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Old 2nd June 2003, 03:57 AM   #1
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Default 5V PSU question for Aanalog

Bulding a preregulated +- 10V for analog stage to be finally regulated to +-5V.

I'm looking for a very clean preregulated supply so I set to build a CRCLC-Reg-C.

In order is 4700u film bypassed, 10 Ohms, 2200u, 1.3H, 2200u LM317T/337T and 4700u. The adj terminal of the reg y bypassed with a 10u lytic and a .47u film cap.

The final regs will be a simillar to LM431 at the analog pins of the chips.

The problem is that I can't have a completly clean line at the scope when using it wide open (200 Mhz). When a low pass filter is applied up to 20KHz the line appears clean. When wide open the line is thicker at 5/10 mv scale (using a 10X probe)

I have tested different transformers, bypassing and up to 4 inductors but still the line is thicker.

Am I dreaming to have a clean supply to the MHz?
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Old 2nd June 2003, 04:18 AM   #2
jwb is offline jwb  United States
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Your probably dreaming if you think your scope has that kind of sensitivity. I always use my scope with the 20MHz low-pass engaged if I have the vertical set below 20mV/division. The scope just has too much noise, and the high-impendance probes pick up stray emissions from everywhere.

If you really want to measure below 20mV/div, use coax and the 50Ω input termination instead of a regular 10X probe.
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Old 2nd June 2003, 04:54 AM   #3
PMiczek is offline PMiczek  United States
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I agree. You are measuring the noise on the exposed wires and the probe, as much as noise in the power supply. If you have to use a 10x probe, put something like a 40-100uH hash choke in series with the probe. Also, engaging the BW limit and or the inline choke on your scope will let you see any remaining 120Hz ripple w/o the HF noise.
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Old 2nd June 2003, 05:05 AM   #4
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Thanks for the input.

With the 20MHz low pass engaged I have a clean line all the way.

I was actually trying to look for any inestability at hi freq.

BTW, what's a hash choke?
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Old 2nd June 2003, 04:50 PM   #5
PMiczek is offline PMiczek  United States
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A hash choke is a small value inductor wound on a ferrite core. Search Digikey for the words "hash choke", and you will find some.
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Old 2nd June 2003, 05:48 PM   #6
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100uH hash choke should do the trick!!!
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Old 2nd June 2003, 06:31 PM   #7
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Default Re: 5V PSU question for Aanalog

Quote:
Originally posted by apassgear
In order is 4700u film bypassed, 10 Ohms, 2200u, 1.3H, 2200u LM317T/337T and 4700u. The adj terminal of the reg y bypassed with a 10u lytic and a .47u film cap. *SNIP*
Am I dreaming to have a clean supply to the MHz?
You will find the supply to be "cleaner" without the LM317/337 regulators, or if you have to use them to get the noise down to the microvolt level you need a quieter error amplifier and voltage reference than the one resident in these devices. The Jung and Sulzer setups have been discussed frequently on this site.

wrt measurement -- you can directly solder a piece of coax to the terminus at the power supply and a BNC connector on the other end. also check the scope AND power supply to see that they are both plugged into the same outlet !
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Old 2nd June 2003, 08:12 PM   #8
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Default Re: Re: 5V PSU question for Aanalog

Quote:
Originally posted by jackinnj


You will find the supply to be "cleaner" without the LM317/337 regulators, or if you have to use them to get the noise down to the microvolt level you need a quieter error amplifier and voltage reference than the one resident in these devices. The Jung and Sulzer setups have been discussed frequently on this site.


Actually I have a Jung +-15V reg installed on the CDP for the analog section but my gut feeling tells me that they don't sound very good. Though I have to confess that the OpAmp on these are NE5534. With this setup they produced a lot of oscillations when trying to set up a LM6181 I/V analog stage with the TDA1541A S1 DAC.

For this reason I wanted to try the described supply which seems an overkill as a preregulator. I added the LM317/337 just to get rid of the voltage fluctuation of the mains line and have the final shunt regs, LM431 type, have an easier job at the now beeing construted OPA660 I/V buffer.

I don't care about the size or cost of this pre reg, up to a point of course but want a clean output and that's why is taking so long on the contruction and "testing".

At this moment I'm using only one inductor (1.3H, 33 DCR) at each rail between two 2200u caps. The other two were intended for use at the ground side. Using them at that position they seem to worsen the noise using my crapy test setup.

Is ther a better way of using the spare inductors, say in place of the 10 Ohms resistors to form a CLCLC before the regs?

Which would be a better way to get rid of the mains voltage fluctuation?

Thanks for the input!!!
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Old 2nd June 2003, 08:52 PM   #9
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Default Re: 5V PSU question for Aanalog

[QUOTE]Originally posted by apassgear
[B]Bulding a preregulated +- 10V for analog stage to be finally regulated to +-5V.

---------------------------------------------
Why not use a 6V battery; an 18VA one lasts forever!
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Old 2nd June 2003, 08:54 PM   #10
PMiczek is offline PMiczek  United States
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If by mains fluctuation you mean ripple, I don't see how there could be enough left after two stages of regulation to affect the output. Not so you can easily say the output stage does not sound good. The ripple and any line variation should be a fraction of 1mV after the first 317/337 stage, and if your Jung regulator works at all the way it is supposed to, down to less than 100uV at the output stage. (btw, you said +/-10V in your first post, and +/-15 in the last one).

I would look elsewhere, but just to make sure the power supply is not the cause, how about replacing it with two 9V batteries temporarily, only at the output stage? Meaning take the regs out of the loop and see what it sounds like. If you decide to try that, leave any local decoupling in place though.

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