Noise issue with DIY variable PSU and audio amplifier

[Jolly 7]

Originally Posted by Al (astral highway)

Oops. My bad- you mention 21VDC, Jolly. These various voltages began to confuse me.

However that’s an off-load figure with your 12V xformer outputting 16VAC.

I think you’ll get about 15.5VDC after the bridge and smoothing capacitor under load, which could give you regulated max 12V DC output.

No worries.

Actually I do read 18V DC on my multimeter and it comfortably powers up my Hacker Sovereign II too. So, as it serves my purpose, I am happy the way it is.

You were absolutely right saying that replacing the amplifier circuit wouldn't make any difference. I made a TDA2822M based circuit, powered from AC with a 21V to 5V stepdown converter, and took out the old board, but the noise persisisted. I'll now be looking at modifying the circuit as per Dave's diagram in post #40.

[Jolly 7]

[QUOTE=G6ONEDave;1519769]I've attached my thoughts on a circuit for you that might do the job.

Thanks for the detailed analysis and the circuit diagram. I think my UPC1212C IC has survived my experiments so far as the amplifier circuit still works normally when powered from another external source.

I will try to modify my circuit based on your diagram or else build a completely new one.

[G6ONEDave]

I hope that the circuit works as expected for you. Do check how much head room you need for the 7809 to work correctly. If I remember correctly the low drop out varieties don't need as much head room as the bog standard types.

I've not checked the data sheet for the UPC1212C to see how much current it draws, possibly 250mA or less but once you know the normal operational current you can work out the dropper resistor value using ohms law.

Dave

[Al (astral highway)]

Please note from mfr data sheet, "....input voltage must remain typically 2.0V above output voltage even during the low point on the input
ripple voltage".

To protect against this happening (likely at switch-off), install reverse diode connected cathode to pin 1, anode to pin 3 of VR.

(Thanks to Ed Dinning for pointing this out to me recently. You'll see it on commercial designs using these voltage regulators.)

[Jolly 7]

My problems with audio distortion and extraneous noise while feeding the amplifier are now solved. Here's what I did:

1. Installed my own amplifier board based on a TDA2822M chip. There was nothing wrong with the Grundig board, but my DIY board is smaller and was easier to customise

2. Instead of tapping of the power transformer AC secondary, I took two parallel taps off the rectified 21V DC, one for the regulator circuit and another one for a Mini 360 buck converter. I then set the output voltage of the converter to 5V DC to feed my amplifier circuit. This outputs audio that's loud enough from a 4 or 8 ohm speaker.

3. Some enthusiasts elsewhere have reported success by using mains stepdown transformers as audio input transformers. I used an old 6-0-6 volt mains transformer from a Reader's Digest tape recorder, with the primary connected to my external audio source via a 3.5mm mono jack and the secondary to the amplifier input. The audio is now crystal clear and there is no attenuation of audible frequencies.

There is still some hum which may be from the close proximity of my new 'audio transformer ' to the mains transformer, but this can easily be made inaudible by increasing the volume of the audio input signal.

Here are some pictures.

[Jolly 7]

Here's a close-up view of the mini-360 stepdown voltage converter attached to the old Panasonic bridge rectifier board.