Phono Section, Preamp, gain matching for noise
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 |
The reason the SNR specs for the Pulsus (and most other phono stages) worsen as the gain setting is increased is probably that they are defined relative to a reference level at the output of the phono stage that is the same, regardless of gain setting. Therefore the measurement for higher gain settings will be taken at a lower input signal level, reducing SNR in the circuitry at the front end of the phono stage. That in turn figures to generally be the most critical point in the signal path with respect to overall SNR performance, since signal levels are lowest there. In contrast, your experiments involved a fixed input signal level. So the SNR differences you obtained for the various gain settings presumably resulted from differences in the relation between signal levels and noise contributors at points in the signal path between the phono stage's gain adjustment provisions and the volume control in the preamp (inclusive). As you indicated, the net result of all of that is probably not analytically predictable. Regarding comparisons between SNR specs for the phono stage and the preamp, while in general it can be expected that the SNR performance of a good line stage will be superior to that of a good phono stage, I would comment that a direct comparison between the numbers is pretty much meaningless in the absence of specified reference levels. Unfortunately, SNR specs are commonly presented without any indication of what signal levels they are referenced to. That is the case for these particular components, as can be seen here and here. Regards, -- Al |
Therefore even though the signal is attenuated almost all the time I am getting the benefit of an active preamp, would you agree?Yes, certainly. Although there are some rare exceptions, nearly all active preamps have the volume control mechanism at a point in their internal signal path that is "ahead" (upstream) of their output buffer stage. So for nearly all designs turning the control down will have no effect on output impedance, and will not convert the active preamp into a passive one. Which is not to say that the setting of the volume control won't have any effect on sound quality, particularly if it is set to introduce a lot of attenuation. When cranking up the volume the noise I hear is pink in nature. I won’t call it pink, since it covers a limited frequency spectrum, It seems like all frequencies are being crossed over to the tweeter. I have no buzz or humSo it sounds like the ground loop effects I indicated as a possibility are probably not a significant factor. A simple way to verify that, though, would be to use a "cheater" (a 3-prong to 2-prong adapter) to temporarily isolate the AC safety ground pin of the power plug of either or both of the components. Some minor corrections to your math. With respect to the amplifier, what is directly proportional is the relation between output voltage and input voltage. Therefore output power varies in proportion to the square of input voltage. So 0.28V in would result in about 61W out. Also, 125W/87.5W is about 1.5 db, not 3.1 db. 125W/61W is about 3.1 db. For a given load, as you probably realize: db = 10log(P1/P2) = 20log(V1/V2), where P is power and V is voltage. Also, keep in mind that 0.28 mv represents the output of the cartridge under specific test conditions, and that figure may reach significantly higher levels on the peaks of some recordings, as well as often being at much lower levels. Continued good luck in your experiments! Regards, -- Al |
