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Old 9th May 2017, 11:07 pm   #1
indigo.girl
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Default How to cure a noisy Bardwell amplifier?

Hi everyone,
I have one of these old Bardwell stereo amplifer circuits described in this thread : https://www.vintage-radio.net/forum/...d.php?t=113530

I have put it inside a wooden case and connected up a 4" speaker and a bluetooth module to provide the input signal. (I'm only using the LH channel btw as I can only fit one speaker in the box I'm using).

I want to put the variable pots on the front of the box so I have unsoldered from the PCB and reconnected them with some shielded cable.

However, despite using the shielded cable I am still getting a fair bit of volume-dependant hiss which is made worse when I touch the cables or speaker etc.

Is there anything more I can do to protect the signal from picking up unwanted noise? Its this an earthing issue? (Its not the fault of the bluetooth module as it happens even if I just inject a wired signal into the circuit input points).
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Old 10th May 2017, 5:46 am   #2
Diabolical Artificer
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Default Re: How to cure a noisy Bardwell amplifier?

I noticed a couple of things. Firstly, please lose the insulation tape, it looks amateurish, sorry if that sounds harsh.I used to use it when I started out doing electronics but after finding out about heat shrink tubing I've not used it since, only for temporary insulation. Heatshrink is far better and looks neater, and does a better job of insulation.

Second thing I noticed is the blue wire going to the pot, it splits into three, why? Your better to return all earths to one point and minimise ground wiring. It helps to draw a quick sketch/schematic so you can really see what is going where.

There's a trick you can try by using a metal plate earthed and move it around over an input stage say to see if that improves things. That and the previous suggestion is more about reducing hum though. It sounds from the hiss you describe that the input has too much gain and would need reducing.

Andy.
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Old 10th May 2017, 9:33 am   #3
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Default Re: How to cure a noisy Bardwell amplifier?

Hello , your amplifier is in a wooden box and the pots are attached to it, also. Normally both would be inside a grounded metal case and the metal cases of the potentiometers would thus be grounded. You amplifier doesn't 'know' which are good signals to amplify from erroneous ones. Domestic environments are 'noisy'. See for example what happens if you put an input wire close to virtually any large object.

Put the amplifier in a grounded metal enclosure with the pots' cases grounded too and see how you get on.
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Old 10th May 2017, 9:40 am   #4
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Default Re: How to cure a noisy Bardwell amplifier?

I once had similar problems with a Sinclair Project 60, even though it was in a metal case. The hum pickup was, as might be expected, worse on the magnetic cartiridge i/p, despite the use of screened cables, even for the wiring to/from the on/off switch (part of the function selector switch bank) The only cure was not to route the mains wiring anywhere near the function switches, by fitting a seperate on/off switch about 6" away. A case of bad layout design from the start, IMHO. The amp was built according to the instructions in the Project 60 instruction book, BTW!
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Old 10th May 2017, 10:36 am   #5
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Default Re: How to cure a noisy Bardwell amplifier?

There are a couple of transistors in there as well as the chips. One or more of those may have gone noisy. They are probably NPN general purpose types, so you could replace them with a BC549 (ideally), any BC54x type, a 2N3904 or any other reasonably low noise type you have to hand. Check the leadout arrangements of any replacements.

I would not expect a wooden case, mains cable routing, use of vinyl tape or any of the other earlier suggestions to cause hiss, but you never know and it is good advice in other respects.
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Old 10th May 2017, 1:52 pm   #6
indigo.girl
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Default Re: How to cure a noisy Bardwell amplifier?

Thanks for all the suggestions - I will work through them and see what helps. Is the idea of the inner metal box to shield the innards using the same principle of the shielded wire (I think I read its called a Faraday cage). Or is it acting like a large sink for charge like my body does when I touch the metal of the pots or speaker to reduce the noise?
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Old 10th May 2017, 2:00 pm   #7
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Default Re: How to cure a noisy Bardwell amplifier?

I also seem to be getting some clicking noises (about 300 clicks per minute)
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Old 10th May 2017, 5:56 pm   #8
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Default Re: How to cure a noisy Bardwell amplifier?

One thing I found with mine is (apart from instability when you turn the volume right up) is that the volume control isn't right at the input of the amplifier. This means that even with the volume at minimum, if the input level is too great, distortion can result. All you need to do to avoid this is to make sure the output level from the bluetooth or whatever source you are driving it from isn't turned up too much. Make sure the power supply is big enough for the job and be careful with wiring layout, keeping leads as short as possible. Don't use thin wire for power and earthing, and keep input leads well away from speaker connections. Mine seemed to behave itself once put together properly, and is in a plastic box, so no screening there either.
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Old 10th May 2017, 6:28 pm   #9
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Default Re: How to cure a noisy Bardwell amplifier?

I'm using a 12V power supply for this. At high volume I get distortion as it can go very loud.

I don't have much room inside my wooden box. Could I glue tin foil to the inside surfaces of the box perhaps?
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Old 10th May 2017, 6:51 pm   #10
David G4EBT
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Default Re: How to cure a noisy Bardwell amplifier?

Amazing little amps, and 10 Watts per channel too.

Here's what they sound like without the hiss:

https://www.youtube.com/watch?v=5gZXleVOTIs

How good is that At a fiver or thereabouts, that's 25p a stereo Watt!

Good luck in sorting it.
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Old 10th May 2017, 7:11 pm   #11
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Default Re: How to cure a noisy Bardwell amplifier?

If they can deliver that amount of power I would guess that a 12 Volt power supply needs to be able to provide about 3 Amps to keep everything on track. If the power supply is underated then that could potentially cause instability/strange behavior and distortion, particularly at high volume peaks.
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Old 10th May 2017, 10:34 pm   #12
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Default Re: How to cure a noisy Bardwell amplifier?

If you're using a CCTV power supply (guessing by the screw plug adapter) that's probably a noisy switch-mode power supply and not intended for audio.
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Old 10th May 2017, 11:28 pm   #13
indigo.girl
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Default Re: How to cure a noisy Bardwell amplifier?

Quote:
Originally Posted by OscarFoxtrot View Post
If you're using a CCTV power supply (guessing by the screw plug adapter) that's probably a noisy switch-mode power supply and not intended for audio.
Thanks Alan and Oscar - regarding the power supply its a 12V / 2Amp AC/DC adapter. What kind do I need for audio?
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Old 10th May 2017, 11:37 pm   #14
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Default Re: How to cure a noisy Bardwell amplifier?

The problem is that switch mode PSUs like that one tend to generate electrical noise, though they don't all do it. Most people prefer to use a linear supply for audio - a conventional transformer based supply.

That said, SMPSU noise isn't a hiss, more of a buzzing noise. I still think transistor noise is the most likely explanation for your hiss. The Bardwell amps are a long way from hifi and some audible hiss at high volumes may be normal.

Try running the amp from a PP3 battery - if the hiss is still there, then it's not coming from the PSU.
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Old 11th May 2017, 11:28 am   #15
indigo.girl
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Default Re: How to cure a noisy Bardwell amplifier?

Quote:
Originally Posted by David G4EBT View Post
Amazing little amps, and 10 Watts per channel too.

Here's what they sound like without the hiss:

https://www.youtube.com/watch?v=5gZXleVOTIs

How good is that At a fiver or thereabouts, that's 25p a stereo Watt!

Good luck in sorting it.
Thanks David - what is the bank of capacitors that has been attached to it?
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Old 11th May 2017, 11:55 am   #16
indigo.girl
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Default Re: How to cure a noisy Bardwell amplifier?

Quote:
Originally Posted by Biggles View Post
If they can deliver that amount of power I would guess that a 12 Volt power supply needs to be able to provide about 3 Amps to keep everything on track. If the power supply is underated then that could potentially cause instability/strange behavior and distortion, particularly at high volume peaks.
Hi Alan - I've been looking at the datasheet for TCA940 to find the current requirements. It quotes 3A max for 24V but not sure how this would reduce for 12V (I'm doubting these things are as simple as a linear correlation!). There is also a graph showing max working conditions. I've copied them and pasted below. Do these specs help tell us what max current I should go with and whether I do need 3A for my set up? Ta
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Old 11th May 2017, 12:08 pm   #17
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Default Re: How to cure a noisy Bardwell amplifier?

Quote:
Originally Posted by paulsherwin View Post
There are a couple of transistors in there as well as the chips. One or more of those may have gone noisy. They are probably NPN general purpose types, so you could replace them with a BC549 (ideally), any BC54x type, a 2N3904 or any other reasonably low noise type you have to hand. Check the leadout arrangements of any replacements.
Hi Paul - just to clarify - are the transistors in the circuit are inherently noisy or has one gone noisy over time. I could swap over the transistor from the unused channel if the latter. Its a BC 239.
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Old 11th May 2017, 12:18 pm   #18
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Default Re: How to cure a noisy Bardwell amplifier?

Here is a short video of the background noise and clicking. I have the volume turned up to the max so this is at its worst for the demo. Overall, at the volumes I'll use it at the noise is not really a big issue but I'd like to get it as good as possible and to learn the principles.

https://youtu.be/gINoKJ3p1JE

I also get audio distortion when I play it loud but I presume that the speaker is limiting here because the amplifier is so powerful at 5W. This is an old 4 ohm cone speaker I picked up as a spare that I doubt was made to handle 5W.
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Old 11th May 2017, 12:22 pm   #19
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Default Re: How to cure a noisy Bardwell amplifier?

The BC239 is a low noise transistor from the BC108 family. A modern BC549 is a direct equivalent. It won't have been noisy originally unless the amp is very badly designed. You can swap the two BC239s over, but there's a chance they've both gone bad (though they don't have a reputation for this).

Any general purpose NPN transistor should be a drop in replacement, but as I said earlier, check the leadouts - they aren't all the same.
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Old 11th May 2017, 12:42 pm   #20
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Default Re: How to cure a noisy Bardwell amplifier?

The noise in the video definitely isn't caused by faulty transistors. It sounds to me like a combination of mains hum, which I'd expect from such an amplifier with the input not connected to anything, and a rhythmic clicking which sounds like it might be coming from the switch mode power supply. Try a different power supply and see if the clicking goes away.

Replacing transistors is definitely not going to help.

Chris
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