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#1 |
diyAudio Member
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so i finished building this amp http://uti-zadar.hr/quad/25wamp.htm
it works, with some minor issues. i have around 300mV DC on output, i can adjust my P1 and P2, but when i reduce that DC voltage on the output below 300mV then i have to sound at all at the speaker. so it works okay, just there is this soft humming in the background... how do i eliminate this humming? also i connected the output from my pc soundcard to the amp input. when i turn up the volume on the pc, i get a lot higher output, but the quality worsens... any help cheers |
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#2 |
diyAudio Member
Join Date: Aug 2005
Location: K-town
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Hi
300mV seems a bit much for 25W. Adjusting P2 should have no effect on this because it sets output bias current. What is DC bias through Q12/Q13? If it is too high, then more PS ripple will be on the rail voltages. Maybe use two PS caps per rail seperated by a small wirewound resistor in the PS? I think it might be better to have the DC offset control at the LTP than adjusting VAS bias. Perhaps place a pot in series with R6 that is 20% or so of the resistor value to fine tune the LTP leg currents so they are equal AND the DC offset is 0V. Just a thought ![]()
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#3 | ||
diyAudio Member
Join Date: Dec 2005
Location: the north
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![]() Quote:
I have used the same method as in this amplifier several times. But for more extreme precision and ultimate amplifiers I would use other ways. This would include selective matching of input stage NPN PAIR, as well as input base current compensation for input pair. ============== A trick you could use, is to try to switch the input pair Q1+Q2. Let them change place with each other. Sometimes this will give less DC-offset at output .. sometimes more! You can also try to find 2 transistors for Q1 + Q2 that will give better result regarding output DC level. Ideally, P1 should be set, so that same amount of current goes in each of Q1 and Q2. Theoretically 1.0+1.0mA. This is exactly 50%-50% sharing of the current coming from Q3 (2mA) From the designer article (link in pudge667 first post): Quote:
Here is the schematic. We can see P1 close to negative supply rail in Second stage. P1 adjusts the current through Q9 and so indirectly will alter the sharing % in input Q1,Q2. This way does not mean going for 50%-50% in input primary, but change to maybe 55%-45% to achieve zero Output Offset. The best thing is to get 50%-50% current in Q1, Q2 and at same time get ZERO DC output. This is called trimming the amplifier and is necessary for all 'prefectionists'.
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