Rogers S 100 amp

[MrHill]

Hello people, well I turn to you again for a little info on Rogers s100c and s100a amps, I am not sure of how these will work in a 4ohm 8ohm and 16ohm setup, if they can be use for all 3 in a speaker cabinet, or only 1 type ohm per unit. As a rule I have always been taught only 1 or the other but not 2 or even 3, but what I have read which is very little it appears that these amps are built for that so I need to have so real facts on this!!?

[David Anderson]

First, a minor point: The company is Rodgers, with a 'd.' This will help you in searches for information.

What gets confusing when talking about amps and load impedances is that tube and solid-state amps follow very different rules.

For SS amps, the speaker impedance limits are related to how much current the output stage can supply before it damages itself.

Say a SS amp can supply 20 Watts into a 16 Ohm speaker. That same amp might theoretically supply 40 Watts into an 8 Ohm load, and 80 Watts into a 4 Ohm load. But that 4 Ohm load is making the power supply and output stage work a lot harder (and hotter) to supply the necessary current. Past a certain current limit, and you can blow your output transistors or your power transformer.

The problem with musical instrument amps is that they didn't publish their ratings like you'd find with a separate commercial power amp where they would tell you in the instructions what the minimum acceptable load is. So with something like a Rodgers SS amp, you might want to look at the load it was connected to in a Rodgers tone cabinet. You could safely go higher (16 Ohm) in load impedance. It's at lower impedances where you might run into problems.

Looking at an S-100C schematic, it has a 4A fuse for the voltage rails, so that's a limit set by Rodgers. Running a couple of quick & dirty calculations, it seems to me that the amp would be safe at full power into an 8 Ohm load, but would exceed the 4A fuse rating into a 4 Ohm load. Maybe that answers the question.

[toodles]

The Rodgers S100 series of amplifiers can operate 4, 8, or 16 Ohm series, or any combination that provides a minimum load of 4 Ohms. Two 8 Ohm speakers in parallel will provide a 4 Ohm load.

The issue with combining speakers of different impedance on the same amplifier is that the lower impedance speaker will draw more current, and then there may be a balance difference between the two speaker systems--that might be a problem or might not be, depending upon the installation..

Note that sometimes within a single speaker system, drivers of different impedance can be used, for example, 8 ohm tweeters and midranges are sometimes used with 4 ohm woofers. It works because woofers are not usually as efficient as midranges and tweeters, and a good speaker system designer takes into account the differences. The system is rated for the lowest impedance of the drivers in the system.

[David Anderson]

When I ran the numbers, I was getting the highest possible Watts output with an 8 Ohm load. With 16, you are limited by the supply voltage, and with 4 Ohms, you are limited by the 4 Amp supply rail fuse -- unless I'm doing the math incorrectly.

[MrHill]

Thanks guys for your help with this , I will use 2 amps and not worry about the impedance problem. I have a 4 ohm woofer I will setup with 1 amp and my 8 ohms with another , I had plans to use the 2nd amp for another project but I will use something else for that. Thanks again.