Ok, so I have just bought a pair of Sherwood Alto 621 speakers, wanting to connect them to my computer - as they require speak

you need an AMP not a toy, that's just 2x20W, your speakers are capable of 30-100W it won't be able to drive them very well, it'll make noise, but it won't be good.

Dragos Manea

Mar 30, 2015

Looking at that amp i dont think it is 2 x 20W RMS, more likely 2 x 12W RMS.

EDIT: How i said, i found this
Package Content:
1x Lepy LP-2020A Amplifier
1x 12V 3A Power Supply
1x User Manuel

Taking in consideration that power is Voltage multiplied by current, 12V x 3A = 36W. A class D amp has 70% efficiency that means 36W x 0.7 = 25W audio RMS power, this divided by 2 because there are 2 channels, how i said 12W RMS on channel.
*we need an expert though, because I'm a bit confused. I saw a bundle of some speakers with the N22 amp:

bundle: https://www.amazon.com/Audioengine-P4-Bookshelf-N22-Amplifier/dp/B00DWH7IOY

Same link at Audioengine's site. So the N22 is only 40W peak per channel, but the P4 speakers go to 125W per channel.

The P4 speakers are roughly similar (100W per channel for the ones above) so the N22 specs should be fine?

Plus: https://hometheaterreview.com/how-to-pick-the-right-amp-for-your-speakers-or-vice-versa/

"You can blow up speakers with virtually any size amp, or you can use them safely with virtually any size amp, depending on how you drive them." To make his point, Sandy told me that he's currently driving his pair of GoldenEar Triton Two towers (which carry a recommended amplification rating of 20 to 500 watts) with a 22-watt-per-channel tube amp from Line Magnetic. "I had David Chesky over recently to listen to the setup, and he was blown away by how good it sounded," he told me. "I've even driven the Triton Two with amps as low as six watts per channel, and it did a pretty good job. You'd be surprised."

I got the impression two factors also matter:
1) quality of amp, and
2) don't drive speakers above distortion

The $25 amp linked above by the poster doesn't sound great based on reviews.

This one may be too expensive, but at a quick glance I didn't see anything for about $50 that looked good:

**I am absolutely NOT an expert. In fact the above amp may even be a bad choice or not even be compatible. I find amps and speakers a bit confusing.


Jan 10, 2006

So it's got an amp that is designed to go with the speakers, that's important.

Tube's react differently at the distortion point, and work at a higher voltage. You will also find that sensitivity of the speaker is important. https://www.seymourduncan.com/forum/showthread.php?88098-So-why-ARE-tube-amps-louder-than-solid-state

if you've got what appear to be roughly $200 speakers, a $25 amp is not the right choice.
If all you can spend right now is $25 then it will play the speakers. It won't play them very loud so you have to be careful.
When the amp is over driven it will cause distortion that will damage the tweeter. The damage can accumulate over time or happen all at once. Plan on an upgrade when you get some more money together.
Part of the problem is the somewhat deceptive power ratings on most of the low cost digital amps. You have to dig through the spec sheet until you find the lowest number and even that could be bogus.
Regular consumer amps have to go through an FTC mandated procedure before the power can be measured into a standard load. Even that has limitations since it doesn't take into account how the amp reacts to a complex load rather than a pure 6 or 8 ohm load. Cheap amps usually have low current. Power is voltage times current so you can have a high voltage low current amp with the same rating as a low voltage high current amp. Depending on the load the power output can be vastly different.
Tube amps add other factors like the way they clip when reaching their maximum power.
If your budget is limited I would suggest a used integrated amp or receiver. An older Marantz AV receiver without HDMI doesn't go for much and they sound quite decent. The amps have good current and the preamp sections are clean.
They also have the advantage of built in DACs so if you have digital audio output you can bypass the PC DACs. Maybe run you $100-125.

Dragos Manea

Mar 30, 2015

Wrong, voltage is a fixed value for certain powers amps no matter what impedance you use, what gives you the power of the amp is the resistence for which is made. P = (V x V) / R where V is the output voltage of the amp and R is the speaker impedance. So no, a high voltage low current is made for 8 ohm or 16 ohm speaker and a low voltage is made for 2 ohm or 4 ohm speaker, it is clearly rated in the amp manual, the speaker impedance and the power outputted on that impedance. Most amps you will see that they have 2 ratings, one for 2 ohms and one for 4 ohms, or one for 4 ohms and one for 8 ohms because the voltage is a fixed value and cannot be changed and the outputed power depends on the speaker impedance. Never put a load of a lower impedance compared for what the amp was build, if the amp says a minimum of 4 ohms never put a load of 2 ohms it will fry the amp, it forces the amp to output 2 times the power it is designed and it will not stand that load.

Edit: @americanuadiophile
You said that power is:
P = U x I
but I = U/R so replacing I in the power formula the power becomes P = (U x U)/R, this is the recommended formula for power calculation in Audio amps.