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Old 24th December 2014, 10:18 PM   #1
djduck is online now djduck  Estonia
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Default Reducing distortion in this class B circuit

Okay so I have this industrial amplifier that uses this circuit: http://www.kn34pc.com/sch/sch_ampl/e...mc_1511_cr.jpg
I am using it with B&W DM310 speakers and sources are laptop, tuner and turntable.
I like the overall sound of this amplifier. Fast and punchy bass with very good lows, even with the 8 inch woofers. Generally a very detailed sound also.
More info on this amplifier can be found here: KN34PC-Silistra
Now this is supposedly a class B amplifier with a class A pre-amplifier. Now this of course bring in the issue of crossover distortion. And this amplifier has it. It's not too bad at lower frequencies, but mid-highs sound very fatiguing.
For an example on the song "Wishing Well" by Airborne Toxic Event, the beginning has this... synth?? tune and it sounds very harsh. That is just one example, but anything like that sounds bad with this amplifier.
So what I want to know is if there is any way to improve this circuit to reduce crossover distortion? Because I really like the overall sound of this amplifier and I don't have enough money currently to buy another amplifier.
I've already re-capped it and put in new wiring. This did help with overall SQ but not with distortion.
Also, the input of this circuit is a bit odd. First you have a 3.9K resistor, a 470p input to ground bypass cap, then a 470n coupling cap, the feedback resistor, and then another coupling cap. Is this good or should it be changed?
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Old 24th December 2014, 10:52 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by djduck View Post
Okay so I have this industrial amplifier Now this is supposedly a class B amplifier with a class A pre-amplifier. Now this of course bring in the issue of crossover distortion. And this amplifier has it. It's not too bad at lower frequencies, but mid-highs sound very fatiguing. So what I want to know is if there is any way to improve this circuit to reduce crossover distortion? the input of this circuit is a bit odd. First you have a 3.9K resistor, a 470p input to ground bypass cap, then a 470n coupling cap, the feedback resistor, and then another coupling cap. Is this good or should it be changed?
This amplifier may not be suitable for other than a subwoofer or LF biamping, since the heat sinks are likely not adequate for higher bias current.
I wouldn't change the input circuit, other than perhaps installing better small capacitors.

Last edited by rayma; 24th December 2014 at 10:58 PM.
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Old 24th December 2014, 11:17 PM   #3
djduck is online now djduck  Estonia
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Originally Posted by rayma View Post
This amplifier may not be suitable for other than a subwoofer or LF biamping, since the heat sinks are likely not adequate for higher bias current.
I wouldn't change the input circuit, other than perhaps installing better small capacitors.
We could try higher bias, since the heatsinks are pretty huge and as it is now, this amplifier emits next to nothing in terms of heat, even at high output for long periods of time (yes, I like it loud )
How do I set higher bias? I think I have some sort of a clue but I'd rather hear from someone smarter than me, so please, share with me your wisdom
I replaced input electrolytics with film. Same size as original, 470n and then 1uF. Should I go higher maybe?
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Old 24th December 2014, 11:22 PM   #4
rayma is offline rayma  United States
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Originally Posted by djduck View Post
How do I set higher bias? I think I have some sort of a clue
It appears that the bias in not adjustable in this unit, not surprising in a class B amplifier.
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Old 24th December 2014, 11:29 PM   #5
djduck is online now djduck  Estonia
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Originally Posted by rayma View Post
It appears that the bias in not adjustable in this unit, not surprising in a class B amplifier.
Any way to "make it adjustable"?
We can butcher this thing as much as we need.
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Old 24th December 2014, 11:33 PM   #6
rayma is offline rayma  United States
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Originally Posted by djduck View Post
Any way to "make it adjustable"?
We can butcher this thing as much as we need.
The bias transistor circuit seems to be centered on V138.
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Old 24th December 2014, 11:44 PM   #7
djduck is online now djduck  Estonia
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Quote:
Originally Posted by rayma View Post
The bias transistor circuit seems to be centered on V138.
Yes, I noticed.
Let me point out that I have basic knowledge about electronics and how amplifiers work. So I basically know what most of the components do. But I'm still learning.
So how should I change the bias circuit?
V168 looks to be another part of the bias circuit.
Also are there any other changes that could be done to this circuit to get better sound? Other than bias.
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Old 25th December 2014, 12:31 AM   #8
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That schematic is an eye bulger. The bias circuit would take a lot of analysis and reiteration to change just right, and would not get done without the output section running away at least once, which could easily leave you better off starting from scratch. That is a class G amp with a Sziklai pair on one side for all NPN quazi-complimentary power outputs. The sinks have been sized to handle the possible dissipation expected with the bias the way it is. The whole design (including layout) is contrary to requirements for stellar HF performance. The same circuit with modern parts could probably perform much better but would require that every value be changed. Anyway, if you're that fresh and don't care whether you lose what that museum piece is currently capable of, here's a nice article I recently saw on the kinds of things you have to think about for stable class AB bias: http://hifisonix.com/wordpress/wp-co...es-V1.0231.pdf

Last edited by Andrew Eckhardt; 25th December 2014 at 12:41 AM.
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Old 25th December 2014, 12:39 AM   #9
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Quote:
Originally Posted by djduck View Post
Also are there any other changes that could be done to this circuit to get better sound? Other than bias.
Almost certainly reducing the impedance of the supply. More caps, close-in to the output devices in other words.
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Old 25th December 2014, 10:26 AM   #10
djduck is online now djduck  Estonia
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Thank you for the link, Andrew. It's a bit of a longer read so I'll read it a bit later, busy with the holidays right now
Anyways, I think maybe I should take the amplifier modules out and measure all the resistors, etc... in the bias circuit to make sure everything in within tolerance and working to it's full potential?
Also, should I bump the input coupling cap size? I have some nice 4.7uF K73-16V caps that I could throw in there.
This amplifier's power supply has separate caps for both modules. 27V lines have 2000uF (1000x2) per rail and 48V rails have 4700uF per rail. I think all the caps are fine because this thing has some serious bass performance and there is next to no output hum. I could be wrong tho, so I guess I'll replace the 1000uF caps. The bigger ones should be okay. They are rated for 72V, so there is a lot of headroom there.

Last edited by djduck; 25th December 2014 at 10:29 AM.
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