3 way impedance match - Page 1
Forum section: electronics / beginners

author: rahulrahullll
author_rank: Contributor
timestamp: November 23, 2017, 05:26:17 am
content: i have---60 w 4 ohm woofer40 w 4 ohm mid30 w 12 ohm tweeter4 ohm 3 way crossover basic type---2 inductor and caps, resistors-------tda 2050 amp---30 w at 8 ohms/ 50 w at 4 ohms--------WHAT IS THE IMPEDANCE OF THIS 3 WAY TOWER SEEN BY TDA 2050 METHOD OF CALCULATION ?

author: Brumby
author_rank: Unknown
timestamp: November 23, 2017, 05:50:10 am
content: 3 onions and a canary.Seriously, there is not enough information to give a definitive answer. A full circuit diagram of the entire crossover, including speakers would be necessary to calculate an answer - and if you want to get really pedantic, at least the frequency vs impedance characteristics of each of the speakers.With a bit of guessing and assumption, we could offer a ball park figure - but I've not been into crossover network design and it's been years since I've even looked at them that closely.PS. Don't use all caps - unless you want to be interpreted as SHOUTING.

author: rahulrahullll
author_rank: Contributor
timestamp: November 23, 2017, 06:35:49 am
content: thanks----------i have another doubt--------how measure ac current output of amplifier by dc multimeter(in my multimeter there is only dc current mode) just wanted to calculate powerhttp://www.ashopbd.com/wp-content/uploads/2017/02/Kros-s-Pocket-Digital-Multimeter-SDL326809972-1-97f3b.jpg

author: Brumby
author_rank: Unknown
timestamp: November 23, 2017, 07:50:16 am
content: 1. Get a resistor to match the amplifier output - 4 ohms, 8 ohms or whatever - with a high enough power capacity to handle the amplifier output.2. Measure this resistance as accurately as possible.3. Connect to amplifier and feed the amplifier with (ideally) a sine wave of no more than 400Hz (limit of your DT830D)4. Turn the volume up on the amplifier and measure the AC voltage across the resistor.5. Use Ohm's law to calculate current.General recommendation: Do NOT run your amplifier at elevated levels for any longer than youabsolutely need to.Any full power measurements are going to stress the amplifier. It will be your responsibility to ensure that such stress does not exceed the limits of the amplifier or you could damage it.

author: macboy
author_rank: Super Contributor
timestamp: November 23, 2017, 03:21:35 pm
content: Quote from: rahulrahullll on November 23, 2017, 05:26:17 amIf your crossover expects a 4 Ohm tweeter, then in order for it to work correctly, you need to add a 6 Ohm resistor in parallel with the tweeter. This provides a 4 ohm total impedance, which will make the crossover point correct. Otherwise, the tweeter will produce sound at a much lower frequency than expected. There is usually not much power in the highest couple octaves, so a 5 Watt resistor is quite big enough.Even though you have three 4 Ohm speakers (after adjusting the tweeter to 4 Ohms as above), the load to the amplifier is still just 4 Ohms. The crossover effectively splits the signal between the speakers based on the frequency content. The TDA2050 is rated to drive 4 Ohm loads, so you should be OK. Keep in mind though, that speakers are complex electro-mechanical devices, and the "4 Ohm" rating is only "nominal" or approximate. The real load may drop to 1/2 of nominal (~2 Ohm) at some frequencies and may go very high (tens of Ohms) at other frequencies. The amplifier might have difficulty if the woofer drops much lower than 4 Ohms.Consider upgrading to a better crossover. A 3-way unit with a 2nd order (12 dB/octave) slopes will have 4 inductors, 4 caps, and maybe some resistors.

author: rahulrahullll
author_rank: Contributor
timestamp: November 23, 2017, 03:27:59 pm
content: thank you-----Brumby ---actually i dont know how 400 hz sine wave sending , dont have 4 ohm--30 w resistor--quite long process----is there any shortcut method to measure ac current by dc multimeter----any approximate method maybe----http://www.ashopbd.com/wp-content/uploads/2017/02/Kros-s-Pocket-Digital-Multimeter-SDL326809972-1-97f3b.jpg

author: rahulrahullll
author_rank: Contributor
timestamp: November 23, 2017, 03:35:37 pm
content: thank you--macboy------so according to you what should be the speaker resistance for woofer,mid,tweeter and crossover for 4 ohm amp and 8 ohm amp-------------

author: Bud
author_rank: Super Contributor
timestamp: November 23, 2017, 03:42:50 pm
content: BTW you have the dash key stuck.

author: jmelson
author_rank: Super Contributor
timestamp: November 23, 2017, 05:58:52 pm
content: Quote from: macboy on November 23, 2017, 03:21:35 pmFREQUENCY?? No, not possible. Yes, the tweeter will get the wrong amount of power, and possibly the balance of sound would be wrong. I'd suspect the tweeter would be not producing much output, but the LC components in the crosover could do just about anything with this mismatch.Oh, maybe I get your meaning, the tweeter would start getting power from the crossover at a lower frequency than intended. Yes, could be, assuming the crossover just uses a series cap for the tweeter. But, with the 12 Ohm impedance, it might never get much output, compared to the other speakers.Jon

author: macboy
author_rank: Super Contributor
timestamp: November 23, 2017, 08:50:11 pm
content: Quote from: jmelson on November 23, 2017, 05:58:52 pmThe power in the tweeter at any given high frequency (well above the crossover frequency) is the same, with or without the resistor! The problem is that the crossover frequency is shifted down because the resistance of the speaker coil is part of the filter itself. Higher R means lower Fc.\$F_{c}=\frac{1}{2\pi RC}\$The crossover described by the OP is a 1st order one, and for the tweeter, is just a simple capacitor. It, together with the resistance of the speaker/tweeter (12 ohms) forms a one pole high pass filter. At very high frequencies, the capacitor is low impedance and the speaker sees the full voltage (very little voltage dropped across the cap). At very low frequencies, the capacitor is high impedance and the speaker sees very little voltage (it is almost all dropped across the cap).The shift in the frequency of the crossover point means that the tweeter will be exposed to more musical power and might burn out or be physically damaged by large excursion.(edit)Even if it isn't damaged, there will likely be a hump in the frequency response of up to +6 dB due to the overlap of the mid and tweeter spanning about 1.5 octaves.(/edit)FYI, the woofer has a coil in series with it forming a LR lowpass filter, and the midrange has a coil and a capacitor in series with it, forming a RLC bandpass filter. (A much better 2nd-order crossover would add shunt elements as well, but the OP said his had 2 coils and 2 caps, so it can only be 1st order).Since the OP said the crossover was designed for 4 ohms, it needs a 4 ohm load for the crossover frequencies to be correct. That is why the 6 ohm resistor is added in parallel to the 12 ohm tweeter.(edit:)It does not attenuate the tweeter response at all (a series resistor would). The 12 ohm tweeter might or might not be efficient enough to provide a flat(ish) response in combination with the other drivers. Randomly picking a woofer, mid, and tweeter will not usually yield a 3-way speaker with a good flat response. Usually, one or two drivers need to be padded (attenuated).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2017, 02:28:26 am
content: Speakers are terrible loads. The impedance is complex versus frequency, making filter design awkward. The best approach is to measure the speaker impedance, with the speaker in the enclosure, and use that to design the filter.On the upside, the flatness spec is terrible, too (a speaker might be +/-10dB, and listeners acclimate very easily to worse), so it doesn't much matter -- how accurate or how well designed the filter is.Tim

author: Brumby
author_rank: Unknown
timestamp: November 24, 2017, 02:29:10 am
content: Quote from: rahulrahullll on November 23, 2017, 03:27:59 pm- You can get Apps for mobile devices.- You can use the sound card on your computer and use a program or search for "sine wave generator" for an online source, such ashttp://onlinetonegenerator.com/- You can build a simple sine wave oscillator. It doesn't have to be low distortion - just reasonable.The fact of the matter is, if you want to measure the output of the amplifier, you are going to have to feed it a signal. Ordinary audio - like a song - is not steady enough for measurement by a multimeter and would have too high a crest factor anyway. So, if you want to feed a signal that your equipment can measure with any confidence, then a sine wave is the one to use.QuoteGet creative. 30 120ohm 1W resistors in parallel; A long piece of wire; Some nichrome wire; Salvage a piece of the element from an electric heater.QuoteIn all seriousness, this is NOT a long process. However, what do you expect when you don't have the right equipment?QuoteDo you have a diode and a capacitor? Create a simple peak detector. This will give you a maximum volts as a DC value - but you will have to add the forward voltage drop of he diode to get the actual peak voltage. Again, use a sine wave as your signal source and you can divide the peak volts by 1.414 to give you an equivalent RMS value of the AC. To get the current, you will need to divide this by the impedance of the load at the test frequency used.In short, by using test gear that cannot directly measure the quantity, you are going tohaveto use indirect methods and some (simple) mathematics to get an answer.

author: Audioguru
author_rank: Super Contributor
timestamp: November 24, 2017, 04:44:29 am
content: Why do you want to measure AC current with a DC multimeter? To calculate the power?Why do you need to calculate the power when the datasheet shows a graph of the power into a 4 ohm load (that you have) with different supply voltages.If your supply is plus and minus 15V (30V total) then the power is a little less than 20 Watts. The output power is 50 Whats (with horrible 10% distortion) into 4 ohms only when the volume control is turned up too high and the supply voltage is the maximum allowed voltage of plus and minus 22.5V (45V total) then the real power is a little less than 40 Watts at a fairly reasonable lower distortion.The output impedance of the amplifier is not 4 ohms or 8 ohms. Its load can be 4 ohms or 8 ohms and the impedance is not matched. The output impedance of a modern amplifier is 0.04 ohms or less so that it can damp the resonances of speakers. A woofer would sound like a one note bongo drum if its resonance is not damped by the very low output impedance of an amplifier. Midrange and tweeters also resonate but at higher frequencies and sound much better when properly damped.EDIT: Here is the graph of power: