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Old 16th Dec 2008, 12:16 pm   #9
Kat Manton
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Join Date: Jul 2005
Location: West Yorkshire, UK.
Posts: 1,700
Default Re: Musical Fidelity B1

Hi Mark,

I've been experimenting some more, I just couldn't leave it alone

I wasn't too happy with it running hot due to those rather high standing currents. It's supposed to be a class AB amp but I'd got it biased moderately into class A.

So I looked for another way to set it up.

Originally Posted by mhennessy View Post
As you've found, it's important that the offsets match each other so that the net DC offset is close to 0V. So I wonder if one should measure the voltages across R64 and R63 with respect to ground? This would make sense were it not for the extra transistors, and note that these are not included in the feedback! Yet their contribution to the standing current should have an effect on the THD, so I would guess that on the production line, these would have been lined up using a distortion analyser as well as a voltmeter.
That's what got me thinking. I'm now fairly sure that it must've been originally set up with a distortion analyser and can't really be done without one.

This is the latest setup procedure I've devised.

Start with all 'set bias' presets set for minimum bias.

With the front towards you, from left to right:
  1. Fully clockwise
  2. Fully counter-clockwise
  3. Fully clockwise
  4. Fully clockwise

(Yup, one of 'em works the opposite way to the other three, just to add to the fun...)

For one channel:
Feed in 1kHz sine wave, adjust volume for 1V across 10R load (100mW.)

Alternating between the two presets, slowly increase the bias while keeping the DC offset somewhere around 0V.

The volume control will need slight adjustment to maintain 1V output.

Watching the output on one trace and the distortion output of the analyser on the other trace, slowly but surely the crossover distortion can be seen to fall.

Eventually, it's possible to minimise the crossover distortion while still having some; in other words the amp isn't biased into class A.

There's a slight dip in distortion on the meter at this point. If the bias is increased beyond this point, the distortion rises slightly then falls off rapidly as the amp is now running in class A.

With a bit of careful tweaking it's possible to get this 'dip' to coincide with 0V DC offset.
Repeat for the other channel.

After setting it up using this method, I've measured the voltages across the four 0R47 resistors.

I now have what seems like a more reasonable 23mA total through the upper half and 30mA through the lower half, on both channels. The discrepancy will be due to R58.

Distortion is still within specification (actually 0.12% up to around 15W), it's running a lot cooler and doesn't sound any different.

I'm now of the opinion that it isn't possible to set up a B1 properly without a distortion analyser. It must be how they were originally set up; just tweaking the bias presets for some voltage measured somewhere doesn't really work.

This is unlike the more 'typical' class AB amplifiers I've worked on. You usually get two presets, one for DC offset and the other for bias. Setting up involves nothing more than adjusting the bias preset for some mV across a resistor and the DC offset preset for zero.

I think that's it... Let's see how long I can leave it alone for now...

Cheers, Kat
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