Recently I purchased this amp https://www.sparkfun.com/products/9612 and a bluetooth audio board https://www.sparkfun.com/products/11777.
The amplifier works great when I just give it an analog signal from an audio cable. The bluetooth module gets an audio signal fine that I can output to headphones. However, the problem begins when I try to use both together; that is, tying the output lines of the receiver to the input lines of the amplifier. When I try to connect the receiver and amplifier to the same ground I get a very quiet output but when I power them from two separate sources (still having the outputs tied to the inputs) the signal comes through fine and works as I expected.
I was wondering if it is possible to make these devices work together when using the same ground because I plan on powering my project from a single source. I believe the problem is arising from the amplifier circuit tying the negative input to ground whereas this is not done on the bluetooth module.
The RN52 has balanced outputs and neither should be connected to ground. Consult the RN52 datasheet to see how to connect an amplifier with balanced inputs, using capacitors. An example schematic is shown on page 13, Figure 2-7 of the datasheet.
For the amplifier I have, the negative input is tied to ground on the board, as shown in the schematic. https://www.sparkfun.com/datasheets/Kit … %20v11.pdf
For the amplifier you mentioned, this is not the case. It looks like the solution shown relies on the ground not being tied to a negative input, but rather gets tied to the output, like in an op amp circuit.
Is there any way to connect my inputs (maybe pass through a buffer amplifier to make the signals like the amp input wants)?
Is there any way to connect my inputs (maybe pass through a buffer amplifier to make the signals like the amp input wants)?
Yes, you could use a unity gain differential amplifier to convert the differential output to single-ended output. Just about any op amp would work, especially with a bipolar power supply (e.g. +/- 5V). With some thought, the input circuitry of the LM358 preamp in the STA540 could be changed appropriately.
Another option to try would be connect both (+/-) SPKR outputs through suitable load resistors (say 220 ohms - 1K) to ground, and then use just one of the outputs as input to your audio amp. I imagine that the average voltage at each SPKR output is one half the RN52 supply voltage, but with a reasonably high impedance load like the suggested 220 ohms, the output amplifier might tolerate this abuse. If that doesn’t work, try the 1K load resistors. In either case, connect the RN52 and STA540 grounds together.
I could not find any information in the RN-52 data sheet about the characteristics of the audio output circuitry. What piss-poor documentation!
Yes, you could use a unity gain differential amplifier to convert the differential output to single-ended output. Just about any op amp would work, especially with a bipolar power supply (e.g. +/- 5V). With some thought, the input circuitry of the LM358 preamp in the STA540 could be changed appropriately.
Does the power supply need to be bipolar? I tried it with just using the supply given to the board (+3.3V) and I just got noise out of the difference amplifier. The unity gain differential amplifier circuit is like this http://www.electronics-tutorials.ws/opamp/opamp18.gif This does not make sense because it should be taking the (-) input, inverting it and adding it to the (+) input to make the resultant signal, right? I was thinking, it works like this: http://www.wolframalpha.com/share/clip? … qi7337gbid, where the green wave is my output. Instead, it seems to be cancelling the waves out; leaving only noise. This is one of the times I really wish I had an oscilloscope, haha!
You should verify that with no signal, the SPKR outputs of the RN52 are at about 1.6 V. If so, you don’t need a bipolar power supply and the circuit you linked (opamp18.gif) should work. Set all resistors to be 10K. However, you will need an op amp that runs from a single-sided power supply of 3.3V and many don’t. I like the ICL7612 from Maxim for such purposes. Maxim may even send you a free sample.
I’ll check the outputs when I get home, thanks! Out of curiosity, why won’t a regular op amp (LM358) work and what is the difference in single sided supply op amps?
The LM358 will work at 3.3V, however the inputs are not “rail to rail”, which means that they cannot be driven above Vcc - 1.5 V (in your case, about 1.8V) without introducing a great deal of distortion. Other op amps offer rail-to-rail operation. Consult the data sheets for details.
This and other problems crop up when you try to use low voltage power supplies with op amps, but not so much with higher voltages, or with split supplies of +/-5 V or greater.
There is really no practical difference between single-supply and dual-supply op amps, but the circuit design depends heavily on which type of power supply you choose.
