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Old 10th August 2008, 06:59 PM   #1
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Default Linear Regulators

Hello,

For a pre-amp I am designing, I will be needing a few rails.
Below is a list, and how I thought of achieving them.
All are fed through a 2x18Vac supply (probably Hammond 182L18), rectified with MUR860, bypassed by some small 100nF capacitor.

- 12V for input signal relays (LM7812)
- 5V for the digital circuity (microcontroller, LCD. Microcontroller will achieve its 3.3v using an LDO) (LM7805)
- +/-5/12/15V (TBD) - Analog PSU. Here I thought about using LM317 as from what I understood it yields better performance which might be noticeable
- +/-15V VU Meter driver circuit - LM7815

Would this topology do the job?
What would be the best suggested method for providing this many rails?
(Going with only LM317/LM337's make the PCB quite large so I'm trying to avoid that).
Note that this pre-amplifier do not provide a gain stage, this will be done via a tube part with its own PSU.
Volume control is achieved using a DS1666 IC.

Many thanks
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Old 10th August 2008, 08:03 PM   #2
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Shalom, I've been under the impression that one could get a more precise and less noiseless voltage out of the adjustable regulators compared to the fixed ones. This information should be available in the datasheets or application notes.

The most important design constraint is the required current with each of the supply voltages. These ubiquitous mentioned regulators commonly allow maximum of 1.5 amps output current, but I wouldn't dare take that much out of them. At least the performation (stability, noise issues) might be compromised.

I see nothing inherently wrong with the use of these regulators, maybe someone else has different opinions?
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Old 10th August 2008, 09:42 PM   #3
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Shalom

What you said about noise levels is correct - but does it really matter for relay/digital (uC) supplies?

About current requirements - ~500mA on each rail is probably all I'm going to need, unless there are current requirements for an opamp buffer (for the vu meters) I am unaware of
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Old 10th August 2008, 11:14 PM   #4
Alexsch is offline Alexsch  Israel
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What may be the reason for using ultrafast diodes at 50Hz? I would suggest, regular types may give you less dissipation. And the lower the diode voltage rating, the less its conduction loss.
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Old 10th August 2008, 11:28 PM   #5
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Good point, Alex
The main reason the MURs popped to mind was because I have a few of them lying around..
But I guess another few diodes are not going to be noticeable on the BOM of a tube preamp
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Old 10th August 2008, 11:35 PM   #6
Alexsch is offline Alexsch  Israel
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A TUBE preamplifier? Are we out of old good JFET's?
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Old 10th August 2008, 11:38 PM   #7
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Everyone seems to brag about the magnificent sound of those tubes.
I could go buy one, or build my own and experiment with it, learning in the process. So I was thinking, why not?

For fun, and not for profit
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Old 11th August 2008, 06:10 AM   #8
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Quote:
Originally posted by eranrund
Shalom

What you said about noise levels is correct - but does it really matter for relay/digital (uC) supplies?

I think you can have a bit more noisy supply for relay, but not for sensitive digital devices! Personally I would try filter the digital supplies well, and pay attention to the PCB layout as well. That means, a separate ground for analog and digital is recommended.
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Old 11th August 2008, 07:13 AM   #9
Alexsch is offline Alexsch  Israel
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If you are building tube preamplifier, where are the HV rails? You are talking about low voltage rails so far. And what digital circuit has to do with it anyway? And relays?
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Old 11th August 2008, 08:53 AM   #10
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Tube PSU is on a different board, being fed from its own power transformer.
The digital circuity is for volume control (through DS1666), infra-red remote control and input selection (hence the relays - used to choose from several input sources).

About noise reaching digital devices - Except the DS1666 which is fed with its own +/-5v rail, the other devices are not very sensitive and some supply bypass capacitors are all they need.
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