Phono Section, Preamp, gain matching for noise
Al: I think he is measuring mainly white thermal noise as opposed to 60/120/240 Hz hum (via the tweeter). I found something similar with the PS Audio GCPH. That pre-amp has a low noise input stage and a gain cell buffer/output stage. The gain on the input stage is determined by a set of fixed gain settings (48/54/60/66 dB) and the gain cells have a volume control. So the input stage is analogous to your the pre-amp and the gain cell similar to the line amp. I found that the lowest noise (as measured by my Tektronix 7L5 spectrum analyzer) was found at the higher input gain settings, minimizing the output stage gain. In other words, the output buffer gain stages were noisier at higher gains than the input stage was. In fact, with gain increases of 6 dB of the output gain cells, I gained 8-12 dB in noise. But this trend was very non-linear, and this effect occured only over the last 6 dB of gain in the gain cells. Below that, the noise was dominated by input stage gain. |
I'm not sure exactly what the question is myself, but I do use an Avid Pulsus. You need to use at least the 60db setting (or even the 70db setting) on the phono stage for a cartridge with that low an output. I would forget about the "10:00 - 2:00" rule and use what's best for your cartridge. It also sounds like your amp produces more noise than the Pulsus. |
05-22-12: Dhl93449Hi DHL, I realized that he is not measuring hum. Note that I said "even though no hum occurs." Ground loop effects can cause noise at high frequencies, as well as low frequency hum. And it is conceivable to me that the former could happen in the absence of the latter. If high frequency distortion components and/or noise that are on the AC line were to couple through stray capacitances onto the chassis and/or the circuit grounds of each component, to a degree that is unequal in the two components, and if the components have their chassis and circuit grounds connected together (or connected together through a low impedance), and if 60 Hz and its lower order harmonics are not able to so couple (to a significant degree), that is what would happen. See this reference, and keep in mind that the interface between the particular components that are involved is unbalanced: http://www.jensen-transformers.com/an/an004.pdf Note this statement: The noises originating with the power line are generally described as EITHER [emphasis added] "hum", which is predominantly 60 Hz, or "buzz", which consists of a mixture of high-order harmonics of 60 Hz. These harmonics are the result of power line waveform distortion, which commonly reaches 5% THD and is caused by many types of non-linear power line loads. Because the human ear is much more sensitive to frequencies in the 2 kHz to 5 kHz range at these very low levels, buzz is usually more audible than hum, even though the hum level may be electrically larger.RFI/EMI pickup in the cables is also a possibility, as I indicated. Best regards, -- Al |
Thanks for the responses, of interest, and an indirect question, was based on the SNR published for the Acurus RL-11 of 95db A Weighted. The Avid Pulsus on the other hand has a noise spec from -81db to -67db. I have plenty of gain so operating the Pulsus at 48db of gain, even though the XX-2 is 0.28mv, still gives me the “typical” preamp range of 10:00 – 2:00. If strictly going by that rule, I would have set up the Pulsus gain that way. If you look at the SNR in that configuration it is SNRdb = Psignaldb – Pnoisedb. SNRdb = 114db – 60db = 54db. In the Pulsus 60db gain configuration. SNRdb = 114db-52db = 62db. Looking at the specs, Pulsus being noise floor at gain, and Acurus being SNR, it is not apparent to me how you would surmise this result analytically, without actually measuring. It looks like Dhl93449 hit it on the head when he said “I found that the lowest noise (as measured by my Tektronix 7L5 spectrum analyzer) was found at the higher input gain settings, minimizing the output stage gain.” A related question, has anyone actually measured their SNR using this method? I believe I have a quiet system and I also believe 62db is a good result, but what is a reasonable expectation? |
