Flat Balanced Input Moving Coil Phono Preamp Construction Information

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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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mediatechnology
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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.)

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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.

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This is what the finished board should look like after installation.
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