Tuesday, July 14, 2009

Notes on Audio Op-Amps

I am getting back into modifying some gear and I figured I would post the notes that I have collected on Op-Amps over the years.
Just some random collected information about certain OpAmps.
Sites with OpAmp Reviews: http://tangentsoft.net/audio/opamps.html
http://audioreviews.blogspot.com/2005/10/flavors-of-audio-op-amps.html
http://www.rock-grotto.co.uk/opamp.htm
Designing with OpAmps
DIY Audio Thread... Best OpAmp
OPA2604/OPA2134 good replacement for TL072 however be careful of powersupply overtaxing
TL052,TL062 higher quality TL072 at low voltages
TL0xx are FET input, dc offset irrelevant. But can cause problems when replacing with BJT input opamps.
FET for BJT NOT BJT for FET
TL062 often better than TL072
OPA2227 for TL072 - not cheap
MC33078 for TL072
OPA627 better than NE5532
OPA627 sounds tube-like
OPA2132 is good as well
OPA404
MC33079 ro LM837
NJM2114 supercharged NE5532
LM4562 very netural, recommended
LM6172 with some help for ART Tube MP mod how about TI burr brown ...like opa4132 (seems to be an excellent audio quad), opa4134 (not really the same but very good as well), opa4227 (the ...227 series sound VERY good as replacement opamps in my modded SPL vitalizer from 1994!)...just a thought... Very Happy
Better parts are available for summing. You want very large open loop gain in the audio band to resolve all those channels with low noise. The LME49720 is much better as is the AD 8599. The LM6172 has too great a bandwidth and too much noise to use as a summing opamp, it will have stability issues.
Jim Williams
Audio Upgrades
Digikey apparently knows that the LM4562 and the LME49720 are the same part.
Contact Dave Hill at Jensen for the exact transformer model. It's designed for this preamp as Jensen designed the preamp for Presonus. It's a great value, only around $30, about 1/2 what their premium transformers cost. You will need a Hakko 808 or similar desoldering tool to remove the old Chi-com transformer without pulling out plated through holes of the pcb. If the zorbal network also needs to be changed, Jensen provides the parts for that with the transformers. Best parts for opamps are the LT1357 and the OPA211. The OPA827 is a great fet input amp, better than the overpriced and dull sounding OPA627. Don't forget the quad surface mount MC33079 for the inserts, I use a LT1359 or a LM49740, a BB OPA4134 will also work. The balanced output opamp is a through hole 5532, I use the very fast high current LM6172. Replace the output coupling caps with 220 uf 25v Panasonic FM's. Presonus power supplies are set to + - 20 volts, too high for these 18 volt parts, there is a screwdriver trim pot to reset to 18 or 17 volts.
Jim Williams
Audio Upgrades
Hi there. Thanks for interest in the Eureka and in modifying... The Eureka ships with a pretty good transformer, our own model, and TI (Texas Instruments) op-amps, which are pretty much industry standard and fairly quiet and transparent. Though it's based on the same preamp circuit as our VXP, MP20, and M80, I would characterize it as being slightly brighter, perhaps. I never thought of it as edgy, but that may be a fair description. I think it's slightly brighter because the normal impedence setting (2500) for the Eureka is slightly higher than the open impedance of the transformer, which provides the input imdepance for the older models, as they don't have the added impedance selector circuit. I could be wrong, but I suspected that's what accounts for the slightly different sound. Because the impedance is set before the mic hits the transformer, the transformer mod is a bit more subtle/harder to hear on the Eureka than in our older units. It's also significantly more difficult to disassemble than our older models. For that reason, I usually suggest that people pass on transformer-modding the Eureka. Of course, if you really want to, you can. The only 2 direct-replacement drop-in alternatives that exist are the Audisar Plus-8 and the Jensen JT-11K8-APC. Both are very fine parts. Email me for info if you need to know how to obtain these. PreSonus doesn't carry upgrade components, but I don't have a problem in pointing people in the right direction. I'm a tweaker/mod guy myself, so I can definitely understand the urge to improve stuff.
What's alot easier to do is to replace the op-amp chips and there are several on the market. The most popular is the Burr Brown BB-OPA627. Our stock chip is actually among the quieter ones available, and so though you'll sacrifice a few db in noise performance by taking ours out, most people claim the BB part is 'bigger, beefier, better heft on the bottom end', etc.. Second most popular is the Analog Devices AD-797AN, which sonically, people say is pretty much like our stock part but just performs a bit better. Faster transient response, greater clarity, clearer image, better attack, better detail, etc. ... but I am talking subtleties. Another chip option is the Linear Technologies 1357 (Digi-Key full part number: LT-1357CN8-ND). This chip is possibly the most quiet one on the market, super noise and distortion performance. It's actually also one of the most affordable. Only $4.63 each from Digi-Key. This is the piece I use myself, so I can capture the cleanest, most accurate sound and then give it color later if I choose to. If you want surgical accuracy, this is the way to go. If you want added warmth/bottom, the BB part is the way to go. The TI and AD lie somewhere in the middle. I hope this info is useful, have fun!
Chad Kelly
PreSonus Audio

Here is a great link I found in regards to OpAmps, great stuff!
http://nwavguy.blogspot.com/2011/08/op-amp-measurements.html

1 comment:

googel456 said...

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