Hello everyone, I hope I am posting this in the correct forum. I run a Hafler DH 101 preamp and a DH 220 amp that I am very happy with but recently have noticed a slight hum out of one of the speakers, it sounds like a typical 60 hz type of hum. I have also notice that the hum changes slightly if I rotate the bass control on the preamp. The only way the hum stops is if I unplug the output of the preamp tha runs to the amp input so I assume this hum is coming from the internals of the preamp. Curious if there are any Hafler gurus here that may be able to point me in the direction of where to look inside the preamp. I do have some schematics for the preamp. Thanks in advance.
Hafler DH 101 Preamp issue
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62vauxhall
Addicted Member
Not a guru, not a tech, just a hobbyist but I've been picking away at a DH101 for maybe ten years and in that time corrected wiring mistakes and replaced a number of parts including all of the electrolytic capacitors. Most recently a wholesale replacement of TO92 style transistors. That was to eliminate spurious noise that manifested once the pre-amp had been on for a while.
Someone had assembled mine from a kit. How about yours? A slow careful exam of all solder points using a magnifier would not hurt.
If I had your issue, I would have the pre-amp connected to the power amp but disconnect all signal sources to see if the hum you notice disappears. That would show if the hum is in fact being generated within the pre-amp or by a component connected to it. If the hum persisted, I would then run through all the input selector buttons to see if the hum manifests no matter what input is chosen or if it is isolated to one input selection.
I can categorically state that the RCA jacks David Hafler used for DH101's were absolute crap. The cheapest way to construct a jackpack I've ever seen. With my example, the entire affair had failed over time. I do not know what metal the center pin receptacles and outer barrels are made from but a brief glance is all it takes to see that they are low quality. Something on the order of tin can material. The possibility of oxide coated contact surfaces is high, Plus, the fiberboard strips where the RCA jacks are mounted is flimsy. In the case of my DH101, those strips were broken into several pieces.
If you find that hum is present on all inputs, I would look to the power supply.
EDIT: The reason adjusting the bass control alters the hum is because 60Hz is within the bass control's adjustment range. Either boosting or cutting the frequency range it is intended to affect.
Someone had assembled mine from a kit. How about yours? A slow careful exam of all solder points using a magnifier would not hurt.
If I had your issue, I would have the pre-amp connected to the power amp but disconnect all signal sources to see if the hum you notice disappears. That would show if the hum is in fact being generated within the pre-amp or by a component connected to it. If the hum persisted, I would then run through all the input selector buttons to see if the hum manifests no matter what input is chosen or if it is isolated to one input selection.
I can categorically state that the RCA jacks David Hafler used for DH101's were absolute crap. The cheapest way to construct a jackpack I've ever seen. With my example, the entire affair had failed over time. I do not know what metal the center pin receptacles and outer barrels are made from but a brief glance is all it takes to see that they are low quality. Something on the order of tin can material. The possibility of oxide coated contact surfaces is high, Plus, the fiberboard strips where the RCA jacks are mounted is flimsy. In the case of my DH101, those strips were broken into several pieces.
If you find that hum is present on all inputs, I would look to the power supply.
EDIT: The reason adjusting the bass control alters the hum is because 60Hz is within the bass control's adjustment range. Either boosting or cutting the frequency range it is intended to affect.
Last edited:
MuseChaser
Super Member
I'd also replace the jacks with this kit....
https://www.updatemydynaco.com/documents/DH101JackFieldReplacementsAssemblyManualRev1p0.pdf
That's the pdf manual for installation of the jacks. Dan's store site is down right now to catch up on orders,but should be back up again in a couple days. Great products.
The previous poster is correct ... The jacks on the DH101 and DH110 are absolutely hideous. I built the 110 many decades ago, and have had to revisit the jack board several times over the decades. I spoke with Dan about the possibility of a replacement kit for the 110, but it wasn't commercially feasible in his opinion.
https://www.updatemydynaco.com/documents/DH101JackFieldReplacementsAssemblyManualRev1p0.pdf
That's the pdf manual for installation of the jacks. Dan's store site is down right now to catch up on orders,but should be back up again in a couple days. Great products.
The previous poster is correct ... The jacks on the DH101 and DH110 are absolutely hideous. I built the 110 many decades ago, and have had to revisit the jack board several times over the decades. I spoke with Dan about the possibility of a replacement kit for the 110, but it wasn't commercially feasible in his opinion.
Thank you. I have disconnected all signal sources and the only thing connected was the output wire from preamp to amp and the hum was still there. If I then disconnect the preamp output the hum stops. I have also ran through all the selector buttons and they make no difference to the hum. I agree the rca jacks are a cheap and poor design, junk in fact! On my list to do's is to change them out if I am going to keep running this preamp.
Interesting. Thanks.
The site is back up so will be ordering the rca upgrade kit for the 101 very soon.
I did not have a hum problem with my DH101 but I agree with the RCAs being awful. I had been using the 101 as a phono preamp but the RCA connections were so unreliable that I finally removed it and threw it into a box out in the garage. It sat there for a few years. Just recently, I bought some cheapie RCAs from Amazon (they were still way better than the originals). They were very small and looked like they would fit with the proper spacing on that fiberboard. These were threaded with a nut to hold them onto the fiberboard. I removed the originals, used a step drill to open the holes up a little and screwed the new pieces to the fiberboard. The fiberboard is very thin and consequently not very stiff but I persevered. Long story shortened, the 101 is back in my system, again as phono-stage preamp for my 2nd TT setup. I just wish there was more room for some beefier RCA jacks but it all worked out well.
Look up in this thread and you will see a good update kit. I will be doing this soon. After what you did do you still have the hum?
I had a hum problem only when the cables moved due to the worn-out RCA jacks. Other than that, I had no hum issues. Now, the 101 operates as if it was new. BTW, that was a nice upgrade kit. It would have saved me a little time.
So I received the rca upgrade kit from updatemydynaco and was pretty impressed with the quality of parts. Took me a few hours to install but was pretty straight forward with his good instructions. Will be seeking out other DH 101 parts in the near future.