AMPLIFIER DESIGNS FOR REDUCED IMD
We all would like our new amplifier to offer a clear display of the individual instruments, imaged perfectly and in place with distinct bass. Electra-Print Audio Company has developed new low IMD circuitry and reviewers say it sounds like twice the power, with better imaging, authoritative bass and does not show harshness at high volumes. Intermodulation Distortion (IMD), is the resulting sound heard of musical instruments of a lower frequency, modulating or mixing with, instrument(s) of higher frequencies during playback. This combination of mixed tones, depending on the music and level changes, results in a harsh, dissonant screech, as music increases in volume. This is commonly mistaken as Total Harmonic Distortion (THD) due to IMD is never indicated in the amplifier specs.
Electra-Print Audio has been investigating the effects, causes and reduction of IMD in tube amplifiers. The following is a list of causes and cures and low and low-rise IMD circuitry, in a practical application format. IMD being the modulation of high frequencies by the lower frequencies with a resulting sum and difference of many odd combinations of audible products. The standard measured value of IMD for very good performance of an SE amplifier is less than 2% at 1 watt and about 4% at the amplifier's mid-power level. For very low power amps, such as the 45 tube, 2% at .5 watt is used. All of our tests are done with, two tone, 60Hz/6kHz, 4:1, SMPTE standard signal. These amplifier designs do not use global negative feedback, common in push-pull amplifiers. Those amplifiers that use negative feed back show results with very low IMD as well but also generate artificial bandwidth and its resulting signal compression. This is at the expense of output power with a dependency on Nyquist phase analysis so that the amplifier will not oscillate.
Causes and Cures:
Cause #1 - Output transformers, SE design: The output transformer when combined with the output tube gives highest generation of IMD. The core mass of an SE output is in play with the lower end of the bandwidth. The core has a slight magnetic lag at these low frequencies and as the high frequencies are induced, this lag modulated these highs. The core is not in use above 2-3 kHz but does steer the field inwards for the coil and this keeps the efficiency high. The resulting IMD is a product of the individual cadence of core and induced high frequencies.
Cure #1 - is to reduce the interwinding capacity so high frequency phase shift is minimized and increase core mass for less motion at power level used. Also, loading power output tube to a little higher primary impedance so tube is not subjected to its own nonlinearity below it optimized power operating point. The interwinding capacity will shunt the primary and lower the impedance above 10 kHz and output tube's nonlinearity will add to the transformer IMD generation. Electra-Print Audio has been building variations on SE output transformers with Low IMD type windings.
Cause #2 - Output tubes will generate IMD in many ways. For example, when the plate load is too low an incorrectly selected tube operating point, a low value bypass cap across self bias resistor and grid bias voltage in motion with the signal due to a too high value grid resistor. Any combination of the above mentioned problems will result in two or more different motions in a signal path. This increases IMC. Driver and voltage amp tubes with un-bypassed self bias resistors used for decreasing THD and/or used for local feedback for lowering gain, will increase IMD due to the many different motions at their cathodes.
Cure #2 - Heavy bypassing of this resistor so all motion is at a very low frequency which will not be too noticeable. Bias source impedance is reduced as low as possible so it remains in control of the tubes linear operating point. Using a fixed bias or bias supply and grounding its cathode or filament, would be the best combination for this. With driver and voltage amp stages an algebraic summing resistor can be used to null IMD motion slightly but measurably, also to lower gain slightly (local NFB). Very low THD will follow the lowering of IMD product generation, not the other way around. We have measured a very low THD with very high IMD measured at same operating level.