4. https://www.ebay.co.uk/itm/266783777308 (SMD + a bit of THT)

Number 4 is interesting as it appears to have through holes to add a pot in line with R5 and R7 - perhaps to add volume control? If my understanding of op-amps is correct (unlikely, but play along), the the amplification can be controlled by one of the resistors so bridging the right resistor on any of the boards should do that?

The board comes with surface mount resistors installed for a gain of 10 (20 K and 2.2 K). The 2.2 K resistors have the thru-holes so you can install a resistor in parallel to decrease the gain.

The AD828 is an interesting choice for an audio module. It is designed from the ground up to be a video amplifier. In an audio application is has decent noise and excellent slew rate. The board runs on a single DC supply, and generates a mid-voltage "ground" for the circuits. There are coupling caps in and out, so the output is referenced to the DC input return (VCC-).

ak

 

Attached, a very noddy block diagram... Please don't laugh!

The board comes with surface mount resistors installed for a gain of 10 (20 K and 2.2 K). The 2.2 K resistors have the thru-holes so you can install a resistor in parallel to decrease the gain.

Which is what I was hoping... I have some 10K audio pots but I guess I'll have to test if they decrease the gain sufficiently.

The AD828 is an interesting choice for an audio module. It is designed from the ground up to be a video amplifier. In an audio application is has decent noise and excellent slew rate.

Curious, but if it works... Advice on which module is module is best (if at all), is probably above and beyond, but if there is advice on one(s) to avoid, it would be much appreciated.

The board runs on a single DC supply, and generates a mid-voltage "ground" for the circuits. There are coupling caps in and out, so the output is referenced to the DC input return (VCC-).

ak

So my question is, do these modules limit their outputs to line-level so I don't damage any amp or active speaker? If they do, how do they do it? If they don't, how do I do it? Eg, if my input is +5V, then VCC- would be -2.5V, so the output wold be +2.5V? (ie -2.5V + 5V = +2.5V, within line-level voltage)? Is +5V sufficient 'headroom' to prevent clipping?

I've not included the optional balanced modules on the schematic, because I think they'd need a +/- power supply. There is one question on the schematic regarding providing the mono sum to both left and right channels.

So many questions, my apologies.

 

I have some 10K audio pots but I guess I'll have to test if they decrease the gain sufficiently.

A variable resistor with the series feedback element is a gain control, not a volume control. With the pot turned all the way down (dead short), the gain will be reduced from 10 to unity. The output voltage will equal the input voltage, not 0 V. In this circuit, there is no negative gain possible.

So my question is, do these modules limit their outputs to line-level so I don't damage any amp or active speaker?

No. The peak output voltage is limited by the power supply voltages minus the opamp output stage headroom requirement.

If they do, how do they do it? If they don't, how do I do it? Eg, if my input is +5V, then VCC- would be -2.5V, so the output wold be +2.5V? (ie -2.5V + 5V = +2.5V, within line-level voltage)? Is +5V sufficient 'headroom' to prevent clipping?

My vote is to build a system that is clean and non-clipping throughout with lotsa headroom. Then add signal limiting circuitry. This can be as simple as a few diodes and resistors for a soft clipper, or a full-blown AGC circuit for no clipping. Note that the AGC circuit will pump up and down as it rides the peaks of a loud waveform.

I've not included the optional balanced modules on the schematic, because I think they'd need a +/- power supply.

Depends on the module.

On your block diagram, the Input switching circuit is incorrect.

ak

 

No. The peak output voltage is limited by the power supply voltages minus the opamp output stage headroom requirement.

My vote is to build a system that is clean and non-clipping throughout with lotsa headroom. Then add signal limiting circuitry. This can be as simple as a few diodes and resistors for a soft clipper, or a full-blown AGC circuit for no clipping. Note that the AGC circuit will pump up and down as it rides the peaks of a loud waveform.

Depends on the module.
ak

Effectively, I would have to amplify up the input with lots of headroom (ie +/- 15V) and then use gain control to reduce each output back down to an appropriate level. This is clearly not an 'easy win', so I'm going to go away and think about it (and why I thought it would be 'easy'!).

I have just acquired an old Extron MDA 3A distribution amplifier which will partially fulfill my requirements but I will also study it to see what I can learn.

Many thanks for your help.

 

With a single distribution amplifier it should be rather simple to add simple adjustments to reduce the signal level to appropriate values for each output channel.