Help lowering line voltage to imput voltage.

maxximus

Member
2014-02-22 2:11 am
Help lowering line voltage to input voltage.

I am in the process of building a Bluetooth speaker assembly. I am using this amp/Bluetooth board to drive 2 3" speakers. It has a 8 watt output max. http://www.amazon.com/TSA2110A-Clas...&redirect=true&ref_=oh_aui_detailpage_o01_s00

I am looking to drive a 8" sub with this amp here TDA7492 Digital Audio Amplifier Board 2x50W

However the output voltage would be a bit to high for the second amp. I am hoping to get it just right so the first amp controls the volume correctly for the second. I am unable to get input specs for the second amp, however they state line voltage is to high. I may just run it off my phone and use a dmm to approximate the voltage if I have to.

What would be the simplest way to lower the voltage? If it is simply a matter of wiring in a pot, what ohm value would be suggested. The guy I was talking to at P.E. was completely unsure on this.

Any help is appreciated.
 
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Wait ! I mean..don't run; the amplified signal that is present at the outputs of the TPA3110 isn't referred to ground, so you can't just put an attenuation net ( 2 resistors, or a pot ). As usual the splitting of sub-frequencies and full-range ( above 80-100 Hz) should be done at line level, not power.
Moreover, the signal must be "buffered" in order to not overload ( or...underload ) the previous stage. So, looking at the TPA3110 datasheet, the L and R input are at pin 3 and 12 ( the signal is decoupled by a capacitor, before). Those are line level signals. From there, they should be buffered, then summed, then low-passed>> to have a sub-mono-signal
:eek:
 

maxximus

Member
2014-02-22 2:11 am
There are no input level points that I can tap on the board. I already looked. the solder points are too small. I'm hoping I can bring the output signals that are wired into the two 3" speakers down to a voltage the sub amp can work with. Is this not possible?

I already have crossovers made to handle amplified signal. If I can't do this easily I may just skip the idea of wiring in a sub and amp.
 
Ok, maybe you can obtain line level ( from 150 mV to 1.5 V ) from those (floating, not referred to ground ) outputs.
The fact is that the signal is amplified and we don't know yet the bandwidth of the amplifier and THD at low frequencies. This will add up to the original signal and will be furthermore amplified :mad:

Probably, you can start with a set of 4 resistors: 1 kΩ in series to each output, then join the two couples in each channel; then another resistance to ground sets the reference ( 680 Ω ??? ). Also (10 X) like 10 kΩ and 6.8 k Ω should do.
 

maxximus

Member
2014-02-22 2:11 am
I think I found what I am looking for its called a speaker to line level converter. I found one for 5 Bucks that should work.

I'll have to post pictures of this when it is done. It should be approximately a 10" cube doing 2.1 sound. 2 3" speakers and one 8" sub/woofer I know there are many compromises with this, but the sound quality and output should beat any off the shelf system at a fraction of the price.
 
I think I found what I am looking for its called a speaker to line level converter. I found one for 5 Bucks that should work.

remember that the outputs are not referred to ground----:rolleyes:

BTW I was looking at the PCB of the amplifier, and the 3 & 12 pins are easily detectable...there are the two SMD caps interrupting the path from the BT module, the L & R channels...I know it's problematic to desolder the two tiny caps and make a derivation from there, but it's the only solution.