Follow Up: Lounge Audio LCR Mk III measurements

[Ecaroh]

Yes but this is an internal current limit spec and the opamp will be very non-linear way before that. Doug Self has some measurements on it in SSAD, page 146 (my 2nd Ed.). 10mA peak should be about the max in linear operation, but that still is huge compared to the usual output current requirements.

Thank you, Jan. The Self measurements are great to have.

 

[Ecaroh]

LT1028 is good, AD797 even better, especially if you use the distortion cancellation feature (although many designers don't even bother, the distortion is already far below the other non-linearities in a phono chain).

Any advantage in an MM-only design to be reaped from AD797 vs. 5534? Other than a chance to spend $11/chip ;-) ? Specs are great.

 

[LTig]

Thanks for this, SIY. I wouldn't be using the MC input, so nothing to address there...

The MM part looks very close to my DIY preamp, except that I have much more gain in the first stage (MC).

Just by a coarse look at the gain setting networks I think that the gain for MM is less than 40 dB at 1 kHz (~16 dB for Z1/101 and ~20 dB for Z2/102) which is a little low for my taste. A typical MM pickup with 5 mV output results in just 310 mV. But this can be fixed by increasing R3/R103 or decreasing R4/104 in the first stage, there is a lot of reserve in stage 1.

 

[SIY]

Any advantage in an MM-only design to be reaped from AD797 vs. 5534? Other than a chance to spend $11/chip ;-) ? Specs are great.

No, the 797 is excellent for MC, but too much current noise for MM. The guy who designed the 797 uses a FET diff amp for his own phono stage.

edit: Here's a nice trick You can calculate the optimum source resistance (i.e., minimum noise figure) for an opamp from the formula Ropt= en/in. When you plug the numbers in for the 797, you get 450 ohms. That's fine at low frequencies, but as the inductance kicks in in the midrange and treble (where you hear the hiss), the 797 doesn't look as good.

 

[LTig]

Any advantage in an MM-only design to be reaped from AD797 vs. 5534? Other than a chance to spend $11/chip ;-) ? Specs are great.

As SIY said the AD797 is not suited to MM (~ 1kOhm source impedance) but very well suited to MC (~5..20 Ohm source impedance). If you really want to burn money on MM try the OPA227 (unity gain stable) or OPA228 (gain higher than 5, should work in your case).

 

[SIY]

MM source Z is actually higher than 1k as you get into the midrange and treble- remember that there's like a Henry of inductance, so less of the 47k load is shunted.