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Old 19th May 2017, 01:59 PM   #1
shrad is offline shrad  Belgium
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Default Question on interstage transformer with class D drive

Hi all,

I have an idea to challenge which is to use a small class D module such as the cheap PAM8403 boards from china...

I drove a quick test with some ferrite cores I have at hand which are giving the following results :

primary: 5 turns
secondary: 35 turns

signal at the input of class D amp at line levels max volume, I have around 16Vpp at the toroid output with a 390 ohm load R and a small piezzo to "simulate" a tube grid and see if everything survives abuse

as there must be many people here involved with transformer design for audio, could anyone point me in the right direction to find docs to optimize my design a bit?

I fear this is far from optimal besides reflecting a correct impedance at the output of the PAM module, and I'd like to know if I'm in the right direction or not...

I plan to test directly driving a push pull with a dual secondary transformer to minimize cost and size of a small amplifier like a 10W EL84 PP or something alike

Thanks a lot if anyone is able to help!
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Old 19th May 2017, 03:14 PM   #2
hooman is offline hooman  Iran
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good can you tell abut benefit of class d ic drive push pull el84?
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Old 19th May 2017, 04:33 PM   #3
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You don't need a power amp to drive the EL84's, just a voltage amp will do.
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Old 19th May 2017, 04:34 PM   #4
shrad is offline shrad  Belgium
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well, you spare some tubes and components for the whole input stage and phase splitter, and the overall cost is ultra low

I don't want to compare benefits, just fiddle and learn by experimenting

in this case, gaining a better knowledge and understanding of ferrite transformers for audio use and trying a new hybrid topology is the benefit
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Old 19th May 2017, 04:37 PM   #5
shrad is offline shrad  Belgium
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in this case I use a 3W class D amp so basically all thep ower is used to drive the toroid transformer to obtain higher voltage and more importantly insulation

if I have input insulation with a class D drive and output insulation with an OPT then I can have a small amplifier with two tubes in PP with a cheap OT, so basically cut costs down at maximum for crude amps
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Old 19th May 2017, 06:55 PM   #6
DF96 is offline DF96  England
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The ferrite must have very high permeability if 5 turns is sufficient for an audio frequency primary! What are you seeing at the secondary: audio or just switching spikes?
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Old 19th May 2017, 07:02 PM   #7
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You'll need 100 times the turns to get anywhere near full audio band through a ferrite transformer unless the cross section is absolutely monsterous, like suitable for several kilowatts of power.

Even if you specifically designed the class D driver and just dispensed with the transformer, this "topology" has no practical application.

This sounds like one of realflow's troll projects.

Last edited by Andrew Eckhardt; 19th May 2017 at 07:11 PM.
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Old 19th May 2017, 07:38 PM   #8
sser2 is offline sser2  United States
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Correct me if I am wrong, but I don't think Class D transformer works in audio band. Depending on the encoding system, it passes square impulses of 48 or 196 kHz in the PCM system, or variable frequency in DSD. The requirements for such transformer are similar to those in switch mode power supplies. Few turns, but leakage inductance and parasitic capacitance must be kept at minimum to avoid waveform distortion, and insertion loss at such high frequencies may be a problem. Transformer performance can be evaluated by observing square wave at specified frequency in an equivalent circuit. There is a lot of info on the internet regarding design of switch mode transformers.
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Old 19th May 2017, 07:53 PM   #9
sser2 is offline sser2  United States
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Originally Posted by hooman View Post
good can you tell abut benefit of class d ic drive push pull el84?
This is the most straightforward way of reproducing digitally-encoded sound. No DAC is involved, as encoding pulses are directly fed into amplifier input, and amplified to desired level. Digital-analog conversion occurs at the output of the amplifier due to inability of output transformer to pass carrier frequency.

In a traditional reproduction chain, digital signal is converted into analog by DAC, and then analog signal is amplified by audio amplifier. The elegance of Class D is a very simple and short signal path.
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Old 19th May 2017, 08:37 PM   #10
shrad is offline shrad  Belgium
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I see most of people have fallen into the easy trap

I don't want to pass audio through ferrite by inducing audio frequencies through it, I want to pass the high frequency pulse train as this is a filterless integrated amplifier with totem pole outputs and filtering is cared for in the internal modulator

I don't use a digital mode like PCM or serial audio, just plain crude old line audio fed into the input of the class D module

I don't need explanation about how a SMPS works, I know the subject and I know that it is at least not the total part of my problem here

what my setup does is basically a first stage of integration/voltage gain via a small 5:35 ferrite transformer which will enable me to isolate the input section from a power stage and raise the voltage to a level acceptable for driving a push-pull stage

my issue is to ask if people know of a usual mean of optimizing primary turns for a small class D output (for presenting a sufficient load) and optimizing turns ratio for correctly adapting impedances

my goal is to produce a simple amplifier without a power transformer, using class D to isolate input from power section and toroid to isolate power section from output

this is just to challenge an idea I had, not to deny any practice or frustrate anyone I would just like to go as far as possible to a functional amp, whether it would be good or not
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