This is a review and measurements of my Openamp phono preamplifier. The project was released in 2012 as an open project.
Openamp is a preamplifer for MM phono cartridge. It uses operational amplifiers, a monolithic buffer and a feedback RIAA correction. DC servo is used to remove DC voltage from the output. Output stage/buffer of the Openamp works at 20mA idle current. It remains in class A for all amplitudes of output voltage for loads 600 ohm and more.
The preamplifier is supplied from 2 x 15V to 2 x 18V regulated power supply.
Gain is 40dB
Openamp Measurements
Let's start with the output noise plot, with the input shorted by 50 ohm. Our sample uses OPA134 opamps, which are not the parts with the lowest noise, however the JFET input opamps are better for the MM phono preamp, one of the reasons is the absence of the input bias current flowing through the MM cartridge.
We can see that the unweighted output noise over 16Hz – 45kHz band is 86uV, this goes down to 78uV over 22Hz-22kHz unweighted and 45uV when A-weighted. So the 1kHz SINAD at 5mV/1kHz input (500mV output) cannot be better than 76dB (22Hz-22kHz). Let's see.
Now the usual measurement with 5mV/1kHz input:
We get THD = -106dB (excellent) and SINAD = 73.9dB (not so good, noise limited. OPA627 as input opamp is about 5dB better in noise).
Let's investigate distortion near clipping point:
At 8.41V output, which means 84.1mV input, we get THD = -97.7dB and SINAD = 94.5dB. This is a very nice overload margin.
“Most important measurement in a phono stage is frequency response”, as @amirm used to say, so here we go:
Now the THD and THD+N vs. output voltage plots
at 1kHz
We can see the distortion “knee” at 9V.
And at 5kHz
Unfortunately my DAC + output divider does not yield enough noiseless voltage at 5kHz (remember RIAA curve makes lower gain at higher frequencies) and if I omit the output divider, I get too much DAC noise. However, oscilloscope shows there is no problem with clipping level at high frequencies, it remains the same as at 1kHz.
Conclusions
This project was given to the public 10 years ago, together with schematics and Gerber files. I believe it made a very good MM preamp, with very high output drive capability due to output LT1010 buffer used. One may load it with 100 ohm without problem, with high overload margin.
Openamp is a preamplifer for MM phono cartridge. It uses operational amplifiers, a monolithic buffer and a feedback RIAA correction. DC servo is used to remove DC voltage from the output. Output stage/buffer of the Openamp works at 20mA idle current. It remains in class A for all amplitudes of output voltage for loads 600 ohm and more.
The preamplifier is supplied from 2 x 15V to 2 x 18V regulated power supply.
Gain is 40dB
Openamp Measurements
Let's start with the output noise plot, with the input shorted by 50 ohm. Our sample uses OPA134 opamps, which are not the parts with the lowest noise, however the JFET input opamps are better for the MM phono preamp, one of the reasons is the absence of the input bias current flowing through the MM cartridge.
We can see that the unweighted output noise over 16Hz – 45kHz band is 86uV, this goes down to 78uV over 22Hz-22kHz unweighted and 45uV when A-weighted. So the 1kHz SINAD at 5mV/1kHz input (500mV output) cannot be better than 76dB (22Hz-22kHz). Let's see.
Now the usual measurement with 5mV/1kHz input:
We get THD = -106dB (excellent) and SINAD = 73.9dB (not so good, noise limited. OPA627 as input opamp is about 5dB better in noise).
Let's investigate distortion near clipping point:
At 8.41V output, which means 84.1mV input, we get THD = -97.7dB and SINAD = 94.5dB. This is a very nice overload margin.
“Most important measurement in a phono stage is frequency response”, as @amirm used to say, so here we go:
Now the THD and THD+N vs. output voltage plots
at 1kHz
We can see the distortion “knee” at 9V.
And at 5kHz
Unfortunately my DAC + output divider does not yield enough noiseless voltage at 5kHz (remember RIAA curve makes lower gain at higher frequencies) and if I omit the output divider, I get too much DAC noise. However, oscilloscope shows there is no problem with clipping level at high frequencies, it remains the same as at 1kHz.
Conclusions
This project was given to the public 10 years ago, together with schematics and Gerber files. I believe it made a very good MM preamp, with very high output drive capability due to output LT1010 buffer used. One may load it with 100 ohm without problem, with high overload margin.
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