hey bluebird-
you hit on a pivotal point, the place where you are most often going to hear the un-subtle differences between amplifiers is when they begin to distort, my pet monkey can tell the difference between amps then. Other stuff however is more subtle, some amps are softer and others stiffer, and this is all about the amplifier itself. The amount of slew distortion and what it is doing to transients is another place you will hear differences. The speed at which an amp will react to transients is yet another place, although you can lump this in with slew distortion if you really wanted to. Power supply and headroom issues is yet another place where you will hear some differences, although its hard to pinpoint exaclty that thats the deal... I think the quad eight stuff at +/- 28 volt rails is much bigger and open sounding compared to an api 325 at +/- 18 volts. Is it the opamp design, or is it the power rails? I think if you have a snare drum hitting a decently designed amp with +/- 28 volt swing, its going to be easier to get a BIG clean sound out of that than with an opamp of equal efficiency running with a smaller voltage swing. I dont know enough about electronics to defend that position, but thats my speculation.
In practical comparison, the hamptone Jfet pre I find to sound incredibly stiff with really edgy distortion. This amp has ruled in a way on bass because of that like no other amp I have ever used. That magic is DEFINITELY the amplifier and not the transformers.
A UA 1108 on the other hand, is the mushiest fet amp Ive ever heard and if you hit the front end really hard, you can hear the fet overloading and compressing and again, thats totally the amplifier adding the color, not the transformers alone.
Comparing a 283 AV amp to a 340 amp, the 340 can get really "spitty" sounding and can deliver a really cool "help me, Im totally overloaded" sound that Ive never heard a 283 AV do. The 283 is just BIG sounding and Im not refering to the transformers... I used to take those output transformers and hang them off of my ghost, not really the best driver in the universe, but it gave a good enough idea of what those transformers do on their own. Hearing the 283 drive it on stuff with super fast transients, a snare drum for instance, the 283 will gradually slow down the high end before it completely turns it into a square wave. On the class ab neve opamps, they sound way more consistent over a wider operating range, but go into a square wave much more abruptly on a snare and it doesnt have that pleasant slowing, it gets spitty and then distorted, and thats the opamp.
Looking at the JLM 99v, Ive been tracking with it now for a few weeks and like the 283, it has a slowing in the upper mids as it gets pushed, but it doesnt completely clamp down on the high end, there is still a nice amount of air that passes as it gets pushed harder, but gets a nice aggressive edge that isnt spitty at all when pushed to the brink. I honestly dont know what to compare this thing to, its really unique, if you can imagine the speed of a 2520 on top but with the sound of a 283, its hard to describe.
When selecting a pre for tracking, its good to think about the opamp selection in terms of behaviour and then the transformer that its driving in terms of color and control. Yes, the transformer will obviously add a tremendous amount of color to what you put through it, but take the case of the neve a/b opamps and a 2520 or a 1731, the neve is going to get spitty spitty spitty in the high end as it approaches a square wave where the 1731 will remain pretty stiff in the the highs until VERY abruptly clipping, the 2520 behaves kind of the same but is a little more gradual with the clipping on transients.
View most of my comments here in regards to transients, drums, etc. Thats where I always hear the edge of this stuff the MOST, its easy to hear what the baseline of the amplifer is at a usable gain and than judge how the opamp freaks out when you overload it for an instant with a transient, like a snare drum. You might want to push the 283 WAY hard on a snare to get a really cool effect, but with the same micing setup, it might be much cooler to record the same thing giving yourself a lot more headroom if you are using something with a 1731 opamp.
Telefunken V276 amps have a really smooth midrange but the neumann v476 is waaaay less grainy sounding, and Id put that off completely to the difference in the opamps. Those amps both have about the most unusable distortion out there, these arre circuits you want to run with plenty of headroom to make the most of, where something like a neve, on a rock record, can benefit from being run into the red, constantly for effect.
There was talk a while ago about the transformers poor headroom in a spectrasonics 610, and that very well may be the case, my experience with that box is that its clean clean clean clean then completely distorted. Ive always put this off on the transistors in the box, not the transformers, its not gradual saturation at all. From the older guys I know, they all refer to the SS amps as having poor headroom. I would also lump the neotek series two dual transistor mic pre in this category, clean clean clean then unusable squarewave distortion, but nice sounding up to the square wave. Think off it like, nice sounding, then louder, louder, louder, then unusable distortion. The top of the amp sounds just like a louder version of the bottom gain range of the amp. I find these types of amps totally boring and lifeless for rock, if I was recording classical music, however, I would want to use amps like this exclusively. Compare that to say, a 2520 where the bottom of the gain range is cool. Its gets louder, but the sound gets a little more 3-d as the lower mids get furry, more gain goes from furry to growl, more gain, high end starts to compress a little, and THEN you get some nasty distortion, but its not so nasty where it goes KRAKLE KRAKLE and ruins the track, you've got a good way to go in that "ok its really distorting now" area before KRAKLE, where the neotek type amp or the spectrasonics, its all nice and then pretty much instantly KRAKLE.
I guess one of the ways I decide upon an amp is the usable window of distortion that it can handle when you look at practicality. I cant make a 1272 or an 1108 sound bad. amps like that are like the level sponge, they can just absorb level and still churn out a pleasant sound, even if its completely distorted, its still pleasant and maybe not what you wanted, but still usable if you have to use it. Other amps sound good and get to that krakle point eventually, and others get to the krakle point pretty quickly. Some amps will give you a different frequency response (mids seem to peak more) as they overload where other amps are really linear until they go krakle. If you are doing location recording where unpredictability is a big part of the job, one type of amp might benefit you more than another, and of course other amps might be better to use if you are in a studio and can hit a talkback button and ask for another take because of operator error, many reasons to chose a pre...
A good place to see how an opamp can freak out is with a distressor. Mess around with that and you can hear the inner and outer limits of what an amplifier can handle and how it will distort when pushed over the edge. That box is transformerless, so all the distortion which you'll hear is all about the design of the amplifier blocks.
To bring this to a more maddening level, if you are thinking about tracking in terms of saturation, you can view amplifier selection when mixing in terms of presence. An amp that can pass a whole bunch more high end is often going to put stuff towards the front of a mix. An amp that has a lot of distortion in the high end can bring a bass forward in your mix. An amp that slows down the high end will seemingly bring stuff deeper into your mix, and then theres the playing with midrange distortion to move stuff around in between. Yes of course transformers will play into those decisions in all the same way, but by patching your tape out into this or that amp can very drastically change the place where your source is sitting in your mix, depending on the transient content of the source, the pre its patched to and the headroom that pre has.
Oh, and then theres tubes. anytime you need to turn something into a mess of undefined mush, run it through a tube. When I need an undefined midrange that is just a round blob, most old tube amps will get me there. Im sure a tube head will step forward right now and extoll the virtues of a new tube design to counter weigh what Im saying here, but Im talking about an LA2A or a Pultec EQ or an Altec 436, those get patched when I need the round orange blob. If you record on a computer, this can be the most awesome thing.
And then there's shitty chips. The 741 may have a demonic hate by folks out there, but I swear, a stock LA4 is about the most awesome tool for taking an edge off a vocal, not because of the wonderful smoothing power of the optical compressor, but because of the horrid slew distortion in the amplifier which will take a sharp transient and smear it all over the place. I have a hybrid QE eq which has chips in the EQ, so its really smeary sounding BUT, it uses an AM-10 on the output, so its got this MASSIVE punch to it. Good for those instances where you need a smeary punch like a click clacky low tuned bass where the player was sloppy, the slow smeary junk of the chips in the EQ can do a lot to soften the fret and finger noise without having to surgically excise those freq's with a notch EQ. Some chips however, can be fast as all hell, I think that true systems precision 8 used a burr brown chip and that mic pre was without question the fastest mic pre I have ever heard in my life, awesome awesome awesome sounding box that would IMMEDIATELY put whatever you put through it right on top of everything in your mix. Something like that would just absolutely RULE for recording flutes with ribbon mics, the tracks would be completely 3-D sounding, that box was transformerless.
hows that for a comparison? Short answer- the circuit matters a TON.
Im sure someone else can speak to other opamps, but those are the ones I use the most at my place. Add an am16 to this list which is just the wierdest amp Ive ever loved. wierd behaving little amplifier.
dave