noise floor w/ and w/o input signal

[gearheadgene]

Just wondering how these amps work in real life. At the moment, I am simulating them to get an understanding. Anyway, during simulation with 0V input the noise floor is real flat around -120 dBV (up to around the switching freqency). But, change the input to something real, say 2khz tone, the noise floor jumps up drastically to around - 50dBV.

Is it my simulation, and maybe I am missing something, or do Class D amps behave this way in real life?

Oh, at the moment, the simulation is using all ideal components and no feedback.

[classd4sure]

You mentioned the test frequency, but neglected everything else like gain, power level, type of amp (self oscillating etc)

At higher power the noise floor might increase slightly, but I don't think it's any kind the sort of offender a class B amp would be.

Possibly due to increased EMI, especially with a bad layout. More likely due to parasitics being worked a little harder at higher power levels, which is what a simulator might actually be showing. In reality your filter inductor /core material would play a major roll in how it changes with power levels.

Either way on an extremely dynamic recording with a good D amp, you can crank the volume right up and the noise floor you end up hearing is still that of the recording itself. So that of the amp must be a good measure lower than the majority of recordings out there.

[gearheadgene]

I left out the details because I was really just looking for a real world comparison vs simulation. Since you ask:
1. not self oscillating, 400Khz triangle wave feeds comparator.
2. Open loop. No feedback until I understand this portion of the design.
3. All components are "ideal".
4. Both 1/2 bridge and full BTL topologies.

Can you tell me how much of the switching noise leaks out to the speaker? This test circuit isn't optimized by any means, but with no audio input, the amplitude of the switching noise (at the speaker) is around 0.5 Vp-p. Seems high to me, but I have no point of reference.

[classd4sure]

.5V pk-pk is great.

That would be the switching noise leaking out to the speaker there. It should also be at fairly low power.

The speaker's impedance at that frequency is high, time constant of it is long, there's no physical way the speaker can react to it at all, anymore than you could hear it if it did.

The biggest problem of that high frequency ripple is EMI from the speaker wires. If you have long speaker wires or just want to attenuate that ripple even further to help prevent EMI you can simply employ a common mode choke on the speaker outputs.

Some of the older threads in this forum really cover alot of material, even if they're kind of long, worth reading, should help in your quest alot.

[gearheadgene]

.5V pp is good, eh? Well that's good to hear (so to speak)! Again, it's in simulation with ideal components.

how would a common mode choke work in a single-ended app (half bridge)? If I use an L-C filter with more poles, that should help for single ended.

I will look through those posts - thanks.

gene

[classd4sure]

Single ended still has a ground return from the speaker. So you just use the two speaker wires, power out and ground.

[Pafi]

With ideal elements you can't get -50 dB noise floor at 2/400 kHz. There must be some simulation mistake. But for a more or less proper noise floor result you must simulate it with ~100 ps time step! (And this allows only ~-120 dB noise floor with 1024 points FFT.)

[gearheadgene]

Originally posted by Pafi
With ideal elements you can't get -50 dB noise floor at 2/400 kHz. There must be some simulation mistake. But for a more or less proper noise floor result you must simulate it with ~100 ps time step! (And this allows only ~-120 dB noise floor with 1024 points FFT.)

my time step is set at {.001/16384}, or around 62ns. which works really well for single tones. I will try 100ps, but you are going to make my 600MHz cpu work hard

[gearheadgene]

Originally posted by classd4sure
Single ended still has a ground return from the speaker. So you just use the two speaker wires, power out and ground.

ok, but I don't think it will attenuate the residual switching noise because it's not common mode. The choke will help with common mode stuff though.

[classd4sure]

with your usual length of speaker wire much of it will become common mode