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Old 9th May 2007, 05:00 PM   #1
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Default 25w amp completed

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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Old 9th May 2007, 05:45 PM   #2
CBS240 is offline CBS240  United States
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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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Old 9th May 2007, 08:57 PM   #3
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Default Re: 25w amp completed

Quote:
Originally posted by pudge667
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
There are several ways to come very close to 0.00V at output.
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:
P1 is used to adjust the DC level on the output.

The adjustment might seem a bit unusual, as it in fact adjusts the bias current through Q7,
but it works quite effectively without adding a lot of additional components.

With standard components it should be
no problem to get near 0 VDC on the output,
and still get close to 5 mA through Q7.
.

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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