General Class D push pull amp output question - diyAudio
Go Back   Home > Forums > Amplifiers > Class D
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 21st April 2017, 06:07 PM   #1
diyAudio Member
 
Join Date: Nov 2016
Location: Shropshire, England UK
Default General Class D push pull amp output question

I have built several Class D push pull output low frequency RF amps of up to 1kW on 136kHz. To get a nice sinewave output relatively free of harmonics I have to use a low pass filter network after the output transformer. in the audio world does a similar design of amp need any filter network before the load (speaker)?

I ask as I am wondering about doing this:

Push pull output, Class D low frequency (136kHz) RF amp, circa 500 Watts. If I want to test it for a few hours into a dummy load does it matter if the low pass filter network usually used to get a sine wave, harmonic free output into an antenna, is not between the output transformer and the dummy load? I want to leave it running overnight and I am modding the LPF and it's in bits until next week. Thanks.
  Reply With Quote
Old 21st April 2017, 08:32 PM   #2
DUG is offline DUG  Canada
diyAudio Member
 
Join Date: Jan 2003
Location: mississauga ontario canada
Quote:
Originally Posted by Chris Wilson View Post
I have built several Class D push pull...

Do you mean bridge?
__________________
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
  Reply With Quote
Old 22nd April 2017, 10:13 AM   #3
diyAudio Member
 
Join Date: Nov 2016
Location: Shropshire, England UK
Quote:
Originally Posted by DUG View Post
Do you mean bridge?
No, here's the schematic of one of them
Attached Images
File Type: jpg 1kw-amp-schematic.jpg (212.6 KB, 106 views)
  Reply With Quote
Old 22nd April 2017, 01:15 PM   #4
DUG is offline DUG  Canada
diyAudio Member
 
Join Date: Jan 2003
Location: mississauga ontario canada
You have a flip-flop for dividing the frequency and a driver for the FET's.

I see how it is push-pull but where is the PWM for the "class D" function?

Looks more like class B. (or even class C)
__________________
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."

Last edited by DUG; 22nd April 2017 at 01:19 PM.
  Reply With Quote
Old 22nd April 2017, 10:47 PM   #5
diyAudio Member
 
Join Date: Sep 2016
There are filterless class-d audio amps but usually for low power and where the amp is very close to the speaker. In higher power amps, filters are required to reduce both the radiated EMI from the speaker cables and to prevent the HF ripple current from heating the voice coils. You should be able to run a dummy load without a filter but you won't have the reactive currents due to the filter so there will be less stress on the output devices.
  Reply With Quote
Old 22nd April 2017, 11:37 PM   #6
diyAudio Member
 
Join Date: Jan 2010
what you show is some sw transmitter power stage.
No audio input
No pwm modulator
No audio output
  Reply With Quote
Old 23rd April 2017, 09:38 AM   #7
diyAudio Member
 
Join Date: Nov 2016
Location: Shropshire, England UK
Quote:
Originally Posted by voltwide View Post
what you show is some sw transmitter power stage.
No audio input
No pwm modulator
No audio output
That's exactly what I said I was showing
  Reply With Quote
Old 23rd April 2017, 09:38 AM   #8
diyAudio Member
 
Join Date: Nov 2016
Location: Shropshire, England UK
Quote:
Originally Posted by JohnnyInBris View Post
There are filterless class-d audio amps but usually for low power and where the amp is very close to the speaker. In higher power amps, filters are required to reduce both the radiated EMI from the speaker cables and to prevent the HF ripple current from heating the voice coils. You should be able to run a dummy load without a filter but you won't have the reactive currents due to the filter so there will be less stress on the output devices.

OK, that's very helpful, thanks Johnny!
  Reply With Quote
Old 23rd April 2017, 02:01 PM   #9
DUG is offline DUG  Canada
diyAudio Member
 
Join Date: Jan 2003
Location: mississauga ontario canada
Quote:
Originally Posted by Chris Wilson View Post
I have built several Class D push pull output low frequency RF amps ...

Push pull output, Class D low frequency (136kHz) RF amp, circa 500 Watts...
Quote:
Originally Posted by Chris Wilson View Post
No, here's the schematic of one of them

Quote:
Originally Posted by Chris Wilson View Post


Quote:
Originally Posted by voltwide View Post
what you show is some sw transmitter power stage.
No audio input
No pwm modulator
No audio output
That's exactly what I said I was showing
?

I fail to see where the class D comes into this...Please explain.
__________________
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."

Last edited by DUG; 23rd April 2017 at 02:03 PM.
  Reply With Quote
Old 24th April 2017, 10:56 AM   #10
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
Yes, that is not a Class D amp. It is merely a switch-mode PA. Exactly which Class it runs in will depend on exactly what load is applied. It could be E or F or some variant of these. Or it could be just a naive switcher of no particular Class and so have not particularly high efficiency.

A square-wave output at the intended frequency filtered to a sine wave is not Class D.

Sorry, I ought to answer the question too. Running a soak test of a switch-mode PA with the output filter missing will not tell you much about how it will operate with the filter present, as the filter is an integral part of how the circuit operates. With no filter, you could more or less heat dissipation in the output devices. You could get bigger or smaller voltage excursions etc.

Read up about Class F (and possibly Class E too). Don't assume that because someone describes his circuit as Class D that it really is Class D. In the case of Classes E and F you don't just need the right circuit; you also need the right tuning.

73 G8HQP

Last edited by DF96; 24th April 2017 at 11:02 AM. Reason: answer OP's question
  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
Push-Pull fixed bias and output power question LATUBEGEEK Instruments and Amps 30 27th February 2015 02:55 PM
Vintage V-FETs in class-A amplifier with complementary push-pull output R-K Rønningstad Pass Labs 25 25th October 2010 01:40 PM
A question about Push-pull class-a mutter Pass Labs 8 17th July 2010 07:04 PM
Push pull class A bias question fragman56 Tubes / Valves 30 24th December 2003 07:23 PM
Push Pull Output Stage Question 316a Tubes / Valves 4 10th February 2003 01:04 AM


New To Site? Need Help?

All times are GMT. The time now is 09:07 PM.


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
Wiki