Help understanding amp ohms and watts
mikes...@gmail.com
speakers at 8 ohms. Because it is a rack-mount amp inside the combo
unit, I can easily disconnect the speaker and use the amp to power a
cab unit.
I don't quite understand how ohms relate to watts. For example if I buy
a cab that has 4-8" speakers in it that operates at 4 ohms and is rated
at 600 watts, how well would my amp run this? If I understand right, as
ohms are doubled, watts are cut in half so my 8 ohm amp would actually
be powering this cab at 300 watts. Is this correct? Other than that,
what would the consequences of this be? Am I at risk of toasting my amp
in the process? With the sound quality be real lousy?
Mike
<><
Jim Carr
news:1163463561.0...@m73g2000cwd.googlegroups.com...
> I don't quite understand how ohms relate to watts. For example if I buy
> a cab that has 4-8" speakers in it that operates at 4 ohms and is rated
> at 600 watts, how well would my amp run this? If I understand right, as
> ohms are doubled, watts are cut in half so my 8 ohm amp would actually
> be powering this cab at 300 watts. Is this correct? Other than that,
> what would the consequences of this be? Am I at risk of toasting my amp
> in the process? With the sound quality be real lousy?
That formula is a rough and optimistic estimate. Check the specs on your amp
to find out what it puts out and different Ohm ratings. If 4 Ohms is not
listed, assume that your amp won't function against that load. The manual on
this amp says don't run it at anything but 8 Ohms, so don't. It's not just a
matter of power. There's a whole lot of stuff that goes into how an amp and
speaker react with one another. It's extremely critical on a tube amp and
less so on a solid state amp, but it's an issue nonetheless.
Amp makers *want* to be able to list various Ohm ratings, especially lower
ones because the wattage will be higher - that's great for marketing. If the
manual doesn't say it goes down to 4 or 2 Ohms, don't try to run it that
low. Bad things might happen. At the very least you could end up with your
mother-in-law recovering from surgery at your house.
So, if you want an external cabinet, get an 8 Ohm cabinet. The Hartke 410 is
8 Ohms, for example.
Middle C
I think you may be a little misguided here. The watt rating of the cabinet
is how much power it will handle before it will be subject to damage. It's
the ohm rating of the cabnet that affects the power output (Watts) of the
amp. So in your case, if your "amp" is rated at 120w with an 8ohm(speaker)
load, the amp will roughly be 230w or so with a 4 ohm (speaker) load. As
a general rule of thumb, reducing the 'speaker' ohms will double the power
output (watts) of the amp. But remember, more power also means more heat
in the amp and not too many bass amps are well suited below of 4 ohm load.
All that said, the Hartke manual does not reccomend the use of an external
cabinet for this amp. I suspect it is not capable of disapating heat with
anything less than an 8ohm load.
Middle C
Benj
mikes...@gmail.com wrote:
> I don't quite understand how ohms relate to watts. For example if I buy
> a cab that has 4-8" speakers in it that operates at 4 ohms and is rated
> at 600 watts, how well would my amp run this? If I understand right, as
> ohms are doubled, watts are cut in half so my 8 ohm amp would actually
> be powering this cab at 300 watts. Is this correct? Other than that,
> what would the consequences of this be? Am I at risk of toasting my amp
> in the process? With the sound quality be real lousy?
Ohms relates to watts ONLY in solid state amps. The reason is that in
those amps the max voltage is constant, therefore as you lower the Ohms
you increase the current which means more watts. HOWEVER, Every amp has
a rated current limit and that current limit is usually expressed in
terms of how low the Ohms of a speaker it drives can be. For many amps
that rating is 4 ohms. But in your case the amp was designed for a
combo and since the combo uses an 8 Ohm speaker, you MUST assume that 8
Ohms is the lower limit for that amp, unless you can find specs
somewhere that indicate otherwise.
So the bottom line is that putting a 4 Ohm cab on that amp is much more
likely to result in toast than 300 watts (250 is probably closer to the
truth if it actually is rated for 4 Ohms).
Note that watts rating of the speakers is irrelevant so long as it is
more than the max output of the amp in question. It only indicates how
much power the speakers can take without damage.
Bottom line is you had the right idea and it would work OK, IF and only
IF the spec of that combo amp says it can drive a 4 Ohm load. So until
you get a note from Hartke telling you that 4 Ohms is an OK load for
that amp, you had better not go that way.
Benj
(Who notes that in tube amps the situation is more complex and the
power tends to stay the same regardless of Ohms (taps on output
transformer match load))
Misifus
equal ohms times amps times amps.
That part is simple, it's applying it in the real world that gets
complicated. The Watts rating on a speaker cabinet is merely a
suggested limit. It says, "these speakers won't be harmed by wattages
up to this limit". So, you can power a 400 watt cabinet with 50W, or
100W, or 200W, or even 400W. The point is the watts of the cabinet
don't enter into a calculation of watts amps and ohms. Those all have
to do with the particular amplifier in use.
Theoretically, and in practice, the lower the ohms the power amp sees,
the more current (amps) it will push through the cabinet and the more
watts generated in the cab. However, as has been said, the type of amp
has a lot to do with how it will react to different loads (ohms).
Just remember, if it was easy, everybody would do it. <g>
-Raf
--
Misifus-
Rafael Seibert
mailto:raf_s...@cox.net
blog: http://rafsrincon.blogspot.com/
Photos: http://www.flickr.com/photos/rafiii
home: http://www.rafandsioux.com
--
Posted via a free Usenet account from http://www.teranews.com
Brian Running
The manual for your amp specifically states that it should only be used
with an 8-ohm speaker cabinet. If you use a 4-ohm cabinet, there will
not be enough electrical resistance to the flow of current out of the
amplifier, and too much current will flow. This will eventually
overheat wires inside of the amp and melt them. You don't want that.
If you use an external cab, make sure it's an 8-ohm cab, and not less.
JohnO
> At the very least you could end up with your
> mother-in-law recovering from surgery at your house.
And with a little bad luck, it's *female* surgery. So, don't you dare
connect that amp to a 4 ohm load.
-John O
S&y
12", Aluminum frame, Neodymium magnet). I am weary carrying this 45 lb
box around to rehearsals & small venue gigs.
<dodges small objects & jeers throw by owners of 60 lb plus cabinets>
I made a cover for the amp chassis from some perf-metal sheet I got at
Home Depot. It can now be converted to a 10lb portable with a carry bag.
I believe the heat issue & caution from Hartke to run just one 8 ohm
speaker stems from the combo enclosure. The amp's power transistors are
mounted on the rear panel which has a large heat sink. The heat sink
sits about 1" inside the box's recess with access to cooling air from
only one direction. There is no fan, no inlet or outlet for air flow.
Only the small vent under the handle to avoid trapping hot air in the
cabinet.
Cooling should be much improved with the amp sitting in the open on top
of the speaker. I plan to, carefully, try running the 12" & 15" speakers
with the 120 w amp but, haven't found a situation where I would need it.
The amp has a nice quiet DI & the combo makes a great stage monitor when
a PA is on hand.
Time will tell,
S&y
Benj
S&y wrote:
> Cooling should be much improved with the amp sitting in the open on top
> of the speaker. I plan to, carefully, try running the 12" & 15" speakers
> with the 120 w amp but, haven't found a situation where I would need it.
> The amp has a nice quiet DI & the combo makes a great stage monitor when
> a PA is on hand.
>
> Time will tell,
The "removable" amp thing is a great idea, but be very careful about
the lower Ohms cab. There is much more to this that is involved than
just heat sink dissipation. For example the output transistors must
have double the current ratings needed for 8 Ohm drivers AND the power
supply must be able to deliver that double current level without damage
as well. There is a possiblity that these things can do this or at
least try to do it in a way where droop and other overload
characteristics limit the output in relatively safe ways...but you
never know so be careful when overdriving lower Ohms cabs.
Benj
S&y
venues with no PA. It'll be easier to carry without the amp in it & that
is the point.
S&y
saw3...@yahoo.com
The three elements of Ohms' law.
V, I, and R
V (volts) is the measure of force pushing the current through a
conductor.
I (intensity) is measured in Amperes (Amps) and is the measure of
current moving through a conductor.
R (Resistance) Is the measurement of opposition to movement of current
through a conductor.
If you know two of these parameters you can deduce the third by using
ohms' law.
This website gives good information about Ohms' law.
http://www.csgnetwork.com/ohmslaw.html
Resistances in series are simply added together to obtain total
resistance, for instance, if in a speaker cabinet housing four 8 ohm
speakers, the speakers are wired in series,that is the output of the
power transistors goes to speaker1 and through it to speaker2 and
through to speaker3, through speaker3 to speaker4 where it emerges and
then returns to the common (ground) of the output transistor thereby
completing the circuit.
+ -------wwww---------wwww---------wwww---------wwww------ -
When resistances are wired in parallel the output of the power
transistor goes to the input of speaker1 then to the input of speaker2
(it does not go through speaker1) then it goes to the input terminal of
speaker 3 (it doesnt go through speaker2), then to the input terminal
of speaker4 (it doesn't go through speaker3) In other words the Plus
(+) wire comes from the output then to the input terminals of speakers
1,2,3 and4 and ends there. The return (-), also called common side of
the output, wire does the same as the (+) wire did except it goes to
the (-) side on each speaker and ends at speaker 4.
Impedance (total resistance) of a parallel wired speaker set is a
little more complicated to deduce. the formula is 1 divided by the sum
of 1 divided by R1, plus 1 divided by R2, plus 1 divided by R3, plus 1
divided by R4.
Lets do an example: Say we have four eight ohm speakers wired in
parallel. Then according to the formula we need to know the sum of
1/R1+1/R2+1/R3+1R4 so we can divide 1 with it. Since 1/R1 (1 divided by
8ohms) equals .125, then 1/R1+1/R2, +1/R3, and +1/R4 is the same as
.125 + .125 + .125 + .125 which equals .5 . Now we can divide 1 by the
sum of deducted. 1 divided by .5 is 2 , yes 2ohms. Four 8ohm speakers
wired in parallel equals 2 ohms of resistance aka impedence. The same
four 8ohm speakers wired in series would equal 32 ohms of resistance.
We now need to calculate the impedence of speakers 1 and 2 wired in
series and speakers 3 and 4 also wired in series and the two pairs
wired in parallel.
We can treat the two 8ohm speakers in series as a 16ohm speaker. So
using the formula for two speakers in parallel R1 x R2 divided by R1 +
R2 we get: R1 (16ohms)(remember we are treating two 8ohm speakers wired
in series as a single 16 ohm speaker). So R1 (16 ohms) times R2 (16
ohms) equals 256 (16x16=256), now we divide this by R1 + R2 so R1 (16
ohms) plus R2 (16 ohms) equals 16 + 16 = 32. and 256 divided by 32
equals 8 ohms. Yes this arrangement presents an 8 ohm load for the
output to work into. So one can see that we do have some extra choices
for impedence when we use the formulas.
Misifus
It might be easier to say - speakers wired in series, as you described,
add the resistances. So, four eight ohm speaker in series make 32 ohms
(4+4+4+4=32).
In parallel, if the speakers are all the same value, divide by the
number of speakers. Four eight ohm speaker in parallel make 2 ohms (8/4=2).
Benj
Misifus wrote:
> Thorough, if not easily followed.
>
> It might be easier to say - speakers wired in series, as you described,
> add the resistances. So, four eight ohm speaker in series make 32 ohms
> (4+4+4+4=32).
>
> In parallel, if the speakers are all the same value, divide by the
> number of speakers. Four eight ohm speaker in parallel make 2 ohms (8/4=2).
...And if they are in parallel and not all the same value go find some
tech who knows how and have him do it for you! Anyway, that case
usually isn't important in bass because if the speakers are NOT all the
same value then the power is unevenly distributed between the speakers
and that is usually not a good thing.
Benj
saw3...@yahoo.com
> Thorough, if not easily followed.
>
> It might be easier to say - speakers wired in series, as you described,
> add the resistances. So, four eight ohm speaker in series make 32 ohms
> (4+4+4+4=32).
>
> In parallel, if the speakers are all the same value, divide by the
> number of speakers. Four eight ohm speaker in parallel make 2 ohms (8/4=2).
>
> -Raf
----------------------------------------------------------------------
Yes, much easier said that way. But not as entertaining for those who
like the conceptual details. Its easily followed by slowing down and
taking time to absorb as you go. I guess I write that way because I
like to read the detailed descriptive articles. For those who like to
speed read and scan an article for pertinent points and don't want to
be bothered by cumbersome details, now they have your post so they wont
be unnecessarily held up by mine. You are very good at distilling
information .
Misifus
Too many years of teaching this stuff to the unwilling.
-Raf
--
Misifus-
Rafael Seibert
mailto:raf_s...@cox.net
blog: http://rafsrincon.blogspot.com/
Photos: http://www.flickr.com/photos/rafiii
home: http://www.rafandsioux.com
--
BW
though some manufacturers say that their products can handle down to 2
ohms. It's a marketing department exaggeration, and asking for trouble.
Or, to put it another way:
"If you consider the equation 4 x 87.36 (with a + or - variable of
.21), if extrapolated to the maximum exponential quadrant, results in a
coefficient of 739.55. If you add this coefficient to the initial
reading for the plectrum (72 x 3.49 to the third power), your results
will indicate the appropriate voltwattohmage. Of course, if you fill up
with a higher than 87 octane, all these readings may change."
I hope this has been useful.
BW
saw3...@yahoo.com
> >
>
>
> Too many years of teaching this stuff to the unwilling.
>
> -Raf
Ah! A teacher. Good! You can keep us on the right track when we stray
too far afield.
Speaking for myself, and others here I'm sure, just let me say that a
lot of us are self taught amatuers and tinkerers that are always
looking for that elusive just right bass sound. The sound that is
always so near yet so far away.
And when we do get to enjoy that just right mix for a little while the
soundman will unerringly ruin it by opening an unused mic that
resonates with one of the notes in our pattern thereby rendering
compression useless.
We realize that impedence matching plays a big part in getting that
"you know it when you hear it" sound.
Best wishes. s3d
saw3...@yahoo.com
LOL! You have a very creative imagination BW. I know thats what some of
the technical stuff can look like at first glance. I just went through
a harrowing experience trying (and failing) to figure out how to make a
call with my new Trac phone. It turned me into a babbling idiot and I
ended up in the fetal position sucking my thumb. I'm waiting for one
of the kids to come by and show me how to work it. s3d
Middle C
"Misifus" <raf_s...@cox.net> wrote in message
news:4560e279$0$21176$8826...@free.teranews.com...
> saw3...@yahoo.com wrote:
>> Misifus wrote:
>>> Thorough, if not easily followed.
>>>
>>> It might be easier to say - speakers wired in series, as you described,
>>> add the resistances. So, four eight ohm speaker in series make 32 ohms
>>> (4+4+4+4=32).
>>>
.... this new math is just killing me. :o
Brian Running
You know, Raf, if you were trying to teach them that 4+4+4+4=32, it's no
wonder they were unwilling. :-)
Benj
Middle C wrote:
So, four eight ohm speaker in series make 32 ohms
> >>> (4+4+4+4=32).
> >>>
>
> .... this new math is just killing me. :o
dum, dum, it's new math, new, hoo, hoo, math!
It won't do you any good to review math!
It's so easy, so very easy,
that only a child can do it!
(Tom Lehrer)
Deputy Dumbya Dawg
"Benj" <bja...@iwaynet.net> wrote in message
news:1164911977.8...@l39g2000cwd.googlegroups.com...
:
: Middle C wrote:
: So, four eight ohm speaker in series make 32 ohms
: > >>> (4+4+4+4=32).
Um that would be sixteen ohms nominal combined
impedance, until one blows then it will be infinity
ohms combined impedance.
dawg
Middle C
> : " Misfus " wrote:
> : So, four eight ohm speaker in series make 32 ohms
> : > >>> (4+4+4+4=32).
c..
"Deputy Dumbya Dawg" <Deputy_Du...@whiteehouuse.gov> wrote in message
news:etKbh.4939$sf5....@newsread4.news.pas.earthlink.net...
Misifus
Misifus
Brian Running
>> no wonder they were unwilling. :-)
>
> Too clearly.
If that's the worst mistake you ever make, you'll still be doin' pretty
good... :-)
JohnO
Misifus wrote:
> Brian Running wrote:
> >> Too many years of teaching this stuff to the unwilling.
> >
> > You know, Raf, if you were trying to teach them that 4+4+4+4=32, it's no
> > wonder they were unwilling. :-)
>
>
> Too clearly.
>
If you had done that in the classroom, you might have ended up
resorting to this:
http://www.dvorak.org/blog/wp-content/uploads/2006/11/ha.jpg
(don't know if that's real, but it's good on several levels.)
-John O
Misifus
Neither the worst I am certain, nor the last. <g>
Misifus
As a retired teacher I will say that if a teacher can't admit that they
make mistakes, they're dead meat in the classroom. When the kids catch
you in a mistake it makes you more human and teaches them to trust their
own understanding.
Middle C
> Brian Running wrote:
>>>> You know, Raf, if you were trying to teach them that 4+4+4+4=32, it's
>>>> no wonder they were unwilling. :-)
>>>
>>> Too clearly.
>>
>> If that's the worst mistake you ever make, you'll still be doin' pretty
>> good... :-)
>
>
> Neither the worst I am certain, nor the last. <g>
>
> -Raf
>
> --
Don't sweat it - we know what you meant. Just could help having a little
fun with it.
Misifus
I would hope so.