A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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

Post by ricardo » Tue Mar 22, 2016 6:39 am

ricardo wrote:I've used a small piece of Cu cut from pipes to clip two transistors together, gunking with heatsink compound and finishing of with a bit of heat shrink for quick & dirty applications.

This was a ribbon mike 'cloudlifter' type device for ribbon mikes where it was necessary to keep Vbe matched.

Worked well.

For http://www.aes.org/e-lib/browse.cfm?elib=3941 which required rather better thermal match, the devices were clamped between two small square Aluminium rods with more heatsink compound. Can't remember if we covered the assembly with thermal insulation.

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

Post by mediatechnology » Tue Mar 22, 2016 7:15 am

I've used a small piece of Cu cut from pipes to clip the transistors together, gunking with heatsink compound and finishing of with a bit of heat shrink for quick & dirty applications.
I've considered that and would like to epoxy them to make "custom" ( :lol: ) dual matched low noise transistors.
The question is, what low-cost readily-available epoxy can I use?
The minimum order for Wakefield Delta Bond (1 oz) would be a lifetime supply and is about $50: http://www.alliedelec.com/wakefield-152-1b/70236725/

I'm seriously entertaining the thought of using regular epoxy or JB-Weld.
I used Super-Glue one time on LEDs and I think it degraded them.
Solvent contamination of the internal die concern me.

Maybe I'll try conventional Zinc/Titanium Oxide heatsink compound with a metal tube as ricardo suggests and heatshrink over the whole thing.

Paul mentioned that it would make service difficult but it isn't any more difficult than unsoldering a dual transistor.
If the heatsink goo doesn't ooze from the ends when the heatshrink is heated it ought to make a nice package.

I'm not sure how much air current induced DC drift matters in a sealed box but having them coupled better does make the servo work less.

BTW I listened again at 56 dB gain for any sign of popcorn noise and there was none.
The ZTX851 seem to be very quiet, very consistent and the rbb well under 2Ω.
https://ka-electronics.com

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

Post by ricardo » Tue Mar 22, 2016 7:29 am

mediatechnology wrote:I'm seriously entertaining the thought of using regular epoxy or JB-Weld.
I used Super-Glue one time on LEDs and I think it degraded them.
I've used hardware store Araldite and it works well but I didn't find any advantage over H/S compound and you couldn't take it apart if you had too. I think the Zetex packages (as are most TO92) are epoxy anyway so should be compatible

I wouldn't use SuperGlue.
BTW I listened again at 56 dB gain for any sign of popcorn noise and there was none.
The ZTX851 seem to be very quiet, very consistent and the rbb well under 2Ω.
These would now appear to be the devices of choice for very LoZ LN applications :)

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

Post by mediatechnology » Tue Mar 22, 2016 7:43 am

These would now appear to be the devices of choice for very LoZ LN applications
+1. Very much so.
I need to try the ZTX951 PNP and directly compare it to the 2SB737 samples I have.

I'll try regular epoxy and let you know.

Also looked at Artic Silver: http://www.amazon.com/Arctic-Silver-Pre ... B001IMN6Y6
It's non conductive: http://www.arcticsilver.com/arctic_alum ... hesive.htm
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Gold
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Gold » Tue Mar 22, 2016 11:09 am

I use Locktite brand adhesive epoxies. The dispenser gun is $20 and the cartridges are about $!5. There are many different epoxies to choose from. The 3M system gun starts at $85 and the epoxies are more expensive too.
http://www.mcmaster.com/#high-strength- ... s/=11ng2e7

Maybe the top of a tin can would work.

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

Post by mediatechnology » Tue Mar 22, 2016 12:49 pm

I found some 9/32" brass thin wall tubing in my stash that is perfect.
Aluminum would be better: viewtopic.php?f=12&t=796
I may visit Home Depot today and see if they have any aluminum tubing or short spacers with a 9/32" internal ID.

The ZTX851 are inexpensive enough that I can experiment.
I would like for people to be able to solder the transistors on the board, test it, then dab on goo and slip a metal sleeve over it.
After the epoxy cures then set DC balance.
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Gold
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Gold » Tue Mar 22, 2016 12:58 pm

Another idea I had was to use a little bar stock. Drill it out to the minimum diameter (9/32"), then use a small reamer or tapered bit to taper the hole. Then you could slip it over the transistors until it's tight. It requires a few more tools than a tube but it may work better.

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JR.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. » Tue Mar 22, 2016 1:59 pm

Back in the day for thermal bonding biasing diodes to power amp heat sinks so the class AB bias voltage would track the power transistors with temperature, we would just route the diode(s) through round clearance holes in the heat sink, perhaps a dab of heat sink compound was added to make a better thermal contact. As I've mentioned before, if there isn't major heat flow (power dissipation) the thermal resistance device to sink is less significant.

If these two devices are running at relatively constant power dissipation, perhaps two clearance holes in a common piece of aluminum stock and some extra thermal compound would provide adequate thermal tracking.

I guess you could measure this, or just do whatever you feel like. :lol:

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by mediatechnology » Tue Mar 22, 2016 4:00 pm

Introducing the ZTX-851 differential pair in the "TO-1/4-20" package.

Image

This works far better than the heatshrinked pair of ZTX851.
I had to taper ream a 1/4-20 nut and then dabbed 5 minute epoxy onto the top.
The epoxy filled the voids.

I also made a pair simply epoxy'd together which is about the same as the heatshrinked pair.

The TO-1/4-20 (aka nut) passes the hand wave air current test with only minimal short term drift.

I did have to expand the DC balance trim range to accommodate ZTX851 Vbe differences.

Since this works it looks like a TO-5 heatsink will also work.

EDIT: An M7 nut works even better and does not require reaming to fit 2X ZTX851 in the e-line package.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Gold » Tue Mar 22, 2016 6:34 pm

Nice!

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