do tweeters need to be protected when making measurements on them?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
A cap is always a good idea. Particular care needs to be taken around the tweeter's resonance, as even a very low input can result in a substantial cone displacement. If the tweeter's resonance is, say 1,000 Hz, a 1 uF cap will be ~160 Ohms, plenty of attenuation at 6 ohms. But at resonance, some tweeters' impedance will reach 50 ohms, so that there is a lot less attenuation. So even a cap has protection limitations.

At higher frequencies, the tweeter's operating range, a one volt signal will be fine, about 100-200 mW, with no danger at all of the voice coil frying.

At resonance, start at a very low level signal, and increase it slowly until you have the minimum level needed for your measurements. If the nature of the sine wave out changes audibly, (or on the scope if you are using one) back down to perhaps half that level.
 
Thanks for the reply curmudgeon. I was going to use speaker workshop for FR measurements of my vifa XT25TG30-04 tweeter. I think it uses an MLS signal so I'm not sure if you can limit the frequencies you test at. I can get away without a cap if I'm careful with the volume levels and only go up as high as needed to get a usable measurement?
 
yes, with a carefully limited signal. What is the source impedance of your signal source? A very low source impedance as from a power amplifier will keep the voltage constant, regardless of tweeter impedance. A higher impedance, such as from a signal generator, will show a marked increase in V across the tweeter at resonance, which is not really what's wanted...;)
 
For the frequency response measurement I'll be hooking the speakers up to my power amplifier and measuring with a mic. For impedance measurements, I'll be driving them with the sound card.

That's exactly what I do with SW. Impedance measurements using only the sound card's grunt - no external amplifier.

for tweeter amplitude (frequency) response, I set the amp at 1V output for all my tweeters. Haven't fried one yet. The Vifa D19 3/4" dome I tested was fine with a 1V output level.

Just put a DMM set to AC voltage measurement across your amp speaker terminals and play an MLS tone. Increase the volume on the pre-amp until you read 1V and you're ready to go.

Don't change the volume level when switching to woofer measurements, otherwise you won't know how much to pad the tweeter (unless you scale after the fact)

David.
 
diyAudio Chief Moderator
Joined 2002
Paid Member
Re: Okay then...

Undefinition said:
I haven't used any protection on any tweeters I've tested yet, and haven't had any ill effects. However, I have heard it's a must on ribbons and planars. That may be, but it makes me wonder this, then:

Won't putting a cap in-line with the driver cause a 90 twist in the phase of the measurement? How would one account for that?


Yes its a must on Ribbons and planars especially with Pink noise.

Yes it will turn the phase. You can compute minimum phase using only the SPL data though. Soundeasy has Hilbert, LEAP has Minimum Phase Transform and tail correction in its Processing menu.
 
Re: Okay then...

Undefinition said:
I haven't used any protection on any tweeters I've tested yet, and haven't had any ill effects. However, I have heard it's a must on ribbons and planars. That may be, but it makes me wonder this, then:

Won't putting a cap in-line with the driver cause a 90 twist in the phase of the measurement? How would one account for that?

If you are using a two channel MLS system lie SoundEasy, Speaker Workshop, LoudSpeaker Lab etc, place the probe between the cap and the driver and the effects of the cap (freq resp and phase) will be normalized out.

Also some systems such as SoundEasy or LspLab allow you to use pre-emphasis when measuring tweeters and this lowers the level of the low frequencies in the MLS signal while still giving accurate frequency response.

Regards,

Dennis
 
diyAudio Chief Moderator
Joined 2002
Paid Member
This is the one:
 

Attachments

  • screenshot19.gif
    screenshot19.gif
    14.7 KB · Views: 158
Thanks Ray, I know its around there.
For some reason when I tried to calculate I kept
getting the incorrect answer.

Next question, does it matter if wired in series
with positive terminal
or negative terminal.

I was thinking negative line because that is the current
flow?

I'm sure simplest to just solder it to the tweeters terminal?
and not melt the tweeter....

When I use the tweeter in the speaker cabinet,
I can adjust the phase from the active crossover, I believe.

It's in there somewhere I just have found the correct module though.

I'm thinking then with a 20uf cap with rolloff at about 2K,
I should be fine using this setup.
 

Attachments

  • 01. CutOff 10 250 2500 inf.jpg
    01. CutOff 10 250 2500 inf.jpg
    85.3 KB · Views: 62
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.