I'm just a self-taught hobbyist, not a trained engineer, but I've been at this a while and I believe I understand a few basic things. (Although if anyone wants to correct me, please go right ahead. 
)
One thing we know is true is that different amplifying devices have different headroom capabilities. Some popular opamps, for instance, cannot swing output signal voltage all the way from 'rail to rail'. That means that if you have an opamp phono stage with +/-12VDC supply, the opamps may not be able to swing the signal beyond about 20V peak to peak, which is 10V peak, which is about 7V rms. (I think I got that right. Anyone who knows better, please correct me! Thanks.)
Let's say we have a MM cartridge with 5mV nominal output signal. Let's also say that a major click/pop excites the needle enough to produce a pulse that is 16X that level (+24dB), so 0.005*16 = 80mV pulse. (This was measured and found to be likely in real life, in M. Jones "Valve Amplifiers Third Ed.")
Let's say our phono preamp has gain of 40dB, which is 100X.
If that pulse is cleanly amplified by the phono preamp, it's going to be 8V at the output.
As you can see, that pop will clip the opamp preamp, by a volt, or over 10% past its overload threshold. It's important that the preamp be able to handle this overload gracefully, and without lasting effects that make the overload more audible (and annoying) than it should be. Once the overload condition has ceased, the phono preamp should return to normal operation as instantly as possible (instant recovery from overload).
The pulse of signal voltage caused by this click/pop should not be a big deal for a traditional line stage or integrated amp being driven by the phono preamp because there will usually be a volume control (attenuator) between the phono preamp output and the amplifier input. Presumably you will not be running the volume control pinned to 10. If you have your volume control set to -20dB attenuation (output 10x less than the input) then the amplifier will see 0.8V from that big pop on the record. That's survivable. If you have the volume control set to a much louder -10dB attenuation (output 3.2x less than the input) then that pop will send 2.5V to the input of the amplifier, which will be very likely to overload it. So if you play your LPs with the volume control almost all the way up, you'll need to go to pro level gear capable of accepting a 4V input signal without overload. That's probably not necessary for home hi-fi.
Another limiting factor regarding headroom in budget phono preamps is how the RIAA EQ circuit is implemented. Some preamps use a single opamp per channel with the EQ in that opamp's feedback loop. This means the opamp must drive both the load (the following preamp or amplifier input) *and* the EQ network in its feedback loop, at the same time. Many contemporary desktop amplifiers have very low impedance inputs, some with slightly lower than 5k ohm impedance (a 5k ohm volume control). This presents a 'heavier' load to the phono stage opamp than would an amp with 50k ohm input impedance (for instance). Also, the opamp with active EQ in its feedback loop will have a rising frequency response above the audio band (it will amplify frequencies higher than 20kHz more than it needs to). There's lots of high frequency noise caused by scraping a stone on the end of a super-light stick through a lumpy vinyl ravine. How well does that opamp handle trying to amplify all that HF noise? How much HF noise is there in that click or pop? Lots, right?
As was pointed out to me in a different thread (thank you restorer-john), contemporary amplifiers and home theater receivers use a microprocessor to adjust volume inside their DSP circuitry, controlled by a motorized potentiometer, which is in turn controlled by a remote control. This means there is no resistive attenuator knocking down signal levels to the input of the amplifier. The amplifier sees the full amplitude signal voltage from the source, no matter what the amp's volume control setting is. In the case of one example amplifier, its inputs overload with an approximately 4V signal applied. Our example phono click/pop far exceeds that. The amplifier will clip on this, no matter what. What will that sound like? (I don't know...)
I guess this all means that there is more going on in a phono stage than simple THD and SINAD can describe. These are useful metrics, as you do want low distortion and low noise, of course. But they don't tell you about overload threshold (which you'd like to be high) and overload behavior. If you read Amir's phono stage reviews, he does account for overload threshold. He'll tell you if a preamp has low headroom in his estimation.
Looking at the Schiit Mani 2 review, it looks like everything is pretty good except the headroom at very high frequencies.
The clipping threshold (i.e., headroom) is barely adequate up to 10kHz (25mV), but then worsens dramatically at 15kHz, and more so at 20kHz. Fortunately, signal amplitudes at these high frequencies are always very low (musical instruments don't make high amplitude 15kHz tones). But, what about sustained groove noise up there? Stylus tip resonance when hit by a click/pop?
Looking at the same measurements on the Parasound Zphono, we see that its headroom is independent of frequency, and the headroom is higher overall:
That's actually pretty impressive (max input = 42mV). The clipping threshold at a super-high 20kHz is pretty much the same as at midband frequencies. So, while the SINAD and THD scores are worse for the Zphono, its broadband headroom measures much better.
Which of the above two will 'sound better' playing records?
All other things being equal, will the Mani 2's lower noise and distortion result in better perceived sound quality from records?
Or will the Zphono's immunity from high frequency overload allow it to 'sound better' playing records, even though its noise and THD levels are worse than the Mani 2's?
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)One thing we know is true is that different amplifying devices have different headroom capabilities. Some popular opamps, for instance, cannot swing output signal voltage all the way from 'rail to rail'. That means that if you have an opamp phono stage with +/-12VDC supply, the opamps may not be able to swing the signal beyond about 20V peak to peak, which is 10V peak, which is about 7V rms. (I think I got that right. Anyone who knows better, please correct me! Thanks.)
Let's say we have a MM cartridge with 5mV nominal output signal. Let's also say that a major click/pop excites the needle enough to produce a pulse that is 16X that level (+24dB), so 0.005*16 = 80mV pulse. (This was measured and found to be likely in real life, in M. Jones "Valve Amplifiers Third Ed.")
Let's say our phono preamp has gain of 40dB, which is 100X.
If that pulse is cleanly amplified by the phono preamp, it's going to be 8V at the output.
As you can see, that pop will clip the opamp preamp, by a volt, or over 10% past its overload threshold. It's important that the preamp be able to handle this overload gracefully, and without lasting effects that make the overload more audible (and annoying) than it should be. Once the overload condition has ceased, the phono preamp should return to normal operation as instantly as possible (instant recovery from overload).
The pulse of signal voltage caused by this click/pop should not be a big deal for a traditional line stage or integrated amp being driven by the phono preamp because there will usually be a volume control (attenuator) between the phono preamp output and the amplifier input. Presumably you will not be running the volume control pinned to 10. If you have your volume control set to -20dB attenuation (output 10x less than the input) then the amplifier will see 0.8V from that big pop on the record. That's survivable. If you have the volume control set to a much louder -10dB attenuation (output 3.2x less than the input) then that pop will send 2.5V to the input of the amplifier, which will be very likely to overload it. So if you play your LPs with the volume control almost all the way up, you'll need to go to pro level gear capable of accepting a 4V input signal without overload. That's probably not necessary for home hi-fi.
Another limiting factor regarding headroom in budget phono preamps is how the RIAA EQ circuit is implemented. Some preamps use a single opamp per channel with the EQ in that opamp's feedback loop. This means the opamp must drive both the load (the following preamp or amplifier input) *and* the EQ network in its feedback loop, at the same time. Many contemporary desktop amplifiers have very low impedance inputs, some with slightly lower than 5k ohm impedance (a 5k ohm volume control). This presents a 'heavier' load to the phono stage opamp than would an amp with 50k ohm input impedance (for instance). Also, the opamp with active EQ in its feedback loop will have a rising frequency response above the audio band (it will amplify frequencies higher than 20kHz more than it needs to). There's lots of high frequency noise caused by scraping a stone on the end of a super-light stick through a lumpy vinyl ravine. How well does that opamp handle trying to amplify all that HF noise? How much HF noise is there in that click or pop? Lots, right?
As was pointed out to me in a different thread (thank you restorer-john), contemporary amplifiers and home theater receivers use a microprocessor to adjust volume inside their DSP circuitry, controlled by a motorized potentiometer, which is in turn controlled by a remote control. This means there is no resistive attenuator knocking down signal levels to the input of the amplifier. The amplifier sees the full amplitude signal voltage from the source, no matter what the amp's volume control setting is. In the case of one example amplifier, its inputs overload with an approximately 4V signal applied. Our example phono click/pop far exceeds that. The amplifier will clip on this, no matter what. What will that sound like? (I don't know...)
I guess this all means that there is more going on in a phono stage than simple THD and SINAD can describe. These are useful metrics, as you do want low distortion and low noise, of course. But they don't tell you about overload threshold (which you'd like to be high) and overload behavior. If you read Amir's phono stage reviews, he does account for overload threshold. He'll tell you if a preamp has low headroom in his estimation.
Looking at the Schiit Mani 2 review, it looks like everything is pretty good except the headroom at very high frequencies.
The clipping threshold (i.e., headroom) is barely adequate up to 10kHz (25mV), but then worsens dramatically at 15kHz, and more so at 20kHz. Fortunately, signal amplitudes at these high frequencies are always very low (musical instruments don't make high amplitude 15kHz tones). But, what about sustained groove noise up there? Stylus tip resonance when hit by a click/pop?
Looking at the same measurements on the Parasound Zphono, we see that its headroom is independent of frequency, and the headroom is higher overall:
That's actually pretty impressive (max input = 42mV). The clipping threshold at a super-high 20kHz is pretty much the same as at midband frequencies. So, while the SINAD and THD scores are worse for the Zphono, its broadband headroom measures much better.
Which of the above two will 'sound better' playing records?
All other things being equal, will the Mani 2's lower noise and distortion result in better perceived sound quality from records?
Or will the Zphono's immunity from high frequency overload allow it to 'sound better' playing records, even though its noise and THD levels are worse than the Mani 2's?
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