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Author Topic: vintage JBL amp: balanced input issues  (Read 1780 times)

Jan Koelmans

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vintage JBL amp: balanced input issues
« on: December 03, 2012, 08:43:56 am »

Hi,

I'd like to use two revised JBL 6012 mono amps (~1978) to drive my Tannoy Hpd315 monitors (Devon cabs) from the same era. Not being an expert at all, the instructions in the manual are not quite clear to me (http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/JBL-6011~6012.pdf).

I'd like to use the balanced inputs of the amps, but the amps don't have the matching/bridging transformers installed (nor output transformers), and the manual seems to indicate they are neccesary for using the balanced inputs.

Would it be advisable to connect the amp's balanced inputs to a balanced 600 ohm source and drive 8 ohm speakers without matching/bridging transformers (and output transformers)? What negative effect may come from the missing matching/bridging transformers? Just some loss of gain or no/bad sound? Extreme SPL's are not required but I am hoping to get good sound from them..

Any other considerations I should be aware of?

Thanks a lot for any ideas on this!
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Ivan Beaver

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Re: vintage JBL amp: balanced input issues
« Reply #1 on: December 03, 2012, 11:51:13 am »

Hi,

I'd like to use two revised JBL 6012 mono amps (~1978) to drive my Tannoy Hpd315 monitors (Devon cabs) from the same era. Not being an expert at all, the instructions in the manual are not quite clear to me (http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/JBL-6011~6012.pdf).

I'd like to use the balanced inputs of the amps, but the amps don't have the matching/bridging transformers installed (nor output transformers), and the manual seems to indicate they are neccesary for using the balanced inputs.

Would it be advisable to connect the amp's balanced inputs to a balanced 600 ohm source and drive 8 ohm speakers without matching/bridging transformers (and output transformers)? What negative effect may come from the missing matching/bridging transformers? Just some loss of gain or no/bad sound? Extreme SPL's are not required but I am hoping to get good sound from them..

Any other considerations I should be aware of?

Thanks a lot for any ideas on this!
The input and output transformers are for completely different uses.

You will need a LINE LEVEL (NOT MIC LEVEL) transformer for the input.  Just put it close to the amp-but depending on the transformer you might want to have it a couple of feet away to help reduce hum induction into the transformer.

The output transformer (for solid state) is for stepping the voltage up to a 70V line.  So no need for it with 8 ohm speakers.
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Tim McCulloch

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Re: vintage JBL amp: balanced input issues
« Reply #2 on: December 03, 2012, 12:33:08 pm »

The input and output transformers are for completely different uses.

You will need a LINE LEVEL (NOT MIC LEVEL) transformer for the input.  Just put it close to the amp-but depending on the transformer you might want to have it a couple of feet away to help reduce hum induction into the transformer.

The output transformer (for solid state) is for stepping the voltage up to a 70V line.  So no need for it with 8 ohm speakers.

Also, there is no such thing as a "balanced" loudspeaker.  Loudspeakers are inherently single-ended.
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Patrick Tracy

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Re: vintage JBL amp: balanced input issues
« Reply #3 on: December 03, 2012, 01:23:38 pm »

Hi,

I'd like to use two revised JBL 6012 mono amps (~1978) to drive my Tannoy Hpd315 monitors (Devon cabs) from the same era.

Seems like odd gear to use for live sound.

Jan Koelmans

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Re: vintage JBL amp: balanced input issues
« Reply #4 on: December 03, 2012, 02:43:46 pm »

The input and output transformers are for completely different uses.

You will need a LINE LEVEL (NOT MIC LEVEL) transformer for the input.  Just put it close to the amp-but depending on the transformer you might want to have it a couple of feet away to help reduce hum induction into the transformer.

The output transformer (for solid state) is for stepping the voltage up to a 70V line.  So no need for it with 8 ohm speakers.

Does that mean that the amp will not accept balanced input without an input transformer? Because I get the impression from the manual that it will... Or is the Matching/bridging transfomer that it mentions actually some kind of balun/DI box, meaning that the amp is in fact not suited for balanced input as it is?

Bear with me here..
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Jan Koelmans

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Re: vintage JBL amp: balanced input issues
« Reply #5 on: December 03, 2012, 02:46:42 pm »

Also, there is no such thing as a "balanced" loudspeaker.  Loudspeakers are inherently single-ended.

I'm aware of this, sorry if I suggested otherwise...
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Jan Koelmans

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Re: vintage JBL amp: balanced input issues
« Reply #6 on: December 03, 2012, 02:56:09 pm »

Seems like odd gear to use for live sound.

Yes, sorry about that. I have no such intention ofcourse. May have come to the wrong place, but appreciate any help. Hopefully it does qualify as Pro gear, FWIW...
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Patrick Tracy

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Re: vintage JBL amp: balanced input issues
« Reply #7 on: December 03, 2012, 03:07:40 pm »

Yes, sorry about that. I have no such intention ofcourse. May have come to the wrong place, but appreciate any help. Hopefully it does qualify as Pro gear, FWIW...

The sister forums, R/E/P, is probably a better fit.

Tim Perry

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Re: vintage JBL amp: balanced input issues
« Reply #8 on: December 03, 2012, 03:08:12 pm »

Back in the days we were watching Fred and Wilma roll into the drive-in some amplifiers (like these) came with sockets for optional plug in balancing transformers.   At they time they were fairly expensive options.

If you need balanced and are not concerned about keeping it vintage, simply add a radio shack 'ground loop isolator' . This will easily plug in to the exiting RCA inputs.

In many cases no adverse effect will occur by simply plugging mixer out to unbalanced in.
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Patrick Tracy

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Re: vintage JBL amp: balanced input issues
« Reply #9 on: December 03, 2012, 03:10:15 pm »

And of course there's ye olde Rane Note 110.
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