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Hafler DH-110 Tone Control Pop
Hafler DH-110 Tone Control Pop
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Old 15th September 2020, 05:27 PM   #1
rickcl is offline rickcl  United States
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Hafler DH-110 Tone Control Pop
Default Hafler DH-110 Tone Control Pop

Hello all. Before I throw in the towel on this problem, I thought I would run it by all the pros here.
What I have is a Hafler DH-110 preamp that has a loud pop when the tone circuit is activated. I have checked out all passive components and replaced all transistors with matched models.
Its a simple baxandall circuit that is either is in the feedback circuit or around it. A tone filter also ties the circuit to ground. Thos caps are fine.
Pressing the tone switch slowly can bring about -21.00v spikes as noted in the schematic attached.
My guess is that it is inherent in the design. Even the manual makes mention of spikes if the button is pushed too slowly.
Even pressing the button quickly results in quite a pop if hooked up to a power amp.
Muting circuit Fets are new, nothing weird there.
Thank you all for looking
Attached Images
File Type: jpg DH-110-tone11.jpg (114.4 KB, 77 views)

Last edited by rickcl; 15th September 2020 at 05:28 PM. Reason: additional info added
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Old 16th September 2020, 03:15 AM   #2
PRR is offline PRR  United States
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Hafler DH-110 Tone Control Pop
When S6b is in transition, the NFB loop is broken. It is a HIGH gain amplifier. It is sure to slew to a rail faster than the switch mechanics move. Don't switch slow!!

There's surely some other way to do this with other trade-offs, but I don't have the DH-110 memorized.
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Old 16th September 2020, 04:12 AM   #3
bansuri is offline bansuri  Italy
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Maybe the bypass switch can be exchanged into a make_before_break type, which would put both feedback paths in parallel for a moment, reducing the gain instead of latch-up. Try it out before by bridging 1/3 and 2/4 and see what happens.
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Old 16th September 2020, 06:33 AM   #4
rickcl is offline rickcl  United States
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Hafler DH-110 Tone Control Pop
I hadn’t thought of the feedback loop being interrupted. I did wonder about a make before break and will do some testing.
Thank you both for your input.
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Old 16th September 2020, 03:07 PM   #5
rickcl is offline rickcl  United States
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Hafler DH-110 Tone Control Pop
It looks like a MBB or shorting switch Is not an option. If anyone can think of a method to keep feedback loop from being broken I would be glad to try.

Thanks!
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Old 17th September 2020, 12:27 AM   #6
PRR is offline PRR  United States
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Hafler DH-110 Tone Control Pop
Tack 1Meg across any two terminals of S6b. Better?
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Old 17th September 2020, 12:53 AM   #7
rickcl is offline rickcl  United States
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Hafler DH-110 Tone Control Pop
Funny I was wondering about that. I didn't know a value. I will check it out.
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Old Yesterday, 09:13 PM   #8
rickcl is offline rickcl  United States
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Hafler DH-110 Tone Control Pop
Partial solution: Connecting a 820k resistor across pins 5 and 6 , and 7 and 8 of S6B reduced the pop by about 60%. Before it was a shock that could damage speakers. Connecting resistors across terminal of S6A did not improve the noise.
I hope this partial fix might help others.

Thank you to contributors here
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