Effects of MC Loading
- Thread starter JP
- Start date
Jim Hagerman
Member
Exactly. No measurable difference in frequency response. That leaves only one possible explanation (that we've come up with so far), and that's braking (velocity damping of stylus), which happens outside of the above measurements.
It makes perfect sense and follow's the laws of Faraday, Lenz, and Lorentz. It's observable in other electrical generator systems, for example, regenerative braking in a Tesla car. The effect is proportional to velocity, so clearly impacts treble region the greatest (thanks to equalization), and that's exactly what many think they hear.
What else could it be? Bias holds that the louder of two signals is often thought of as superior, but the opposite is happening here, as many prefer the sound of lower loading resistance.
nailer
audionerd
In the subjectivity of imagination ...
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JP
7480 74111110101115
Do the controlled tests of your laybacks yet?
flavio81
Turntable technician
... plus whatever things could be happening on the magnetic system (permanent magnet, yokes' hysteresis, etc) as a consecuence of the change in input impedance...
The datasheet shows curves for impedances ranging from zero to 600 ohms. LOMC cartridges range from about 3 ohms to about 40 ohms. A cartridge with a source impedance of 47k? Even MM cartridges don't have a source impedance anything like that. 1k (roughly) plus inductance is more like an MM source impedance.
I performed an unintentional experiment once.
When I first built my DIY phono stage, I installed the loading resistor AFTER the input coupling capacitor.
This caused rolled-off highs, and even mistracking.
So I played an album several times, cleaning it between plays with a velvet pad spritzed with alcohol. After each play I raked the album with a flashlight and observed whether the stylus was digging anything more out of the groove.
Once new dust stopped appearing (about 6-7 plays), I shorted across the coupling capacitor and played the same album. Not only did the high frequencies return, there was a fresh debris field of fine dust (quite a bit), and the mistracking stopped as well.
My takeaway was that the loading can influence both electrical and physical performance characteristics of the cartridge. Maybe there is something to Jim's cantilever braking thoughts.
BTW, I just recently removed those shorter-across coupling capacitors.
When I first built my DIY phono stage, I installed the loading resistor AFTER the input coupling capacitor.
This caused rolled-off highs, and even mistracking.
So I played an album several times, cleaning it between plays with a velvet pad spritzed with alcohol. After each play I raked the album with a flashlight and observed whether the stylus was digging anything more out of the groove.
Once new dust stopped appearing (about 6-7 plays), I shorted across the coupling capacitor and played the same album. Not only did the high frequencies return, there was a fresh debris field of fine dust (quite a bit), and the mistracking stopped as well.
My takeaway was that the loading can influence both electrical and physical performance characteristics of the cartridge. Maybe there is something to Jim's cantilever braking thoughts.
BTW, I just recently removed those shorter-across coupling capacitors.
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Are the resistors now in the correct position?
The turntable has a stand-alone ground wire to the phono pre-amp. Correct?
Yep. Everything has been wired as it should be since the 2nd or 3rd day I built it.
I was aware of the resistor after coupling capacitor issue pretty much right after I finished assembling it. I wanted to hear how it sounded. After hearing it, I decided to see if the rolled-off highs and mistracking were causing the cantilever to be restrained.
And I'm not saying I know this to be the case, but it sure seems to be the case.
There is no ground wire between the phono stage and turntable, the ground is carried by the RCA cable on my current turntable.
flavio81
Turntable technician
Yes, very small source impedance, but also consider the input impedance that the preamp designer wants to design into the circuit. The complete system is what the opamp will "see". If we, for example, create a preamplifier with 100 ohm loading, we're not connecting the cartridge straight to the opamp input. (To simplify my point).
Now, my only point was that in a real-life preamp, the circuit might not work exactly with the same performance parameters if the loading (not source impedance! but loading as a preamp setting!) is modified.
This might or not make a difference. I'm just saying it is a factor to take into account into the model.
I think the last post from Jim Hagerman probably points to the main factor that might make an audible change.
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Yes, the cartridge goes straight to the opamp input. The load impedance isn't in series between cartridge and opamp.
That was talked about on page one of this thread. See post #9.
There's no denying the existence of the magnetic braking effect, at least in theory, but whether the effect is audible remains to be proven. Certainly, nobody has yet produced any measurements to prove it.
flavio81
Turntable technician
You're right... Oops my bad... Too little sleep today. Maybe i should post when I can think more clearly.
flavio81
Turntable technician
There is more than one way to prove it. Listening tests can help too. I understand JP has tried to give us one listening test, with his files. However, my point, the point we were repeating, is that there is more than one variable. If nobody can reliably distinguish between JP's files, this would mean that for the cartridge (and preamp) he is using, there is no audible difference.
wyn palmer
Addicted Member
Thanks. It's a difficult task to describe the distortion mechanisms of opamps to someone who seemingly has, at best, a less than adequate understanding of their internal operation.
To put it crudely.
The sensitivity to source impedance is due to the modulation of the input bias conditions due to the varying common mode voltage at the input.
With bipolar this is mainly a change in the input transistor current gain, resulting in a change in the input bias current, which creates a nonlinearly varying input dependent voltage açross the source impedance - hence the distortion.
The source dependent distortion is worse as the swings at the input increase, and hence, for a given output swing, it will reduce as the closed loop gain increases because the input voltage swing falls, although the distortion due to the reduction of the feedback factor, will actually increase.