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Author Topic: 8-ohm vs 70V distribution  (Read 2451 times)

Peter Kowalczyk

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8-ohm vs 70V distribution
« on: March 07, 2018, 07:20:42 pm »

Hi Folks,

Apologies if this has been asked / beaten to death before....

I'm working on a restaurant system design with a few zones and many speakers in each zone.  In addition to BG music, we plan to send live program material to these speakers from performers in an adjacent space.  Music is a part of the restaurant's brand identity, and the owner gives a sh!t, so audio quality is an important consideration.   

I'm wondering about whether to plan to operate the speakers (QSC Acoustic Design series, various models) in their 8-ohm mode, or in their 70V mode.

Obviously, 8-ohm would require more amp channels and more complex wiring.  However, I gather that audio quality can suffer greatly in 70V mode.

- Can you recommend a good resource for learning about and planning 70V systems?

- What 'threshold conditions' would cause you to plan for 70V distribution? 

- For a large number of distributed speakers in their 8-ohm mode, can series/parallel wiring be effectively used to reduce the number of amp channels required? (Assuming the amp's voltage is adequate for SPL required...)

- What else am I not considering?

Thanks all!


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John Roberts {JR}

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Re: 8-ohm vs 70V distribution
« Reply #1 on: March 07, 2018, 08:08:46 pm »

Quality should be comparable, and only situation where I might lean 8 Ohm vs 70V would be for dance/trance music because of heavy bass content that is stressful for magnetics that saturate from too much bass content.

There are too many resources to list, have you considered doing a search? (kidding of course not).

JR
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duane massey

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Re: 8-ohm vs 70V distribution
« Reply #2 on: March 09, 2018, 03:02:29 pm »

70v, use quality speakers and processing. If you need more bass, use 8ohm ceiling subs in each room.
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Duane Massey
Technician, musician, stubborn old guy
Houston, Texas

Peter Kowalczyk

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Re: 8-ohm vs 70V distribution
« Reply #3 on: March 09, 2018, 10:49:39 pm »

Thanks for mitigating my concerns.  High-impedance distribution will certainly be easier to implement.

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Hal Bissinger/COMSYSTEC

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Re: 8-ohm vs 70V distribution
« Reply #4 on: March 10, 2018, 11:55:14 pm »

70v, use quality speakers and processing. If you need more bass, use 8ohm ceiling subs in each room.

I second the installation of 8 ohm subs with their own processing because of the live music. Otherwise you could get away with 70v subs.

-Hal
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Josh Millward

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Re: 8-ohm vs 70V distribution
« Reply #5 on: June 13, 2018, 10:09:45 am »

Yes, they are right, 70V systems do not have to suck. However, they are often less than "HiFi" because the cost is reduced too much. Often, the installed wire is too small and the transformers on the loudspeakers are also too small. After all, these are the most expensive parts of the system.

If you use good transformers with wide frequency response and do not try to run sub-sonics through them AND you use good sized wire like a 12 AWG size instead of 18 AWG AND you use decent quality loudspeakers, there is no reason your 70V system can't sound really good.

If you need some low frequency boom in the rooms, then run a low impedance subwoofer to fill in under your 70V distributed loudspeakers. A couple subs can make a world of difference for a sub-par 70V system.
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Josh Millward
Danley Sound Labs

Ivan Beaver

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Re: 8-ohm vs 70V distribution
« Reply #6 on: June 13, 2018, 11:34:40 am »

70V gets a bad rap, but sometimes it is deserved, due either to the gear or the install.

Basically 70V is just a higher impedance speaker, so you have less loss across the line and can hook up more in parallel.

Allow about 10% overhead is a good idea.

There are some good 70V products out there, along with some really cheap/bad ones.

Don't blame "70V", but rather how the concept is used.
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David Buckley

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Re: 8-ohm vs 70V distribution
« Reply #7 on: June 14, 2018, 06:19:28 pm »

Others are right, there is nothing wrong with 70V as a concept, its just the implementation of products is variable, tending towards bad.  The issue is the transformer in the speaker, and sometimes the transformer in the head end, though strictly, one does not need a transformer at the head end, just a big enough amplifier.

These transformers limit, and often seriously limit, both the power handling of the speaker, and the low frequency response.

If you are willing to spend more than the minimum dollar, and prepared to put the wiring effort in, then one can get a very workable solution without the compromises of 70V transformers by using series/parallel wiring arrangements.  Four 8R speakers in series/parallel is still 8R, as is nine speakers in series/parallel, as is sixteen speakers.  Just need an amplifier that can deliver the required wattage into 8R.
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Luke Geis

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Re: 8-ohm vs 70V distribution
« Reply #8 on: June 14, 2018, 10:50:26 pm »

I don't believe 70v is inherently bad, it is just misunderstood and often setup incorrectly due to logistical limitations. I.E. 70v is often setup in places where standard 8-16 ohm application may better be suited. To boot, zip wire is used and often too many speakers are ran or improperly set, making things become problematic. Insertion loss is one factor and I can bet that not many installers actually measure the impedance of the speaker leads to ensure that they are in fact within spec. Often the belief is that you can run as many 70v speakers as you wish provided the total wattage  that all the taps are set to don't exceed the amplifiers available wattage. This is untrue if there is enough insertion loss, or you simply have so many speakers that the actual impedance seen at the amplifiers terminals is too low.

The bandwidth limitations of 70v speakers is a very real thing, but again cost and application comes to mind. I would not use 70v in situation that require significant volume at high bandwidth ( such as dance halls ). Often the low impedance counterparts don't fair much better in similar situations with similar designed speakers. For all intents and purposes however, if fidelity and SPL is a need, then 70v is not ideal. If you need high speaker counts with less SPL and amplification, then 70v wins. The weak link with 70v systems is the transformers. Often the speakers within them are the same as the standard low impedance options. Many install speakers actually come so you can choose between 70v and low impedance in one unit. The EV Evid comes to mind as one such unit.

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Hal Bissinger/COMSYSTEC

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Re: 8-ohm vs 70V distribution
« Reply #9 on: June 15, 2018, 05:14:43 pm »

70 volt gets a bad rap because it can be and often is installed as the cheapest alternative using the cheapest material available. Spend some money on it and it will rival any 8 ohm system.

Quote from:  David Buckley
If you are willing to spend more than the minimum dollar, and prepared to put the wiring effort in, then one can get a very workable solution without the compromises of 70V transformers by using series/parallel wiring arrangements.  Four 8R speakers in series/parallel is still 8R, as is nine speakers in series/parallel, as is sixteen speakers.

While that is certainly possible I wouldn't want to be the one to install or service a series/parallel distributed speaker arrangement.  >:(

-Hal
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