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Old 5th Dec 2022, 1:09 pm   #21
Jolly 7
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Default Re: Noise issue with DIY variable PSU and audio amplifier

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Looking at the pics in thread 17, the amplifier pcb appears to have a bridge rectifier on it, so why is there also what appears to be a bridge rectifier circuit on the pcb seen below the transformer in your first pic?

You only need one bridge rectifier for this build unless there is something else attached somewhere that is not showing in the pics. I would suggest the removal of the pcb below the transformer and the voltage regulator. Then take the transformer output to the bridge rectifier that is on the amplifier pcb.

Something else to check is if the amp chip is stereo or mono. If the amp chip is a stereo type then both inputs will need a signal and both outputs will need speakers to be attached. If there is only one input of a stereo amplifier circuit being given a signal then the other channel could be picking up interference that would produce noise in that channels output.

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There are two full wave bridge rectifiers because one was already built in to the Grundig radio amplifier board and another into the Panasonic AC rectifier board. I kept both as it was the easiest solution for me. The aim of my build is to have both the variable DC supply and amplifier simultaneously available, using one of the secondary transformer windings for the rectifier and one for the amplifier. I can also switch on or off either board depending upon what I'm testing on the bench and how I'm testing it.
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Old 5th Dec 2022, 2:19 pm   #22
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Default Re: Noise issue with DIY variable PSU and audio amplifier

I think you must have some circuit interaction using the unconventional power supply arrangements. Either provide isolated supplies and then just one common earth point or provide the appropriate dc rails from one source and one 0V.

If it still hums after you've turned the power off (I assume drawing power from from the reservoir capacitors) then we can rule out stray induced hum via the rather close proximity of the unearthed mains transformer.
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Old 5th Dec 2022, 4:01 pm   #23
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Default Re: Noise issue with DIY variable PSU and audio amplifier

Looking at the pics of the transformer it seems that you are powering the amp from one half of the secondary and the voltage reg is getting it's supply from both secondaries being in series. Is this correct? because if so you have an imbalance on the transformer secondaries. A transformer with seperate 12v and 6v secondaries would be a better bet.

A possible solution with what you have is to use one bridge rectifier across the 6-0-6 and add a second voltage reg of 9v to supply the amp. That should keep the transformer outputs balanced.

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Old 5th Dec 2022, 4:55 pm   #24
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Default Re: Noise issue with DIY variable PSU and audio amplifier

[QUOTE=Jolly 7;1518878]
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I think I'll try one of them instead to see if that gets rid of the noise problem. This is probably the easiest way for me. If it doesn't, it's back to the drawing board 😄
.. the amplifier sounds great when feeding it with an audio source not powered by the same transformer e.g. an android tablet or battery operated radio.
So that's a good start. It eliminates certain problems right away.

The problem is almost certainly to do with a) your grounding arrangements or b) something getting in the signal pathway when it shouldn't.

Effective grounding isn't as simple as which wires to connect to others.

Takes quite a lot of hands-on experience and examining successful builds of circuits to see how it's done. Same with diagnostic fault-finding.

It may be -- because you haven't demonstrated it otherwise yet -- that this is quite new terrain for you...

Afterthought. There's a really good reason why there aren't commercially available combined variable voltage power supplies in the same package as an audio amplifier. It's a really bad idea! . If they did exist, they wouldn't be sharing the same transformer and they'd be completely isolated power supplies and there's be a lot of physical space and shielding between them.

But nothing ventured, nothing gained...

So, if you're open to some redesign ideas, which may include increasing the footprint of your assembly, it may be AOK in the end.
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Old 5th Dec 2022, 6:58 pm   #25
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Default Re: Noise issue with DIY variable PSU and audio amplifier

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Looking at the pics of the transformer it seems that you are powering the amp from one half of the secondary and the voltage reg is getting it's supply from both secondaries being in series. Is this correct?
This is important. I too can see two wires at the end of one secy, one from the centre-tap and one from the other end. If the two rectifiers are fed with different voltages, then on alternate half-cycles there will be a voltage between the DC negative of the PSU and the DC negative of the amplifier. Connecting a source that has its audio output common with DC negative to the PSU and to the amplifier, will then short-circuit this AC voltage via the signal ground and cause disastrous effects. Put a voltmeter between the PSU negative output and the amplifier signal ground - do you see anything AC or DC?

Even two rectifiers fed in parallel from the same AC winding, with interconnections on their DC sides, can cause circulating hum currents due to the difference between their forward drops. I would never attempt to use the same secondary for both applications. It could be made to work, but probably cause much more trouble than it is worth, compared with using separate transformers.
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Old 5th Dec 2022, 11:54 pm   #26
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Default Re: Noise issue with DIY variable PSU and audio amplifier

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Quote:
Looking at the pics of the transformer it seems that you are powering the amp from one half of the secondary and the voltage reg is getting it's supply from both secondaries being in series. Is this correct?
This is important. I too can see two wires at the end of one secy, one from the centre-tap and one from the other end. If the two rectifiers are fed with different voltages, then on alternate half-cycles there will be a voltage between the DC negative of the PSU and the DC negative of the amplifier. Connecting a source that has its audio output common with DC negative to the PSU and to the amplifier, will then short-circuit this AC voltage via the signal ground and cause disastrous effects. Put a voltmeter between the PSU negative output and the amplifier signal ground - do you see anything AC or DC?

Even two rectifiers fed in parallel from the same AC winding, with interconnections on their DC sides, can cause circulating hum currents due to the difference between their forward drops. I would never attempt to use the same secondary for both applications. It could be made to work, but probably cause much more trouble than it is worth, compared with using separate transformers.

Thanks for enlightening me on this. I am reading both AC and DC voltages when connecting my multimeter between the PSU negative output and the amplifier signal ground. The AC reading is 3.9 volts and the DC 5.46 volts. Does this indicate a fault that shouldn't be there at all ?
I am no engineer, but if this is a fault, short of changing the transformer, could an extra diode or two be used anywhere along this circuit to cure this ?
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Old 6th Dec 2022, 12:32 am   #27
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Default Re: Noise issue with DIY variable PSU and audio amplifier

If you are full-wave rectifying two different voltages from the same secondary, then the two DC circuits can never be interconnected as the voltage between them is unavoidable. The circuits must either run from the same rectifier or have independent secondaries isolated from one another.

To get multiple voltages from the same secondary that can share a DC common terminal, one must either use full-wave rectification with a centre-tapped winding (which is also the DC common), or resort to just half-wave which has its own problems.

Even so, I wouldn't make a monitor amplifier that shared a power source with anything else. There's too much scope for interaction which reduces its usefulness for troubleshooting. You want your testing tools to be beyond reproach so that any abnormality must be in the device under test.
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Old 6th Dec 2022, 6:10 am   #28
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Default Re: Noise issue with DIY variable PSU and audio amplifier

Thanks Lucien, I said this in post 3 or 4.

[QUOTE=joebog1;1518539]Sounds to me like its coupling back through your earth and negative of the power supply. I am assuming of course that they are common to the single transfomer and rectifier, audio amp, and power supply.

You could fix that by using a transfomer on the input of the audio amp. It will need to be primary and secondary NOT an auto transfomer.


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Old 6th Dec 2022, 10:50 am   #29
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Default Re: Noise issue with DIY variable PSU and audio amplifier

You did indeed, but I was expanding the point that it's not just noise or a ground loop, but an actual pulsating DC voltage that is present between the two different DC negative rails, if one rectifier is connected to half the secondary and the other to the whole (which hasn't been explicitly confirmed.)
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Old 6th Dec 2022, 9:26 pm   #30
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Default Re: Noise issue with DIY variable PSU and audio amplifier

[QUOTE=joebog1;1519095]Thanks Lucien, I said this in post 3 or 4.

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Originally Posted by joebog1 View Post
Sounds to me like its coupling back through your earth and negative of the power supply. I am assuming of course that they are common to the single transfomer and rectifier, audio amp, and power supply.

You could fix that by using a transfomer on the input of the audio amp. It will need to be primary and secondary NOT an auto transfomer.


Cheers

Joe
Hi Joe..I tried an old audio input transformer, similar to an LT700, with isolated primary and secondary windings, but that did not get rid of the issue. I fed the audio signal to the primary and connected the secondary to the amplifier's signal input and ground.
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Old 6th Dec 2022, 10:03 pm   #31
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Default Re: Noise issue with DIY variable PSU and audio amplifier

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I think I'll try one of them instead to see if that gets rid of the noise problem. This is probably the easiest way for me. If it doesn't, it's back to the drawing board 😄
Jolly , itís so unlikely to be a fault with a board.

Itís likely to do with layout and grounding. You will probably just replicate the same outcome.

As suggested earlier, please will you annotate your photos so we can see which wires are carrying a signal and which are connecting the boardsí power supplies snd how. It looks to me like you have one board with a bridge rectifier one the left , an amp to picture right and a linear voltage regulator on a small board behind it , with a heatsink ? Is that right ?

Correct. Three boards in total. The amp board has its own built-in bridge rectifier too.

And you identified the yellow wire clamp as being on one of the signal leads , which is connected to the wire from the ferrite suppressor with the brown lead? Is that right?
Correct.

Here's a photo with some annotations which might help explain. I have now got rid of the yellow and turquoise wires to the audio amp board and am trying to power it from the same AC source I have used for the voltage regulator circuit. It shouldn't rob too much additional current. I think the transformer is rated for 3 watts, so there should be enough power for both circuits connected in parallel, I reckon.
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Old 7th Dec 2022, 10:21 am   #32
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Default Re: Noise issue with DIY variable PSU and audio amplifier

Ok, give that a try as part of the experience , no harm done ó and interesting to see what you find. Itís only a few minsí work.

But realistically, what Lucien says in #25 (last paragraph) and patiently in #27 (middle paragraph) needs to be understood and remedy made accordingly if you want to fix this.
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Old 7th Dec 2022, 11:13 am   #33
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Default Re: Noise issue with DIY variable PSU and audio amplifier

Also, take a look at Daveís practical solution in post #23. Simple to rig and reliable.
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Old 7th Dec 2022, 2:35 pm   #34
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Default Re: Noise issue with DIY variable PSU and audio amplifier

Looking at the pic in post 31, I'm a little confused re the quoted voltages. The transformer is labelled as 2 secondaries at 6-0 & 6-0 (6 volts ac each winding), so how can you have 12vac across one of the secondary windings and 21vac across the two in series?

Either the transformer is wrongly labelled or you have a faulty ac voltmeter. The output voltages on the transformer secondaries might get as high as 9vac each off load but will definately only give 6vac when on load at the amperage rating of the transformer. If the transformer is actually producing 12vac per winding then the regulation rating of the this transformer is rather poor or it's wrongly labelled, in my opinion.

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Old 7th Dec 2022, 5:46 pm   #35
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Default Re: Noise issue with DIY variable PSU and audio amplifier

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Looking at the pic in post 31, I'm a little confused re the quoted voltages.
So am I. I made a quick sketch, colour matching best as I can. It's oriented same way as Jolly's Xformer in the photo.

Jolly, what you've got here is a centre-tapped 12V winding. You've made the centre tap with the pink wire.

So...logical explanation is that you've simply transposed the numbers to get 21V AC instead of 12V AC. If not, what Dave says...

You can see from my sketch that there is 12V across the green wires. Two windings in series with a centre-tap.

There is 6 VAC across the turquoise and yellow wires, not the 12 V you say for your amp. That's only available across the green wires.

These things need to be really clear before you can do any diagnostics.

On top of that arises another problem. You are now drawing uneven amounts of current from the RHS and LHS windings of the Xformer.

The 12V circuit across both windings causes draws one amount of current to flow through both windings.

The amplifier circuit across the RHS winding draws an additional amount of current to flow through that winding alone. Boom! Tiny transformer here, more load on one side, so there now just can't be exactly the same voltage output across each winding. It is some amount lower on the RHS.

And so now what happens to the (nominally) 12V output across the two secondaries in series? Suddenly things are a bit more complex than meets the eye.

Please note that the green wire 1 and green wire 2 aren't 'carrying [X] volts' as you label them. The voltage is across the pair of wires.
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Old 7th Dec 2022, 6:51 pm   #36
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Default Re: Noise issue with DIY variable PSU and audio amplifier

Apologies for the errors. The 21V I annotated is actually the rectified DC voltage. Before rectification it is 15.9V AC across both green wires. That is how I am able to get up to 18 volts DC from the LM317T regulator.

Please see a photo of the now altered transformer secondary wiring. I tried to run both circuits in parallel but still got amplifier noise, although somewhat less. I am building another amplifier circuit as I have all the components and want to give it a go to see if it makes a difference. Will post updates here.
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Old 7th Dec 2022, 10:07 pm   #37
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Default Re: Noise issue with DIY variable PSU and audio amplifier

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Before rectification ...it is 15.9V AC across both green wires...That is how I am able to get up to 18 volts DC from the LM317T regulator.
Ahah but that's an illusion... please read on. It might look that way, but typically you might have about 24V in to get regulated 18V out.
You can't get 18V out of 16V with a voltage regulator.

Say you had the classic voltage divider formed of two resistors. They are in some fixed ratio.

Let input voltage V=24. No regulator. Say, just for example, R1 = 3R, and R2 = 9R (with whatever R units multiplier) and ta da! you'd get your 18V out.

So long as they're in that ratio, that's what you'll get.

Your LD 317 has a slightly different formula, Vout=1.25 * (1+R1/R2) + an amount that is negligible and can be disregarded.

But it needs an input voltage by some margin higher than the output voltage, all the same. Otherwise it isn't regulating at all.

So how are you getting your 18V out? Well, if you haven't already tried this, hook up your transformer (nominal 16V output) to a bridge rectifier and the smoothing capacitor that you'll be using and see what DC voltage you read.

The power supply is the building block of all such circuits. When you're trying to make a stable, predictable test instrument as you are, that becomes pretty obvious.

I wouldn't go down the rabbit hole of getting another amp to compare until you know what voltages are involved here and how regulation is happening, or not.

Then you can unpick the noise a bit more, if you're prepared to do the basics first
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Old 8th Dec 2022, 7:52 am   #38
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Default Re: Noise issue with DIY variable PSU and audio amplifier

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

However thatís an off-load figure with your 12V xformer outputting 16VAC.

I think youíll get about 15.5VDC after the bridge and smoothing capacitor under load, which could give you regulated max 12V DC output.
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Old 8th Dec 2022, 9:02 am   #39
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Default Re: Noise issue with DIY variable PSU and audio amplifier

The thing to look for on a regulator is "headroom" or voltage dropped across the device; some "low headroom" types are better than others. There is then the "regulation factor", which is due to the current being drawn by you circuit.
These all add up and cause the regulator to drop out of regulation and cause hum.
Unless you are absolutely certain of you calculation and circuitry you need at least 5V more from the RMS output (not peak rectified) of the transformer than the regulated output voltage you require.
This may mean more heating of the reg, so be sure it is on a heatsink. Experience will eventually show what you can get away with in terms of headroom.

The final problem will be from mains regulation. If your local main voltage varies (which it will) then the output of the transformer will also vary

Have fun sorting that lot out, but it will eventually work

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Old 8th Dec 2022, 3:06 pm   #40
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Default Re: Noise issue with DIY variable PSU and audio amplifier

I've attached my thoughts on a circuit for you that might do the job. I say might because it is load sensitive. Also before you proceed much further, I suggest that you test the Grundig audio amp on an external 9vdc supply to check that it is working normally. My reason for this is down to the voltage abuse that it has probably been subject to.

According to the datasheet for the UPC1212C, this audio output chip has a maximum voltage rating of just over 11vdc, the recommended working voltage is 9vdc. Your measured voltages have exceeded the highest maximum for the chip, hence why you need to check it now before proceeding any further.

If the audio chip has survived OK then then I suggest you re-configure to something similar to my diagram. The capacitor values are not super critical, I just went with some typical values. Make sure that the voltage rating on the capacitors is higher than the expected circuit voltage. For 21v across the 4,700uf you could use a capacitor rated at 25v but I would choose a 35v to give a safety margin that will allow for any possible surges. The 2,200uf should be rated at 25v for the same reason, the 470uf can be a 16v. The 22 ohm resistor is there to drop some voltage to the 7809 regulator but may need altering higher or lower according to how many amps the audio stage is taking.

The buzzing interference that you are most likely getting will be down to 3 possible reasons. One of which has been discussed in relation to the imbalance on the transformer secondaries. The second probable reason is due to the close proximity of the audio amp to the transformer. This interference is down to the magnetic field of the transformer being induced into the audio amp. The third possibility for interference is the audio input to the amplifier. The wiring from the phono input socket should be screened (coaxial), also the wiring to the volume control should likewise be screened. Bear in mind that the phono socket (unless you have a special type) does not short the signal path to 0v when there is nothing plugged in. This means that the audio amp will still try to amplify anything that the wiring picks up.

As for a layout, I would leave the transformer where it is but move the mains input socket to be at the side of it. Then I would fit the rectifier and regulator circuits about where the pcb with the bridge and fuse already is. I would mount the audio pcb at the opposite end of the box to where the transformer is. This keeps the audio amp as far away from the transformer as is possible and will limit the amount of induced interference. Keep the audio input socket and volume control as far away from the transformer as practicable given the size of the enclosure.

I realise that all this means a total rebuild but it should be worth it in the end. You can always rats nest it it to start with just to prove the point. The audio amp could be mounted on end as it were to the opposite end of the enclosure.

Voltage regulators often require 3v higher input to the output to work correctly, so a 9v type will require 12v input. You can increase a bit more on the inputs but also bear in mind that if the input is excessive either the voltage regulator will run very hot, as it has to dissipate the difference of voltage, or the voltage reg will be destroyed due to overvoltage. Fortunately the specifications for voltage regulators are freely available, so you can make an informed decision.

Hope this all makes sense.

Dave
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