25w amp completed

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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
 
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:)
 
pudge667 said:
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):
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'.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.
 
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