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 Post subject: Two amps one set of speakers
PostPosted: Apr Tue 28, 2020 3:08 pm 
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New to the forum. As a kid I built three Heathkit amplifiers and recently decided to build a cabinet for all of my equipment. I have a Heath AR15, AR1500, AA1640, BIC turntable, TEAC A4010S real to real and a set of JBL Century L100 speakers.
The question I have is how I can use both amplifiers for different tasks. Currently the AR15 is not being used. I'd like to power the turntable with it running it through the JBLs. It doesn't have a pre amp out so I can't run it though the 1640 power amp. Wondering if I could wire each amplifier separately to the speakers and install diodes in the speaker leads to eliminate any feedback into the amplifiers. I don't want to risk any damage to this 70's equipment, hoping someone one on the forum will agree or tell me I'm crazy.
Thanks


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 Post subject: Re: Two amps one set of speakers
PostPosted: Apr Tue 28, 2020 3:34 pm 
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Heathkit wrote:
New to the forum. As a kid I built three Heathkit amplifiers and recently decided to build a cabinet for all of my equipment. I have a Heath AR15, AR1500, AA1640, BIC turntable, TEAC A4010S real to real and a set of JBL Century L100 speakers.
The question I have is how I can use both amplifiers for different tasks. Currently the AR15 is not being used. I'd like to power the turntable with it running it through the JBLs. It doesn't have a pre amp out so I can't run it though the 1640 power amp. Wondering if I could wire each amplifier separately to the speakers and install diodes in the speaker leads to eliminate any feedback into the amplifiers. I don't want to risk any damage to this 70's equipment, hoping someone one on the forum will agree or tell me I'm crazy.
Thanks


You can't do it that way, but it's possible to do it some other ways.

Simplest method is to use a manually operated switch to connect the speakers to whichever amplifier you are using at the moment.

It would also be possible to install a relay (or more than one relay) to automatically switch the speakers to whichever amplifier was on.

What you have to be very careful about, is not to apply power to the other amplifier while no speaker load is connected to it. If that's a concern, you also must include a method to prevent that from happening, or a method to connect load resistors to the output of the amplifier which isn't connected to the speakers in case someone does turn it on.

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 Post subject: Re: Two amps one set of speakers
PostPosted: Apr Tue 28, 2020 3:38 pm 
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Joined: Apr Tue 28, 2020 2:34 pm
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Thanks Dennis
I'll look into building a switch box.


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 Post subject: Re: Two amps one set of speakers
PostPosted: Apr Tue 28, 2020 5:17 pm 
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Location: Seattle WA US
A switch box is an invitation to disaster. You need to avoid a situation where you can disconnect an amp from a speaker while the amp is delivering full power. BOTH amps should be set to minimum gain, or powered off, before you change speaker connections.

Suggestion: connect your best performing equipment in a single string, with good connections. Put the remaining gear on a shelf in a nice dry place.

-Chuck


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 Post subject: Re: Two amps one set of speakers
PostPosted: Apr Tue 28, 2020 5:52 pm 
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You could get an external phono preamp, and I would suggest that if that would NOT require speaker switching.

Switching speakers has to be done very carefully as has been noted. Of course, if you can remember to turn the volume on each amplifier DOWN all the way before switching speakers, that might be safer, but you still run the risk of momentarily cross-connecting commons (grounds) with possible smoke-releasing results. You'd have to switch both + and - connections to each speaker and to each amp, with break before make contacts.

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 Post subject: Re: Two amps one set of speakers
PostPosted: Apr Tue 28, 2020 8:57 pm 
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Based on the advice given I might be better off locating a set of vintage speakers and run the AR15 and turntable completely separate from the AR1500 and AA1640. Thanks everyone for your advice.


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 Post subject: Re: Two amps one set of speakers
PostPosted: May Fri 01, 2020 9:47 pm 
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Location: Boston, MA USA
Back in the day every audio salon or listening room -- even those within Radio Shack -- had a large switch panel that could connect any turntable to any amplifier to any speaker. You could play with different components by manipulating the switches. I remember auditioning speakers that way. There was obviously a way to do this safely.

-David


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 Post subject: Re: Two amps one set of speakers
PostPosted: May Fri 01, 2020 10:37 pm 
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Location: Sunbury, Ohio 43074
Yes, there is a way. The switch matrix to switch the amps and speakers was pretty expensive :)

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 Post subject: Re: Two amps one set of speakers
PostPosted: May Sat 02, 2020 5:39 pm 
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This might work
https://glass-ware.stores.yahoo.net/inseswandpcb.html


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 Post subject: Re: Two amps one set of speakers
PostPosted: May Tue 12, 2020 11:18 pm 
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All those fine amps and only the JBL for speakers? Nothing like a stacked pair to make it even better! :D

I had an AA15 from a pawn shop back when I was still living at home. Best amp I'd heard at that point in time. Too bad I blew the thing up when an errant tool drop shorted it out. My dad was never happier to see me wreck something. :roll:

Back in 2005, my wife was gone for a week and I had fun with hooking up all my gear at once. Every speaker pair had an amp driving it, something like 800-900 watts adding up all ten channels. :mrgreen:
-Ed


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 Post subject: Re: Two amps one set of speakers
PostPosted: May Wed 13, 2020 3:16 pm 
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I was amazed when I researched JBL L100s. I can buy new ones for $2k each and used ones for $500 each. Hard to justify when my recliner is positioned in the perfect position for a single pair of JBLs. But it sure would be fun to do what you're suggesting.


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 Post subject: Re: Two amps one set of speakers
PostPosted: May Tue 19, 2020 10:20 pm 
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Hello. First post on this forum but have been looking at stuff for a long time. Tons of great info on here!

Anyways, here is a solution to your two amp multiple speakers problem. I've had this switch box for a long time and never had any troubles switching two amps (tube and tran) and multiple speakers. It has power resistors in the box to prevent the amps from being unloaded when you switch.

I attached a couple of pics and a copy of the owners manual.

The original company was acquired by another company which was acquired by another company. This might be the right website as the Woods logo is the same as on the box. Looks like you have to sign up to view their products...geez. www woods com

They might come up on eBay occasionally.


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 Post subject: Re: Two amps one set of speakers
PostPosted: May Fri 22, 2020 12:31 am 
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Joined: Apr Tue 28, 2020 2:34 pm
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Cool,
Thanks


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