Silly question

MTXMAN

Well we all have to ask these questions some time or another so here it goes... What exactly is THD percentage and how does that relate to sound quality/ power handling etc... I've gotten the impression that lower percentage is better but what exactly does this difference in percentage translate to... Thanks for the help with a question that many of you probably feel should be common knowledge... Just trying to learn what I can so one day i can be considered an "expert"

PoweredByDodge

THD = total harmonic distortion.

however most amp companies will also figure in noise - random noise that comes from the amplification process and random noise due to electronics inside the amp.

thus, THD = total harmonic distortion + random noise associated with that amp.


this is given in a percentage which is supposed to be a certain percent of the "output" signal to your speakers.

basically, think of it this way... you know how you have a picture ( a bmp / jpg / gif) on your computer? -- then you go and blow it up, try to make it twice the size of what it used to be... well the edges on the faces aren't as sharp and it can be blurry in places... blow it up a lot and you'll see the image look like abstract art instead of an actual picture.

well the same goes for sound... when you amplify it, you get a certain amount of distortion of the original signal... true 0.0 % thd is in reality impossible, although i'm sure someone somewhere is theoretically working on a way to make it happen... i still dont think its possible though.

well the lower the percentage of distortion (also figuring in random noise from the amp which will also "color", distort, or otherwise **** up your audio signal) the "cleaner" your amp is going to be (output sound wise).

anything under 0.5 for subs is "very good" for your off the shelf regular retail stuff.

anything under 0.1% for highs for off the shelf stuff is good as well.

if i could pick and choose i'd try to shoot for below 0.1 for subs / 0.05 for highs... but i'm a poor man and i use MTX with 2% (sheets come out 1.3 - 1.7) for subs and 0.3 and lower for highs.

your better not mass marketed stuff with a sick pricetag is going to have much better ratings -- well under that 0.1 mark... most under hte 0.05 mark.

this is not the "tell all" of an amp -- an amp with a very very low THD can have a **** **** bad bad bad slew rate and horrible damping factor which will give you a sound just as "blah" as an amp with a great slew factor and high distortion.

you've got those three criteria to balance and try to find an amp that has a decent rating for all three... thus giving u good overall performance.

Systems

<----note name (I forgot to sign in ;D)

Ok so for a 4 ch amp with 125x4 RMS at 4 ohm with 0.04THD, what slew rate and dampening factor would make this a great amp... Also can you give me the basics on slew rate and dampening factor??? (slowly moving away from looking solely at the wattage for an amp ;D) with a $1000 price tag what slew rate and dampening factor would make the amp i listed in the other post worth while?

Thanks a ton for the info PBD :D:D

RuSsMaN

A small excerpt posted on the forum from a man that was into audio while most of us were still an itch in our daddies pants.

"As for those high THD levels, humans (real humans) cannot discern THD figures as high as .5%. There will probably be someone who comes along and says they CAN discern lower levels than that. They will be full of **** (or not human).

You can get figures low /........../ by adding massive amounts of negative feedback into the design, which is NOT a good thing. Trust your ears, not the figures...."

Cheers,
Russ

MTXMAN

Ouch i just looked at the eclipse 4 ch i was thinking about getting and it has a .1% THD that's sad...

RuSsMaN

1/10th of 1%, what an outrage.

MTXMAN

for a 4 ch it's not that great considering that the Salesman was trying to get me to buy that amp... He knew that cost didn't matter and I wanted great sq but he recomended an amp that didn't compare to some of the other one's they carry :mad: I think i'll be speaking to someone else next time

PoweredByDodge

as far as that guy who said u can't hear it above 0.5% if u're human... i want to see his research and double blind tests to prove this.

common belief used to be u couldn't discern anything under 2%.... that was quickly disproved.

i'm sure that a trained ear on true double blind tests of similarly matched amps with all other factors virtually idnetical EXCEPT thd, somebody who knew their audio **** could tell the difference.

for what its worth, i can tell the difference between 0.3 and under 0.1

... and i'm not full of ****.

0.1 is not bad at all -- its "average" for an off the shelf class AB amp run in its affixed channel mode (ie, a 4 channel run in 4 ch mode or a 2 chan run in 2 ch mode --- when u bridge them those numbers change a little)

as far as what slew rate makes what amp good.... dude that's realllllyyy relative to the amp / price / etc.

for a 1000 dollar amp... with a power rating of 125 x 4 @ 4 ohm / ~500 x 2 at 4 ohm bridged .... i'd expect a damping factor of at least 500, and a slew rate of at least 30-35 v/ us.

that's about common. i mean RF and MTX make amps like that... 125 x 4 at 4... with like less than 0.1% or i think RF is 0.05% thd... mtx lists a >200, which when u really put the thumb screws to them they'll tell u the AB's are more like 350-ish, and a slew of 20... while the RF is standard 500 or better across the board with a slew of 30 or better as well.

so... considering you're paying a grand (the mtx is only like 700 -- the RF more like 900?) then i'd expect the numbers more around the RF... if u're payin retail that is.

i'd try to compare it to something of better quality but the problem is the **** that's better quality i only know 'net prices on it.... i've never bothered to retail price any of it...

as far as what slew/damping factor ratings are good.

the higher the better.

bare **** minimums are sadly right around what you'll get out of an RF /mtx... 20 or so on the slew rate is about bare **** minimum.... it becomes more important as power output goes up... a high slew rate in a 2,000 watt amp is more important than on a 20 watt amp. you've got greater ground to cover there. it may get hard to find a reasonably priced amp with a rating over 50 or 60... but its not impossible.

a damping factor of 50 is supposed to be the "technical" minimum to control a sub... i wouldn't use anything under 150 unless i got it and the sub i was putting on it for free... ...upwards of 250-300 for a sub would be a good budget range.... 500+ is nice... get into better amps you'll see them up in the 750 - 1000 or better range.


but u know -- u can't just judge an amp on ratings...

like russman said, your ears matter too.

so does durability.

if you LIKE how an amp sounds... and its a well built amp that u know isn't gonna conk out on you after a month, then you're money is not wasted, and it is a good product that u can rely on.

that's what's important.

people usually just start giving a **** about slew/damp when you start getting into very high pwoer applications or you're going for "ultimate" SQ and using equipment (speakers) that are costing u a small fortune and u want an amp worthy of pushing them.

the other time u start worrying about it is when the ratings are DISGUSTINGLY low... i mean like BAD bad.

for most stuff -- u just want something reasonable -- and the old "listen to it, sounds good, aint gonna break" rule applies.

PoweredByDodge

oh and as far as what are slew / damping factor.

well i posted it in these boards a lil while ago and its like 12-30 at night so i'm too lazy to go cut and paste it (it was a nice explaination)... but here's the cliff notes...

slew rate:

since voltage cannot change instaneously, it must change with a rate. basic electronics study will show you that if you have a 5 volt battery, and a coil(i'd say a cap but a coil is more akin to a speaker), u hit the coil to the battery and it wont instantaneously pull the current necessary to hit 5V acros its terminals. it'll take some time... which may be like a teeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeny weeny micro micro milli micro itsy bitsy second. ... but it is time.

so you've got your amp putting out voltage to your speaker... now the amp is putting out analog signal, so the signal information itself is constantly changing.. but also, as you get different strength "bumps" or "whistles" or whatever.. drums, guitar, quiet guitar, loud guitar, different notes... u get the idea... as it goes from one to the next it needs to adjust te voltage. maybe Snoop is pumpin out the bass hard for a second, then comes back with a quiet note... well when it goes to the quiet note from the loud one, it has to drop voltage, well the speed at which it drops voltage is the slew rate.

if its too low of a rating then the amp may not be able to drop all the wya down to that lower note... for example, and i know you'd have to have super fast techno **** with a very high power amp that had a bass note that went from full power to silence, back to full power in an utter split second... but.. given a situation like that, if you had an amp with a **** slew rate.. i mean dog **** **** low... then it maynever get that silence in there through the speaker.. the amp may put out a full strength note, then a half strengh (instead of silence) then a full strength again.

lets look at it the other way -- super high slew rate -- the higher it is, the more absolutely accurate your sound reproduction will be. for SQ competitors high slew rates are a must... if you have speakers good enough to put out perfect sound, u better have an amp capable of making it.

damping factor:

this is probably the shittiest definition in the world, as i'm gettin real tired but... its bascially teh amp's ability to control the speaker .... it is how tight of a grip the amp has on the speaker's movement. it keeps it from getting sloppy. very low damping factor = tendency to get sloppy. and anything under 50 in spl comp may result in an inability to control the woofers.