Allowable DC Current for a Phono Cartridge

[djoffe]

Does anyone know of a reference that discussed the allowable DC current for a phono cartridge. The context is cases where the phono cartridge is direct-coupled to the base of a bipolar transistor. It might have 1 uA of DC current flowing. Does anyone know if this is a problem?

[djoffe]

Walt Jung, on page 6.19 of this reference,

http://www.analog.com/library/analog...h6_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.

[djoffe]

TI has an app note:

http://www.ti.com/lit/an/snoa586c/snoa586c.pdf

that shows direct coupled LM833's for a phono preamp.

LM833 input bias current ranges from 500nA to 1000 nA.

So...it seems like 1 uA wouldn't be a problem?

[oshifis]

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.

[Bonsai]

Don't DC couple your cart unless you are sure the current is well defined under all conditions.

Personally, this is one place I think a good quality cap is worth it.

[djoffe]

I sent the question to Shure. They basically "recoiled" in horror at the thought of any DC current going thru the cartridge.

[h_a]

1 uA

That's pretty much the same as the leakage current of elcos:

http://www.tadiranbatteries.de/pdf/BothEng.pdf

I'm sure there are plenty of cheap phono stages out there that AC couple with cheap elcos, so that shouldn't be much of an issue.

Still, I don't remember to have heard or read a definitive tech reply from a manufacturer, so please post it there!

[Calvin]

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

[bulgin]

Originally Posted by oshifis

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]

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!