Intermittent powered speaker, woofer impedance at 1.7ohms! - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: July 25, 2021, 08:33:22 pm
content: Hi guys,I've got a mackie thump15 powered speaker in for repairs. The issue was the sound cuts out when the speaker is turned up loud. My initial thought was dry joints and intermittent connection due to vibration.When I got it in and after removing the amp I measured the resistance of the drivers and the woofer measures at 1.7ohm!. That didn't look right. The marking on the speaker states that its suppose to be 8 ohms. So then my thought was the speaker coil have shorted partly and the amplifier chip was detecting a fault when driven hard and was cutting out. The amplifier chip used is TDA8924TH.But here is where things got a bit interesting. To check whether the amplifier was functioning I connected my test woofer rated at 8 ohms and it was working fine no issues (my test woofer was not attached to the enclosure). To see whether the sound cuts out, I then checked with the faulty woofer and it worked perfectly fine :S and same as before the woofer is not attached to the enclosure.I haven't got a working mackie thump to measure the actual resistance of the coil of the woofer. The label on the speaker says 8 ohms though I know sometimes labels can be misleading. If some one can confirm the resistance of these would be greatly appreciated. Can the TDA8924TH be paired with a 2 Ohm-ish load and work?Thanks in advance for your input.

author: TimFox
author_rank: Super Contributor
timestamp: July 25, 2021, 08:46:50 pm
content: Eight ohms should be the AC impedance (not constant over frequency) that results from the coupling of the speaker and the air. The DC resistance should be less: otherwise, the system is very inefficient.You could measure the speaker impedance (including acoustic load) by placing 1 to 4 ohms in series with the ground lead to measure the current when driving the disconnected speaker from an audio signal generator (maybe 50 or 600 ohms source impedance) and measuring the AC voltages across the coil and across the resistor with an oscilloscope or AC voltmeter. Make sure the voltmeter is good at your frequency range: they all have a high frequency limit, and for a subwoofer the AC response may fall off at low frequency. With an oscilloscope, you may have problems with a common ground, and measure the total voltage (coil plus resistor) and do some complex (phase sensitive) math.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 25, 2021, 08:55:28 pm
content: Thanks @TimFox, normally with an 8 ohm rated speaker I do get around 6-7ohm DC resistance. If I'm measuring around 1.7 ohm DC resistance could it still be considered ok?

author: TimFox
author_rank: Super Contributor
timestamp: July 25, 2021, 09:09:28 pm
content: I don’t have specific examples, but I would suspect that a subwoofer would have lower resistance (compared with nominal impedance) than mid-range or tweeters. Also, speaker systems with crossover networks have extra resistance in the networks. Efficiency is probably more important for subwoofers. Many multi-speaker systems have passive variable attenuators for the high-frequency units. The AC impedance is a “good thing”, since it couples your amplifier’s output power to acoustic power, but the DC resistance is only an inefficiency.After writing that, I looked at some Jensen guitar speakers, around 12 to 15 inch, and found that the 8 ohm units did have spec values of around 5 ohms for DC.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 25, 2021, 09:47:37 pm
content: An 8 0hm speaker measuring 1.7 ohms resistance is f**ked.Your normal measurements of 6-7 ohms is what you should expect.

author: TimFox
author_rank: Super Contributor
timestamp: July 25, 2021, 09:50:09 pm
content: Is that still true for the driver in an active subwoofer circuit? Perhaps with current feedback?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 25, 2021, 10:00:17 pm
content: This is not a Sub, it is a conventional driver plus tweeter.I doubt any kind of 8 ohm driver is going to measure anything like 1.7 ohms resistance, it is f**ked.By the way, Yamin, before buying a generic 15 inch driver to replace it, you'd better measure the existing speaker accurately.You will probably find nothing you can buy will fit the cabinet or the mountings.

author: TimFox
author_rank: Super Contributor
timestamp: July 25, 2021, 10:25:04 pm
content: 6 ohms DC for an 8 ohm speaker seems to be true, possibly a requirement to avoid low load on the amplifier, but that would mean that a damping factor > 10 is a waste of effort, since the 6 ohms is a parasitic resistance in series with the "motor" and the output impedance of the amplifier..

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 26, 2021, 11:08:03 am
content: I may be wrong about the Thump 15A speaker size not being standard.The SRM350 and SRM450 range definitely use a slightly smaller speaker and nothing else will fit easily.Check carefully if you are going to replace by a non Mackie driver, though you should really do this in pairs.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 26, 2021, 12:19:43 pm
content: Quote from: Audiorepair on July 26, 2021, 11:08:03 amThanks @Audiorepair, I did do a dry fit with a yamaha 15" driver and it does seem to fit.I wanted to ask - why do you think that the faulty woofer worked? I didn't run it for long as I didn't want to cause further problems. The woofer didn't cut out on me like it did for the customer for the couple of minutes I passed signal through. Thats why I started having second thoughts about that maybe the label of the speaker was wrong and it might be like I first thought dry joints.Did you work on thump15 before?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 26, 2021, 06:35:14 pm
content: Here's a weird thing, I got 2 of these in for repair today!Both speakers worked at very low volumes, but any increase and the amp cut out.Took out both drivers, and both measured around 0.3 ohms.Clearly the voice coils here have shorted turns, yours have less shorted turns than mine, but even a couple of working turns would be enough to get some sound out of the driver.The amp will cut out once it exceeds its current limit, it doesn't sense low ohmage.And yes I have worked on these before, there are lots of them out there and people break them.My customer claims they were fine before he stored them in the garage for a long time, so maybe the voice coils have corroded into shorted turns.I will have to cut them open and have a look.And yes, I also offered up an old 15" driver to check if it would fit, and it does, so it looks like Mackie decided not to go the unobtanium route on alternative drivers and sourced more generic ones.The SRM models I mentioned are around 12 inch drivers, but not quite.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 26, 2021, 08:54:41 pm
content: Quote from: Audiorepair on July 26, 2021, 06:35:14 pmHehehe nice coincidence. The drivers are rated for 8 ohms right?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 26, 2021, 09:58:31 pm
content: Will check that tomorrow.I have come across speakers that use one impedance driver in the Active version, and a different one in the Passive cab.I think it was Peavey who did that.So you could double up the 8 ohm passives to give 4 ohms on your own amp, but the active version was only capable of its own 4 ohm driver, giving a higher SPL than an 8 Ohm would.

author: andy3055
author_rank: Super Contributor
timestamp: July 26, 2021, 10:11:35 pm
content: Just a couple of things:1. I hope you checked the speaker with everything removed from the terminals-at least from 1 terminal of the speaker as any crossover components connected will throw our readings all over.2. If the speaker truly has a burnt voice coil, before hooking up a replacement, make sure there is no DC present on to the terminals. ie., check the amplifier output for any DC coming out.A good voice coil also can be checked with a 1.5-3 volts DC applied. There will be a good movement of the cone back/forth when you reverse the polarity. I say this because some speakers are designed to be very low resistance in their voice coils and that is normal. Bose makes speakers like that and they also use Aluminum voice coils! But as one of the posts above mentions, yours might be a regular speaker and should have upwards of 6 ohms for the woofer.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 26, 2021, 10:31:31 pm
content: Thanks @andyQuote from: andy3055 on July 26, 2021, 10:11:35 pmYes I measured the resistance of the voice coil with the connectors removed and the driver on my bench.Quote from: andy3055 on July 26, 2021, 10:11:35 pmThis crossed my mind too and I had planned on doing it. Now that you have mentioned it I wanted to ask of a question which I had - I see very less people talking about checking for DC present on the speaker outputs on Class D amplifiers compared to Class AB amplifiers why could that be? and what could be an 'acceptable' range of DC (I know it should ideally be 0).The second question is - I am under the impression that its not a good idea to turn on class D amplifier without a load, I know its not as detrimental as a valve amplifier running with out a load but there is a chance of destruction of class D amplifier due to ringing and such. Please correct me if I'm wrong. So by placing a dummy load across the terminals I won't be able to measure the true value of DC present if there is any.

author: EHT
author_rank: Frequent Contributor
timestamp: July 26, 2021, 11:36:50 pm
content: Couple of things... if there is any DC present in your amp's output, that is a fault. It should have protection circuitry to shutoff a relay in this case. If it doesn't, its not a great amp to be using. You could test for that with a resistive load. Fire it up with no input signal, there should be no output (no DC, no AC noise).If you want to check the speakers, use a frequency generator, a series resistor and a scope. The DC resistance is misleading. You will get a good picture of it if you sweep through the audio range. You should see a peak at its resonant freq.

author: andy3055
author_rank: Super Contributor
timestamp: July 27, 2021, 01:48:12 am
content: There should be no DC on the outputs at all. Most manufacturers use matched pairs of output devices so that the balance will be good to achieve this. Even then there can be very minor drifts and some negligent DC might be present. In worst cases if there is DC, very often you can hear a Hum. When we repair stuff, it is hardly possible to get matched pairs as end users. Some will sell but can they be trusted?Your thinking of the tube amps is wrong, unfortunately. You should never power a tube amp without a load. That will destroy the output transformers. Believe me, I have done that in my young days when I did not know what I was doing. Transistor amps do not have that issue but any imbalance in the output stages will present DC on the speaker terminals. I cannot say I have repaired many class A amps as they are not that common like AB amps IMO.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 27, 2021, 06:19:40 am
content: Quote from: andy3055 on July 27, 2021, 01:48:12 amThat's exactly what I said in my post, should never run a tube amp without a load it was going to be destroyed right away. I wasn't sure that it would be the case for class D amps as well. Sorry my wording might have been all over the place in my previous post.

author: TimFox
author_rank: Super Contributor
timestamp: July 27, 2021, 01:41:46 pm
content: Your original wording was correct.The situations are a bit different, however:The ideal Class-D amplifier comprises hard voltage switches driving the load through inductance to extract the low-frequency (audio) waveform. With insufficient inductance, the high-frequency current (at the switching frequency) is too high for the switches.In a linear vacuum-tube amplifier, the plate voltage swings both ways from the quiescent point (voltage equal to power supply) as the plate current swings through the impedance transformed from the load to the primary winding. If the load resistance is too high (or open-circuit), the positive voltage swing on the plate can become dangerously high.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 27, 2021, 06:23:55 pm
content: There are no markings on my drivers that indicate impedance.When I Googled for replacement drivers I came across an ad that said the Mackie part they offered was good for the Passive Thump15 AND the Active Thump 15A.This may or may not be true.I could not find any genuine Mackie drivers available for love nor money, they are discontinued, apparently.I'm not going any further with this as I am not going to put any third party driver in the two I have, but if you mistakenly put in a 4 ohm driver and the power amp blows up because its been designed for 8 ohms, then you are going to have a problem.There was no significant DC from either of my amp modules, which worked fine on my test speaker.If there was any DC I think it would be rather difficult to not notice it when you connected and disconnected the driver, it would make rather unsavoury noises in the process.Ears are good tools.Or, you could put a lightbulb in series with the mains input, so see if connecting and disconnecting the bass driver showed any additional power draw.Or a Wattmeter would show you the same.It is possible that a faulty Class D amp (or output filter) might be putting inaudible HF switching hash through the speaker, this might be noticable with the bulb or wattmeter, but I think might also have artifacts the ears pick up too.When a valve amp feeds back ultrasonically, you can hear it in the power supply changes, you don't have to hear the feedback itself.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 27, 2021, 06:36:46 pm
content: Oh, Yamin, if you are able to cut the cone out of your damaged one ond post photos of the voice coil here, I think it would be very interesting to see the failure mode.I can't do that yet with mine, they belong to the customer and I am not going to replace them.I suspect a design issue rather than just being blown up by excessive power, maybe a hot spot shorts the turns.Neither of my voice coils are rubbing on the magnet, which usually happens when speakers get melted.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 27, 2021, 07:03:12 pm
content: Quote from: Audiorepair on July 27, 2021, 06:36:46 pmYes sounds like a good plan. I've already updated the customer of the issue waiting for him to let me know how he wants to proceed.Quote from: Audiorepair on July 27, 2021, 06:23:55 pmLet me be very noob and ask this- is it possible to measure DC on speaker terminals of a class D amplifier the same way one would measure dc offset of a class A/B amplifier that is with no input signal and no load connected and measuring across the terminals.Thanks so much for the handy tips about the light bulb/wattmeter.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 27, 2021, 07:25:27 pm
content: I'm not really a Class D expert, others here are far more qualified to comment, I've noticed you've asked this several times on different threads.I suspect you havent had an answer because it is rather a grey, complicated and design specific area, and nobody wants to post and be wrong.I'm not sure going DC is actually that likely or possible on a Class D amp anyway.

author: TimFox
author_rank: Super Contributor
timestamp: July 27, 2021, 09:39:38 pm
content: Class-D amplifiers are normally DC-coupled at the output, relying on feedback to keep the DC level close to zero. Otherwise, they would need a huge, expensive capacitor in series with the load. Therefore, a failure of the servo (op-amp) or one of the switching devices (MOSFETs) can result in a damaging DC level across the load.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 27, 2021, 10:47:06 pm
content: Surely a short of one Mosfet would destroy the other Mosfet in a few cycles, you would not get permanent DC from such a failure that would take out a speaker?

author: TimFox
author_rank: Super Contributor
timestamp: July 27, 2021, 10:51:59 pm
content: The failure could be less severe than a short, maybe not enough to blow a fuse, and put a moderate amount of DC on the output, beyond the capability of the feedback to correct. Presumably, the audio would sound terrible at that level of malfunction.

author: perieanuo
author_rank: Frequent Contributor
timestamp: July 28, 2021, 05:54:10 am
content: Quote from: Audiorepair on July 27, 2021, 06:36:46 pmhi, i rewind in young age some tweeters with that type of fault, one end of the wire was not properly secured (the external one, when the coil was finished by the machine, let's call it the 'end') and in time the insulation between the coil end and the coil 'start' was deteriorating and caused low impedance.cause i'm lazy, i just eliminated that final loop or maybe 2 loops, the tweeter was ok for another 5 years. what do you want, students have no timethe problem may be like i described maybe, maybe not.finding another amp to test the speaker should be not so complicated if frequency generator or another testing gear is not available, of finding another good speaker to test the amp...

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 29, 2021, 12:51:16 pm
content: Just measured the bass driver from another Thump 15A I forgot I had here (hasn't been picked up from ages ago).Measured 5.6 ohms, so that will be an 8 ohm driver then.

author: Yamin
author_rank: Frequent Contributor
timestamp: July 29, 2021, 07:18:54 pm
content: Quote from: Audiorepair on July 29, 2021, 12:51:16 pmThanks so much @Audiorepair for the confirmation.Really do appreciate it.