Hi Geoff I currently have a 6000 which is fine in every respect but you are welcomed with a thud at standby 'on'. I cannot see how this can be reduced, due to the fact we are dealing with a single rail supply whose mid operating point (at the amplifier output, about +30v to ground) is isolated via those large capacitors.

Those capacitors on initial power up will, no matter what you do let current flow through to the speakers. Now unless I have missed something I cannot see how this initial 'surge' is controlled, and how your thud can be the result of a fault, especially if it is the same on all channels!

One thing that could result in a quieter thud is output capacitors that are faulty and as a result have a higher ESR...?

Maybe this has more to do with speaker sensitivity and / or impedance? Also when you had the boards in the service position are you sure it wasn't a loose wire that resulted in the silence, not the other way round!?

BTW your switch mod really is superb Laminated film can be made up with different numbers of layers for various thickness's.. Use a guillotine & scalpel to cut up and it takes very little time... Am going to try this on my Begrams I think!

If there was an award or a payment I would give it. What a great technical resource.

Hopefully you will be inclined to do another at some point.  This thread needs to be archived safely.

Graham and Eugene

Thanks for your positive response.  I too like reading this type of thread.  When the more experienced people contribute their wealth of knowledge, these documents can become invaluable sources of information and ideas.  Glad you enjoyed.

Regards

Geoff

Hello Olly

I definitely agree with your thoughts on this type of amplifier and have been wondering myself how the design can ever work without causing a “thud” on startup.  I have used the scope to have a look at the voltage changes and noted the following points. 

The DC voltage on the output capacitors builds up very slowly, taking up to 10 seconds to ramp up to 30V.  This in itself should not cause a “thud”.

During a one second period immediately after the relay activates, the voltage rises sharply and drops away.  This voltage peak is hard to pick up but could be in the range of 5 to 10V before it melts away into the normal DC ramp.

This machine immediately goes to “Standby ON” when the power is applied.  Hence the relay is activated while the 60V supply is being established.  I thought that the 60V might be causing the problem but then found that the same noise occurred when the unit has been in “Standby OFF” for a period of time.  During this time, the 10000uF cap is kept fully charged so it is unlikely to surge when the unit is being switched back to “Standby ON”.

As I said, I have now run out of ideas.  Some days it seems that the intensity of the thud is becoming less but I think that is an illusion caused by gradually becoming accustomed to it.

Glad to hear that you are going to try some shims on your beograms – I hope they work as well as these ones have done.  A very simple solution isn’t it.

Regards

Geoff

Hi Geoff, how are things doing with your 6000? I designed and built the circuit I mentioned works very well and fits neatly between capacitors and pre-amp board, I will pm with details if you want. I also tested the switch mod on a 4004 with lovely results, very happy!

Anyway, give us an update :)

Hello Olly

Sorry for the late reply but I have been out of the loop for a while, overseas on business actually.

Thanks for sharing your video showing the delay circuit on the BM6000 speakers.  The “thud” that I heard was very similar to mine and I am sure that you will have fixed that issue.  I found the video hard to work out the circuit and would appreciate it if you could post a simple circuit diagram with a list of the components.  That would be a great help.

Unfortunately my machine has been packed away for the time being because I didn’t manage to get much further.  As you will remember, I got to the stage where everything was working and I only had the “thud” to sort out.  I tried hard to work out where the fault was located but to no avail and every time I switched it on, the noise seemed to get more aggressive.  Finally, there was another bang (without the olufsen!!!) and I lost the left rear amplifier yet again.  As this was the third set of ICs that I had managed to burn out, I wasn’t in the mood to buy some more and blow them up as well.  That Beomaster has now been sent to solitary confinement and I don’t really know what I am going to do with it.  I suppose that the big issue is that there is probably something wrong with the amplifier PCB but it is so damn difficult to work on.  Every time you pull it out to gain access to the components, you put more stress on the connections and there is probably an intermittent contact somewhere in the circuit that is playing up.  One day when I am totally bored, very rich and have an endless supply of MJ2501 and 3001s ……… I might start again.

Thanks for all of your help and I was very pleased to learn that you had some good success with those shims.  My ones worked brilliantly on the 6000 – for the little time that I had to play with it.

Regards

Geoff

Ah sorry to hear that! I guess if things gradually got worse resulting in a bang (without the olufsen hehe) then there is obviously something else a-miss... The thud in the video sounds worse than it is because the mic would have been on max gain, and the mechanical sound of plugging in produced more noise than I realised!

Do you like the led mod BTW?

I will send the circuit details to you I will have to scan them in to the puter not enough time at the moment!