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Title: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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!
Title: Re: vintage JBL amp: balanced input issues
Post by: Ivan Beaver 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.
Title: Re: vintage JBL amp: balanced input issues
Post by: Tim McCulloch 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.
Title: Re: vintage JBL amp: balanced input issues
Post by: Patrick Tracy 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.
Title: Re: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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..
Title: Re: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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...
Title: Re: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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...
Title: Re: vintage JBL amp: balanced input issues
Post by: Patrick Tracy 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 (http://repforums.prosoundweb.com/), is probably a better fit.
Title: Re: vintage JBL amp: balanced input issues
Post by: Tim Perry 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.
Title: Re: vintage JBL amp: balanced input issues
Post by: Patrick Tracy on December 03, 2012, 03:10:15 PM
And of course there's ye olde Rane Note 110 (http://www.rane.com/note110.html).
Title: Re: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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
Title: Re: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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?
Title: Re: vintage JBL amp: balanced input issues
Post by: Ivan Beaver 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".
Title: Re: vintage JBL amp: balanced input issues
Post by: Tim Perry 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).


Title: Re: vintage JBL amp: balanced input issues
Post by: Mac Kerr 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
Title: Re: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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..
Title: Re: vintage JBL amp: balanced input issues
Post by: Jan Koelmans 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
Title: Re: vintage JBL amp: balanced input issues
Post by: Geoff Doane 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