Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: free on December 31, 2013, 06:10:50 AM
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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?
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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
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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.
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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.
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"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?
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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.
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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 ;D 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?
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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.
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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
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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)
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galyons, that's disappointing ;D
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.
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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.
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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 ;D
I pass on the old comment....I am biased! ::)
;D
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See post above 6.25mm(1/4") but check just in case yours is different.
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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).
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galyons, that's disappointing ;D
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.
The volume pot acts as a voltage divider. The incoming voltage, let's call it 2V, is across the whole volume pot resistance be it 10k, 50k or 100k. The wiper changes the ratio of voltage dividing.
What Paul Birkeland suggests is a series resistor for each channel which limits the voltage fed to the volume control. So if you have 2V coming in you can cut that to 1V maximum on the volume control, or 1.5V maximum, or 3/4V maximum, you get the picture.
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Ah, ok, hm I might try that out. So I would just add two (one per channel) 50k or so resistors and that would work? If it works with such low voltages then this must really work :D Most resistors are for that kind of low voltage :)
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Is this the same procedure as adding the resistors in line with the RCA as per in the Crack sticky section but using a much higher value? If so it is very easy to do.
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Yes, it is a logarithmic function. A 100k in series will cut the volume by 3dB.
I would suggest listening to the lowest recorded music you have and determining the highest volume level you want to listen to.
Then turn the Crack off and measure the resistance from the input to the wiper and from the wiper to ground on one of the channels. This gives you the ratio of resistance that you need.
Post this back and we can suggest a proper value for the series resistor.
Yes, it is the same thing as in the sticky.
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Thanks I'll do that. Can take some time until I receive the Crack, ordered just today.
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Yes, it is a logarithmic function. A 100k in series will cut the volume by 3dB.
I would suggest listening to the lowest recorded music you have and determining the highest volume level you want to listen to.
Then turn the Crack off and measure the resistance from the input to the wiper and from the wiper to ground on one of the channels. This gives you the ratio of resistance that you need.
Post this back and we can suggest a proper value for the series resistor.
Yes, it is the same thing as in the sticky.
AH HA super this is much clearer to me now and I now know how to measure for wiper resistance. Which I didn't prior to your post thank you.
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OK this should help a picture example of resistors added inline with the RCA left and right channel inputs this should make it much clearer as you have not yet built your Crack. Sorry about the poor pic it was rather warm and I only my old camera to hand which doesn't focus to well.
(https://www.head-fi.org/image/id/5497829/width/900/height/900/flags/LL)
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Ah, ok, hm I might try that out. So I would just add two (one per channel) 50k or so resistors and that would work? If it works with such low voltages then this must really work :D Most resistors are for that kind of low voltage :)
No, no, no. As I keep saying, you can't protect your headphones without having some idea of how much limiting you need to do. 50K resistors will knock you down 3.5dB, but we have no way of knowing if that's enough, or too much!
The bolt idea is a good one. You can always loosen the knob on the volume control to adjust its mounting position and fine tune the setting.
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Thanks JamieMcC for the picture, this helps.
Caucasian Blackplate, I would of course have tested if this ohmage would be enough (there will be one DAC to which the Crack will be connected, not any other so that can be sorted out, but the bolt solution wouldn't help on this source problem either) :)
Bolt idea is not bad either.
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I don't get the sense that the HD600 ever gets so loud that it could be damaged by a Crack. It's the low impedance cans that can blast really loud with the Crack. But to do what you want you are looking for a 300 ohm variable L Pad at the output to waste output power. You will have to make one, they aren't readily available. Or you could just put a big resistor in series with the headphone jack T and R pins, like a 600 ohm 5 watt.
I'm not saying it will sound good, but it will save you from having to ask people to be careful with your gear. I would instead be inclined simply to not let people use your gear whom you can't count on to be careful with it.
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I'm not going to build my own 300 ohm variable L Pad, but I'd be happy with a simple resistor solution. Interesting that you suggest to put the resistor at output. I read that a resistor can lower the sound quality, is this true for Crack? (oh indeed you're saying this too? "I'm not saying it will sound good") Otherwise I'd chose the bolt solution.
"I would instead be inclined simply to not let people use your gear whom you can't count on to be careful with it."
This is out of the question since the people can be my family members :) They/or visitors are (already) careful with gear but you know one can forget(at certain age)/not know not to use headphones as speakers ;D
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So, Doc, if the resistors will lower the sound quality (you don't need to reply if this is true) I'd go with the bolt solution.
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The answer is that it might lower the sound quality, or it might not. You would have to try it. The stop for the volume control could work for one source. At least until you find a recording that has such a low level that the stop won't let you turn the volume up enough for that song. It's diificult to know what the ideal position would be without quite a bit of trial and error.
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Thanks. Yes know is not perfect but fortunately it'll only be the HD600.