What causes motorboating with 4 ohms? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

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
Reply
 
Thread Tools Search this Thread
Old 13th February 2005, 09:37 PM   #1
tmblack is offline tmblack  United Kingdom
diyAudio Member
 
Join Date: Jan 2005
Location: UK
Default What causes motorboating with 4 ohms?

I have repaired an amplifier and it works correctly with 8 ohm loads.
When I connect a 4 ohm speaker the amplifier starts to softly buzz and then grows into a full motorboat sound.

I addes more caps across PS and this does not help. Adding 10 ohm resistor at output does allow it to drive 4 ohms but what a waste of power.

Is the PS too weak?
Any suggestions?

Thanks

Tom
  Reply With Quote
Old 13th February 2005, 11:08 PM   #2
diyAudio Member
 
Join Date: Sep 2001
Location: Fairbanks, Alaska
Default Additional trouble shooting is required

I would suspect that the amplifier is unstable even with an 8 ohm load. Loading it with 4 ohms puts it over the edge and the oscillation that the instability produces becomes audible as motor boating and an also as a rise in noise level, buzzing as you describe it. This buzzing noise is probably due to the onset of nigh frequency oscillation.

The instability may be due to an inherent design problem which may have been aggravated by having replaced active parts with ones with higher gains or changes in other characteristics. It is also possible that the amplifier may still have a part installed that was slightly damaged by whatever caused the amplifier to fail originally. Further trouble shooting may be needed to find such a part. Donít rule out a potentially damaged trace on a circuit board or other problem that may affect circuit continuity, grounding or a change in the value of a part such as a resistor.

I doubt that your problem is caused by the power supply. An oscilloscope would be nice to have so that you could see what is happing at the output of amplifier at various signal levels and loading. Care should be taken since oscillation in a power amplifier can lead to failure of the output devices and also damage tweeters. A variac with an AC current meter would also be nice to us so that the mains AC input current to the amplifier could be monitored during trouble shooting. Many amplifiers will show an increase in power consumption when oscillation starts.
  Reply With Quote
Old 14th February 2005, 02:05 AM   #3
tmblack is offline tmblack  United Kingdom
diyAudio Member
 
Join Date: Jan 2005
Location: UK
Thank you for your suggestions Alaskanaudio.
I used an AC voltmeter than is 0 when there is no load. I'll get an oscilloscope to confirm your suspections.

Tom
  Reply With Quote
Old 14th February 2005, 03:03 AM   #4
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
If the output devices of your amplifier are TO-3 metal can MOSFETs whose cases are used as source terminal [like 2SJ50 and 2SK135] and the circuit is a source follower, then : Try grounding the heatsink to a point near the local supply decoupling capacitors of each channel
  Reply With Quote

Reply


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
Motorboating in Dynaco ST-70 neddyboy Tubes / Valves 10 2nd July 2007 02:38 AM
motorboating jayme Tubes / Valves 16 2nd May 2007 12:56 PM
ISOBARIC 16 ohms load powered by 8 ohms Amp eddi0 Subwoofers 24 21st October 2006 03:29 PM
Is my reg motorboating (as the bishop said to the queen)? Triophile Solid State 8 2nd April 2006 10:57 PM
Motorboating in leak 20 bronto64 Tubes / Valves 5 13th August 2004 10:49 AM


New To Site? Need Help?

All times are GMT. The time now is 08:53 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2