View Full Version : JBL 6012 amp: balanced input issues

12-05-2012, 04:00 AM

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 JBL-UREI Electronics/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 possible 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)? IF so, 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!

12-05-2012, 09:28 AM
Shouldn't need the output transformer unless you want more than 50w/ch (assuming nominal 8 ohm speakers).

As to connecting a "600 ohm balanced source" directly to the HI and LO balanced inputs...
you would hear essentially nothing without an input transformer. If you were to connect
the source to the 50K input and ground, it might work, you might damage your source,
it might not work at all... it rather depends on the design of the source component or preamp.

If you are bent on using these amps and with balanced inputs, I'd start looking for
the appropriate input transformers (noting that other manufacturers may offer workable

12-05-2012, 09:41 AM

Your amp does not have electronically balanced inputs, so You would need a transformer. Good input transformers are expensive, cheap ones actually are just output transformers. See the website of Jensen Transformers.


12-06-2012, 09:11 AM
Hmmm looks like I'm having to let go of the balanced input thing...

Shame, would have liked to put the amps right by the speakers and drive them through balanced XLR!

Thanks anyway