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Old 8th March 2015, 01:30 PM   #1
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Default Heath Williamson type W-3M

I purchased a pair of these several years ago, the capacitors had been replaced so I felt no pang of guilt in probing around the innards. Found that many of the carbon comp resistors were more than 20% off spec, so replaced with Panasonic 3 waters, or Vishay RN60.

The feedback is different from the Heath manual, will describe when I trace it out.
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Old 8th March 2015, 01:44 PM   #2
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About 0.12% distortion, not bad. A little tweaking and you could null out that predominant second.
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Old 8th March 2015, 02:18 PM   #3
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At what power level the FFT was measured ?
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Old 8th March 2015, 02:44 PM   #4
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W-3M
The W-5M puts an extra series R-C damper (Zobel network) across the secondary.
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Last edited by smoking-amp; 8th March 2015 at 02:50 PM.
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Old 8th March 2015, 09:23 PM   #5
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Does the PT CT go to cap neg or to star gnd? The PT CT really should be fused, if you've jumped the moral hurdle of changing out resistors.
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Old 9th March 2015, 12:52 PM   #6
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Quote:
Originally Posted by artosalo View Post
At what power level the FFT was measured ?
5 watts into 8 ohm.

The feedback network is 12k||100pF rather than 6.8k||33pF. This unit also has the 47k load resistor on V1 paralleled with 1k+510pF. I've seen this in other Williamsons, so Heath may have had stability issues with the early models.

I replaced the carbon comp 27k resistors for V2 with 27k Panasonics matched to 0.1% The originals were a little worse than 1%

Each 5881 is running @60mA.

The output of the amplifier was also attached to the 16R tap, but was driving 8R. This is perhaps the most significant change.
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Last edited by jackinnj; 9th March 2015 at 12:58 PM.
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Old 10th March 2015, 12:51 PM   #7
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Little more tweaking -- changed the DC balance a bit and the 2nd is down to -79dBRa. Also included the schematic with the parts which differ from the original W3:
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Old 10th March 2015, 04:54 PM   #8
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OK, I'm done with this one:
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Old 10th March 2015, 08:30 PM   #9
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Was post #7 also at 5W ?

The 3rd harmonic is almost below the mains harmonic modulation of the signal. I guess a contributor to the rise in THD with output power would be a rise in the mains harmonic modulation level as the power supply gets worked harder.
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Old 10th March 2015, 09:48 PM   #10
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Quote:
Originally Posted by trobbins View Post
Was post #7 also at 5W ?

The 3rd harmonic is almost below the mains harmonic modulation of the signal. I guess a contributor to the rise in THD with output power would be a rise in the mains harmonic modulation level as the power supply gets worked harder.
Yes -- that's the point at which the Heath W-3M manual specifies THD%

As you see, I ran it up to 20W.

I could do a low frequency FFT at different power levels, 1Hz to 300 Hz
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