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  #1  
Old 10th February 2006, 01:22 PM
Rocky Rocky is offline
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Default Kel84 Hum

Hi all, as you can see I am a newbie to this forum and I am also a newbie to valve amplifiers although I have been into Hi-Fi for some 35 years.
I have just finished building a Kel84 amp that I bought some two years ago and would like to ask you experts some questions.
The mains transformer on the amp is humming quite loudly and I would like some advice on how to maybe eliminate/reduce this. There are two plastic spacers on the top and bottom of the transformer which should isolate it from the chassis but it still hums.
Secondly, I am driving Quad 10L speakers which have an sensitivity of 84dbs and 8 ohm impedance. This is not the optimum I know but the room is not too large and I get a reasonably loud sound. I am however a little worried that I may drive the amp into clipping and damage my speakers. What is the audible effect of clipping so that I may recognise it when it happens?
Thirdly, to be a pain, another question. I have a slight hearing problem in that my right ear is not as good as my left and on other amps use the balance control to adjust the output. There is no balance control on the Kel84 and was wondering if I could add a resistor somewhere in the left channel to give the same effect.
Thanks in advance for any advice forthcoming.

Allen
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  #2  
Old 10th February 2006, 03:11 PM
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Rob042 Rob042 is offline
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Default Re: Kel84 Hum

Allen
welcome to this 'mad' world.

transformer noise - is there humming coming throught the speakers? if not and all the noise is comng ftom around the tx I would - 1. check the incoming voltage and the voltages leaving the tx, the transformer could be worked to hard. while at it I would double check all the test point voltages to ( I find this best done with the amp plugged into where it will live as the voltages to the workshop are on ave 5% higher than in the house ).If that fails to show any problems check the rectifiers and earth wiring. Once you have done all that and you still get noise ... hmm

2. clipping - you'll know when you hear it, distorsion , a nasty sharping of the sonics. you'll know.

3.I have tinitus in one ear but have found that adjusting the angles of the speakers to be more effective than adding a balance control.Try that first, just in case it helps. Then maybe a balance pot.

hope this helps
Rob
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Old 10th February 2006, 03:12 PM
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Rob042 Rob042 is offline
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Default Re: Kel84 Hum

Allen
welcome to this 'mad' world.

transformer noise - is there humming coming throught the speakers? if not and all the noise is comng ftom around the tx I would - 1. check the incoming voltage and the voltages leaving the tx, the transformer could be worked to hard. while at it I would double check all the test point voltages to ( I find this best done with the amp plugged into where it will live as the voltages to the workshop are on ave 5% higher than in the house ).If that fails to show any problems check the rectifiers and earth wiring. Once you have done all that and you still get noise ... hmm

2. clipping - you'll know when you hear it, distorsion , a nasty sharpening of the sonics. you'll know.

3.I have tinitus in one ear but have found that adjusting the angles of the speakers to be more effective than adding a balance control.Try that first, just in case it helps. Then maybe a balance pot.

hope this helps
Rob
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  #4  
Old 10th February 2006, 04:11 PM
Rocky Rocky is offline
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Default Re: Kel84 Hum

Hi Rob

Thanks for the welcome and response to the post.

There is no hum coming through the speakers and I did check all the test voltages when I had completed building the amp and they were well within tolerances. However, I will check them again and look in the area you have suggested.

I will also try adjusting the angle of my speakers to see if I can alleviate the balance problem.

Allen
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  #5  
Old 10th February 2006, 04:31 PM
John Caswell John Caswell is offline
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Default Re: Kel84 Hum

Hi Allen,
First the level problem, is the amplifier a plain amp or does it have input selection?

Regards

John aka Dr John
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  #6  
Old 10th February 2006, 05:47 PM
Rocky Rocky is offline
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Default Re: Kel84 Hum

Hi John

The amp has input selection.

Regards
Allen
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  #7  
Old 10th February 2006, 06:41 PM
Richard Richard is offline
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Default Re: Kel84 Hum

Hello and welcome Allen,

There's a balance control here, you just need a 250k linear 2 gang pot,

http://www.world-designs.co.uk/forum...read.php?t=148




Rich

Last edited by Richard; 11th February 2006 at 09:07 AM. Reason: add pic
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  #8  
Old 11th February 2006, 12:10 PM
Rocky Rocky is offline
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Default Re: Kel84 Hum

Hello Richard

Many thanks for pointing out the FAQ on a balance control and for the point to point diagram. I shall incorporate this into my amp.

Rob,

I have checked the voltages etc. around the area of the mains transformer and choke but cannot find any problems. I think it may be mechanically induced hum. I have now trawled the old WAD forum and found that one member had success in eliminating mechanical hum by sitting the transformer on rubber grommets. I will try this in the next few days and let you know if it cures the problem.

Overall, apart from the hum, I am quite impressed with this little amplifier. I bought the kit on a whim but doubted that it would sound much good therefore didn't rush to complete it. How wrong I was, it has just walked all over my Meridian 501/555 set up. I think I may now be a valve convert and looking to move upwards as my son is also impressed and offered to buy the Kel84 when it is sorted out.

Regards
Allen
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