dc offset at phono inputs

[mmerig]

Will some short-term, DC offset (< 1 second, about 250mV max) at the phono input damage a moving coil cartridge? After a second, there is zero DC offset.

[jackinnj]

how do you get that much input bias current (x R = voltage) on an MC input ?

[mmerig]

how do you get that much input bias current (x R = voltage) on an MC input ?

I don't know, and I asked about some potential ways of reducing it on the Solid State forum a while back, but no one offered a solution.

What I reported in my question was voltage, not current, and I cannot remember what the current was at a reasonable resistance. Anyway, the head amplifier that has the DC offset problem is very old (25 years plus), and I think I will just start from scratch with a newer design and components.

Thanks for your attention, jankannj.

[AndrewT]

Hi,
assuming a 10r coil resistance the power coming back from that DC pulse is about 6mW. That is unlikely to do any heat damage.

But as J said, where could 5 to 10mA come from? out of the bases of the input transistors? Unlikely.
Has something gone faulty? does it amplify? properly?

[jackinnj]

you usually have an input bias voltage as some current flows FROM the input pins -- the input usually has a shunt resistor of anyhwere from 50 ohms to a few hundred ohms -- so I am surprised that you would have that much voltage on the input.

You might want to just try try a low-noise, low input bias current opamp to replace the one in there now. First, though, make sure that there isn't a lot of acumulated dust and grime around the input opamp.

[mmerig]

There are no opamps, and I don't see a shunt resistor (but I may be mis-understanding the circuit).

I made a schematic directly from the PCB layout and the components on it. I could attach this, or just look at my old post about it in the solid state forum (Head amp DC offset adjustment), where it is already attached.

Any suggestions would be appreciated. I reduced the DC offset with a 10uf cap and 100K resistor, as implemented at the outputs of the board in its original state.

I have not used the head amp, as I was not sure it would ruin a phono cartridge. I could try it with an old moving magnet cartridge, just to see if anything comes out of it.

[jackinnj]

The DH-102 and DH-112 were "black boxes" -- I gave my DH-110 to #2 son after upgrading the power supply a bit. Never saw the schematic. I purchased mine new in 1981 -- so there have been improvements since that point I would assume.

[lineup]

Originally posted by mmerig
Will some short-term, DC offset (< 1 second, about 250mV max) at the phono input damage a moving coil cartridge? After a second, there is zero DC offset.

might possibly happen in some amplifier
before the power supply electrolytic capacitors are loaded fully
that some offsets will be right at power on

in a phono amplifier, with very high voltage gain (sometimes x1000 )
you would expect there could be such temporary offset at output

but of course some offset might be at phono amp input, too
some amplifier may be counstructed in a special and a bit unusual way
and so have this 'feature'

especially if there is NO SOURCE connected
This is what I wanted to know
if that 250mV value is measured with input OPEN, unterminated
if so
you may try using a resistor across the input terminals
and see if not you get a much much lower temporary offset

I guess you could try with 1kohm or 10kohm resistor.


My idea comes from that fact, that even a very small offset current
will cause a large voltage, if the input resistor has very high value.

What tells us this is NOT the case,
is that this phono is a MC, Moving Coil, amplifier
and they usually have very low input impedance.
Can be like 100-220 ohm, if I know what talking about.

[lineup]

here we can download
HAFLER pre- and power amplifier service manuals
There is no special manual for your phono amp
but the phono schematic may be in some of these manuals.

At least I could find the full schematic to several line amplifiers.

Library Archive

On-Line Documentation and Data Sheets
Hafler provides on-line documentation consiting of owner's manuals and printed data sheets.
.

And manuals for these Amplifiers, for example, can be downloaded:
DH-120, DH-200, DH-200
.. as well as many other pre-amps, power-amps
and Loudspeaker Monitors, Subwoofers.

PDF Download page:
http://www.hafler.com/techsupport/index.asp?ID=3


lineup

[lineup]

.
here for example is the schematic for the Phono Amplifier section
in HAFLER DH-101 PreAmplifier
As there is also a full parts list with component values, in the PDF Manual,
we know what we need to know to understand this HAFLER Phono amplifier.

see attachment

lineup