Allowable DC Current for a Phono Cartridge

Walt Jung, on page 6.19 of this reference,

http://www.analog.com/library/analogDialogue/archives/39-05/Web_Ch6_final_I.pdf


says that a direct coupled phono cartridge should have a bias current of <<100 nA, citing offset voltage with a 1000 Ohm cartridge. I'm not sure that's an issue...1000*1uA=1 mV. The offset voltage for the opamp is probably that much...

So, I guess I'm still looking for an answer tied more to the cartridge, I guess.
 
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It might also depend on the cartridge construction (MM, MC). I know that MC (and probably also MM) cartridges use about 0.5 mil (12.5 micrometer) wire. A small heat could smoke this tiny wire. At least I managed to break my delicate MC pickup with ~1 mA :-(
I think the biggest danger is the switch-on transient, not the steady-state current.
 
Hi,

the <<100nA bias in the Jung-Article refers to the resulting high output offset voltage, but isn´t a reference for the allowable current through the cartridge itself. One could calculate a safe absolute maximum current value from the diameter or crossectional area of the coil wires which could be as thin as 15µm.
1/10 the calculated value should give sufficient safety headroom.
From experience even high bias and offset parts like the Instrumentation amps from ADI, TI or That, namely the SSM2015-2019 or INA103/163 never destroyed a pickup in more than 20 years of DIY and commercial useage.

I regard 1µA of input Offset current (or input bias current) as a non-issue even for lowest output finest wire pickups.

jauu
Calvin
 
It might also depend on the cartridge construction (MM, MC). I know that MC (and probably also MM) cartridges use about 0.5 mil (12.5 micrometer) wire. A small heat could smoke this tiny wire. At least I managed to break my delicate MC pickup with ~1 mA :-(
I think the biggest danger is the switch-on transient, not the steady-state current.

Hi

You are slightly out of spec with cartridge coil wire guages. More like 2 to 2.5micron.

bulgin
 

ashok

Member
2002-06-06 4:43 am
3RS
I have always used Opamp based RIAA networks with direct coupling. Never damaged a cartridge coil so far or had any operational problems. However there was one accident many years ago. A MC coil popped out of it's rubber holder when I powered up the preamp which had an electrolytic cap at the input. I attribute it to charging currents of the cap. But with direct coupling this has never happened . Maybe the cap had residual charge on it .

I still have that AT31E cartridge assuming I can push back the coil into the holder. I didn't have a small enough tweezers at that time. Must think it out carefully before attempting anything. If I'm careless I'd loose an unused cartridge!
 
Hi,

it certainly does, at least as long You choose a common source topology. Things change if You use a common base topology.
Also, since MCs are low-impedance sources, the voltage-noise component dominates and this is where JFETs are infireor to BJTs. To achieve sufficiently low noise figures You need to parallel devices. This asks for source-resistors that add noise again, or very tight matching. So as usual ... one advantage comes with a disadvantage ;-)
Still though I´d prefer JFETs in a paralleled common-source topology as input devices.

jauu
Calvin
 
Hi,

I'm rather uneasy with servoes intended to null out an input offset. Their behaviour needs to be carefully controlled at power on and -off. Due to the long time constants involved the input may take several seconds to settle at the specified operating point. Enough time to have fried Your PickUp well done.
Also almost all simple servoes are a guarantee for Offset Voltage. Typically smaller than without the servo, but still not 0V.
And lastly ... doesn't a servo do away with caps in the signal path, but actually adds complex circuitry to it. Since a servo is typically drawn sideways like an addon in many schematics, it seems to have popularized the myth of not beeing part of the signal path.
As such it seems to hold truth to the sentence: "A cap not seen is a cap not heard".
A possible cap value reduction is the main if not only advantage I see.
A servo may create problems of which a simple cap maybe free by nature.
So it all dependes on applcation and implementation. There are good and bad examples for both.

jauu
Calvin
 

franzm

Member
2010-12-09 11:53 am
... one (I at the least :)) can think of a servo based system to remove DC current from the cartridge. I know that not everyone likes servo's, but it (if the problem exists) may be a way to solve it. My question is, has anyone ever seen a system like that (removing DC cartridge current using a servo)?
P. A. Sjöström (hope i spelled it right) has an interesting circuit using chopper op amps. Sjöström Audio - QSXM2 The extreme phono (RIAA) amplifier