[Punnisher]

I'd like to make a simple safeguard for headphones and tube amps using a timed relay so that the headphones are not connected at the time of startup.

Is this not the ideal solution? If not, what are some simple ways to switch on and off an audio signal? It would be a bonus if it's automatic after a period of time. I guess the issue would be the amplified audio signal would be passing nearby to a magnetic field created by the coil in the relay.

For example, you turn on the amp, and 10 seconds later the headphones are engaged. You turn off the amp, and the headphones are immediately disengaged and then a short time later, power fades from the tubes.

Is this possible with very simple components?

I was thinking that if the time it takes for the relay to disengage is less than the power for the amplifier, it would work. Of course it depends on how large of a capacitor you have before the amplifier.

Nate

 

[hopeless]

If the relay helps to save or lengthen the life of your headphones (or speakers), a relay is the way to go, even if there was a microscopical decrease in sound quality.

To answer the question of the thread title... I guess it depends on the quality (and proper sizing) of the relay

but more in terms of reliability I'd say.

 

[tomb]

There is no quality lost in going through the relay. Many DIY headphone designs use this. AMB was probably the first to come out with a ready-made PCB design for a headphone-delay relay with his E12. Other delay-relays have been incorporated into the Millett MAX, the Millett MiniMAX, SOHA II, and the CTH.

There are other issues with using relays and signal wiring, but not with this one.

 

[Punnisher]

Ok, sounds good.

Though it's difficult to find an adjustable timed relay that's very small. The few I have found are very expensive.

I think I'll just use a PIC and program it to fire two relays. I may be able to do other functions as well like voltage reading and a DC offset cutoff.

If you guys have recommendations on tiny timed relays that are cheap, let me know!

 

[Juaquin]

A small PIC or AVR (I'm a fan of the ATTiny13) with a small-signal latching relay would probably be the best bet. Then you could do other cool things too, like use the PGA volume control chip instead of a normal potentiometer so that the channels are exactly matched, etc.

 

[Pars]

Look at amb's e12 The ε12 Muting / Protect Circuit. Provides a delay as well as DC offset detection. This can also be done with such things as a 555 timer if you need longer delays, etc.

 

[hopeless]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif There is no quality lost in going through the relay. Many DIY headphone designs use this. AMB was probably the first to come out with a ready-made PCB design for a headphone-delay relay with his E12. Other delay-relays have been incorporated into the Millett MAX, the Millett MiniMAX, SOHA II, and the CTH. There are other issues with using relays and signal wiring, but not with this one.

The relay does add some (very small) resistance to the output impedance of the amp, doesn't it? With speakers that can matter.

 

[Punnisher]

I really doubt it, as the amount of material you're adding is very small. It's just a couple flat copper pieces that contact each other. I was more worried about the charged coil next to the signal. Perhaps the interference won't be in the audible spectrum.

I was aware of that particular AMB circuit, but I wanted something simpler. Though the PIC idea makes things more complex than just a relay so I will consider both.

 

[amb]

Quote:

Originally Posted by hopeless /img/forum/go_quote.gif The relay does add some (very small) resistance to the output impedance of the amp, doesn't it? With speakers that can matter.

You just need to choose a relay that sized properly for the application. The contact reisistance is inversely proportional to the contact current rating.

 

[hopeless]

Quote:

Originally Posted by amb /img/forum/go_quote.gif You just need to choose a relay that sized properly for the application. The contact reisistance is inversely proportional to the contact current rating.

Yeah, sure. Also you can use one relay per channel with twin contacts put in parallel (like in my Pioneer amp).

 

[_atari_]

Quote:

Originally Posted by hopeless /img/forum/go_quote.gif Yeah, sure. Also you can use one relay per channel with twin contacts put in parallel (like in my Pioneer amp).

I got two tips, I want to share with you on this issue:

First: Using two relays (which is meant more for power applications, but wont hurt for headphones), both double throw - so that you can switch on or off both signals at once. The two relays are added in parallel, the second closing after the first has already closed. So if here is any contact degeneration by switching (e.g. tiny sparks - it is meant for power applications), the second relay closes after an connection has already made and will not receive any contact degeneration, providing a better contact. The second relay has to switch of first - to not receive any contact degeneration by switching off (the contact is still made by the probably a bit degenerated first relay), after that the first relay switches off.

Second: Use latching relays, so that you do not have to power the coil al the time, but just send short impulses for switching (if those impulse interfere with the signal quality it will be milliseconds before switching on or off - sounds bearable to me

)

But I have on question: Is there a safe (i.e. distortion free way) to replace the relays with some concrete components, like MOSFETs or so?