sub tuning effecting amplifier temperature

[Ian Macoomb]

Greeting RAC'ers.

Can someone please explain to me how the tuning frequency of a box can cause
the amplifier to run hotter? Or how if tuned improperly, how it could cause
the amp to run hotter? I can't see how it matters.

And while you're at it, what's the max effiency of a class A/B amplifier
(theoretical and typical)?

Just trying to settle an argument.

--
----------------------
Ian Macoomb,
Ottawa, Ontario
'97 SHO
14.966 @ 91 mph

[Mark Zarella]

> Greeting RAC'ers.
>
> Can someone please explain to me how the tuning frequency of a box can
cause
> the amplifier to run hotter? Or how if tuned improperly, how it could
cause
> the amp to run hotter? I can't see how it matters.

In practice, I'm not so sure it matters much. However, the tuning freq of
the box can have an effect on the motion of the cone/vc given the same input
signal. This in turn can produce a different "level" of back emf
(hmmm...it's not really back emf in the classical sense of the term - it's
just Faraday's law i guess). This different level can contribute to a
difference in effective impedance of the driver.

> And while you're at it, what's the max effiency of a class A/B amplifier
> (theoretical and typical)?

Typical: around 50%. Or less. Theoretical: depends on the input signal.

[Ian Macoomb]

Yeah, but if it's mistuned, won't the acoustic impedance be too low? If
this is so then it's possible that the subs could be underdampened and thus
more efficient in some frequency range. Wouldn't that make it run cooler
for a given output level?




"Mark Zarella" <seesigfile> wrote in message
...
> > Greeting RAC'ers.
> >
> > Can someone please explain to me how the tuning frequency of a box can
> cause
> > the amplifier to run hotter? Or how if tuned improperly, how it could
> cause
> > the amp to run hotter? I can't see how it matters.
>
> In practice, I'm not so sure it matters much. However, the tuning freq of
> the box can have an effect on the motion of the cone/vc given the same
input
> signal. This in turn can produce a different "level" of back emf
> (hmmm...it's not really back emf in the classical sense of the term - it's
> just Faraday's law i guess). This different level can contribute to a
> difference in effective impedance of the driver.
>
> > And while you're at it, what's the max effiency of a class A/B amplifier
> > (theoretical and typical)?
>
> Typical: around 50%. Or less. Theoretical: depends on the input signal.
>
>

[Mark Zarella]

> Yeah, but if it's mistuned, won't the acoustic impedance be too low?

Depends on what you mean exactly by "mistuned". :)

> If
> this is so then it's possible that the subs could be underdampened and
thus
> more efficient in some frequency range. Wouldn't that make it run cooler
> for a given output level?

Hmmm. Makes sense, unless there's a phase issue involved. I'll think about
it later and look through some literature. I'd love to know the answer as
well.

Anyone else have any insights?

[Soundfreak03]

>Oh well. I was just having an argument with an "installer" on a local ricer
>board. He suggested checking the box tuning when a guy was asking why his
>old Rockford amp was running hot. Gee, ya think he could be running 2 ohms
>mono?
>

Probably running them in 2 ohms mono, which the amp may not like, then again he
may live here in Texas and it is just going to do that because everything is
hot.
I have never encountered a situation where the box tuning could make an amp run
hotter, it may be possible theoretically but practically? I dont think so. I
think the "installer" is just trying to make himself look smart.

Les

[Ian Macoomb]

To quote the "installer":
"It's a bit more involved than that, drivers with certain EBP really don't
like being in certain boxes.

1) Take a driver that hates ported boxes, at the tuning freq. impedence may
rise as high as 50 - 60 ohms, and just as with over loading an amp by going
with too low of a load, the amp will get quite hot playing into much higher
impedences that it was not designed for. Amps are designed to work at a
certain impedence, the closer the speaker matches the amps internal
resistance, the more power it makes and the cooler it runs. Try running a
good set of tweeters (4-ohm) with a passive cross-over at say 5K. Run them
in stereo and see how your amp gets hot. Thats because those 4 ohm tweets
playing over 5K will be about 10-15 ohms a piece.

2) Has to do with overly high tuning frequencies and over-excursion below
that, but I'm tired now. "

He's got just enough knowledge to be dangerous.


"Mark Zarella" <seesigfile> wrote in message
...
> > I'm surprised there's not a big response to this one. Where have all
the
> > old farts gone? I see Lizard is still around. And GeO.
> >
> > Oh well. I was just having an argument with an "installer" on a local
> ricer
> > board. He suggested checking the box tuning when a guy was asking why
his
> > old Rockford amp was running hot. Gee, ya think he could be running 2
> ohms
> > mono?
>
> Yeah, checking your box volume isn't exactly the first thing that comes to
> mind when presented with those symptoms. The guy's clueless.
>
>

[Mark Zarella]

> To quote the "installer":
> "It's a bit more involved than that, drivers with certain EBP really don't
> like being in certain boxes.
>
> 1) Take a driver that hates ported boxes, at the tuning freq. impedence
may
> rise as high as 50 - 60 ohms, and just as with over loading an amp by
going
> with too low of a load, the amp will get quite hot playing into much
higher
> impedences that it was not designed for.

That sentence in and of itself should make the rest of his post suspect.

> Amps are designed to work at a
> certain impedence, the closer the speaker matches the amps internal
> resistance, the more power it makes and the cooler it runs.

The amp's "internal resistance", aka its output impedance, is usually a
fraction of an ohm. I don't know of many amps that will run cool with that
kind of load.

> Try running a
> good set of tweeters (4-ohm) with a passive cross-over at say 5K. Run
them
> in stereo and see how your amp gets hot. Thats because those 4 ohm tweets
> playing over 5K will be about 10-15 ohms a piece.

They shouldn't be. At least not "4 ohm" tweeters.