Installing a volume on my front audio.

Installing a volume on my front audio. I have a stereo hooked up to the soundcard output at the rear. My computer also has a socket at the front I often plug headphones into. Rather than having to change the volume on my computer when I switch between the two, it would be cool to put a knob on the front to change the the volume on the headphones. Looking in my Jaycar catalogue they have two small logarithmic pots available in 10kohm and 50kohm - any idea what value resistance I would need?

geek_cybertao, Jan 1, 6:03 pm

Sounds too big.. to me. I seem to recall a figure of 32 Ohms is the speaker/earphone output. Your pot needs to be somewhere around that value.

geek_gbbrot, Jan 1, 7:29 pm

Http://is.gd/ekyF .

geek_kevin16, Jan 1, 7:37 pm

Headphones With volume control would be easyer

geek_swivel, Jan 1, 7:37 pm

I have volume on the earphones Simple.The headphone out should be
cutting al other output,so speakers
will be silenced.Unless you have a
monitor/playback card for mixing.Or
else your wires are crossed X.

geek_deodar, Jan 1, 7:43 pm

# 1 Is the h/phone socket at the front on the front panel of the CD drive ??

geek_biggal, Jan 1, 7:50 pm

Realtek HD (onboard soundcards) do not cut out if both (front & rear) outputs are connected.

geek_lostdude, Jan 1, 7:52 pm

No. It's a socket at the front that it connected to the motherboard. I want to put a pot inline and install it in the side of the tower. I'd much rather do that than buy another set of headphones.

geek_cybertao, Jan 1, 7:56 pm

For those interestedIt's a DFI Lanparty motherboard with a Karajan audio module that feeds the front jacks.

geek_cybertao, Jan 1, 8:01 pm

My way Little box with 1000 ohm log pot inside. Lead with jack at one end and jack socket coming from centre pot connection at other end.

geek_biggal, Jan 1, 8:11 pm

Disconnect speakers Unless you need both.

geek_deodar, Jan 1, 8:26 pm

Thinking more on it. The headphones do provide an impedance of 32ohms. If I simply increase the resistance with a pot, it will mess with the frequency ranges, no?

geek_cybertao, Jan 1, 8:31 pm

NoNo reactance/impedance involved apart from that of the phones

geek_biggal, Jan 1, 8:58 pm

Yes a resistor in line with headphones will stuff it up, it will ruing the damping factor which will change the bass response majorly.

On both machines I have used recently with a realtek hd audio, it has cut off the audio but only when the case had hd audio front jacks, which my old case did not have so they didnt turn off the rear jacks, its an option in the realtek control panel if the rear mutes when the front is connected, and you can make it a seperate audio device which is what I do since it means I can have my headset in there and use it for voip while playing other stuff.

geek_richms, Jan 1, 10:52 pm

You could make up a stereo headphone amp consisting of a few LM386 amp chips so you can adjust the volume without screwing with the impedance of the headphones or affecting responce ect and assuming you can solder otherwise i could make it and list it as an auction there are circuits on the internet avalible why cant you just use the amp's headphone socket and just adjust the amps main volume?

geek_pc_nut, Jan 1, 11:13 pm

Yeah, that's what I was thinking. Do you know what I need in the way of a headphone driver? I'm not all that concerned about quality, but something simple that would be better than just a resistor. Some sort of op-amp?

geek_cybertao, Jan 1, 11:17 pm

Ooh, thanks pc_nut. Yeah, I could plug them into the stereo. But I'd like to take the time to kit out the front socket, it would be more convenient and I won't have to rely on the stereo - just to 'future proof' and be more flexible.

geek_cybertao, Jan 1, 11:19 pm

http://www.hobby-hour.com/electronics/lm386-power-audio.php

make up two of these puppies and you've got a stereo headphone amp

geek_pc_nut, Jan 1, 11:23 pm

Cool, thanks man. I remember those chips from when I had a dick smith funway kit. I think it was from the second series...many, many blue shaded moons ago.

geek_cybertao, Jan 1, 11:39 pm

Look at the cmoy headphone amp - its quite a popular low cost schematic - I don't know how it would like sharing a ground with the PC power supply tho, that will but your biggest obstacle when dealing with audio between things. Some coupling caps on the input would probably solve it but its being about 10 years since I have looked into anything like that.

geek_richms, Jan 2, 2:05 am

You could get a Behringer Headphone Amp 1/4" input though.

geek_deodar, Jan 2, 7:13 am

I was thinking about the amp diagram when I was drifting off to sleep(yeah, I have weird dreams). The input to the amp controls the volume by passing the input siganl...through a variable resistor! So the amp would suffer the same bass frequency dampening. That's why they have the circuit with a bass expander. And using an amp will amplify the noise in the computer like rich mentioned. I'm going to use biggal's suggestion and make an external box to try different things with before I commit to an in-case mod. Thanks for the ideas, guys.

geek_cybertao, Jan 2, 11:08 am

# 10 I should have stipulated a double gang pot for stereo, difficult but not impossible.
Damping factor, normally used in reference to spkrs, is a murky subject. The effect on bass response can be easily checked with a mock up of the 32 ohm phones and resistors.

geek_biggal, Jan 2, 12:02 pm

Yes, I realise that. Thank you for bringing it to my attention. I hope my local Jaycar is open today.

geek_cybertao, Jan 2, 12:05 pm

Sweet as! I got a 10kohm(smallest they had) log pot from Jaycar and wired it inline. It's a bit too high so I don't get much sensitivity from the knob, but I can fix that by adding a couple of resistors to it. A potentiometer has three terminals as it can be used as a voltage splitter, the load on the circuit is always the same, the percentage that goes to the headphones is controlled. Godflesh still has all the bass.B-) Good to read the soundcard documentation as well, it seems there are supposed to be jumpers shorting the two channels for the rear output to work, but if you take them off and plug in the front port the rear is disabled. The cables for the front ports on my case have two plugs per channel, acting as a jumper as well(so both ports are always active. So the next project will be to wire up some transistor gates to disable the rear port while something is plugged into the front port.

geek_cybertao, Jan 2, 7:05 pm

Resistance inline to an amp will not change the response since the amp is a linear resistive input, wheras speakers are a dynamic load with inertia which causes them to generate. A low impedance connection is essential so that when the signal changes the diaphram will slow and stop quickly. Its like the difference between a cordless drill with a brake and one without - the braking one puts a short across the motor so it stops dead, the one that has no brake just leaves it open circuit coasting along. resistance between the amp and the speaker makes it more like the crap drill with it moving on its own after the signal stops.

geek_richms, Jan 2, 10:17 pm

Interesting. So the effect would be less with headphones, as they are lighter with a smaller movement range? And the effect is less at higher volume(with just the pot) due to less resistance, than at lower volume with higher resistance? Lower frequency ranges are effected the most?

geek_cybertao, Jan 2, 10:30 pm

The effect is different across the range since impedance is hardly constant for a speaker, and usually it is more of a muddying of bass then a rolloff of the bass. Poor damping is described as musical by some people, its a problem that valve amps have which is part of their sonic signature (second only to the harmonics normally) - so you may not hear it as bad, but certainly its not sounding the way that things were intended by the artists.

Thenagain, if you are playing pop music on it, forget it, wont make any difference since its already destroyed sonically.

geek_richms, Jan 2, 10:36 pm

If I was that concerned and uptight about quality I wouldn't be using an analog output on the front of the computer with mid-priced headphones to listen to mp3s and games.

geek_cybertao, Jan 2, 10:49 pm

Yeah, the front outputs seem to be pretty noisy with RF which bugger up feeding it into a headphone amp, I find the back is remarkably better. Even on my trashy philips phones and the onboard sound there is a huge difference between turning it down with the inline volume control and the windows mixer.

geek_richms, Jan 2, 11:06 pm

The noise used to be terrible and practically unusable. A week ago I put a ferrite ring on the CPU fans power lead with, remarkable improvement. Nothing audible, at least as far as I can hear.

geek_cybertao, Jan 2, 11:11 pm

I dont get anything audible out other then in a dead quiet room with the fans on slow as they go, but there is so much ultrasonic crap on the output its not funny. Put a scope on the old computer and it had about 30mV of crap at 20,40,60 etc kHz from something. Actually there was some noise when I used the front USB port which I assume shares a ground since its on the same PCB on the case. But taking the front audio into a headphone amp with its high input impedance was a shocker - Not suprising considering the cables in the case are just unshielded stuff.

geek_richms, Jan 2, 11:14 pm

I have left the front USBs unplugged for the same reason. Mine has really cheap wire too. I guess it would be worth replacing it with some shielded network cable if I feel motivated to build the switching mechanism I mentioned earlier to disable the rear output, and then it should be worth while building the amp as well if it cuts the noise down.

geek_cybertao, Jan 2, 11:20 pm

One of my old cases has the old AC97 front jacks, but I like the case since its a slimline desktop one that's 17" wide so good for the home theater. Cannot find the bare jacks _anywhere_ to make my own front panel, and all the other front panel solutions I have found are either AC97, just extend to 3.5mm plugs to stick in the back, or are just bare headers that you have to assemble onto the headers yourself with no sense wire for HD audio jack detection.

geek_richms, Jan 2, 11:26 pm

Cybertao You say you put the pots inline with the headphones. You might get a better effect if the head phone wires are connected across the outer two pot tags and the head phone speaker is connected from the wiper to one of the outside tags. That way you will have control from full on to full off

geek_ruki1, Jan 2, 11:36 pm

Sorry, could you elaberate? I've got the ground wire connected to the first pin of both gangs(as there is only one ground for both channels). Then the left line-out is connected to the second pin, and the jackpoint line connected to the third. Same with the right channel on the second gang.

geek_cybertao, Jan 2, 11:47 pm

Not too sure exactly which connection is which from you explanation. You should have the "output" and "earth" at opposite sides of the pot. The center connection is the wiper and should be connected to the headphones. If you use the center connection as the one that connects to the "output", when you turn it down to minimum will end up shorting to earth, and that's not good. The above connection just forms a voltage divider and should work fine.

geek_pheonix, Jan 3, 12:05 pm

The left terminal is the earth the centre terminal is the input, and the right terminal is the output. When the knob is turned up to 11, the earth/input will have the full 10k resistance, while the input/output has none. You're right that when the knob is turned right down, the 10k resistance will be between the input/output and the input/ground will be effectively shorted. That's okay, as the volume is never right down - I unplug the headphones when not in use. It works, I've learned a lot from the experience, and will be replacing it at a later stage. Thanks for the warning though.

geek_cybertao, Jan 3, 12:28 pm