DC Offset on the input of a Threshold SL10

[hifi-hospital]

Does anyone have experiance with DC offset on the MC phono stage of a Threshold SL-10? No problem with MM Phono Stage or line stage and no DC on the output. The unit has low distortion and is very quiet. I'm dieing to listen to it but don't want to magnatize my phono cartridge.

Thanks

[sreten]

Your MC's coils are likely to melt before any discernable effect on your
MC cartridge, which by definition is permanently magnetised one way.
(Very much like a loudspeaker driver)

Generally its a choice of the difficulty of having high quality coupling
capacitors due the capacitance values required versus leaving them out
and having some harmless DC offset current through the cartridge.

What is the DC offset and your cartridges DC resistance ?

sreten.

[hifi-hospital]

Hello Sreten,
The MC inputs have approx 3.4v open circuit. When I hook up my test generator, it drops to .24v. Since the preamp is 24 years old, I was checking it out completely before hooking it up to my Threshold SA 3.9e, which is also DC coupled. I didn't want to nuke my speakers.

[Nelson Pass]

The Threshold M1 circuit used bipolar transistors operated
in common-base mode, so that they cascoded the output
of the MC cartridge. The Emitters were driven, the Bases
were grounded (AC) with a DC bias, and the output came off
the Collectors. No feedback, very low noise, low distortion.

The current from the Emitter went through the coils, but it
was only a few mA, and did not cause problems. You could
easily argue that it in fact biased up the wire connections,
magically improving them.

[sreten]

I thought you were taking about a much smaller offset,
3.4V is a lot and you should be very wary of attaching
a MC to it.
Does your signal generator have low DC impedance ?

Certainly try connecting a resistor equivalent to your
cartridge, low/medium MC's are 3R to 10R usually,
and see what the offset voltage value is.

From your description it seemed that it is working and
presumably both channels have no output offset and
similar input offset ?.
It does seem a lot.......

sreten.

[sreten]

I hazarded a guess that your sig gen was ~ 50 ohm at DC
and that this would mean all the input device current was
going through the cartridge, seems to be the case.

I believe 3 to 5 mA would be normal for this case,
so a maximum of 0.05V across a 10R test resistor.

sreten.

[hifi-hospital]

Thank you Nelson and Sreten,
I use a Sound Technology 1500 Analyzer System which I believe is 50 ohm or lower output impeadence. Hooked up to the generator, the offset drops to 0.24v.
It is comforting to know that the S-10 is working as designed since I'm anxious to give it a listen. Now I just need to get a new Level control. Evidently the original one wore out and the previous owner took it to a TV repair shop. The control in there now looks like it belongs in a '50s HiFi Console.

Does anyone know the design value of the control? If I remeber correctly my old NS-10 used a 10K, wire-wound, Spectrol control.

Thanks again,
Doug

[Nelson Pass]

The circuit was not critical of the value of the volume pot -
anything from 10 to 50 K will be fine.

[hifi-hospital]

Thank you Nelson!!

I'll be on the lookout for the latest & greatest "audio approved" pot. Er. . . Ah . . Potentiometer

I'll probably look for an Alps Black Beauty.


Thanks again,
Doug

[pjmont]

Hello Nelson, i have a question for you,
i use a Threshold SL10 preamp and a Threshold TA300 poweramp very nice component, now i have a problem in SL10 preamp.
I have found (with multimeter) in left output channel a DC 0,12Volt and this is a problem for my poweramp and woofer (a strange and unusual movement at burn in ), the pre sound well (or i think well!) but i'am worry for the poweramp and woofer safe.
From what to be caused this? can you help me?

thank you
Pierluigi (Italy)