Hello!
My apologies for taking so long to jump in on this topic, due to the holidaze... I thought I'd take a moment to explain myself...
When I've got a box on the bench, one of the first things I do is run a sine wave through and look at the overload characteristic - no load and 600-ohm load - and then apply a square wave under similar conditions. These two tests quickly tell me if there are any obvious problems (like a bad class AB output stage, bad caps, transformer ringing etc.). And, since I know what the rise time of a square wave is supposed to look like, I tend to investigate when things look a little "slow," if only to satisfy my own curiousity.
Take, for example, the 100k output pot on the 1176 (attached). I was scoping things out when I noticed a difference pre and post pot - keeping in mind the vulnerability to cable capacitance. UREI used a generic shielded wire from the pot wiper to the amp. For the hell of it, I tried a higer grade, low-capacitance belden cable and realized a slight improvement. I kept the original wire, just in case the customer objected, but he was happy and gave me other work.
In the studio, by contrast (and, you could even say, to contradict myself) I find that lots of things are too bright - or brittle - and while I wish I had an arsenal of ribbon mics to compensate, I make do with what's available.
One day I was recording sax and chose to use a modified Pultec MB-1. The circuit is very similar to the Altec 1566, but with negative feedback, so adding a feedback control allowed me to hear it both ways - and to demonstrate the effect feedback has on frequency response, the overload characteristic and square wave response to my studio maintenance class.
Now, understand I like the way 78-era recordings make the reeds sound and when the sax mic was opened up it was NOT what I was looking for so I tried the MB-1. Initially, it to had too much detail, so I turned the feedback OFF and attenuated the secondary of the input transformer to compensate for the additional gain. The "defocused" sound was an improvement to me and to those listening.
My point here is not to say whether feedback is good or bad - for opamps, it's obviously necessary. And, for opamps, I do prefer mine to be neutral. ALL of you have detailed how challenging that can be, both technically and psychologically.
In the case of the UREI LA-4, for example, I think we can all agree that the 4136 is neither transparent nor does it color the sound in a complimentary way AND that almost any newer opamp would be an improvement. Especially in this case, when a transformerless circuit degrades a squarewave like the stock LA-4 does, I tweaked it until it looked more like the input. And, I might add, not from an expensive piece of test gear but from a $60 battery powered oscillator.
In this digital era, we have come to rely upon / expect a wider range from our sonic palette and this is the reason there are so many options from so many manufacturers. On one hand, there is a definite need to pursue the more realistic path of high fidelity, of precise capture and reproduction. On the other, I think simple discrete circuits without feedback have an equal but opposite contribution to make.
Our ears are all different and so pleasing them at the same time is not an easy task. I rely on my ears, but I also take stock in what I see on the 'scope and the distortion analyzer - we've got to at least attempt to correlate these things - me in my crude way and others with their greater depth and arsenal of tools.
My hat is off to Bruno and anyone who can correlate the math with what we see via schematic and test equipment, as well as what our ears tell us. I also apologize for whatever shortcomings my columns may have. They are not easy to edit and I am often late in getting them in on time, making it harder to zoom out and get a better perspective (until they appear in print, when it's too late). I am thankful to all those who take the time to read and attempt to make sense of them. I also welcome your criticism and feedback, positive and negative.
Best wishes to all in the coming year.
sincerely,
eddie ciletti