A (silly?) summing box idea...

[Marc Girard]

Hello there,

I'm new around here and I'd like to say that this board is very cool. I'm building my GSSL these days and the amount of information available thru this board is simply great! Can we be friends!?!? :grin: :roll:

Now, I'm no electronic expert but I have that funny idea in my mind... And since I"m not an expert, I wonder if it would work. So here it is:

What about a little "stomp-box" that would have a DB-25 connector (Tascam wiring) at one end and two XLRs at the other end? It could be a little box we could carry in our pocket to do some external summing... And send the final output to a nice Neve or tube preamp for make up gain.

Pretty much like the Folcrom thing I'd say, but more portable, only 8 channels and hard wired pairs (like, all the odds summed to the left and evens to the right).

I read somewhere that a mic-pre like to see "200R" and that I simply have to multiply my number of channels I need. Say, 4 channels x 200R = 800ohms. Does that make any sense?

Would it be as easy as soldering 800ohms resistors to the DB25 pins and solder all the ends directly to the XLR connector? It's almost impossible, I'm sure there's something I'm missing here, since the Folcrom sells for around 800$ US...

Thanks for reading!
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[Emperor-TK]

Hey Mark, welcome to The Lab! I remember you from the Paris users group, so I thought I'd roll out the welcome mat this time. I can't answer your questions specifically, but there have been a lot of summing mixer designs floating around here. One that comes to mind is a design from a member named NY Dave. I'm sure it shouldn't be too hard to fing the thread with the search function.

Regards,
Chris Wargo

 

[NewYorkDave]

Chris, that "Sleeper" icon is a riot! That was a great movie.

Marc, I hope you didn't go crazy searching for posts from this nonexistent username "NY Dave." :wink: Anyway, the idea of a "pocket mixer" is not crazy at all. Matter of fact, the idea has been kicking around in the back of my mind since I saw your inquiry several days ago. I've got an idea for you which I'll draw and post sometime soon.

Moderators: it seems this thread rightfully belongs on the Drawing Board.

 

[Sammas]

Hi there!

Since we are throwing up Summing ideas, i'll add mine instead of creating a new thread (hope you don't mind MG! :guinness: )

I've been thinking about summing some summing :green:

that is 8 channels of summing purely for something like drums with an API style preamp for makeup gain, maybe even a Jlm99V. But with a twist, having another 10 channel summing circuit in which the stereo output of the api style drum summer feeds into the inputs 1&2 and then the remaining 3-10 inputs are for the rest of the mix and a cleaner/less colouring preamp is used for the final makeup gain, maybe a Jensen based design.

Of course you could just put the entire passive circuit for all 16inputs in one box but have the first 8 inputs running with the resister values for 8 channels then fed through any preamp pair with a send/return configuration which is in theory just like a hardwired 1&2 input for the second summing stage. The second stage would run on resister values for 10 inputs.


Im sure i made little sense then, so the idea is running an 8 channel summing box into a 10 channel summing box with the ability to choose the preamps/compression for each the first and overall stage of the summing.

Would it be better to run 2 independent "buckets" of 8 channels that are then summed into a stereo signal? so having 3 stages of makeup gain? or could i do it successfully (soundwise) running a series kinda summing setup.

 

[Marc Girard]

Chris: Thanks for the warm welcome! My PARIS reputation follows me where ever I go! Jokes apart, I read a few things here and there about passive summing but I gotta admit, I'm a bit confused, I'm still researching on the matter...

Dave: I'm happy that you find that I"m not that crazy after all! Let me know if you have ideas, we could join forces and rule the galaxy... (?!?)

Sammas: It's too late for me to undersstand your concept, I'll have another read tomorrow!

 

[Sammas]

[quote author="Marc Girard"]Chris: Thanks for the warm welcome! My PARIS reputation follows me where ever I go! Jokes apart, I read a few things here and there about passive summing but I gotta admit, I'm a bit confused, I'm still researching on the matter...

Dave: I'm happy that you find that I"m not that crazy after all! Let me know if you have ideas, we could join forces and rule the galaxy... (?!?)

Sammas: It's too late for me to undersstand your concept, I'll have another read tomorrow! [/quote]


thats ok :green:

I dont think i even understand it... i might have to draw a picture :wink:

 

[bcarso]

Is that a mixer in your pocket or....

Thanks for reminding me of what that avatar is from. Ah the orb....

 

[quadwould]

my personal idea... though excessive i think..and probably very expensive to build.....thogh maybe not... depending on design....

a summing box based around the BA183/283

am i nuts :? :thumb:

 

[pstamler]

[quote author="Marc Girard"]I read somewhere that a mic-pre like to see "200R" and that I simply have to multiply my number of channels I need. Say, 4 channels x 200R = 800ohms. Does that make any sense?[/quote]

It does from the output end, but not from the input end. If you used four inputs, then each input would see an impedance of 800 + (800||800||800||Rp), where "||" means "in parallel with" and "Rp" is the input impedance of the preamp. Let's say that's 1500 ohms, so the total load would be 1026 ohms. That's on the low side for the output of (presumably) a multichannel D/A converter. Even for 8 channels, the input impedance of each would be 1798 ohms. Some D/A's can drive that happily, some won't be so happy.

The secret is to use larger summing resistors, say 10k or 20k. Let's say you choose 20k; for an 8-input mixer, still assuming a 1500 ohm input impedance for the preamp, input inpedance for each channel will be just under 21k, which is fine, but (assuming low-impedance sources) the output impedance will be 2500 ohms, way too high. So you add a termination resistor to ground; 215R for a 200R source Z, 158R for a 150R source Z. Presto.

Except that now your insertion loss is greater: 41dB in the case of a 200R output Z, 43dB for a 150R output, as opposed to only 18dB for a design without a termination resistor. So the makeup gain needs to be higher.

That's why Neve, etc. use high-current driver amps and a low-impedance summing bus.

Peace,
Paul

 

[NewYorkDave]

I've been rather ill and have also been working some marathon shifts, so I haven't had a chance to come back to this till now. Paul has already done a good job of explaining the particulars, but here's a ready-to-use design template that should meet your needs.

8.5kB PDF

In the interest of keeping it simple--and thus possible to build into a small box at minimum expense--I did not include any kind of loudness/input impedance compensation for "L+R" assignment as I have in some designs I posted here previously. Considering how these "passive summing" boxes are normally used, it doesn't really seem necessary.

 

[electric]

ny dave,
cool info!!!:thumb:
how would you add pan pots to the scenario though? i would be very interested in this.

the reason i ask is because if you use external summing with a lot of outboard eq/compressors you may want to pan certain mono stems somewhere else in the stereo field besides up the middle. (not all the time but it happens) this way you would not need to use 2 eq's,2 compressors to process the stem. (to do panning w/ daw you need to send stereo stems which unnecesarrily requires twice the outboard to process. this gets expensive with api's/neve's etc ). also it would be cool if you could bypass the pan circuit if you dont need it to preserve sonic integrity. imo now you would have a kickass passive mixer!!!!!(my neve pres are salivating :green: )

does this make sense? this is why i see a product like the dangerous mixer as a valuable summing option. (almost buy 2 of them every week and then stop myself) however 2 linked dangerous mixers go for about 5000. i have tons of great pres in the shop. if i could build a nice passive mixbus w/ panning and use the pres for makeup gain i would be thrilled!!! (and so would my pockets)

cheers :guinness:
electric

 

[NewYorkDave]

With currently available parts, continuous panning in a fully balanced passive mixer is not easily done. The best approach, it seems to me, would be to build the mixer internals unbalanced and do the balun conversion at the front end if needed.

 

[higginsrob]

Hi guys, love the site. I'm new to the world of DIY electronics, this is my first post.

My question is on the quality of resistors. I?m building a passive summing unit out of a patch bay. I recently went to a local store that specializes in resistors. They tried to sell me non-inductive resistors that cost $5 each. They said they usually serve satellite and missile manufactures, but once in a while audiophiles will come in wanting the non-inductive resistors, due to the fact they are quieter and sound better.

Does anyone know about non-inductive resistors, or have any information about the quality of resistors in audio circuits?

Thank you!

 

[bcarso]

Welcome to the forum.

Wirewound resistors tend to be inherently inductive unless done with a special non-inductive winding construction. But they are quite rare these days, although famously stable and low-excess-noise.

When metal films are made they typically start with a blank with a certain bulk resistivity coating from one end to the other. These go into a machine which measures the end-to-end continuously while the blank is rotated and a spiral groove is cut. When the desired resistance is reached the machine stops and spits out the grooved cylinder. Typically, the bulk resistivity blank is selected to take practically an entire end-to-end spiral for the desired value.

The spiral adds a bit of inductance, but usually it is small compared to the resistance at any audio or even much higher frequency of interest.

But, one could in principle cut a series of grooves parallel to the axis of the cylinder. I have seen this done for particularly low resistance values, when the final part was enclosed in a hermetic glass case (your tax dollars at work---these were primo aerospace-quality parts). This construction has lower inductance.

The bulk metal foil resistors from Vishay have a different construction and a different trimming technique, and are almost wholly non-inductive. They are also very pricey.

 

[PRR]

> non-inductive resistors that cost $5 each.

For almost any audio purpose: Carbon-Film, $0.12 each

These are "non-inductive" far beyond the audio band, in any reasonable value. They are also pretty noise, low voltage-coefficient, low inductance, bla bla bla.

Wire-wound are excellent audio resistors and were THE standard high-precision audio resistor for decades. Everything has inductance and "wound" increases inductance, but it really is not a problem.(*)

Metal-Film is also an excellent audio resistor.

The "problem" with wire-wound and metal-film, for audio, is that they sound and work 99.9% the same as carbon-film, and cost much-much-much more. Money that might better be spend somewhere else, with more audible effect. Yes, some folks hear improvement with costly resistors, others don't.

For vintage-sound guitar amps, some folks prefer the old Carbon Composition. Carbon-comp is coal-dust mixed with clay. I remember when these were $0.12; before Carbon-Film escaped from microwave labs into MASS production (driving out Composition). They have a slightly gritty quality that adds flavor to Fender guitar. They have been out of production for years. Huge stockpiles must exist, but to get a couple of a specific value you have to haunt guitar-amp sites and pay $0.20-$2 each.

In very low values, 10Ω and below, you may need to think about inductance. I have a workhorse 8Ω 44 Watt wire-wound resistor I use as a dummy-load for speaker-amps; when I measure up to 100KHz I have to remember that it gets up to 8.5Ω impedance up there. The 0.27Ω resistors used as ballast in speaker-amps may be inductive within the audio band (but mostly we use wire-wound anyway: the inductance is not enough to hurt anything). If you build a 60dB 1KΩ attenuator, the bottom resistor is 1Ω and if it has 8uH inductance then your "-60dB" could be only -57dB at 20KHz. But generally a carbon-film resistor has no more inductance than its leads: keep leads short in low-impedance work. If in doubt: parallel! Ten 10Ω resistors in parallel gives 1Ω, and (if you keep the leads very short) about 1/8th the inductance of a single resistor.

(*) The idea that a wire-wound resistor has "significant" inductance is quickly refuted if you wind some air-core chokes. You have to wind a LOT of turns around air (or anything except iron) to make a choke that does anything at all at audio frequencies and impedances. 10 turns around a resistor doesn't approach 10Ω until far past the top of the audio band. If you want to wind 8Ω at 2KHz, as for a loudspeaker crossover, air-core, you end up with about a pound of wire.

 

[bcarso]

One problem I've had recently was that the inductance, although as PRR says, low at audio frequencies, of some 4 ohm sand-filled resistors I use for amp dummy loads, have enough L to cause marginal chip amps to oscillate at much higher frequencies. Bad news and shame on them, but in many cases the L of the intended speaker load is enough less that the amp behaves in its intended application.

Since these are usually very cost-sensitive designs and the speaker will be captive to the amp, it's necessary to use the actual transducers for testing rather than make the amp have guaranteed stability.