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Old 21st August 2006, 04:11 PM   #1
TomSSS is offline TomSSS  United States
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Default Preamp Power Supply Suggestions

I'm in the process of building a preamp for my vehicle. I've built a prototype, and I'm having problems with noise. Attached is a block diagram of the system. In the block diagram the voltage regulator is shown separate from the the preamp, but in reality everything after the battery is on the same PCB. During initial testing the 100 Ohm resistor shown was actually 0 Ohms. This could be the reason why I was picking up noise. Now, I have replaced the 0 Ohm resistor with a 100 Ohm resistor as shown in the diagram, but I haven't tested it out for noise yet.

Please give suggestions/comments about this design. My main concern is with the power supply portion (linear regulator) and reducing/eliminating noise.

Regards,
Tom
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Old 21st August 2006, 04:22 PM   #2
anatech is offline anatech  Canada
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Hi Tom,
That will not make you happy. What you really should have is a supply that transformer couples, you can get a bipolar supply at the same time. The 100 R resistor should not have any current running through it. It is used to break ground loops.

So try to make a little inverter that will allow you to break the supply ground.

-Chris
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Old 21st August 2006, 04:38 PM   #3
TomSSS is offline TomSSS  United States
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Chris,
Thanks for the reply. In order to do what you are talking about, am I going to have to build a switcher? I really wanted to stay away from that and make the PS design as simple and cheap as possible.

*Edit* But then again I do want a clean audio signal, so if I must build something different thant what I currently have I'm up for it.
*/Edit*
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Old 21st August 2006, 04:45 PM   #4
anatech is offline anatech  Canada
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Hi Tom,
You need to build a switcher for the best performance. There can be other ways to do this, but they are no where near as nice as a switching power supply.

-Chris
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Old 21st August 2006, 04:48 PM   #5
TomSSS is offline TomSSS  United States
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Chris,
Thanks again. I'll look into building a PS with transformer coupling. In the meantime what are "the other ways to do this" you mentioned?

Regards,
Tom
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Old 21st August 2006, 05:10 PM   #6
anatech is offline anatech  Canada
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Hi Tom,
There are capacitive coupled things that never really worked well. Again, you want to isolate the ground. That's important.

-Chris
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Old 21st August 2006, 09:21 PM   #7
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Default Isolated Switcher

Tom,

Check out these links:

http://cache.national.com/ds/LM/LM2577.pdf
http://cache.national.com/ds/LM/LM2587.pdf

It's the National Semiconductor SimpleSwitcher series of switcher ICs. Using an LM2577 - 3A chip (or an LM2587 - 5A chip), you can make a nice bi-polar output isolated flyback switcher that is reasonable simple (hence the name SimpleSwitcher ), and excellent in performance. Of particular interest to you would be p.14 of 27 on the LM2587 Datasheet.

Cheers,

Steve
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Old 21st August 2006, 11:53 PM   #8
TomSSS is offline TomSSS  United States
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Steve,
Thanks for the input. So, I could basically take the circuit in figure 11, then just isolate the grounds and use an opto-isolator for the feedback to the control IC? What if instead of an opto-isolator I use a 100 Ohm resistor to separate primary and secondary grounds? Or is the opto-isolator the better (cleaner) approach?

Regards,
Tom
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Old 22nd August 2006, 01:27 AM   #9
TomSSS is offline TomSSS  United States
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One more thing that I should mention is that in the preamp circuit shown in the block diagram, signal reference is at +7V (= 3+(11-3)/2).

So if the noise floor is 1V and my signal range is 7V +/- 4V will I still have noise problems?

Regards,
Tom
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Old 22nd August 2006, 02:32 AM   #10
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How elaborate is the preamp (crossover, multi-band EQ...)?

You mentioned that you wanted to do it as cheaply as possible. Price range?

A transformer based switching power supply is probably the best solution (depending on how you define best). It gives you isolation and a bipolar supply as mentioned. A capacitor based switching supply can give you the bipolar output but not the isolation.

To get isolation from the chassis ground, you can use a transformer (ground loop isolator) to break the ground loop. If you want to make a slightly more complex preamp, you can build in a circuit (op-amp based) that will cancel the noise that would be associated with ground loops.

Old PAC devices (simple crossovers and such) grounded the devices via the shield ground of the head unit. To get the same benefits, you can ground your preamp directly to the head unit's case. Since there would be no difference of ground between the head unit and the preamp, there should be no ground loop.
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