Ordered Crack today -- and restricting volume to protect headphones

[free]

Hi, ordered Crack today
Question: how does one restrict the volume to a certain level to protect the HD600 from being destroyed if volume is turned accidentally to the maximum value?

[Paul Birkeland]

I think you'll find that this is a very uncommon feature on headphone amps.  At maximum power, it will take a bit of time to blow out a driver, and the situation will be loud enough that you'll hear it from across the room, notifying you to go turn the amp down.

Though it may not be particularly confidence inspiring, we have not heard about this happening in all the years that the Crack has existed.  (One or two folks thought this might've happened, but it ended up being electronics related) 

If you have only one source and only plan to use the HD-600's, there is potentially a way to limit the maximum output voltage by adding a set of resistors at the input, but as soon as you changed sources or headphones, you'd have to recalculate these values.

-PB

[free]

It will only be HD600 and there will be other people using the Crack so I think to have the restriction may be important. Adding resistors at the input -- and I have no idea if that will work good -- sounds like a solution to me. Anyone else who can confirm this solution? (I do not want the maximal volume even be there for seconds because I think this may already damage the driver in some kind of sound)
There is this potentiometer which regulates the volume?, maybe using one with double the ohm (or half) is a solution? IDK.

[Paul Birkeland]

There is this potentiometer which regulates the volume?, maybe using one with double the ohm (or half) is a solution? IDK.

It doesn't quite work like that.  As I said, you need to know the peak output voltage of the source that you plan to use, and that has to be your only source.

[free]

"you need to know the peak output voltage of the source that you plan to use"
Isn't that the Crack? and everything about its voltages/amps is known (by e.g. measuring it) The designers of the Crack knew the volts/amps and chose a potentiometer accordingly to it? Or did I misunderstood you answer?

Can I not rotate the volume knob until I decide that will be the max. allowed value and by that I chose another potentiometer which only goes to that value?

[Paul Birkeland]

Let's say you feed a 1.5V output iPhone into the Crack, then later plug in a hifi DAC with a 5V output.  Each source will produce a different output level from the Crack for the same volume control position.  The same goes for headphones - the same source and level position will produce more power into 600 Ohm headphones than 150 Ohm headphones.

You cannot buy a potentiometer that won't go to -0dB, though other types of attenuators can exhibit this behavior. 

I still would suggest not worrying about this and not modifying the amp in this manner, as it is remedying a problem that has never presented itself, and is not addressed in other designs. 

[free]

Ah now I understand what you mean by..1.5v vs 5v. Well as already said HD600 will be the only headphone. The problem: the people who will be using the volume knob are old and will forget that they may not use the headphones as loudspeakers and can destroy the driver that way. Furthermore there can be new persons too. So you kind of understand my problem I guess 
The volume restriction doesn't need to be very precise but I'd like to restrict the value to approx. 60% or so. So I'm curious (independent from 1.5 or 5v), what would happen if I'd use a potentiometer with a) double the ohms and b) half the ohms?

[Paul Birkeland]

So I'm curious (independent from 1.5 or 5v), what would happen if I'd use a potentiometer with a) double the ohms and b) half the ohms?

I'd expect the difference between a 200K and a 50K pot to be less than 1dB with most sources.

[galyons]

So I'm curious (independent from 1.5 or 5v), what would happen if I'd use a potentiometer with a) double the ohms and b) half the ohms?

Nothing much.  The attenuation will be close to the same regardless of the impedance of the potentiometer.  The various impedance levels are for load matching between components and have nothing to do with the attenuation level.

I pass on the old comment....I am biased!   

Cheers,
Geary

[JamieMcC]

Perhaps a solution would be to use be a chicken head pointer knob and use a small socket head 3mm bolt tapped into the top plate, cut the threaded part flush with the underside making a sort of grub screw but better looking. The bolt head would act as a volume stop. The bolt could easily be unwound and taken out if needed.

I have seen some nice alloy ones the type used on vintage receivers, these type examples

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.ebayimg.com%2Ft%2F2-x-chicken-head-pointer-knobs-for-vintage-valve-amp-effects-pedal-UK-seller-%2F00%2Fs%2FNDY2WDU1MA%3D%3D%2Fz%2FS7MAAMXQWzNSjAP7%2F%24_35.JPG&hash=d62a13c0b19147bce67fe9457ed98de7d94d2b25)

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.ebayimg.com%2Ft%2FTen-x-Echoplex-Style-Pointer-Knobs-For-Effects-Pedal-Like-Chicken-Head-%2F00%2Fs%2FMTIwMFgxNjAw%2F%24%28KGrHqF%2C%21pcE7BcvkHhDBO2kPKQlM%21%7E%7E60_12.JPG&hash=a58ccd93250d01bbfbdb77ee22cf39f5476bb277)

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.ebayimg.com%2Ft%2FM3-M4-M5-M6-M8-Aluminium-Socket-Cap-Screw-Pro-Bolt-Alloy-%2F00%2Fs%2FNTAwWDUwMA%3D%3D%2F%24%28KGrHqJ%2C%21hwFBdfwCisDBQoppTp73g%7E%7E60_12.JPG&hash=3ca32727d5cb7a7240171c4c62f0913223146e51)

[free]

galyons, that's disappointing 
Thanks JamieMcC, that's the solution I will use  if no one will post a better solution. Where I can buy it for the Crack? What's the name for this knob (diameter) so I can find a German dealer here.

[JamieMcC]

Ebay  chicken head pointer knob

Ebay  Aluminium Socket Cap Screw

Plenty to choose from I think the cracks pot shaft is 6.25mm(1/4") best to check yours.

[free]

I see that I can measure the diameter from the original knob I'll receive then buy accordingly one from ebay. Thanks. These colorful bolds don't look too bad also 

I pass on the old comment....I am biased!   

[JamieMcC]

See post above 6.25mm(1/4") but check just in case yours is different.

[free]

Ok thanks -- all
This bold-solution is probably even better because if I want to drive 600ohm headphones I just need to remove the bold and not solder or something (yes I have the hole then but that's ok).