50 years ago, it was usually done by a signal transformer. The signal transformer converted the balanced signal to an unbalanced one and an unbalanced amplifier took it from there. The transformer could also transform the impedance to a more convenient level, if needed or desired. The signal transformer was just one of the components of the microphone amplifier, a rather expensive component if it was any good.
Nowadays the input stage of a microphone preamplifier with balanced input is usually designed to do it all by itself, without any signal transformer. If you want to understand in more detail how, look up terms like differential pair and instrumentation amplifier.
Meanwhile I've been making some progress. Thanks to your help the MIC's have similar volume compared to the other channels of the mixer 🙂
However there's a new issue I cant seem to resolve. When I use a JBL GO2 small portable speaker (amp inside) and connect the mixer output the sound is pretty decent.
When I connect the mixer output via an external PAM8403 amplifier board to a JBL GO2 speaker driver (disconnected from the internal amp) the sounds is terrible. Here's a video to hear the difference.
I'm a bit lost on what's going wrong here... Does anyone have an idea? Is this amp that bad or is something going wrong in the way I hooked things in the circuit?
However there's a new issue I cant seem to resolve. When I use a JBL GO2 small portable speaker (amp inside) and connect the mixer output the sound is pretty decent.
When I connect the mixer output via an external PAM8403 amplifier board to a JBL GO2 speaker driver (disconnected from the internal amp) the sounds is terrible. Here's a video to hear the difference.
I'm a bit lost on what's going wrong here... Does anyone have an idea? Is this amp that bad or is something going wrong in the way I hooked things in the circuit?
So you are also Dutch? Nice that it works well now via the GO2's internal amplifier!
For some reason, it sounds hard clipped via the PAM8403. I don't really have a clue why yet. Does it also easily clip on music?
For some reason, it sounds hard clipped via the PAM8403. I don't really have a clue why yet. Does it also easily clip on music?
I'm living in Delft. Where do you live?
Yes its also clipping on music, especially the bass. I guess I'll first try to hook a new PAM8403 (even though I've tried another one already) and see if something went bad because of my beginner soldering skills.
Maybe there could be something with the power supply. The JBLGO2 works on its own battery, while in the other situation the 5v on the PAM8403 shares the common with the rest of the circuit which is on 12v.
Also I think it might be a good idea to see how it sounds if i connect my phone directly to the PAM with the speaker to see if the problem might be in the mixer. I think it is doing a 2 times amplification so maybe this is too much for the PAM while the jblGO2 amp can still handle. Weird thing is that also on low volume the sound seems off....
Yes its also clipping on music, especially the bass. I guess I'll first try to hook a new PAM8403 (even though I've tried another one already) and see if something went bad because of my beginner soldering skills.
Maybe there could be something with the power supply. The JBLGO2 works on its own battery, while in the other situation the 5v on the PAM8403 shares the common with the rest of the circuit which is on 12v.
Also I think it might be a good idea to see how it sounds if i connect my phone directly to the PAM with the speaker to see if the problem might be in the mixer. I think it is doing a 2 times amplification so maybe this is too much for the PAM while the jblGO2 amp can still handle. Weird thing is that also on low volume the sound seems off....
Have you got a schematic of the amplifier board and if so, could you post it?
If you only had a problem at high volumes, it could simply mean that 1.4 W into 8 ohm (or 2.5 W into 4 ohm) isn't enough. Weird that it also seems to clip at low volumes.
If you only had a problem at high volumes, it could simply mean that 1.4 W into 8 ohm (or 2.5 W into 4 ohm) isn't enough. Weird that it also seems to clip at low volumes.
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Do you have a background from the Delft university? And you’re working at one of the companies at the Tech campus?
I studied electronics at the Delft university and I work at the Delftechpark.
Nice. I studied Industrial Design Engineering in Delft. Unfortunately the electronics courses were very basic but I’m eager the learn more myself. I went to the elektrotechnise vereeniging at EWI yesterday, they have a nice workshop and some stock of components that is also open for hobbyists. It had a nice atmosphere and the people there are happy to share information I found out. By coincidence it seems like we’re working at about the same location. I’m renting a small office at the Delftechpark where I can do some woodworking and electronics hobby projects.
Does that mean it sounds clipped at low volume or is there some other problem with the sound at low volume?
I finally had a good look at your wiring photograph. I have a couple of remarks, but I haven't a clue if they have anything to do with the problem:
-As you already mentioned, the ground wiring is rather convoluted due to the shared supply. The amplifier that drives the loudspeaker will have a distorted ground return current that should not cause voltage drops that get added to the microphone signal. I haven't a clue if that requirement is met at the moment. Can you do an experiment with a separate supply for the PAM8403 board, with only one ground connection to the rest of the circuit, namely at the PAM8403 board's signal input?
-It might help to place the microphone amplifier at some distance from the amplifier and loudspeaker. It's a filterless class-D amplifier that is bound to radiate modulated ultrasonic magnetic fields from its output wires and loudspeaker, I don't know if those can do harm when picked up by the microphone preamp.
-The 4700 uF capacitor is rather far away from the amplifier board. The purpose of decoupling capacitors is to short wire inductance, but that only works well when you place them as close as practical to the circuit that needs decoupling.
I also had a look at the schematic of the PAM8403 board and the PAM8403 datasheet. The volume potmeter is straight at the input of the board, so you should always be able to ensure it doesn't clip by turning down its potmeter sufficiently. If you turn it down too much, you may end up driving your mixer into clipping to get some sound out.
Setup 2 shows how things are wired now.
So do I understand correctly that i should wire as in Setup 1?
In Setup 1 the amp has a separate power source for the 5v +, the audio signal minus is connected to the 12v circuit's minus.
I don't know, because I don't understand your picture for set-up 2. There is no wire from the mixer to the amplifier and I don't know what the board with the micro-USB connector is for.
That board is a jq6500 soundboard. You can connect 5 buttons to play 5 sounds. I want to use these as sound effects buttons and they occupy one of the 4 channels on the mixer.
I forgot the signal wire in setup 2 but its there, from the mixer to the amp.
I was thinking of something like this, at least as an experiment. Apologies for the crappy drawing.
When your power supply is in its default independent mode and there are no banana cables shorting the negative outputs, you have three independent supplies in one. Connect the supply and ground of the amplifier board to the + and - outputs of one of these channels (channel 3 in the drawing) that you only use for the PAM8403 and only make a connection between the ground of the PAM8403 board and the ground of the rest via the shield of the amplifier input signal cable - or via a ground wire twisted around the signal wire, if you use separate wires rather than shielded cable.
When your power supply is in its default independent mode and there are no banana cables shorting the negative outputs, you have three independent supplies in one. Connect the supply and ground of the amplifier board to the + and - outputs of one of these channels (channel 3 in the drawing) that you only use for the PAM8403 and only make a connection between the ground of the PAM8403 board and the ground of the rest via the shield of the amplifier input signal cable - or via a ground wire twisted around the signal wire, if you use separate wires rather than shielded cable.
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I’m now replacing the alligator clips with the final soldered wires. Once I’ll have everything hooked I’m thinking of using the oscilloscope to compare the input and output signal of the pam8403 hopefully somewhere this week.