|Solid State Talk all about solid state amplification.|
Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
||Thread Tools||Search this Thread|
|9th May 2007, 06:00 PM||#1|
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...
|9th May 2007, 06:45 PM||#2|
Join Date: Aug 2005
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
All the trouble I've ever been in started out as fun......
|9th May 2007, 09:57 PM||#3|
Join Date: Dec 2005
Location: the north
Re: 25w amp completed
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.
P1 should be set, so that same amount of current goes in each of Q1 and Q2.
This is exactly 50%-50% sharing of the current coming from Q3 (2mA)
From the designer article (link in pudge667 first post):
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'.
|Thread Tools||Search this Thread|
|Thread||Thread Starter||Forum||Replies||Last Post|
|Another Zv9 completed||Peter Daniel||Pass Labs||60||8th August 2007 11:53 PM|
|I completed my amp!||peppennino||Tubes / Valves||21||6th May 2006 04:29 PM|
|first completed sub||chainenoble||Subwoofers||4||11th June 2005 06:23 PM|
|HELP! Completed sub is bad!||pne123||Subwoofers||20||17th February 2004 11:34 AM|
|New To Site?||Need Help?|