With the thermal nuts added there's a less "1/f-looking" response.

Some of the higher gain but low rbb ZTX devices on this board along with some modifications might make a good mic preamp.