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[Craig Hauber]

I have a customer that wants slide-type volume controls (I guess they have them in their home)  Does anybody know of any 70V types?  I see many 8-ohm style home ones but can't seem to find any slider-type 70V ones.
Another possibility is to use the 8-ohm types on the 70V line.  I figure if the jumpers are at the 1X position that they would be 1:1 at full volume so they may actually work with a 70V system.  (I'll try it out on the bench tomorrow)
-any ideas?

Thanks

[Hal Bissinger/COMSYSTEC]

I have no idea what impedance they will present to the amp and speaker though I suspect much too low. I have never seen a schematic for those things either with their magic jumpers and all so I know little about them except that the one shown is stereo.

-Hal  

[Craig Hauber]

Okay here's my un-scientific experience when experimenting with these type of volume controls.

I did an impedance sweep of a new 70V ceiling speaker (single-driver bose can, set at 32W)

I then connected 1 channel of the VC and impedance swept it in the off position.  with the little jumper set at "4X" it approximated the lowest impedance I saw with the speaker by itself.  It however did definitely have a rising curve with it's own peaks -much like the response of a speaker and not at all like a straight resistor would have.

(I would post screenshots but they are on a windows laptop 40mi from here)

I connected up the speaker and hooked it to a new 400W @ 70V contractor amp I have here (EV PA-2400T)

At low volumes it worked, but definitely was attenuating the highs considerably compared to the speakers native response  The VC did work though and had a decent even range between off and full.  At it's highest setting it still was a couple dB quieter than no volume control at all -so max setting still has windings inserted and not a straight bypass like 70V attenuators -probably something to do with those 2X,4X,8X jumpers on the unit.

At higher volumes (amp flashing the -10 LED) the volume control actually moved!  It was vibrating so much from the windings of it's coil I didn't dare keep it at that level for more than a few seconds -the amp was also making noises from it's transformer so I shut it down fast.

The signal I was using was band limited from 75Hz to 12k, cant imagine what kind of vibration would occur with lower frequencies.

So In my brief tinkering I've come to my conclusion that these controls are garbage and I wouldn't want them even in an 8-ohm home stereo application like they are intended.  (This also must be the reason why decent documentation on these products doesn't exist!)

[Jason Lavoie]

Craig Hauber wrote on Tue, 07 July 2009 02:36

Okay here's my un-scientific experience when experimenting with these type of volume controls. I did an impedance sweep of a new 70V ceiling speaker (single-driver bose can, set at 32W) I then connected 1 channel of the VC and impedance swept it in the off position.  with the little jumper set at "4X" it approximated the lowest impedance I saw with the speaker by itself.  It however did definitely have a rising curve with it's own peaks -much like the response of a speaker and not at all like a straight resistor would have. ... At low volumes it worked, but definitely was attenuating the highs considerably compared to the speakers native response  The VC did work though and had a decent even range between off and full.  At it's highest setting it still was a couple dB quieter than no volume control at all -so max setting still has windings inserted and not a straight bypass like 70V attenuators -probably something to do with those 2X,4X,8X jumpers on the unit.

Whoa there.. you're mixing apples and walnuts..

that volume control is made for 8ohm speakers, and won't do you much good with your 70V speaker and amplifier.

the idea behind the 2x,4x.. switch is so that when you put it in the 2x mode it will take an 8ohm speaker and make it seem like a 16ohm speaker to the amplifier (letting you put 2x the speakers that the amp would normally take)
in 4x it would make it seem like 32ohm, letting you put 4 speakers in parallel *on an 8ohm amplifier*

70V is another thing altogether. and the reason the volume controls aren't interchangeable is the same reason that you can't just mix and match 70V and 8ohm speakers or amplifiers.

Jason

[Lee Douglas]

Just speculating here- never actually tried it but couldn't you put the transformer right before the volume control, using the volume control as a standard eight ohm volume control before the speaker?  Not as intended I'll admit but it might solve the problem.

[Craig Hauber]

Jason Lavoie wrote on Tue, 07 July 2009 06:55

70V is another thing altogether. and the reason the volume controls aren't interchangeable is the same reason that you can't just mix and match 70V and 8ohm speakers or amplifiers. Jason

There's not that big a difference between "8-ohm" or "70V"  It's the same signal but requires different impedances (-until you get up into the 600W range, then the two start to really get similar.)
An 70V amp that can handle an 8-ohm load will be around 600W.  The 400W amp I was using can easily handle a minimum impedance of about 16-ohms (I can't remember the exact math) And that VC was showing a solid 150ohms at 50Hz and rising to well over 5kOhm at 8k.
There was definitely a reaction of some kind between the transformer in the speaker (which is questionable in itself -it was a bose after all and I've never seen a 32W coil that small!), the volume control and the output toroid in the amplifier.

My customer is just S.O.L. on 70V slide volume controls unless I use the resistor-style 8-ohm ones between the speaker transformer and the actual driver -but that would negate the advantage of hundreds of feet of 18ga that 70V distribution provides!

[Jason Lavoie]

Craig Hauber wrote on Wed, 08 July 2009 02:59

There's not that big a difference between "8-ohm" or "70V"  It's the same signal but requires different impedances (-until you get up into the 600W range, then the two start to really get similar.)

at the wattages you're probably working with (judging by the 8-ohm volume control) there is a big enough difference.
if you had a 600W speaker and a 600W volume control then yes, it would all work out fine.

Quote:

My customer is just S.O.L. on 70V slide volume controls unless I use the resistor-style 8-ohm ones between the speaker transformer and the actual driver -but that would negate the advantage of hundreds of feet of 18ga that 70V distribution provides!

your only option is to use an 8-ohm speaker with your slide volume control, then put a 70V transformer before the volume control. so you set your tap where the wire comes in, then it's all 8-0hm from there.
Unless your volume controls are closer to the amp than the speakers. then you'd have to use an 8-ohm amp, then volume controls, then transformer it up to 70V and send it off to the speaker.


Jason

[Andrew Welker]

You could always use 2 transformers, one before the volume control, and one after it, if the location isn't convienent to put it between the amp and a transformer or a transformer and the speaker. Probably would degrade the quality too much, unless nice, high quality transformers are used.

[Hal Bissinger/COMSYSTEC]

Sounds like a shoemaker's approach to get by with using these cheap controls which will probably fail under constant use anyway. And what happens if you have multiple speakers off one control? I suppose the two transformer method would work but where do you put them?

Sounds to me like you need to convince the customer that sliders are not an option.

-Hal