An alternative to the 2520 for 312 projects - Test Results

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Category 5

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Jul 24, 2004
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Knowing how hard real 2520s are to come by, and knowing that substitutes like the Avedis and OSA op-amp are cleaner and not really 2520 like at all, SCA is now offering this.

http://www.pbase.com/sbushman/image/35613078/original.jpg

Thanks Tim for letting me be one of the first to receive these. The potting job looks great. Now I need to install and test them. I'll post my findings here.

According to Tim these run just fine at anywhere from +/- 18v to +/- 24v. I'll be running them at 18v to compare them to my 2520s (the 2520s are at 18v now), and then I'll run them at 22v to see what kind of benefit there is (if any).


Shane
 
Nice, but the $75 a piece price will keep me from ordering in a hurry. With my Wall-Mart greet-at-the-door job I have to work ten hours for that opamp.
 
Very interesting and exciting, but I agree tk, kinda expensive. Can't you get a API 325 line card with a 2*5*2*0 with output tranny for about the same price? Regardless, I still can't wait to see what you think about it.
 
I think they ran the originals on +-16V.

I'm interested in your reports Shane - keep us updated. Do you have any Melcor PCBs....would love to hear a comparison between the 1731, 2*5*2*0 and the SC25?

Cheers Tom
 
I don't have any Melcors, but if someone wants to loan me a pair I will be glad to set up a controlled test. It would be interesting to find out once and for all how much the different op-amps will acually change the sound of the 312 circuit. After all, the transformers are probably the most responsible for the sound.

As for the 2520s, at D Alexanders website you can download the 2520 spec sheet. It is listed as running from +/-15 to +/- 20 volts, obviously gaining headroom with voltage increase.

Shane
 
[quote author="Category 5"] After all, the transformers are probably the most responsible for the sound.[/quote]

In this case, I wouldnt place a bet on that at all...

dave
 
I did some unscientific listening tests and the 2520 and SC25 do sound a bit different to my ear. I looked as some frequencey curves while I listened back and they look remarkably close to identical, but while the 2 2520s will almost completely phase cancel with my matched MXL 603s they didn't come close to canceling with 1 2520 and 1 SC25.

The difference in the curves seems to be in the roll-off below 63 Hz. The 2520 seems to slope a little steeper for a higher overal reduction (not by much though). Funny, however, that the 2520 seems to have more pronounced low end above 63 hz, so it sounds fatter. The SC25 seems to have a more pronounced upper-mid range than the 2520 which makes it sound clearer and more present...but not in the highs like the N72, it is definitely in the upper mids. I want to say around 3.5-4.5k but I can't really tell by looking at the frequency graph (Waves PAZ). I need better frequency measurement equipment to get a better visual. I think the SC25 might help a solo instrument poke out of a busy mix a little better than the 2520. This is speculation though, since my tests were quick and not in the context of a mix.

So the 2520 (at least the old model) and the SC25 certainly offer different flavors. I can't imgaine why they sound so different though, and look so close to identical on the frequency plot. I guess my ears are getting more sensitive to these types of differences. That, or the frequency plot isn't worth a damn. I am thinking of keeping the 2520s loaded and building another A12 pair for the SC25s.

This says a lot about the flexibility of the a12 as far as color goes. You could probably have a full rack of A12s and have 4 completely different sounding pairs! I imagine I will find the same true of the J99 when I get it.

Shane

Update - I did some tests with Right Mark analyzer, although I'm not sure how my setup affected the accuracy. I simply ran the balanced out of my RME into the mic in of the A12s. I had to turn the output fader on the soundcard to near it's minimum position to get the levels low enough. i am not sure if the impedance difference will skew the test considerably. The noise figures are almost completely useless since I was certainly amplifying the soundcards noise a far amount.

the curves do look REMARKABLY close, but not identical. The 2520 is about .05 dB hotter up to 1kHz, and then the SC25 takes over. The SC25 is about .05 - .08 dB hotter up to about 12 khZ where it starts to become about .1 dB hotter. Both taper off considerably after that and go about 3dB down at 35kHz. There is a noticable bit more harmonic distortion in the 2520 than the SC25 (about 25-30% on average) but the peaks are all at the same points. I imagine the SC25 is probably built around the later model 2520, and is probably pretty close to an exact clone if there is such a thing. I certianly feel they both have value, but after listening to an acoustic guitar track for a while I began to prefer the SC25. It seemed clearer and more focused. It lacked a bit of low end that the 2520 had, but it only seemed to add a bit of mud so it was not missed. The SC25 certainly had enough low end of it's own though.

Both Op Amps produced the most pronounced peak at about 15hz. There is a huge bump there, probably from the transformers and not the op-amps though. I wonder if this affects things in the audible range? Could it help add punch somehow?

Funny, in headphones there seemed to be some annoying upper mid frequencies in the SC25 track that were missing from the 2520. In my monitors this was not the case (Audix Studio 1a).

For kicks I compared the A12 with the 2520 to the N72 1272 clone. I was surprised to see that the 1272 drops about a decibel at 20k, with the slope starting around 16k. The 2520 held flat all the way up to 35k where it was opnly down a few dB. I always thought the N72 sounded like it had more high end than the A12, but I'll chalk it up to harmonics. Anyone care to help explain?

Shane
 
They are only $40 if you buy them with the kit. $75 isn;t that much. If he is building them in small quantities his production costs could be high. I can tell you the modules are potted and packaged pretty much like any other commercially available op-amp. real 2520s are $75 from API, and that is with an exchange op-amp. Without a bad 2520 you can't even get one.

THat's why it's nice to see someone coming out with a 2520 alternative that emulates the behavior of the real 2520. Short of making your own there aren't a lot of alternatives. the Avedis is a bit different.

Shane
 
[quote author="Category 5"]I did some unscientific listening tests and the 2520 and SC25 do sound a bit different to my ear. [/quote]

well considering an old 2520 and a new 2520 sound pretty different...

dave
 
[quote author="soundguy"][quote author="Category 5"]I did some unscientific listening tests and the 2520 and SC25 do sound a bit different to my ear. [/quote]
well considering an old 2520 and a new 2520 sound pretty different...
dave[/quote]

Fortunately, no company makes the same crappy semiconductors that were made back then. Nobody uses the same processes or process equipment that were used thirty years ago either.
 
[quote author="soundguy"][quote author="Category 5"]I did some unscientific listening tests and the 2520 and SC25 do sound a bit different to my ear. [/quote]

well considering an old 2520 and a new 2520 sound pretty different...

dave[/quote]

I would love to try to compare to some newer 2520s, or Baumans 1731 and 2520 projects. All in all I would have to say that the two are remarkably similar, even if my first impressions indicated otherwise.

I am pretty sure the SC25 is modeled after the new 2520s, so that's probably where the difference comes into play.

Shane
 
Here are the frequency curves

White 2520 - Green SC25

http://www.pbase.com/sbushman/image/35865632/original.jpg


White 2520 - Green 1272

http://www.pbase.com/sbushman/image/35865631/original.jpg


White 1272 1 - Green 1272 2

http://www.pbase.com/sbushman/image/35865633/original.jpg

Shane
 
[quote author="tk@halmi"]Fortunately, no company makes the same crappy semiconductors that were made back then. Nobody uses the same processes or process equipment that were used thirty years ago either.[/quote]


This is probably worthy of another thread, but did old stuff sound "old" when it was new? IE, does the sound of a semiconductor change over time, like 30 years?

What is our current stuff we build gonna sound like when it is "vintage"?

dave
 
[quote author="Flatpicker"]This is great info, Shane. Thanks for all the work! :thumb:[/quote]

Sure. I only hope my tests are accurate enough to draw meaningful conclusions. If I get ahold of some other op-amps I will repeat the tests.

Shane
 
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