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Old 1st October 2012, 03:08 AM   #1
JZatopa is offline JZatopa  United States
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Default Has anyone played around with this as an amp front end?

IF you are a tube purist, this post is not for you. If you are interested in the many ways we can reach higher levels of audio nirvana, please come on in.

The O2 headphone amplifier I built got me thinking about how I may have not given opamps a fair shake. The amp is one of the most transparent pieces I have ever used. This got me thinking about using an opamp in the front end of a tube amp. My research led me to this interesting little guy. MSK-130. Looking at this opamp, I think I could make an interesting amplifier. It looks like it has enough gain to make an amplifier with just output tubes.

With the output voltage swing of +/-95V it looks to me like it just falls short of driving a 300B, depending on what voltage you are at. Am I right about this? Is it ok to fall short here?

Are there any other interesting things we could use this for? Maybe an interstage instead of a front end (similar to powerdrive)?

I am still a newbie when it comes to designing amplifier instead of building them. So if I missed some fundamental reason this couldn't be done, please explain it to me so I don't make the same mistake twice.
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Old 1st October 2012, 03:26 AM   #2
cotdt is offline cotdt  United States
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It's a great idea and definitely worth trying, though I would never say that opamps sound transparent.
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Old 1st October 2012, 03:32 AM   #3
TheGimp is offline TheGimp  United States
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It would make for a great grid driver in a curve tracer.
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Old 1st October 2012, 03:49 AM   #4
ChrisA is offline ChrisA  United States
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Originally Posted by DJNUBZ View Post
IF you are a tube purist, this post is not for you. If you are interested in the many ways we can reach higher levels of audio nirvana, please come on in.

The O2 headphone amplifier I built got me thinking about how I may have not given opamps a fair shake. The amp is one of the most transparent pieces I have ever used. This got me thinking about using an opamp in the front end of a tube amp. .
All tube amps have op-amp front ends if you consider the recording studio a "front end". Most of the music we hear has gone thought many TL072 (or the like) op amps. Not saying this is good, just that it is the way it is.

I've never understood why n opamp inside an audio power amp does not work. I've tried simulations and the results are poor. You'd think that with such a huge amount of gain the NFB would work well to correct out any problems but I can never gt the total distortion numbers to be reasonable if I include an op-amp in the loop.

As line level devices they are nearly perfect, just can't get them to work in a power amp. Could be that I'm simply not smart enough, but I'm not alone in that.
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Old 1st October 2012, 03:56 AM   #5
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It would make for a great grid driver in a curve tracer.
Yes, it looks similar to the LME49810 driver I am using on my tracer. It works quite well. I am planning to use a pair to drive 6336 in SE.
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Old 1st October 2012, 06:30 AM   #6
JZatopa is offline JZatopa  United States
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Originally Posted by cotdt View Post
It's a great idea and definitely worth trying, though I would never say that opamps sound transparent.
I am not saying that opamps are transparent per se. I am saying that the implementation of the opamps in the O2 are transparent. If you have not built one and tried it yourself, I highly recommend it.
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Old 2nd October 2012, 08:48 PM   #7
Fenris is offline Fenris  United States
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Somewhere on the Tubecad site several years ago, Broskie had a circuit of a standard op-amp driving a PP EL-84. Simple enough. I've thought about doing it, but then you need a +/- 15v power supply as well, and I'm not sure if there would be any benefit.
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Old 2nd October 2012, 09:20 PM   #8
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Nothing new under the sun:
Elektor Special Audio 4, November 2010: See Attachement (0.5Mb )

You know a source for the MSK-130 ?
Attached Images
File Type: jpg OpDriverTubePower.jpg (547.6 KB, 350 views)
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Old 2nd October 2012, 09:27 PM   #9
disco is offline disco  Netherlands
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Originally Posted by ChrisA View Post
All tube amps have op-amp front ends if you consider the recording studio a "front end". Most of the music we hear has gone thought many TL072 (or the like) op amps. Not saying this is good, just that it is the way it is.

I've never understood why n opamp inside an audio power amp does not work. I've tried simulations and the results are poor. You'd think that with such a huge amount of gain the NFB would work well to correct out any problems but I can never gt the total distortion numbers to be reasonable if I include an op-amp in the loop.

As line level devices they are nearly perfect, just can't get them to work in a power amp. Could be that I'm simply not smart enough, but I'm not alone in that.
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Old 2nd October 2012, 10:56 PM   #10
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I bet that chip costs near $1000. The big high voltage opamps from Apex used to cost $150 or so...
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