John Curl's Blowtorch preamplifier part II

[PMA]

Please note that OPA637 is stable for gains 5 and higher. So it would not work with integrator-like networks (active MM compensation) that require UGS (unity gain stable) opamps. In such case, OPA627 to be used.

[Jay]

Electrolux, it sucks!

[PMA]

Originally Posted by john curl

There is no such thing as a 'JC-2 Mk-1' design, in my experience. Would you like to clarify your accusations?

Richard means this design (that you have posted):

[PMA]

Originally Posted by john curl

For the record, the IC op amp that I use in place of the AD797 has to amplify over 30 dB or 40 times and has a final load of 800 ohms above 2KHz. Not so easy as some might guess.

They way I tested AD797 (and other opamps) yesterday was quite similar or probably more stressing. Gain 40dB and linear load of 1kohm.

How did you like the National Semiconductor opamp you have used to replace the AD797? BTW, in my measurements the AD797 has lower noise than 4562 and is more resistant to HF interference.

[john curl]

Thanks PMA, I didn't draw the schematic. I think of it as the JC-3, but this was an early version.

[jan.didden]

Originally Posted by Chris Hornbeck

Thanks very much to PMA for the excellent comparative measurements and for the striking conclusion, that despite these tiny numbers an ordering of sound qualities is still possible.[snip]Thanks again,
Chris

Ohh? I thought we had agreed here that measurements say nothing about sound quality? Are you now saying that the rank of 7th determines the rank of sound quality, even if it is below the -110dB audibility threshold John gave?

To paraphrase: dazed and confused ...

jan

[PMA]

Today I have prepared a 2nd set of opamp measurements. Now it is CCIF2, 19+20kHz. Only difference tone (1kHz), i.e. 2nd harmonic component, is calculated in 0.000X% results. But we have the FFT spectra.

Test conditions: Gain = +101 (40dB), Vout = 2.9Vpp, loaded with feedback network 1kohm + 10 ohm. The schematics is in the attached file. I have also added the measurement limit - card loopback measurement. It is another card than yesterday, for it performs better in CCIF2 test.

One can see that AD797, AD844 and OPA637 have had excellent results in this CCIF2 test. OPA134 has higher 3rd order products - the distortion lines close to 19kHz and 20kHz spectral lines.

Regarding THD1kHz and CCIF2 measurements, the winner is AD797. But, as correctly described by Richard, this opamp needs much more care regarding PCB and decoupling layout and is more sensitive to RF interference than JFET input opamps and than AD844 CFA opamp.

[jan.didden]

Pavel I wonder how much of this is CM distortion?
Your feedback network is probably to low in value to try in inverting mode?

jan

[PMA]

You are perfectly right, Jan. My setup erases influence of CM distortion and non-linear capacitances.

1) Resistors in -IN and +IN are only in tens of ohms. Too low to see non-linear capacitance effect.

2) Gain +101 and output voltage slightly above 1Vrms results in 10mVrms input voltage only. Again, influence of CM distortion is suppressed.

I wanted to measure noise (SNR) - thus low resistance values.
If I measured at gain 1 - 10, the result of good opamps would be below my distortion and noise measurement threshold limit.

So it goes, nothing is perfect.

[jan.didden]

Originally Posted by PMA

So it goes, nothing is perfect.

Some of those opamps are close enough, technically.
The next frontier is marketing!

jan