BM8000 idle current 0.07mv?

Hi Guys,

I've just been reading beo bills page about the BM8000 and on it he says...

"Our final area is the output circuit of staged push-pull power transistor outputs. These are the warm "Transistorless Sound" Bang&Olufsen patented power outputs for left and right channels. The secret to these once everything else has been put right is to set for dead-on idle currents of 0.07millivolts and then balance the push-pull circuitry to absolute null or zero volts!"

This doesn't make sense to me... Can anyone explain? I've emailed him to ask for clarification but he never replied.

Lee

Hi Lee,

referring to the manual, the idle current has to be around 50 mA, which corresponds to 18 mV (0,018V) over the low-ohm-resistors (TP200 to TP201). So 0.07mV is fairly low. To measure the volts is much more convenient, but when you read the amperes in series, it's more accurate,  At the speaker output itself - the speaker switch has to be on, but no speaker connected- , the voltage should be zero. This is the position, where plus and minus supply voltage meet through the transistors and emitter resistors to cancel each other out. So this section is "business as usual"

Chris

Hi Chris,

Yes I would've thought that 0.07mV would be so low as to cause crossover distortion. Maybe beo bill measures somewhere other than across those resistors.

Lee

Hi Lee,

now I know what you are talking about, never knew this site. And yes, he doesn't say, where he measures it. And that he connects so many speakers to it... i have no words. 

Lee:

Hi Chris,

Yes I would've thought that 0.07mV would be so low as to cause crossover distortion.

this can be heard at lower volume, but it's not harmful.

Chris

ProGram:

Lee:

Hi Chris,

Yes I would've thought that 0.07mV would be so low as to cause crossover distortion.

this can be heard at lower volume, but it's not harmful.

Chris

Sorry Chris, but that's rubbish.
Most people won't notice crossover distortion at LOW volumes but I can assure you that it can easily be heard at higher volumes.
And I can also assure you, that's it's definitely harmful! Not to the amplifier (though it could theoretically run a bit warmer) but to the tweeters in your speakers, due to the large
content of 2nd, 3rd etc. harmonics (which typically are too high for us humans to hear but at a considerable level and it all gets
filtered to the tweeters).

Adjust according to the manual and you'll be fine.
I see no reason at all to set the idle current any different.

Martin

Just to clarify. I'm not going to be doing any adjustment to my system. It's been fully over-hauled by Olly including all new caps, trimmers and even the LEDs in the displays so I'm sure it's all spot on.

I just read that page and thought it didn't sound quite right... I knew for certain that the 0.07mV idle current wasn't the B&O recommended level and wondered if anyone would know where that figure was "plucked" from.

Dillen:

Sorry Chris, but that's rubbish.

Most people won't notice crossover distortion at LOW volumes but I can assure you that it can easily be heard at higher volumes.And I can also assure you, that's it's definitely harmful! Not to the amplifier (though it could theoretically run a bit warmer) but to the tweeters

Martin

Martin,

It's the amp, I was talking about. But I thought, the distortion is more at low volume, sorry for that. It is really long time ago since I heard such fault.

Chris

Dillen:

 

Adjust according to the manual and you'll be fine.
I see no reason at all to set the idle current any different.

Agree.

 

Each transistor has its "working space". Setting it up below or to high will affect this.

Setting it up with a low idlecurrent, the interaction between the positive and negative transistors will be affected and result as crossover distortion.

Setting it up to high will result in distortion at higher levels as the signals clipps against the powersupply. With damaged transistors or drivers as result.

The mentioned site sounds interesting for the novice, but accually referres to adjusting the idle-current and offset-voltage. But with wrong values.

Adjusting the offset to 0 mV (plus or minus 5 mV) is needed as it is a DC coupled design. It lacks an output capacitor (not needed and less distortion)