A12 Gain?

[thearnicasync]

Hi folks,

I just got my a12 back from Tim...I messed it up testing it.

It sounds RAD. Question, though: anyone else noticing obvious less gain than the N72?

I've got the A12 dimed for strummed acoustic guitar (soundelux u195), and at +4 it's pretty tough to peak out the meters in Logic. If I switch to -10, it's easy to peak it, but, well....I was just curious.

I thought the a12 was designed to have the same gain as the n72...

kb

[tele_player]

A couple of nights ago I compared my N72, J99 and A12, using a U195 and acoustic guitar. At about 12 inches, each preamp had several notches of gain remaining at appropriate levels in ProTools LE using a Digi 002 set for +4 input levels.

The U195 has a somewhat low output, compared to, for instance, a TLM103, but I definitely didn't crank the gain up on any of the preamps.

I'll test it again next time I get the chance, my Seventh Circle box is currently out on loan.

[Category 5]

The reason you have the gain all the way up is the low output of the mic. Plug in something hotter and you'll sit right at about 6 or 7 on the dial.

FWIW gain will probably vary based on the op-amp you use, but compared to my N72s the A12s don't seem to have as much gain.

Shane

[tele_player]

Cat5 - re-read what I wrote above? I was comparing the three preamps, and had the gain switch on all three in about the same position (6 or 7, I think). Yes, using a U195.

The A12 runs the op amp in a negative feedback loop. This greatly reduces the effect of various op amps 'open loop gain' on the overall gain. I haven't gotten around to plugging in the new SCA25 (replacing the Avedis 1122), but I don't expect a big change in overall gain.

Note: be sure you don't have the PAD engaged on the U195.

[thearnicasync]

Funny. I went back and tested last night...comparing gain on N72 and A12. There wasn't anymore than 5 dB or so of difference. Man, the A12 sounds rad. A little cleaner than the Old School Audio thing I had, but still nice and crunchy.

Cool...

kb

[tele_player]

Amazing how different transformers make such a difference, eh?

[tpryan]

I had one pair each of A12, N72, and J99 running side by side yesterday and took the opportunity to make some precise maximum gain measurements. The test signal was a 2KHz sine wave at -10dBu, attenuated by 40dB for the A12 and N72 and 50dB for the J99s. Measured gains were:

A12-a: 70.94
A12-b: 70.99
N72-a: 72.11
N72-b: 72.23
J99-a: 76.95
J99-b: 77.13

So the N72s are a little hotter than the A12s, but only by about 1dB. The J99s are a lot hotter, and they're supposed to be. The J99s were jumpered for 1000 ohm input impedance. Jumpered for 250, they'll produce another 7dB of gain.

[Category 5]

I just did some measurement on my A12s and N72s

Keep in mind I am using API 2520s, not the Avedis, and they are the older ones.

I am getting about 72db of gain with the N72s, which are within 1/25th of a decibel of each other at all gain settings, and at most settings within 1/50th (pretty closely matched!).

I am getting about 68db of gain with the A12s, a full 4db lower than with the N72s. I am running my 2520s at +/-18v (which I think should only affect headroom, not gain).

So there you have it. It seems that your Avedis op-amps give about 3db more gain than the 2520s. I have been to lazy to plug in the SC25s but plan to do complete comparison between them, the 2520s, and hopefully some Melcors if I can borrow some.

Shane

[tpryan]

I'm really suspicious of the discrepancies you're measuring between the N72 and A12. There should be no significant difference in maximum gain between the 2520s and 1122s, since the gain is determined by the external resistor network, not the op-amp. If the difference is real, it implies the 2520s you're using have an open loop gain less than 45dB or so, which is way too low and suggests a bad part.

[Category 5]

Hmmm...interesting.

I am certain the measurements are real. I will perform the test again today and give exact figures.

Could it be because the 2520s are the older (1973) version? I mean, they sure do sound good, and both of them have identical gain.

Shane

[Category 5]

I just redid the measurements, and I tested the null measurements too (without the preamps) to make sure there wasn't any error introduced by the soundcard (RME Multiface).

I get

N72 1 - 72.62 dB

N72 2 - 72.68 dB

A12 1 - 68.50 dB

A12 2 - 68.58 dB

So yes, there is a difference of about 4.12 dB between my A12s and my N72s.

I can't believe there is a problem with the op-amps because they are so close in actual gain. They are within .08 dB of each other at maximum gain and are within 1/10 of a decibel of each other at every gain setting. If they are defective it is certainly remarkable that they both have the exact same defect! Also, they sound good, and except for a couple of settings having a DC offset of -8mV at gain setting 6 (and -6mV on settings above and below) they sound and perform good. The offset is when the op-amp is nulled to 0.00 dc offset while at maximum gain, and I figured it was just because of the age of the parts.

I am getting a swing of 5.82 and 5.91 dB respectively in the 6 dB trim faders.

What should the actual open-loop gain of a 2520 be? The sheet at Dan Alexanders site gives it in V/mV (200 v/mV) and I don't know how to translate that. I know there were different versions of the 2520 and mine are definitely the older ones. The console they were pulled from was a 1973.

Shane

[tele_player]

That's 200v/mV . Whew!
OK, that's 200,000 - about 106dB, if I grabbed the right formula.

[Category 5]

That's 200v/mV . Whew!
OK, that's 200,000 - about 106dB, if I grabbed the right formula.

Corrected above. Yes, you are right 200v/mV. I don't think it is 106dB though! That would be insane!

Shane

[tele_player]

That's open loop gain, not insane at all. Op amps are not run open loop, ever. Well, not in any linear audio application, anyway.

[Category 5]

Tim,

I just replaced one of the 2520s with an SC25. Curiosity got the best of me. The gains are identical, so I am sure there is nothing wrong with my 2520s. In fact, they were within .02 - .07 dB of the 2520 in the same preamp, and I chalk that difference up to measurement system error too.

I also did some unscientific listening tests and the 2520 and SC25 do sound a bit different to my ear. I looked as some frequencey curves 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

P.S. I just noticed that you did your gain measurements with a 2kHz sine wave. i did mine with a 1kHz sine wave, so I'm not sure but this may have something to do with the differences. - I just tested again and it did not.