OPA627 as a Filter/Buffer? - diyAudio
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Old 8th November 2004, 06:36 AM   #1
D_GR8_1 is offline D_GR8_1  Australia
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Default OPA627 as a Filter/Buffer?

I am using some OPA627s as active filter in my GC design, so I was wondering whether I need to add another stage after them for the buffer or implement the filter/buffer all in one.

Also is it better to leave it as it is or bias it into Class A?

What are the benefits of Class A biasing?
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Old 8th November 2004, 07:48 AM   #2
garbage is offline garbage  Singapore
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hi D_GR8_1

you might want to check out my past posting here

there is a link to a BB white paper regarding using buffer after the opamp.

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Old 9th November 2004, 03:22 AM   #3
D_GR8_1 is offline D_GR8_1  Australia
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Default I read all of it and now...

Hi,

maybe Carlosfm can answer this if he reads the post.

Let's say I design the pre-am the way he described it his thread.

Then I feed the signal to a IGC (LM3886).

Can I integrate the HPF into the pre-am? i.e. add components to the OPA627 that will filter the signal as well as act as the buffer?!

Or, do I need extra OPA672s in the signal path from the source to filter the signals -> pre-am -> power amp???

That's my confusion!
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Old 9th November 2004, 07:25 PM   #4
troystg is offline troystg  United States
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Hi-

Actually I'm looking at the BB white paper on the OPA627 and the examples they have for circuits.

Am I missing something or is there only one device involved when using the 627 as a buffer?

Please see attached picture from the BB whitepaper:

http://www.ociw.edu/instrumentation/...rts/OPA627.pdf
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File Type: jpg 627.jpg (8.2 KB, 343 views)
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Old 9th November 2004, 08:57 PM   #5
D_GR8_1 is offline D_GR8_1  Australia
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Default OPA627+BUF634

If you read "COMBINING AN AMPLIFIER WITH THE BUF634" an application note from Burr-Brown, you will see that that the reason for adding the BUF634 to the OPA627 is to improve output current characteristics.

The OPA627 on its own as most op-amps doesnt have excess output current, in fact it is only about 45mA. By adding the BUF634 the current output can be increased upto 500mA, so almost a tenfold.

The benefit of this (from my understanding) is that this allows you to drive a power amp regardless of the distance between the pre & power amps without any signal loss!

The idea is that this buffer can be used as a Headphone Amplifier as well.
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Old 9th November 2004, 09:29 PM   #6
Mr Evil is offline Mr Evil  United Kingdom
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What sort of power-amp needs more than 45mA to drive it?!

Unless there is a possibility of needing to drive a particularly low impedance load, then adding a buffer after an op-amp filter is not required. OPA627 is a fine op-amp, capable of driving most normal loads, such as chip-amps.

However, you may need to put a buffer before the filter, if the preceeding device (whatever it may be) might have trouble with the reactive input impedance of the filter.
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Old 9th November 2004, 11:45 PM   #7
D_GR8_1 is offline D_GR8_1  Australia
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Ideally, this combination is mainly intended as a headphone amp.

However, carlosfm has used it for making his pre-amp and he reports a very good performance.

So, all I can say is try it without BUF634 then try it with BUF634, see which configuration sound better.

That's all.

I think this thread has sidetracked a bit as my main concern was whether I can add extra components to the buffer design to implement a filter/buffer with one op-amp.

And it's still not answered...
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Old 10th November 2004, 12:17 AM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Well, the point of a buffer is to isolate the source from the load. Usually this means presenting a high input impedance and an adequate current output capability. An op-amp based filter will satisfy the second, but as I mentioned before, it doesn't quite satisfy the first since the input impedance will be reactive (for most normal filter topologies). Whether this means you need a buffer in front of the filter or not depends on what the preceeding stage is capable of.
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