STK 4038 II mono amplifier - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Chip Amps Amplifiers based on integrated circuits

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
Old 13th October 2012, 11:53 PM   #1
diyAudio Member
Join Date: Jun 2012
Default STK 4038 II mono amplifier


I have built a small mono amplifier based on the STK 4038 II amplifier module.

I have built a prototype and while switching the power ON, there is some DC voltage at the amplifier output and the cone of the connected speaker moves noticeably outwards and also makes a subtle noise. After the power has been ON for about 2..5 seconds, the speaker cone slowly returns back into its 0 position.

Is this kind of behavior a sign of a problem in the amp design? The amplifier circuit & PCB are both probably from some old Elektor design, but unfortunately I don't have the schematic at hand at this time. The power supply has a 2x22 VAC/100 VA transformer, 35 A diode bridge and two 10'000 F capacitors.

After this brief start-up issue this prototype amplifier appears to function very well. Even though the wiring in general is not very tidy and the signal wires are not of shielded type, there aren't any extra noises or other issues. I'm planning to add an active lowpass circuit and drive a multimedia subwoofer (2 x 6" woofers) with it. I just used the 12" to play some tunes from my laptop.
Attached Images
File Type: jpg IMGP3030.JPG (812.5 KB, 142 views)
File Type: jpg IMGP3023.JPG (815.5 KB, 135 views)
File Type: jpg IMGP3032.JPG (603.2 KB, 112 views)
  Reply With Quote
Old 14th October 2012, 12:21 AM   #2
diyAudio Member
nigelwright7557's Avatar
Join Date: Apr 2008
Location: Carlisle, England
Sounds like capacitors taking time to charge up.
Murton-Pike Systems PCBCAD51 pcb design software.
  Reply With Quote
Old 14th October 2012, 10:55 AM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
you have crippled the heatsink by lying it on it's side.

The output offset is typical of many amplifiers during start up.
Some get it all done in the first fews tens of milliseconds. Others take much longer. Your amp seems to be in the "longer" category.

I'd suggest you use a speaker isolating relay with a time delay that is longer than the time the offset takes to reduce to near zero mVdc.
I'd also suggest you use this relay to isolate the speaker if the amp ever go faulty and sends an output offset that can destroy your speaker.

Ah, a thought. Is your amplifier AC coupled and using a single polarity supply?
That topology uses an output capacitor. That capacitor has to charge up during start up. That could be what your are seeing as aht speaker movement. The DC current running through the speaker to charge up the output capacitor to (roughly) half the supply voltage.
Again an output relay to isolate the speaker. But you must add a resistor to allow the capacitor to charge up before connecting the speaker.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 14th October 2012 at 11:06 AM.
  Reply With Quote
Old 15th October 2012, 11:06 PM   #4
diyAudio Member
Join Date: Jun 2012
Thanks for suggestions.

I might re-locate the heat sink into the back side wall of the enclosure, at least it would be in free air although it would still be horizontal. It would also free up a little bit of space inside the enclosure for the lowpass and protection circuits.

The power supply is a dual polarity type with +/- 33 VDC no-load voltages and the speaker output does not have a large capacitor in series... Attachment shows a page from the amplifier module datasheet. The circuit in my amplifier is not much unlike it, except that in my amp there is some kind of a pre-amplifier circuit using LF353 op-amp.

I need to look for a suitable DC protection circuit for use with this amplifier.
Attached Images
File Type: png STK4038II.png (34.3 KB, 90 views)
  Reply With Quote
Old 15th October 2012, 11:40 PM   #5
Einric is offline Einric  United States
diyAudio Member
Einric's Avatar
Join Date: Feb 2011
Location: Bozeman, MT
My tube preamp makes my LM4780 amp do the same thing if I don't wait for the tubes to fully warm up before switching on the amp.
  Reply With Quote
Old 15th October 2012, 11:50 PM   #6
diyAudio Member
KatieandDad's Avatar
Join Date: Nov 2011
Location: UK
If the two PSU caps are different the rails wont start up equally.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
stk 4231 II protect esteban2008 Chip Amps 6 24th February 2012 09:26 AM
stk 4192 II rs1026 Chip Amps 8 3rd February 2012 09:19 PM
Stk-4182 Ii? Arran Chip Amps 1 2nd August 2005 10:29 AM
Bridging stk 4192 II chip rs1026 Chip Amps 0 30th August 2004 02:41 PM
hint: STK 4221 II till Swap Meet 0 18th June 2004 11:55 AM

New To Site? Need Help?

All times are GMT. The time now is 12:40 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2