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Advice: Idle current for HK PM640 VXi and PM645 VXi

wkloppen

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Mar 9, 2021
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Hello gents,

I've read a lot about a much discussed topic > the idle current. I pretty much know how to adjust it warming up the lady first, minimizing the volume putting the speakers on with no signal applied.
What I find kinda strange is that all some of these experts talk about idle current and then mention milli volts. Doing the math is easy but still..
Both manuals "say" 40 mv +/- 2 mv for each channel. I also read manufactures tend to be a bit over conservative preventing noob users frying their end stages.

I don't wanna start a whole know discussion as I understand there are (duh) different opinions:). With 40mv across the double resistor (2 x 0,27 ohm.....yes I know the scheme says 0.22 but the scheme is wrong) would result in an idle current of 74 ma.

Any advice on what to apply on these oldies would be appreciated.

willem

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AnalogSteph

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Optimum bias levels, at least for 8 ohm loads, are generally accepted to be between 13 and 26 mV (Vt/2 and Vt). There's an old paper from the early '70s that attempts to compute this numerically, and Douglas Self has simulated these things quite extensively with results presented in his power amplifier book.

In practice, things are a bit more complex... first of all, distortion only tends to rise fairly slowly when going into overbiased territory, things are shifting a bit when we are looking at lower-impedance loads, and bias actually is not exactly fixed but rather fairly dynamic, since a power transistor die will be heating up much more quickly than typical thermal compensation can follow. A brief power peak could make nominally ideal bias go up significantly, until thermal compensation catches up and eventually pulls things down into underbiased territory before things start to level out again.

If you really want to know, you would need to conduct some measurements - SMPTE IMD tends to be telling, for example. This typically takes an audio interface with low distortion balanced inputs (mic level: perhaps a Focusrite Scarlett 2i2, line level: Tascam US-2x2HR), a suitable 4 or 8 ohm dummy load or two for the amplifier, a custom-made balanced attenuator with 4 resistors per channel to bring down full power speaker output level to something more palatable to the device in question, and possibly some more adaptation to reconcile the audio interface's generally balanced outputs with the amplifier inputs (if there isn't an extra pair of RCA outs or a separate DAC or soundcard available).
 
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