Rolls Passive Summing, 150 Ohm Output?

[RANDY]

The Rolls Folcrom Passive Summing Box Has A Balanced 150 Ohm Output.how Is This Possible If It Is Passive? They Say It Has No Active Circuitry Or Transformers Or Caps! What's Up? Thanks!

[Steve Conner]

Reading between the lines, it probably uses a 10k resistor from each input to a summing node, and a 75 ohm resistor from the summing node to ground. (being stereo balanced, that circuit is repeated with 4 summing nodes) Therefore:

The outputs will only be balanced if you drive it with balanced inputs.

There will be 42dB of loss through it, which is to say that the output voltage is 1/134 of the input. (in fact I worked back from the quoted 42dB and 10k input impedance to determine that the summing node was 75 ohms)

You could build it yourself in a weekend for $200 including the fancy rack enclosure.

[Mark Black]

Here's a pretty in-depth overview of a summing box:

http://www.rane.com/note109.html

I've used the method outlined in the note many times with good results.

[sryan]

- You could build it yourself in a weekend for $200 including the fancy rack
- enclosure.

Steve's basically correct in how this device works.. It's a cascade of summing circuits. Justin's box also allows an input to be assigned to either or both sides of a stereo bus, so there's also possibly dummy loads are engaged when a channel is engaged to one or the other or both output busses. Although I'm not sure of this. Having built a few things like this, I'm not sure if $200 is doable unless you already have all the chassis punches, tools, etc and don't mind having a lot of remnant 100 packs of resistors left over for your next project perhaps.

This sort of thing is a rats nest to wireup without a routed board. Especially considering the switching capability. There is also possibly high precision parts or hand matched as well.. And if you want to make a company of it then there is of course overhead and dare I say profit.. <chuckle>. But if you're in need of a project it's certainly doable and not nearly as high of a dweebfactor as an amplifier.


YAS
(yet another Steve)

[Steve Conner]

Hmm, I guess you're right sryan. The last fancy rack enclosure I bought cost $200 just by itself

[sryan]

Comments from Justin regarding the Folcrom..

Steve, yeah the price of enclosures is rediculous..

Here's some comments regarding this topic from Justin.. Here's just
up the road a piece from me. He's a great supporter of DIY/RAP/etc..
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1. "Rolls" isn't the company that makes the Folcrom. (It's Roll Music Systems,
a different company than 'Rolls')
2. The Folcrom uses those switches both for flexible routing abilities, and to maintain low source impedance on unused channels in order to preserve good crosstalk performance when less than all of the inputs are in use. The switches aren't cheap, but as far as I'm concerned they're very necessary.
3. You could probably build a similar device for $200, but it would take most people a lot more than a weekend. You could build it in a weekend, but it would cost more than $200. You could build something fast and cheap, but it wouldn't be comparable to the Folcrom. Cheap, fast, good: Pick two. Most intelligent people who seriously consider building their own clone of a product like this usually realize that saving money isn't a good reason to do it (unless your time is worthless). Learning stuff, having fun, and customizing the circuit or cosmetics for your own needs and desires are all much better reasons to get into DIY.

Unless you happen to have access to a well-equipped metal shop and some serious skills, the enclosure is going to eat up most of that $200 budget, will take a long, long time to build, and when you're done it'll still look homemade. The switches will cost more than the other half of your budget and will require some precision in the chassis construction, and you still have to pay for connectors, hardware, and a bunch of other components. The DB25 connectors do save money, but I would hate to have to solder wires to them by hand. Printed circuit boards make life so much easier. Unfortunately, having a single circuitboard manufactured will cost more than your $200 by itself, and unless you already do that kind of thing (for a living), it'll take at least a few weeks to lay it out.

Meanwhile, the Folcrom sells through dealers for $775. That's significantly less than any of the other legitimate summing devices on the market, and the overwhelming consensus is that it's a fantastic bargain for a unit that does what it's supposed to do and does it very well.

Cheers,
-----
Justin Ulysses Morse
Roll Music Systems Inc
Minneapolis, MN
www.rollmusic.com
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[Steve Conner]

Yeah, those are all valid points. I do actually design electronic equipment for a living, and in a commercial context I have to agree that the Folcrom thingy probably isn't overpriced. If he can sell it for $775, it can't cost him more than about $250 to build, so the build quality and parts quality look remarkably good.

However, my point was that it doesn't actually have any complicated electronics in it that would defeat a hobbyist. I still think that if you wanted to experiment with passive summing, you could build a device that did the same thing for a few hundred dollars. In my case, I do have a metal shop and a buttload of tools and punches, and I don't care if my gear looks homemade either.

I'm also skeptical of the premise that it's based on, that analog summing sounds better than digital summing. I think this is just the same effect seen by those hi-fi tweakheads who stack up 64 TDA1541 DACs for their CD players. The SNR gets better the more DACs you connect in parallel. This is essentially what the Folcrom does, connect your DACs in parallel, and so I argue that this is where the sonic benefit (if any) comes from, especially if you're using the older 16-bit gear.

If this theory were correct, then spending the $775 on a real good 24-bit DAC and mixing in the box would work the same. So would mixing in the box, sending the mix to all of your DACs, and paralleling them with the Folcrom. But that's a whole other issue.