Distortion Tests - HifimeDIY T2 / LJM L20D / SURE TDA7498 - Page 7 - 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 18th August 2012, 05:44 PM   #61
trevmar is offline trevmar  United States
diyAudio Member
 
Join Date: Jan 2011
Smile Amplifier and Speaker protection

The L20D has some protection, under and over-voltage.
This is one of the reasons I have been using my modified TDA7498 boards in my 'stereo' here. The integrated chip amps do have more protection for the speakers built-into the chip. Over-current, supply rail levels, load impedance, etc. But the L20D has a cheap, easily replaceable output chip. At least, I think that is what LJM was thinking.

IMO, there is a limit to how accurate the protection on a component board can be, in any case. You almost certainly want to invest in a speaker protection board.

What I ended up with was documented here:
Trevor Marshall - Class D Audio Amplifier Design - TDA7498 Output filters
and here:
Trevor Marshall - Ground Effect Omnidirectional HiFi Loudspeakers

Sorry I took so long to reply to the posts above. I am totally happy with my setup now, and don't read the board very often.

Trevor..
  Reply With Quote
Old 18th August 2012, 07:11 PM   #62
dgta is offline dgta  United States
diyAudio Member
 
Join Date: May 2011
Trevor, in reading the datasheet it seems to me the TDA7498 doesn't have any specific internal mechanism for sensing DC at the output. Is the current limiting what you depend on for speaker protection? Or do you use the 7498 board with separate speaker protection?

While DC at the output would cause massive output current, I'm not sure most speakers can take the 7A current limit of the 7498. Now with 32V supply and 8 ohm speakers, you're probably limited by the dc resistance of the speaker itself, which will probably allow less than 7A. So no benefit from the current limiting.
  Reply With Quote
Old 18th August 2012, 07:24 PM   #63
trevmar is offline trevmar  United States
diyAudio Member
 
Join Date: Jan 2011
Oh Heavens - I remember looking up this scenario once, and deciding it was dealt with in the chip, but I can't remember how. Let's look at the data sheet...

OK, I remember - I decided that the over-temperature protection was really quite important at minimizing problems, and because the amplifier was bridged, with both halves of the bridge in the one chip, the speaker would only be harmed if the TDA7498 "half-failed"

I did test over-temperature by running without a heatsink. It shut down cleanly. Took quite a bit of power to overheat the chip, though, the spkrs were quite loud...

So not a really rigorous protection, but I wanted the amp really small and lightweight, sitting here in my tiny office. I did buy a few spare Peerless speakers, just in case They are an amazing driver... (I can easily get spare SEAS woofers, don't need spares of them).

Last edited by trevmar; 18th August 2012 at 07:26 PM. Reason: typo
  Reply With Quote
Old 18th August 2012, 08:15 PM   #64
dgta is offline dgta  United States
diyAudio Member
 
Join Date: May 2011
Quote:
Originally Posted by trevmar View Post
because the amplifier was bridged, with both halves of the bridge in the one chip, the speaker would only be harmed if the TDA7498 "half-failed"
If an output device in either amp fails and shorts to either rail, you have DC on the speakers. Yes the other amp is still switching, and with 32V B+ and 50% duty cycle it only puts 16VDC across the speaker. That's better than 32V, but still not desirable. Might be OK with some 8R speakers.

The only speaker I ever lost was connected to a well respected piece of Luxman gear. Playing at moderate volume, running cold. One of the final transistors failed short and put the rail voltage on the speaker. Amp had overtemp protection, etc. but no DC sensing.

Yes, disposable speakers make a difference, but not my favorite protection method
  Reply With Quote
Old 18th August 2012, 08:37 PM   #65
trevmar is offline trevmar  United States
diyAudio Member
 
Join Date: Jan 2011
Quote:
Originally Posted by dgta View Post
Yes, disposable speakers make a difference, but not my favorite protection method
For me, Performance, Size and Weight were most important, roughly in that order. I really didn't have any room for the speaker protector boards (those relays are massive) and so decided to go the expendable speakers route.

I did expect to be blowing up those tiny Peerless, but I haven't had that problem (yet). My ears would give out first, I think. Which is kind of interesting, and surprising...
  Reply With Quote
Old 18th August 2012, 08:49 PM   #66
dgta is offline dgta  United States
diyAudio Member
 
Join Date: May 2011
Thanks Trevor. I am also specifically looking at this type of board mostly for the performance and size, although weight is not important to me. But I will be using it with nice speakers and the external protection boards basically screw up the "size" part of my requirements. Oh well...
  Reply With Quote
Old 4th October 2012, 04:57 AM   #67
diyAudio Member
 
panson_hk's Avatar
 
Join Date: Aug 2005
Blog Entries: 10
I also got one SURE TDA7498 board recently. I found that output snubber RC on my board is not in series connected output + and -. Both R and C are connected to + and - (in parallel). When I put 0805 22 R there, the resistors smoked during test.
  Reply With Quote
Old 4th October 2012, 05:12 AM   #68
diyAudio Member
 
panson_hk's Avatar
 
Join Date: Aug 2005
Blog Entries: 10
I believe TDA7498 is a self-oscillating class d. The switching frequency decreases to zero when output near clipping as shown in the figure below.

CH1 & 2: output + and - (after output filter); CH 3: inductor current; CH 4: output PWM waveform. Math channel = CH1 - CH2

As can be seen, PWM frequency drops to DC during output clipping.
Attached Images
File Type: jpg TDA7498 output.jpg (146.6 KB, 717 views)

Last edited by panson_hk; 4th October 2012 at 05:19 AM.
  Reply With Quote
Old 4th October 2012, 07:09 AM   #69
diyAudio Member
 
panson_hk's Avatar
 
Join Date: Aug 2005
Blog Entries: 10
Zero Hz is better term than DC.
  Reply With Quote
Old 7th October 2012, 08:26 AM   #70
diyAudio Member
 
panson_hk's Avatar
 
Join Date: Aug 2005
Blog Entries: 10
I am going to test the SURE TDA7498 board in parallel bridge operation. Although data sheet does not mention this operation mode explicitly and availability of TDA7498E, the board layout shown in the data sheet was prepared for this mode. Are TDA7498, TDA7498E and TDA7498MV share the same silicon?
  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
hifimediy T2 mods? bdbender Class D 20 12th August 2011 12:08 AM
WTB: Hifimediy T2 Frosteh Swap Meet 1 7th August 2011 08:03 PM
Improving Hifimediy t2 jtjy Class D 3 20th July 2011 05:02 PM
Power supply for Hifimediy T2 eladmi Power Supplies 0 10th March 2011 07:14 PM
Hifimediy T3 and T2 amps... oppinions? jaistanley Class D 6 30th November 2010 10:36 PM


New To Site? Need Help?

All times are GMT. The time now is 01:40 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