Inside op-amps

The difficulty in using decompensated op-amps in an RIAA stage, is that unity gain is reached at some point. If not unity, then often a lower gain than 3 or 5 WRT the NE5534 and OP37.

These are not the only op-amps featuring decompensation, so watch out for so called "recommendations", and get to know op-amps (they're not like valves).

Experience shows that phase which is made to "kick back up" to preserve margins, are the op-amps that sound poor. However, that doesn't seem to put off the "rollers". Ignorance is bliss...

Headphone amps don't need much voltage gain, often less than 3 or 5 is quite sufficient, so unless you're able to work out the poles and zeros that lead to a stable noise gain, steer clear.

Basically, three stage op-amps are going to give you grief. There might be the odd exception, such as a rail to rail device, but is it suited to audio? Op-amps are not exclusively made for audio, but if all you ever do is frequent audio forums, perhaps you'd never know?

As an example, one op-amp application I worked on was a HGV rolling road, used to test brakes on tractor units as well as trailers. These op-amps were at times used for audio by some, and they have to go right out to high frequencies to be able to have high voltage gain at DC. These too had to be compensated, otherwise the readings on the meters would be in error. They amplify a tiny voltage from a strain gauge bridge, which shows braking effectiveness.

Other uses might be in temperature measurement, and also in medical electronics. Op-amps are often designed with a specific main use, but the manufacturer will also want to sell it into other industries.

A decompensated op-amp could be compensated for RIAA use by adding resistance in series with the 2122Hz pole capacitor, but that seriously affects the 20kHz gain, so must be compensated for after Riso, using an RC filter to continue the roll-off. The problem here is the overload margin has to be increased to cater for it.

This is why the experienced designer has to way up all things, and choose the most appropriate op-amp. If the designer has to redesign a product to use a different op-amp, then component values change. So, if somebody wanted to "roll in" an AD711 to an AD817 designed Era Gold V, or an AD817 into an AD711 designed Era Gold V, then there will be a noticeable difference in the bass "quantity" as well as quality. If then that somebody preferred one above the other, then that somebody ought to seriously consider tone controls!!!

The LF156 (356) and LT1056 feature all the great things you'd ever need in audio, except equal slew-rate.

Mr Jung puts it down to the p-channel input stage, and he's probably right, but the output stage - complicated by short circuit proofing - is almost the NE5534 output stage (spot the Shaw diode!).

Quasi complimentary outputs like these also have unequal positive and negative slew rates.

As long as they're made to behave (using "VCCS GOSIT" in simulating the case of the 156/1056), as output stages quasi's can sound great, but you'd not be advised to use a quasi where capacitors are used for EQ.

The charge rates differ because the slew rates differ.

The audible effects can be minimised by using low DA capacitors, which is why the NE5532 Gram Amp 2 trounced the cheaper NE5532 Gram Amp 1 (the Gram Amp 1 used polyesters).

The LF411, however, has near equal slew rate, and just about right for EQ such as RIAA, provided it's high enough impedance (not much output current). Again, the downside is there is no boasting about S/N, but oh, the sound!

Same with the Solo. The 24V is actually used as +12V and -12V?