Bridged ClassD 800W in 4 ohms - oscillation problem

[Zoors]

Hi all,

I do have a problem with undesirable oscillation noise in output.
The amp is a subwoofer amp for a 18" setup.
It is two Class D power amps connected in bridge.
Two separate oscillators, that is.

Now, even if the preamp is filtered LPF around 100Hz, these ClassD twins have different oscillation frequency!
One is like 102 kHz and the other 98kHz.
That means I get the difference signal in output, i.e. 4kHz.

I have tried my best to figure out what is wrong with thi design, and I should want a synchronizing connections between the two amps...

But is there another solution? Could it be that the Q value of those large LPF's at output has deteriorated? So too much of the carrier is present?

Any help in this matter would be very appreciated!
I would be very grateful for some comments from people with insight in this area.

[Pafi]

Hi!

The freq. difference alone is not a problem, unless something (inside the amp) demodulates it. Demodulation is possible if two switching signal mixed in the PWM modulator (and other conditions are presented). The "alien" switching signal can come via:
- feedback path (post-filter FB is more sensitive then pre-filter FB)
- power supply (high capacity, LowESR on-board caps, and independent power supply wiring can help)
- ground loop
- capacitive or inductive coupling (proper signal levels and impedances, proper layout).

If no signal comes from other channel, then no interference.

High loop gain, good, balanced comparator, and symmetrical switching times have also good influence.

If you need a specific advice you must tell what have you built.

[Zoors]

Thanks for your input!

Well to me its rather obvious that it is the differnece frequency.

Problem is that the carriers before th LPF are also different in amplitude, not much, but some.

A working amp (that I compare with) has a bit better Q-factor in the output LPF and are therefore more silent.

I think the problem is that the LPF corner frequency are the same, but the frequencys that are to be cancelled are 1. a bit differnet in frequency (4kHz) and amplitude of carrier is a little different. I guesss the 4,7u polyprops capacitors are too bad...
Or... the frequncys must be adjusted to be the same at silence.

[Zoors]

Ok, heres the schematic ...

It is just basic... this is one of the two bridged amps.
Both are on the same PCB and are powered from one common PSU.

[Pafi]

Zoors!

You are searching the problem in a wrong place, but I can't tell for sure where it is. This schematic is almost unusable, and exact method of connection of two sides is very important too.

The problem is not from 1 place. It is a product of a complex process, therefore it can be eliminated by more then one method.

Different amplitude is just a side-effect, it's not a reason, nor a consequence.

I saw a bridged ClassD amp wich had an awful (seen at oscilloscope), very strongly amplitude modulated output because of slightly different switching freq, despite of this there were absolutely no audible interference.

[Pafi]

Sometimes the whistling noise produced by the preamp. There is no general reason for interference. It depends on actual realisation.

Well to me its rather obvious that it is the differnece frequency.

Of course! I didn't tell other! But what you hear is not directly the difference! If you have only sum of 2 signals, 98 kHz and 102 kHz, they are still inaudible! The only way to make them audible is connecting both of them to a nonlinear network, wich makes many other frequency, for example the difference freq. This is called demodulation. If you can disable demodulation in some way (eliminate nonlinearity, or eliminate some of the input signals), you won't hear interference.

[Eva]

The EMI of one amplifier is disturbing the other and vice-versa. Consider either synchronization or intentionally mismatching the frequencies by 20Khz or more to bring the intermodulation products above the audio band.

[Zoors]

Well my theory is this:

The output LPF (20mh in series, 4.7uF to ground) is not working on one side as well as on the other.

Remember this is a bridge setup, therefor we have one 18" loudspeaker driven by tw identical poweramps with two of these, suppose to be identical, LP filters.

The LP filters are leaking HF into the speaker coil, where they collide and a ringmodulation effect occurs. Resutl frequency is not the sum, but the difference of the input HF power PWM scuarwaves when no audio are input (silence). The power squarewave frequencys syuppose to be 100kHz each here, but they are not. They have a 4 kHz difference, and that signal is very clearly heard when amp is silent.

Solution =

1. better Q value of LPF filter. Matching of caps, hi quality hi stabilty caps etc. With a resonance frquency that matches the carrier power HF as exact as possible.

2. better synchronicity between the two bridged amps feedback loops (something I learned here, thanks!). This could be done by have some sort of link from one amp output (master) to the others feedvback loop (slave)


(#1) will minimise the ringmodulator effect probability, and (#2) will make the amp totally silent in this cas, as the difference of two absolutely identical input frequencys will be Zero. (silence)

This is my theory and solution proposal, and I would love to have it challanged!

[Pafi]

20mh in series

??? Millihenry? It would be extraordinary huge! If you think "mh" means microhenry, then: 20 uH is a little small for this low fsw.

and a ringmodulation effect occurs.

Do you know anything about ring modulators? Do you see any ring modulator on the output? If yes, then cut it out!

better Q value of LPF filter.

Do you know what is Q of an LC filter? I don't know what do you think.

With a resonance frquency that matches the carrier power HF as exact as possible.

The resonance freq. must not match to carrier. It have to be much lower. LC filter must have only small influence on switching freq, especially in an amp with pre-filter feedback.

better synchronicity between the two bridged amps

This can be a solution indeed, but you asked another solution in your first post.

And synchronisation can turn interference into distortion if you don't do it properly.

[acid_k2]

Originally posted by Eva
The EMI of one amplifier is disturbing the other and vice-versa. Consider either synchronization or intentionally mismatching the frequencies by 20Khz or more to bring the intermodulation products above the audio band.

Eva, I agree with you.

Zoors, you need:

- the same carrier frequency for the two amps

OR

- a difference between carrier frequencies greater than 20Khz: for example, 85 KHz and 115 KHz will give you the beating frequencies at : 115+85=200kHz (who cares? ) and 115-85=30Khz (out of audio band)

Another "raw" solution: a post- lowpass filter (on the speaker) at 200 Hz will give a lot of attenuation at 4Khz...