Homemade LPL

eulogious · Post by **eulogious** » Thu Jun 17, 2010 4:23 am

Hello all,

So I decided that I wanted to build my own LPL because it a)Looks really easy, and b)It looks like I can get the parts for really cheap, but I ran into some questions that were asked but never answered.

I found this:

http://download.phoenixphorum.com/TechS ... L_Info.pdf

Which pretty much explains the LPL, but leaves out a couple things I was curious about.

I also found this:

http://phoenixphorum.com/firewall-vt311 ... w=previous

But it still doesn't answer my questions.

My main questions is this. What type of pot is the LPL, linear or logarithmic? I have yet to find an answer to that...

I found an offhand post on some weird forum saying that a linear pot will work, but you can't always trust the info you find in some random forum that I never heard of before.

So anyways, I plowed on ahead and found the pot that I think will work:

http://www.parts-express.com/pe/showdet ... er=023-550

I like this one because it's cheap, has a splined shaft, and almost looks exactly like the pot in my LPL that I have now. It's a 50K ohm 1/2 watt linear taper potentiometer. So does this pot sound like it should work? I have emailed parts express for some more info on this pot, specifically asking about the shaft diameter so I can find a knob for it, so we shall see what comes of it.

So inorder to make my LPL, all I need is a 50k ohm pot like the one above, 20 or so feet of 4 line phone cable, and regular phone connectors to connect it to the amp, and then a knob to control the pot, correct?

Then connecting the phone cable to the pot looking at the back of the pot the wires are like this:

Black wire to left pin
Red and Green wires to center pin
Yellow wire to right pin

Correct?

It looks like it's that simple, but I just wanted to make sure before I go buying parts. So sorry if this all seems redunant, I just could not find the answers to my questions and I would like to know for future reference as well. That way when someone like me goes looking again, it's all answered. If this has been answered before, I apologize, I searched and found nothing conclusive.

If it's that easy, it looks like I can source all the parts for a LPL for about $5 or so. That's alot better than $25. I also need to make 2 more for my 2 other cars at some point, and 2 buddies of mine are also getting PG amps soon, so they will want a LPL as well. So buying a bunch of parts to make them will be TONS cheaper than buying 4 seperate LPL's ($20 compared to $100). That and I can charge my buddies $10 each and make some of my money back

Thanks all!

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 4:36 am

Just made mine, work flawless!

I bought a logarithmic 50k pot over fleebay, pack of 5, with matching knob, pack of 5 also. I also buyed a telephone crimper tool, and 50 terminal. all of theses cost me like 18$ shipped. The phone cable was free, took it at job.

eulogious · Post by **eulogious** » Thu Jun 17, 2010 4:42 am

So you are using a logarithmic one? I wonder if it makes a difference?

You got a fleebay link, I am going to search now, but if you got the stuff from a certain seller, I will just go to them as well

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 4:44 am

OK Here are the items from ebay :

Pot : http://cgi.ebay.ca/ws/eBayISAPI.dll?Vie ... 0593082896
Knob : http://cgi.ebay.ca/ws/eBayISAPI.dll?Vie ... 0339785500
Crimper : http://cgi.ebay.ca/ws/eBayISAPI.dll?Vie ... 0538068972
Connector : http://cgi.ebay.ca/ws/eBayISAPI.dll?Vie ... 0561300304

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 4:45 am

Do you want a picture of the final install on my dash? Can go out to take one for you, as i forgot to take one for my install log anyway

eulogious · Post by **eulogious** » Thu Jun 17, 2010 4:50 am

str3atwarrior wrote:Do you want a picture of the final install on my dash? Can go out to take one for you, as i forgot to take one for my install log anyway

That would be sweet dude!

Thanks for the link, those pots look great! You can't shake a stick at the price as well. Thanks for that!

So it works great for you then, just like a "real" LPL?

Again, I know redundant, I just am curious about the difference between logarithmic and linear...

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 4:57 am

Can't say about just like a real LPL, didn't heard one, but i know that when the knob is all the way off, no sub boom at all, and when it's all the way on, damn it shakes like hell! And that might sound dumb, but i totally love this little mod, since some tunes i'm listening to sounds wayyyy better with just a little more or a little less boom, so it's easy to adjust with this!

Here are the pics of the install:

The third one is just to show the actual size of the knob. The little white bar is nice on the knob. It looks clean enough to me!

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 5:01 am

Just asked my friend wikipedia for this, here is his output :

Logarithmic potentiometer

A logarithmic taper potentiometer has a resistive element that either 'tapers' in from one end to the other, or is made from a material whose resistivity varies from one end to the other. This results in a device where output voltage is a logarithmic function of the mechanical angle of the pot.

Most (cheaper) "log" pots are actually not logarithmic, but use two regions of different resistance (but constant resistivity) to approximate a logarithmic law. A log pot can also be simulated with a linear pot and an external resistor. True log pots are significantly more expensive.

Logarithmic taper potentiometers are often used in connection with audio amplifiers.

Linear taper potentiometer

A linear taper potentiometer has a resistive element of constant cross-section, resulting in a device where the resistance between the contact (wiper) and one end terminal is proportional to the distance between them. Linear taper describes the electrical characteristic of the device, not the geometry of the resistive element. Linear taper potentiometers are used when an approximately proportional relation is desired between shaft rotation and the division ratio of the potentiometer; for example, controls used for adjusting the centering of (an analog) cathode-ray oscilloscope.

So i guess logarithmic was exactly what was needed! But i may be wrong, and we all know Wikipedia can be too, so maybe we should wait for a pro's input on this...

eulogious · Post by **eulogious** » Thu Jun 17, 2010 5:12 am

Ya this is what I found... Linky

What is taper? It's just the ratio of the resistance already passed as the pot turns to the total resistance of the pot, described as a curve. For instance: we want to make a variable power supply with an adjustment pot that smoothly varies the voltage from one to ten volts, so we want a control that lets us do that. We have no idea whether we'll want mostly low voltages or high voltages, so we want to adjust it equally well anywhere in the range. In this case, it's most natural for the control pot to have an equal change in resistance or voltage divided per unit of rotation - we want the control to feel linear. This much of a turn is one volt, no matter whether it's near 0V or near 10V.

Volume controls are different. The human ear does not respond linearly to loudness. It responds to the logarithm of loudness. That means that for a sound to seem twice as loud, it has to be almost ten times the actual change in air pressure. For us to have a control pot that seems to make a linear change in loudness per unit of rotation, the control must compensate for the human ear's oddity and supply ever-increasing amounts of signal per unit rotation. This compensating resistance taper is accurately called a "left hand logarithmic taper" but for historical reasons has been called an audio or log pot. In these pots, the wiper traverses resistance very slowly at first, then faster as the rotation increases. The actual curve looks exponential if you plot resistance or voltage division ratios per unit of rotation.

If you used an audio/log taper pot for the control of the power supply we mentioned, the output voltage would increase very slowly at first, creeping up to maybe 10% of the final output at 50% of the pot rotation. It would then blast the other 90% in the last half of the rotation - very hard to control. Likewise, if we used a linear pot for volume control, the volume would come up dramatically in the first half of pot rotation, and then do very little change in the last half.

The dark horse taper is reverse audio, or more strictly "right hand logarithmic" taper. This taper traverses resistance very quickly at first, then more slowly as it is turned further. It's the inverse of the audio taper. This is used in some bias circuits and in controlling the speed of certain RC oscillators, which is where the audio tinkerer runs into it most.

The following diagram shows the three main kinds of pot tapers, along with one common approximation to an audio taper. Curve 1 is linear taper. If we clip one lead of our Ohmmeter (Hey! There he is again!) onto the leftmost lug, and the other lead on the center lug, then the resistance we read as we rotate the pot clockwise will fall on the curve that goes diagonally upwards. The proportion of the total pot resistance we traverse as we turn the pot is linearly proportional to the amount of rotational travel we turn.

Curve 2 shows what happens with an audio or logarithmic taper. As we turn the shaft, the proportion of resistance we traverse increases slowly at first, more slowly than the percentage of rotation. As we get past half the available rotation, the rate of resistance traversed speeds up as we get closer to the furthest rotation. This compensates for the human ear by increasing sound levels very slowly at first, then faster as the ear's sensitivity falls off at higher sound levels.

When we buy "audio taper" pots, we usually get something like Curve 3. For less expensive pots, manufacturers use a two or three-segment approximation to Curve 2. It's not perfect, but it usually works OK. Curve 4 is the typical resistance versus rotation curve for reverse log pots. In real life - that is, if you ever found one of these in real life - it is usually a two or three segment approximation, too.

That pretty much sums it up. Logarithmic is what is needed. Linear will work, it just won't have a nicer transition when turning it up.

eulogious · Post by **eulogious** » Thu Jun 17, 2010 5:19 am

I like that knob. I was unsure at first, but after seeing it installed I like it. And for the price! Can't beat that at all. I will have to order these up next month when I gots more money. Thanks for the links and the pics man, really helped me out!

So for about $20 shipped, including everything needed (pot, knob, heatshrink, cable, and cable end), I can make 5 LPL's. That's a good deal!

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 5:25 am

I have 4 left, so if you wanna try one before ordering, i don't mind sending you a pot and a knob! But of course that means when i'll go down in the US you'll HAVE TO pay me a beer :p

eulogious · Post by **eulogious** » Thu Jun 17, 2010 5:31 am

str3atwarrior wrote:I have 4 left, so if you wanna try one before ordering, i don't mind sending you a pot and a knob! But of course that means when i'll go down in the US you'll HAVE TO pay me a beer :p

It's all good man, I trust ya, but thanks for the offer! I have a LPL installed in my car now, so I am in no hurry to build my own, I just wanted to have the info and parts handy when I did. I just really didn't want to spend anymore money if I didn't have too

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 5:33 am

No trouble

eulogious · Post by **eulogious** » Thu Jun 17, 2010 5:37 am

I will add that if you are ever in the Seattle area, I will buy you a beer anyways! That goes for any forum member

Well, to a point. If 50 of you guys show up looking for a free beer, I might not have the money

But you get the point

Post by **ttocs** » Thu Jun 17, 2010 7:25 am

What ya wanna kill me? I became allergic to almost everything a few years ago and cut beer out as my diet as I have a hard enough time not puking from water and normal food.

Post by **stipud** » Thu Jun 17, 2010 7:26 am

Pretty sure the standard pots are linear, but you can get away with logarithmic just fine.

eulogious · Post by **eulogious** » Thu Jun 17, 2010 7:35 am

ttocs wrote:What ya wanna kill me? I became allergic to almost everything a few years ago and cut beer out as my diet as I have a hard enough time not puking from water and normal food.

well, then I will get you a glass of water if you are in the area! Hell, maybe even a bottle of perrier

That sucks you are allergic to just about everything, that totally blows dude!

stipud wrote:Pretty sure the standard pots are linear, but you can get away with logarithmic just fine.

Is there anyway to test them to tell the difference? I will have to look into it. Since I have a LPL already…

I am just trying to get the most useful info I can for this, and to try to duplicate PG as much as possible

Honestly you probably won't be able to tell the difference between the two, but I am sure PG choose the type they did for a reason, and that's why I would like to copy them the best I can, if at all possible

eulogious · Post by **eulogious** » Thu Jun 17, 2010 7:50 am

This just got me thinking… Since I have an actual PG LPL, and the question is what's the difference, maybe I should just test them

So st3at, you mind if I take you up on that offer and send me a pot and and knob for this test? YGPM

dwnrodeo · Post by **dwnrodeo** » Thu Jun 17, 2010 8:54 am

stipud wrote:Pretty sure the standard pots are linear, but you can get away with logarithmic just fine.

x2 for linear taper.

Post by **stipud** » Thu Jun 17, 2010 9:22 am

If you have an LPL why are you building your own?

shaheen · Post by **shaheen** » Thu Jun 17, 2010 9:34 am

This brings back memories as my first interaction with Larry F was a phone call asking him about a LPL. I wrote it down and still have the book some 15 years later.

50Kohm Linier Taper Potentionmeter.

4 wire cable
2 outers to the outer pins and the 2 inners on the inner pin.

Been building them for years with great results.

Thanks Larry

str3atwarrior · Post by **str3atwarrior** » Thu Jun 17, 2010 10:47 am

Pm sent!

Never had a real one, so can't say if there is a difference, but the one i made gives me a pretty good control over my bass adjustment! I'm lovin' it!

eulogious · Post by **eulogious** » Thu Jun 17, 2010 11:07 am

stipud wrote:If you have an LPL why are you building your own?

I gots myself 2 other cars that will have PG gear in them soon, my wife's Kia that I already have 1 amp for, and then my MR2, and I got one buddy who already got the other 400.2 from me to power a sub, so I am sure that he will want one, and then I have yet another friend who is looking for gear for subs as well, so I am sure he will want one too. So that is a total of 4 LPL's right there. So instead of spending $50 on 2 for me, I will just charge my friends what it cost me for parts, and get the LPL's for free for me

I just don't want to spend money on something that litterally costs a few dollars to make, when they retail for a little over $30. I like PG on my stuff and all, but on my LPL it doesn't say PG anywhere, so for $30 I didn't even get the name. That's the main reason for making my own

shaheen wrote:This brings back memories as my first interaction with Larry F was a phone call asking him about a LPL. I wrote it down and still have the book some 15 years later.

50Kohm Linier Taper Potentionmeter.

4 wire cable
2 outers to the outer pins and the 2 inners on the inner pin.

Been building them for years with great results.

Thanks Larry

Sweet dude! Thanks for verifying what PG used! That's awesome! I really like having good and proper info. Man, I wish I got into this stuff "back in the day". Sounds like it was good times!

I still want to try a log pot just to see the difference in the two, and to see which I think is "better". Why not? At this point I am really curious, and it's super cheap to try!

St3eat is going to send me the pot he bought, so I will build my LPL and then test it against the one installed in my car now.

Thanks all for the info, and it looks like all my questions have been answered! Now I just need to wait for my "new" LPL to show up and test it out!

akwizeguy · Post by **akwizeguy** » Thu Jun 17, 2010 12:30 pm

eulogious wrote:
stipud wrote:If you have an LPL why are you building your own?
I gots myself 2 other cars that will have PG gear in them soon, my wife's Kia that I already have 1 amp for, and then my MR2, and I got one buddy who already got the other 400.2 from me to power a sub, so I am sure that he will want one, and then I have yet another friend who is looking for gear for subs as well, so I am sure he will want one too. So that is a total of 4 LPL's right there. So instead of spending $50 on 2 for me, I will just charge my friends what it cost me for parts, and get the LPL's for free for me I just don't want to spend money on something that litterally costs a few dollars to make, when they retail for a little over $30. I like PG on my stuff and all, but on my LPL it doesn't say PG anywhere, so for $30 I didn't even get the name. That's the main reason for making my own

shaheen wrote:This brings back memories as my first interaction with Larry F was a phone call asking him about a LPL. I wrote it down and still have the book some 15 years later.

50Kohm Linier Taper Potentionmeter.

4 wire cable
2 outers to the outer pins and the 2 inners on the inner pin.

Been building them for years with great results.

Thanks Larry
Sweet dude! Thanks for verifying what PG used! That's awesome! I really like having good and proper info. Man, I wish I got into this stuff "back in the day". Sounds like it was good times!

I still want to try a log pot just to see the difference in the two, and to see which I think is "better". Why not? At this point I am really curious, and it's super cheap to try!

St3eat is going to send me the pot he bought, so I will build my LPL and then test it against the one installed in my car now.

Thanks all for the info, and it looks like all my questions have been answered! Now I just need to wait for my "new" LPL to show up and test it out!

i was just looking up info for building an LPL myself this week, since they are way over priced on ebay. Can ya report back the difference in sound or anything when you do your comparison?

Post by **ttocs** » Thu Jun 17, 2010 12:32 pm

I don't think it will make a difference other then the lower end will probably not have much adjustment, and the top end will have a feathers touch and be very peaky. THey are both 50k ohm pots it is just the ratio to which it turns and has its resistance.