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How to build a three-mic passive combiner?

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How to build a three-mic passive combiner?

Postby DC-Choppah » Sun Mar 07, 2021 3:56 am

I use 8 mics to record the drum kit. I would like to combine the three tom mics into a single channel and just record one mono track for all three tom mics. This will free up 2 channels so I can record keys and bass at the same time to lay down some grooves with the whole rythm section together.

So I went to look for a passive box that I could plug in three mics and combine to one output, but can't seem to find one. Seems like a good project to design/build my own and teach the kids some more skills.

The three tom mics are Sennheisser e604 dynamic mics. So I don't need phantom power.

These mics sound great, so I don't want the box to have an effect on the way the mics sound.

The mics have a nominal impedance of 350 Ohms. I would like to make the box specifically to be used with these mics since that is the only thing it is for.

These are the mics: https://en-us.sennheiser.com/instrument ... ents-e-604

Anyone have a notion of the right kind of circuit to get these three tom mics combined into one output?
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Re: How to build a three-mic passive combiner?

Postby ef37a » Sun Mar 07, 2021 8:19 am

I am sure you know DC that the best way to do this is a small mixer? However a passive combiner is perfectly possible.

The simplest way is to just parallel the 3 input XLRs pin to pin and take that to the output XLR.
Some might object to that, raising the point that one mic is 'loading' the other and that will affect its frequency response. If that possibility bothers you, each mic could be isolated to a degree with a resistor in each 'leg' (to preserve balance) I would try 150 Ohms times two per mic . This will result in some loss of level but...1) You have 3 sources combining and 2)Toms are loud!
However should you find the level is badly affected you can always short the resistors out or shunt with another one.

As you say, phantom power not required but I am a fussy old sod so I would include two 100uF 63V capacitors in the output circuit? One day you might be glad to have a 'spook juice stopper' to hand!

I have found this, https://reverb.com/item/13940522-mini-x ... in-seconds Not sure you can get it here. It is a bit of money but I doubt I could buy all the bits and get a drink out of it for that price! There are dozens of passive 4 way mixers on Zon but they are all jacks and probably not balanced .

Just seen that the mics, at 1.8mV, are a dB or so more sensitive than an SM57 so I doubt level will be a problem.

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Re: How to build a three-mic passive combiner?

Postby The Elf » Sun Mar 07, 2021 10:19 am

I'd just get a bigger mixer.

It seems a horrible compromise to sum all the tom mic's into mono.
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Re: How to build a three-mic passive combiner?

Postby Mixedup » Sun Mar 07, 2021 12:00 pm

I'm no fan of wide panned progtastic toms, but if it's a stereo recording, ie with stereo overheads or room mics, I'd still avoid recording the toms in mono like this, as they won't line up with the overhead image... If pressed for inputs, you can get a decent recording of the kit with 2-3 mics anyway, which should leave plenty of room for reinforcement mics or triggers.
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Re: How to build a three-mic passive combiner?

Postby CS70 » Sun Mar 07, 2021 12:45 pm

I too have made recordings with a 8-pres mixer (it took a while at the beginning to realize I had additional line inputs so I could use external pres :headbang: )

I concur that the best is to get a bigger mixer if you want quality recordings.

But if you can't or don't want to, and all you want is to get a quick and dirty mix to jot down ideas with the band, there's a much better way than trying to save channels by monoizing the toms (which if you keep the overheads stereo will sound.. special).

Just get your self a cheap and cheerful small additional mixer (like the 2-pre, 4 lines small Yamaha), and feed the "drums" stereo out from the mixer you have to that, which will leave you the two pres on the smaller mixer free for keys and bass.

Unless it's for the fun of playing with an electronics project, of course.
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Re: How to build a three-mic passive combiner?

Postby Hugh Robjohns » Sun Mar 07, 2021 1:47 pm

Passive mic combiners were abandoned in the 1950s, for very good technical and practical reasons. And while you could simply parallel the connections, as Dave suggests, it will inevitably affect the sound quality/character to some degree. The panning issue is also a valid one.

The better solution is either a bigger mixer with sufficient channels, or a second small mixer to handle some (or all) of the drum mics, feeding as a stereo pair into the instrument mixer and/or interface as necessary. Small mixers are so cheap these days its pointless and too compromising to do any thing else.
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Re: How to build a three-mic passive combiner?

Postby Tomás Mulcahy » Sun Mar 07, 2021 2:20 pm

Hugh Robjohns wrote:Passive mic combiners were abandoned in the 1940...
There is a curious historical exception to this. Right up til they installed the TG1234, Abbey Road had 3 channel passive mic mixers in regular use. These were used on almost all of the Beatles sessions for the drums because the REDD mixers did not have enough inputs.

I cannot remember the technical details, as to whether Abbey Roads impedance matched 600ohm system mitigated the compromises involved, but it was still a compromise as Hugh rightly points out. It's all detailed in Recording The Beatles (which is a fantastic book I no longer own).

So ya. Don't do it :)
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Re: How to build a three-mic passive combiner?

Postby ef37a » Sun Mar 07, 2021 3:30 pm

In the same vein as "long ago" I had a similar problem in picking up a chorus line of kids for repro' and recording (OR tape). None were above about 4ft tall and, being an 'Am Dram' production they were not belting kiddy pros nor was there any money!

We solved the problem to a degree buy buying (6 iirc) identical very cheap Japanese dynamics of the type used with the small mono cassette recorders of the day. These were modded with a bit of brass tube to add weight and rewired to balanced and the whole lot paralleled and hung on a lighting barrel and fed up to the gods on a single balanced cable to the kit.

Thus the mics could be 'dropped in' when the kids did their piece but pulled up for the adults.

As Hugh says, the response must have been screwed rotten but better than not heard at all!

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Re: How to build a three-mic passive combiner?

Postby Hugh Robjohns » Sun Mar 07, 2021 3:52 pm

Here's one just for you, Dave. ;-)

The BBC OBA/9 equipment -- the portable OB sound gear from the 1950s, which included a number of three-channel passive mic mixers (MX/29) and a pair (main and backup) of OBA/9 valve amplifiers to provide the 70dB of necessary make-up gain.

The complete BBC Engineering Department description and schematics are here: http://www.bbceng.info/ti/eqpt/DU_1.pdf It's a lovely read!

And there's a photo of a rig in use below with a five 3-channel mixers on the right-hand side.
Image

The standard rig included two OBA/9 amplifiers as main and spare, shown at the top of the middle rack, each with a PPM on the front. The bottom two units in the middle stack are their power supplies.

An output distribution unit to handle selection of the OBA/9 amplifier and all the routing controls to various output lines, as well as the talkback/cue feeds is in the left hand stack, along with a speaker monitoring unit.

Thems were the dayz... :D
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Re: How to build a three-mic passive combiner?

Postby resistorman » Sun Mar 07, 2021 3:53 pm

DC, have you tried using less mics? There are lots of good reasons to do so... maybe you should give it a go for fun?
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Re: How to build a three-mic passive combiner?

Postby forumuser840717 » Sun Mar 07, 2021 7:07 pm

Personally, I'd either mic the kit with fewer mics or get a bigger mixer or submixer rather than mess about with turning the toms into a single mono channel. Perfectly useable mixers are so cheap these days, especially secondhand, that it isn't worth messing about with passive combiners and the resultant sacrifice in flexibility and quality.

However, if one really needs a basic 3:1 mic combiner then the RDL ST-UMX3 3:1 Mic/Line Mixer module works well. Like other RDL products it's a solidly designed and ime very reliable little widget but RDL stuff is far from a cheap option and the ST-UMX3 costs as much as a conventional full-function budget audio mixer.

RDL do a large range of useful boxes and 'project block' modules covering a load of different requirements. I've used quite a lot of them for various quick builds or bodge boxes (buying them used on eBay can give savings big enough to actually make them good value). They used to do a ST-AMC3 - 3:1 Active Mic Combiner, some of which I used in a session talkback system I built but I think they're now discontinued. They also do some passive combiners but for all the reasons given by previous posters, I really wouldn't go that route in this case.
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Re: How to build a three-mic passive combiner?

Postby DC-Choppah » Sun Mar 07, 2021 8:15 pm

If I got a larger interface I wouldn't have to do this.

Currently, I only have 8 channels into the DAW at a time with my 8 channel interface. That is what limits me. If I got a 16 channel interface I could record everything at once the way I like it now.

Hmmmmm

I run a Yamaha 16 channel analog mixing board so I can route mixes around the project studio into the interface in any arrangement on the fly. I really like that.

The mixer has 8 buses so those feed the interface into the 'line in' inputs of the interface. I don't use the mic preamps on the interface (M-Audio Ultra 8r). This interface cannot be updated anymore either. The last software update was many years ago. Perhaps it is time to upgrade?


If I got a 16 channel interface, I would want to plug the 8 permanent drum mics into the interface and use it's 8 mic preamps for the drum mics. I like to keep those set up and not touch them. But then use 8 more inputs on the interface as 'line in' to record the 8 busses from the analog mixer the way I do now.

So I would need an interface with:


16 input channels
8 with mic preamps for the drum kit
8 line inputs (from the Yamaha mixer for all the other stuff)

4 stereo monitor outputs (used for 3 headphone mixes and outboard gear)

Headphone output to monitor what is going into the DAW

MIDI

USB connection to the PC (Windows 10)

Compatible with ProTools 11+

19" rackmount - single space

Onboard reverb to add into monitors while recording



The above is exactly what I have now with the M-Audio Ultra 8r, but expanded to 16 channels instead of 8. So it should be easy to insert the new interface into my system.

Any thoughts on a suitable interface is appreciated. So much stuff out there, it's a bit bewildering.
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Re: How to build a three-mic passive combiner?

Postby Trevor Johnson » Sun Mar 07, 2021 8:27 pm

Thems were the dayz...

Many thanks Hugh, those twenty odd pages were a great read on a rather cold and wet Yorkshire afternoon. One note: some heaters at 6.1V, others at the 'standard' 6.3V. I loved the battery bit too!
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Re: How to build a three-mic passive combiner?

Postby Sam Spoons » Sun Mar 07, 2021 9:48 pm

DC-Choppah wrote:If I got a larger interface I wouldn't have to do this.

Currently, I only have 8 channels into the DAW at a time with my 8 channel interface. That is what limits me. If I got a 16 channel interface I could record everything at once the way I like it now.

Hmmmmm

I run a Yamaha 16 channel analog mixing board so I can route mixes around the project studio into the interface in any arrangement on the fly. I really like that.

The mixer has 8 buses so those feed the interface into the 'line in' inputs of the interface. I don't use the mic preamps on the interface (M-Audio Ultra 8r). This interface cannot be updated anymore either. The last software update was many years ago. Perhaps it is time to upgrade?


If I got a 16 channel interface, I would want to plug the 8 permanent drum mics into the interface and use it's 8 mic preamps for the drum mics. I like to keep those set up and not touch them. But then use 8 more inputs on the interface as 'line in' to record the 8 busses from the analog mixer the way I do now.

So I would need an interface with:


16 input channels
8 with mic preamps for the drum kit
8 line inputs (from the Yamaha mixer for all the other stuff)

4 stereo monitor outputs (used for 3 headphone mixes and outboard gear)

Headphone output to monitor what is going into the DAW

MIDI

USB connection to the PC (Windows 10)

Compatible with ProTools 11+

19" rackmount - single space

Onboard reverb to add into monitors while recording



The above is exactly what I have now with the M-Audio Ultra 8r, but expanded to 16 channels instead of 8. So it should be easy to insert the new interface into my system.

Any thoughts on a suitable interface is appreciated. So much stuff out there, it's a bit bewildering.

Why not just buy an interface with 16 mic/line inputs and dispense with the mixer? Simplifies the system and removes two old pieces of kit?
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Re: How to build a three-mic passive combiner?

Postby DC-Choppah » Sun Mar 07, 2021 10:30 pm

ef37a wrote:I am sure you know DC that the best way to do this is a small mixer? However a passive combiner is perfectly possible.

The simplest way is to just parallel the 3 input XLRs pin to pin and take that to the output XLR.
Some might object to that, raising the point that one mic is 'loading' the other and that will affect its frequency response. If that possibility bothers you, each mic could be isolated to a degree with a resistor in each 'leg' (to preserve balance) I would try 150 Ohms times two per mic . This will result in some loss of level but...1) You have 3 sources combining and 2)Toms are loud!
However should you find the level is badly affected you can always short the resistors out or shunt with another one.

As you say, phantom power not required but I am a fussy old sod so I would include two 100uF 63V capacitors in the output circuit? One day you might be glad to have a 'spook juice stopper' to hand!

I have found this, https://reverb.com/item/13940522-mini-x ... in-seconds Not sure you can get it here. It is a bit of money but I doubt I could buy all the bits and get a drink out of it for that price! There are dozens of passive 4 way mixers on Zon but they are all jacks and probably not balanced .

Just seen that the mics, at 1.8mV, are a dB or so more sensitive than an SM57 so I doubt level will be a problem.

Dave.

Thank you Mr. Dave! The resistive mixer approach. Thanks for that! Simple enough. Thank you for the detailed design considerations. Folks are saying that it compromises the sound.


I have an ART 2-mic passive combiner. I just tried it on the tom 2 / floor tom mics.

I recorded the two toms first regularly into two channels. Then I combined them and put them into one channel and boosted the gain by 6 dB. The levels check out to be the same.

The sound of the two channel mics is definitely better. The drums are more distinct and they seem to ring longer, have more clarity.

I guess when combining there really is some stuff that gets cancelled out or lost and never makes it into the DAW.


OK. Gonna scrap the combiner idea and record more channels instead.
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