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mmerig 25th April 2006 06:07 AM

Rail voltage drop in preamp
I am trying to adapt a Hafler DH-102 moving coil head amp to my Hafler 110 preamp. The 102 has an input votage of +- 18 V, while the rail voltage on the Hafler 110 is +-23. I made a voltage divider to drop the volatge to 18 volts and the proper current, but it drops the preamp rail voltage way down (the divider works well on a high-current bench-top power supply). The preamp transformer is supposed to be large enough to handle a head amp. Hafler made newer versions for the 110 but I doubt they were that much different than the 102.

The power supply filter caps on the 102 are rated at 25 volts, but I think 20 volts would be safer. Or, would it be okay to run the 102 at 23 volts?

The rails on the preamp (110) are regulated (LM317/LM337), and an easy option would be to change the reference zener diodes from 22 V to 18 V. This should make the rail voltage about 19. The preamp has a lot of gain, so I could lose some rail voltage and probably not miss it. Are there any problems that could arise by dropping the rail voltage?

AndrewT 25th April 2006 07:05 AM

what value of divider resistors are you using?
They should not cause so much draw that they drag down the regulated rails. Normally a voltage divider string should run with about 10 times the current that the circuit needs to minimise modulation of the tapped off voltage as current demand changes. This may be your problem.

A better solution would be a dropper resistor feeding an 18V Zener and use this to feed the lower rails of the head amp.

All you need to do is remove the lower resistor and replace it with the Zener. You need to check the Zener stays in conduction at full head amp current and does not overheat at minimum head amp current. Carefull adjustment of the dropper resistor should achieve this. A 500mW Zener can run with a range of current from 2.8mA to 27mA. This maximum might be a little low so 1300mW Zener might be better or use a Zener referenced transistor as your lower rail voltage adjuster. You will need a decoupling cap on the Zener to help minimise abrupt voltage changes on the headamp supply, something in the range 100uF to 1000uF.

The gain of the 110 pre has nothing to do with the selection of the rail voltage. Rail voltage determines the maximum output voltage sent to the power amp. Gain determines the sensitivity to input voltage for a particular level of output voltage.

More important is the required input voltage to the power amp and the resistance of the pre amp to spiky interference. These combinations will determine a suitable maximum preamp rail voltage.

sreten 25th April 2006 01:20 PM


isn't a simple RC decoupling filter the best solution for this case ?

It could easily drop the 5V and would also reduce noise and ripple.


richie00boy 25th April 2006 01:49 PM

You could probably even just run the head amp straight off the higher rails. Just check your capacitor voltage ratings. Any 35V or lower might be pushing it.

Your potential divider failed because the head amp makes the impedance of the lower dropping resistor swamped.

mmerig 25th May 2006 04:12 AM

The solution turned out to be simple. There was a resistor (same value on each rail of course) on the main board in series with the rail taps that I used as inputs for the voltage divider. These resistors had too high a value. Once I changed them to an appropriate value, I got the current and voltage needed for the head amp. The original resistors are probaby correct for the head amps meant for the Hafler dh110 (the dh111 or dh112). The dh102 head amp (the one I have) is for the dh101 preamp.

Just the same, thanks for all of the advice. I should not have missed those resistors.

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