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.
P.S- I don't hear any coil rubbing when driven with signal or pushed and pulled by hand
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.
P.S- I don't hear any coil rubbing when driven with signal or pushed and pulled by hand
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Service manual here: MACKIE THUMP-15 SCH Service Manual download, schematics, eeprom, repair info for electronics experts
It shows 280W / 6Ω / 41Vrms at the amplifier output, which seems to follow the power law P=V2/R correctly. The Mackie schematic shows ±38V supply rails.
The data sheet for the TDA8924TH is here: TDA8924TH datasheet(15/35 Pages) PHILIPS | 2 x 120 W class-D power amplifier
It shows maximum output current as 12A and maximum power in BTL as 240W. from P=I(2)*R, R=P/I(2) = 1.7Ω. The data sheet shows maximum supply rails of ±30V, not 38V!
One of them must be wrong.
It shows 280W / 6Ω / 41Vrms at the amplifier output, which seems to follow the power law P=V2/R correctly. The Mackie schematic shows ±38V supply rails.
The data sheet for the TDA8924TH is here: TDA8924TH datasheet(15/35 Pages) PHILIPS | 2 x 120 W class-D power amplifier
It shows maximum output current as 12A and maximum power in BTL as 240W. from P=I(2)*R, R=P/I(2) = 1.7Ω. The data sheet shows maximum supply rails of ±30V, not 38V!
One of them must be wrong.
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I had a pair of the 12" Thumps and both woofers died, and yes they exibited an intermittent sound problem before giving up completely.
Thanks @johnmath. Is it a coincidence that I'm measuring DC resistance of 1.7Ω across the coil? Where the rated impedance according to the label should be 8Ω. Typically I have noticed that with an 8Ω rated speaker I would measure around 6-7Ω dc resistance. Could I assume that this driver is ok?
Thanks @conanski. Did you get a chance to measure the coil resistance of the new drivers that you replaced?
There's quite a large disparity between the owner's manual which claims 400Wrms and the service manual which claims 280W and the chip data. The chip is rated BTL into 4Ω speaker load, so a driver DC resistance of 1.7Ω is quite low.
You should get 5.5 - 6.5 ohms using a mulyimeter. 1.7 ohms is far too low. My guess is that part of the voice call has shorted.
You SURE it´s TDA8924?
It does not meet supply voltages or speaker impedance AT ALL.
By contrast, TDA8954 is very much used, and meets/exceeds all specs.
Including 420W at 8 ohms by datasheet, rails up to +/-41.5V, etc.
Besides that, the woofer is shot or improperly measured.
Or:
Some of those woofers have 2 voice coils, if 4 ohm each = 8 ohm total; if parallel 2 ohm so 1.7 ohm dcr is reasonable.
We need woofer pictures, from behind and sideways to see magnet and frame, and a label closeup.
Woofer might be a replacement, of course, and previous owner/tech might have used a "car audio" one, very low impedance, which will "work" at the bench but trigger protections all day long at any significant power.
Add a TDA89** closeup showing label.
It might even be the wrong type, replaced by a former service Tech, go figure, now and then I have found worse mistakes.
Also sometimes Factories recycle or keep a popular model name , it helps on sales, but different years show different guts.
The fist Mackie Thump (was it the 18" one?) used a crazy circuit with a *single* high voltage supply, think 90V DC or so, and switched it with MosFets to either feed the positive or negative half cycle, it was equivalent to crazy high +/- 90V rails, 180V PP availble .
Woofer was 12 ohm , they blew all the time, and were typically replaced by 16 ohm Eminence ones which lasted way more.
That said, the THUMP name was well deserved, they were literally chest pounding.
If not remembering wrong, they were capable of 600W RMS (into 12 ohm).
A real tour de force, impressive design but way over the top.
I was surprised at finding a "weak" (by Mackie standards 😉 ) meager +/-38V Class D amp here.
It does not meet supply voltages or speaker impedance AT ALL.
By contrast, TDA8954 is very much used, and meets/exceeds all specs.
Including 420W at 8 ohms by datasheet, rails up to +/-41.5V, etc.
Besides that, the woofer is shot or improperly measured.
Or:
Some of those woofers have 2 voice coils, if 4 ohm each = 8 ohm total; if parallel 2 ohm so 1.7 ohm dcr is reasonable.
We need woofer pictures, from behind and sideways to see magnet and frame, and a label closeup.
Woofer might be a replacement, of course, and previous owner/tech might have used a "car audio" one, very low impedance, which will "work" at the bench but trigger protections all day long at any significant power.
Add a TDA89** closeup showing label.
It might even be the wrong type, replaced by a former service Tech, go figure, now and then I have found worse mistakes.
Fully agree, that´s why I am asking for more data.
Also sometimes Factories recycle or keep a popular model name , it helps on sales, but different years show different guts.
The fist Mackie Thump (was it the 18" one?) used a crazy circuit with a *single* high voltage supply, think 90V DC or so, and switched it with MosFets to either feed the positive or negative half cycle, it was equivalent to crazy high +/- 90V rails, 180V PP availble .
Woofer was 12 ohm , they blew all the time, and were typically replaced by 16 ohm Eminence ones which lasted way more.
That said, the THUMP name was well deserved, they were literally chest pounding.
If not remembering wrong, they were capable of 600W RMS (into 12 ohm).
A real tour de force, impressive design but way over the top.
I was surprised at finding a "weak" (by Mackie standards 😉 ) meager +/-38V Class D amp here.
Yes TDA8950 or TDA8954
I remember servicing a Thump with a TDA8950 and I have have seen them with TDA8954. These are excellent chips but if I remember they used the surface mount ones. The through hole chips had better heat dissipation but even so were never going to reach the quoted figures. Also the figures quoted by NXP were at 10% THD +N.
Again from memory the TDA8920 and TDA8924 is rated at +/-30V max but without a signal. Working max is +/- 27V. In all cases to get the output power the load has to be 8 ohms. Running at 4 ohms would bring in Over Current and probably Over Temperature protection.
I remember servicing a Thump with a TDA8950 and I have have seen them with TDA8954. These are excellent chips but if I remember they used the surface mount ones. The through hole chips had better heat dissipation but even so were never going to reach the quoted figures. Also the figures quoted by NXP were at 10% THD +N.
Again from memory the TDA8920 and TDA8924 is rated at +/-30V max but without a signal. Working max is +/- 27V. In all cases to get the output power the load has to be 8 ohms. Running at 4 ohms would bring in Over Current and probably Over Temperature protection.
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Thanks guys, I'll snap some photos of the woofer and the chip. The chip is on the underside of the board so would have to remove the board to get a shot of it.
They were replaced under warranty(boxes weren't 1yr old) so I never saw them, but afaik they are a single coil 8ohm driver.
DC resistance is usually slightly less than speaker impedance.
So your speaker is partly shorted.
So your speaker is partly shorted.
I have had that fault on Mackie and a couple of similar cabinets. Loud crump at high volume. Measuring the coil resistance as I moved the coil gave a short at various positions; new driver cured each one.
I think there might be a slight error. On the manual it shows that its +/-28V power rail.
The compression driver has it's own amp doesn't it? At this end of the powered speaker spectrum manufacturers like to add together the amp power for both drivers and then double or even quadruple that for the advertising fairlytail.
Having owed these boxes as well as a bunch of other similar powered and passive PA speakers I can confirm that they produce the amount of sound that would be expected from a 250w speaker.
Sounds like max square wave power (marketing BS basically) v. rms sinewave power.
If the voice coil is 1.7 ohms maybe its burnt off its insulation, warped a bit and started welding itself together? Gross damage like this should be visible if you push the cone out and peer behind the spider at the voicecoil emerging from the gap.
Its classic burnt coil.
I had 2 speakers and after a heavy session one was less loud than the other.
So I measured DC resistance and one was about 6 ohms and the other 2 ohms.
So tore bad speaker apart and the voice coil just fell apart with burnt windings.
I had 2 speakers and after a heavy session one was less loud than the other.
So I measured DC resistance and one was about 6 ohms and the other 2 ohms.
So tore bad speaker apart and the voice coil just fell apart with burnt windings.
Thanks guys for the confirmation and the inputs. Time to look for replacement woofers... Any idea if it would be possible to get a reconing kit for this particular model?