Flat Balanced Input Moving Coil Phono Preamp Construction Information

Construction information for DIY projects, including the MS Mid Side Matrix, Elliptic Equalizer, Mastering Console, Phono Transfer System, Insert Switcher and the Dual Class-A Amplifier. You can post your baby pictures here.
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mediatechnology
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Thermally Coupling the ZTX851 Input Pairs

Post by mediatechnology » Tue Nov 26, 2019 4:58 pm

mediatechnology wrote:
Wed Jul 10, 2019 5:09 pm
I wanted to find an easy way to thermally-couple the input devices and finally decided on using a common "acorn" or "cap" nut.

A 1/4-20 or 1/4-28 nut requires drilling out the threads slightly since the fit is snug. An M7 acorn nut, if you can find them, should fit nicely.

The transistors, after installation and test, were first glued together with 5 Minute epoxy. I used a pair of needle nose pliers with a rubber band wrapped around the handle to hold the devices together while the epoxy set. After the transistors were bonded together I filled the voids in the nut with epoxy and coated the outside of the transistors. I let the epoxy set slightly so it wasn't runny. I then set the nut onto the transistors and let it cure. As the epoxy cures the thermal expansion and hydraulic forces inside the dome of the acorn nut will try to force the nut off the transistors. As it cures it needs to keep being pushed down.

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

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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:

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

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Moving Coil Phono Preamp Balanced In Balanced Out with ZTX851 "Chrome Dome" Input Pairs.
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Thermally Coupling the ZTX851 Input Pairs: Part 2

Post by mediatechnology » Wed Dec 04, 2019 5:59 pm

Use two part "5 minute" epoxy. This is some JB-Weld that I got at Home Depot.
Do not use plastic glue or super glue or the transistors may be damaged.

(The little black square thingy in the plunger is the cap.)

Image

https://www.homedepot.com/p/J-B-Weld-0- ... /303710929

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First glue the two transistor pairs together. Use a rubber band wrapped around the handles of needle nose pliers to improvise "epoxy-stats."

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Once the epoxy sets on the input pairs acorn nuts are installed.

I used 1/4-28 fine thread acorn nuts. https://www.homedepot.com/p/Everbilt-1- ... /204604730

The 1/4-20 acorn nuts stocked by Home Depot are a little taller and would have a bigger void to fill. For the transistors to clear the threads of a 1/4-20 or 1/4-28 nut a 15/64" drill should be used to clear the threads. You can use a 1/4" drill but will have a little more float to deal with when setting the nut.

Coat the transistors with epoxy, fill the void in the nut dome and coat the threads with epoxy. Lower the acorn nuts onto the transistors until the bottom of the nut is flush with the bottom of the transistors. Its best to use too little epoxy, test fit the nut, and then add more if needed. If a little epoxy flows out of the bottom its OK just don't use too much and glue everything to the board.

It may be necessary to hold the nuts in place for a few minutes. I've found that the final step is a little easier if the epoxy is mixed and allowed to partially cure until it is gel-like. It's more viscous and the nut tends to stay in place.

Image

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This is what the finished board should look like after installation.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by JP. » Fri Dec 06, 2019 3:22 pm

Hey Wayne,

I've been looking forward to this release for some time. Finished putting together one of my units last weekend.

I used one of the MM boards to balance the signal from my generator with a 1:100 pad at the output to get .775 VRMS to the two 4K99 series resistor at the input of the MC board. I'm not able to get the same output measurements as the build guide:

J5,6,7 -3.958dBu
J5,6 -9.509dBu
J5,7 -13.24dBu
J6,7 was -24dBu with trim set to 13R

Aside from a quick check of resistor values I haven't troubleshot this, rather I've just been listening to it with my XL-88D. Very quiet.

IMG_2971.jpeg

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Fri Dec 06, 2019 3:54 pm

Glad it's quiet and working well for you.
Thanks for joining us!
That's a nice build. What chassis did you use?

I'll need to think about it a little more but you might want to make sure that the jumper shunts aren't shorting to each other.
They have windows on one face to retain the pin and when they are installed side-by-side with the windows facing inward they can sometimes short together. J4/J5 and J6/J7 are where those opportunities are.

Are the output level measurements balanced?

I'll try to zoom in on your pic to look at the resistor values.

EDIT: I think I see the shunts installed with the windows inward.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by JP. » Fri Dec 06, 2019 5:30 pm

This chassis: https://proaudiodesignforum.com/forum/p ... t=30#p9224. I bought 4 when I did the flat MM build. Glad I did too as I see that size isn't available from them anymore. As I was in a hurry to get the build done before I left the house for the week I swapped the board in to one of my MM builds.

Perhaps over the upcoming break I'll work on a new chassis. I wanted to incorporate Jan Didden's silent switcher as the supply. Likely more specmanship than anything as I can't say the Meanwell bricks have been problematic.

J4/5 shunts had the windows inward, J6/7 were not. I've a second unit on the bench for calibration - when I do that I'll check this one again too. I've swapped the shunts for ones with pull tabs just now. For some reason this last batch from Mouser is impossible to pull off the headers by hand.

Output measurement were taken across - and +.


IMG_3009.jpeg

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Fri Dec 06, 2019 5:37 pm

I thought that chassis looked familiar.

I'll double-check my gain numbers on a board here.
The resistors looked right.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by JP. » Fri Dec 06, 2019 6:01 pm

In practice the MM setup has more noise. Not in practicality, but at full volume 60Hz and harmonics dominate from the cartridge. Not so with the MC setup.

One recommendation - put the 4K99 resistors in the BOM. I hadn't read the build guide in-depth so I didn't know to order them. Luckily I've a good stash of extra components.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by billshurv » Fri Dec 06, 2019 7:17 pm

Hi JP,

Fancy seeing you here! Reminds me I need to send you a PM with some Sony MC related questions.

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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by mediatechnology » Fri Dec 06, 2019 8:47 pm

JP. wrote:
Fri Dec 06, 2019 6:01 pm
One recommendation - put the 4K99 resistors in the BOM. I hadn't read the build guide in-depth so I didn't know to order them. Luckily I've a good stash of extra components.
I think that's a good idea.
I might want to increase the quantity of the ZTX851 as well so there are some extras for matching.
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Re: Flat Balanced Input Moving Coil Phono Preamp Construction Information

Post by JP. » Sat Dec 07, 2019 11:50 am

JP. wrote:
Fri Dec 06, 2019 6:01 pm
In practice the MM setup has more noise. Not in practicality, but at full volume 60Hz and harmonics dominate from the cartridge. Not so with the MC setup.
This is solved. The headshell on the EPA-501H is composite, and the AT-150MLX lower body is not electrically connected to the upper. I used the the original ground jumper and a piece of 32 magnet wire to ground the lower body to the armwand.

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