Help in adding tweeter to horn speakers

[ngccglp]

Hi,

I've seen people adding JBL hi freq driver (2402, 2405 etc.) to horn speakers by connecting a capacitor in series to the +ve terminal of the driver.

May I know:

1. Whether the capacitor in series is performing the freq cut off function

2. If so, may I know the value of the capacitors to achieve 18 khz, 20khz and 22 khz crossover respectively?

Thanks.

[johnaec]

Why such a high frequency? The 2402 is virtually useless above 18K, and the 2405 barely...

John

[Don Mascali]

I use 2344A horns and 2426H drivers for HF and have added a 2404 UHF with a 2.2 mfd cap in series. JBL says the minimum impedance for these is 7.2 ohms. I'm not sure what the formulas say as to the "correct" value, but I've tried several values and these sound the best to me.

I like the gradual (6db/octave) roll off as it seems to work well with my horns. I actively cross all of the other points so I don't have the HF roll off compensation for the 2344A/2426H combination which the guys have written tons about in the 4430 thread.

Good luck,
Don M

[Mr. Widget]

May I know:

1. Whether the capacitor in series is performing the freq cut off function

2. If so, may I know the value of the capacitors to achieve 18 khz, 20khz and 22 khz crossover respectively?

It isn't quite that simple. I single capacitor in series with a speaker will provide a first order cut off... that means you will get a 6dB per octave roll off with the -3dB point typically considered it's cutoff frequency. To determine the value of capacitor for a given frequency you will need to know the impedance of the speaker (any type including horn tweeters) and refer to a chart or get the formula... do a google search there are dozens of places to find this info.

Off the top of my head here is an example. For an 8 ohm speaker a 4 uF cap will give you a -3dB point of 5KHz, 2 uF will be 10KHz, 1 uF will be 20KHz.

A first order crossover has such a gentle slope that even the 1uF cap will allow a significant amount of acoustic energy be produced by a tweeter that only has a response to 20KHz.

In answer to the second half of your question, obviously all three values would be around 1 uF but I am confident you won't hear a difference between these three values... certainly not at those frequencies.

It is convenient to talk about these networks as blocking the lows or cutting off low frequencies, but in reality it isn't that simple. If you really want to study the matter you will discover that different order crossovers have different effects of the phase of the drivers, and will affect their polar plots, and will have differing amounts of driver interaction.

Widget

[frank23]

it is of course more complex than simply adding a capacitor in series

but if you want to do it the simple way you have to consider 2 things:
- cross over at least 1 octave from the impedance peak caused by resonance
- match the sound level by adding a resistor deviding network between capacitor and driver

if this simple method is performed right, it can be quite effective, especially because your ears are a lot less sensitive at 15kHz than at 1.5kHz, so any errors are not so prominent

greetings, Frank

[ngccglp]

Thanks guys.

My Altec A7 mid and bass drivers are all 8 ohms, but the tweeter is 16 ohms, can I still use the tweeter or must I really change the diaphragm to a 8 ohm?

[Mr. Widget]

The name plate may say 16 ohms but it is quite possible that at the crossover region the driver isn't actually 16 ohms... in any case it's impedance relative to the other drivers is immaterial. It's impedance must be taken into account in regards to designing it's high pass filter. (Choosing a cap in this case.)

Widget

[ngccglp]

Thanks Widget,

You are spot on, the value of the capacitor used by my friend is 1 uF.

Any idea what value of the capacitor I should use if the driver is 16 ohms. I've been hunting around, so far can only find 2405 that are 16 ohms.

Thanks.