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
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
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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.
There's surely some other way to do this with other trade-offs, but I don't have the DH-110 memorized.
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.
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.
Thank you both for your input.
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!
Thanks!
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
I hope this partial fix might help others.
Thank you to contributors here
I want to thank Rickcl for their work. The resistor truck mentioned in their last post worked well for me! The biggest resistors I had were 47k, so I ran two of them in series for each channel. It reduced the noise from a frightening pop to a merely annoying crackle.
Could someone with more knowledge help me understand the significance of the value of these resistors? What changes when I go up to 1meg? Could I keep going g to 2meg and continue to see improvements?
Could someone with more knowledge help me understand the significance of the value of these resistors? What changes when I go up to 1meg? Could I keep going g to 2meg and continue to see improvements?
The lower they are the more they warp the response of the Baxandall circuit I would think, so you want them large, but too large and the spikes get big, so its a compromize.
Unfortunate circuit design, its no more complex to do it properly I'd have thought.
Unfortunate circuit design, its no more complex to do it properly I'd have thought.
I have on hand 660K and 2.62M resistors, any suggestions as to which would be the best fit? Or should I just try both and see what works?
Bumping this thread because I'm wrapping up my DH-110 project and I don't fully understand the significance of the value of the resistor across pins 5/6 and 7/8.
I see direct improvement as I decrease the value of the resistor, with 26k nearly eliminating the noise completely. I've done some listening with the resistor across one channel only, using the tone controls, and I don't notice an immediate difference. Mostly, I'm concerned about this statement by Mark:
Would someone be so kind as to help me understand what he's referring to here? I just want to have a better understanding of the effects of these changes.
I see direct improvement as I decrease the value of the resistor, with 26k nearly eliminating the noise completely. I've done some listening with the resistor across one channel only, using the tone controls, and I don't notice an immediate difference. Mostly, I'm concerned about this statement by Mark:
Would someone be so kind as to help me understand what he's referring to here? I just want to have a better understanding of the effects of these changes.