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Openamp MM phono preamplifier

pma

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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_pcbassy.jpg


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.

Openamp_noise_E1DA_2.png


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:

Openamp_0.5V_E1DA_2.png


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:

Openamp_8.4V_E1DA_2.png


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:

Openamp_FR_E1DA_2.png



Now the THD and THD+N vs. output voltage plots

at 1kHz

Openamp_FR_THDN_1kHz_2.png


We can see the distortion “knee” at 9V.

And at 5kHz

Openamp_FR_THDN_5kHz_2.png


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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pma

pma

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Let me show a less conventional look at distortion plots:

Openamp_THDN_dBFS.png


It is the same plot as THD and THD+N vs. voltage plot at 1kHz as shown in post #1, but now in dBFS scale.

The top trace shows rising amplitude of fundamental frequency, we go from -40dBFS to 0dBFS. Below, from top, is a THD+N trace and it reflects just noise, THD+N is here defined just by noise.

Below the THD+N trace is the THD trace. This is distortion only, without preamp noise component. But, we can see the system and FFT "noise floor", which is somewhere at -130dB. So the real distortion can be seen just above 1V and we can see it rises from -130dB at 1V to -100dB at 8V.
 
OP
pma

pma

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20kHz voltage swing. Again, there is no limited swing at this high frequency. Show me the second phono preamp with such behaviour :). This sample had 2x17V power supply, thus the 30Vpp swing. With 2x15V supply, the swing is 26Vpp.

1665172923520.gif
 
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mlloyd1

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Jul 5, 2018
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Hi PMA,
You may recall I mentioned previously at "the other place" how much I liked my build of your preamp.
It's nice to see impressive measurements supporting the quality I've heard.
:cool:
mlloyd1
 
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