Detailed guide in pictures to how not to build an amplifier.
You have been warned, this story will show how I did my first single-chip amplifier and PSU, and how you do not want to build it. For example there was a small explosion and other crappies, result is somewhat unexpected and described at the end of the page…
And now a bit of midnight electronics horror.
As I wrote yesterday, I somewhat implement single-channel single-chip trivial chip amplifier. To test the implementation I need a power supply unit. It should be simple enough, since LM3886 has voltage stabilizer built-in. So just transformator, four diodes to create a rectifier (I used Schottky diodes, it is possible to use usual ones, one can shunt them with capacitors), several huge capacitors to smooth out the resulted voltage, and resistor which will be used by capacitors to charge out when there is no load. Simple.
Simple PSU. Version 1.0
Since most of the chip amplifiers use differential voltage, you likely have to have two PSUs, which will be connected in the middle point (+ of one and – of another) to create a ground. It is possible to use single supply, but it is rarely used.
Closer look at power supply unit
Noticed different look of the PSUs? This is because they are different. Very different. What could be simple than 5 details scheme? Nothing, but even there one (read: me) can break things. Since I’m stupid enough not to properly count all needed details in advance, I got not enough Schottkey diodes and did not get any usual one at all. Actually I got exactly 8 Schottkeys, but I did not pay attantion that they are coupled with common cathode. Which basically means that 4 boxes can not create two rectifier bridges… So, I created one PSU with 3 diode couples, and still have single coupled Schottkey diod. Sounds as a deadlock? Not at all. As we know, each transistor somewhat consists of a pair of diodes. They can not be used simultaneously, since base controls current between collector and emitter, but I happend to have two yet unused transistors, so I connected their bases and emitters as diodes into rectifier bridge. I also shunted them with two small capacitors. That’s how it looks.
Detailed PSU view
Ok, both PSUs are ready, let’s turn them on and check the output voltage.
Turned on PSUs
Connected, turned on, attached voltmeter. Good. Almost. Actually not that good. Really bad I would even say. Voltage differs more than 10 volts, while AC voltage should be the same according to manufacturer. Since it is not stabilized PSU it could not that bad, but now it is not possible to connect them together to get ground point, since it will have about +10 V voltage. I was curious where and why exactly there is that big difference, so tried to put voltmeter probes to the transistors and managed to short-circuit one of the shunt capacitors to the input (note, that capacitor shunts single diode, so effectively it is connected to input and one of the outputs), so it got charged by more than 30 V AC. It could not be that bad, except that capacitors I used were only capable of working with 15 V. Result is quite expected.
Exploded capacitor in one of the PSUs
Solder them out and replace with 250 V capacitors. Should be rock solid now. Probably, since I do not remember if base-emitter can be connected to 30 V AC. That’s how new version looks.
PSU. Version 2.0
I also made a simple voltage devider using two 8 (iirc) kOhm resistors connected one after another, so that middle point could be a ground. With above changes I got about 29 V DC and thus about 14 V as a differential signal.
Connected to the amplifier, soldered connectors, started a signal from the notebook (audio output with about 20-30 mV amplitude) …
PSU and amplifier
… and got silence output (well, there some noise about 10 mV which does not depend on input signal and was present even if there were no input signal at all). Removed all output filters, so that scheme essentially looked as simple operation amplifier with negative feedback to the inverting input, but still no changes.
Its time to look into the datasheet… And find out that LM3886 requires input differential voltage minimum 20 volts, I checked differential signal when amplifier was connected to the PSU and found that it is about 22-23 V, so it is close to the limit, but it should work.
Hopeless. No ideas. Finita la comedia. But I’m lucky. Some electronics gods decided to move my eyes couple of lines below input voltage description to the mute function of the chip. It happend that open or grounded mute pin means mute and 0.5 mA current means pass-through amplification. At least I wanted to believe that, since I actually was not sure how to translate that lines exactly. So, simple resistor to the pin and V-, turn it on and…
Amplifier output
yellow is input and blue is output signal for some amplification ratio. I used my drop-headphones as load, and they do produce the amplified sound. Curious about its quality? Guess that…
It is awful :) So far I do not know where the crap interference sneaks in, but I will think, likely it is somewhere at the input chain or voltage circuit, since with and without the output filters results are the same.
But it is already very different story…
LM3886 amplifier. Take 1. Passive OS fingerprint module update.
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I don’ think that using a voltage divider to get your ground is a very good idea. It may work, but you may get very bad results with it, especially using something like 2 x 8Kohm resistors. With these values you effectively only have a 29V/16Kohm ~ 2ma draw which is nice from a power conservation standpoint, :) but it means that if anything is likely to draw even a small amount of current form that ground it will drastically change its value! Since it is your signal ground, it may very well be the cause of your noise.
How are you connecting your 2 PSUs together in the first place? Are they both about 14V and you just connected them in series? If so, you already have a midpoint between them at the connection point, why do you need the voltage divider?
Of course it is a bad idea, but it was the only way to create differential source with my details :)
Yes, current will be rather small, but it is enough to show that scheme works, and deviations will be comparable with the current itself. Voltage deviation should also not be a problem (it drops by 3-4 V when amplifier is connected), since it is still in the allowed range (although too close to the limit).
PSUs are not connected, since they were built with different details (the one I used for testing even uses base-emitter part of the transistors as rectifying diodes), and theirs output characteristics differ noticebly (for example voltage difference is about 10 V), so if I will connect them in series, I will not have a ground point, since voltage on the differential ends will not be the same.
I will create better PSU of course, maybe with stabilizer (since preamp may require that) and/or powerful output transistor (I think mosfet will be a good choice), but what I created so far was enough to test what was made.
Next I will experiment with single-supply amplifier and will test it against my second PSU, which is supposed to have better characteristcs than which was tested with differential output. I will use reference design from the lm3886 datasheet.