Thanks Hans, this is what I thought I saw. I'll have a look through my Picoscope measurements to see if they are any better. Otherwise, there is no use in trying to get a meaningful SNR measurement with either of these set-ups. A last option (if I want) is to change the head-amp to +30 or + 40dB, and see waht that brings. Otherwise, I can only use this set-up if I want to see if I can get rid of the 15-20 kHz junk (and the 50 Hz hum), once I know that this isn't aliasing I am looking at. My thoughts are that the turntable (which is direct drive) is generating noise, or that my tone-arm grounding leaves room for improvement. Having said that, the record's own groove noise is much higher than what is coming from the RIAA pre-amp / cart combo.
Previously I was using the Picoscope in a valve environment to trim phase-inverters and balance cathode currents on PP outputs to minimize 2nd harmonic distortion, for which it worked well, since the distortion levels are much higher than the noise floor.
Previously I was using the Picoscope in a valve environment to trim phase-inverters and balance cathode currents on PP outputs to minimize 2nd harmonic distortion, for which it worked well, since the distortion levels are much higher than the noise floor.
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Is the annotation "Soundcard + 20 dB head-amp, input soundcard shorted" correct? It seems more logical to short the 20 dB preamplifier input.
At 1 kHz:
-112 dBV with shorted phono preamplifier input and 20 dB of extra amplification
-125 dBV with (presumably) shorted 20 dB amplifier input
Subtracting noise powers: 10 dB log10(10-11.2 - 10-12.5) ~= -112.2233 dBV with shorted phono preamplifier input and 20 dB of extra amplification, corrected for measuring equipment noise
Corrected for the extra 20 dB: -132.2233 dBV at the phono preamplifier output with shorted phono preamplifier input
44.1 kHz sample rate, DFT length 65536 samples, Hann window, presumably scaled such that a tone in the middle of a bin is shown with the right voltage -> noise bandwidth 1.5 * 44100 Hz/65536 ~= 1.00937 Hz, I think
Noise density at the phono preamplifier output when its input is shorted: 10-132.2233/20 V/sqrt(1.00937 Hz) ~= 243.67 nV/sqrt(Hz)
Equivalent input noise voltage density: 243.67 nV/sqrt(Hz)/1030/20 ~= 7.7057 nV/sqrt(Hz)
Theoretical estimate: thermal noise of 953 ohm plus thermal noise of 100 ohm plus op-amp voltage noise: 6.4844 nV/sqrt(Hz)
All in all, the measured noise with shorted input is close to the calculated noise with shorted input. It would be interesting to see how much worse it gets with the real cartridge connected again.
At 1 kHz:
-112 dBV with shorted phono preamplifier input and 20 dB of extra amplification
-125 dBV with (presumably) shorted 20 dB amplifier input
Subtracting noise powers: 10 dB log10(10-11.2 - 10-12.5) ~= -112.2233 dBV with shorted phono preamplifier input and 20 dB of extra amplification, corrected for measuring equipment noise
Corrected for the extra 20 dB: -132.2233 dBV at the phono preamplifier output with shorted phono preamplifier input
44.1 kHz sample rate, DFT length 65536 samples, Hann window, presumably scaled such that a tone in the middle of a bin is shown with the right voltage -> noise bandwidth 1.5 * 44100 Hz/65536 ~= 1.00937 Hz, I think
Noise density at the phono preamplifier output when its input is shorted: 10-132.2233/20 V/sqrt(1.00937 Hz) ~= 243.67 nV/sqrt(Hz)
Equivalent input noise voltage density: 243.67 nV/sqrt(Hz)/1030/20 ~= 7.7057 nV/sqrt(Hz)
Theoretical estimate: thermal noise of 953 ohm plus thermal noise of 100 ohm plus op-amp voltage noise: 6.4844 nV/sqrt(Hz)
All in all, the measured noise with shorted input is close to the calculated noise with shorted input. It would be interesting to see how much worse it gets with the real cartridge connected again.
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Marcel,
The 243nV/rtHz@1kHz corresponds perfectly with the simulation in #119.
And with the 30dB gain this calculates of course into 7.7nV/rtHz@1kHz EIN.
It shows again how reliable noise prediction can be with LTSpice and proves that the Amp works perfectly.
Arjen got the spectrum (almost) right in the end after earlier attempts that were way off, bravo.
For the perfectionist a bit more gain from the second amp would also have shown the spectrum’s high end correctly
Hans
The 243nV/rtHz@1kHz corresponds perfectly with the simulation in #119.
And with the 30dB gain this calculates of course into 7.7nV/rtHz@1kHz EIN.
It shows again how reliable noise prediction can be with LTSpice and proves that the Amp works perfectly.
Arjen got the spectrum (almost) right in the end after earlier attempts that were way off, bravo.
For the perfectionist a bit more gain from the second amp would also have shown the spectrum’s high end correctly
Hans
The noise at high frequencies will get much stronger with the cartridge included, so that helps.
Yes, the caption should have read "3. Soundcard + 20 dB head-amp, input head-amp shorted"
Below are some more spectrum plots with cart attached.
1. Cart + RIAA amp + 20 dB pre-amp. This is with the RIAA amp fed from the internal 17V7 zener supply & DD motor/platter running
There is effectively - 80 dB / 50 Hz hum. I might try to run the RIAA amp from 2 x 9 V batteries and / or disconnect the DD motordrive and / or play around with my tone-arm grounding. There is a difficulty in that the DD drive electronics consist of two dedicated chips. The chip that drives the DD motor has a heatsink. This heatsink is internally connected to the V- supply. It is heatsinked to a subchassis (unisolated) and that subchassis is grounded to the bottom metal plate of the turntable together with the tone-arm ground. I have had to connect V- from the RIAA pre-amp, not to the minus of the zener in my supply (as you would normally expect to do), but close to the minus of this chip that drives the DD motor, to get rid of (low but) audible noise (with the volume turned all the way to max.) when switching the rpm's over from 33 -> 45 and v.v. The current draw has a spike in this moment. I can tell from the ripple voltage on the main power supply smooting cap and the supply to the motor drive drops by a volt or so for a moment until the rpm's have stabilized. I think the gain of the darlington combo may be running low in this case (due to beta-droop). I also see the voltage on the zener dropping when switching the rpm's around. It might deplete the zener current in this moment.
So, there are some low-level issues, however:
2. Cart + RIAA amp + 20 dB pre-amp, playing a quiet passage of a record
What can be seen is that all the pre-amp noise and hum drowns in the record's own groove noise.
It's worth considering.
Up to 16 kHz, I'd say this is the case. The junk from 16 - 20 kHz just disappears. I can still only hear to 12 kHz.