LDR3X Power Supply Question

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33na3rd

LDR3X Power Supply Question
« on: 29 Oct 2013, 04:55 am »
I have a question regarding the power supply options for the LDR3X.

First of all, please forgive my ignorance as I am not a regular DIYer, but I do own a decent soldering iron and a modest multimeter.

As an owner of a Melos SHA Gold since 1995, I've been a big fan of this type of volume control. My SHA finally started having the familiar Photentiometer problems 2 years ago. The original Photentiometer is somewhat fragile, and not many people want to work on them these days.

I decided that I would like to bring my Melos back to life, with the LDR3X board. I took out the old Photentiometer board & figured all my connections, except for the power supply options.

I have access inside the chassis to 120 VAC, and 24 VDC.

So my question is should I,

1) Tap the internal 24VDC and use a DC to DC converter to get 5 or 12VDC? I have no idea how much amperage is available from the 24 VDC supply that originally feed the Photentiometer board.

2) Tap the internal 120VAC and install a 5 or 12 VDC power supply inside the chassis. I'm guessing that one really wouldn't want a switching power supply inside a line level preamp?

3) Use a separate external 12VDC power supply and connect it to the back of the chassis with a jack?

I apologize to all the advanced builders, if my question seems dumb!

Thanks,

Jeff

tortugaranger

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Re: LDR3X Power Supply Question
« Reply #1 on: 29 Oct 2013, 11:55 am »
I have a question regarding the power supply options for the LDR3X.

First of all, please forgive my ignorance as I am not a regular DIYer, but I do own a decent soldering iron and a modest multimeter.

As an owner of a Melos SHA Gold since 1995, I've been a big fan of this type of volume control. My SHA finally started having the familiar Photentiometer problems 2 years ago. The original Photentiometer is somewhat fragile, and not many people want to work on them these days.

I decided that I would like to bring my Melos back to life, with the LDR3X board. I took out the old Photentiometer board & figured all my connections, except for the power supply options.

I have access inside the chassis to 120 VAC, and 24 VDC.

So my question is should I,

1) Tap the internal 24VDC and use a DC to DC converter to get 5 or 12VDC? I have no idea how much amperage is available from the 24 VDC supply that originally feed the Photentiometer board.

2) Tap the internal 120VAC and install a 5 or 12 VDC power supply inside the chassis. I'm guessing that one really wouldn't want a switching power supply inside a line level preamp?

3) Use a separate external 12VDC power supply and connect it to the back of the chassis with a jack?

I apologize to all the advanced builders, if my question seems dumb!

Thanks,

Jeff

Hi Jeff,

Hardly a dumb question.  :scratch:

The LDR3x has an on board OnSemi MC7805CTG DC voltage regulator that delivers 5 VDC to the board. This regulator is rated up to MAX input voltage of 35V. Incoming voltage to the board is also seen by a Supertex VP3203N3-G PNP mosfet switch that provides the "trigger out" voltage signal when the unit is turned on. This mosfet has a max source-drain breakdown voltage of 30V.

Based on these specs you should be able to use the existing 24 VDC supply.  Just to be sure I just now applied 25 VDC to the board as a test and as expected there was no problem. 

However, please note that since the "12V Trigger Out" feature of the LDR3x is predicated on using a nominal 12 VDC supply, the trigger out would be 24 V (pass through of the supply) when the unit is on which may not be compatible with trigger inputs on other external components.  If you don't use or care about the trigger out feature then this caution is irrelevant.

Also, the actual current draw of the LDR3x with no external connections is no more than 20 ma even though the board specs mention 250 ma. That higher level was recorded on an earlier vintage development board that was attached to multiple LEDs, encoder, external relay board etc.

Cheers,
Morten

33na3rd

Re: LDR3X Power Supply Question
« Reply #2 on: 29 Oct 2013, 03:11 pm »
Hi Jeff,

Hardly a dumb question.  :scratch:

The LDR3x has an on board OnSemi MC7805CTG DC voltage regulator that delivers 5 VDC to the board. This regulator is rated up to MAX input voltage of 35V. Incoming voltage to the board is also seen by a Supertex VP3203N3-G PNP mosfet switch that provides the "trigger out" voltage signal when the unit is turned on. This mosfet has a max source-drain breakdown voltage of 30V.

Based on these specs you should be able to use the existing 24 VDC supply.  Just to be sure I just now applied 25 VDC to the board as a test and as expected there was no problem. 

However, please note that since the "12V Trigger Out" feature of the LDR3x is predicated on using a nominal 12 VDC supply, the trigger out would be 24 V (pass through of the supply) when the unit is on which may not be compatible with trigger inputs on other external components.  If you don't use or care about the trigger out feature then this caution is irrelevant.

Also, the actual current draw of the LDR3x with no external connections is no more than 20 ma even though the board specs mention 250 ma. That higher level was recorded on an earlier vintage development board that was attached to multiple LEDs, encoder, external relay board etc.

Cheers,
Morten


Hi Morten,

That's great news about the 24 volts!

I'll be ordering the LDRX3 soon, and will post pictures when it's done.

Thanks again!

Jeff

33na3rd

Re: LDR3X Power Supply Question
« Reply #3 on: 3 Nov 2013, 05:51 pm »
After further investigation, I discovered that my 24VDC is actually +/- 12VDC dual rail. I am in like Flynn!

It is good to know about the LDRX3's voltage tolerance on the 12 volt input though. I've got a 15 volt power supply available in an old Dynaco Pat-5. I'm starting to get ideas!

tortugaranger

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Re: LDR3X Power Supply Question
« Reply #4 on: 3 Nov 2013, 07:02 pm »
After further investigation, I discovered that my 24VDC is actually +/- 12VDC dual rail. I am in like Flynn!

Just stay positive Jeff. Stay +!  :D

33na3rd

Re: LDR3X Power Supply Question
« Reply #5 on: 23 Nov 2013, 08:23 pm »
My Melos is up & running again!

I posted pictures on the Customer Projects thread page.

Thanks again Morten!

Jeff