Best phono stage?
- Thread starter awsmone
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Again, what is the signal source that excites this MHz+ range resonance? Do we have any real world measurements of it in an audio signal chain?
Any tonearm has portions of unscreened wiring. Wood/carbon fibre tonearm tubes leave the tonearm wires completely exposed for a foot or so. Twisting the wires does a lot in reducing induced EMF, especially if a high CMRR circuit follows, only CMRR is a function of frequency and pretty much nonexistent in the MHz region. So, yes, i am quite certain that such frequencies are abundant in the signal coming off a cartridge and this has little to do with the cartridge itself.
That's some theory. 5 MHz signals are abundant coming off the cartridge due to one foot of exposed wire - not an antenna mind you. Whew! What evidence do you have to back it up? Sorry, but I don't buy it.
What I really don't care for here are two ideas: (1) that it must be accepted 5 MHz can happen in the signal chain at a level high enough to resonate at +32dB, despite there being no answer to a direct question as to its source, and (2) that most phono designers don't know how to deal with this *mysterious* signal. It sounds more like ad copy than science. Let's not forget that a significant number of phono stages employ SUTs which don't have any ability to propagate 5 MHz forward.
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The energy of the cartridge itself is enough to drive the peak into excitation. This article explains how audio frequencies are able to drive the resonance into oscillation; pay attention to the phrases
and
https://en.wikipedia.org/wiki/Electrical_resonance
***The energy needed to start the oscillation need not be the frequency of the oscillation itself!*** It really is that simple.
If it were any other way, oscillations due to layout errors in audio circuitry could never get started; it seems that this is the bit that is tripping you up. That is why I mentioned earlier that a simple DC pulse is enough to set off the oscillation. This principle is used in a number of ways in other fields, for example a resonance is used in a self-oscillating class D amplifier to set the switching speed. But the amp doesn't have a built in oscillator and isn't oscillating when you turn it on; the act of turning it on with the resulting DC states is enough to set off the oscillation.
OK so, something (maybe a DC pulse) from the cartridge excites a momentary peak which causes poorly designed phono circuits to oscillate. The cart resonance, *if* excited, will not be sustained by itself. So if changing from a 47k load to something heavier becomes audible, then the phono is at fault for failing to prevent these MHz oscillations because they are responsible for the audible difference. I think that's it, in a nutshell? I suppose if one does hear a difference from a change in load and your theory as to why is actually correct, then these momentary peaks (and MHz oscillations in bad circuits) must be commonplace. So can you share any measurement data on this? The regularity of their occurrence so as to be audible should make them readily measured. It'd be a great demonstration of the unconditional stability of your product. But also, it would help to put to bed the alternate theories which can be found in the threads on this topic around the internet, which at least to me thus far, seem more plausible.
I used the DC pulse as an example of how easily a high Q peak can be driven into excitation. This is not something that cartridges do- if you read and understood the Wiki link I provided, you would then know that any energy from the cartridge can potentially trigger oscillation in the peak.
The cartridge itself does not resonate!
The inductance of the cartridge in parallel with the capacitance of the tonearm cable creates the electrical resonance.
Reducing the loading resistance is audible on several accounts:
1) If RFI from the resonance is messing with the phono section, it manifests usually as brightness which is reduced by loading, especially if the loading successfully detunes the resonant peak
2) the loading causes the cartridge cantilever to be harder to move (stiffer). This could affect high frequency performance
3) the loading causes the cartridge cantilever to be harder to move (stiffer). This affects the mechanical resonance of the cartridge in the arm.
You are correct in that this is a commonplace problem. If you see loading switches on a preamp, its a good bet the designer of the phono section hasn't taken this all into account. IME none of the phono sections in import amps and receivers of the 1960s, 70s and 80s did. I have shared measurement data already in two links already (one in a post that I quoted). The other is one I commonly link:
http://www.hagtech.com/loading.html
Anecdotally, about 35 years ago I attempted to design a box that when connected to the cartridge, would make it easy to determine the correct loading for LOMC cartridges. To this end I ran some squarewaves through a few cartridges, and discovered they did not modify the squarewave at all- no rounding, even at 10KHz! Now this is easy to replicate, you just have to be careful to keep the squarewave generator at a very low level (meaning your oscilloscope is going to be running at high gain). Essentially you put the cartridge in series with the output of the generator and observe the result on the 'scope. My plan at the time was to place a resistance (the load) across the cartridge; but obviously at the time what I wasn't taking into account was the capacitance of the tonearm cable. But try it- so many audiophiles think the loading is like a tone control; what we see with this measurement is that the coils of the cartridge are entirely unaffected.
So something else is afoot. From my own work designing phono sections for a living, I found that the internal stability of the phono section (its resistance to self-oscillation; this is a layout, power supply bypass and stopping resistor issue) played a role, but further that the phono section should not be sensitive to RFI (in any form) and particularly should not be vulnerable to overload from the RFI caused by the cartridge/cable resonance, as the latter causes ticks and pops depending on the state of resonance in the peak from second to second. Until recently, its been a bit of a trade secret, because if you make a phono section that seems to exhibit less surface noise, people like that. So I've never published any measurements.
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Hagerman shows examples of resonance for MM carts, which happen near the audio band. If this was happening with MC carts, then I'd be concerned. But we don't have any measurements of the entire phenomenon being described here with MC carts. And it's a bit disingenuous to say you've never published measurements in order to protect your intellectual property, while saying these events are commonplace as I can think of many ways to protect your IP while demonstrating your theory. As it stands, you leave too many iffs lying around. In some ways, though, that's a clever thing to do. Meanwhile, the IAR 5 Moncrief article which I know you're aware of, offers a plausible explanation as to what's going on when we load a cart and backs it up with actual measurements. On the subject of tics and pops, they can certainly cause large outputs in ~100kHz range which could obviously temporarily overload some phono stages - though not yours!
Hagerman shows what happens with MC cartridges in the article at the link. Its the same info that JCarr presented in his posts at the other link which I posted (from a quote).
Obviously you don't believe me, so if you know any engineers, ask them how and audio circuit would behave if an RF signal that is up to 1000 times stronger than the audio signal (30dB) were to be applied to the input of the audio circuit at the same time as the actual signal. How would the audio circuit react? Could it act any differently if designed to reject the RFI? How differently would it behave? How could the peak be reduced, if the audio circuit was unable to cope with the RFI?
And then see what he says.
Nope. Hagerman's treatise does not accurately deal with MC carts. Jim is off by orders of magnitude for MC cartridge inductance. MC cart inductance is in the uH range. Look at the specs for yourself. Lyras in the 10 uH range, for example. There is variance, but none of the MC carts seem to exceed 50 uH at the absolute high end. So when you enter real MC values into his calculator, then you'd see that the peaks exceed 1 MHz. Plot the MM and MC together as below, and the picture is very clear. The rightmost peaks simulate the realistic range of MC inductance as 50uH and 5uH. The leftmost 3 peaks are in Jim's article. And a decade of difference well beyond the range of audibility is no small amount. Please stop twisting Jim's info into something it's not.
As to audio circuit behavior from "a signal 1000x stronger than the audio signal applied at the input". Are you talking about power factor, now? +30dB is 32x signal amplitude. Maybe you're not talking about a signal applied at the input, but after the MC gain stage? It's hard to know what you're talking about the way your wording tends to obfuscate things. But speaking of obfuscation, this completely misses the point anyhow. To stay on point, the right question to ask a (mechanical) engineer would be whether or not it is physically possible for a cartridge to generate any harmonic content that could excite a resonance in excess of 1MHz. I suspect it's not. And you can only provide iffs, without real world measurements. So the question stands.
As to audio circuit behavior from "a signal 1000x stronger than the audio signal applied at the input". Are you talking about power factor, now? +30dB is 32x signal amplitude. Maybe you're not talking about a signal applied at the input, but after the MC gain stage? It's hard to know what you're talking about the way your wording tends to obfuscate things. But speaking of obfuscation, this completely misses the point anyhow. To stay on point, the right question to ask a (mechanical) engineer would be whether or not it is physically possible for a cartridge to generate any harmonic content that could excite a resonance in excess of 1MHz. I suspect it's not. And you can only provide iffs, without real world measurements. So the question stands.
Sorry, I'm not reading anything into Jim's article that isn't there. As you rightly point out though, some LOMC cartridges have much lower inductance than others. There are some things you're not taking into account; first is the capacitance in parallel with the cartridge, which is a variable depending on the input capacitance of the phono section and the capacitance of the tonearm cable, which definitely are not fixed- they are variables!
The second is you also seem to be thinking that since the Lyra is much lower than shown in the article, that somehow it doesn't apply; the simple fact is that if the inductance is less, the peak is at a higher frequency. If you think that those peaks up in the MHz region don't affect audio circuits you are quite mistaken! RFI is well-know to cause havoc in audio; google it.
I don't know how I could have been more clear: there is no obfuscation. It does seem that that you either don't want to accept or are simply unable to understand my answers. Let's take this example, since you seem stuck on the idea that a cartridge needs to have MHz response for this to be an issue:
While you are correct that no cartridge can go that high, the cartridge itself does not need to in order to excite the electrical resonance. As I pointed out before, a simple DC pulse is enough to excite a high Q resonance; I suggest you re-read the Wiki link I provided earlier on electrical resonance.
The second is you also seem to be thinking that since the Lyra is much lower than shown in the article, that somehow it doesn't apply; the simple fact is that if the inductance is less, the peak is at a higher frequency. If you think that those peaks up in the MHz region don't affect audio circuits you are quite mistaken! RFI is well-know to cause havoc in audio; google it.
I don't know how I could have been more clear: there is no obfuscation. It does seem that that you either don't want to accept or are simply unable to understand my answers. Let's take this example, since you seem stuck on the idea that a cartridge needs to have MHz response for this to be an issue:
While you are correct that no cartridge can go that high, the cartridge itself does not need to in order to excite the electrical resonance. As I pointed out before, a simple DC pulse is enough to excite a high Q resonance; I suggest you re-read the Wiki link I provided earlier on electrical resonance.
I discovered the thread here. At first I didn't want to read it, because everything must be outside my price range. Then I read it for fun and discovered Audiospecials Phonolab. Although Audiospecials is only 30 minutes away from me by car, I did not know the device. So I borrowed the Phonolab and I have to say I am thrilled. Now I only have an Omtec Antares and a Nagra BPS for comparison. The Phonolab runs very well with my Simon Yorke S9 with Jan Allaerts MC1 Boron. I love it. I will probably have to buy. There is no way around it.
Thanks
Dirk
Thanks
Dirk
Simon York S9. No doubt, extraordinary in sound and it looks like a sculpture.
Micro Seiki is the more tecnical look.
groovemaster
Ralph, the point I'm making re: the Hagerman treatise is that your average person is going to look at it and say "Holy shit! A 20dB peak at 150KHz is what Ralph is talking about!" (when he goes on about tics and pops). However, the fact is, Hagerman is off by a factor of 100 to 1000 on his estimation of the L for an average MC. So it's very easy to mislead the common reader by siting this article - whether or not it's intentional, and I'm not saying it is. Sure, one can plug in the actual values in his calculator and see that it actually happens in the MHz range but the pic speaks volumes of incorrect info about the true scale of this. Because, again, a decade difference well beyond audibility is nothing to shake a stick at.
Now, saying that Hagerman applies is fine when the resonance is at 150Khz as this is a frequency that can be generated by a cartridge. But extrapolating this to 1.5Mhz and admitting the cartridge cannot generate that type of signal but saying it can come from elsewhere completely ignores the fact that a new model is required. Back to Jcarr's sims -- the key element is the inductance that is causing the resonance. It is part of a low impedance source driving a high impedance load. His results do not apply to any signal not generated by the cartridge. If the signal comes from outside the "black box" that is the cartridge, then suddenly that inductance appears as load shunted to ground and everything changes. The only way I can get the resonance excited in this case is to drive it from a very high impedance source and the only way to get your purported behavior is to drive it from a current source. Below are four more pictures. One is the original Jcarr model with a 1R, 10K, and 1M source Z for reference. The only meaningful plot of this graph is the blue one which represents a 1R source. The other two take the input signal to points in the circuit outside external of the cartridge. The last single trace one is if the external RFI is a current source, and good god almighty, an 81dB peak. The peak and dip at 50-80Mhz are the LC resonances of the cable which for some reason are hidden in the Jcarr graphs. I guess the next thing that needs to be addressed is what is the entry point and impedance characteristic of the mythical RFI so a better model can be made.
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Now, saying that Hagerman applies is fine when the resonance is at 150Khz as this is a frequency that can be generated by a cartridge. But extrapolating this to 1.5Mhz and admitting the cartridge cannot generate that type of signal but saying it can come from elsewhere completely ignores the fact that a new model is required. Back to Jcarr's sims -- the key element is the inductance that is causing the resonance. It is part of a low impedance source driving a high impedance load. His results do not apply to any signal not generated by the cartridge. If the signal comes from outside the "black box" that is the cartridge, then suddenly that inductance appears as load shunted to ground and everything changes. The only way I can get the resonance excited in this case is to drive it from a very high impedance source and the only way to get your purported behavior is to drive it from a current source. Below are four more pictures. One is the original Jcarr model with a 1R, 10K, and 1M source Z for reference. The only meaningful plot of this graph is the blue one which represents a 1R source. The other two take the input signal to points in the circuit outside external of the cartridge. The last single trace one is if the external RFI is a current source, and good god almighty, an 81dB peak. The peak and dip at 50-80Mhz are the LC resonances of the cable which for some reason are hidden in the Jcarr graphs. I guess the next thing that needs to be addressed is what is the entry point and impedance characteristic of the mythical RFI so a better model can be made.
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Sorry, the frequency response of the cartridge has nothing to do with the resonance. I've mentioned this several times to you already and you keep coming back to the idea that if the cartridge doesn't have the bandwidth, the resonance can't happen. This is false. The bandwidth of the cartridge has nothing to do with it. The inductance does, and so what if its a lower inductance than you see in a particular example- this simply places the resonant peak at a higher frequency; that doesn't change the fact that its still there and still able to be driven into oscillation.
What I suggest you do is look into radio frequency circuits and the nature of resonance, since you seem stuck on this cartridge bandwidth thing. A bell is a pretty good example- it rings at a particular frequency (its resonance) even though struck by something not operating anywhere near that resonance. Electrical circuits do the same thing. So the bandwidth of the cartridge has nothing to do with it. The fact that the resonance is there though has profound effects on the input circuit of the phono section.
Whether the peak as at 150KHz or 3MHz doesn't matter all that much. What matters is how well the phono section can handle that sort of frequency at that sort of level at its input. The bandwidth of the cartridge has nothing to do with it.
If you think that I might have mention the fact that the bandwidth of the cartridge has nothing to do with it a bit often here, that is because I want to make clear to you that the bandwidth of the cartridge has nothing to do with it. This is easy to sort out if you look into the nature of resonance- in particular the bit of how a peak can exist and go into excitation, triggered by an event that is nowhere near the resonant frequency. This is the same process that is used in self-oscillating class D amps when they start up.
So the bottom line here is that the bandwidth of the cartridge has nothing to do with the frequency of the resonant peak of which we are speaking.
The bit that seems to be holding you up is understanding that a resonant peak can exist that has nothing to do with the bandwidth of the system. The other bit seems to be that an electrical event outside the frequency of the peak can, when applied to the circuit in which the resonance exists, can set the resonance into excitation (oscillation). If you can get these two facts to sit nicely in your head, then you will understand what I'm talking about here.
What I suggest you do is look into radio frequency circuits and the nature of resonance, since you seem stuck on this cartridge bandwidth thing. A bell is a pretty good example- it rings at a particular frequency (its resonance) even though struck by something not operating anywhere near that resonance. Electrical circuits do the same thing. So the bandwidth of the cartridge has nothing to do with it. The fact that the resonance is there though has profound effects on the input circuit of the phono section.
Whether the peak as at 150KHz or 3MHz doesn't matter all that much. What matters is how well the phono section can handle that sort of frequency at that sort of level at its input. The bandwidth of the cartridge has nothing to do with it.
If you think that I might have mention the fact that the bandwidth of the cartridge has nothing to do with it a bit often here, that is because I want to make clear to you that the bandwidth of the cartridge has nothing to do with it. This is easy to sort out if you look into the nature of resonance- in particular the bit of how a peak can exist and go into excitation, triggered by an event that is nowhere near the resonant frequency. This is the same process that is used in self-oscillating class D amps when they start up.
So the bottom line here is that the bandwidth of the cartridge has nothing to do with the frequency of the resonant peak of which we are speaking.
The bit that seems to be holding you up is understanding that a resonant peak can exist that has nothing to do with the bandwidth of the system. The other bit seems to be that an electrical event outside the frequency of the peak can, when applied to the circuit in which the resonance exists, can set the resonance into excitation (oscillation). If you can get these two facts to sit nicely in your head, then you will understand what I'm talking about here.
I've stuck to it because you haven't demonstrated via measurement nor simulation not any empirical method that an audio circuit containing a cartridge as source can excite resonance in the MHz range. I don't dismiss it as a possibility, but I do consider it a significant extrapolation.
The reason I've not done that is that there is no reason to- the simple fact is that this principle is widely understood in electronics. It does not have to be re-proven any more than Ohm's Law.
Well done Ralph and thanks for explaining things [some of us already know] so well. The bell analogy for resonance excitation is really a good one. And yes, the better Phonos will deal with it better. This electrical resonance is an unwelcome and inevitable property of any LC circuit, and can be exited by any amount of energy in the circuit.
Funny, my 16 year old son started asking about short circuits over the weekend and the discussion evolved to how I was recently able to load at 47k with major sonic success and at max phono gain of 76dB, in my attempt to tell him that a 10 ohm load was close to a short, and then demonstrated how dead the sound with it was and why
Funny, my 16 year old son started asking about short circuits over the weekend and the discussion evolved to how I was recently able to load at 47k with major sonic success and at max phono gain of 76dB, in my attempt to tell him that a 10 ohm load was close to a short, and then demonstrated how dead the sound with it was and why
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Too bad this couldn't be a more productive discussion. Bells don't ring spontaneously. They also sound different depending upon where and how you strike them. You've effectively used the simplest model to describe the resonance but you're relying on an as of yet undefined model to excite it. Sure there's theoretical possibility, but the theory chain is only as strong as its weakest link.
No one said that bells ring spontaneously. Yet another twisting of what's actually being written. What Ralph said is that any frequency - and much lower, in fact - than resonant can excite the ringing, when the bell is energized. Said LC resonance exists by virtue of the circuit, and is excited by virtue of applying electric current. The resonance is NOT there when there is no current flowing through the circuit, just like a bell does not ring at rest. These are not hard concepts to grasp.
