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Old 28th May 2007, 03:14 PM   #1
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Hi everyone

This is my first post at DIYaudio. Apologies, in advance for any breach(es) of protocol. ;-) This post relates to op-amps in (high-gain) phono stages and (low gain) pre-amps.

Firstly, I run a 60dB gain (low-output MC) dual-mono phono stage that utilizes 2 pairs of DIP-seated Burr Brown OPA37GP op-amps. Sonically it's fine but I'm unrelentingly curious. Does anyone know if Burr Brown OPA637's are direct electrical drop-in replacements, and if so, what sought of changes might occur? Any other more modern designs that could also suit?

Secondly, shortly I'm about to road test another dual-mono phono stage which utilizes 2-stage gain comprising (don't gag, not my design!) one pair TI TL-071 followed by another pair Burr Brown OPA660 or unknown brand CA3280. How easy is it to swap out the TL071 for a more modern op-amp design?

Thirdly, if one was designing a premium quality line-level op-amp-based pre-amp, maybe combining functions of headphone amp and USB DAC as well, which op-amp(s) would suit best?

Fourthly, is the ultimate low-ripple, noise-free DC power supply design based upon Walt Jung's 'super-regulator' principles? Should one use Schottky diodes above HEXFRED and other fast-soft recovery types?

I should add that I've listened to countless devices incorporating op-amps, most of which sound harsh and distorted. Of those components that seem to deliver the sonic goods, I've found all are made by Burr Brown.

At the risk of creating undue controversy, it appears that each op-amp brand has a vaguely familial sound. AD op-amps almost always sound brittle and artificially forward, even their pricey, premium ones.

And that brings me to some of DIYaudio more animated technical debates. It's crucial to focus on basic electrical parameters in designing circuits with op-amps, and graphs and data have their place. However at the end of the day, one should always relate their activities back to what each component contributes to the overall sound quality, viz, how closely an original recorded event is reproduced/replicated by your audio equipment. This process will automatically disqualify anything to do with digital compression (MP3 and the like). How any audio print media can dare to seriously review MP3 players and computer sound cards as viable audio devices is beyond me! :-)

nightcap
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Old 28th May 2007, 03:50 PM   #2
anatech is offline anatech  Canada
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Hi nightcap,
Good thread topic.

It seems to me that all op amp based signal amplifiers may have a "sound". Whether you can hear it depends a great deal on your system, and you. I think PCB layout is just as important as the op amp you use.

This seems to be a topic worth exploring.

Cheers!

-Chris
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Old 28th May 2007, 03:55 PM   #3
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This is my basic "first look" to see if op-amps are plug compatible. First, download the datasheets for both parts. Is the supply voltage of the new part greater than what's being supplied? Are the basic input, output, and power signals on the same pins? What's the minimum stable gain, and is the circuit set up for that or greater? Finally, look at the compensation and offset pins. Usually they're not used, but different op-amps use different polarities and configurations for offset. If it's used, make sure it's wired like the data sheet. Ditto any compensation used. If the new part meets the above, it will usually work well enough to dig deeper into how to optimize it and how it sounds. I've used the OPA627 in scientific instrumentation, and it's a fantastic part in terms of low offset and low noise. The OP37 is a higher bandwidth version of the old OP27, and they're both aimed primarily at precision lower frequency applications. I think they're bipolar, but would have to check. IMO, they're pretty old designs. Again not sure, but I think the 627 is jfet or cbfet or something, but it has way lower input currents, and IMO has little in common with the old 27/37 parts. Unfortunately, it's quite expensive, and may or may not be any better sound-wise than (man is my memory shot- forgive p/n errors) the 833/797 parts commonly used.
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Old 28th May 2007, 08:12 PM   #4
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Thanks Chris and Conrad.

Certainly will compare specs, and agree that overall circuit design, materials and parts selection, and attention to detail optimise sonic outcomes.

It'd be good to hear from DIYers who might share brutal first-hand experiences, whether in fact, there is any merit in swapping out older op-amps; if so, any gain, and how much pain? Also those related aspects of DC power supply and pre-amp design......

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Old 29th May 2007, 02:34 AM   #5
sandyK is offline sandyK  Australia
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OPA2134,LM4562 and AD8066 are worth looking at. The OPA2134 is a good choice for replacing devices such as NE5532, 4558 etc. LM4562 is more revealing than the OPA2134, but worth trying. It is still the "flavour of the month" (I think)
AD8066 is another very revealing opamp, but only comes in SOIC .
It requires close bypassing. It usually sounds better using normal 100nF bypass capacitors instead of the recommended 100nF ceramic, PROVIDED that voltage regulators are fairly close by,for stability reasons.
SandyK
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Old 29th May 2007, 10:58 AM   #6
sandyK is offline sandyK  Australia
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Nightcap

If you want more information on op-amp "rolling" and subjective reports on the effect of using different types of op-amps. Have a look at the Rock Grotto Audio Forum. They have tested most of the better opamps in various types of equipment.
SandyK

http://rockgrotto.proboards39.com/index.cgi
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Old 29th May 2007, 01:06 PM   #7
anatech is offline anatech  Canada
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Hi sandyK,
Interesting forum that's just starting up. Sounds like their heart is in the right place and I wish them well.

If you search on this site, you will see many op amp comparisons and discussions. The results from your search may be more than you wish simply due to the number of members posting here, so refine your search and read in little bits.

Are we the other "big forum"? Anyway, everyone is welcome and membership in other forums is a good thing. It helps everyone share knowledge, so don't be shy about it.

-Chris
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Old 29th May 2007, 08:28 PM   #8
sandyK is offline sandyK  Australia
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Rock Grotto isn't a new forum, just a small chattier forum . It is more hands on than theory. I am a member of both forums, because I also like the more indepth technical side as well.In particular Solid State amplifiers , preamplifiers etc.

Regards
SandyK
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Old 29th May 2007, 10:19 PM   #9
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Hi Sandy,
Nice little place you have there. It's like a small club where everyone knows each other.

Hands on is more than welcome here too, as you know.

-Chris
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Old 29th May 2007, 10:40 PM   #10
sandyK is offline sandyK  Australia
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Chris
I mentioned Rock Grotto because some of the less experienced
people may feel intimidated by the sheer size and more technical side of this forum. There is also an unfortunate tendency here for some members to "snipe" at some of other peoples designs, although most members try to be helpful. I originally registered as a member on Rock Grotto because I wished to post a project about modifications to the Musical Fidelity X-DAC V3 without being subjected to minor technical criticisms. But as someone who has been constructing electronics projects for >30 years, I do enjoy the technical excellence of DiyAudio Forum.
BTW, I have previosly visited this forum on and off, for several years.

Kind Regards
SandyK
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