70v System Versus Impedance Matching switches?

[Blenton]

So, given a few somewhat limited operating conditions, are there any real advantages to setting up a 70v system over using a small Class D amplifier (like a Topping 100 watt/channel) with an impedance matching circuit (like a rotary volume knob)? Here are some system requirements and specs:

- background music only in a 6000 sq ft office/warehouse (about 75x75 with a slanted open ceiling from 18-24 feet)
- no paging needed, just music
- good frequency response
- Wire runs will be less than 100 feet to the furthest speaker
- enclosure type is anything expect for in-ceiling as there is no acoustic ceiling, just insulation
- hoping for decent, some-what balanced coverage. But it doesn't have to be 100%.
- no need for expandability (number of speakers will be fixed)

I have some ideas as to what I'd like to build, but thought I'd go cast a net out and see what comes back. You know, make sure I'm not completely bonkers . With the relatively small number of speakers we are thinking of using (less than 10), I think I'm more inclined to just wire them up series/parallel to get the right load on the amplifier than set up a 70V system. Or use an impedance matching switch (like what would be used for a volume control in separate zone of a 70V system) to match the impedance to the amp. The 70V is definitely more flexible since I could expand it without needing to add amps, as well as the fact that if 1 speaker goes down it doesn't affect the rest of the system. But I don't think the speakers will see more than 4 watts each, with maybe 10 watts each if they get crazy. That means that if I keep a Topping amp at <80% power as you would with a 70V system, each speaker would see the right amount of power and could get louder than needed. It's also a lot easier to hide a small amp like that than it is to find a rack to mount most 70V amps. And I really like the class D amps I've played with, so they are quite appealing to me for this project.

I've even thought about using something as small as NS3's with a small tweeter (maybe 2 or 4 drivers per speaker) to help with directionality, beaming, and keeping a low profile. But I'm thinking a single 5 1/4" driver would be well suited for each speaker (also with a tweeter). An appropriate amount of baffle step, combined with low power levels, would hopefully keep xmax in check while providing sound down below 100 hz. Each speaker coverage would be about 25x25' (with a large ceiling) which is about equivalent to a mid to large family room. I'm not aiming for 30 hz that will pound your chest, so I don't think I'm asking too much of these smallish drivers. But maybe I am. Thoughts?

[Sydney]

Re: 70v System Versus Impedance Matching switches?

The area is 126,000 cubic feet.
The speakers would be displacement limited and have insufficient power.

[jinjuku]

Re: 70v System Versus Impedance Matching switches?

70V solves the problem of long runs. You could use a rule of thumb of a speaker every 750 sq feet as a loose guide.

[Blenton]

Re: 70v System Versus Impedance Matching switches?

Interesting... It seems those two posts contradict each other

@jinkjuku - Thanks for the general rule of thumb! It appears that I was following it without even knowing that I was doing so; I was planning on 8 speakers. The room is roughly 6000 sq feet and 6000/750 = 8 I was leaning towards 70v, but the added expense may not be worth it since the system will not be designed around flexibility or expandability. Maybe I'm not looking in the right place, but it seems that the appropriate taps are ~$20 per speaker, but the transformer for the amp is even more. Or, it seems an amp that was designed for 70V is much more expensive and has a higher $ to watt ratio.

@Sydney - could you elaborate, please? Your comments seem to be in direct opposition with jinjuku's. I offered several musings (such as using a gaggle of NS3s or a 5 1/4" woofer) - to which are you referring? I understand that anything short of using a plethora of rock star pro audio gear will be excursion limited when I decide to host my favorite band for karaoke night but we are just aiming for background music.

Perhaps my math isn't right here, but if my speakers (system, not individual drivers) each have a conservative reference of 87 db @ 1w/1m, 8 watts would give me 9 db of gain, putting it at 96 db. If I place them so that I am roughly 4 meters away, at most, I would lose ~12 db, putting me at ~84 db. AFAIUnderstand, normal conversation in an office is usually between 60 and 70 db (this office is a little bit on the noisier side, but not noisy enough that I have to speak loudly to be heard). That would make it more than loud enough for background music, IMO.

As for being underpowered, aren't 70v taps usually less than 10 watts? I know PE has some that are 15watts, but most I've seen are 10/5/2.5/1/.5. So what's the difference between wiring 4 speakers in series parallel to each channel of a medium sized digital amp and using a 70V system? I understand the length of the wire runs has much to do with it as well, which is really the main advantage to a 70V system. But after closer examination, it looks like 60 feet will be the longest run. With such low voltage running through the wires, 16 or 14 gauge wire appears to be sufficient for that run.

I'm not trying to be combative or snooty, just trying to learn. If I were being snooty, I would point out that the VOLUME of room is ~126,000 feet^3 - the AREA is still ~6,000 ft^2 :D

[Sydney]

Re: 70v System Versus Impedance Matching switches?

...I'm not trying to be combative or snooty, just trying to learn. If I were being snooty, I would point out that the VOLUME of room is ~126,000 feet^3 - the AREA is still ~6,000 ft^2 :D

There is no contradiction - jinkjuku is correct; a 70V system reduces wire size by increasing voltage.
FWIW: Distributed system aren't for high SPL but rather a desired SPL above ambient noise level.
Basically Sound is about pressuring air and the room volume of air needs to be considered NOT just floor footage ( a common oversight ). The height above listener is a variable - closer is louder but smaller coverage area, higher is quieter over greater area
The drivers you mentioned are not large and do not displace a lot of air - Add up the surface area ( Sd ) and then compare to larger drivers. ( 8" 70v full range for instance ).
Consider a speaker designed for a high voltage system distributed system has greater than average sensitivity than other speaker. That means a small fraction of a watt is sufficient to raise above noise floor. With a less sensitive speaker the power applied has to be greater and working the speaker harder, pushing it closer to non-linear behavior with greater distortion and leaving little or no reserve for crest factor. A reason why heavily compressed music ( ie like musak - dynamic range compression ) is often used in such a system.
BTW: There are design guides provided by Distributed Sound Manufacturers that would be helpful.