Effective slew rate and output inductors - 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 28th September 2012, 04:44 PM   #1
diyAudio Member
 
lordnoxx's Avatar
 
Join Date: Feb 2011
Default Effective slew rate and output inductors

Hi all,
I am currently designing/building my own Class-AB power amp.
Thus I came across selecting a proper output inductor. Spice simulations have shown that I need a 2H air inductor.

So in simulation as well as in reality I recognized that the inductor followed by the nominal 4 ohms load (resistive) reduces the effective slew rate that is seen by the load. That is, measured in front of the inductor my amp is capable of producing 72V/s into 4 ohms load. But under the same conditions measured right behind the inductor the slew rate is just about 25V/s.

I am now wondering when reading about amplifiers that have slew rates of >100V/s and also have quit large output inductors.

How can that be? Do manufacturers measure in front of or behind the output inductor? Or: How to achieve >100V/s slew rates behind the inductor with a 4 ohm load present?

Hope you can help me with this because searching the internet as well as reading books from Bob Cordell or Douglas Self could not help me with this.

Thanks!

PS: Please apologizes my not perfect English. Greetings from Germany

Last edited by lordnoxx; 28th September 2012 at 04:55 PM.
  Reply With Quote
Old 28th September 2012, 06:28 PM   #2
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
The inductor will certainly make a lowpass filter at the output of the amp. The fastest it will slew, if everything in front is infinitely fast, is R/L volts per second per volt of output step. That slew rate is only observed at the beginning of the transition, as later in the transition, it will slow down as it completes the exponential approach.

Using your values, the fastest you'd expect it to slew is 4/2e-6 = 2V/us per volt of step amplitude. So, if the amplifier output before the inductor slewed from -15V to +15 Volts, you would see at most 60 V/us across the 4 Ohms.

I think most people would measure the slewing on the amp side of the inductor, since the lowpass action of the L-R filter would limit the observed slew rate. That shouldn't be an issue, since it's just a linear filter, with a rolloff at 318 kHz. The slewing test is a way to see that the amp is fast enough not to slope overload. Looking at the output of the L-R filter would kind of obscure that measurement.

Finally (getting too long a post here), many people would shunt the L with about 10 Ohms. That would increase the output slew rate, and also damp hi-Q resonances that might otherwise cause a very low impedance to appear across the output of the amp when driving capacitive loads.

Akitika GT-101

Update My Dynaco

Last edited by djoffe; 28th September 2012 at 06:29 PM. Reason: factor of 2
  Reply With Quote
Old 28th September 2012, 07:20 PM   #3
sregor is offline sregor  United States
diyAudio Member
 
Join Date: Jan 2006
Location: massachusetts
IMO it's not as important as you think. First - there is a difference between "slewing" and rise time. Slewing is when one of the stages is usually when one stage of an amp is in a state where the voltage can't increase any faster and additional signal is lost (and usually other misbehavior) IMO it seems to be a form of "current clipping" in one stage. Good examples can be seen in most op amp data sheets. The limitation in HF response caused by an inductor will not result in other signal loss or other misbehavior.

Also, output inductor won't be a factor with most speakers, as tweeter inductance is usually significantly higher, making output indctance a moot point.
__________________
Steve

Last edited by sregor; 28th September 2012 at 07:27 PM.
  Reply With Quote
Old 28th September 2012, 11:50 PM   #4
diyAudio Member
 
lordnoxx's Avatar
 
Join Date: Feb 2011
Thank you djoffe and sregor. Your both answers helped me a lot. Now I know that I am on the right way with my little amp. And I know where I will measure the slew rate when it comes to writing the specifications sheet ;-)
  Reply With Quote
Old 29th September 2012, 01:04 AM   #5
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

A properly designed amplifiers slew rate is hypothetical. The section that
limits slew rate is isolated and the designs max "slew rate" established,
is usually how it works, but you then set about designing the amplifier
so that slew rate limiting will never occur under normal operational
conditions for the amplifier and it is a gross overload condition.

5V/uS is adequate for 100W/8ohm/20KHz, (200W/4ohm/20KHz).
10V/uS is adequate for 400W/8ohm/20KHz, (800W/4ohm/20KHz).

Slew rate, power and bandwidth are inextricably linked at high frequencies.

A good amplifier might need say 5V rms input at 40KHz to induce
slew rate limiting in an intermediate stage, though this is clearly
going to cause gross overall amplifier clipping and distortion.

The output inductor doesn't really come into practical considerations.
You will never see a good amplifiers claimed slew rate at the output.

rgds, sreten.

Of course, badly designed amplifiers can have slew rate problems.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 29th September 2012 at 01:24 AM.
  Reply With Quote
Old 29th September 2012, 04:53 AM   #6
cbdb is offline cbdb  Canada
diyAudio Member
 
Join Date: Oct 2008
Location: Vancouver
Slew rate is measured in the amps non linear state, and has very little to do with the amp playing music. As long as its considerably faster (2x) than the numbers sreten has provided (which are easy to accomplish) you have no worries. The Zobel network is not part of the equation.
  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
slew rate and power output. JBL Everything Else 2 16th September 2001 11:51 PM


New To Site? Need Help?

All times are GMT. The time now is 05:08 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