Stereo to mono summing......
Old 7th May 2005
  #1
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Stereo to mono summing......

Aloha,

I'm interested in a simple small signal analog L/R summing circuit..... balanced. Done discreet? Or, what? Also, whats the best way of going about an on/off send for HR824's ? BTW--Cool to see E/E's among forum members...

--Thomas
Old 7th May 2005
  #2
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If you can tolerate approx. 6 dB of loss, this should work:
Attached Thumbnails
Stereo to mono summing......-monosummer.gif  
Old 7th May 2005
  #3
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Into a highish impedance (why hasn't this keyboard got a "greater than or equal to" symbol?) 10k? you don't even have much of a loss to consider- signal in phase equal level left & right (= phantom center) sees hi z- equal level on output to either input. Signal one side only= -6dB = exactly what you want
Old 7th May 2005
  #4
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Thread Starter
Aloha guys,

Thanx for the summing network ideas. -6db should be fine. And, can I drive both speakers from this output? So, what about the stereo on/off circuit? should I use relays?

Mahalo,
Thomas
Old 8th May 2005
  #5
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Tim Farrant's Avatar
 

Quote:
Originally Posted by BoomChuk
Aloha guys,

Thanx for the summing network ideas. -6db should be fine. And, can I drive both speakers from this output? So, what about the stereo on/off circuit? should I use relays?

Mahalo,
Thomas
So what you are really looking for is a switchable stereo/mono circuit you can insert between your desk monitor output and speakers, right?

Tim
Old 8th May 2005
  #6
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Thread Starter
Yeah Tim,

The whole shootin' match. Mono summing to both speakers, on/off and a/b switch.
But I'll take whatever you'd like to discuss or draw up.

Mahalo,
THM
Old 12th May 2005
  #7
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Saucyjack's Avatar
 

Bump
I could use something like this as well.
Old 15th May 2005
  #8
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Yeah Tim................. You there?
Old 17th May 2005
  #9
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Thread Starter
..............Hello?
Old 17th May 2005
  #10
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Quote:
Originally Posted by brianroth
If you can tolerate approx. 6 dB of loss, this should work:
this might be exactly what I'm looking for as well. would it make sense to use this circuit coming out of a stereo RCA send from my mixer into a mono RCA in on a powered subwoofer? right now I'm using a radio shack Y cable but I've always suspected there was a better way to sum the signals.
Old 18th May 2005
  #11
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Quote:
Originally Posted by twitme
this might be exactly what I'm looking for as well. would it make sense to use this circuit coming out of a stereo RCA send from my mixer into a mono RCA in on a powered subwoofer? right now I'm using a radio shack Y cable but I've always suspected there was a better way to sum the signals.
For that you just need two resistors and you might use higher values, say 2.2k, just in case the output impedance is on the high side. Connect all 3 grounds together. The L mixer out goes to one end of one resistor, the R mixer out goes to one end of the other resistor. Connect the other ends of the 2 resistors together and to the sub in. It's like your Y cable, but with resistors forming the Y.
Old 18th May 2005
  #12
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Yeah, I picked the 1K values in the original assuming it was driven by a "pro quality" balanced output. I could have gone lower, but figured the value was a good compromise. As drawn, and assuming the output impedance of each output is very low (like 100 Ohms or less), each channel will be loaded by the other via the 1K resistors, or approx. a 4K load. That shoudl be safe even for output stages that use chips like TL07x.

4.4K might be too low for "RCA" outputs. Even if they can supply the current, output coupling caps are likely too small, which would result in LF rolloff.

I *believe* most gear with RCA outs can drive a 10K load, so that would suggest something like 4.7K R's. But, keep the cable from the output of the "summing node" to the input of the device being driven short in length, since the cable capacitance combines with the series resistors to make a low pass filter. That shouldn't be a problem for a subwoofer feed, however.

Bri
Old 18th May 2005
  #13
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wow thanks guys.

along similar lines, I'm curious if there's an equally simplistic circuit to sum 8 mono signals down to a stereo [or dual mono] signal? [or this could be looked at as 4 inputs to one output; running two such circuits in parallel]. a really basic summing mixer. it wouldn't need to have level controls or switches or anything, just ins/outs and internal analog mixing.

what I'm working with is a delta 1010LT that has 8 unbalanced RCA outs, and it'd be nice to have a simple analog summing circuit. it could either sum to line level outs, or mic level if that's possible/easy, that way I could run it into a pair of preamps before dropping it as a stereo signal onto my destination.

if this is really complex no worries, I was just wondering if it could be accomplished in a method as simple as the circuit you guys just worked out for stereo to mono summing?

ps: I'm a layman when it comes to electronics, but I'm a competent solder-monkey and could piece together a simple circuit.
Old 19th May 2005
  #14
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Brian, I agreed with your 1k choice in the sketch, that's what I often use for +4 passive summing. I picked 2.2k for the RCA version only because he said it would be connected to mixer outs. If it was a CD player or something, I'd have said 4.7 or 10k. But without knowing the model number or spec we're both just guessing, I guess.

Twitme, in theory you can passively sum as many signals as you want, but you wind up losing more and more level, and needing more and more make up gain. For 2 signals you lose 6db, like Brian said. Not too bad; you can often just turn up the line trims or raise the faders a bit. For 4 signals it's 12db, more of a problem. With 8 you've got 18db loss, and you start needing a carefully designed amplifier to put it back.

Problems when summing a lot of signals include hum, buzz, and RF, capacitance and instability at the summing point, noise in the gain stage, etc... The more signals you are trying to sum, the more critical these things become. This is one of the big challenges in designing a large console. The DIY approach is possible but tricky. For bigger applications the summing circuit needs to be tailored and fine tuned to the gear around it and it's hard to give useful advice on the internet, because there are so many variables.
Old 19th May 2005
  #15
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David, I sometimes think we're twin sons of different mothers!

When I see "RCA outs" I tend to 'panic" and recall Teac cassette decks, hence my gut reaction with the higher valued R's. You are absolutley correct, though...many things like older Tascam mixers had their shoddy <g> little opamp's output tied directly to the RCA, so your lower R value should be fine.

And, for passively mixing 8, 16, whatever, now we're definitely wandering into that "twilight zone" with mic preamps used for makeup gain.

Interestingly, I walked a friend through a passive summer (I think it was 16 channels) over the phone, but he is a bit of a tweak-head and messed with it for awhile on his test bench.

Bri
Old 20th May 2005
  #16
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I might be about to make a fool of myself here, but I thought I'd ask.

would I be able to measure the impedence of the outputs from my delta 1010LT using a multimeter? and chose resistors for my simple summing circuit [4 mono signals to one mono signal] based on that information? I wouldn't be bothering you all with this [if I really want to start doing DIY electronics I should start reading some books etc] but this circuit seems so simple that I'm compelled to get it right.

thanks!
Old 21st May 2005
  #17
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Twitme, no a multimeter won't give you an accurate answer. I suggest referring to the manufacturer's spec sheets.

Bri
Old 24th May 2005
  #18
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Thread Starter
Aloha Twitme,

The term impedance is an AC expression. Though it's units are expressed in ohms, measurement is done using AC circuits/loads and equipment. Say for example you'd like to match a pair of 8 ohm speakers for a cab and you've got one speaker you like the sound of. Assume this speaker to be a perfect 8 ohm reference, then hook it up to a 1kc source with the second speaker in series with it (daisy chain). Now put your meter on "AC" and hook it across each speaker. You should read the same AC voltage across each speaker. Not very accurate using a meter, better to use a o--scope. But hey, use what'cha got. Anyway, just tryin' to help ya figure.

Hey Brian, you did'nt answer my question about driving two hr824's from the summing node. I'm driving this from the outs of a Korg d32xd. +6 max. Quickly becoming discouraged with Tim and this site-- I posted this thread two weeks ago. Bail him out, if you would.

Mahalo,
THM
Old 24th May 2005
  #19
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BoomChuk, I had to go back and see your original question. Whew, it wasn't directed to me personally <g>.

I'm unfamiliar with the Korg you mentioned. Is it balanced out? If so, it sounds like all you would need to add are some additional switches to turn speakers on and off and insert/remove the summer, if I understand the application correctly.

One thing...there IS that 6 dB drop through the summer. That may or may not be an issue for you.

So, give me some more "specs" on exactly how you would want this device to operate and I'll try to draw up something.

Bri
Old 24th May 2005
  #20
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Thread Starter
Aloha Brian,

Balanced out from a korg Digi 32 track DAW to 824's. Yeah, a P/N for DPDT rocker preffered, But I'll take whatever you dig up-- Detent, push or toggle. I'm thinkin' DPDT so I can thru--put it out or flip it to the sum mode. A series DPST could take care of the on/off duties. But hey, what do I know.....Do it how you like it. My days of analog theory are long past. Might be useful to design in a 6db pad for the through--put, keeps drivers in the cabs, if you catch my drift. Thanx for your prompt reply.... (JAB)

Mahalo,
Thomas
Old 25th May 2005
  #21
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Here's a scrawl for the mono/stereo switch. "The "Sum" block is the drawing from earlier in the thread.
Attached Thumbnails
Stereo to mono summing......-mono-stereo-switch.gif  
Old 25th May 2005
  #22
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Here's the circuit for one speaker on/off switch:
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Stereo to mono summing......-spkr-select.gif  
Old 25th May 2005
  #23
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You can tie two of the speaker on/off switches together by commoning the L+, - etc together between two sets of the switches.

All switches are 4PDT for balanced stereo. For ease of wiring and mounting, I'd suggest C&K (now sold via ITT) mini toggles (widely available) with gold contacts.

Bri
Old 25th May 2005
  #24
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Thread Starter
Thanx Brian,

Easy enough, can I bug ya' for the 6db pad I spoke of?

Aloha,
Thomas
Old 26th May 2005
  #25
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Quote:
Originally Posted by BoomChuk
Thanx Brian,

Easy enough, can I bug ya' for the 6db pad I spoke of?

Aloha,
Thomas
At first, I missed yer question...please be OVERLY informative <g>...but now comprehend.

Getting things RIGHT with passives can be complicated with unknown source and load impedances.

SOOOO, I'm gonna cover my ass (since you've now forced me into untested circuits!!) with this:
Attached Thumbnails
Stereo to mono summing......-monotweaker.gif  
Old 26th May 2005
  #26
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The ckt goes into the "sum bypass" path on each channel. Adjust the 5K on each channel to come up with an "audible level match" when going from mono to stereo. If desired, replace the pot with a fixed R.

Bri
Old 26th May 2005
  #27
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Thread Starter
Aloha Brian,

Thanx very much for taking the time to noodle all this good stuff out for me. Others will use this info, I'm sure. I also want to share this with other Korg d32xd users if that's okay with you. I'll burn it in first. The source impedence is 150 ohms, load is >10k. Can you revise the pad R values now?

Mahalo,
Thomas
Old 27th May 2005
  #28
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Thomas, since a 6 dB pad cuts the voltage in half, then the "variable" shunt R would be 2K in the example I posted. If load is much greater than 10K, then it can be essentially ignored. I picked the 5K tweaker since I recall having some in my parts cabinet! <g> 2K5 would be a better tweaker choice.

Looking back at all of this, I'd be inclined to make all the R's perhaps 1.5x or 2x the values as originally shown in order to prevent "loading" the Korg's outputs.

But I'm just shooting from the hip here! If it doesn't work, don't yell at me! LOL

Bri
Old 27th May 2005
  #29
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Thread Starter
Can I yell For you? Lol) Thanx a million Brian! I'll wip out my iron.

Mahalo,
Thomas
Old 28th May 2005
  #30
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Question Selecting resistor values when summing

Hello,

I understand that a given output must not 'see' an load impedance lower than it's own output impedance.
I also recall that an equal impedance connection is called 'matching' and load impedances 10 times or higher is called 'bridging'.
And also that impedance is called 'Z'

I walked thru the Z specs of most of my outboard, and their input Z's are all in the 20K to 100K range. Output Z varies from 100 - 600 ohm.

Now a number of questions appearing to my ignorant mind.

1. whats wrong with selecting resistors in the 10K range ? (instead of 1K - 2.2K) Which leads to the next question:

2. Is the matching scenario the ideal - if so, why is so many input Z's in the bridging range ?

3. A mixer output spec says 'Actual source Z = 150 ohm. For use with nominal 600 ohm lines' and different output (aux) spec says ''Actual source Z = 150 ohm. For use with nominal 10K lines'. Both outputs are balanced - Are there any differences, or should I take precautions when summing ?

4. How do you calculate the summing networks source Z, or rules of thumb selecting the resistor values ?

BTW, I was looking in to this topic because I need to combine aux's and bus outputs from two mixers into the same fx's and dynamics.

Ciao,

Henrik Munch
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