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Basic power amp question when running 2ch in stereo

B

Bassment Goblin

Mar 19, 2025
9
3
#1
This is a probably n00b question...

When running my 2ch PA in stereo mode (which I will do until I can find a 4 core speakon to TS cable so that I can use bridged mode), do I need to put a load (dummy or otherwise) on the channel that I am not using?

The PA's manual gives a value for 'one channel driven', if that helps, but I am not sure if that is just an 'in theory' spec vs something the PA supports... I am not sure of the class of amp it is - it is an Inter-M M500
 
#2
No, the other channel does not need a load.

You do not need a 4 core cable, 2 core with an NL4 plug, tip wired to 1+ and sleeve wired to 2+. Do not let anything touch the sleeve/barrel of the plug because that could short channel 2 output to ground.
 
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#5
I had a Fender Rumble 500 head for a while, which is always in bridge mode. The manual even mentions the shock hazard. So I measured the voltage on the plug cover and sure enough, it was hot. I found some D'Addario speaker cables with plugs sheathed with some rubberized material. Buy a cable like this, cut off one plug and replace it with a Speakon.

da_pw-cspk-05_main.png
 
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#6
@brianrost has a point. A quarter-phone termination on the bridged output of two amp channels is an accident waiting to happen. This is true of any amp. regardless of the circuit topology -- Class D, Class AB, or whatever. Besides insulating the connector's shell with tape or heat-shrinkable tubing, I'd advise never plugging or unplugging the quarter-phone end while the amp is powered up -- that would help minimize the risk of shorting the plug's shell connection to a ground reference.
 
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#8
Yeah... I might give this a miss.

Thanks for all of the tips (no pun intended). I think I might just live my life on the stereo channel - 130W into a 400W cab is better than frying myself (also, there are small children in the house).

Maybe modding my cab to speakon is a better option in the longer term? I note your earlier post:
Redbrangus said:
A lot of Class-D amps use a bridging topology at the output stage; this means that neither speaker cable conductor is at ground potential and shorting either conductor to a grounded conductive surface (like the chassis of the amp itself) is likely to fry the output devices. The fact that one of the conductors on a quarter-phone is on the shell of the connector makes it especially vulnerable to exactly this kind of unintentional, casual shorting to ground. If you have a well-made Speakon-to-quarter-phone speaker cable, you'll be OK as long as you never (i.e., NEVER EVER!) plug or unplug the quarter-phone end while the amp is turned on. Putting a layer of insulating tape or heat-shrink tubing around the barrel of the quarter-phone plug is also advised.

A far better (and safer) solution would be to change out the connector on the speaker cab -- it's easy enough to do and costs almost nothing. If you go that route, the NL4-MPR is the one with the large, round flange -- it mounts directly to an enclosure more easily and securely than the smaller, rectangular ones (NL4-MP).
Click to expand...

Out of curiosity, can you still daisy chain speakon to a second speakon cab? I'd assume so as it is just a connector. I calculate that I'd be able to get either 2 x 130W in stereo mode with 2 x 8 Ohm cabs (one on each channel), or 1 x 250W into 2 x 8 Ohm cab (4 Ohms total) if chained.

Final question: should I be worried about the current on my +1 and -1 speakon to 1/4" TS tip while in stereo (not bridged) mode? While I will definitely avoid plugging and unplugging while it is hot, could I still zap myself on that one? (again, small children in the house...)

Redbrangus said:
Something else to keep in mind: if that amp is rated for a 4Ω load per channel, you can only connect an 8Ω (or higher) load to the bridged output of the two channels.
Click to expand...

I read it as minimum 4 Ohm on the stereo, so I'm pretty sure I am ok. Correct me if I am wrong (manual below)

1743770354257.png


1743770425240.png
 
#9
Bassment Goblin said:
Maybe modding my cab to speakon is a better option in the longer term?
Click to expand...
Yes, if you can do that, it would be much better. I considered suggesting that in my earlier response, but I didn't want to complicate things.

Bassment Goblin said:
Out of curiosity, can you still daisy chain speakon to a second speakon cab? I'd assume so as it is just a connector. I calculate that I'd be able to get either 2 x 130W in stereo mode with 2 x 8 Ohm cabs (one on each channel), or 1 x 250W into 2 x 8 Ohm cab (4 Ohms total) if chained.
Click to expand...
You can still daisy chain speakers, but that's a parallel connection, so your two 8Ω cabs would be a 4Ω total load. You can plug both of those speakers -- daisy chained -- into one channel of that amp, but not into the bridged output of the two channels because that's only capable of driving an 8Ω load. It kind of looks like your best option is to just run one speaker on each channel in the stereo or parallel mode.

I wouldn't worry too much about the voltage hazard -- in fact, if you're not bridging the output of the two channels, the shell of a quarter-phone speaker cable isn't 'hot' in any way; it's at ground potential.
 
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#10
Redbrangus said:
in fact, if you're not bridging the output of the two channels, the shell of a quarter-phone speaker cable isn't 'hot' in any way; it's at ground potential.
Click to expand...

That's good to know. I already have a cable for that purpose (I used it this afternoon for the first time), so I will keep using that as my primary way of connecting my PA to my cab for now.

The longer term plan will be to move to speakon for my cab (if possible, which it should be). I think I'll be aiming for cabs with speakon already included in the future.

This whole speakon to TS thing is something that I've never considered before. As a guitarist, I've not had to deal with these sort of voltages (500W) before. 120W is the max that I regularly use, and I'd never hot-swap a speaker while the amp was on anyway.

Thanks to everyone for both answering my initial questions and for those really important safety tips (of which I was blissfully/dangerously unaware!)
 
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#14
Bassment Goblin said:
The PA's manual gives a value for 'one channel driven', if that helps, but I am not sure if that is just an 'in theory' spec vs something the PA supports..
Click to expand...
That allows them to pretend that the amp is rated for more power than it really is and in my view is a sign of a power supply that's not up to the task of driving both channels to rated power at the same time.
 
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#15
Redbrangus said:
I wouldn't worry too much about the voltage hazard -- in fact, if you're not bridging the output of the two channels, the shell of a quarter-phone speaker cable isn't 'hot' in any way; it's at ground potential.
Click to expand...

My voltmeter told me otherwise. I measured from the metal shell on the plug to a grounded point on the Rumble. I forget what I saw but it was significant voltage, not ground. Why would the manual mention a shock hazard if the shell is grounded?

Screenshot 2025-04-04 at 1.35.48 PM.png
 
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#18
edwinhurwitz said:
That allows them to pretend that the amp is rated for more power than it really is and in my view is a sign of a power supply that's not up to the task of driving both channels to rated power at the same time.
Click to expand...
Not at all, it’s a very important spec for biamp applications as well as stage monitor applications (and any non-coherent signal actually).
 
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#19
I though I remembered something strange about InterM amps from try to piece a bands PA together years ago.
I found your amp's manual and a rear picture. In bridged mode that amp does not use what is a "normal" Speakon connection of 1+ and 2+, it looks like it uses pin set 2 just and only for bridged operation connection using 2+ and 2-.
For the binding post connections it does use channel 1+ and 2+.

All said stay with operating that amp in stereo or parallel mode.
Keep in mind with an 8 ohm cabinet per channel you have ever so slightly more power per cabinet than two 8 ohm cabinets on channel.
Given that the is more of a MI grade amp the specs may be a little loose!
m500 amp back.jpg
M500 amp.jpg
 
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