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Old 12th March 2014, 10:20 PM   #1
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Default Phono pre share P/S with Main Pre?

Hello all,

I am considering my options for a phono pre-amp and the one I am particularly interested in is the RJM VSPS.

Currently, I use a modified HLLY AMK-II pre-amp, however it does not have a phono input.

What I am wondering is, can I share the power supply of the AMK-II with the VSPS?

More specifically, the AMK-II has an R-core transformer with 15vac-0-15vac (+ 9vac tap, but that is irrelevant). The VSPS has its own 12v regulators (LM7812 and LM7912). Based on the specs of the LM7x12 regulators, feeding it 15v (or a bit more after rectification) should not be a problem at all. So, theoretically, I could take leads off the two 15v, run them through rectifiers and then straight to the VSPS (as per RJM Audio - Construction Guide).

The question is, of course, is this a workable solution? Does sharing a power supply like this introduce unwanted noise or other anomalies that I am unaware of?

I would perhaps try to take it one step further, and fully integrate the VSPS into the AMK-II such that the AUX input of the AMK-II becomes a permanent PHONO input.

Worthy of further discussion or simply crazy?
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Old 13th March 2014, 03:26 PM   #2
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Crazy, I guess.

(but I'm going to try it anyway)
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Old 14th March 2014, 03:36 AM   #3
Zero D is offline Zero D  United Kingdom
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Hi, the rectified + - DC voltage from a 15V tranny will be about 21V. Depending on the load that the AMK-II takes though i expect it would be less. I would test the rails with a Volt meter & see.

LM7812 and LM7912 will be ok after rectification on that tranny, but i would put a heatsink on them.

If the AMK-II is regulated @ say + - 15V you could feed the LM7812 and LM7912 from there = even better smoothing

Quote:
fully integrate the VSPS into the AMK-II such that the AUX input of the AMK-II becomes a permanent PHONO input.
Indeed you could, which would be neater etc.

Whatever option you choose, pay CLOSE attention to ground connections, otherwise you could end up with hum etc ! Let me know what you decide & i'll give you more info

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Worthy of further discussion or simply crazy?
Not crazy @ all
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Old 14th March 2014, 03:55 AM   #4
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Wow, cool, thanks for the response.

My VSPS kit is one the way. Once I have it assembled, I will determine if the AMK-II chassis has enough room for a full integration. I think it does, as long as the caps on the VSPS are not too tall. I may end up substituting shorter, fatter caps if I need to. I may even be able to use the chassis (which is aluminum) as a heatsink for the LM7x12s, if I get creative during assembly of the VSPS.

I will certainly post back here for further advice before commiting to anything.

Thanks again!
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Old 14th March 2014, 04:00 AM   #5
Zero D is offline Zero D  United Kingdom
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Quote:
Wow, cool, thanks for the response.


Quote:
I may end up substituting shorter, fatter caps if I need to
Good idea

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I may even be able to use the chassis (which is aluminum) as a heatsink for the LM7x12s
Make SURE you use insulators, if you do.

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I will certainly post back here for further advice before commiting to anything.
OK ! Might be worthwhile sending me a PM when you do, just in case i miss your post
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Old 5th April 2014, 03:26 PM   #6
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Success!!!

My VSPS kit arrived and I assembled it a few days ago (thanks RJM!).

I studied the AMK-II schematic and discovered that there are test points for both +15v and -15v, and a nice big connection point for common, so I decided to attempt full integration of the VSPS.

I used small side cutters to snip the 3 leads from the AUX RCA jacks to the AMK-II main board. Then I soldered input and output leads (solid core CAT5e) to the RCA jacks and the main board, respectively. Next, I soldered those leads to the appropriate input and output points on the VSPS. Then I did likewise between the +/-15v and common points on the AMK-II and the corresponding power input points on the VSPS. Finally, I ran a lead from the input ground of the VSPS to an internal case screw which conducts to an external screw, via a copper board peg (for an external phono ground screw).

The VSPS is small and light enough that the various CAT5 leads can handle basically suspending it upside down over the AMK-II board. I used industrial electrical tape to provide insulation in appropriate areas, and then a couple of pieces to stabilize the VSPS.

I tested it before reassembling, and all seemed in order, so I put it all back together and I' m happy to say my AMK-II now has a permanent, perfectly functioning phono input!

Here are some terrible photos I took during the process

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Last edited by cogitech; 5th April 2014 at 03:30 PM.
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