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Old 23rd September 2006, 12:44 PM   #1
maxw is offline maxw  United Kingdom
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Question OPA627 with tube buffer....

What do you think of using an OPA627 at a gain of about 2-5 and then using a 6DJ8 buffer stage?

I'd make the opamp part my self and use this kit for the tube buffer part.

Here is the schematic (biggest pic I could find):
Click the image to open in full size.

Click the image to open in full size.

I like the idea because I like the sound of OPA627, I'd like to try something different and it only needs 12VAC unlike most designs which means I wouldn't need to find another transformer.
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Old 23rd September 2006, 01:37 PM   #2
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The OPA627 is one of the better sounding opamps. But I have to ask why you’d want a tube “buffer”, or any buffer, if you have already decided to use such an opamp. You didn’t state your goals – is this a line stage? The opamp will have a far lower output impedance than a tube buffer. In other words, it’s better “buffer” from a driving source perspective. Don’t get me wrong; I’d prefer an all tube design, but once you’ve settled on using an OPA627, then why add anything else, solid-state or tube? The added circuit can only add distortion no matter what it is. The schematic is too small for me to read and to know what gain it has, etc. It appears to be a balanced-in and balanced-out design, so you’d need more than one opamp upfront to keep the stage balanced throughout. By the way, if you make sure that the gain exceeds 5, use the OPA637. It lacks the internal compensation cap and sounds subtly silkier. If it were me, I’d either go all-tube or all opamp here.
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Old 23rd September 2006, 09:33 PM   #3
maxw is offline maxw  United Kingdom
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Quote:
Originally posted by Brian Beck
But I have to ask why you’d want a tube “buffer”, or any buffer, if you have already decided to use such an opamp.
Well, the OPA627 is popular when using the BUF634 as a buffer so I was just thinking of replacing the BUF634 with the tube buffer.

Quote:
Originally posted by Brian Beck
You didn’t state your goals – is this a line stage?
Yep, I want a simple linestage with a gain of about 2-5 and I wanted to use tubes somewhere in it, or all of it.


Quote:
Originally posted by Brian Beck
The added circuit can only add distortion no matter what it is.
hmmm, point taken.


Quote:
Originally posted by Brian Beck
The schematic is too small for me to read and to know what gain it has, etc. It appears to be a balanced-in and balanced-out design, so you’d need more than one opamp upfront to keep the stage balanced throughout.
Well that is not what I want! there is very little documentation of this kit, I think I'll give it a miss.

Quote:
Originally posted by Brian Beck
By the way, if you make sure that the gain exceeds 5, use the OPA637. It lacks the internal compensation cap and sounds subtly silkier.
The OPA637 is hard to get stable though!

Quote:
Originally posted by Brian Beck
If it were me, I’d either go all-tube or all opamp here.
Well I'd like to go all tubes but its hard choosing a design when I want one that is well tried and tested. Everyone in the tube forum seems to have contrasting oppinions about everything!

Thanks for your reply Brian.
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