Audio Amplifier Question

[TallPaul]

I have a 100 watt stereo automotive amp (Radio Shack Optimus) that may date to 70s or 80s. It has MOSFET technology FWIW.

The question: Is this a single 100 watt amp that drives two channels at 50 watts each, or is this two 50 watt amps, each driving a channel?

Since one channel has a loose connection internally and cuts out at times (usu over big bumps), I was thinking of running mono (have adapter to combine the stereo signal) into the one good channel and hook both speakers to that side, leaving the other side unused (but powered).

If I do this:

1. Will I lose volume by having two speakers (assume 8 ohm each) hooked to one side (will it total 16 ohm?)?

2. Or will all 100 watts drive the one channel being used?

You might think I should just buy a new amp, but I am a cheap son of a gun (paid all of $2 for this amp at a garage sale and it gives me great pleasure to run it, besides, the sound is fine with me--loud and clear to my ears).

Go ahead, help me out, shoot me down, laugh, whatever, just keep the thread going and hopefully I'll come out the wiser.

 

[exranger06]

If you're hooking up 2 speakers to one channel, you can connect them either in series or parallel. Connecting in series would cut the power of the amp (2 8-ohm speakers make 16 ohms total). Connecting in parallel would basically double the power (2 8-ohm speakers are 4 ohms total). Most car speakers are 4 ohm, not 8 ohm. So, connecting them in parallel would give you 2 ohms. The amp may not be 2 ohm stable, and you would be stuck with wiring them in series. I would Google the model number to try to find more info on it.

 

[97tbird]

hm- at least in home audio world, 2-ohm stable amps are beasts!! and not very common...don't know how it is in the car audio world..

 

[Jimmy9190]

It's two separate amps in one chassis. The 50 watts may or may not be the real output power, it is probably more like 30 or 35 watts at a decent volume level. The higher the volume the greater the distortion, especially with amplifiers of that vintage.

If you use an adapter and connect 2 low-level RCA inputs to the one working channel of the amp, it won't really be a mono output. It will be either a right or left channel only, depending on which channel of the amp is still working. You will lose stereo separation and a lot of music will sound really bad, especially older recordings. Parts of the musical program will be missing because they are engineered and recorded in the missing channel of your amp.

Monaural signals are a summation of left and right into one channel, which is done to increase the output power of the amp. On older amps it usually very dramatically increased the distortion too, depending on the volume level. Mono signals are almost always used for subwoofers only, because of their increased output power.

If you connect 2 speakers to the one working channel of your amp it will actually increase the output power in watts. Distortion will also increase. Volume is a logarythmic function, not a linear one. That is to say, to increase the sound in decibels by two times as loud, you need to increase the output power in watts by ten times.

Two 8 ohm speakers wired in parallel on the one working channel will show the amp a resistance, or load of 4 ohms. All you have to do is apply Ohm's Law, and multiply the resistance of each speaker then divide that by the sum of the total amount of resistance of the speakers: 8 x 8=64/16=4 ohms total resistance. The Optimus amp might handle the load OK but it's not a good idea electronically speaking to continue to drive the amp with one channel being inoperative.

If you want to keep the amp, a good car audio shop could probably repair it for you, or at least give an estimate on repairs. It could be a simple problem like a cold solder joint on the circuit board and not cost much to fix it. OTOH, good quality amps nowadays don't cost much brand new.

 

[TallPaul]

Yeah, I guess a new amp makes a lot of sense, but I don't think they even have small ones of 100 watts anymore.

I would use a stereo to mono adapter that has electronics to combine the two channels into one. I listen to mono on my mp3 player all the time in one ear and it sounds fine.

The loose connection is where the RCA input block mounts it wiggles and the bad connection is under it where you can't easily get to it. I wedged it and it was good for a while, but one big bump upset things. Maybe try a bigger wedge.

Yeah, what happens if an amp is powered and not used, is it bad? Seesm not much differrent than running music at very low volume.

I'll have to check the ohms and all that stuff tonight.

 

[Papa Bear]

Some amps are bridgeable, what model is it ?

 

[Jimmy9190]

Sounds like either there is a cold solder joint or a bad RCA jack on the circuit board for the input. Either one would probably not cost much for parts and labor to fix it.

 

[TallPaul]

Oooooh, I totally forgot to look at the amp last night. I'll look tonight. There is a kid down the block that is very hand with electronics, maybe should have him look at it.

 

[TallPaul]

It's a Radio Shack Optimus 100 watt Stereo Amplifier,
Model 12-1971A. Suspect that may be the date too.

 

[Propflux01]

Most RS amps of that vintage were made for 8ohm speakers ( which RS seemed to be the only ones selling auto audio in 8 ohm) but would run a 4 ohm speaker quite well. If you look at the distortion ratings for those years vs today, you won't see too much difference, really. If its the RCA input thats giving you problems, it should be a relatively easy fix which you could prolly do yourself with a solder iron. I wouldn't run it as a 'mono' amp, I would fix it and use it as intended.

 

[TallPaul]

There is another kind of input on the amp, forgot what it's called, but not sure it would work for me. I am running a AAA battery operated mp3 player direct into the amp. I am amazed the OEM speakers in the '95 F150 take all I can dish out with this amp. I figure am not putting out much more than 20 watts per channel or so. ( Tested it with small fuzes like 2 amp and 3 amp, turning up till they blew to calculate the wattage drawn. Am running a 7 amp fuse even though it takes a 15 amp.)

 

[PandaBear]

Maybe what you can do is open up the chasis and see where the "loose connection" is and resolder it, or if you aren't handy just wait till you find another amp that has 1 broken channel and use one amp per speaker.

Unless you turn your volume up very high, running them in parallel would still be ok.

 

[TallPaul]

The problem is that the loose connection is under the plastic block that has the RCA inputs. No way to get at it, maybe replace the whole block. I re-wedged it a little tighter so maybe it won't cut out any more or much, I can live with it. I certaily will keep my eyes open for more garage sale stuff, but you can see why this amp was at the garage sale.

Thanks Panda. I also could protect the amp as I am by running a lower amperage fuse that would blow well below it's capacity.