For experimental purposes, I would like to connect an audio amplifier module (see typical circuit below) to various non-conventional inputs and loads. For example, unpowered air coils with very low resistance/inductance. Without load compsensation these would be a dead short.

My concern is how to satisfy the input and output impedance specs of the amp, and effectively make it bomb-proof while still getting a degree of amplification.

Would this best be achieved by inserting mini coupling transformers, e.g. 3K:3K input to ground and 8R:500R across the output? In this case, would C1 still be needed?

Or can I simply place a 3K and 8R resistor in series with the "loads" to ground at the in and out respectively?

If both would work, I would like to please understand what are the pro's and con's of each approach in terms of the amp's performance. Or is there a better method?

 

You copied a completely wrong schematic from "the other side of the world"
On some of the modules, the negative pin might wrongly be marked "GND".
On this side of the world the TDA2030 and its replacement TDA2030A are obsolete and not available.

 

It is just a "typical" circuit to help anyone here who might like to address my general question. Because I am old and obsolete myself, I have a box full of these modules. When everything but class D is banned due to climate change they will be worth a lot of money.

They do work fine without a negative supply by the way. Negativity is never a not good thing.

https://components101.com/ics/tda2030a-pinout-features-datasheet

 

It is just a "typical" circuit to help anyone here who might like to address my general question. Because I am old and obsolete myself, I have a box full of these modules. When everything but class D is banned due to climate change they will be worth a lot of money.

They do work fine without a negative supply by the way. Negativity is never a not good thing.

https://components101.com/ics/tda2030a-pinout-features-datasheet

That does not sound like a viable retirement plan, at least not with corporate bonds yielding 6% all of a sudden.

 

The output impedance of a modern solid-state amplifier is almost zero ohms so that is damps speaker resonances.
The TDA2030 was designed for 4 or 8 ohm speakers.
The max allowed current and max allowed heating must be observed.
The size of your module is fairly small so it probably has poor cooling that limits its max heating.

Here is another "typical" circuit that is full of errors:

 

I am not too concerned about errors because I already have working modules.

To rephrase my OP., and referring to your latest correct circuit.

1) If I connect the secondary of a 3K:3K transformer between the input and ground, would that compensate for low resistance across the primary?

2) If I connect the primary of an 8R:500R transformer in place of the speaker would that compensate for low resistance loads across the secondary?

By "low" I mean less than an ohm.

If "yes" to any of the above, could the transformers be replaced with resistors of similar value to ground?

 

When everything but class D is banned due to climate change they will be worth a lot of money.

Perhaps they need to think about speaker efficiency first! 78.5% for a class B amplifier rather better than 0.25% for some loudspeakers.

 

For experimental purposes, I would like to connect an audio amplifier module (see typical circuit below) to various non-conventional inputs and loads. For example, unpowered air coils with very low resistance/inductance. Without load compsensation these would be a dead short.

My concern is how to satisfy the input and output impedance specs of the amp, and effectively make it bomb-proof while still getting a degree of amplification.

Would this best be achieved by inserting mini coupling transformers, e.g. 3K:3K input to ground and 8R:500R across the output? In this case, would C1 still be needed?

Or can I simply place a 3K and 8R resistor in series with the "loads" to ground at the in and out respectively?

If both would work, I would like to please understand what are the pro's and con's of each approach in terms of the amp's performance. Or is there a better method?

View attachment 277923

View attachment 277922

To drive things like air-cored coils I would recommend a resistor between coil negative and ground, and take the feedback from the resistor, so it behaves like a constant current source.
However, you only have to look at a TDA2030 in a funny way and it will go unstable, so it might not take kindly to messing about with its feedback.
For the input, there's much more latitude- R2 can be much higher than 1k.

 

I will try the feedback resistor which I assume is also for current limiting. So no output transformer would be used.

On the input side, what is the smallest impedance the TDA2030, or similar IC's, can work with? The actual input coil has negligible resistance so some kind of impedance matching is needed. What would you recommend?

 

On the input side, what is the smallest impedance the TDA2030, or similar IC's, can work with? T

Zero

 

So, I can in effect short the inputs for zero signal. What happen as the resistance/impedance across the input is gradually raised? How would it affect the performance of the amp? Is there some sweet spot I should be aiming at?

 

So, I can in effect short the inputs for zero signal. What happen as the resistance/impedance across the input is gradually raised? How would it affect the performance of the amp? Is there some sweet spot I should be aiming at?

Generally, lower is better. As the resistance is raised, offsets due to the bias current start to be noticeable. If you were aiming for really low noise (which obviously you aren't in this case) then noise would increase with the square root of resistance.

 

1) If I connect the secondary of a 3K:3K transformer between the input and ground, would that compensate for low resistance across the primary?

A transformer and the amplifier are designed for AC and work with AC, not DC.
You do not need a power amplifier to use a weak 3k load.
If the load AC impedance is less than 4 ohms then the amplifier will become overloaded and burn if the output level and supply voltage are too high.

2) If I connect the primary of an 8R:500R transformer in place of the speaker would that compensate for low resistance loads across the secondary?

Again, the amplifier works with an AC load of 4 or 8 ohms, not DC.
The 8 ohms to 500 ohms transformer will stepup an amplifier output voltage of 12V RMS to 750V RMS if the 500 ohm winding is not overloaded.

 

Here is perhaps a more straightforward example. This amp module has a stated input impedance of 30K.
https://www.kemo-electronic.de/en/L...er/Modules/M032N-Amplifier-12-W-universal.php
https://www.kemo-electronic.de/en/L...er/Modules/M032N-Amplifier-12-W-universal.php

I understand the output situation. In place of a speaker, I would just add an 8R series resistor to load up the air coil.

But how should I best match the amp's 30K input impedance to the very low resistance air coil on the input side? Coupling transformer? Resistor (where, value)?

I realize I am not using the amp for what it was designed for. Its sensitivity is 80mV. But what arrangement would give me the best possible outcome for lower voltages? Can anyone please offer a specific suggestion?

 

What is an "air coil"? Is it a coil of wire without a magnetic core, just air? Then how does it produce an audio signal?
What does an "air coil" do at the output of an audio amplifier?

An amplifier might have an input sensitivity of 80mV but its input device is a microphone with an output of only 5mV. Then you add a "preamp" between them with a gain of 16 or more.

Input transformers are expensive today and are not used anymore but 60 years ago they were the only preamps available. Today an inexpensive opamp is a preamp.

 

At input, the air coil couples with an applied external field. Output is a weak EMF being whatever "noise" happens.

Any way, I will use a 3K:3K mini coupling transformer on the amps' input (about $5 last I looked), and a load resistor at the output.

Thanks for the replies received, which have helped me consolidate my admittedly unusual thinking.