# WTD588D-U sound module to TDA7297 amp equals big distortion

#### WShawn

Joined Apr 9, 2017
16
Hi:

This is my first post here, so I hope you'll bear with me.

I'm attempting to connect a sound board (built around a WT588D sound module) to an amp powered by a TDA7297 Dual Bridge Amplifier, but the resulting sound is very overmodulated.

I offered to help an acquaintance install an electronic kit into a Ghostbusters Proton Pack he's building. The kit includes a sound board built around a WT588D-U voice/sound module. The module has several sound effects programmed into it, along with the Ghostbusters theme, trigger by various switches. This sound board runs off of 3 volts, so I'm powering that with 2 AA batteries for now. The board has + and - outputs you're supposed to connect to an amp (not included in the kit). I had him buy this amp package, built around a TDA7297 amplifier.

The amp runs off a 12V Li-ion battery. I have a pair of 50W Boss 4 ohm speakers connected to the amp. When I connect the sound board to the amp (via alligator clips clipped to a 3.5mm plug and cable plugged into the 3.5mm jack on the amp) the resulting sound is very distorted. Turning the amp's pot almost all the way down makes the sound quieter, but you can still hear the distortion.

I think this accurately represents how the sound module is connected to the amp:

As a test I routed the outputs of the sound board into the 3.5 mm AUX inputs of a Bluetooth speaker and an old boom box, and in both cases the sound is perfect, no distortion on those devices even with the volume cranked up all the way.

As another test I plugged an iPhone (and then iPad) into the amp using the 3.5mm cable and played the Ghostbusters theme with the iPhone and iPad volume cranked to maximum, and the audio sounded fine, no distortion with the amp audio turned up to the point of being uncomfortable.

So these two individual units appear to work just fine when connected to other amps or sources. It's only when I connect this sound board to this amp that the audio is distorted. Research on the GBFans forum indicate others have had issues with this particular sound board outputting distorted sound.

I've also tried this method of connecting the sound board to the amp, tying together the + PWM output to the wipers of both input pots on the amp and the - PWM to the ground sleeve of the plug (and thus the ground of the amp). That's just as distorted:

My only guess is that the sound board is outputting its signal in a way that the amp can't deal with. I know enough about electronics to read and build simple schematics, but I don't have a great understanding of the underlying electronic principles of how everything works together. I have a 40-year-old Radio Shack multi tester.

Here's a link to a PDF data sheet of the WTD588-D sound module:

https://www.dropbox.com/s/zc323uo3f8583uf/WT588D-U Voice Module 28 pin.pdf?dl=0
And here's a link to a PDF data sheet of the TDA7297 amp chip:

https://www.dropbox.com/s/d6aktgd3qgdngim/tda7297.pdf?dl=0
Perhaps someone smarter than I can look at these spec sheets and tell whether there's some incompatibility between the sound module and amp that can be corrected with different wiring or adding in some simple, additional components. Like would I get a better result tying the DAC output of the sound chip to the amp (with common grounds)? That's not how this sound board has been wired, though. Something else?

If it came to it we could buy a different amp, but I don't want to just randomly buy another type and risk that that wouldn't work either.

Thank you!

Shawn Marshall
Portland, OR

#### TeeKay6

Joined Apr 20, 2019
572
Hi:

This is my first post here, so I hope you'll bear with me.

I'm attempting to connect a sound board (built around a WT588D sound module) to an amp powered by a TDA7297 Dual Bridge Amplifier, but the resulting sound is very overmodulated.

I offered to help an acquaintance install an electronic kit into a Ghostbusters Proton Pack he's building. The kit includes a sound board built around a WT588D-U voice/sound module. The module has several sound effects programmed into it, along with the Ghostbusters theme, trigger by various switches. This sound board runs off of 3 volts, so I'm powering that with 2 AA batteries for now. The board has + and - outputs you're supposed to connect to an amp (not included in the kit). I had him buy this amp package, built around a TDA7297 amplifier.

The amp runs off a 12V Li-ion battery. I have a pair of 50W Boss 4 ohm speakers connected to the amp. When I connect the sound board to the amp (via alligator clips clipped to a 3.5mm plug and cable plugged into the 3.5mm jack on the amp) the resulting sound is very distorted. Turning the amp's pot almost all the way down makes the sound quieter, but you can still hear the distortion.

I think this accurately represents how the sound module is connected to the amp:

View attachment 191880

As a test I routed the outputs of the sound board into the 3.5 mm AUX inputs of a Bluetooth speaker and an old boom box, and in both cases the sound is perfect, no distortion on those devices even with the volume cranked up all the way.

As another test I plugged an iPhone (and then iPad) into the amp using the 3.5mm cable and played the Ghostbusters theme with the iPhone and iPad volume cranked to maximum, and the audio sounded fine, no distortion with the amp audio turned up to the point of being uncomfortable.

So these two individual units appear to work just fine when connected to other amps or sources. It's only when I connect this sound board to this amp that the audio is distorted. Research on the GBFans forum indicate others have had issues with this particular sound board outputting distorted sound.

I've also tried this method of connecting the sound board to the amp, tying together the + PWM output to the wipers of both input pots on the amp and the - PWM to the ground sleeve of the plug (and thus the ground of the amp). That's just as distorted:

View attachment 191881

My only guess is that the sound board is outputting its signal in a way that the amp can't deal with. I know enough about electronics to read and build simple schematics, but I don't have a great understanding of the underlying electronic principles of how everything works together. I have a 40-year-old Radio Shack multi tester.

Here's a link to a PDF data sheet of the WTD588-D sound module:

https://www.dropbox.com/s/zc323uo3f8583uf/WT588D-U Voice Module 28 pin.pdf?dl=0
And here's a link to a PDF data sheet of the TDA7297 amp chip:

https://www.dropbox.com/s/d6aktgd3qgdngim/tda7297.pdf?dl=0
Perhaps someone smarter than I can look at these spec sheets and tell whether there's some incompatibility between the sound module and amp that can be corrected with different wiring or adding in some simple, additional components. Like would I get a better result tying the DAC output of the sound chip to the amp (with common grounds)? That's not how this sound board has been wired, though. Something else?

If it came to it we could buy a different amp, but I don't want to just randomly buy another type and risk that that wouldn't work either.

Thank you!

Shawn Marshall
Portland, OR
First, the WT588D-U does not produce stereo sound; it's monophonic only. I suggest that the best way to use the WT588D is to use its DAC output. As a start (other component values may work better; I don't have any hands-on WT588D experience), connect the DAC output through a resistor (e.g. 1K) to a capacitor (e.g. 0.1uF) to ground (to filter out high frequencies as the DAC changes it output). From the junction of that resistor and capacitor connect a second capacitor (e.g. 0.1uF) to ONE input of the TDA7297. Connect ONE speaker to the output of the TDA7297 channel that you used. Connect nothing to the other output channel of the TDA7297.

#### WShawn

Joined Apr 9, 2017
16
First, the WT588D-U does not produce stereo sound; it's monophonic only. I suggest that the best way to use the WT588D is to use its DAC output. As a start (other component values may work better; I don't have any hands-on WT588D experience), connect the DAC output through a resistor (e.g. 1K) to a capacitor (e.g. 0.1uF) to ground (to filter out high frequencies as the DAC changes it output). From the junction of that resistor and capacitor connect a second capacitor (e.g. 0.1uF) to ONE input of the TDA7297. Connect ONE speaker to the output of the TDA7297 channel that you used. Connect nothing to the other output channel of the TDA7297.

Hi TeeKay6:

Thanks for taking the time to reply; I appreciate it.

I'm going to draw up what I think you've described to make sure I'm understanding your advice, but I have a question. In this setup would the first capacitor be tied to the ground of the WT588D or to the ground of the TDA7297 amp? Or should the grounds of the sound board and amp be tied together?

Cheers!

Shawn

#### TeeKay6

Joined Apr 20, 2019
572
Hi TeeKay6:

Thanks for taking the time to reply; I appreciate it.

I'm going to draw up what I think you've described to make sure I'm understanding your advice, but I have a question. In this setup would the first capacitor be tied to the ground of the WT588D or to the ground of the TDA7297 amp? Or should the grounds of the sound board and amp be tied together?

Cheers!

Shawn
Here's a concept sketch that is my best guess as to what the very poor datasheets say. I have copied some from the Applications, Sec 9, of the WT588D-U datasheet. I have drawn only the minimal amount for concept; you must add missing pins, values, etc.

Good luck. I know nothing more about these devices than is given in the datasheets.

#### WShawn

Joined Apr 9, 2017
16
Hi:

Thank you for taking the time to draw out this sketch; you've gone above and beyond the call of duty, and I appreciate it.

FYI, the WT588D chip used in this sound board is built into a package that looks like this one:

I'm assuming this package includes the components you've indicated in your sketch (I haven't been able to find a schematic), and I'll just have to add the components relating to the DAC output.

Could you hazard a guess as to why this sound board works fine when connected to the Aux inputs of a Bluetooth speaker or boom box but NOT with this TDA7297 amp?

Cheers!

Shawn

Joined Feb 20, 2016
3,537
You will need to ground the lower ends of the pots for a start.
Maybe have a 100K series resistor from the sound chip to the pots too, with a 10nF cap across the pot. (just guesses for the values. Try some and see what changes)

#### WShawn

Joined Apr 9, 2017
16

I've tried two different means of connecting the sound card to the amp. Could you clarify which one I should be using and between which exact inputs and outputs I should try adding resistors and caps?

The lower ends of the pots ARE grounded on that amp package we bought from Amazon; I neglected to indicate that on my crude schematic (which is based on my observations of the traces on its PCB).

This is the corrected diagram (though it doesn't include the components on the WTD588 package I linked to above).

Here's the ALT version which ties the PWM- to the ground of the amp.

The instructions that came with the sound board were very vague as to how the outputs of the sound board connect to the amp:

Note that the developer of this kit has ceased support, so he will be no help.

Cheers!

Shawn

Joined Feb 20, 2016
3,537
The "speaker" outputs are PWM so there will be 3V pulses on them. An R/C filter will turn the PWM into analog signal, but have you tried the DAC output instead? That is the output you should be using.
Once again, run a 10K or some other value to set the max volume, resistor from the DAC pin to the pots. Then a 10nF cap across the pots. Fiddle with the values to suit.

#### TeeKay6

Joined Apr 20, 2019
572
Hi:

Thank you for taking the time to draw out this sketch; you've gone above and beyond the call of duty, and I appreciate it.

FYI, the WT588D chip used in this sound board is built into a package that looks like this one:

I'm assuming this package includes the components you've indicated in your sketch (I haven't been able to find a schematic), and I'll just have to add the components relating to the DAC output.

Could you hazard a guess as to why this sound board works fine when connected to the Aux inputs of a Bluetooth speaker or boom box but NOT with this TDA7297 amp?

Cheers!

Shawn
A PWM signal would normally be passed through a low-pass filter to recreate the analog signal. Without filtering the PWM signal is just a high frequency rectangular wave that the TDA7297 is not intended to accept.

#### WShawn

Joined Apr 9, 2017
16
Thanks to those who have been responding with their expertise. As you can tell by the photo of the actual soundboard it was designed to be plug and play for people with no knowledge of electronics or soldering. Just screw down the wire connections as indicated.

I found what appears to be the entire schematic of the WT588D-28P PCB package, and this (and my continuity tests) indicate that pins 8 and 9 are tied together, meaning the PWM + and DAC are the same signal. Does this impact some of these proposed solutions? Apologies for not posting this earlier.

As I noted, in the plug and play Heavy Props soundboard pin 8 of the WT588D-28P chip is going to the Speaker + terminal and pin 9 is going to Speaker - terminal.

Cheers!

Shawn

#### TeeKay6

Joined Apr 20, 2019
572
Thanks to those who have been responding with their expertise. As you can tell by the photo of the actual soundboard it was designed to be plug and play for people with no knowledge of electronics or soldering. Just screw down the wire connections as indicated.

I found what appears to be the entire schematic of the WT588D-28P PCB package, and this (and my continuity tests) indicate that pins 8 and 9 are tied together, meaning the PWM + and DAC are the same signal. Does this impact some of these proposed solutions? Apologies for not posting this earlier.

View attachment 191972

As I noted, in the plug and play Heavy Props soundboard pin 8 of the WT588D-28P chip is going to the Speaker + terminal and pin 9 is going to Speaker - terminal.

Cheers!

Shawn
Searching the Web I have found at least 4 versions of WT588D-x-x datasheets...all of them are poor datasheets in that they give incomplete and conflicting information. Take your pick as to which you think is correct. It is quite likely that the designers of the Heavy Props soundboard had the benefit of having actual hardware to evaluate and test, and they likely know more about the chips than is revealed in the datasheets. If you have a Heavy Props board, then evaluate whether it works as you wish. If so, copy it.

#### WShawn

Joined Apr 9, 2017
16
Searching the Web I have found at least 4 versions of WT588D-x-x datasheets...all of them are poor datasheets in that they give incomplete and conflicting information. Take your pick as to which you think is correct. It is quite likely that the designers of the Heavy Props soundboard had the benefit of having actual hardware to evaluate and test, and they likely know more about the chips than is revealed in the datasheets. If you have a Heavy Props board, then evaluate whether it works as you wish. If so, copy it.
Hi TeeKay:

Thanks for the response. As far as I can tell Heavy Props is one guy who bought a batch of these WT588D boards, programmed in the sound effects and song, and had boards fabricated with screw terminals that the WT588D could plug into. The Heavy Prop board (pictured above) has no components on it, just traces leading to the screw terminals.

I'm not trying to copy it; I'm just trying to make it work with the amp I suggested. It works with other amps (Bluetooth speaker, boom box) but not this one. I'd tell my acquaintance to buy a different type of amp if I was sure it would work. I'm still baffled why it works with these other amps but not the TDA 7297.

Best.

Shawn

#### TeeKay6

Joined Apr 20, 2019
572
Hi TeeKay:

Thanks for the response. As far as I can tell Heavy Props is one guy who bought a batch of these WT588D boards, programmed in the sound effects and song, and had boards fabricated with screw terminals that the WT588D could plug into. The Heavy Prop board (pictured above) has no components on it, just traces leading to the screw terminals.

I'm not trying to copy it; I'm just trying to make it work with the amp I suggested. It works with other amps (Bluetooth speaker, boom box) but not this one. I'd tell my acquaintance to buy a different type of amp if I was sure it would work. I'm still baffled why it works with these other amps but not the TDA 7297.

Best.

Shawn
Since we have no schematics for those other amps that you claim work properly, we can make no more than guesses, wild guesses. I have explained why I believe that the TDA7297 may not work well when driven with a PWM signal. If the other amps have no problem being driven by a PWM signal rather than an analog signal, then use one of them. If you want to use the TDA7297's PWM, then filter the PWM to an analog signal before applying it to the input of the TDA7297. The TDA7297 was not designed to be directly driven by a PWM signal.

#### Audioguru again

Joined Oct 21, 2019
1,508
You said that the WT588D board works with other amps but not with the cheap Chinese TDA7297. Then maybe the TDA7297 amp is fake or defective? Did you try another signal source to feed this amp?

#### WShawn

Joined Apr 9, 2017
16
You said that the WT588D board works with other amps but not with the cheap Chinese TDA7297. Then maybe the TDA7297 amp is fake or defective? Did you try another signal source to feed this amp?

Thanks for taking the time to reply. I wrote in the initial post that I'm able to plug an iPhone into the TDA7297 via a 3.5mm cable (and the Lightning headphone dongle) and that TDA7297 amp works fine. I crank the volume up full on the iPhone (and an iPad Air 2 I tested), and the amplified audio is perfect, no distortion.

Shawn

#### WShawn

Joined Apr 9, 2017
16
Since we have no schematics for those other amps that you claim work properly, we can make no more than guesses, wild guesses. I have explained why I believe that the TDA7297 may not work well when driven with a PWM signal. If the other amps have no problem being driven by a PWM signal rather than an analog signal, then use one of them. If you want to use the TDA7297's PWM, then filter the PWM to an analog signal before applying it to the input of the TDA7297. The TDA7297 was not designed to be directly driven by a PWM signal.

Apologies for coming off as dense or whatever. I appreciate your patience; I know it's not fun trying to explain things to a noob.

You write that the TDA7297 was not designed to be directly driven by a PWM signal, so that's new information to me and good to know, thanks.

Looking at the software manual for the WT588D (section 2.6 of the attached PDF) it looks like there's an option to set the sound output to DAC or PWM. I'm betting the developer set that to PWM. As I don't have the encoding module required for reprogramming this package I think I'm stuck with PWM.

So I guess I have two options to make this work:

1. Filter the PWM to an analog signal this particular amp can cleanly read.

2. Buy an amp that can ingest an unmodified PWM signal. I don’t know what I’d look for in a description to be sure I’m getting such an amp.

This post on the Arduino forum has a simple schematic to filter PWM (for a totally different application). Would that work? What about connecting the PWM to a DAC, as seen farther down in the thread?

https://forum.arduino.cc/index.php?topic=518945.0
I'm guessing that Adafruit DAC wouldn't work as I'd have to route in clock signals and such, and I probably can't do that.

Thanks again for taking the time to help. I honestly am trying to troubleshoot as much of this as I can by myself.

Best.

Shawn

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#### TeeKay6

Joined Apr 20, 2019
572
Apologies for coming off as dense or whatever. I appreciate your patience; I know it's not fun trying to explain things to a noob.

You write that the TDA7297 was not designed to be directly driven by a PWM signal, so that's new information to me and good to know, thanks.

Looking at the software manual for the WT588D (section 2.6 of the attached PDF) it looks like there's an option to set the sound output to DAC or PWM. I'm betting the developer set that to PWM. As I don't have the encoding module required for reprogramming this package I think I'm stuck with PWM.

So I guess I have two options to make this work:

1. Filter the PWM to an analog signal this particular amp can cleanly read.

2. Buy an amp that can ingest an unmodified PWM signal. I don’t know what I’d look for in a description to be sure I’m getting such an amp.

This post on the Arduino forum has a simple schematic to filter PWM (for a totally different application). Would that work? What about connecting the PWM to a DAC, as seen farther down in the thread?

https://forum.arduino.cc/index.php?topic=518945.0
I'm guessing that Adafruit DAC wouldn't work as I'd have to route in clock signals and such, and I probably can't do that.

Thanks again for taking the time to help. I honestly am trying to troubleshoot as much of this as I can by myself.

Best.

Shawn
@WShawn
Learn the difference between analog and digital. A Web search for "analog vs digital" will link you to many explanations.
A PWM signal is a digital signal; it has only two values: low and high, also called "0" and "1". Viewed on an oscilloscope the PWM signal is a series of rectangular pulses of varying width (typically; there are other forms of PWMs). Wide pulses have a large average value; narrow pulses have a small average value. When a PWM signal is applied to the input of an analog amplifier (such as the TDA7297), the amp interprets each pulse as two analog values, min and max, that are rapidly alternating from min to max to min... The pulses typically have an amplitude in the 1V to 5V range. Thus, one pulse may have a value of 0V for 10uS and then 5V for 100uS and then 0V for 12us, etc. To a TDA7297 such a string of pulses looks like a very high frequency signal (PWM frequencies can range from 10pulses/sec to 1,000,000pulses/sec) of enormous amplitude--the amp expected max values of perhaps tens of millivolts but receives values of (e.g.) 5V. This will immediately overload the input stages of the amplifier, producing unexpected and undesired effects, including what you are calling distortion. Analog amplifiers work only with analog audio signals; they do not work with PWM signals. PWM and other digital circuitry works only with digital signals (0's & 1's; lows & highs); digital circuitry does not accept analog signals. All that I have stated is simplified; real electronics is more complex and embraces combinations of analog and digital. Hence, it is in fact possible to drive some analog loudspeakers with some PWM signals and get acceptable results.

#### WShawn

Joined Apr 9, 2017
16
@WShawn
Learn the difference between analog and digital. A Web search for "analog vs digital" will link you to many explanations.
A PWM signal is a digital signal; it has only two values: low and high, also called "0" and "1". Viewed on an oscilloscope the PWM signal is a series of rectangular pulses of varying width (typically; there are other forms of PWMs). Wide pulses have a large average value; narrow pulses have a small average value. When a PWM signal is applied to the input of an analog amplifier (such as the TDA7297), the amp interprets each pulse as two analog values, min and max, that are rapidly alternating from min to max to min... The pulses typically have an amplitude in the 1V to 5V range. Thus, one pulse may have a value of 0V for 10uS and then 5V for 100uS and then 0V for 12us, etc. To a TDA7297 such a string of pulses looks like a very high frequency signal (PWM frequencies can range from 10pulses/sec to 1,000,000pulses/sec) of enormous amplitude--the amp expected max values of perhaps tens of millivolts but receives values of (e.g.) 5V. This will immediately overload the input stages of the amplifier, producing unexpected and undesired effects, including what you are calling distortion. Analog amplifiers work only with analog audio signals; they do not work with PWM signals. PWM and other digital circuitry works only with digital signals (0's & 1's; lows & highs); digital circuitry does not accept analog signals. All that I have stated is simplified; real electronics is more complex and embraces combinations of analog and digital. Hence, it is in fact possible to drive some analog loudspeakers with some PWM signals and get acceptable results.
It's obvious I've aggravated you, so I apologize for that. I do appreciate your help and insight.

I know the difference between digital and analog in general terms, but your explanation of why this PWM output is creating distortion in the amp is clear and helpful, thanks.

Believe me, I hate asking for help. I've spent 10-15 hours trying to troubleshoot this stupid thing, researching online, trying different components, etc. In my posts here I've tried to be as thorough and clear as possible.

And this isn't even my project; I'm trying to help someone I barely know. I could have just told him the sound board you have is junk. Spend $70 on one that has a built-in amp that we know would work. AFAIK I'm stuck with the PWM outputs of this board. As I wrote above, it's now a matter of either: 1. Building or buying a filter to convert the digital PWM signal to a line level analog one. For instance, a Google search of PWM to line level amp gives me this on the Arduino forum: https://forum.arduino.cc/index.php?topic=9510.0 I'll give this simple RC filter a shot to see whether it might work here. If it doesn't perhaps I'll start a separate thread asking this exact question. 2. Find an inexpensive amp that can deal with PWM signals. I know they exist; I have two in the form of that Bluetooth speaker and boom box. But it doesn't make sense to install a$120 Bluetooth speaker or boom box when a \$10 amp could probably work with some modifications. Obviously those commercial products have some sort of circuitry in them that makes their amps compatible with PWM inputs. Perhaps it's a matter of deducing what type of circuitry that could be.

Best.

Shawn

#### Audioguru again

Joined Oct 21, 2019
1,508
The Arduino forum shows a volume control, not a filter. Your amplifier already has a volume control.
Instead you need a lowpass filter. Use a series 4.7k resistor from the WT588D PWM output feeding the input to the amplifier's volume control AND feeding a 22nF filtering capacitor to ground. Turn down the volume control and slowly turn it up then tell us how it sounds. It will probably sound without distortion but very muffled then try smaller capacitor values.

Joined Feb 20, 2016
3,537

2. Find an inexpensive amp that can deal with PWM signals.
Why?
As has been mentioned a number of times, just add an R/C network to convert the PWM or DAC signal to analog.

Obviously those commercial products have some sort of circuitry in them that makes their amps compatible with PWM inputs. Perhaps it's a matter of deducing what type of circuitry that could be.
Probably a resistor and capacitor!

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