Hi all! Owner of an RNHP (Rupert Neve Headphone Amplifier) here. The amp is known to generate significant noise/current from capacitor charge/discharge during power up and power down which passes loud noise and likely DC straight to the headphones. Neve advises unplugging headphones every time the unit is powered on or off, but my home studio is somewhat automated and the RNHP is in a rack out of reach. I also feel like an amp at this price point shouldn't require the user to disconnect and reconnect sensitive headphones before power on/off to avoid damage.
I thought I could just put the headphone plug on relays I could automate to switch in and out, but this apparently isn't functionally the same as disconnecting the plug for some reason. I've detailed below.
I feel like headphone protection is relatively underdiscussed and I'm trying to use this as a learning opportunity. I'm wondering if anyone with more electrical engineering knowledge than I have can weigh in on the feasibility of these ideas or others. There is significant information available about the design and circuits in this thread.
Here are some of the ideas I've tried:
I'll endeavor to post any updates from my ongoing testing.
Thanks,
MI
I thought I could just put the headphone plug on relays I could automate to switch in and out, but this apparently isn't functionally the same as disconnecting the plug for some reason. I've detailed below.
I feel like headphone protection is relatively underdiscussed and I'm trying to use this as a learning opportunity. I'm wondering if anyone with more electrical engineering knowledge than I have can weigh in on the feasibility of these ideas or others. There is significant information available about the design and circuits in this thread.
Here are some of the ideas I've tried:
- Connect each of the 3 headphone conductors to their own 2-way relay. Open relays during power on/off and close the relays after a delay.
- This seems to me like an automatable way to "unplug" and "replug" the headphones without literally having to do it.
- All of my experiments with this yield an explosive pop sound when I switch in the headphones the first time after power on, presumably due to the current accumulating somewhere in the circuit and then being released into the headphones all at once
- I'm not sure if there's a solution to be had here with a 2-way relay and capacitors to drain or shunt the current but I haven't had success with my attempts.
- Any signal degradation from the relays? Not sure how to pick the right ones for headphone audio signals
- Disconnect headphone plug from jack. Power up. Connect headphones. Disconnect them again before powering off.
- Inconvenient, but it seems to work PERFECTLY.
- This doesn't produce ANY noise upon connection of the headphones. Why does this work when my relay solution doesn't? I'd think they're functionally identical.
- Adapt headphone output from RNHP and headphone plug conductors to BNC. Use a logic-controlled video editing router to switch the headphones in and out.
- This works perfectly with no explosive pop sound upon switching, but ONLY if I engage the 75 ohm termination switches on the router.
- I am not sure why this matters. Any ideas?
- Don't know how to measure the signal path in the editing router to assess sound quality
- Utilize an inline headphone protection circuit such as the E12 muting/protection circuit
- Presumably a circuit like this would handle whatever current is generated during power on/off without sending it to headphones
- I can't seem to find any prebuilt solutions for this
- Not sure if this adds components to the signal path which I feel like isn't necessary
- Modify a speaker protection circuit like this one for use with headphones
- Perhaps switching it in and out via 2-way relays so it is not inline?
I'll endeavor to post any updates from my ongoing testing.
Thanks,
MI
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Background information with many options:
https://sound-au.com/project33.htm
The solution in the link you provided uses uPC1237HA which is a de-facto solution for your needs. A very old IC design that still works perfectly. Now, the question is: has it been implemented correctly..? The only way to know is to try. I think it will work. Note: for it to work correctly, the IC needs to sniff the pulsating 50/60Hz mains, i.e. needs to have access to the transformer's secondary windings. Does that headphone amp have AC-in or DC-in? If it is a DC-in.... then you may have a problem, in which case I'd use a 555 timer
https://sound-au.com/project33.htm
The solution in the link you provided uses uPC1237HA which is a de-facto solution for your needs. A very old IC design that still works perfectly. Now, the question is: has it been implemented correctly..? The only way to know is to try. I think it will work. Note: for it to work correctly, the IC needs to sniff the pulsating 50/60Hz mains, i.e. needs to have access to the transformer's secondary windings. Does that headphone amp have AC-in or DC-in? If it is a DC-in.... then you may have a problem, in which case I'd use a 555 timer
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If the pop persists when you use a relay, even if the relay is delayed significantly, you have DC on the hp output.
No way of disconnect finesse will fix that.
There should not be any DC on the output, there may be a defect in the amp.
Do both channels have it?
Jan
No way of disconnect finesse will fix that.
There should not be any DC on the output, there may be a defect in the amp.
Do both channels have it?
Jan
At first, I thought that the two 4700uF Caps were 24V DC rail splitters,
but looking at the picture, it seems that the power supply is a single power supply and these Caps are used for output coupling.
In this case, it must be sufficiently charged so that the output terminal side becomes 0V within the delay time of the relay.
I think that's why it's useless without a 75Ω terminator when using a video switcher.
It is recommended to use the 2C type for the output delay relay, as follows:
A relay for small signals with a contact capacity of about 1A or more is sufficient.
good luck
but looking at the picture, it seems that the power supply is a single power supply and these Caps are used for output coupling.
In this case, it must be sufficiently charged so that the output terminal side becomes 0V within the delay time of the relay.
I think that's why it's useless without a 75Ω terminator when using a video switcher.
It is recommended to use the 2C type for the output delay relay, as follows:
A relay for small signals with a contact capacity of about 1A or more is sufficient.
good luck
That's a current feedback amp. It will oscillate if what you are suggesting is implemented.
I am aware that the TPA6120A is a current feedback type.
What part of my suggestion would induce oscillation?
I just suggested adding a resistive load to discharge (should call it charg?) Cap during the delay time.
This question came to my mind when I looked at the photos again.
This type of HP jack has a terminal with a switch structure that is connected when the plug is pulled out, and it seems that a resistor for charging and discharging the output coupling cap is connected there.
Therefore, when the plug is inserted after the power is turned on, the pop disappears, and the resistor is disconnected from the circuit while the plug is inserted, which is different from switching with an external switch or relay.
* The jack also has a switch on the sleeve as well as the tip and ring. although it's not on the circuit symbol.
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I want to thank everyone for the extremely helpful responses thus far. I am learning from all of them.
Extreme_Boky - thanks a bunch for the very informative link. This amp has DC in, to answer your question. Sounds like this might invalidate the headphone protection circuit I mentioned? I see your and dreamth's references to a 555. I've done a bit of digging on this component but can't quite tell how it would relate - is it as simple as a on-off timer? Or is there more to the functionality than that?
jan.didden - thank you much for your response. Compared to having the headphones connected during a full power up, which produces noises and some pops, my relay solution produces in fact a MUCH worse, explosive, DC-sounding pop all at once the first time I close the relays after a power up. It does appear to be across both channels. This explosive noise does not occur if I remove the headphone plug at power on/off, nor is it an issue with the 75 ohm terminated BNC switcher. Per mason_f8's excellent analysis, it appears that built into the jack's functionality are resistors that adequately offload this DC when headphones are disconnected. Perhaps what's needed is for me to place similar resistors that route the DC to ground on one side of double-pole relays?
mason_f8 - I especially appreciate the time you've put in - highly informative and a great learning experience for me. I never would have guessed that the amp DOES in fact have resistors in place to handle the offending power on/off voltage, just that they're tied into the physical operation of the jack. This functionality isn't mentioned in the documentation. Perhaps I can switch to an automated or time-delayed double-pole relay with external resistors directed to ground on one side and my standard headphone conductors on the other? Or, maybe there's a way to use the existing resistors, perhaps with a dummy/modified plug?
I appreciate your polite reply.
Usually, the document does not describe the circuit-like function, but there may be a description that "(to avoid noise) Please plug in after turning on the power , and unplug before turning off the power".
It is possible, but it is not recommended to modify the inside of the device, so it is recommended to create a box like the one below.
In your first post, it seems that the main power is managed collectively, so I think it is possible to configure the relay delay circuit without needing to access the RNHP.
If you prefer to do it manually, 2-circuit, 2-pole switch can be used instead of the relay, but contacts is more reliable a sealed type relay .
Usually, the document does not describe the circuit-like function, but there may be a description that "(to avoid noise) Please plug in after turning on the power , and unplug before turning off the power".
It is possible, but it is not recommended to modify the inside of the device, so it is recommended to create a box like the one below.
In your first post, it seems that the main power is managed collectively, so I think it is possible to configure the relay delay circuit without needing to access the RNHP.
If you prefer to do it manually, 2-circuit, 2-pole switch can be used instead of the relay, but contacts is more reliable a sealed type relay .
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