Amp for 20 year old speakers?
August 5, 2004 3:49 PM   Subscribe

I've inherited some Polk Audio speakers - old ones from the 80s, and I need to get an amp to drive them. I don't know much about stereos. I think they are Monitor 7 series speakers, based on the model number inscribed on them. I'm taking them to Burning Man, so the amp should be cheap, but capable of making the music loud. I don't want to spend lots on a high-end amp. Just something that will work. What does "Recommended Amplifier Power 20-150 w/channel" mean?
posted by scarabic to Sports, Hobbies, & Recreation (12 answers total)
The upper end of the "Recommended Amplifier Power" is the maximum number of Watts you should inflict on them. You can go higher, especially if you're using a quality amplifier, but you do so at your own risk. The lower limit is more or less meaningless unless possibly with less than 20 watts the voice coil doesn't generate enough force to move the speaker very linearily.

Go to a pawn shop and get an amplifier that's 150 Watts or less (or bigger, just don't turn up the volume past the point where the speaker starts distorting).

Different amps of the same actual power output won't make a difference in sound volume. A better amp will be cleaner but it doesn't sound like you care about that.
posted by substrate at 4:42 PM on August 5, 2004

A note (and forgive me if I'm stating something you consider obvious):
You cannot plug a CD-player, Cassette deck, or any other source, directly into an amplifier. Instead, the source is connected to a pre-amplifier, and then the pre-amp is connected to the amplifier, which is then in turn connected to the speakers. If you don't have a pre-amp, and don't want to buy one, you can purchase an integrated amp (pre-amp + amp) or a stereo reciever (tuner + pre-amp + amp).

Also, I definitely agree with substrate that buying used equipment is definitely your best bet.
posted by kickingtheground at 4:57 PM on August 5, 2004

Response by poster: Ah - I guess I've only ever encountered amps that have built-in preamps, kickingtheground. If it has RCA plugs in the back of the amp, is that all I need? How can I tell if that 150W amp at the pawn shop has an integrated preamp? Is the presence of a radio tuner inside it any indication? How about the presence of a phono/aux/tape selector? Will that tell me anything?
posted by scarabic at 5:06 PM on August 5, 2004

Response by poster: also -

What does the pre-amp do?

I'm mainly planning on plugging in my MP3 player via , line-out jack to a miniplug/RCA stereo splitter. Any special considerations?
posted by scarabic at 5:09 PM on August 5, 2004

You cannot plug a CD-player, Cassette deck, or any other source, directly into an amplifier.

1. This is most certainly NOT true with many if not most modern components. You will want a preamp and a dedicated amp if you have many sources (e.g., a CD player and a tape deck) because a dedicated amp has one, sometimes two inputs. Otherwise, unless your source's line voltage levels are lower than normal, you can plug it right into the dedicated amp. Besides, if you're not specifically in the market for a dedicated amp, it is likely that most amps you encounter are integrated anyway.

2. All speakers have an impedance rating, which is a joint measure of its resistance and capacitance. Most speakers are rated at 8 Ohms, but some (big ones?) are 4 or 2 ohms. All speakers with bare wires coming out of them should have a rating printed on the back. All amps also have such a rating printed on the back. A speaker with higher impedance plugged into an amp rated for lower impedance is no problem; the reverse can potentially fry the amp.
posted by azazello at 5:27 PM on August 5, 2004

almost any amp with a volume control, input sockets, and the appropriate power rating will do.

a special preamp was needed for vinyl to correct for non-linear cartridge responses. otherwise, it's just extra amplification and volume/tone controls (easier to change the tone at low power and then amplify the signal you want to hear than try to mess with a high power signal).
posted by andrew cooke at 5:30 PM on August 5, 2004

(in case that wasn't clear - if it has a volume knob, it has a preamp)
posted by andrew cooke at 5:32 PM on August 5, 2004

Response by poster: Ah, thank ye all. I think I'm ready to go shopping!
posted by scarabic at 5:46 PM on August 5, 2004

2. All speakers have an impedance rating, which is a joint measure of its resistance and capacitance.

OK, this isn't quite relevant but... impedance is frequency-dependent resistance to current flow. It's the effective resistance (in ohms) that a signal encounters, and depends on the resistance, capacitance, and inductance of the speaker, along with the frequency of the applied signal.

Sorry. Sometimes the pedant in me takes control.
posted by lalas at 6:19 PM on August 5, 2004

Response by poster: Electricty baffles me (probably why I barf on speaker-specs pages). So many of the measurements we encounter are combinations of or relationships between otherwise unintuitive, seemingly arbitrary concepts like "resistance." It's hard enough assimilating a concept like resistance without having to twice-bake it into impedance and work with that, too.

Thanks for the note, though!
posted by scarabic at 6:40 PM on August 5, 2004

Most amps will have the impedance on the back. Most consumer amps are 8 ohms and so are the speakers you linked to. Don't worry about what impedance means unless you're really interested. If you match the numbers you'll do ok.
posted by substrate at 7:36 PM on August 5, 2004

All you need to know about impedance: you can damage or destroy an amplifier by trying to drive a speaker load that is below the amp's minimum impedance rating.

This is a notoriously murky topic, but here is a good way to think of it - and I'd consider it a favor, if I have this wrong, for someone to explain it to me properly. Think of impedence as electrical resistance. Suppose an amplifier is designed to deliver 200 watts of power into a load of 8 ohms. Now, connect a speaker load of 4 ohms and turn the amp up. It will keep trying to drive a bigger load than is actually connected to it, and may overheat and let the magic smoke out or something.

As substrate says, this is not usually an issue with consumer stereo equipment, since it is usually all compatible.
posted by crunchburger at 8:25 PM on August 5, 2004

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