Cheap DAC + Class D differential amplifier

[knicklicht]

Hello all,

I am trying to build a very cheap circuit that contains a DAC as well as a Class-D amplifier. As an input I have a Zedboard FPGA. I want to generate a 1.8 V PWM signal and bypass the analog input stage typically found in the circuits that I found. The output load is a 4 ohm speaker and I am looking for around 50 W of RMS power output. I think my lab power supply will be sufficient at first. Most amplifier circuits that I saw use around +/-15 V. However, it only supplies positive voltage. That's why I chose to build a differential amplifier. Since I have a digital circuit background I am unsure about the components that I need. I found a design that uses very few components. It basically consists of an analog input stage that generates the PWM signal (which I don't need). The PWM signal is passed to an SN74LVC2G86 XOR gate to generate the differential signal. Then it uses two MP8040DN high current integrated half bridge + drivers. I was able to source the XOR gate but the MP8040 is not readily available in Europe (only on AliExpress). Is this IC the right choice here or are there alternatives that are more popular? I do not need a drop in replacement as long as I have a cheap design that fulfills my requirements. I am also trying to keep the output low pass filter as simple and cheap as possible. I was thinking of 25V 1mF electrolytic capacitors and 15uH SMD CD75 power inductors. Will this work?

Thanks

 

[KlausST]

Hi,

A bit of confusion...

You talk about DAC and you talkk about PWM. For me: Either the one or the other.

Most amplifier circuits that I saw use around +/-15 V

Sorry to say that: Most Class D amplifiers i´ve seen use single supply.

why I chose to build a differential amplifier.

I guess you mean "BTL" = bridge tied load amplifier

I found a design that uses very few components.

If you do an internet search you don´t find "a" circuit, you may find many thousands.

Go to a marketplace you will find farily cheap ready to buy Class D modules. You probably can´t built it on your own for that low cost.

If you want to build it on your own:
* just go to an electronics distributor and do a search for "Class D amplifier ICs". Farnell lists more than 300. About 50 of them above 50W.
* go to a Class D amplifier semiconductor manufacturer. Read the datasheets. They also provide application notes, design notes, example designs, PCB layout examples ... and every thing and every information you need.

There are rather simple circuits, but there also are rather advanced circuits.

Klaus

 

[knicklicht]

Hey Klaus,

thanks for the quick response. As I said I am new to analog circuit design and don't have the lingo down. Yes, I think a BTL amplifier is what I am looking for. I really only want to use a cheap PSU and I have one that produces positive voltage.

The first thing I did was search for an IC that does the exact job I need it to do. But because I do not really know what to search for I didn't find anything. Yes, there are many class d circuit designs and even ICs. However they all seem to have an analog input and I do not want to first convert my digital FPGA signal to an analog signal just to create a PWM signal and then convert it to analog. I thought that would impact the sound quality to much and would increase circuit complexity increasing cost. If there are ICs that can process a digital signal from my FPGA that would be great. It is important though that for a single audio channel I only need a single digital output from my FPGA. I would be very thankful if you know of an IC that can do this.

 

[FvM]

You are essetially searching for a full bridge, e.g. 4 MOSFETs and suitable gate drivers. There are many possible solutions, you should have an idea of intended switching frequency and audio bandwidth.

 

[knicklicht]

I found this commercial single bit audio encoding technique called Direct Stream Audio. It seems to me that it is pretty much PWM. They are using switching frequencies of around 12MHz and state state that signal frequencies up to 80kHz can be reproduced. So I think that this would also be sufficient parameters for my use case. However, they also talk about noise shaping above 20KHz but I think that only has to do with how the PWM signal is generated.

 

[BradtheRad]

I was thinking of 25V 1mF electrolytic capacitors and 15uH SMD CD75 power inductors. Will this work?

See LC filters in my simulations of class D amplifier. Carrier is popular commercial 44.1 kHz. Incoming signal is converted to SPWM, then applied to a filter and 4 ohm load. (I resorted to simple bipolar AC.)

I played with L & C values until the resulting analog waveform appears workable.

Compare performance for sine waves at two different frequencies (100 Hz & 10 kHz).

 

[knicklicht]

I have found this 4 channel half bridge IC called "STA515W". I think it is exactly what I need. The datasheet provides a sample circuit.

I would like to build it this way, but I am confused about C21. It says 2200uF but the symbol is not a polarized capacitor. Is this just a mistake or do I need something else than an electrolytic capacitor? Just to be sure all other non polarized capacitors are ceramics, right? If I choose to implement the circuit using SMT how do I determine which footprint I need?

 

[KlausST]

Hi,

If this is what you need, then the headline and first post really was misleading (to me).

Output power, channel count, no DAC ...
We don't know about your speakers, the applications requirements, wiring ...

Your target for "minimal filters":
Please consider that your amplifier, wiring, speaker may act as "HF transmitter" if not designed properly. In worst case your amplifier may interfere with your local "emergency authorities" radio...

Mind that PWM causes overtones, mainly depending on the rise rate of the power bridge output. The overtones are much higher than the fundamental PWM frequency. So: not 20kHz (HiFi audio limit) , not 80kHz (application audio limit) , not 12MHz (PWM fundamental) .... your filters need to be able to suppress in the 100MHz range. --> PCB layout, part selection and so on need to be suitable.

Klaus