Bridged Speaker Output to Line In

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,653
Ok, so I have this small audio effects module, its output is 2 watts bridged class D. After designing a project using this module I have come to the realization that it's not powerful enough for my needs, but contains the functions that I need, such as digital volume control etc.

So what I really need to do is connect a more powerful amp to the bridged speaker outputs, so my main questions is...

Is this even close to what I need?

AAC_Speaker_2_Line_in.jpg

Any and all input welcome. (except for changing the effects module)

The Effects Module Adafruit Audio FX Sound Board + 2x2W Amp - WAV/OGG Trigger -16MB ID: 2217 - $29.95 : Adafruit Industries, Unique & fun DIY electronics and kits

The Amp I want to use Amazon.com: 2Pcs Digital Amplifier Board,TPA3116D2 Dual Channel Audio Stereo AMP High Power Digital Subwoofer Power Amplifier Board 2x50W 5V 12V 24V for Store Solicitation Home Theater Square DIY Speakers: Home Audio & Theater
 

Deleted member 440916

Joined Dec 31, 1969
0
As both outputs contain the same signal you only need to attach to one. I think you can assume that either output will be ground referenced in some way, usually bipolar power rails will be referenced to ground so you dont need the galvanic isolation you have shown. Just tap off the output into a reasonably high impedance attinuator before applying to the following amplifier, if that has an AC coupled input fine, otherwise include a series capacitor in your circuit as well.
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,653
The signals wont be the same because it's stereo, and I'm pretty sure I need the isolation because the effects module output is bridged.
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,653
Hi.

So you are saying I can combine the two negative outputs of the bridged outputs? (or just use one of the outputs)

I have heard that is a big no no.
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,653
Perhaps I am using the wrong terminology here.

Forget about the term "bridged" just let me say that the negative outputs of the effects module are not common.
 

Ian0

Joined Aug 7, 2020
2,178
How about a couple of 1:1 audio transformers? Do the outputs to the speakers have series inductors? If not you may need to add some - you don't want the input to the next stage to be PWM. The transformer may filter out the high frequency PWM but it may need a little help with a series inductor and parallel capacitor.
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,653
Now I'm really confused, yes I intend to use two 1:1 audio transformers as in the image in my OP, then divide the output, while creating a ground for the amp.

I don't see any inductors or caps in the schematic.

i see what you are saying about feeding the PWM to an amp instead of speakers, what values would recommend and which side of the transformers would I place them on?

I think the PWM is 12MHz.
 
Last edited:

crutschow

Joined Mar 14, 2008
27,172
A bridged output may have a DC voltage to ground so I suggest a series capacitor if you want to connect it directly to an attenuator.
I see no particular need to use a transformer if a single-ended output works.
 

Ian0

Joined Aug 7, 2020
2,178
If it's Class-D then the output will be high-frequency PWM at a couple of hundred kHz. You really don't want that in the input to your amplifier.
If you are lucky you can put 4.7nF across the 1k resistor to make a low-pass filter to filter out the PWM, You really need to know the leakage inductance of the transformers to know what the response of the filter will be. You can measure leakage inductance with an inductance meter by measuring the primary inductance with the secondary shorted out.

One thing to note: Be very wary of "audio" transformer specs. Sometimes "audio" refers to telephone bandwidth, and it's only good for 300Hz to 3.4kHz not 20Hz to 20kHz.
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,653
I'm also confused by this statement from Adafruit...

"All the way to the right is the Class D stereo amplifier. This is for connecting to a pair of speaker! The outputs are
bridge-tied-load (BTL) so do not connect R+ to L+ and R- to L- to get more power, it could damage the chip. If you only
need one speaker, simply connect the channel you want."

Does this mean they have taken a stereo amp and turned it into a mono amp?
 

Ian0

Joined Aug 7, 2020
2,178
I'm also confused by this statement from Adafruit...

"All the way to the right is the Class D stereo amplifier. This is for connecting to a pair of speaker! The outputs are
bridge-tied-load (BTL) so do not connect R+ to L+ and R- to L- to get more power, it could damage the chip. If you only
need one speaker, simply connect the channel you want."

Does this mean they have taken a stereo amp and turned it into a mono amp?
No it means that if you've only got one speaker then you're just listening to the left channel of the stereo!
Even if it wasn't bridge-tied-load then connecting the right output to the left output would be a really good way of blowing it up!
 

Ian0

Joined Aug 7, 2020
2,178
A bridged output may have a DC voltage to ground so I suggest a series capacitor if you want to connect it directly to an attenuator.
I see no particular need to use a transformer if a single-ended output works.
Agreed - provided that the amount of DC voltage to ground is fixed or constant. If the power supply varies with load and the DC offset is half of the power supply, then that adds some rather odd low frequency components to the signal.
 

Thread Starter

ElectricSpidey

Joined Dec 2, 2017
1,653
So...transformers needed or not, if I add a filter for the PWM...will the circuit in the OP work or not?

I need to get something started on this.

And thanks for all of the input so far.
 

Ian0

Joined Aug 7, 2020
2,178
Now I'm really confused, yes I intend to use two 1:1 audio transformers as in the image in my OP, then divide the output, while creating a ground for the amp.

I don't see any inductors or caps in the schematic.

i see what you are saying about feeding the PWM to an amp instead of speakers, what values would recommend and which side of the transformers would I place them on?

I think the PWM is 12MHz.
At 12MHz (which sounds a bit high to me, but may be true) the leakage inductance of the transformer would form an inductance in a second-order LC filter, so putting the C across the secondary will complete the filter. If the PWM frequency (12MHz)) is above the self-resonant frequency of the primary, then a series L would be a good idea, otherwise the power amplifier is seeing a capacitive load.
 
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