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Moving Coil Cartridges: Mobile Phone Interference

Posted by Graham
June 12th, 2016
Audio interference

Interference-free High Fidelity Listening

The hi-fi industry was (and still is) unprepared for cellphone interference when it comes to the high gain circuits essential to phono preamplification for moving coil cartridges.

The problem is that diode junctions rectify radio frequencies, just like a "cat's whisker" radio receiver, but to get a good signal to noise ratio, transistors, which include such junctions, are almost invariably used.

If the roaming mobile phone signal hasn't yet broken into your listening pleasure, it is most likely that you don't as yet have an offending cell phone in the vicinity. However, most moving coil cartridge users live with the threat of this disturbance.

Circuits claiming complete RF interference protection can no longer make that claim since the proliferation of mobile-phones over recent years have proven that assumption wrong.

The culprit? The hi-fi enthusiast will see the phone as the culprit, but the accumulating legislative directives don't see it that way. It then falls to the moving coil preamplifier circuit designer to make circuits that are immune to interference.

At one time all that was required was good shielding. A 9 inch metal arm tube offers effective shielding up to 30MHz which should keep the local taxi firm radio transmissions at bay. However, today's average GSM phone uses over 20 times that frequency - at which the single ended pick-up arm shielding serves no purpose at all - the arm wires inside become pick-up antennas!

At the same time there has been a trend toward lower and lower output moving coil cartridges, as these are often advertised as being superior (but are they really?).

Low output requires much more gain (voltage amplification) and junction transistor (BJT) inputs get used to keep hiss levels relatively inaudible. That's OK until a GSM phone in the vicinity checks its roaming frequency. Its radio frequency interference (RFI) is immediately rectified and the loud squawking and rumbling noise can't be good for one's speakers.

Greater immunity can be achieved, and in most cases no more GSM mobile phone breakthrough will be heard...

Firstly, the use of an inverting input amplifier places resistance in the signal's path. The construction of any resistor is slightly inductive eating-up some of the radiated energy. But it also has capacitance which can allow RFI to jump over. However, by splitting the resistance and placing a capacitor to ground, the input can swallow up more GSM phone interference energy before it hits the amplifier input stage.

The second means of greater immunity is the use of an FET input instead of the junction transistor: input linearity above its bias point is at least an order of magnitude more than with a BJT so there's much less chance of rectification.

Now look at 1 and 2 together and using the scissor or see-saw analogy of an inverting input op-amp circuit, it can be seen that the op-amp input is a virtual zero - a signal "black-hole". If RFI does the same as the signal it will be greatly attenuated, and the JFET input linearity is massive by comparison. It has far less chance of being rectified, and it can't be amplified because the cell-phone frequency is well beyond the amplifier's capability.

So that's the answer then? Well, it would be if not for the hiss.

Immediately we hit a snag! All resistors are noise sources (think white noise). A 1000 Ohm resistor generates more noise than the input of a good low noise BJT input operational amplifier or op-amp. That noise being in series with the amp input will be amplified along with the signal.

Another snag is the equivalent input noise voltage of a JFET op-amp. It too gets amplified along with the wanted signal.

It seems such a shame that this excellent method (possibly only real way) of rejecting mobile phone interference cannot be used... ?

But it can! By cutting down the input resistance to a maximum of 500 Ohms the resistor noise is reduced to that of many a ultralow noise op-amp. By including parallel resistors that can be switched in, a range of resistive loads up to 500 Ohms should suit virtually all conventional MC cartridges.

And use a new generation ultralow noise JFET op-amp and the equivalent input noise will be little more than resistor noise.

But with the trend for ever lower output moving coils, the S/N ratio is still going to be problematic. However, it doesn't take much more output for the signal to be far enough out of the noise for the hiss the electronics produces to become acceptable.

Wouldn't it be worth it to be rid of the cell phone breakthrough risk?

The newly developed Accession MC phono stage input circuitry for moving coil cartridges isn't only excellent at rejecting GSM phone interference, it has the revolutionary phono preamp technology only the Accession can do.

Choose your MC cartridge wisely by going for a minimum output of 0.25mV and the Accession MC input stage will transform your vinyl listening pleasure in a way you've never known before.

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Yet another great article, thanks!

My Denon DL-103R > Lundahl LL1933 Stepup > Reflex M > Musical Fidelity A3 isn't suffering from mobile phone interference?!

Is it just luck or does the SUT actually acts as a filter?

Graham Slee

At lower radio frequencies the SUT will act as an RFI filter, but because of winding capacitance it will eventually become a low impedance path to RFI at some much higher frequency. The Reflex M has an FET input which is much less susceptible to RFI