Amplifier Quesions

[TallPaul]

Hi, I just installed an old Radio Shack Optimus 2x50, 100 watt amp in my pickup. It is a model 120-1971 and I think it's pretty old, but it works very well, great sound (to my ears anyway //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif), and has full MOSFET power supply.

I am running a portable AAA battery powered mp3 player with the headphone lead plugged directly into the amps Low Level RCA inputs. So the only volume control I have is the one on the MP3 player.

1) Do I have to abide by the 2/3 volume rule for maximum output (to avoid overdriving the amp), or is this not a typical case? It seems to sound fine at higher volume levels like 80 percent of max. I would think there is no preamp in that the mp3 only has an AAA battery and it is hooked direct to the 100 watt amp, so I should be able to run the mp3 at full volume and not hurt the amp, right?

2) What should I set the amps input level control to, lowest, 2/3 of max, max? does it matter?

3) The amp can run 4 or 8 ohm speakers. I have two 4 ohm 5x7s on it now. Can I run 4 ohm on one channel and 8 ohm on the other (use one channel for heavier bass in a small cabinet speaker from household stereo)? It only has outputs for two speakers.

4. I have the amp powered anytime the ignition is on or in accessory. Is it harmful to the amp to have it on and not play music?

5. Amp says max current draw is 15 amp. I am running a 10 amp fuse on it and it has not blown even at 80 percent max volume. would it be correct to assume the 10 amp fuse would blow before the amp exceed 2/3 of it's maximum output, and if that's the case, then I should be able to turn the volume up all I want and the fuse would protect the amp?

Other specs for the amp:

10% THD Output Power @ 14.4 VDC 1kHz at Stereo Mode is 50W x 2

freq response is 20 - 20,000 Hz +/- 3 dB

Input impedence is 10K ohms

Input Sensitivity for 10% THD Output Power is

250mV for Low Input and 2.5 V for High Input.

Speaker Impedance is 4-8 ohms

I am running it though the factory stock 5x7 pillar speakers in a '95 F150. I think Ford put speakers that would handle their higher end stereo as they seem to handle this amp fine, so far, but only have been running it a couple hours of driving time so far.

Thanks in advance for all your help.

This setup cost me $20 //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif : $2 amp (garage sale), $6.50 plus $6 shipping for MP3 player (ebay), and $5 for headphone to RCA adapter. The rest I had laying around the house.

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[DNick454 10+ year member]

As long as the gain on the amp is set so that the amp is putting out its full power at full volume on the mp3 player and no more... then you're safe. Kind of a weird setup, but whatever works, works I guess.

 

[TallPaul]

Yeah its weird because I have no bass and treble controls other than selection of the equalizer on the MP3, which I have set to DBB (I think it means double bass boost as it's the heaviest bass of any setting on this MP3).

Someone told me I can measure output voltage at the speaker terminals (disconnect speakers) at full volume and then resistance, from which I can calculate wattage to see if it is exceeding. I suppose it is only 25 watts RMS per channel.