Basic class D amp questions - diyAudio
Go Back   Home > Forums > Amplifiers > Class D

Class D Switching Power Amplifiers and Power D/A conversion

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 9th May 2011, 10:54 PM   #1
diyAudio Member
 
Join Date: Dec 2009
Default Basic class D amp questions

I have looked elswhere in the forum, but I can't seem to get a clear answer to the following questions:

1. Does a class D amp turn an anologue signal into a digital one? (I am a vinyl freak and see no point in losing analogue sound);
2. I see that all class d amps have an operating range that cuts off at 22khz - unlike tube and class a designs. Now I appreciate that I am not a bat, but I am curous as to why this is the case.

I am asking these questions because I am thinking about building a three-way active speaker
  Reply With Quote
Old 9th May 2011, 11:28 PM   #2
diyAudio Member
 
sofaspud's Avatar
 
Join Date: Nov 2005
Location: San Antonio
1. No. Class-D uses binary switching for efficient amplification, as opposed to biasing the transistor output in the linear region. I suppose "digital" might fit since it's an on/off output, but it only serves to cloud the real goings-on IMHO.
2. The output must be low-pass filtered to regain the desired signal. I've not noticed that all are cut off at 22kHz so I can't comment on a specific frequency.
I'm sure others can provide more info. Class-D has made huge inroads the past 5-10 years and worth an audition for your project (again IMHO). You might like it.
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
  Reply With Quote
Old 10th May 2011, 12:17 AM   #3
diyAudio Member
 
theAnonymous1's Avatar
 
Join Date: Feb 2004
Location: Anonymityville
Quote:
Originally Posted by loheswaran View Post
I see that all class d amps have an operating range that cuts off at 22khz - unlike tube and class a designs. Now I appreciate that I am not a bat, but I am curous as to why this is the case.
Not all Class-D designs are the same, but a lot of them do start to roll-off not much past the ~20kHz mark. I don't see this as a problem IMO.

I think the reason this is the "norm" is a combination of the switching frequencies used (200-300kHz) and the fact a first order filter is used on the outputs. If you push the corner frequency too far up, then there won't be enough attenuation at the amps switching frequency (causing too much radiated noise). Multi pole filters cause other problems.

^That's how this layman understands it anyway; a pro is welcome to correct me.
  Reply With Quote
Old 10th May 2011, 12:44 AM   #4
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by theAnonymous1 View Post
Not all Class-D designs are the same, but a lot of them do start to roll-off not much past the ~20kHz mark. I don't see this as a problem IMO.

I think the reason this is the "norm" is a combination of the switching frequencies used (200-300kHz) and the fact a first order filter is used on the outputs. If you push the corner frequency too far up, then there won't be enough attenuation at the amps switching frequency (causing too much radiated noise). Multi pole filters cause other problems.

^That's how this layman understands it anyway; a pro is welcome to correct me.
You can run class d at higher frequencies than 200-300KHz.
You can also shift the filter frequency up a bit as the carrier will be rejected by the speaker anyway. Or use a dual filter. In fact you can now get filterless class d amps.
  Reply With Quote
Old 10th May 2011, 01:03 AM   #5
diyAudio Member
 
theAnonymous1's Avatar
 
Join Date: Feb 2004
Location: Anonymityville
Quote:
Originally Posted by nigelwright7557 View Post
You can run class d at higher frequencies than 200-300KHz.
Yes, I realize there are >1MHz Class-D amps. The average for a lot of designs falls in the 200-300kHz range though.

Quote:
Originally Posted by nigelwright7557 View Post
You can also shift the filter frequency up a bit as the carrier will be rejected by the speaker anyway.
As I understand it, it has more to due with RFI being emitted by the speaker cables. I don't think it's recommended to have much less than -40dB attenuation at the switching frequency for that reason.

BTW, I meant to say 2nd order filter in my first reply.

Last edited by theAnonymous1; 10th May 2011 at 01:21 AM.
  Reply With Quote
Old 10th May 2011, 01:06 AM   #6
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by theAnonymous1 View Post
Yes, I realize there are >1MHz Class-D amps. The average for a lot of designs falls in the 200-300kHz range though.



As I understand it, it has more to due with RFI being emitted by the speaker cables. I don't thing it's recommended to have much less than -40dB attenuation at the switching frequency for that reason.
I use 200KHz on my amps and they sound great.
I tried 400KHz but the class d chip got hot so I went back to 200KHz.

The actual frequency content at the speaker leads isnt as bad as might be first thought. Its is a 200KHz sine wave not a square wave so really doesnt radiate that badly.
  Reply With Quote
Old 10th May 2011, 02:58 AM   #7
TICUL38 is offline TICUL38  Canada
diyAudio Member
 
Join Date: Jun 2005
Location: Ste-Therese, Quebec
Unnecessary speaker coil heating with HF signals? would this not be a reason for cutting off frequencies beyond what human ears can use?

Louis
__________________
All those "bits" give me indigestion!
  Reply With Quote
Old 10th May 2011, 11:03 AM   #8
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by TICUL38 View Post
Unnecessary speaker coil heating with HF signals? would this not be a reason for cutting off frequencies beyond what human ears can use?

Louis
But the inductance of the speaker coil will reject such high frequencies.
  Reply With Quote
Old 10th May 2011, 05:37 PM   #9
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
The input filter is just idiot proofness. Many class-D amps (analog of course) react histericcaly to slew overload.
There's basically no problem in designing switching amp of, say, 100kHz small signal bandwidth.
  Reply With Quote
Old 10th May 2011, 09:22 PM   #10
TICUL38 is offline TICUL38  Canada
diyAudio Member
 
Join Date: Jun 2005
Location: Ste-Therese, Quebec
Can someone suggest technical reference(s) for justifying
the little importance I sense here, for filtering out the high frequency "hash"...
Thank you,

Louis
__________________
All those "bits" give me indigestion!
  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
Massive amp modification - basic questions - Rotel RB991 donor inteificio Solid State 19 1st December 2010 08:13 AM
Basic op amp optical compressor questions LarsXI Solid State 0 1st July 2008 11:32 PM
Some basic questions about s-5 amp kit mods fenpark15 Tubes / Valves 15 22nd December 2007 10:56 PM
Basic sub/amp questions Katokahn Subwoofers 3 1st November 2006 05:03 AM
some basic questions (ProAudio & horns) tech.knockout Multi-Way 2 11th April 2005 11:46 AM


New To Site? Need Help?

All times are GMT. The time now is 03:31 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