Tweeter midrange protection devices/calculation

Archived from groups: rec.audio.tech (More info?)

Hi all,
I'm trying to build in some PTC's into my 2 way speakers (used for bass
guitar) so as to protect the midrange/tweeter in this 2 way system.
I don't know what exactly the power rating of the tweeter is but I've been
assured that for bass guitar it will work fine with my EVM 15B 200Wrms.
From what I understand, the EVM is rated up to 1.5 kHz so according to this:
http://sound.westhost.com/tweeters.htm
200W RMS actually translates to a "real world power handling of about 150W
RMS with 50W available to drive the tweeter above that frequency.

I know what crossover to use in this application and that the tweeter will
have an approx -2.5dB pad on it leaving about 28 or so watts that will
actually "drive" it.

The confusing thing for me is how the PTC's are rated and how you go about
selecting one for the application - non-destructively...

My catalogue lists PTC's as having a "Switching current" so does this mean
that for the 28W into 8 ohms I need a 1/2 amp PTC? Does this also mean that
these switch at the same current regardless of voltage?

The secondary reason for having a closer look at PTC's is the possibility of
using them to limit inrush current for a valve amplifier - if they switch
fast enough...

A penny for your thoughts please...
cb

Archived from groups: rec.audio.tech (More info?)

In <c5rvdn$4dl$05$1@news.t-online.com>, on 04/17/04
at 09:10 PM, "Chris Berry" <christoforos@Notmail.com> said:

>Hi all,
>I'm trying to build in some PTC's into my 2 way speakers (used for
>bass guitar) so as to protect the midrange/tweeter in this 2 way
>system. I don't know what exactly the power rating of the tweeter is
>but I've been assured that for bass guitar it will work fine with my
>EVM 15B 200Wrms. From what I understand, the EVM is rated up to 1.5
>kHz so according to this: http://sound.westhost.com/tweeters.htm
>200W RMS actually translates to a "real world power handling of about
>150W RMS with 50W available to drive the tweeter above that frequency.

Many speaker manufacturers have had success using "Polyswitch" PTC
devices. They come in a variety of ratings, their response speed is
reasonable for protecting tweeters, they reset automatically, and are
relatively inexpensive.

That said, I don't think you will find a simple solution that will
offer absolute protection. You need a device that will track the
temperature of the voice coil and react accordingly. True, tweeter
heating is related to the voice coil current, but the amount of heating
is somewhat related to frequency. A simple PTC device is relatively
frequency independent.

Since manufacturers have started using Polyswitches, the number of
tweeter failures I've seen has dropped dramatically, however, the hard
core abusers still manage to destroy both the tweeter and the
Polyswitch (repeatedly). Polyswitches are rated for a certain number of
switching cycles. The Polyswitch offers excellent protection from an
occasional transgression, but is not a good long term solution for
chronic abuse. One slight problem is that the result of Polyswitch
action is a bit too subtile for the chronic abuser to detect. Since the
sound continues, the abuser assumes that there is no problem and the
abuse continues. Some of the more observant abusers will complain that,
when they push the system, the highs become muted. This abuser feels
that the speaker is at fault. (These are the same "smart" abusers who
would solve the "problem" of a constantly blowing fuse by replacing it
with a larger value.)

>I know what crossover to use in this application and that the tweeter
>will have an approx -2.5dB pad on it leaving about 28 or so watts that
>will actually "drive" it.

Appropriate PTC devices are usually inserted in series with each
driver.

>The confusing thing for me is how the PTC's are rated and how you go
>about selecting one for the application - non-destructively...

>My catalogue lists PTC's as having a "Switching current" so does this
>mean that for the 28W into 8 ohms I need a 1/2 amp PTC? Does this also
>mean that these switch at the same current regardless of voltage?

Yes, as long as the operating voltage is within the device's rating,
current is current.

A nondestructive iterative approach would be to start with a Polyswitch
with a somewhat lower rating and work with the speaker system. If you
find that the device is triggering too often, move to a higher trip
current. Continue raising the trip current till you reach the driver's
published limit or you stop having nuisance trips.

I'm not sure what your "28W" figure means. It doesn't seem like it is a
driver rating, rather, it's the available power in your application.
Don't assume that the original design guaranteed the drivers' safety.
Driver ratings are usually a bit vague anyway. The rating could be
"forever" at any frequency (within its intended operating range), or
there may be some time limits. The "forever" limit could be somewhat
lower.

>The secondary reason for having a closer look at PTC's is the
>possibility of using them to limit inrush current for a valve
>amplifier - if they switch fast enough...

TV sets have used PTC's for decades. While it's a bit more trouble and
expense, I prefer a relay switched series resistor to limit inrush
current.

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Archived from groups: rec.audio.tech (More info?)

"Peter Larsen" <SPAMSHIELD_plarsen@mail.tele.dk> wrote in message
news:4088BCD3.226EE0D8@mail.tele.dk...
> Please post all relevant facts up front.

Ah there's the rub, non technical people don't know what facts are relevant
until you tell them.

TonyP.

Archived from groups: rec.audio.tech (More info?)

Chris Berry wrote:

> The EVB 15B is running fullrange. Mounted coaxially is a grundig
> full range driver (about 3 1/2" ) with a little felt behind it.

Ah. Very different from what most people would expect when the word
"tweeter" is used. Generally a horn driver working above 2500 Hz would
be assumed when a treble unit is indicated, this is not a treble unit,
it is a cone type midrange unit.

> This is a 5.2 ohm driver.(this was measured with a
> proper impedance meter at different frequencies)

> The driver is from the German Bundestag so if it can
> deal with Helmut Kohl....:-)

Hot air is one thing, Musical Instrument use another.

> In series with that driver are 2 5.6 ohm (20W) resistors
> wired in parallel giving an 8 ohm load.

Aha. Is the attenuation at all required?

> a 15uF capacitor is wired in series with the 8 ohm load.

You should imo make a bit more out of this, i.e. determine Fs for the
midrange unit and compensate its impedance varitions accordingly. Here
is why: the units impedance will be higher at Fs. Because of that the
cross-over and attenutation resistors will fail to attentuate the input
signal at and around Fs of the driver. Worst case, and worst case does
not always apply, is that the largest thermal stress on it results from
signal around its Fs and not from "high range" that it is intended to
reproduce.

> I've no idea what the midrange driver's response is BUT
> a friend has been using it successfully with his evm 15B
> without blowing it so far.

Makes sense, it probably has a real world ... hmm, this *is* a guess ...
5 to 10 watt long term powerhandling and being mounted as it is provides
it with excellent cooling.

> It seems to provide usable output to 5kHz.

It should, and it should be reasonably smooth up to that range, by its
size, nothing else known, I would expect usable output to 10 kHz.

> Peter, don't get me wrong, I post quite vaguely not so someone
> can just tell me - use that component - but so I can understand
> why and calculate things for myself.

Understood, but the above text would have been _very_ different in case
of a horn type unit, and I did want to know which of two possible
followups to write ... O;-)

> cb

The above is excluded from the applicability of CE standardisation for
mileage specs because those are kilometric, in other words: your mileage
may vary ... oh, and:

I would probably use a 12 dB pr. octave x-over on the unit or use a
smaller cap for say a 2500 Hz first order cross-over.

Using polypropylene caps may be well worth it, but that is just my way
of doing it .... try a 10 microfarad and also a 6.8 microfarad component
value too, I think 10 could be a way better choice than 15.

A 1 ampere slow blow fuse may suffice (x), or the unit may protect it by
blowing up first (xx), I'd start with 500 mA and see if it blows or nor.

(x) suggested by Ohms law

(xx) suggested by Murphy's law.


Kind regards

Peter Larsen

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