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 Post subject: Question about speaker selector
PostPosted: Mar Sat 18, 2017 4:53 am 
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Joined: Jan Thu 24, 2013 11:50 pm
Posts: 2348
Location: Bristol, Rhode Island
Hi,

I am considering buying a speaker selector box to be able to switch between two tube amps (Scott 299B and Stromberg Carlson ASR433) and one set of speakers. I am looking at a Niles SVC4 (see pic).
I don't know too much about speaker selectors and I have a question:
Will this selector place a dummy load on whichever amp I am not using, when both are powered up? Obviously that would be critical as I don't want to run my amps with no load attached but would like to be able to toggle back and forth between them.
The Niles has "protection circuits" and I am wondering if that's referring to a dummy load. I haven't been able to find any information or a user's manual online, but this seems to be a pretty popular selector that sells for decent money on ebay.
Thanks for any insight you might be able to provide.

Ted


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 Post subject: Re: Question about speaker selector
PostPosted: Mar Sat 18, 2017 6:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16077
Location: Albuquerque, NM 87123
That appears to select either of two amplifiers and 4 sets of speakers. Kind of wondering why you would want volume controls, unless it's made for a business like a restaurant where you need to control volume in various locations.
The Niles HDL4 "Protection" circuit is just two 3 ohm resistors; hardly high tech. I think they just have them in series with the speakers in case there's a short.


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 Post subject: Re: Question about speaker selector
PostPosted: Mar Sat 18, 2017 7:53 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 8492
Location: Powell River BC Canada
Use a 4 pole 3 position switch and two 30 ohm resistors.

Under no condition, are the amplifiers outputs connected to anything except a speaker or resistor !

1. speakers are connected to Amp 1, a resistor is connected to Amp 2
2. speakers are connected to nothing , a resistor is connected to Amp 1, and Amp 2
3 speakers are connected to Amp 2 resistor is connected to Amp 1


I spent considerable time wiring demo sound rooms for stereo stores.

At one time Switch craft marketed a switch box especially for that purpose.

Once, a junior tech was dispatched to wire a panel in a plush sound room.
Connections were made by lying on the carpet under the console.

He fell asleep, and the branch manager videotaped him with the new Akai
video system we were selling then.

That story is still rattling around 40 years later.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Question about speaker selector
PostPosted: Mar Sun 19, 2017 4:18 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 381
Location: Fayette County, Pa
2 speakers from one amp is easy; two amps driving the same speakers becomes tricky. Problem is maintaining a load while switching, even if you put a resistor load you still have the instant the switch is transitioning between them and that creates the potential for problems.

The proper way to do this is using a muting switch on the inputs of the amps. switching between amps is done in a sequence. First the mute is applied to both amps to ensure no audio output. At that point, with no signal there will be no voltage spike during the instant the switch has no load on either amp while changing configuration. ( The "no load" condition is not a problem if no signal is present) Once transition is complete the mute is removed. Audio will then go to the speaker and dummy load as expected.


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 Post subject: Re: Question about speaker selector
PostPosted: Mar Sun 19, 2017 6:24 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 8492
Location: Powell River BC Canada
If no load is connected the plate sees a inductance, with a reactance rising with frequency.
Some amps have feedback loops from the output transformer. Invitation to instability.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Question about speaker selector
PostPosted: Mar Sun 19, 2017 1:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12505
Location: Fernandina Beach, FL
radiotechnician wrote:
If no load is connected the plate...
Invitation to instability.

And I have had to replace tube sockets that were burned by arcing, no doubt because of no load and high volume.

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Don


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 Post subject: Re: Question about speaker selector
PostPosted: Mar Sun 19, 2017 4:54 pm 
Member

Joined: Jan Thu 24, 2013 11:50 pm
Posts: 2348
Location: Bristol, Rhode Island
Thanks.

It seems to me that the volume pot on the selector switch is probably there to aid in switching. I would imagine you turn it down to zero when switching, and once you've switched it would turn it back up. Hard to believe there's not much information on this unit online, as I said it seems to be a pretty popular one. I'm going to check it out and test it.


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 Post subject: Re: Question about speaker selector
PostPosted: Mar Sun 19, 2017 6:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3817
Location: Boston, MA USA
Don Cavey wrote:
radiotechnician wrote:
If no load is connected the plate...
Invitation to instability.

And I have had to replace tube sockets that were burned by arcing, no doubt because of no load and high volume.

That's why it is so critical to perform the switching with no signal to the amplifier, as CaveRat suggests. No signal input => no output. No output => no feedback signal, no arcing, no problem.

-David


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