I have built several Class D push pull output low frequency RF amps of up to 1kW on 136kHz. To get a nice sinewave output relatively free of harmonics I have to use a low pass filter network after the output transformer. in the audio world does a similar design of amp need any filter network before the load (speaker)?
I ask as I am wondering about doing this:
Push pull output, Class D low frequency (136kHz) RF amp, circa 500 Watts. If I want to test it for a few hours into a dummy load does it matter if the low pass filter network usually used to get a sine wave, harmonic free output into an antenna, is not between the output transformer and the dummy load? I want to leave it running overnight and I am modding the LPF and it's in bits until next week. Thanks.
I ask as I am wondering about doing this:
Push pull output, Class D low frequency (136kHz) RF amp, circa 500 Watts. If I want to test it for a few hours into a dummy load does it matter if the low pass filter network usually used to get a sine wave, harmonic free output into an antenna, is not between the output transformer and the dummy load? I want to leave it running overnight and I am modding the LPF and it's in bits until next week. Thanks.
There are filterless class-d audio amps but usually for low power and where the amp is very close to the speaker. In higher power amps, filters are required to reduce both the radiated EMI from the speaker cables and to prevent the HF ripple current from heating the voice coils. You should be able to run a dummy load without a filter but you won't have the reactive currents due to the filter so there will be less stress on the output devices.
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I fail to see where the class D comes into this...Please explain.
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Yes, that is not a Class D amp. It is merely a switch-mode PA. Exactly which Class it runs in will depend on exactly what load is applied. It could be E or F or some variant of these. Or it could be just a naive switcher of no particular Class and so have not particularly high efficiency.
A square-wave output at the intended frequency filtered to a sine wave is not Class D.
Sorry, I ought to answer the question too. Running a soak test of a switch-mode PA with the output filter missing will not tell you much about how it will operate with the filter present, as the filter is an integral part of how the circuit operates. With no filter, you could more or less heat dissipation in the output devices. You could get bigger or smaller voltage excursions etc.
Read up about Class F (and possibly Class E too). Don't assume that because someone describes his circuit as Class D that it really is Class D. In the case of Classes E and F you don't just need the right circuit; you also need the right tuning.
73 G8HQP
A square-wave output at the intended frequency filtered to a sine wave is not Class D.
Sorry, I ought to answer the question too. Running a soak test of a switch-mode PA with the output filter missing will not tell you much about how it will operate with the filter present, as the filter is an integral part of how the circuit operates. With no filter, you could more or less heat dissipation in the output devices. You could get bigger or smaller voltage excursions etc.
Read up about Class F (and possibly Class E too). Don't assume that because someone describes his circuit as Class D that it really is Class D. In the case of Classes E and F you don't just need the right circuit; you also need the right tuning.
73 G8HQP
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Hmm, I just called it Class D as all the designers of similar push pull LF amps with a following LPF call them Class D. It works fine, but I built a smaller version and wanted to test it straight into a dummy load as the LPF was in bits. I have finished the LPF now so will test into the load THROUGH the filter, thanks for the explanation!
My understanding is that Class D necessarily involves a switching frequency much higher than the signal frequency, with some form of PWM or similar encoding the signal. Signal frequency square wave into a low pass (or bandpass) filter would be similar to Class F or Inverse F, depending on whether the filter presented a high or low impedance to the third harmonic.
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