DIY Passive Line Mixer?

[mshilarious]

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.

x||y is two loads in parallel, and I was using x : y to show the ratio of resistance that forms a voltage divider. There were three loads I was considering there, but one of them, the 12.7K, is a parallel load itself: the 10K mix resistor of the other channel in series with the parallel load off both sides of the fader, 5K||6K (5K from 1/2 the fader plus 1K output impedance of the input device), so written all the way out: 10K : (10K (other channel mix resistor)+ 5K (1/2 fader)||(5K (other 1/2 fader) + 1K (output impedance of input device)))||1K (shunt)||10K (input impedance of output device).

have a link? or can I get this at radio shack?

I posted one in #38:

http://www.homerecording.com/bbs/showpost.php?p=1830055&postcount=38

frederic suggested the addition of some resistors to bleed off power from the big caps, so maybe put a 1K/1W resistor in parallel with each cap. Also I would add a LED off the +15V side across a 3.3K resistor as a power on indicator. Make it a blue one Finally I always like to put a heatsink on my regulators

You can get most of the parts at RS. However RS doesn't sell a transformer big enough to feed a +-15V regulated supply, and they don't sell negative volt regulators. So you could either go unregulated, or get some parts from digikey. Of course RS doesn't sell quality opamps, so you'll have to order out for those, but if you are just building a test circuit, I believe they have TL082s.

This tranny:

http://www.radioshack.com/product/i...atId=2032058&kw=transformer&parentPage=search

This rectifier (they have a 50V version listed, but this one is always at my local stores):

http://www.radioshack.com/product/i...ctifiers&fbc=1&kw=rectifier&parentPage=search

This cap:

http://www.radioshack.com/product/i...wCatId=2032058&kw=capacitor&parentPage=search

plus the fuse, fuse holder, switch, resistors (and LED!) should get you to between 15-16.5V, so I'd watch the opamps' ratings.

 

[FALKEN]

sweet.

ok.

my plan is going to be to first build this thing passive.

then add the active circuitry shortly thereafter.

can you recommend an output transformer to convert this low-level unbalanced signal to a balanced one?

I've been reading the Jensen website and I guess the JT-11 is used for a lot of line-level applications (input and outputs) but since my output signal might be signicantly lower (more like mic level) should I be looking at something different?

I am going to buy the box today.

I am very excited.

 

[FALKEN]

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.

I've been grappling with this for a few days now. Terminal 3 is usually ground, right? 1 is input and 2 is output? (or the other way, doesn't matter, right?) so the 10k pot will provide a 10k resistance between the signal and ground? and then the wiper affects the signal going to the output? since you would have input and output on terminals 1 and 2, is there no resistance there?

here is what is confusing me. signal needs some resistance to flow right? so if the pot is on 10, the signal is flowing through unaffected (but 10k is sent to ground in paralell). but when the pot is on 0 (no signal flow), is the effective resistance 0 or infinity? it seems that in either case you would get no signal flow.

arrgh I am such a n00b!!!!!! but these questions plague me for days now!

 

[frederic]

If pin 1 is the input signal, pin 2 is the wiper (output) and pin 3 is ground...

fully counterclockwise "0" the input sees a 10K load to ground, and the output is directly shorted to ground.

fully clockwise, "10" the input seels a 10k load to ground, and the output is directly shorted to the input.

Rotating the pot varies between the two.

This is why we've stated that passive mixers reduce the signal... because no matter what, you have at least the value of the pot (10K in your case) shunting to ground. Plus, whatever series resistors are used to isolate.

Resistors resist the flow of electricity - hence the name

 

[FALKEN]

a - ha!


thanks, man.


now I get it.

I am just about finishing finding all the parts.

this thing is going to be really rad.

should I be looking for carbon film resistors or something else?

 

[frederic]

should I be looking for carbon film resistors or something else?

For your prototype in an altoids box, use whatever you have in your hands.

For the final version, carbon is okay, just try to get 5% or better so each channel performs equally. This means non-radio shack parts.

 

[FALKEN]

questions #1,000,001-1,000,003 :

should I pay double the cost for the "anodized" cases? Could they really help that much with RF?

for a mute switch should I use a SPST or a SPDT with one side tied to ground?

also, while on the subject of grounding, is it necessary to ground all of the parts of the circuit together, or can they be grounded to different parts of the box?

I can ground them to the box, right? (for a passive setup)

what kind of wattage to look for on the resistors? 1/2 watt?

thanks thanks again.

 

[frederic]

should I pay double the cost for the "anodized" cases? Could they really help that much with RF?

In my opinion it does nothing for RF. It looks nicer, it's more scratch resistant, and is easy to wipe fingerprints off. And it doesn't rust or corrode.

for a mute switch should I use a SPST or a SPDT with one side tied to ground?

Sure, if you want to put a full load on the input. For a passive mixer I'd use a DPDT switch... use one side to open the input from the pot, and the other side to short the output of the pot to ground.

also, while on the subject of grounding, is it necessary to ground all of the parts of the circuit together, or can they be grounded to different parts of the box?

Technically, for "perfection" one ground is idea. The box will be small enough I doubt it will matter.

what kind of wattage to look for on the resistors? 1/2 watt?

Not even.

 

[mshilarious]

what kind of wattage to look for on the resistors? 1/2 watt?

No, standard is 1/4 watt. Power supplies often need 1W or even larger. I never bother with 1/2W, in between seems fairly useless in terms of stocking parts.

I accidentially ordered 10 1.5 ohm 1/2W resistors last month I'm going to kill off four on them stacked as a 6 ohm 2W resistor

Good time to review Ohm's Law:

V = I * R

I = V / R

P = I * V

I is current in amps, V is voltage in volts, R is resistance in ohms, P is power in watts.

Thus if your max signal level is say 15V, across a 1K resistor that's 15 mA, or .225W. Just shy of 1/4W. It would be unusual to see a current that large in a line level component, certainly that was sustained long enough to cause a problem. Remember that current is determined by total resistance to ground, not any single component, in your design input impedance exceeds 10K ohms so your max current is should always be less than 1.5mA or .03W.

Also each resistor in series will have the same current, but will not have the same power. Instead, the total power is spread among the series components in the same ratio as their resistance. For example if you put 9V across 1K and 2K resistors in series, the current is 3mA for the entire circuit, but the voltage drop in the 1K resistor is 1/2 the drop in the second resistor, so 3V * 3mA = .009W and .018W for the second resistor (6V * 3mA), for a total power of .027W for the circuit.

As far as resistor tolerance, in some places matching is more important than others. You can still buy 5% carbon resistors and use your multimeter to match them more closely for places like balanced inputs and outputs.

And hey! Don't dis the Shack! Their resistors are all 5% carbon film, they just don't sell the sexier 1% tolerance or metal films

 

[frederic]

And hey! Don't dis the Shack! Their resistors are all 5% carbon film, they just don't sell the sexier 1% tolerance or metal films

I dis the Shack because back in the mid-80's while in college, I managed a dying store and brought it onto the map, generating a profit.

Every time I walk into one now, I'm highly disappointed.

 

[FALKEN]

Hey,

thanks a lot!!

this one thing is confusing me though.

Sure, if you want to put a full load on the input. For a passive mixer I'd use a DPDT switch... use one side to open the input from the pot, and the other side to short the output of the pot to ground.

do you mean like this?

(couldn't figure out how to draw the dotted line in excel) :

 

[frederic]

Um, I don't know. You're asking me to think long after my caffiene ran out.

Like this:

The half of the switch connected to the output selects between ground and the wiper of the fader (pot), and the other half of the switch simply disconnects the input signal completely when the first half shorts the output to ground.

You might be able to just open the circuit on the input line with a switch and call it a day, thinking about it. When the switch is open, that's mute. When it's closed, signal passes.

I'm still anti-passive in regards to mixers

 

[mshilarious]

I dis the Shack because back in the mid-80's while in college, I managed a dying store and brought it onto the map, generating a profit.

Every time I walk into one now, I'm highly disappointed.

Well, you know, I'm just thankful that in a county of 30,000 people, there is a store I can walk into and walk out of with parts to build a transformer-balanced all-discrete mic preamp. I ain't saying they are the best parts in the world, but in the retail environment of 2006, that is a near miracle.

Most of the store may be silly electronic crap and sales clerks who'd rather sell me a cell phone contract than the standoffs and DPDT switch I bought today instead, but those drawers, man I love 'em

 

[frederic]

Most of the store may be silly electronic crap and sales clerks who'd rather sell me a cell phone contract than the standoffs and DPDT switch I bought today instead, but those drawers, man I love 'em

Point taken. As far as retail goes, they have some parts that are useful. I often need values (and connectors especially) that they don't have in the stores. On their website, yes.

For example, when I was sync'ing my six akai DR-series recorders, I needed 8-pin din plugs. Discontinued, but four stores within a 1 hour radius of each other each had a few.... so after a lot of driving I was able to make a bunch of akai proprietary sync cables that places like sam-ash want $90 a pop, for about $6 each. rat-shack plugs and a snip of cat-5

But it seems things I want from them get discontinued. They used to sell a 12V 20A linear power supply for under $100, that's gone, which sucks because the one I use in the garage blew up recently. So now I have to figure out what the parts should be which will be interesting considering all that's left of those parts are leads sticking up off the circuit board and a really large black stain on the surrounding parts.

Some of the stores don't even stock CB antennas, mounts and extensions anymore.

Or large power transformers.

There was a huge electronics supply place about 45 minutes south of where I am, and all they stocked were electronic parts, about 15-20 times the size of a radio shack. No speakers, no home theater, no cell phones, nada. Electronics, and tons of pro-audio connectors (switchcraft mostly). Imagine any kind of switch, they'd have it. Not at the best prices, but it was convienent to walk in on a sunday afternoon and buy four XLR jacks, 16 oddball FET transistors, a couple of high density molex conenctors, and enjoy a free cup of coffee

Sadly, they disappeared a few months ago.

 

[FALKEN]

yeah I could see how that would be convenient. I am surprised radio shack still stocks the things they do; I could'nt imagine that many people go in there and buy resistors and whatnot. although its a college town so maybe people do. but it would be nice to look at the stuff before you buy it. you guys wouldn't believe the trouble I'm having tracking down a DP3T and a 4PDT on the web. I'm trying to limit my orders to two places (parts express and futurlec) and I dont think that's gonna happen.

 

[frederic]

yeah I could see how that would be convenient. I am surprised radio shack still stocks the things they do; I could'nt imagine that many people go in there and buy resistors and whatnot. although its a college town so maybe people do. but it would be nice to look at the stuff before you buy it. you guys wouldn't believe the trouble I'm having tracking down a DP3T and a 4PDT on the web. I'm trying to limit my orders to two places (parts express and futurlec) and I dont think that's gonna happen.

mouser, digikey, allied electronics, and so on.

 

[FALKEN]

mouser, digikey, allied electronics, and so on.

the only ones on digikey were like, $8 each if I buy 1,000; otherwise they're $17 each, which is ridiculous. maybe I'm missing something?

I have to look into mouser and allied though.

 

[FALKEN]

one last one....

if I want a selector switch to select from three different stereo busses...

Can I use a DPDT that is "on-on-on" or do I have to get a rotary?

thanks a million!!!!!

 

[FALKEN]

hey guys,

you've taken me so far already,

don't give up on me now!

this has turned into a great thread that wouild probably benefit a lot of n00bs just like me. at least, I hope.

 

[mshilarious]

one last one....

if I want a selector switch to select from three different stereo busses...

Can I use a DPDT that is "on-on-on" or do I have to get a rotary?

thanks a million!!!!!

I don't think so. A DPDT on-on-on usually works like this:

On (up):

o-o x

o-o x

On (center):

o-o-o

o-o-o

On (down):

x o-o

x o-o

Normally the signal would go to the center pins, and the outer pins would be 1 of 2 busses. For 3 busses in stereo, yeah I think you'd need a dual rotary switch.

The 3T toggle switches that are out there, at least the ones I'm familiar with, work like this (single pole):

o-o x x

or

x o-o x

or

x x o-o