Updating a Hafler DH-110 preamplifier from the early-80’s with new electrolytic capacitors, along with a cleanup and then testing.
The Hafler DH-110 came in both a partially assembled kit and fully assembled unit. I had purchased this unit as a kit back in the early 80’s. It was originally paired with a Hafler P‑500 amplifier and Dahlquist DQ-10 speakers.
I had packed this preamplifier away before an out of state move 17 years ago, and decided it was time to return it to service.
The electrolytic capacitors were over 36 years old, so I decided to replace all of them with high quality Nichicon electrolytics. All of the original non-electrolytic capacitors were in excellent shape and were placed back in their original positions and channels.
I had researched what modifications some other audio enthusiasts had performed on their DH-110’s and had decided that Hafler had already optimized the circuitry, and to leave well enough alone. Well almost …
One of the changes that I made was to replace the individual channel supply rail electrolytic capacitors (470 uF @ 25 V) with a non-polarized version of the same capacitance and voltage rating. These capacitors are physically larger and have a wider lead spacing than the originals.
If I was to do this again I would stay with high quality polarized electrolytic capacitors, and maybe increase the capacitance to 560 uF instead.
After replacing the capacitors I cleaned all the switches and potentiometers with the exception of the volume control unit, with DeoxIT D5 contact cleaner. The volume control is a sealed aluminum Noble potentiometer.
I also disconnected the ribbon cable connectors and cleaned those as well as the back panel RCA connectors.
Capacitors C19 and C119 are both on a circuit board module and are comprised of a 0.82 uF (0.86 uF measured) film capacitor with a series resistance of 0.4 Ω and a 4.7K Ω series resistor. The resistor is most likely R69 which is not shown on the PC-14 board layout. This module is located at the input of each line amplifier channel.
I did add a heatsink made from a piece of 3/4″ X 1/16″ aluminum angle stock for IC1 and IC2 the 23.2 volt regulators. I noticed during testing that both of these regulators were fairly hot and could benefit from some additional cooling other than the small section of steel frame they were attached to. I reused the original mica insulating washers and added some high quality thermal paste between all surfaces including the frame.
I used my recently built Ultra-Low-THD Sine Wave Generator for testing the performance of the DH-110. All tests performed were well within the original specifications of the DH-110, and I was extremely happy with the results.
The only thing that I would like to still like to replace are the original RCA jacks which are still usable after cleaning off some very light corrosion, but could benefit from new jacks especially in the phono section.