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Topic: Looking for advice on a modest amp for a pair of scavenged speakers (Read 759 times) previous topic - next topic
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Looking for advice on a modest amp for a pair of scavenged speakers

I have a pair of speakers from an old late-90s PHILIPS all-in-one "hi-fi" thing (CD, radio, double tape, aux). I wasn't interested in the main unit, but couldn't sell it either, as it wasn't in good shape (melted belts on the tape decks, the CD tray had lots of trouble sliding in and out, and the amp was very noisy even a low volume). Can't remember the model now, but it didn't go for much money at all on eBay, so I didn't see the point of going through the trouble and expense of fixing it (learning on the go, at that). But I kept the speakers and now I would like to find a modest cheap amp to make some use of them.

The label says this:
Quote
SPEAKER SYSTEM
FB 332/352
4303 308 40540
IMPEDANCE 3 OHM
MADE IN HUNGARY

I'm sure they aren't anything special at all. I don't really know much about these things and I just stuck to headphones for most of my life. I won't be blasting them very loud at all, living in an apartment building, and not wanting to go deaf early... or breaking them, so I figure I can do with something rather cheap... I guess. I was hoping for something around 20-30 EUR, perhaps a little more if necesary but not much higher, given my finances.
I supose that, at this price range, a unit with both analog input and also a DAC is out of the question? I don't have much of an idea, having never shopped for this kind of stuff.
I just want something modest for a non-demanding kind of use, something that won't die in two months. Add some rubber feet under the speakers, and that's it.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #1
Well, with a nice case and such they usually don't  go for less than 50 EUR. Something like this for example:
https://www.ebay.co.uk/itm/265709090574

You can go cheaper with amps made for cars, but then you'd need a separate 12V power supply of course.
Music: sounds arranged such that they construct feelings.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #2
50 is way too high. I can do without the case. I do have some materials to make one myself if needed.
I'll look into the car amps. Thanks for the advice. Any particular spec I should look for, or avoid?
I think I do have some 12V supply lying around, not sure about the amperes, though.

Actually, that model isn't even that convenient to me. Not that I couldn't make an adaptor, but these speakers have the bare wires you need to put in these mechanical catches, whatever they are called. As for input, it's not much of a problem, but I would prefer a jack, perhaps, though I admit RCA is usually more sturdy.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #3
One thing to watch out for in particular is that the speakers are labeled 3 Ohm. Usually amps like 8 Ohm and most can be persuaded to go down to 4, I don't know about 3. Maybe search the web for "speaker impedance".
Music: sounds arranged such that they construct feelings.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #4
That means they could burn the speakers?

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #5
That means they could burn the speakers?

The amp will be one that gets stressed more and actually more at risk of being damaged.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #6
The amp will be one that gets stressed more and actually more at risk of being damaged.

Oh. Goes to show how little I know about this.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #7
At least back in the old days, car speakers were 4 ohm and car amps were targetting that. I don't know about how the "Class D" (switch-mode) amps fare, but I see things like https://www.aliexpress.com/item/1005001296320298.html (use a laptop power supply?) rated for 4 ohms.
Some of these amps are dirt cheap, and who knows what they are good for. Like, https://www.aliexpress.com/item/1005003617686719.html .

But if there is a local "Craigslist" where you live, you might get something for dirt cheap.
Last two months' worth of foobar2000.org ad revenue has been donated to support war refugees from Ukraine: https://www.foobar2000.org/

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #8
Do you have a local equivalent to Goodwill? Old, working receivers can typically be found there for small change.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #9
Do you have a local equivalent to Goodwill? Old, working receivers can typically be found there for small change.
Well, I could go to some second hand stores in my town here in Spain, yes, but... from what I've been able to see, the situation is hardly equivalent to Goodwill or anything else in the USA.
I don't think I'll be able find what I want at the price I need there... maybe I'm wrong. Also, I don't want a big chunky hifi style component, which is what they usually carry. I don't have room for that. I want one of these small modules. Just input, output, and a knob for gain. If I can only get a bulky hifi type component, I'll sell or trash the speakers instead.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #10
Searching "cheap stereo amplifier" on eBay I see a lot of small items in US under $20.  Note that most of the cheapest options require you separately purchase a 12V DC power supply; another $6.

If you can find it, the Lepai is said to be a good performer and comes with a power supply:
https://www.parts-express.com/Lepai-LP-2020TI-Digital-Hi-Fi-Audio-Mini-Amplifier-with-Power-Supply-310-3000

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #11
It seems all amplifiers are rated at 4ohm minimum, as ktf said above...
How bad can it get plugging those 3 ohm speakers? Can I bring them up to 4 ohm with a 1 ohm resistor, or it doesn't work like that?

EDIT: oh... this one can do down to 2 ohm, but it's already 45 bucks...
https://www.parts-express.com/Lepai-LP-168HA-2.1-2x40W-Amplifier-1x68W-Sub-Output-310-308
Is there anything particular to low impedance that makes amplifiers more expensive to produce?

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #12
I would happily use a 4 ohm rated amp here. "Nominal" speaker ratings is not an exact science - measure it and it varies with frequency - and I bet that a loudspeaker labeled as "3 ohm" would get a "4 ohm" tag from other manufacturers (a lot would stick to "4" or "8").

I don't know how much El Cheapo devices like https://www.aliexpress.com/item/32829367169.html are worth; notice how it does not have a power supply at that price, but can accept a wide range of voltages, so if you have one around - for example from an old computer! - you can wire it up if you are handy.

Expect the usual AliExpress shipping times.
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Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #13
@Porcus Thanks. So you don't think it's a big deal.
Sure I have a bunch of DC adapters lying around, and I have no trouble wiring stuff up.
I never ordered anything from AliExpress, believe it or not. heh.

I guess I'll keep looking, and make a trip to those second hand stores and see what I find.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #14
I agree with Porcus and I'd just go with an amp rated for 4-Ohms,   The odds of burning-out the amplifier are VERY
LOW, especially if you're not pushing it to full volume.  

If you are paranoid you can add a resistor, .   Resistors are cheap but if you have to pay for shipping you're paying for "insurance" that you probably won't need.   (I found a 1-Ohm 1 Watt 5% resistor at DigiKey for 20 cents US, and a 1% resistor for 50 cents.)

The resistor makes a voltage divider "throwing away" 1/4 of the voltage 1/(3+1).    That's a loss of 2.5dB to the speaker which is almost half of the power but not that much of a "loudness" change.   The resistor means less total power, plus some power is wasted in the resistor.    1/4 of the voltage going to the resistor means 1/16th of the power to the resistor so you should be safe with a 1W resistor.

...Since speaker impedance isn't constant over the frequency range the voltage divider is different at different frequencies and this can introduce frequency response variations.    It shouldn't be too bad with a 1-Ohm resistor but it's something to be aware of.  
  Where speaker impedance is higher, more signal goes to the speaker making a relative "bump" in frequency response and where speaker impedance is lower more voltage is lost to the resistor and you get a "dip".

I've never purchased anything from Aliexpress or Alibaba.  I've used eBay once or twice where that was the only choice, and "unknown" 3rd party Amazon sellers.


Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #15
@Porcus, That is certainly a neat looking device, anyone owns one? (Just noticed that this one is named slightly differently, but assuming that the device is the same).
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Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #16
I see many of these take stereo and output 2.1.
I have an el-cheapo desktop speaker set with derived subwoofer too, that I cluelessly bought many years ago. It's OK-ish quality-wise I guess, but when an instrument goes into the lower notes, it moves to the center, away from the side it was on, which is annoying.
Is this typical of modern 2>2.1 amps today too or is there any difference that you know of? I'm asking as a general rule. I understand each model can be very different, of course.
Each device will have a different frequency cutoff, volume offset, mixing... Those desktop speakers I have move the lower end away from the sides into the central "subwoofer", and the cutoff is not that low either; but I guess other models could just and just replicate the lower end in the central speaker for extra bass, while leaving everything meant for the sides as it is,

My impression is I should just avoid those that derive 2.1 from stereo as those speakers probably already have enough response in the low frequencies. And I don't have a stand alone sub/woofer either.

 

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #17
Sorry for double posting. I was also thinking, that these speakers have been sitting there for a long time. They were pretty dusty and grimey. I had to give them a good clean on the outside, but I'm wondering about the inside. Aside from dust, I wouldn't be surprised if there were spiders or moths that found the way in and decided to make them their home for a while.
There doesn't seem to be any obvious way to open them up. Maybe the front screen comes off? I don't know. I'm no stranger to assembling and disasembling stuff, but I never dealt with this kind of thing before, and there aren't any visible screws or anything. Is it possible they are just glued up and not meant for servicing, and I'll just ruin them if I try?
Oh, and I did test them by ghetto-wiring them to the 2.1 desktop thing mentioned above, just to make sure they work too. So that's fine, at least.

Re: Looking for advice on a modest amp for a pair of scavenged speakers

Reply #18
Quote
I have an el-cheapo desktop speaker set with derived subwoofer too, that I cluelessly bought many years ago. It's OK-ish quality-wise I guess, but when an instrument goes into the lower notes, it moves to the center, away from the side it was on, which is annoying.
Is this typical of modern 2>2.1 amps today too or is there any difference that you know of?
That's normal.    The crossover is supposed to block bass to the main speakers and block highs to the woofer.* 

If you send bass to the midrange or tweeter they can distort.      There is more power in the low frequencies, and in this type of design the left & right speakers are not designed to handle the power or the low frequencies.   And, there are usually 3 amplifiers with the bass channel having more power so the left & right amps would distort while the bass channel still has headroom.

But sometimes people add a subwoofer to an existing setup without a crossover.   (The subwoofer usually has a built-in low-pass filter.)

Normally the bass is panned to the center anyway.   That way in a regular stereo setup, both woofers are utilized for low frequencies where you need the most output.    Usually the whole instrument (the kick drum or bass guitar) is centered so it doesn't matter if it's playing some higher notes.     But, sometimes all of bass is blended to mono.    This was (and is) always done on vinyl records because it's easier for the stylus to track mono bass.

And, the crossover frequency is usually lower where our hearing is "non-directional".    This is the main idea of "getting away" with only one woofer (to save space or cost).

----------------------------------------------------------------------
In a home theater setup things "get complicated".    By default the stereo and surround speakers are supposed to be full range and they handle the regular bass and the subwoofer is not at all with stereo music.   ONLY the "point 1" LFE (Low Frequency Effects) channel goes to the subwoofer.    If only have a stereo setup with no subwoofer, the surround channels are downmixed into the left & right and but LFE is lost.   

But since most home theaters don't have 5 or 7 (or more) large full-range speakers with big woofers, audio/video receivers have a "bass management" feature (a crossover) that routes ALL of the bass to the subwoofer.



*There is an overlap.   Both drivers are operating at the crossover frequency and a typical crossover filter has a roll-off of 12dB per octave, but active filters (in active speakers) can have sharper roll-off.

Many cheap 2-way speakers just have a capacitor as a high-pass (6dB per octave) on the tweeter with the full-range signal going to the woofer.