Hi guys!! Just wondering if you could please help me, I am trying to figure out idle current for my old yamaha a-320, hifiengine does not have the manual, the a-300 has different power transistors and idle current is around 20mv, the a400, which has similar transistors cobfiguration, bias at 6.5mv...??? Any help would be greatly appreciated.
Thank you!!
Thank you!!
I design and build my own amps so no bias current to go by.
So I apply a sine wave and monitor the output on a scope.
I turn up bias slowly until crossover distortion just goes.
On my mosfet amps this is usually about 10mA.
So I apply a sine wave and monitor the output on a scope.
I turn up bias slowly until crossover distortion just goes.
On my mosfet amps this is usually about 10mA.
Hi Nigel, wow, I would love to be able to design my own amplifier, but, alas, I definitely lack the necessary knowledge. I also do not have access to an oscilloscope; would you be able to figure out bias based on transistor configuration? the transistors in the a320 are the same pair in the a400. Thank you!!
Thanks Nigel, yes, you are right. Unfortunately I cannot seem to be able to get a copy of the service manual for the a320, only the a300 (which has different transistors), and the a400 (which has same transistor pair), what would you suggest?
Thank again!!
Thank again!!
Buy a cheap scope.
Even a sound card scope would do for setting bias and they not much.
Obviously need a signal generator too.
You should be a able to get a sound card sig gen online too.
Even a sound card scope would do for setting bias and they not much.
Obviously need a signal generator too.
You should be a able to get a sound card sig gen online too.
Aurally at low volume with simultaneous bias control. Reduce first. Then increase it a little.
The signal from an unadjusted FM tuner (noise) is useful.
The signal from an unadjusted FM tuner (noise) is useful.
Hi guys, thank you for your suggestions. Unfortunately, I have no access to cheap equipment here in Australia. Anyway, I ended up finding a suggestion from another diyaudio member indicating 13mv is the ideal idle current for this amp;however, he also mentioned to look out for heat as the heatsink appear rather small to effectively dissipate heat with this value. What do you guys think??
Thank you
Thank you
Note that bias in the A-300 is set by voltage across both emitter resistors, i.e. 10 mV + 10 mV. I haven't looked at the A-400 schematic but suspect it might have 0.22 ohm emitter resistors rather than the 0.33s in the A-300, so 6.5 mV each would ultimately work out about the same in terms of bias current. Around 35 mA is quite a typical value for '80s Japanese amps.
The theoretical optimum is in fact in the 13-26 mV range (a value towards the low end would generally be chosen to account for dynamic bias variation), but heatsinking may not be up to it. The likelihood of secondary breakdown is reduced if the outputs are running cooler, so it's a tradeoff.
The theoretical optimum is in fact in the 13-26 mV range (a value towards the low end would generally be chosen to account for dynamic bias variation), but heatsinking may not be up to it. The likelihood of secondary breakdown is reduced if the outputs are running cooler, so it's a tradeoff.
