Gellfex Posted June 3, 2011 Report Share Posted June 3, 2011 (edited) Hi all, really glad I found this site. I'm building a travel guitar for my son, basically a single pickup Steinberger style headless, but I'm embedding a headphone amp. I've wired up a 4pdt switch so when you turn on the pocket amp it switches the volume pot from external amp mode to internal amp mode. The trouble lies in the fact that the pocket amp wants it's original 100k volume pot and is unhappy with the 500k I've tried, it's range is all in the 1st 30 deg or so. I tried putting a 100k resistor in parallel with the pot, which should make it ~83k, but it curiously seems to have no affect whatsoever. So do I just use the 100k, lose brightness when it's plugged into a real amp, and it is what it is? Or is 100k simply too low? No one expects it to to be the best sounding guitar ever, I'm not a guitarist and this is my 1st build of one. I scavenged the neck and hardware off a pawn shop Epiphone SG and made my own design tuner tailpiece. The odd thing about it was the volume pots were linear and the tone audio! Did some Malaysian worker have an off day or was that deliberate? Any ideas on how to reconcile the 2 systems would be appreciated, it seems the amp pot is doing more than simple voltage dividing, and that's where my understanding of these things peters out. Edited June 3, 2011 by Gellfex Quote Link to comment Share on other sites More sharing options...
Ripthorn Posted June 3, 2011 Report Share Posted June 3, 2011 If you have a link to the schematic for the amp, that would help. There are other possibilities here as well: - Use the 500k pot for guitar volume, but when switched have the switch put in two resistors strapped from the outer lugs to the middle lug such that you get a roughly 100k linear pot - Compromise with 250k pot - Use 100k pot and when switched to guitar mode, put a 100k or so resistor between the pot ground lug and ground - Use two different pots so that he can have completely independent controls I'm sure there are others, those are just off the top of my head. Quote Link to comment Share on other sites More sharing options...
Gellfex Posted June 3, 2011 Author Report Share Posted June 3, 2011 (edited) Thanks for the ideas. If you have a link to the schematic for the amp, that would help. There are other possibilities here as well: No such luck, it's a Ebay special, but pretty well done and functional, it was the only cheap one around with a MP3 input - Use the 500k pot for guitar volume, but when switched have the switch put in two resistors strapped from the outer lugs to the middle lug such that you get a roughly 100k linear pot The amp REALLY wants a audio taper - Compromise with 250k pot worth experimenting - Use 100k pot and when switched to guitar mode, put a 100k or so resistor between the pot ground lug and ground Wouldn't having an overall resistance of 200k instead of 500k reduce the output volume by 60%? Or do I misunderstand the effect of grounding the signal on volume? - Use two different pots so that he can have completely independent controls Small real estate on a travel sized guitar. It's already a challenge with 4 pots, a toggle, and 3 i/o jacks! I'm kinda concerned about routing the cavity too close to the edge and creating a weak body wall I'm sure there are others, those are just off the top of my head. Edited June 3, 2011 by Gellfex Quote Link to comment Share on other sites More sharing options...
Ripthorn Posted June 3, 2011 Report Share Posted June 3, 2011 You could strap resistors to get a roughly log taper (which is what I meant to say). With the amp, you could always put the pot at the end of the circuit and use the pot as a variable voltage divider and then the value of the pot is much less of an issue. I have used 100k's in place of 1M for volume before and the difference is not drastic. The grounding of the signal is all dependent on the ratio of the resistances the signal sees, so having a 200k resistance to ground will only be slightly (perhaps not even noticeably) quieter than a 500k. Here is what I would do: If the amps volume pot is in the circuit somewhere (not on the end) then I would hard wire a full-on control (0 series resistance to the signal with 100k to ground) and put the pot on the output of the amp's circuit (as long as you don't exceed the amp's power output, then put the pot just before the power amp section). If you are concerned about that, then get yourself a dual concentric pot and wire up one of the sets of lugs for the regular pickup and strap resistors across the lugs to get your 100k log taper, then you have independent controls and don't need the 4PDT switch and can get away with a much cheaper DPDT. Quote Link to comment Share on other sites More sharing options...
dpm99 Posted June 4, 2011 Report Share Posted June 4, 2011 (edited) Just to add a couple more ideas, I recently installed a 2w amp in my wife's bass. One problem I had was a built in gain that I wanted accessible but more or less invisible. I made a hole in the back of the guitar, got one of those pots with extremely long shafts you cut off, cut it, cut a slot for a flat head screwdriver, and put a plastic cover over it (on the back) with a small hole to access the pot if necessary. She likes it clean anyway. You could do something like that with a volume pot for the headphone amp. Alternatively, you might consider bypassing the volume pot entirely for the line that goes out to an external amp, especially if your son is like a lot of people, who leave their pots set at 10 all the time. Some people report more tone/sound/level/voodoo with pots bypassed. My experience leads me to believe that's not consistently true, but can be, depending on the system. There are lots of options. Good luck. I'm curious to see what you decide. Make sure and report back! David Edited June 4, 2011 by dpm99 Quote Link to comment Share on other sites More sharing options...
Gellfex Posted June 4, 2011 Author Report Share Posted June 4, 2011 Thanks guys. After listening to it again I think I'm just going to leave it on the 500k for now and finish the guitar. Right now it's just a prototype of MDF. Then if the kid wants tweaks I can do them, the 100k is smaller dia than the 500k anyway. As for volume, he's been playing the prototype which just has the pickup wired directly to the jack and complaining about no volume pot. The prototype is worrisome in that it won't hold tuning. I'm hoping it's just the MDF and not my tuners, and the poplar body will be better. Rip, I actually had the 4PDT in my archives from decades ago, I'm a real packrat! Thursday my son's buddy came over with his strat but forgot a cord, so I said "give me a few minutes". I rummaged deep and found 2 1/4 plugs and an 8 ft piece of yellow shielded 4-18 leftover from a job and whacked it together. The kids were wowed. Quote Link to comment Share on other sites More sharing options...
JohnH Posted June 4, 2011 Report Share Posted June 4, 2011 (edited) You might be interested to add a simple buffer stage after the pickup, then you can drive the 100k pot with no loss. You can build that with a bout $1 of components, being a resistor, a capacitor and a Junction Field Effect Transistor. have a look at the one built into a cable here: http://guitarnuts2.proboards.com/index.cgi?board=schem&action=display&thread=3150 If you build it into the guitar instead of a cable and box, you could omit R2 R3 and C3 if you wish the output would go to your volume pot and amp. Maybe make C2 about 1uF. J Edited June 4, 2011 by JohnH Quote Link to comment Share on other sites More sharing options...
Gellfex Posted June 5, 2011 Author Report Share Posted June 5, 2011 JohnH, REALLY interesting idea! Easy to do as I already have a 9V aboard for the headphone amp. The only downside I see is that it remains powered until you pull out the cable. I'm lucky if my 12 year old shuts off his amp, expecting him to pull the cord out when he's done may be too much. Maybe this too should get kicked down the road as a possible tweak on the final version. Ergonomics have also been occupying me this stage, trying to lay out jacks and pots on a very small field. The 1" Gibson knobs I believe are too big to be set so close, and I think I need to put the 1/8 headphone out jack on the top side of the body where it can't be confused with the 1/8 MP3 in. The amp I bought had RCA L-R inputs but they're a PITA, ugly, and big compared to using a simple male-male 1/8 stereo cord to the MP3. Does anyone know how bad it would be if the amplified MP3 signal accidentally went into the headphone amp output line? Quote Link to comment Share on other sites More sharing options...
ansil Posted December 1, 2011 Report Share Posted December 1, 2011 why not just change the wafer in a dual ganged pot? they come apart quite easily. add a 500k wafer to a dual 100k from radioshack man Quote Link to comment Share on other sites More sharing options...
Gellfex Posted December 1, 2011 Author Report Share Posted December 1, 2011 Thanks Ansil, but the 100k has been fine so far. My main technical problem has been the damn fancy 1/4 jack having loose connections so when the cord is pulled contacts weren't closing and the signal wasn't getting bypassed to the build in amp. I need to get a better quality one, this was the one that came on the pocket headphone amp. I should post a photo of the guitar, right now it still has the MDF body I made to work out the kinks, I have a slab of poplar to make a final body from. Quote Link to comment Share on other sites More sharing options...
Prostheta Posted December 2, 2011 Report Share Posted December 2, 2011 Sorry. On the Android so I've not reviewed previous comments as I'm rushing this. Low value pots can load pickups, most noticeable as loss of top end. It also depends on whether it is being used as a voltage divider or variable resistor. Strapping resistors across the pot means the pot no longer has a consistent resistance across the track motion, also altering circuit impedance. If you add a buffer stage, these losses can be mitigated without altering the subsequent circuit hugely. Sorry if this didn't help. Rushed post as I'm in a seminar. Quote Link to comment Share on other sites More sharing options...
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