How To "Bridge" A Stereo Amp to Just One Channel

[carltonmdb]

As you already know, for bridge mono operation the mono L/R input polarity must be reversed.
1) The "+" from the right amp output (1/4" Tip) connects to the speaker "+", the "+" from the right amp output (1/4" Tip) connects to the speaker "-".
The amp "-" (Sleeve) connections are not used for bridged mono operation.
Each amp side will "see" half the speaker impedance, so the single 8 ohm speaker will be equivalent to a four ohm loads on each amp side.
2) No.
3) The amp "-" speaker outputs are tied to chassis ground if I'm reading the schematic correctly.
4) Check that the two "-" (Sleeve) connections do share a common connection, they should read very near 0 ohms resistance between them.

If the amp has a stereo chorus, it might sound like a tremolo, as in the volume of the signal will rise up and down.

Thanks for the response!


1) Guess I'm not following this; Wouldn't the "+"/tip from the RIGHT would go to the positive terminal of the driver, and "+"/tip of the LEFT channel go to the negative/"-" terminal of the driver. The response reads to me as I'd essentially be bridging just the RIGHT channel across both "+" and "-" terminals, which would be exciting? Maybe a typo in the 2nd half of your first sentence, and it's supposed to be LEFT channel?
2) Got it, sleeve wiring in cable is ignored at cabinet end, meaning nothing hooked to sleeve of jack in cabinet.

3&4) I will confirm before test firing. If they are not, do they need to be in order to work? And is so, can a simple jumper be used, or is it a more complicated (ie designed into circuitry) connection?



I don't plan to use the input on the amp, rather the stereo effect loop return. According to the manual Fender sent, using the FX return as the input will "...automatically disconnects the preamp, reverb and chorus circuitry when used. This is useful when using the M-80 CHORUS as a slave amplifier..."



Thank you again for the response and info, truly appreciated!

 

[JMFahey]

Please state the *full* model name, as sadly there are many absolutely different amps all called M80 , from an 80W 1 x 12" obviously mono amp, to a stereo ??W 2 x 12" Chorus amp to a 160W BASS amp , maybe more.
Or link to its user manual.

 

[carltonmdb]

Please state the *full* model name, as sadly there are many absolutely different amps all called M80 , from an 80W 1 x 12" obviously mono amp, to a stereo ??W 2 x 12" Chorus amp to a 160W BASS amp , maybe more.
Or link to its user manual.

Sure, its a Fender M-80 Chorus, head only version. Links below are a quick g



Manual - FENDER ULTIMATE CHORUS OWNER'S MANUAL Pdf Download.


Schematic (DL at own risk, appears OK for me but...)- FENDER M80 CHORUS SCH Service Manual download, schematics, eeprom, repair info for electronics experts

 

[weltersys]

Thanks for the response!


1) Guess I'm not following this; Wouldn't the "+"/tip from the RIGHT would go to the positive terminal of the driver, and "+"/tip of the LEFT channel go to the negative/"-" terminal of the driver. The response reads to me as I'd essentially be bridging just the RIGHT channel across both "+" and "-" terminals, which would be exciting? Maybe a typo in the 2nd half of your first sentence, and it's supposed to be LEFT channel?

3&4) I will confirm before test firing. If they are not, do they need to be in order to work? And is so, can a simple jumper be used, or is it a more complicated (ie designed into circuitry) connection?

1) Whoops, late night cut and paste, "+"/tip from left and right are used.

3&4) There are amplifiers now built that have already been internally bridged, so may not be bridged- adding a jumper would short out the outputs

 

[PRR]

...a Fender stereo guitar solid state amp (M-80) that I want to use on a single driver (EVM 18B) in a sub enclosure. ...

The linked stereo amp is rated for 8 Ohm loading.

In Bridge Mono you must load with 16 Ohms.

While 16r EVM18s may exist they are not common.

The linked schematic does NOT show the power amp details. Looking at the layout it is just two devices per channel and I do not think it would be wise to ask each channel to drive 4 ohms (half a 8 Ohm speaker).

Second-hand BIG amps are not THAT expensive in most parts of the world. (Populated parts of the nutmeg state must be filthy with rock shops.) One strange option would be to find a 300W+300W amp with one dead channel... it would soak your EVM18 comfortably, while it is less attractive to a band looking to get mucho power per rack. You will have to ask, but most shops have a half-dead amp in the back which resists fixing.