PLX-1804 Parallel

[MarkA]

Hi, all.

I have a PLX 1804. Sometimes I run it with preamps that have an unbalanced, TS (tip-sleeve) output, sometimes with pres that have a balanced XLR output.

Am I correct in thinking that, to parallel the channels, I could use a male XLR to 1/4" TRS (tip-ring-sleeve) cable? If it works as I'm envisioning, I should be able to use it to connect channels 1 and 2 regardless of whether channel 1 is being fed an unbalanced signal via the 1/4" input or a balanced signal via the XLR input.

Am I missing anything? Are there any drawbacks or compromises which I'm not foreseeing?

Would there be any drawbacks to leaving such a cable connected, paralleling the inputs, if only one channel is being used? (If I recall and understand previous posts correctly, the input impedance of the amp would be halved -- not an issue with most pres, but it would be simple enough to connect/disconnect the cable and see if there's an audible effect.)

Thanks,
Mark

 

[Bob Lee (QSC)]

I have a PLX 1804. Sometimes I run it with preamps that have an unbalanced, TS (tip-sleeve) output, sometimes with pres that have a balanced XLR output.

Am I correct in thinking that, to parallel the channels, I could use a male XLR to 1/4" TRS (tip-ring-sleeve) cable? If it works as I'm envisioning, I should be able to use it to connect channels 1 and 2 regardless of whether channel 1 is being fed an unbalanced signal via the 1/4" input or a balanced signal via the XLR input.

Am I missing anything? Are there any drawbacks or compromises which I'm not foreseeing?

Would there be any drawbacks to leaving such a cable connected, paralleling the inputs, if only one channel is being used? (If I recall and understand previous posts correctly, the input impedance of the amp would be halved -- not an issue with most pres, but it would be simple enough to connect/disconnect the cable and see if there's an audible effect.)

Thanks,
Mark

Sure, you can use a cable like that. Then you could run either a 1/4-inch or XLR connector into the amp.

And yes, the two inputs being in parallel mean that the net input impedance is half that of a single input.

 

[MarkA]

Sure, you can use a cable like that. Then you could run either a 1/4-inch or XLR connector into the amp.

And yes, the two inputs being in parallel mean that the net input impedance is half that of a single input.

Thanks, Bob. I figured it would be identical, electrically, to using a TRS-to-TRS or XLR-to-XLR cable, but wanted to make sure I wasn't missing anything.

 

[bucephylus]

There's no reason you CAN'T maybe drill a hole in the chassis and install a DPST switch between the + and - input legs on both channels, if the reason(s) WHY are compelling enough. I wouldn't do it to a unit that is still under warranty, for one thing. And I would find using one of the short cables that BasturdBlaster linked to a LOT more conventient, making the whole mod thing irrelevant.

Thanks Bob.

I bought a couple of 3PDT switches to let me switch between stereo and parallel; but, when I got inside the chassis, I found there were no great solder points without making a mess of things. Wouldn’t have guessed that both sets of jacks were mounted on a little PWB; but, I can see the manufacturing logic.

Anyhow, this is what I ended up with; which works just fine for me, since I only need a 1/4” input.

BTW, GREAT amp!! Really digging the way it sounds with my bass. I suppose the bass market was too small; but, it’s a real shame the unit went out of production.

 

[Bob Lee (QSC)]

Thanks Bob.

I bought a couple of 3PDT switches to let me switch between stereo and parallel; but, when I got inside the chassis, I found there were no great solder points without making a mess of things. Wouldn’t have guessed that both sets of jacks were mounted on a little PWB; but, I can see the manufacturing logic.

Anyhow, this is what I ended up with; which works just fine for me, since I only need a 1/4” input.
View attachment 3479008

BTW, GREAT amp!! Really digging the way it sounds with my bass. I suppose the bass market was too small; but, it’s a real shame the unit went out of production.

That's a pretty tidy solution! Very nice.

 

[MarkA]

^^^ Coincidentally, I just today picked up my 1/4"-TRS-to-male-XLR cable (from the area mom 'n' pop shop that assembled it for me). I haven't tested it yet (and don't have any photos), but I expect it to work in a straightforward enough manner.