Hafler DH-110 Preamplifier Refresh

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.

Checking THD after capac­i­tor replacement

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.

Hafler DH-110 capac­i­tors and rib­bon cables in the phono section
Hafler DH-110 switch­es, capac­i­tors and rib­bon cables look­ing to front

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.

Hafler DH-110 Noble 204Y50KΩX2 poten­tiome­ter

I also dis­con­nect­ed the rib­bon cable con­nec­tors and cleaned those as well as the back pan­el RCA connectors.

Hafler DH-110 pow­er sup­ply section
Hafler DH-110 line ampli­fi­er sec­tion look­ing to front

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.

Hafle DH-110 main board C119 capac­i­tor module

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.

Hafler DH-110 pow­er sup­ply sec­tion added heatsink for IC1 and IC2

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.

Hafler DH-110 long view of main board
Hafler DH-110 main board bot­tom view of phono side
Hafler DH-110 main board bot­tom view of pow­er sup­ply side

7 Replies to “Hafler DH-110 Preamplifier Refresh”

  1. Hi, great pics and work. I’ve got the same DH110 with the same PC14A. I found it very bright and tried to replace the 0.82uf C19, with a Wima MKP 0.68uf. Significant tonal change: less crispy, more medi­um and bass. I may have lost a bit of high fre­quen­cies but I can’t say or mea­sure if this is due to the addi­tion­al medi­um and bass or a real loss in high freq. What would you sug­gest? I have not yet recapped the e caps. Maybe this would restore the high freq. that I may have lost along the way…
    Many thanks for your advice,

    1. a rise in uf in that area will increase the tre­able and low­er­ing it with decrease it; as it hppened to you. I tried the same but lost the right chan­nel! Working on the issue now.

  2. Hi
    I have a Hafler DH-110 Preamp & the Hafler P‑500 Amplifier. I have had it for over 25 years!! I refuse to part with them, as they make my Allison 1 speak­ers a plea­sure to lis­ten to.

    I have nev­er had any prob­lems with them in all this time time, and they are stil func­tion­ing well. My DH-110 def­i­nite­ly needs a refresh though!! It has start­ed to occa­sion­al­ly cut out on the left and right chan­nels while play­ing music. When i turn the Balance or Volume Knobs I get sta­t­ic and cut-outs. It does­n’t hap­pen all the time, but enough to be annoy­ing, espe­cial­ly when you are used to hear­ing such beau­ti­ful sound from this equipment.

    I am just won­der­ing, what you would charge to over­haul my DH-110? I would love tp keep it for anoth­er 10 years at least!!!

    If you would be so kind, I would great­ly appre­ci­ate if you could do it? I will leave my info below

    Regards and Stay Safe!!!


    1. Hi Jeff,
      I under­stand what a plea­sure it is to lis­ten to a Hafler DH-110 Preamp and P‑500 amp com­bo, as that is the set­up that I used to have.
      I cur­rent­ly do not work on any equip­ment that I do not own. But I may con­sid­er tak­ing on a few small projects next year(2021).

      Greg (Barbouri)

      1. Dear Greg:
        I can ful­ly under­stand your reluc­tance to take a ‘few small projects’ in 2021. If you should recon­sid­er, do let me now if you might be inter­est­ed in tack­ling my DH 110. Chances are I may still be in need of help
        Your arti­cle on DH110 impressed me , and earned my trust in your expertise.
        Be Safe!

        1. Hi luke,
          Still not accept­ing any exter­nal projects.
          I cur­rent­ly have more inter­nal and fam­i­ly projects, than I have time to com­plete. I don’t see this chang­ing any time soon.

          Greg (Barbouri)

  3. I need the part# for the t0-92 tran­sis­tors in the hafler 110 , Q3,4,5,6, they’re writ­ten up as 414c 416c, 556b and 546b do you have any idea? Equivalent?

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