Amplifier Saftey

kingneb · 22nd February 2012, 03:49 PM

A tube amp has the potential to oscillate if there is no input connected or a load on its output. I am aware of this but many people are not.

What type of protection circuit should I add to prevent the potential of oscillation with...

Case A. No speaker connected.

Case B. No input connected.

The amplifier is a KT88 push pull parallel unit. Case B would be easy if RCA shorting jacks existed (if they do, feel free to point me to a source).

mississippi · 22nd February 2012, 04:04 PM

You can put a sparkgap between the two anodes of 2,4 x Va.
You can put controled avalance diodes from the anode to ground (also 2,4 x Va)
You can put a high power low ohmic resistor at the secondary side of the outputtransformer, switchable when the speakers are connected.
etc. etc.etc.

Wavebourn · 22nd February 2012, 04:07 PM

Quote:

Originally Posted by kingneb

A tube amp has the potential to oscillate if there is no input connected or a load on its output. I am aware of this but many people are not.

I am not.

But I am aware that output transformers without load can be damaged.

kingneb · 22nd February 2012, 05:20 PM

I have always been under the impression that the oscillation would take out the transformer, not the lack of a load itself.

DF96 · 22nd February 2012, 05:52 PM

The reason many people are not aware of this is that it is not generally true. Valve amps can be damaged by being run with no output load but it is not oscillation but hard limiting which creates the problem. If you can't guarantee a load will always be present you will have to add a Zobel network.

Oscillation with no input connected is probably a sign of poor layout, and possibly poor design with marginal stability. Don't add a 'protection circuit'; correct the design.

kingneb · 22nd February 2012, 06:00 PM

My power amp buzzes badly with no input, but no oscillations. Don't pull the plug while it is in operation!

GloBug · 22nd February 2012, 06:07 PM

Pretty much all amplifier stages "buzz" without an input, SS or tube to some degree.

The exception may be in consumer products with integrated procedures to stop this, or in lower gain amplifiers it may be more or less inaudible.

richwalters · 22nd February 2012, 06:19 PM

Quote:

Originally Posted by kingneb

My power amp buzzes badly with no input, but no oscillations. Don't pull the plug while it is in operation!

I bet it is, but you're not looking in the right place ! If it buzzes, check unscreened input leads and make sure ones lab bench light doesn't have a flourcesent tube. A well designed amp should be able to accept hot input switching between an o/c and a dummy input load should be reflected in a slight noise level difference. The exception is when those nasty RCA input plugs are used.
I'm just finalising my powerful 250+250W amp; no way would I allow such problems !

richy

DF96 · 22nd February 2012, 06:24 PM

One way to get buzzing when no input is connected is to have a huge input capacitor (too high in value, possibly audiophile boutique component) which acts as one plate of a capacitor. The other plate is every nearby conductor, including mains supply, rectifiers etc.

Try a smaller capacitor. Mount it nearer the chassis. Put a few pieces of grounded wire over the top of the cap to act as a crude electrostatic screen.

kingneb · 22nd February 2012, 06:42 PM

So a well designed amp should allow a user to pull an input jack and/or switch it, while being on, without the amp buzzing during the process?

22nd February 2012, 03:49 PM	#1
kingneb diyAudio Member Join Date: Aug 2008 Location: Evanston, IL / Rochester, NY	Amplifier Saftey A tube amp has the potential to oscillate if there is no input connected or a load on its output. I am aware of this but many people are not. What type of protection circuit should I add to prevent the potential of oscillation with... Case A. No speaker connected. Case B. No input connected. The amplifier is a KT88 push pull parallel unit. Case B would be easy if RCA shorting jacks existed (if they do, feel free to point me to a source).

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Electrical Saftey and a ground loop	ccdoggy	Parts	7	21st January 2008 06:43 AM
Wanted: DIY amplifier builder with experience. I need a 0.7 ohm stable amplifier	Audiophilenoob	Swap Meet	34	16th May 2005 11:58 PM
Soldering Saftey	RobPhill33	Equipment & Tools	41	6th August 2004 10:26 AM
Questions about how to decrease the PASS A-40 amplifier into a class AB amplifier.	novtango	Pass Labs	2	21st October 2002 01:50 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

22nd February 2012, 04:04 PM	#2
mississippi diyAudio Member Join Date: Apr 2006 Location: holland	You can put a sparkgap between the two anodes of 2,4 x Va. You can put controled avalance diodes from the anode to ground (also 2,4 x Va) You can put a high power low ohmic resistor at the secondary side of the outputtransformer, switchable when the speakers are connected. etc. etc.etc.

22nd February 2012, 05:20 PM	#4
kingneb diyAudio Member Join Date: Aug 2008 Location: Evanston, IL / Rochester, NY	I have always been under the impression that the oscillation would take out the transformer, not the lack of a load itself.

22nd February 2012, 05:52 PM	#5
DF96 diyAudio Member Join Date: May 2007	The reason many people are not aware of this is that it is not generally true. Valve amps can be damaged by being run with no output load but it is not oscillation but hard limiting which creates the problem. If you can't guarantee a load will always be present you will have to add a Zobel network. Oscillation with no input connected is probably a sign of poor layout, and possibly poor design with marginal stability. Don't add a 'protection circuit'; correct the design.

22nd February 2012, 06:00 PM	#6
kingneb diyAudio Member Join Date: Aug 2008 Location: Evanston, IL / Rochester, NY	My power amp buzzes badly with no input, but no oscillations. Don't pull the plug while it is in operation!

22nd February 2012, 06:07 PM	#7
GloBug Banned Join Date: Jun 2011	Pretty much all amplifier stages "buzz" without an input, SS or tube to some degree. The exception may be in consumer products with integrated procedures to stop this, or in lower gain amplifiers it may be more or less inaudible.

22nd February 2012, 06:24 PM	#9
DF96 diyAudio Member Join Date: May 2007	One way to get buzzing when no input is connected is to have a huge input capacitor (too high in value, possibly audiophile boutique component) which acts as one plate of a capacitor. The other plate is every nearby conductor, including mains supply, rectifiers etc. Try a smaller capacitor. Mount it nearer the chassis. Put a few pieces of grounded wire over the top of the cap to act as a crude electrostatic screen.

22nd February 2012, 06:42 PM	#10
kingneb diyAudio Member Join Date: Aug 2008 Location: Evanston, IL / Rochester, NY	So a well designed amp should allow a user to pull an input jack and/or switch it, while being on, without the amp buzzing during the process?