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Author Topic: vintage JBL amp: balanced input issues  (Read 8216 times)

Jan Koelmans

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Re: vintage JBL amp: balanced input issues
« Reply #10 on: December 03, 2012, 03:38:26 PM »

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.

Ah! So the matching/bridging transformer in the manual is basically a ground loop isolator? And if I hook my balanced source up to the balanced inputs without a ground loop isolator, will there be any adverse effect other than risk of hum?

Hope not to annoy the hell out of everybody
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Jan Koelmans

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Re: vintage JBL amp: balanced input issues
« Reply #11 on: December 03, 2012, 04:01:58 PM »

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

But what about balanced out to balanced in, in this case?
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Ivan Beaver

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Re: vintage JBL amp: balanced input issues
« Reply #12 on: December 03, 2012, 07:02:36 PM »

Also, there is no such thing as a "balanced" loudspeaker.  Loudspeakers are inherently single-ended.
I might argue that a amp/loudspeaker in bridge mode would be "balanced".  At least as much as a mic or signal line would be.

But we are going to get into real technical "situations" regarding answers in that regard.

And it depends on the description of "balanced".
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Tim Perry

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Re: vintage JBL amp: balanced input issues
« Reply #13 on: December 03, 2012, 08:34:55 PM »

Ah! So the matching/bridging transformer in the manual is basically a ground loop isolator? And if I hook my balanced source up to the balanced inputs without a ground loop isolator, will there be any adverse effect other than risk of hum?

Hope not to annoy the hell out of everybody

It does perform that function. Also in the era of this amp tube type mixers were still plentiful and in service.  This usually required a 600 ohm balanced load for proper operation.
the transformer option allow several type of input configurations. (see fig 2 on manual)

It appears to me that the balanced input terminals will not function without the transformer option unless perhaps someone has modified the unit(s).


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Mac Kerr

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Re: vintage JBL amp: balanced input issues
« Reply #14 on: December 03, 2012, 09:11:04 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. 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!

Without the input transformer the amp is unbalanced in only. You may as well use the RCA input. If the input cable run is very long (more than 20') you may want to add an external input transformer to keep the signal line balanced. There are excellent transformers available from a number of sources, Jensen being the most respected.

On the output side it is fine to use the 4Ω output to drive 8Ω speakers. Using a transformer so you don't put too high an impedance load on a tube amp is important, this is a transistor amp, and should be capable of driving an 8Ω load.

I assume the speakers have been restored. They are more than 30 years old and the surround won't have lasted that long.

Mac
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Jan Koelmans

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Re: vintage JBL amp: balanced input issues
« Reply #15 on: December 04, 2012, 06:44:20 AM »

Without the input transformer the amp is unbalanced in only. You may as well use the RCA input. If the input cable run is very long (more than 20') you may want to add an external input transformer to keep the signal line balanced. There are excellent transformers available from a number of sources, Jensen being the most respected.

On the output side it is fine to use the 4Ω output to drive 8Ω speakers. Using a transformer so you don't put too high an impedance load on a tube amp is important, this is a transistor amp, and should be capable of driving an 8Ω load.

I assume the speakers have been restored. They are more than 30 years old and the surround won't have lasted that long.

Mac

Very very helpful, many thanks!

Now I have one last question. I've also been using a BGW 750C amp (again about the same age) with the same balanced source. It also has transformer sockets for balanced operation, just like the JBL's.

Although that amp should have unbalanced inputs without input transformers aswell, I did notice a very significant change in sound (which I percieved as an improvement) when I changed from input through unbalanced phone jacks to putting a balanced signal into its XLR's.

So, A: how might that be explained and B: is there a chance the JBL's might behave in the same way?

I may have come to a point where trial and error is the only way forward but I like to be sure before I connect anything..
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Jan Koelmans

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Re: vintage JBL amp: balanced input issues
« Reply #16 on: December 04, 2012, 06:46:39 AM »

I assume the speakers have been restored. They are more than 30 years old and the surround won't have lasted that long.

Yes they have new surrounds and they sound very good indeed
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Geoff Doane

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Re: vintage JBL amp: balanced input issues
« Reply #17 on: December 04, 2012, 08:41:13 AM »


Now I have one last question. I've also been using a BGW 750C amp (again about the same age) with the same balanced source. It also has transformer sockets for balanced operation, just like the JBL's.

Although that amp should have unbalanced inputs without input transformers aswell, I did notice a very significant change in sound (which I percieved as an improvement) when I changed from input through unbalanced phone jacks to putting a balanced signal into its XLR's.


My recollection of these older amps is that there was usually a dummy plug in the transformer socket so that you could use the XLR input (un-balanced of course).  Depending on the output topology of the device feeding the amp, the different methods of unbalancing the line between the RCA and XLR inputs could account for the different sound.

I'm thinking particularly of a floating transformer output.  If only one leg of that balanced line is connected to the RCA input (along with ground), the sound is likely to be very thin and low in level, compared to using both legs of the "balanced" output, one to "hot" and one to ground at the amplifier.  Using the XLR input with the appropriate dummy plug, lowers the chance of having a wiring error.

As for the speaker, I think perhaps the term "differential receiver" might describe it best.  It works much the same as the primary of a transformer, after all it is just a coil of wire.  :)

GTD
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Re: vintage JBL amp: balanced input issues
« Reply #17 on: December 04, 2012, 08:41:13 AM »


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