Terry Demol wrote on Thu, 07 December 2006 23:30 |
So Bruno, how can -120dB across the audio band sound 'bland'? Or are we just looking for 'not bland'?
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That's always the question. Like the most neutral speaker is bound to lose in a quick shoot-out, the most neutral amplifier is also generally found to be rater middle-of-the-road. Only when one listens longer does one hear that the "correct" amplifier does deliver all the goods, it only doesn't emphasise any of it. So to me, "bland" is not always an indication of a problem.
The 797 seems a different shade of bland though. I don't pretend to know how an amp with these figures could sound anything but "totally not there" - I hope to find out some day. I do have some indications though. In a noninverting circuit, several input stage error sources (small though they may be) are outside the feedback loop. One unbalanced line amp I've been toying with recently consisted of a pot followed by a discrete 10x gain stage. High frequency (6kHz) THD was markedly dependant of the wiper position, even if the input amplitude was adjusted to get the same output each time. As could be expected, THD was worst when the pot was half-way (we're talking about -106dB at 4Vrms). The impedance mismatch at the two amplifier inputs made the effect of the nonlinear input impedance of same stand out.
I then rewired the circuit to inverting, placing the pot in the feedback circuit and grounding the noninverting input. Even though this results in variable loop gain, the distortion became nearly constant (around -120dB) up to +20dB gain. Since both amplifier inputs are held at ground potential, the input capacitance is not modulated.
This line stage is now as undetectable as you can get in an unbalanced setting. In its previous state it was, well "bland", and there was a clear loss of detail when it was inserted.
I plan to do some further experiments with IC op amps to see if this finding can be extended to explaining some of the extreme colourations in IC op amps that nevertheless have good specs. In another context, I have been using AD797's and OPA627's as the first stage in an integrator chain (discrete 1-bit converter) and the difference was extremely small. Again, the inputs were used as a virtual short. Anyone who has toyed with these 2 op amps will attest that in normal gain stages there are no two more different sounding op amps around.
Whether that counts as an explanation is still an open question, but these two bits of anecdotal evidence suggest that inverting (virtual-short) circuits are less sensitive to these colourations, giving some indications as to where they come from.