Avedis 1122 opamp voltages

[tele_player]

Yesterday, I ordered an Avedis 1122 opamp for my A12. I also asked Avedis what voltage he'd recommend running. Here's his response, included with his gracious permission:

Although at Brent Averill's we run the 1122 on +/- 16 vdc in the 312A, I have actually used them up to +24vdc with no problem. But this higher voltage may not necessarily "improve" the audio much. One benefit of higher voltage is higher headroom; for example, the 312A begins to clip at around +28 dbm (600 ohm loaded - this is very important in determining output current of an opamp.. but this is another subject). If I crank up the voltage to around bipolar 20, it will clip at around +30 dbu; if I do +/-24, it clips at around +32 dbu (at levels this high I cannot load it 600 ohm with my audio analyzer - it does not like it). This looks like a good thing - but are the few db's of headroom necessary?

One major disadvantage of running voltages high is that some of the voltage drop in the opamp will dissipate itself with heat. Changing the temperature of the internal transistors and diodes will slightly change it's current handling characteristics. Whether this affects the sound for the better or worse or not at all, is debatable.

Looking at a digital recorder's maximum headroom level, if you look at the specs of an ADAT, it says that maximum level is at +19 dbu, which is true because I measured it around +18 dbu.. close enough. This is the digital "zero" you cannot pass unless you like the sound of digital clipping. I have also read that other digital hardware maximum levels can range from +18 to +24dbu - most max out at +21dbu, though I'd like to verify these levels myself if I ever get a chance. Remembering that the 312A mic pre, with an output transformer set to 1:2 ratio, will clip at +28 dbm with bipolar 16 volts exceeds these digital zero levels. Most of the time, your average median level would hover around +8 to +10 dbu for ADAT's and probably +12 to +15 dbu for some other devices.

Personally, with all the sound and measurement tests I've done, I would run it anywhere from +/-16 to +/-20... no more, no less. This, I feel, is the perfect window for keeping the opamp to its optimum... without much heat that can possibly change the amps characteristics, but still have more than enough headroom for any digital recorder to handle.

I will let you decide.. and if you do any tests yourself, please inform me of what you find and your test procedures - even if it's just listening to a certain CD through it with monitor speakers. Let me know how it sounds and what you find or if there are any corrections in my understanding of digital levels.