[TriEmbed] speaker stuff

[Brian]

A few tips regarding speakers that everybody might already know:

The rated impedance (Z) of a speaker is generally measured at 1 kHz.  DC 
impedance is lower (impedance of a speaker can be modeled as a resistor 
in series with an inductor; the inductor provides an frequency-dependent 
imaginary impedance [jwL]* while the resistor provides a constant real 
impedance [R]; the total impedance is the cross-product of the two 
components--the square root of the sum of the squares.  Back-EMF [a 
speaker is a motor!] is generally ignored).

Speakers do not like DC.  Your amp shouldn't be putting any out, but if 
there's any question, put a series capacitance in the speaker line to 
act as a DC block.  Some speakers get very upset when all the angry 
pixies flow in the same direction.  Consult your speaker datasheet.  At 
best, it's wasted power; at worst, that wasted power heats up the 
speaker enough to burn it up.

Yes, P = I^2 * Z, but Z is frequency-dependent.  Your calculations will 
represent response at 1 kHz.  Lower frequencies at the same amplitude 
dissipate more power; higher frequencies, less.  This is likely why your 
tweeters were blowing up if you weren't using a crossover.

Those PTZ fuses are specifically called out as "not for use with piezo 
tweeters," so I assume yours are dynamic.  PTZ fuses don't care which 
way the current flows; they only care about power dissipation.  The 
dissipation across the PTZ will be the product of the current through 
the PTZ times the potential across the PTZ.  Since the signs of both 
will always match, that product will always be positive.  Looked at 
another way, P = I^2 * R again, where R is the PTZ's nominal "closed" 
impedance.  Since I is squared, sign is irrelevant.

Wait a minute...  David Soper, Pete Soper... COINCIDENCE?!?!?

Anyway, time for me to go back to work.  Have fun!

-B

* - where "w" represents omega, the frequency in radians per second.


On 1/12/22 11:57 AM, Pete Soper via TriEmbed wrote:
> I'm very impressed. Given eight ohm speakers the fuse will open with P = 
> I^2R or .5x.5x8 = two watts going into the speaker for long enough to 
> heat up the fuse and cause it to open. I'm assuming, however, that the 
> fuse operates properly with AC. You should study the datasheet for this 
> part to confirm this. If you hit a snag, holler and I'll ask a guy who 
> knows a lot about this type of device. But well done you!
> 
> On 1/12/22 09:12, David Soper wrote:
>> I have some NP capacitors in the closet wired into the wires going to 
>> my kitchen speakers.  It occurred to me that I don't need those if I 
>> switch my receiver back to crossing over. It was the tweeters I was 
>> blowing up anyways.  I soldered these in this time for the tweeters 
>> but plan to be careful. I had .75 amp ones in before.
>>
>> https://www.parts-express.com/TE-Connectivity-Raychem-RXEF050-0.50A-Polyswitch-071-254?quantity=1&custcol1=TE%20Connectivity%2FRaychem%20RXEF050%200.50A%20Polyswitch&custcol_ava_item=071-254&custcol_ava_incomeaccount=General&custcol_ava_upccode=844632046889&custcol_ava_pickup=F&custcol_disableshopping=F
>>
>> -- 
>> Dave Soper
>>
>>
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> 
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