How to connect the output of a low power amp to a second high power amp

[lewis]

You can put polarised capacitors in reverse-series to make a non-polar cap, get two 220uF caps, put them in series with their negatives connected together, and you've got yourself a 110uF non-polar cap. Which is close enough. Or 2x 22uF in reverse series = 11uF.

There's some 100uF non-polar caps here: http://www.rapidonline.com/Electronic-Components/Non-polarised-85-deg-C-radial-capacitors-78276

Voltage ratings of all capacitors need to be at least that of the power supply.

[SeanB]

Funny I have no problem getting 100, 200uF film capacitors, though they are the size of a cola can and are marked for motor start applications. Voltage rating is 125V Dc continuous or 275VAC for 30 seconds with 6 starts per hour.

[IanB]

Funny I have no problem getting 100, 200uF film capacitors, though they are the size of a cola can and are marked for motor start applications. Voltage rating is 125V Dc continuous or 275VAC for 30 seconds with 6 starts per hour.

They have come down in size a lot then    In the mid-70s I recall the unpolarized ballast capacitors in fluorescent tube fittings were also the size of a cola can and were only about 8 uF (they were spec'd for 250 V AC continuous).

[MichaelKavanagh]

Brilliant. Thanks, also, how would I change the output gain/volume. With a pot I suppose, but where does it go? Something to do with those 4.7K and 47K resistors?
Cheers
Michael

[SeanB]

Funny I have no problem getting 100, 200uF film capacitors, though they are the size of a cola can and are marked for motor start applications. Voltage rating is 125V Dc continuous or 275VAC for 30 seconds with 6 starts per hour.

They have come down in size a lot then    In the mid-70s I recall the unpolarized ballast capacitors in fluorescent tube fittings were also the size of a cola can and were only about 8 uF (they were spec'd for 250 V AC continuous).

Motor start are a lot smaller value for value than motor run units, as they do not have to survive being connected to the mains permanently. Typically I buy values of 35, 40, 45, 50, 55 and 60uF capacitors with a 440VAC rating for airconditioners. They typically will withstand 1kV on test, and if they start to break down with a 30 second test ( you hear the internal arcing) then I replace them, this is typically after 4-5 years of running. Smaller units of 1, 1.5, 2, 4, 8, 10 and 12uF are used for fan motors, as well as for power factor correction on rapid start ballasts. The PFC and fan motor units are typically rated for 275VAC though.

[lewis]

Ah yes....

You're absolutely right about the 4.7K and 47K resistors changing the gain of the amplifier, and if you find you don't have enough volume you can increase the 47K to 56K or 68K or something. But that's probably not the best place to put the pot. This is because when you adjust the pot, the DC gain of the power amp will vary slightly and the speaker cone will wobble in and out slowly. The 2.2uF cap and the 2200uF output cap will negate this somewhat, but you'll still get a bit of DC at the output of the TDA2030.

The best place to put the pot is at pin 1 of the TDA2030. Replace the 100K resistor with a 100K pot, and connect the wiper to pin 1. I'll modify the diagram if you don't know what I mean.

By the way, you will get a thump from the speaker every time you power on the unit because the 2200uF capacitor needs to charge through the speaker. There's a way round it if it bothers you.

[MichaelKavanagh]

Yeah, if the "thump" could be avoided that would be great. And I think I get what you mean, is the attached schematic correct?
Cheers

[lewis]

Perfect! See how you get on with the thump, it might not be that bad.

[MichaelKavanagh]

Okay, cheers, I'll let you know how I get on

[MichaelKavanagh]

Update!
Breadboarded it and bad news I have double checked and triple checked my breadboarding, and am sure it is as per the diagram.
When I apply power without the inputs connected there is a pop, as predicted. http://youtu.be/36a-JNcXPuU
Now, when I connect the sound output from the decoder (but not actually producing any sound yet) and then the power the speaker makes a horrible buzzing noise. Then it sort of just faded away until it was almost unnoticeable. http://youtu.be/_nDE_FE1eUk
Now, when I connect the inputs and turn the sound on, there is something that sounds a little like what its meant to, but its so distorted. This is the sound it makes: http://youtu.be/7KqO2jCMPkg
as opposed to the "raw" output from the decoder which is this: http://youtu.be/rPsp1CLVEoU
I am using the same speaker in each of the videos. Audio taken with my iPhone so perhaps not the best but it should give you a fair representation. The amplifier circuit at the moment is being powered by 12V, and ground is common with everything else
Any help would once again be greatly appreciated.
Thanks
Michael

[lewis]

Update!
Breadboarded it and bad news I have double checked and triple checked my breadboarding, and am sure it is as per the diagram.
When I apply power without the inputs connected there is a pop, as predicted. http://youtu.be/36a-JNcXPuU
Now, when I connect the sound output from the decoder (but not actually producing any sound yet) and then the power the speaker makes a horrible buzzing noise. Then it sort of just faded away until it was almost unnoticeable. http://youtu.be/_nDE_FE1eUk
Now, when I connect the inputs and turn the sound on, there is something that sounds a little like what its meant to, but its so distorted. This is the sound it makes: http://youtu.be/7KqO2jCMPkg
as opposed to the "raw" output from the decoder which is this: http://youtu.be/rPsp1CLVEoU
I am using the same speaker in each of the videos. Audio taken with my iPhone so perhaps not the best but it should give you a fair representation. The amplifier circuit at the moment is being powered by 12V, and ground is common with everything else
Any help would once again be greatly appreciated.
Thanks
Michael

Oh no! Not to worry, sometimes things don't work first time. Try providing the circuit with a known clean audio signal such as the output from a CD player or PC (don't use the iphone, it might have a class D amplifier on the output too). Better still, if you have a signal generator, use that. Basically we want to try to feed the circuit with a clean (no class D nasties) signal to eliminate possible problems and noise caused by the class D amplifier.

It's also possible the TDA2030 amplifier is clipping. Running from a 12V supply means the amp can only throw about 2-3V into the speaker which is not a lot. Try turning the volume pot down to see if the sound quality improves.

Also, do you have an oscilloscope? One would be very handy right now...

I was making so many assumptions about your setup which may not be accurate. We need to troubleshoot possible problems one at a time to track down the cause.

[MichaelKavanagh]

Hmm, I have a CD player with headphone output, but when I connect that (shielding + one of the signal wires) it just makes some constant unchanging low-ish freq buzzing.
Upped the voltage supply to 24V, still the same behaviour but louder.
I have an oscilloscope that was lent to me, how useful it is I don't know, the owner doesn't seem to care for his stuff much. Its an analogue scope, but better than nothing.

[IanB]

An analog scope should be fine for some basic testing. Do you have a model number or any specifications for it?

The idea would be to feed a clean tone (like a sinewave) into the front of the circuit and then trace the signal through each stage looking for any obvious problems like major distortion or clipping.

Possibly you can find and download some WAV files with pure tones in them, or even find some utilities that can generate those for you. Burn these to a CD and play them to get your test signal. I have not done any experimenting in this area but I speculate that if you look you will find such things.

[lewis]

I'll order a TDA2030 and breadboard the circuit myself, give me a couple of days...

[MichaelKavanagh]

An analog scope should be fine for some basic testing. Do you have a model number or any specifications for it?

The idea would be to feed a clean tone (like a sinewave) into the front of the circuit and then trace the signal through each stage looking for any obvious problems like major distortion or clipping.

Possibly you can find and download some WAV files with pure tones in them, or even find some utilities that can generate those for you. Burn these to a CD and play them to get your test signal. I have not done any experimenting in this area but I speculate that if you look you will find such things.

Its an "Advance Instruments OS3000"

I'll order a TDA2030 and breadboard the circuit myself, give me a couple of days...

Wow, that is very good of you! Actually I did order a TDA2050, but I didnt think that should make much of a difference after skimming the datasheet...

[IanB]

Its an "Advance Instruments OS3000"

Wow, does it really weigh 12 kilos?

[MichaelKavanagh]

Wow, does it really weigh 12 kilos?

Yep, is that abnormal for an analogue scope? The friend who lent it to me has a storage scope that you'd need a van and 2 strong men to transport around 

[lewis]

Forgot to mention - if using a CD player or some other signal source that's isolated from the circuit's ground, you will need a 2.2K (or thereabouts) resistor on each input down to ground. Also, the ground of this circuit needs to be connected to the DCC ground when you're using the DCC as the source.

TDA2050 should be fine.

Nice scope!

[MichaelKavanagh]

Hmm, any chance you could provide a diagram for that? What ground are you referring to? CD players output ground or amplifier circuit ground?
Yep, the grounds are connected when using DCC as the source.
Actually, unrelated but was just wondering, is it acceptable to have a 12V regulator, then a 5v regulator on the output of that? I read somewhere that this could damage it, but couldn't see why as long as you don't exceed current ratings.
Cheers.