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Telnet100 5th August 2019 09:01 AM

Class D Questions
Hello All
Looking for the Class D specialists!
At present, I have been working on a Mackie DLM12S sub Amplifier, which the owner told me suddenly became Very Distorted over a couple of days.
Suspecting the 12" Bass Driver, I removed it from the cabinet and test ran it on another amp, and turned out that the driver was good.
Then: on to the Class D amplifier, which in this case is Two Power Amps which are connected to the Driver in Bridge Mode. After Closely measuring everything I could in the Amps and finding nothing amiss, I separated them into Stereo Mode and proceeded to follow the Service Manual Instructions on setting up the Input Oscillators to the correct frequencies. It is here that I ran into something a bit odd; to get these Oscs to set up to the correct 480Khz, (plus or minus 10Khz), I had to rotate both channel trimpots almost a full 2/3 of a turn to get the right frequency. The procedure is that You disable each amp with a Shorting Link while you adjust the other channel. Then You enable both channels and carefully adjust both trimpots to get the 2 channels to they (Mackie) say 'Phase Lock'? How is this possible, because there is no interconnection in that section between both channels, and the oscillators are only one section of an ordinary Opamp?
Anyway, the bottom line to all of this is that after running Each Amp separately with an Audio Signal, and both sounding quite Powerful and Clean, I then reconnected them in Bridge Mode and back on to the Original Bass Driver. But -- no Good -- Still Very Distorted, and it sounds like there are some sort of artifacts in the sound.
Has anyone here experienced this sort of problem? I can Post the Scope waveforms if this will help -- Thanks for all and any help --

DF96 5th August 2019 02:20 PM

I am going to hazard a wild guess that you might get better help in the Class D section of the forum. Click on the red triangle at the bottom left of your post and ask a kind Moderator to move it.

nigelwright7557 5th August 2019 10:47 PM

With stereo class d amps you can get into a strange interference mode if the clocks are a multiple of each other.
So they usually either set them really close or a long way off to stop beating between the clock frequencies.
Its a while since I designed a stereo amp so my comments might not be quite right.

Telnet100 5th August 2019 11:27 PM

2 Attachment(s)
Thanks for all.
In this particular case the clocks are set to exactly the same frequency, eg; 480Khz, and as I explained, there is no way that These 2 clocks can 'Lock' to each other, as they are each only part of an Opamp and are not connected to each other in any way.
Maybe if Mackie had used a Phase Locked Loop, this problem might not exist. The existing Circuit is very simple. I have attached the circuit to help --Thanks for any help --

kevinkr 5th August 2019 11:34 PM

:cop: Moved as requested! :D

Telnet100 5th August 2019 11:39 PM

One thing I was curious about, is, how much of the 480Khz signal should you be able to see at the Audio Output Terminals?
In this case, there is over 1 Volt P-Pk. I understood that the filter Network after the Output Devices was suppossed to remove it all?
Thanks for all --

nigelwright7557 5th August 2019 11:40 PM


Originally Posted by Telnet100 (
One thing I was curious about, is, how much of the 480Khz signal should you be able to see at the Audio Output Terminals?
In this case, there is over 1 Volt P-Pk. I understood that the filter Network after the Output Devices was suppossed to remove it all?
Thanks for all --

Its a juggling act with the output filter.
1/ Filter it enough to get rid of the carrier.
2/ Not filter it so much top end of the audio is lost.

Telnet100 5th August 2019 11:56 PM

Thanks for that --
in this case the 480Khz Clock Frequency is not very stable; Mackie say that You have to set it to 480Khz plus or minus 10Khz, which I did as in the 'Mackie 2' Pdf I attached.
It was actually quite difficult to achieve that, and I followed the instructions given.
Because the Frequency was so wrong at the start, I thought that this might fix the problem, which it did not.
Really, for a Circuit as simple as this, I can not see how the frequency can be that stable -- and how on earth can these 2 lock to each other (as Mackie suggests) when the two are not even connected in any way?
Any comment would be appreciated --

nigelwright7557 5th August 2019 11:59 PM

Its all about beat frequencies between the two clocks.
If the clocks are close the beat frequency is very small and inaudible.
If the clocks are distant the frequency is small again.

If its fairly/moderately close you can hear the beat frequency on the output.

bwaslo 6th August 2019 12:03 AM

1V pk-pk isn't out of the ordinary. I suspect that the remaining hf that gets past the filters is supposed to be able to affect the triggering of the other side forming a sort of "phase lock" (not really a phase lock, but a similar thing to how old analog TVs used to get their horizontal oscillators synched to the received signal by affecting trip point and timing so they end up tracking).

I'm not a class D expert, but 480kHz seems kind of high (at least it's higher than the dozen or so amps I've seen).

Could it be the power supply collapsing and causing the problem? Maybe put a scope on that and see what it looks like when you're bridged.

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