QUASI Amplifier for Beginners

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#822
what about hFE and fT?

8A and 80W for the drivers in a 100W amplifier sounds way over any specification that is necessary.
The 4A 50W mje15034/5 would probably make better drivers.
The 1.5A 20W 2sa649/c669 are very good drivers, if you can get clones that match the original specification.
Fakes for the Hitachies are unlikely to match spec.
 
#823
It's likely that all of the transistors that go into many of these amplifier DIY builds are sub-spec. cloned parts or outright counterfeits. There are good clones of the Hitachi/Renesas 2SD669/B649, for those who feel secure with old reputations but the distributors don't seem to operate in India or most of Asia, for that matter. Taiwanese manufacturer Unisonic makes the best quality I have tested so far in these and several other respected but obsolete drivers. The prices are good from Profusion in Europe, but they are not as cheap after postage rates as the fakes, for obvious reasons.
 
#827
Can somebody explain why we can't just put another diode to bias the output transistor to class AB? That would eliminate crossover distortion. So instead of two diodes on the VAS stage, we put three. With that diode, i can't find any fundamental circuit difference with other class AB amp like, say, P3A by Rod Elliot. The only difference being that P3A uses Vbe multiplier and we are using diodes to achieve class AB bias. Even this B500 by Apex uses CCS on the VAS stage which should be better than bootstrapped load as used by P3A.
 
#828
ballpencil said:
Can somebody explain why we can't just put another diode to bias the output transistor to class AB? That would eliminate crossover distortion. So instead of two diodes on the VAS stage, we put three. With that diode, i can't find any fundamental circuit difference with other class AB amp like, say, P3A by Rod Elliot. The only difference being that P3A uses Vbe multiplier and we are using diodes to achieve class AB bias. Even this B500 by Apex uses CCS on the VAS stage which should be better than bootstrapped load as used by P3A.
Click to expand...

Another diode would give another 0v7 across the output resistors giving 1.4 amps.
A Vbe multiplier is best bet as it can be adjusted accurately.
 
#829
So with the extra diodes, aside from having high quiescent current (class A anyone?) which can be reduced by increasing the output resistors, we can achieve class AB with this circuit without any further changes? How would it perform sonically to P3A for instance?
 
#830
ClassAB runs hotter than ClassB.

That extra heat is acceptable in low power amplifiers.

Many amplifiers in APEX's Thread are of massive power output.
Extra heat would become a killer.
Many of APEX's amplifiers would blow up if converted to ClassAB

An example.
Qp for a 80Vdc ClassAB amplifier with 100mA bias in a single pair of output is ~ 8W
Qp for a 200Vdc ClassB amplifier with 0mA of output bias is ~ 0W
Qp for a 200Vdc ClassAB amplifier with 100mA bias in a 4pr of output is ~ 80W

That is 80W of extra heat that must be dissipated.
 
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#832
AndrewT said:
ClassAB runs hotter than ClassB.

That extra heat is acceptable in low power amplifiers.

Many amplifiers in APEX's Thread are of massive power output.
Extra heat would become a killer.
Many of APEX's amplifiers would blow up if converted to ClassAB

An example.
Qp for a 80Vdc ClassAB amplifier with 100mA bias in a single pair of output is ~ 8W
Qp for a 200Vdc ClassB amplifier with 0mA of output bias is ~ 0W
Qp for a 200Vdc ClassAB amplifier with 100mA bias in a 4pr of output is ~ 80W

That is 80W of extra heat that must be dissipated.
Click to expand...



dear sir, do class d and class h have low heat?
thanks regard masood
 
#835
Hi Apex.Thanks for sharing circuits
It was my second try of your circuits.and again im dissapointed 🙁
Amp worked but as soon as i turn volume higher it gives bad noise distortion.I used high gain transistors bc546 300hfe.mje340/350 150hfe. 2sc4793/2sa1837 150hfe.I dont think this is reason.
It is not for begginers.or you have some secrets 🙂
 
#838
Batson

I suspect your amplifier is oscillating. I'm sorry to say that there is a big difference between 2N3773 and 2SC4793/2SA1837. I suspect that your circuit is oscillating at higher volume settings. With these (much) faster transistors than 2N3773 you may need to reposition the Miller capacitor between the collector and base of the VAS.

John
 
#839
2sc4793/2sa1837 i used them into driver stage.The same results i got with tip41/tip42
Guys guys if it is beginner amp it wouldnt be so much to measure about it right 🙂)))
There is no wrong resistor replaced.
it works ok on middle volume but you cant listen it loud.that all problem.sounds get into distortion very bad.
 
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