Changing the impedence of a speaker driver  - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: May 09, 2019, 05:49:19 pm
content: Hello everyone,I,m planing to make use of a pair of woofers and tweeters lying around in my house. Both are having an impedance of 4 ohms and I believe it creates a net impedance of 2 ohm when connected through a crossover. Since my amplifier could only go down upto 4 ohms, I'm planning to change the impedance of the drivers. I have heard that audio transformers could help to achieve this but I don't know how. Could anyone suggest me a solution?

author: soldar
author_rank: Super Contributor
timestamp: May 09, 2019, 09:53:07 pm
content: Several thoughts.Nominal impedance does not mean actual impedance over the whole range.If you have a proper crossover filter the impedance should not decrease over the whole range.Depending on your amplifier it probably can handle lower impedance.A transformer is really not a good way to do it.It would be better if you can supply more information.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 10, 2019, 04:35:00 pm
content: Thank you very much for your reply. By considering the nominal and actual impedance range you meant that it is possible to run this setup? I have actually wired this setup today and it was working. However by raising the volume above 60 percent the amp automatically turned off. I'm assuming that the in build overload protection triggered it. So does that mean if I keep a volume below 60 percent I could safely operated the amp?If you could specify about what all specs should I give it would be great help

author: GregDunn
author_rank: Frequent Contributor
timestamp: May 11, 2019, 12:54:05 am
content: You're running them in parallel? That's dangerous at any level. Not only does that make the nominal impedance seen by the amp 2 ohms (which is probably what's shutting off your amp) but you are likely to destroy the tweeter with the low frequency content of the signal. Before you run it again, obtain or build a low pass / high pass crossover network and insert it between the amp and the speakers. As an added benefit, this will make the output in the region where the speakers would overlap quite a bit flatter.

author: soldar
author_rank: Super Contributor
timestamp: May 11, 2019, 11:35:09 am
content: Quote from: GregDunn on May 11, 2019, 12:54:05 amI interpreted his post to mean he was already doing this.

author: radiolistener
author_rank: Super Contributor
timestamp: May 11, 2019, 11:59:56 am
content: Quote from: soldar on May 09, 2019, 09:53:07 pmWhy? Transformer gives noise free and distortion free impedance match with high dynamic range. I think transformer is a preferred solution from signal quality point of view. But transformer will be too heavy, so it is not good for mobile solutions, where equipment weight is critical.

author: Zero999
author_rank: Super Contributor
timestamp: May 11, 2019, 12:03:41 pm
content: A transformer will be very bulky and introduce more distortion.Quote from: GregDunn on May 11, 2019, 12:54:05 amYes, they shouldn't be just connected in parallel, unless it's a piezo tweeter, which will have a higher impedance anyway.A basic crossover can be just a capacitor in series with the tweeter. The woofer will have a fairly high impedance at higher frequencies, so won't load the amplifier too much. It's not as good as a proper crossover, specifically designed for the speaker though.

author: radiolistener
author_rank: Super Contributor
timestamp: May 11, 2019, 12:13:48 pm
content: Quote from: Adhith on May 09, 2019, 05:49:19 pmyou're need transformer with impedance ratio 1:2.Transformer impedance ratio is a square of turns/voltage ratio.So, you're need 1:1.41 turns/voltage ratio transformer in order to match 2 Ohms load with 4 Ohms source.The primary coil turn count W1 will depends on low frequency range and transformer core specifications.In your case it should keep 2 Ohm impedance for 20 Hz when secondary coil is loaded with 4 Ohm load.The secoundary coil turn count W2 = W1 * 1.41.Here is example of typical usage of speakers with different impedance and high impedance transmission line:

author: radiolistener
author_rank: Super Contributor
timestamp: May 11, 2019, 12:15:27 pm
content: Quote from: Zero999 on May 11, 2019, 12:03:41 pmI think properly calculated transformer should not introduce distortions.The distortion problem happens when transformer is used outside it's bandwidth and power limit specification.Nice feature of transformer is that you can use it for impedance match with no adding noises and distortions. Amplifier always add some noise and has low dynamic range.

author: soldar
author_rank: Super Contributor
timestamp: May 11, 2019, 01:45:17 pm
content: Designing an audio transformer with a good, flat response over the entire range is quite a challenge. It is not trivial.

author: radiolistener
author_rank: Super Contributor
timestamp: May 11, 2019, 02:30:22 pm
content: Quote from: soldar on May 11, 2019, 01:45:17 pmyes, it's not easy. But all depends on requirementsOn the other hand active amplifier always add some noise, has very limited dynamic range and IMD.Designing amplifier with high dynamic range and low IMD is also quite a challenge and requires expensive components

author: soldar
author_rank: Super Contributor
timestamp: May 11, 2019, 03:25:27 pm
content: Again, a proper crossover filter should take care of the problem. Getting or building audio transformers is the least practical solution. I would rather put a couple resistors in series. What power are we talking about? Who cares if half is wasted?Again, "4 ohm" is strictly nominal. The impedance varies over the range of frequencies. Search for -- speaker impedance curve-- and you will see what I mean.

author: Zero999
author_rank: Super Contributor
timestamp: May 11, 2019, 05:56:10 pm
content: It's also highly likely the amplifier will be able to drive 2Ohm loads with no problem, especially if it's a modern amplifier with over-current protection. The downside is it won't be able to delver as much power, as it would into the rated load.It's quite likely cheaper to buy new speakers with the correct impedance, than a transformer, which will be more expensive.

author: Calvin
author_rank: Regular Contributor
timestamp: May 12, 2019, 04:20:01 am
content: Hi,"...and I believe it creates a net impedance of 2 ohm when connected through a crossover."That won't happen if a proper xover is used.Within its passband the branch of a xover shall not change the speaker drivers impedance.In its stopband the filters impedance rises and dominates the combined driver/filter impedance the amplifier sees.As for example the tweeters passband falls into the stopband of the woofer (and vice versa) the amplifier will only see the woofer impedance up to the transition frequency point and the tweeter impedance above, hence 4Ohms over the whole frequency range.Due to this no impedance matching transformer is required.regardsCalvin

author: dietert1
author_rank: Super Contributor
timestamp: May 12, 2019, 06:33:20 am
content: A contemporary solution with much better results is activation, i. e. another amplifier for the second pair of speakers. Then digital filters can be used as crossover and for room correction, resulting in high sound quality. Let's mention miniDsp as a supplier. Twenty years ago that could be obtained only using extremely expensive analog equipment ("high end"). That kind of analog equipment is obsolete nowadays.Regards, Dieter

author: _Wim_
author_rank: Super Contributor
timestamp: May 12, 2019, 10:21:53 am
content: I would suggest to first measure the actual impedance. You can do this with a normal sound card, a reference resistor (100 ohm for example) and free software:https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.htmlYou can use the same software to actually measure your frequency response (you will need a mike for that) and crossover.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: May 13, 2019, 09:19:33 pm
content: Autoformers...EXCLUSIVE MCINTOSH OUTPUT AUTOFORMERS.An impedance mismatch between anamplifier and loudspeaker can cause distortion and a reduction in power. The legendaryMcIntosh autoformer is a hand-crafted transformer with output connections for 2, 4, and 8 ohms,allowing an ideal impedance match. A McIntosh amplifier with an autoformer can also safelydrive multiple speakers connected in parallel without shortening the life expectancy of the outputstage. There is absolutely no performance penalty with an autoformer. In fact, its frequencyresponseexceedsthat of the output circuit itself, and extends well beyond the audible range.Distortion is so low it is virtually immeasurable.The “Bifilar” winding techniqueused in the making of autoformersearned McIntosh one of its firstpatents. The design is so advancedit is still used today.https://www.audioclassics.com/brochure/MC602.pdf

author: james_s
author_rank: Super Contributor
timestamp: May 13, 2019, 10:25:30 pm
content: You don't need a transformer, you need a crossover. That will route the low frequencies to the woofer and the high frequencies to the tweeter and you'll end up with something near 4 ohms nominal. This is how speakers work, I've never seen one with a built in matching transformer for parallel drivers.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: May 14, 2019, 03:13:10 pm
content: Right...The OP meant well, but his question was "how to change the impedance of a speaker". the real question is how to use High Range, and Low Range drivers together, in a system, without causing an impedance mismatch seen by the amplifier. Because, he is right... without a crossover at some range, the impedance will be too low. Obviously in a properly designed crossover there will be a distributed impedance across the entire frequency range that will provide corrected impedance over that range, and at the very important crossover region of the two drivers, seen by the amplifier. Determining the correct Xover frequency for whatever those two chosen drivers are, becomes critical. -3dB there, for each driver. Then the frequency response of the individual drivers is left. There is always the possibility that the chosen drivers do not even cross at a desirable frequency, or level. Driver mis-match.In a pinch, manufacturing specs could be used, but safer to measure each driver, and both drivers together, properly crossed over in the chosen cabinet, with pink noise, using an RTA with calibrated Mic, to see the acoustic response over the range. Your handy anechoic chamber will help as well.Using an electronic Xover is another easy way to manually adjust the Xover region, and levels (using two amplifiers) as you watch the response. Then, with that information, you can design the right passive crossover for the job, or just set your electronic one, and be done.

author: james_s
author_rank: Super Contributor
timestamp: May 14, 2019, 05:09:33 pm
content: When I was a kid I pieced together quite a few speakers using whatever drivers and other parts I could get my hands on for free. Some sounded much better than others, I didn't really know what I was doing so it was a crapshoot. They all worked to some degree though and none ever damaged the amplifier or caused it to misbehave. In practice the details are not *that* critical, unless you are going for audiophile sound quality. Something as simple as a capacitor in series with the tweeter will probably work just fine.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: May 14, 2019, 07:06:04 pm
content: ...and the crapshoot of choosing that capacitor.