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Improve a Yamaha amp THD by 12 dB!
Yamaha AX590 / AX592 / AX870 and AX890’s are not so bad amplifiers, but come along with some quite hefty flaws and quirks. E.g. there is no input pair’s degeneration. And amongst other, at first sight obscure twists, Yamaha has completely messed up the thermal design in all of these series with their’s smallish internal heatsinks and lowish idling current. Funny also to see how much space was internally wasted inside the bigger housings of the AX870/AX890: The “higher” model’s boxes are big not to contain better/more technical functionality, but only for prestige’s sake. Even the slightly bigger headsinks of the AX870/AX890 would neatly fit into the case of an AX590/592. The buyer just got a prime portion of a lot of air for her/his spent extra cash.
Just for interest, I gave the AX890 a slight twist on the Vbias trimmer, increasing the idle current of the power transistors to a theoretically more correct value. The Re’s are 0.22Ohm, and I increased Vre from much too low “Yamaha-standard” 5mV … 6mV to some theoretical correct value of approx. 26mV. This measure alone has a very decent effect on thd. So this might be a simple, effictient mod:
Measurements were taken with a sinewave signal of 1.5kHz, at 2Vrms out along with an 5Ohm output resistor. THD is 0.0074% at 6mV and 0.0018% at 26mV. The 3rd harmonic in both graphs seems to be a consequence of the non-degenerated input pair. At 6mV, there is a prominent spike of 2nd harmonic distortion, and also a smaller spike of 4th harmonic distortion. All other spikes above 1k5 are measuring artefacts.
Beware: This is a “hot mod”, as at +-60V and 2*120mA each heatsink has to deal with approx 25W … 30W while idling. So with it’s 2 channels, this glutton now draws hefty 60W while idling. I think that the addition of a big, slow rotating, silent ventilator like the one’s found in multimedia PC’s would be necessary to keep things cool enough inside a “productive” amplifier.
In listening tests the “hot” version appeared a wee bit more immersive than the “cool” version. But it is only, while notable, a slight difference. And also autosuggestions plays it’s role, as the test was not blinded.
After having blamed Yamaha in the intro: You may see it this way – Yamaha seems wisely to have chosen 5mV as a compromise value. To avoid the whole system from getting too hot. In this aim, some “poor” 25mA idling current for each output transistor pair might be a “sweet spot”. Because, on the other side, further reducing the idling current gives rise to a whole series of higher order (n=4 …) harmonic distortions. So maybe, looking differently at it, Yamaha after all made a wise choice, at least an ecologically sane one. One of D.Self’s design is the “Trimodal Amplifier”, a switching Class A or B design, in which he jokingly suggests a switch labelling “Winter” and “Summer”. Winter for class A, Summer for Class B. The same might go for this kind of Vbe tweaking.
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