• Get involved.
    We want your input!
    Apply for Membership and join the conversations about everything related to broadcasting.

    After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post.
    If you use a disposable or false email address, your registration will be rejected.

    After your membership is approved, please take a minute to tell us a little bit about yourself.
    https://www.radiodiscussions.com/forums/introduce-yourself.1088/

    Thanks in advance and have fun!
    RadioDiscussions Administrators

RDL STA-1 Floating power requirement question

R

RetroHacker

This is a bit of an odd question, but I figured here would be a good place to ask. I'm wiring up an RDL STA-1 Stick-on line amplifier (datasheet here: https://www.rdlnet.com/downloads/datasheets/sta-1.pdf) for the purpose of converting consumer line level audio output to be able to drive the balanced line level record input on an old AudioCord cart machine.

When I bought the thing, I had just glanced at the specs, saw that it would run on 24V and figured - perfect! I can just power it off the cart machine itself. 24V is available at one of the pins on the remote socket on the back, so I just had planned on using that as a power source. Tonight, however - when hooking it up (after tracing out the pinouts of that connector on the back of the cart machine to find the 24V, since I don't have the correct manual for this model), I decided to reference the STA-1's manual just to be 100% sure this little amplifier runs on 24Vdc. And that's when I noticed the warning - "Do not ground negative". And that the power requirements specified *floating* 24Vdc.

And, of course, the cart machine's power supply is ground referenced, the grounds at the audio input are tied to the ground in the machine, not to mention, my entire stereo setup shares a common ground.

I can, obviously dig around and find myself an isolated 24Vdc power supply - but even there, I have to be careful, since most computer power supplies are again, ground referenced. DC ground is attached to Earth ground, which would then put it at the same ground as the cart machine and the whole rest of the stereo unless I break the ground prong off the other power supply.

Does anyone happen to know *why* this needs floating DC power? Is it simply to avoid potential hum from ground loops, or would I damage something if I were to attempt to power it from a non-floating power source?

Any assistance would be appreciated.

Thank you!

-Ian
 
So, doing some more research on this, and poking at this with my multimeter - the negative on the power input on the STA-1 is definitely not connected internally to the grounds, which is to be expected. It was suggested to me that this might be using a rail splitter internally to generate a +12/-12 supply, which makes total sense now that I think about it. Especially since the datasheet offers how to power this from +12/-12 directly.

For the time being, I found a power supply PCB in my piles of junk that supplies 24V (among other voltages), and while running a huge, bare PCB power supply to give this silly little amplifier module 50ma of 24Vdc is total overkill, it also works just fine. My solution to the problem is to use an isolating DC-DC converter module (24V in, 24V out), that I ordered from Mouser. I'll install that when I get it.

In the mean time, this has let me make recordings, and discover that my left channel was totally dead and did not record at all. Crank the level all the way up and and I could get a tiny bit of sound through, but not much. I poked at it a bit, and found the source of the problem - an open inductor on the record amplifier card. I stole the inductor from the cuing track circuit, and can now record in proper stereo (without cuing, of course). I ordered a replacement inductor as well.

Now, incidentally, I'm working off the schematics in the manual for an AudioCord A series recorder - but this is an AudioCord S series. They're different, but close enough to give me an understanding of how this one works. If anyone happens to have the manual for an AudioCord S series recorder/reproducer, I'd be interested!

I'll update when I get the isolating DC-DC converter and replacement inductor, hopefully I'll be able to make good recordings, with cuing, when this is all said and done. I'm really interested in how carts are cued and how the controls work. I've actually got a fair number of questions about carts, cuing tracks and control hardware, but I'll post a different thread about that, hopefully in a more organized manner.

-Ian
 
Pretty sure what RDL is warning you about, is not to connect the audio minus (balanced) or audio ground connection to the same ground as the DC power supply ground. One doesn't want DC voltage on the little OpAmp inside the plastic brick.

It's true you can run an RDL 'stick-on' or whatever on 24VDC, but they are happier being fed 12-14VDC. There is a voltage regulator inside that will ultimately provide +-12VDC to the OpAmp, but using 24VDC means the regulator will be working harder.
 
So, in case anyone was curious, I solved my floating power requirement problem - I used an IL2424S isolated DC-DC converter module. This is a small plastic block about the size of a 2x2 LEGO brick. I just stuck that to the side of the STA-1 with some double sided tape, and wired it up. This isolated the power supply, so that the audio ground and DC ground are not longer referenced, and I can run the stick on amplifier off the cart machine's power supply. It worked perfectly.
 
Are cart machines coming back?
 
Kelly A said:
Not for actual station use. More as a curiosity to nerds.
Click to expand...

Nerd that I am....I still use ITC 99B's and Delta STEREO units at my Part 15 FM facility...
I'm lucky enough to have an ample supply of tape cartridges (many new old stock!!) and spare machine parts to keep me going for quite a while.....!!:)
 
Status
This thread has been closed due to inactivity. You can create a new thread to discuss this topic.


Back
Top Bottom