|
Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
|
Thread Tools |
9th May 2017, 11:07 pm | #1 |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
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). |
10th May 2017, 5:46 am | #2 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,670
|
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.
__________________
Curiosity hasn't killed this cat...so far. |
10th May 2017, 9:33 am | #3 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
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.
__________________
Al |
10th May 2017, 9:40 am | #4 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
|
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!
|
10th May 2017, 10:36 am | #5 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,970
|
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. |
10th May 2017, 1:52 pm | #6 |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
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?
|
10th May 2017, 2:00 pm | #7 |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
Re: How to cure a noisy Bardwell amplifier?
I also seem to be getting some clicking noises (about 300 clicks per minute)
|
10th May 2017, 5:56 pm | #8 |
Rest in Peace
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
|
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.
|
10th May 2017, 6:28 pm | #9 |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
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? |
10th May 2017, 6:51 pm | #10 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,768
|
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.
__________________
David. BVWS Member. G-QRP Club member 1339. |
10th May 2017, 7:11 pm | #11 |
Rest in Peace
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
|
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.
Alan. |
10th May 2017, 10:34 pm | #12 |
Heptode
Join Date: Apr 2011
Location: Edinburgh, UK.
Posts: 805
|
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.
|
10th May 2017, 11:28 pm | #13 |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
Re: How to cure a noisy Bardwell amplifier?
Thanks Alan and Oscar - regarding the power supply its a 12V / 2Amp AC/DC adapter. What kind do I need for audio?
|
10th May 2017, 11:37 pm | #14 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,970
|
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. |
11th May 2017, 11:28 am | #15 | |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
Re: How to cure a noisy Bardwell amplifier?
Quote:
|
|
11th May 2017, 11:55 am | #16 | |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
Re: How to cure a noisy Bardwell amplifier?
Quote:
|
|
11th May 2017, 12:08 pm | #17 | |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
Re: How to cure a noisy Bardwell amplifier?
Quote:
|
|
11th May 2017, 12:18 pm | #18 |
Retired Dormant Member
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
|
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. |
11th May 2017, 12:22 pm | #19 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,970
|
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. |
11th May 2017, 12:42 pm | #20 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,681
|
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
__________________
What's going on in the workshop? http://martin-jones.com/ |