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.
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.
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.
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.
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.
mmerig said:
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.
Hafler service manuals
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.
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
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.
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
HAFLER DH-101 PreAmplifier - phono
.
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
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.
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
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Attachments
Thanks, I was aware of the Hafler downloads, and have the manual for the DH110 (my preamp) and the DH101 (whatthe DH102 was meant for). I asked Hafler if they had a schematic for the DH102, and they do not.
Without the cap and resisitor that I added, the DC offset remains high. but as "lineup" suspected, my measurements were with no cartridge or load attached.
I installed the head amp in my pre-amp, and have been using it for a while now. It sounds fine.
The dc offset with a 20 ohm load (like the cartridge) was less than 0.1 mV, and the current was about 0.5 uA.
There is a 220uf NP capacitor in series at the input, so I would guess that this blocks most if not all of the DC. The original design had a polar cap (220 uf) in series. Besides being about 30 years old, I figured a non polar cap would be better than the polar one.
Thanks for all of the help.
The dc offset with a 20 ohm load (like the cartridge) was less than 0.1 mV, and the current was about 0.5 uA.
There is a 220uf NP capacitor in series at the input, so I would guess that this blocks most if not all of the DC. The original design had a polar cap (220 uf) in series. Besides being about 30 years old, I figured a non polar cap would be better than the polar one.
Thanks for all of the help.
mmerig said:
analog_sa said:
Hello.
Beside less linearity, at those small small levels
the NOISE of an Electrolytic Capacitor could be considerable.
At least this is what I think.
Changing that 220uF Lyt into
- two Paralled 4.7 uF non-polar Film Caps ( = 9.4uF )
- or one 10 uF dito
would in my opinion be a Huge Improvement
Could be polyester MKT or polypropylen MKP filmcaps.
I would not try Ceramics, Tantal or Electrolytics Coupling to any Input Transistor,
if I have one good alternative.
For Output is different. Because here the SIGNAL is often Bigger than a few uV and mV
and so the eventual noise from Cap is a level BELOW signal.
christmas
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Onvinyl said:
Yes, this is what we would go for!
And what I know, most modern RIAA Phono amp have no input cap.
This is because they are using dual supply, V+ V- and input at Ground level, 0V.
Only one resistor to ground = input impedance
and one small cap in parrallel with this input resistor
to set the desired capacitance needed for the PickUp in question.
For MM pickup usual values are:
Rin = 47kOhm
C = 0-100pF
( value depends how much capacitance the connection cable contributes
and of the recommendation in instruction manual for the PickUp used )
But here we are dealing with a somewhat older type of RIAA Amplifier.
This is why an input Capacitor is used and Let's keep it like this!
Sure there is a valid reason for it!
The designer is not a schoolboy when comes to circuit designs.
It is one professional designer!
-----------------------------------------------
At the output most every RIAA pickup amplifier needs a Cap (maybe 2.2-4.7uF Film Cap).
Because of the high gain of these amplifiers considerable output DC-offset can occur.
On the other hand,
If your RIAA amp output is feed to a preamp/power with input cap
you should not need any output cap in your RIAA amp.
christmas
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