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Old 26th May 2006, 05:05 AM   #1
mmerig is offline mmerig  United States
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Default Head amp DC offset adjustment.

A Hafler DH102 head amplifier (for moving coil cartridge) has the following DC off-sets:

Input: Rt.= -6.8mV Left = +40.4mV

Output: Rt.= 0.0V Left = 0.0mV

The rail voltages are 17.4 and the + and - are within 0.01 V of each other. I can adjust each rail voltage with a trim pot. Doing so changes each input offset, but the total difference does not change (e.g., + 23.6 and -23.6 mV).

I assume that the offset should be very low (say < 0.001 Volts) so that the cartridge coils are not stressed. If so, what are the best options for reducing the offset? My thoughts are:

1. Try matching the transistors, but it may be tough to get the voltage down to near zero, and it may not stay there for years.
2. Install a cap in series to block DC. Perhaps a high quality 4.7 uf NP 6 to 10 Volt electrolytic or a 3 or 4 uf film cap would work. There is room for either type.
3. DC servo?

On the main preamp board phono section, there looks to be a nonpolar cap in series for each channel that would stop DC from getting to the coil, but the head amp is wired into a loop so that the inputs are directly connected to the phono plugs. I assume that one DC-blocking cap is enough, and it should be just after the phono plugs (that's the way it is without the head amp, and the DC offset is 0.0 mV for each channel).

Any suggestions?
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Old 27th May 2006, 04:01 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
any chance of seeing a schematic?
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regards Andrew T.
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Old 27th May 2006, 09:46 PM   #3
mmerig is offline mmerig  United States
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More than a chance. I have the schematic of the preamp (Hafler DH 110), but not the head amp (DH 102). The 102 is fairly simple and it shouldn't take too long to make one using the unit itself.
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Old 9th August 2006, 12:15 PM   #4
mmerig is offline mmerig  United States
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Attached is the schematic. It could be simpler, but I wanted to keep it close to the the PCB layout to avoid mistakes. The circuit to the right of the big ground loop is for muting, so I did not draw these devices in. The switch adjusts the gain.

I replaced C2 & C102 with a nonpolar electrolytic (220uf) and this did not help.

A 10uf nonpolar cap and 100K resistor at the input (like at the output) eliminates the offset, but it takes several seconds for it to go to zero.

Is there a way to balance (within 5 or so mV) the DC offset at the input with a trimpot so that theer is not too much offset at turn-on?

I have values for the resistors etc., and I'll send them separately.
Attached Images
File Type: jpg dh102 circuit.jpg (64.6 KB, 220 views)
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Old 9th August 2006, 12:16 PM   #5
mmerig is offline mmerig  United States
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Here are the values for the resistors, etc.
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File Type: jpg dh102 circuit values.jpg (58.2 KB, 159 views)
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