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Re: Sweetness VS Load Lines

Posted by Naz (A) on August 7, 2006 at 21:34:56

In Reply to: Sweetness VS Load Lines posted by danlaudionut on August 6, 2006 at 15:38:44:

Hi Dan,

The load line is primarily affected by two things - the value of plate load resistance or impedance and the load that the next stage presents; normally the input impedance of the next stage but it could be the reflected impedance from the spkr to an OP tube. DC coupling is not in itself a factor.

In a simple gain stage such as grounded cathode the plate load resistor is normally the major factor and it is nigh impossible to get anywhere near a horizontal load line without resorting to extremely high B+ voltage, which allows the use of a much higher value plate resistor. That's the major benefit CCS loading and indeed the load line can be close to horizontal translating into lower measured distortion. Unloaded IT coupling provides similar benefits to CCS.

Where we differ is that to my ears this lower distortion is precisely what translates to a cleaner sweeter sound. One of my recent posts detailed some of my findings / measurements and observations in regard to distortion and bias, both in driver and OP stages.

In a nutshell most of the distortion is produced in the OP stage in an SE amp and this is primarily 2ndH with 3rd (and progressively higher order Dist) creeping in at high OP levels. Biasing the Op stage to run at higher current measures, and normally sounds, better. However contrary to what I once thought I found that biasing the driver to lower current (less linear operating point) actually sounded appreciably sweeter. Further, on measurement, it actually made sense. Being out of phase, the higher distortion in the driver stage (mainly 2ndH) better cancels with that produced by the OP stage for a lower overall OP distortion. Obviously this would not be the case if the driver distortion exceeds that of the OP stage (should never be the case anyway) and indeed I found that there was an optimal setting ... guess what, it happened to be close to the lowest overall distortion point!

I think that the idea of high levels of 2ndH distortion being the reason for the beauty of SE sound is a total myth which can be very easily proven. I go to considerable lengths to minimise distortion without the use of global NFB and have always found this to sound better than high distortion; 2ndH or other. IMO the method of achieving low distortion is the primary reason for the confusion and doing it through NFB almost never sounds as good as achieving it through good design with zero global NFB.

Rgds,
Naz

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