FET Power Stage only circuit - Dalek Voice modulator - diyAudio
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Class D Switching Power Amplifiers and Power D/A conversion

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Old 8th July 2014, 11:51 AM   #1
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Default FET Power Stage only circuit - Dalek Voice modulator

Hi all,

This is my first post here, to ask for a bit of guidance on a Class D project.

I'm hoping to ask the right people my questions here, to save hours of Googling, so I'll be the first to admit I've not thoroughly done my research, but a have a basic idea of what I want to do...


PROJECT:

I'm building a 1/2 scale Dalek from the pland on www.projectdalek.com.

Part of this project is a voice modulator. Originally these were ring-modulators, then Moog Moogerfrooger guitar pedals in the newer series. However, I'm working to a tight budget, and am having GREAT success with an Arduino based voice modulator! See here for the code: https://github.com/andygrove/arduino..._ringmodulator

Here is sample of my daughter having a go: Toddler Dalek - Arduino Ringmod Voice Modulator - YouTube (just using our TV as temporary amp and speakers in the video)

I'm building a veroboard circuit for the ATMEGA Arduino IC, 5V regulator, mic preamp circuit, flashing dome light FET driver, and would like to add the amplifier to it. Idea being you program the IC on the Arduino UNO board, then transfer it to the veroboard.


IDEA:

The penny dropped ... the output of the arduino is PWM, so why not drive it straight into a class D power stage? A neat solution, but I need some help in designing the circuit please...


SPECS:

I have a voucher for Maplin to spend - they have a limited selection of FET's, so I want to know if the IRF540A is any use for a Class D? http://www.maplin.co.uk/p/irf540a-mos-fet-transistor-to220-case-n10ah

12V battery power supply.

Volume - is this adjusted by adjusting the supply voltage, and hence the voltage across the speaker whent he FET's are on? if so, do you just use an anjustable voltage regulator?

Arduino output base frequency of PWM signal- To be measured...

Output power requirements: a speaker not too loud, just enough to be heard in an open space, or when surrounded by people in a room.

Quality isn't a massive issue, it's gonig to sound like a Dalek!! but I'd like to avoid any humming, clicks / pops etc...

Many thanks in advance, Scott.
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Old 10th July 2014, 12:37 PM   #2
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Hi,

I've just measured the PWM output from the Arduino...

No oscilloscope to hand, but a calibrated Fluke DVM gave these readings under a NO-load condition:

Frequency = 62.7KHz

Voltage = 2 to 2.5V (take with a pinch of salt as measured with a DVM)

Any guidance on building a Class D power stage to operate on this signal will be more than appreciated...

Many thanks, Scott
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Old 10th July 2014, 03:21 PM   #3
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Location: Munich
For your operation from 12V and using normal speakers already one of the stronger gate drivers can form the entire power switching stage. No need for power MosFets.
If you choose a driver which uses internally a fully BJT output stage, you will need additional freewheeling diodes.
If you choose a driver which uses internally MosFets you do not need external freewheeling diodes.
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Old 10th July 2014, 10:20 PM   #4
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Thank you Chocoholic!

You've found the best solution for me! )

Now, to learn about these IC's and select a suitable one... Is there anywhere on this forum where they are discussed?

I assume I'm still going to need to add the low pass filter to the output?

I've just found a couple of small speakers, they are from a car CB radio set, one of them may be suitable for the Dalek.

Thanks again, Scott
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Old 11th July 2014, 08:32 PM   #5
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Join Date: Dec 2003
Location: Munich
There are lots of drivers and many of them are pretty good.
Often the choice depends on what you have on hand or personal taste...

For a single half bridge from 12V into 8 Ohms I recently used an UCC27322P,
while deriving the PWM from an Infineon XMC4500.
If you want to go for bridged operation or lower impedance you should look for something stronger like the UC3710T + external shottkys for freewheeling.
And if you tend to be unsaint - have a look to IXDN430...


Quote:
Originally Posted by ScottBouch View Post
I assume I'm still going to need to add the low pass filter to the output?
Yes, you should use the popular LC output filter.
Please also note that for your operation with a half bridge powered by a single 12V supply you will need an e-cap (i.e. 470uF...1000uF for 8 Ohms) in series to the speaker in order to block the 6V DC which you get from idle at 50% duty cycle.
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