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Old 31st July 2003, 04:35 AM   #11
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scott I agree with you to a certian extent but they are not planning on you using a regulated source but just a raw power source.
sandy I see your concerns and MOST dc power supplies encorparate fold back current limiting wich an amplifier will not like because of voltage drop on the rails....in fact they are perfect for testing because they will go to a dead short and not damage components being tested in most cases

DIRT®
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Old 31st July 2003, 01:30 PM   #12
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Quote:
Originally posted by JOE DIRT®
scott I agree with you to a certian extent but they are not planning on you using a regulated source but just a raw power source.
sandy I see your concerns and MOST dc power supplies encorparate fold back current limiting wich an amplifier will not like because of voltage drop on the rails....in fact they are perfect for testing because they will go to a dead short and not damage components being tested in most cases

DIRT®
Joe,

Maybe you realize this, but I'm reponding for Russel's benefit. A 20dB gain gainclone chip will have decent response past 200kHz. If you look in this area the PSRR of the chip isn't the 100dB or 120dB number the like to put in the data sheet table. It's around 30dB. At 10MHz and above, there is no rejection on the negative supply. None. Luckily at this point the gain of the amp will be less than 1.

Now, regulated supplies keep a nice regulated voltage independant of mains variation. They can also have handy current limiting features that make troublshooting much less destructive. They also have a wider bandwith of noise voltage since for all practical purposes they are a DC amplifier with a loop gain and loop bandwidth product of their own.

For anecdotal evidence, the worst hf instabillity I've ever seen in an audio amp (I've had a 1GHz bandwidth 10 watt RF amp go unstable and that was spectacular. I'm sure I was blocking radio stations for a very long way.) I've ever seen was with a regulated supply in a college lab. These guys didn't have a lick of bypass on an otherwise functional circuit. They added their caps (like the lab manual said too) and no more pffffzpt lost output devices. Now this circuit was a discrete amp with a design that had relatively little PSRR in it.

Scott
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Old 3rd August 2003, 04:53 PM   #13
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Default Sorry for the long delay

Hi!

First of all, I have to apologize for the long delay in my reply. I cannot agree with Sandy more: "Things here seem to stay dormant and then explode with activity when one is not looking. . . ". I thought it was already a dead thread when I saw the somewhat "philosophical" posting from Analog_sa. (no punch intended).

Thank you to all of you for your kind input. From what I have read in this thread, it seems that an important function of the caps is to filter out the high frequency noise. If I do not need a high capacitance to smooth out the DC supply, then can I use a much smaller value, e.g. 10uF, for the high frequecncy noise filtering?

By the way, Mr. Joe Dirt, could you please share with us your experience on the Tantalum caps in GC? How much capacitance have you used?

Once again, thank you all for your input and time in reading this thread.


Best wishes,
Russell SIT
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Old 3rd August 2003, 05:25 PM   #14
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there have been alot of valid points been made here.


Russel I will explain how I have my GC setup for one channel......I have my ps located remotely from the amp with 2 15 000 uf Elna caps (one per rail) bypassed with .047 uf "orange drop" caps close to the amplfier pins I have 1 uf tantulum caps on each rail.
As it stands right now I am not hearing any audible noise with the current layout.....I am trying different arrangements on this one channel right now before I box it up and make it look pretty.
As far as my circuit goes I am using a 100k volume pot with a 1.5uf poly input cap and a 10 k input resistor and a 220k feedback....none of the parts are considered "audio grade".
I used point to point wiring and kept leads as short as possible

Cheers!!The DIRT®
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Old 8th August 2003, 08:38 AM   #15
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Thank you for your reply.

I thought you are using only the tantalum caps without the electrolytic caps. I would certainly give your configuration a try in my GC.


Best wishes,
Russell SIT
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