Phono cartridge load capacitance for dampening? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 25, 2023, 01:39:04 pm
content: Is the load capacitance for phono cartridge, on the preamp input, Is it only for RF filtering or does it also serve the purpose of sibilance reduction / high frequency dampening?Is there a standard value for the capacitance or is it very cartridge specific?Also a thought, wouldn't it be wise to put a resistor in series with an RF suppression cap on the input to lower the Q so it doesn't risk resonating and making things worse?

author: Gyro
author_rank: Super Contributor
timestamp: September 25, 2023, 02:51:45 pm
content: The capacitance also taylors the high frequency response, which could be thought of as sibilance reduction. Capacitive loading is cartridge specific to an extent - I remember Ortofon releasing the 'Cap-210', which fitted directly on the cartridge pins to add some capacitance (210pF) as some of their MM cartridges required a higher load capacitance than was provided by most setups (they needed around 470pF iirc). You need to take account of different turntable wiring and pre-amp input too. Some manufacturers eg. Quad added variable loading options.I think most cartridges rely on winding resistance to provide damping, in combination with the 47k parallel loading.

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2023, 02:55:54 pm
content: Many MM cartridges specify a load capacitance: this value (200 to 500 pF, IIRC) with the mechanical components of the cartridge affects the resonance of the electromechanical circuit to improve the frequency response.I first noticed this specification when Shure introduced the V15 series in the mid-1960s; the V15-III specified a load of 400-500 pF in parallel with 47k\$\Omega\$, 70k\$\Omega\$ max.It does vary between cartridge models.It can also help with reducing RF interference.The nominal 47k resistor specified for almost all MM cartridges does provide damping of the resonance, along with electrical and mechanical losses inside the cartridge itself."Damping" of a resonance requires resistance (electrical or mechanical): capacitance or inductance re-tunes the resonance.

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2023, 05:41:27 pm
content: Continuing:Electromechanical systems are very interesting, but I will refer you to textbooks for detailed understanding.Strictly mechanical systems, such as a car suspension, have a primary resonant frequency from the massmand the spring constantk:w0= (k/m)1/2.To damp that, the shock absorber exerts a force that opposes the velocity to provide mechanical resistance.Note that damping requires a force proportional to velocity, not a frictional force independent of velocity.In the shock absorber, this results from a viscous liquid being pushed through small apertures: another term is "dash pot".For the MM cartridge, the interaction between the (fixed) coil and the moving magnet gives a voltage source (proportional to the magnet velocity), and the current through the coil applies a proportional force to the moving assembly.The voltage induces a current in the total electrical circuit, which includes the electrical inductance of the coil, its electrical resistance, and the external impedance of the amplifier.Now, mechanically, the variables are velocity and force: force times velocity is power.Electrically, the variables are voltage and current: voltage times current is also power.You can convert the mechanical part of the system into an equivalent electrical circuit, replacing force and velocity with current and voltage, and the parameters mass and compliance (inverse of spring constant) with equivalent capacitance and inductance (I forget which order), add the electrical components, and you get two coupled resonant circuits that transform the needle velocity to the amplifier input.

author: floobydust
author_rank: Super Contributor
timestamp: September 25, 2023, 09:29:44 pm
content: Another thing to consider is the TT cable capacitance and I include that in setting the phono pre-amp input cap value.Shure V15-IV optimum load is 47kΩ (max. 70kΩ) with 200-300pF "Total capacitance includes both the tone arm wiring and amplifier input circuit. (Most amplifiers, tone arms and cables meet this requirement)." The cartridge is 500mH and 1,380Ω.I was working on a Thorens and measure total cable capacitance, 263pF and 274pF difference in L and R values is due to the tone arm coax is two runs, one red the other yellow and the construction is different (plastic thickness) so one coax has a bit more capacitance.

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2023, 09:36:05 pm
content: There was some criticism of the earlier V15 models because of low high-frequency response.One of my commercial preamps included two female sockets on the PCB to install a parallel capacitor, if needed.I suspect that the V15-IV was tweaked for the lower capacitance value to match values similar to what you measured including the turntable cabling.

author: TimFox
author_rank: Super Contributor
timestamp: September 26, 2023, 01:43:54 pm
content: Well, anything that removes energy from the system will damp an oscillation.The velocity-proportional damping force in mechanical systems is equivalent to resistance damping in electrical systems.The "valve spring damper" is a rubbing friction.I use the verb "to dampen" for moisture. See:https://brians.wsu.edu/2016/05/25/damped-dampened/from Paul BriansCommon Errors in English Usage, Wm James & Co., 2013.The technical term for rubbing friction is "Coulomb damping"https://en.wikipedia.org/wiki/Coulomb_dampingas opposed to "viscous damping".Coulomb damping of a mechanical resonance is a non-linear phenomenon, and interestingly results in an ill-determined equilibrium position.From that Wiki article:"The position where the object stops, or its equilibrium position, could potentially be at a completely different position than when initially at rest because the system is nonlinear. Linear systems have only a single equilibrium point."That's a fancy way of saying that Coulomb friction can cause the system to "stick" at different places.It's difficult to get proper damping into a phono cartridge mechanically, so the actual damping in use is predominantly from the coil resistance and external load resistance.The nonlinear effect of Coulomb friction is inappropriate for electromechanical transducers such as cartridges and microphones.

author: CaptDon
author_rank: Super Contributor
timestamp: September 26, 2023, 01:59:11 pm
content: Those flat wound springs are dampers for the single main valve spring. They tend to arrest the sideways motion by being a surface to bear against and they somewhat arrest resonances within the main spring by the fluid viscosity resistance (fluid friction) between the main valve spring and the close fitting flat wound damper spring. In the case of resonance damping the flat spring appears as a large flat surface for the main spring to rub against with fluid as the resistance element between the two. Note that the damper spring adds very little closing force. If you see a conventional looking second spring it adds closing force but very little damping factor. To get that additional force from a single spring would come from a much more brittle spring steel prone to fracture during compression which can run nearly 3/4 inch in a two inch spring. Some setups use a main spring, a damper spring and a helper spring. Meanwhile back at the cartridge discussion, When using the N.A.B. 33 1/3 RPM test disc and a Potomac Instruments Audio Analyzer we were unable to reduce harmonic distortion below about 5% using the best of the best trial setups at our broadcast facility. We called in the corporate engineers thinking we had a problem. The answer....The distortion is predominately even harmonic distortion which is musically pleasing to the ear and is one of the reasons vinyl sounds 'richer' than 'sterile' CD's!!! I don't care what the numbers say on the box tops, the Potomac was telling us there were 5% artifacts once the pure tone was nulled out and that was considered excellent quality!!!

author: TimFox
author_rank: Super Contributor
timestamp: September 26, 2023, 02:37:43 pm
content: Distortion in disc recording and analog tape recording is stronger than people think.To be fair, the second harmonic distortion measured is not as troublesome as if it were third harmonic distortion, which would imply 3rd-order IMD which is nasty.On an audio forum, I once stated the usual assumption that 3rd order is worse than 2nd order in this case.Someone agreed with me, stating that he looked up the 3rd harmonic of A=440 Hz, which is 1320 Hz (the E on a higher octave) and could not find it in a table of musical notes; the closest was 1318.50 Hz.I had to explain the "equal-tempered scale" to him.

author: floobydust
author_rank: Super Contributor
timestamp: October 01, 2023, 04:44:59 pm
content: What is the sample rate and resolution #bits of phonographs? Will trade THD for that lol.