mshilarious
Banned
FALKEN said:
Yeah, you're right, I wonder what the point is with the formula, just say it's 220 and be done with it.
A volume pot would be high impedance relative to the shunt resistor, so I don't think that's the way to go.
Yeah, you're right, I wonder what the point is with the formula, just say it's 220 and be done with it.
A volume pot would be high impedance relative to the shunt resistor, so I don't think that's the way to go.
FALKEN
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so I went to radio shack and bought all the parts I need to make a 2 channel into 1 mixer with volume pots. I just wanted to ask two things..
1. what does the resistance of a pot mean? like say its a 10k pot. is that 10k when its all the way open? or all the way closed (vol at 10?)? or neither??
2. if you want a 20k resistor is it better to use 2 10ks or a 22k. that's all they had at radio shack.
thanks again.
1. what does the resistance of a pot mean? like say its a 10k pot. is that 10k when its all the way open? or all the way closed (vol at 10?)? or neither??
2. if you want a 20k resistor is it better to use 2 10ks or a 22k. that's all they had at radio shack.
thanks again.
frederic
New member
FALKEN said:
Pots have three terminals (or pins if PCB mount).
The rating (say, 10K) is the resistance between terminals 1 and 3, which are the outer pins. The wiper, pin 2 (center), will slide between the two outer pins. The resistance between pins 1 and 3 is always the same, approximately what the part is rated for.
FALKEN said:
If you want to be precise, two 10K's in series for a 20K. If it's not super critical, try the 22K.
FALKEN
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mshilarious said:
can you show me the formula you are using to calculate the db loss via the reisistors?
mshilarious
Banned
FALKEN said:
There are three involved:
parallel resistance = 1 / ( 1/x + 1/y + 1/z )
voltage drop across a resistor = R series / ( R series + R ground )
dB = 20 * log (voltage/voltage reference)
So in the first formula, the parallel resistance of the loads is 1 / ( 1/12700 + 1/1000 + 1/10000 ) = 848 (sorry I screwed up my last post, I didn't mean to put the K after that)
voltage drop = 10000 / ( 10000 + 848 ) = 92.18%
Stated another way, the voltage after the resistor will be 7.82% of the incoming signal.
dB = 20 * log ( 7.82% ) = -22.1 dB
FALKEN
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dude...
so I was going to build the 2 channel version this morning...
and I got out my drill
and put three holes in an altoids box.
only the holes are not clean. the metal is all twisted around the hole...so even though I think it is the right size the pots will not fit in.
what am I doing wrong?
And thank GOD I didn't try this on a chassis right away!
Also the pots from radio shack are really tall; I guess you're supposed to cut them to size? what should you use for this?
so I was going to build the 2 channel version this morning...
and I got out my drill
and put three holes in an altoids box.
only the holes are not clean. the metal is all twisted around the hole...so even though I think it is the right size the pots will not fit in.
what am I doing wrong?
And thank GOD I didn't try this on a chassis right away!
Also the pots from radio shack are really tall; I guess you're supposed to cut them to size? what should you use for this?
mshilarious
Banned
FALKEN said:
An Altoids box is pretty thin so it will warp easily. Make sure it is well supported underneath by a little block of wood. Drill a small hole, then a little larger one, work your way up to the pot size.
You'll find a real chassis with a thick aluminum faceplate is much easier to drill
frederic
New member
FALKEN said:
Altoids boxes are thin. Anytime you drill thin metal you need to have a blacking plate, which usually is a block of wood, MDF, or something soft to prevent peel-through.
Or drill the holes undersized, and ream them from the other side.
FALKEN said:
Aluminum shafts, so you can use a bandsaw, a hacksaw, etc. Once you cut the shaft, hand file the surface smooth. I actually made a jig for my bandsaw so I can cut them the same length consistantly, then I walk over to my grinder, smooth the end and slightly bevel the shaft so the knobs go on extremely easy.
FALKEN
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damn i love this website.
thanks, frederic
thanks, frederic
FALKEN
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hey - thanks!
looks like i'll be going to the home depot this morning. the woman is trying to stain a dresser so it'll be an outing.
looks like i'll be going to the home depot this morning. the woman is trying to stain a dresser so it'll be an outing.
FALKEN
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check it out!
now,
before I plug this thing in,
am I at risk of blowing anything up?
now,
before I plug this thing in,
am I at risk of blowing anything up?
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mshilarious
Banned
FALKEN said:
now,
before I plug this thing in,
am I at risk of blowing anything up?
Nope. Nice work so far
FALKEN
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it sounds pretty good. but i need to isolate the components from each other...is a small piece of cardboard asking for trouble?
frederic
New member
FALKEN said:
For an experiment? Naaaah. Just make sure the cardboard isn't soaked in kerosene.
FALKEN
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mshilarious said:
mshilarious,
I have been staring at this post for days (2nd one on this page) trying to figure out what your nomenclature means. I assume the 12.7k refers to a fader at half tilt? I dont know where the 10k vs comes from and what the || means... (in parallell with?) I guess I just don't see how these expressions match up with the diagram.
a little more explanation would be much appreciated.
I am trying to learn how these circuits actually *work* instead of just soldering them together like a monkey.
FALKEN
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I A/B'd the passive mix against one done ITB and I chose the passive one. (might be biased but whatever) However, there was a ton of signal loss and now I am convinced that I will need makeup gain on the busses. and I am convinced that the best way to do this would be using an opamp at the inverting input, in order to keep the volume constant no matter how many tracks are put on the bus. I am also thinking that if I am going to go this far I may as well add a pair of active Master outputs. That way I can do active or passive mixing but not on a switch - don't want to accidently blow up my preamp. I think I can build this.
What do you think would be the easiest way to add opamps into the box? is there a small, commercially available PCB?
And why again can't I use a wall wart from radio shack for the PSU?
Please forgive me for being a noob.
And for asking so many questions. I really appreciate all of the help.
What do you think would be the easiest way to add opamps into the box? is there a small, commercially available PCB?
And why again can't I use a wall wart from radio shack for the PSU?
Please forgive me for being a noob.
And for asking so many questions. I really appreciate all of the help.
frederic
New member
FALKEN said:
Of course not. You might find something remotely close but you'd end up hacking it anyway, so why not just do point to point wiring on a small perfboard.
FALKEN said:
Op amps prefer split power supplies. You'd need two wallwarts then. Wall warts just suck no matter how you look at it. They are noisy power supplies, they usually don't have any venting and prone to overheating, and the wire that comes out of the molded housing often breaks there due to lack of strain relief.
FALKEN
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frederic said:
have a link? or can I get this at radio shack?
frederic said:
okay. is there a simple DIY out there on the web that will power like 5 double-amps (10 total) ?
thanks.
frederic
New member
FALKEN said:
rat shack.
http://www.radioshack.com/search/index.jsp?kwCatId=&kw=perfboard
http://www.radioshack.com/product/index.jsp?productId=2103799&cp=&kw=pc+board&parentPage=search
http://www.radioshack.com/product/index.jsp?productId=2102844&cp=&kw=pc+board&parentPage=search
http://www.radioshack.com/product/index.jsp?productId=2102843&cp=&kw=pc+board&parentPage=search
and so on.
FALKEN said:
http://www.google.com
Here's one I have bookmarked. Good enough for an amp on both +15V and -15V, more than enough for many op amps. Simple too.