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Old 15th April 2004, 11:47 PM   #1
Adam M. is offline Adam M.  United States
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Default One more buffer question...

Can a buffer be powered from the main gainclone power supply, using a voltage divider and resistors of high enough resistance to supply just enough current to the buffer as to not waste power?
THanks,
Adam
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Old 15th April 2004, 11:56 PM   #2
joensd is offline joensd  Germany
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If you´re talking about an opamp-buffer:
you could use zener diodes to reduce voltage for the buffer or something like 78L15/79L15.
If your voltage rails are <24V use OPA604 for example.
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Old 16th April 2004, 09:40 PM   #3
Adam M. is offline Adam M.  United States
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Any suggestions on zener diodes? I am thinking about using an OPA627 whenever I get around to build it... I'm thinking abuot 12v @ 250ma should be enough, does that sound reasonable?

Are there any sonic benefits to using a voltage regulators? My input voltages are going to be right arounf 35V (the transformer is the only part I have in my hands as of yet). The Zeners are so cheap ...3 cents a piece from digikey for 12v 500ma, and that makes me nervous.

THanks,
Adam
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Old 17th April 2004, 01:54 PM   #4
joensd is offline joensd  Germany
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Hi Adam,
do you mean your voltage rails are +-35V or just 35V?
How did you come to a 12V@250mA (again +-rails?) ?

If you really need that "much" current there´s no question you shouldn´t use zeners to regulate due to power dissipation and bad regulation.
Actually even the 7815/7915-regulators for example (1A-version) are dirt cheap (in comparison with the OPA627 especially)and give better regulation.
And using such a brilliant opamp you´ll probably want the best regulation anyway.

Give some more details of your circuit idea; that´d help.

Regards
Jens

PS.: you could also think about using something like a BUF634 (DIP8 or TO220 for greater power dissipation) which is a brilliant sounding buffer that is a bit cheaper than the OPA627)
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Old 17th April 2004, 03:19 PM   #5
joensd is offline joensd  Germany
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One question to you:
Have you thought about a discrete buffer like a source follower?
Probably the purest, cheapest and easiest solution except it definitely needs a regulated supply.
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Old 18th April 2004, 02:44 AM   #6
Adam M. is offline Adam M.  United States
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The 12v @250mA I came up with from about 30 seconds of thinking after seeing what input voltages the opa627 needed and from remembering back to my op amps class... for some reason I remember feeding a chip that much current and didn't want to be current-limited, I have no basis other than that for the current need.

I did mean +-35v on the power....

My current plan (which changes daily it seems as I learn more and more about practical implementations of these circuits) is to build a PA100 directly from an-1192 with a buffered input and a stepped attenuator of some flavor in front of the buffer since I don't have a preamp yet.

The reason I am thinking PA100 is because I will use this amp on occasion to drive both 4 and 8 ohm speakers in a room with a 15' high cathedral ceiling, I want to have enough power on tap.

Thanks for the suggestion on the BUF634, I"ll read up on it (and I'm sure ask 100 questions)

What is a line source follower?

Thanks for the help, hope I gave you the information you needed.
-Adam
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Old 18th April 2004, 02:50 AM   #7
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Sorry if I might have missed something along the way, but what exactly is the point of adding a buffer to the input of a Gainclone?

se
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Old 18th April 2004, 03:03 AM   #8
Adam M. is offline Adam M.  United States
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A few threads back I asked how to control the volume on a PA100 amp and someone suggested putting an attenuator in front of a buffer ...So I tried to learn as much about implementing a buffer on a gainclone in as short of a time as I could... And here I am. If my quest is misguided, let me know. I also wanted to push myself a little from what I did in labs in school, so its a bit of a learning experience here and a bit of ignorance on how these amps need to be set up in a practical application, where the input source isn't a perfect square, sine, or triangle wave coming from a machine designed to send signals into such chips and the supply voltage isn't dialed in on the machine next to the signal generator.
I'm all for learning, please teach.
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Old 18th April 2004, 05:04 AM   #9
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Quote:
Originally posted by Adam M.
A few threads back I asked how to control the volume on a PA100 amp and someone suggested putting an attenuator in front of a buffer ...So I tried to learn as much about implementing a buffer on a gainclone in as short of a time as I could... And here I am. If my quest is misguided, let me know. I also wanted to push myself a little from what I did in labs in school, so its a bit of a learning experience here and a bit of ignorance on how these amps need to be set up in a practical application, where the input source isn't a perfect square, sine, or triangle wave coming from a machine designed to send signals into such chips and the supply voltage isn't dialed in on the machine next to the signal generator.
I'm all for learning, please teach.
Well unless you're doing an inverting version of the PA100, can't think of any particular reason why a buffer would be needed. But if you want to add one just as a learning experience, it can't hurt.

se
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Old 18th April 2004, 05:52 AM   #10
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Quote:
Originally posted by Steve Eddy
Sorry if I might have missed something along the way, but what exactly is the point of adding a buffer to the input of a Gainclone?

se
I have this option in my Gainclone and the point is:

1 Lowering the offset (possibility to this)

2 Low source impedance for the LM3886 (positive thing)

3 Possibility tayloring the frequency characteristics, using small (not huge) coupling caps.

4 Creating high input impedance which sometimes can be a positive thing.

When I read posts about LM3875 and LM3886, many troubles comes from problems with offset and overall bad design around the passive parts. With a buffer you can get a more predictable and equal result.
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