Converting stereo to mono in amplifier circuit

Thread Starter

danawcook

Joined May 12, 2020
22
I found my way to this thread as I am attempting to build a 3-way speaker that is driven by a class-D bluetooth amplifier. Since the amplifier is 2-channel out, I am trying to build this circuit. I am successful except when using it with a much smaller speaker it does not play very loudly before the amp cuts out. Any thoughts? Is this the appropriate circuit? if not, what can I try?
 

Audioguru again

Joined Oct 21, 2019
1,491
The class-D amplifier cuts out probably because you shorted together its 2 output channels.
You asked, "Is this the appropriate circuit?" but you did not attach the circuit.
Please attach a photo or link to the class-D amplifier and show or describe where on it you connected the little speaker.
 

Thread Starter

danawcook

Joined May 12, 2020
22
MrChips attached the circuit to sum the channels. The circuit diagrammed above is applied to the amplifier’s 2-channel outputs that would normally be connected to speakers directly.

The amp is premade, here:

https://www.amazon.com/gp/product/B07XG33WPN/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1

I do not have the circuit diagram.

I am certain it is obvious, but I am not an EE and woefully inadequate in such knowledge.

The load is currently a desktop two-way (tweeter and woofer) that measures 6.1 ohms but the intended load is a three-way custom that measures 8 ohms and is much larger (built into a AR 2a cabinet with 10 inch woofer).
 
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Audioguru again

Joined Oct 21, 2019
1,491
It sounds like you are using two resistors to mix stereo to mono and feeding the weak little LM386 Power Amplifler input the mono. Then you are wrongly shorting together it to both stereo OUTPUTS (?) of the Chinese amplifier??
Amplifier OUTPUTS are NEVER shorted together like that. if you are using the little LM386 amplifier to feed the Aux input of the Chinese amplifier then the LM386 circuit is not needed.
Instead the two resistors that mix stereo to mono feed the Aux input of the Chinese amplifier.

If you are using the Bluetooth input of the Chinese amplifier then the stereo to mono mixer circuit cannot be used.

Since you will have only one speaker then only one channel of the Chinese amplifier will be used. The datasheet for the TPA3116 amplifier IC used shows an output power of 8W per channel when the power supply is 12V up to 30W per channel when the power supply is 24V, into an 8 ohm speaker. Which power supply voltage and maximum current rating are you using?
 

Thread Starter

danawcook

Joined May 12, 2020
22
I am using a 24 volt 5000mA power supply. The intention is to use the built in Bluetooth, but I will not be able to sum the two channels post amplification.

Thus I am understanding that if my primary intentions are to use a single speaker with Bluetooth, I have the following options:

- abandon the project
- ditch this particular amp and go with a separate Bluetooth antenna and sum the signal spot Bluetooth but prior to input into a different amp which feeds speaker through one channel (purchasing amp and bt module)
- build my own bt module and summing circuit and input into this or any other amp

am I understanding correctly?

thanks for all help and guidance. Greatly appreciated.
 

Thread Starter

danawcook

Joined May 12, 2020
22
Understood but defeats goal of single, portable bt device with more reasonable sound As compared to tine speaker. But, I am just playing around with simple stuff to learn a bit. Thank you as well.
 

Thread Starter

danawcook

Joined May 12, 2020
22
Thank you, Bob. Certainly true. Trying to play with with what I have on hand besides the dual coil subwoofer that I want to stay intact. Thank you though.
 

Thread Starter

danawcook

Joined May 12, 2020
22
Or, I can expand the baffle and have a pair of tweeters and a pair of mid range and a pair of woofers but my cabinet is only 24 by 13 by 11 inches.
 

MrChips

Joined Oct 2, 2009
21,126
So basically, you want to drive a middle channel sub-woofer?
Many 2-channel amps allow you to invert one channel so that you can drive one speaker with the two channels.
Since TPA3116 is already a class-D amp with BTL (bridge-tied load) you cannot do this.
However TPA3116 has PBTL (parallel bridge-tied load) where you can drive one speaker with both channels in parallel.

You could hack the amp and find the Left and Right signals from the BT (bluetooth) receiver, bridge them and then set the amp in PBTL mode.

Read the datasheet of the TPA3116.
 

Audioguru again

Joined Oct 21, 2019
1,491
Can you imagine a speaker with two voice coils playing stereo? One channel will produce a loud tone and the other channel might be silent or with opposite phase and they fight each other.

Paralleling outputs using the PBTL mode takes a mono signal input and produces a mono signal output with higher current if extra current is needed. It does not affect one speaker unless it is 2 ohms.
 

Thread Starter

danawcook

Joined May 12, 2020
22
Thank you all for the previous assistance. So, i purchased a BT module, stand alone. Stereo to Mono circuit at its output to create a single output (left-channel) that is the combination of two channel stereo. This is then input into the amplifier (some Dayton Audio that i had from some time ago). Works

1) but there is noise output via speaker when the source is switching from one song to another.

2) If I want to power both of the devices (BT and AMP) from single power supply, and have a clunky beginners kit with various circuit components, how do I measure a Zener diode and determine what is appropriate for the circuit?

The amp has a power supply that is 12V - 2 A and the BT module it typical 5V - 2A.

I can purchase and or have access (who doesn't have 50 wall warts left over in a box some where) to AC-DC power supplies.

Thanks for assistance in advance. Seems as if the amp will have 12V - 2A and then the zener will divide the circuit and voltage down to 5V...
 
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