What load does the supply see for LM1875? - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 4th February 2015, 09:18 PM   #1
diyAudio Member
 
Join Date: Dec 2012
Default What load does the supply see for LM1875?

Hi,
how can I find out what load is presented to the PSU by LM1875 driving a 6ohm speaker.

Thanks
  Reply With Quote
Old 4th February 2015, 10:03 PM   #2
diyAudio Member
 
Join Date: Feb 2011
The chip is less than 78% efficient. supposedly i would say about half way between 50% and 70% or so.
so.. if You would drive something with 10 watt from this IC, it would present a load of roughly 1.6 watt.
an amplifier is not a static load like a pure resistor of a fixed value.
in fact, it is a verry interesting load, it does mather what You play ( a fixed tone, or music) depends on the load driven by the amplifier (speakers are not a fixed pure resistive load, they are complex impedance, even a 6 ohm nominal impedance speaker can have in its range anything from even down to 4 ohms up to 30 or more ohms. freqvency dependant)

so, in practice usually it is best to take a look at the IC datasheet, multiply the maximum repetative current it can deliver, and multiply by 1.5
a powersupply that can deliver this amount of current at the rated voltage You want the amp to be driven with will most possibly be enough.
  Reply With Quote
Old 5th February 2015, 12:21 AM   #3
diyAudio Member
 
Join Date: Dec 2012
Thanks
  Reply With Quote
Old 5th February 2015, 12:26 AM   #4
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 119
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Its a very interesting question - as Arty says the load presented depends very much on what music (or test tones) you play. Also the load is non-linear because the chip is a classAB amplifier - its generating current pulses which have harmonic content far beyond the audio range.

Without music playing the loading is close to being a constant current source in that the quiescent current doesn't depend much on the supply voltage. If you don't have output offset correction then there'll be additional current flowing through the speaker's voice coil.
__________________
A new scientific truth does not triumph by convincing its opponents ... but rather because its opponents eventually die, and a new generation grows up that is familiar with it. - Max Planck
  Reply With Quote
Old 5th February 2015, 09:16 AM   #5
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
With the 6 ohm speaker, LM1875 is 53% efficient, which is the portion that goes to the speaker, with the remainder of 47% to warm the heatsink. The sum (53%+47%) is 100% of the power input. I didn't count power used for warming the cables, bridge rectifier, transformer, caps, resistors, etc. . . but hey I'm just estimating here.

TI assigns a 34W total power budget to LM1875, and that is 34W budget shared/split between speaker and heatsink. In their example, 8 ohm speaker, 60% efficiency is then 20W to the speaker AND 14W to warm the heatsink.
The sum is 34. The 8 ohm speaker gets 20W

I think it could do slightly more, so I'll be using 36W total. But, the amp is not 100% efficient, so we'll have to multiply the efficiency by the total power budget. With the amp efficiency at 53% while driving your 6 ohm speaker, just multiply 36*0.53 to see that there will be 19W available to the speaker and 17W to warm the heatsink.
The sum is 36. That 6 ohm speaker gets 19W

That will take approximately 21+21vdc rails, at most.
VA for transformer is not exactly the same as watts (even though both are volts*amps=watts), but the difference is a minor 10% or so (power factor?).
If 6 ohm speaker, it is possible to use up to 36va~40va 15+15vac transformer for each monobloc without excessive risk of exploding the amplifier.

Those are maximized figures.
Exceeding that, could be sketchy and result in reduced longevity.

Pushing LM1875 to max requires current limited supplies most sensibly done with monoblocs.

P.S.
If you wanted a much more convenient stereo build, go get the 12.5+12.5vac 2A center tap transformer at the nearby radio shack. After power board (as easy as one bridge rectifier and a pair of big caps), that's 17+17vdc rails. The results from that option are 13W to most speakers and the excellent convenience that the datasheet sample schematic is optimal at that voltage.
__________________
Tools, Models & Software for DIYClipNipper boostLM1875 TurboPowerful TDA7293 kitTDA7294 pt2pt ♦ My post has opinion.
  Reply With Quote
Old 5th February 2015, 02:05 PM   #6
diyAudio Member
 
Join Date: Dec 2012
Thanks for you responses.

I am interested in learning, so a "convenient stereo build" is not what I want

I was planning to do single supply of 44V but from your explanation it looks like 42V may be better. I will drop the 58V from the psu to 42V using LM317 regulator. To handle more current I have to use some power transistor.
  Reply With Quote
Old 5th February 2015, 04:52 PM   #7
diyAudio Member
 
Join Date: Feb 2011
ah, this last post helps a lot in understanding what are You interested in :d
http://www.ti.com/lit/ds/symlink/lm1875.pdf
page 3.
see the table, and entry : Current limit
it says a tipical 4A.
therefore, we choose a higher than 4 A regulator.
that would be something that can handle 5A current, or more.
  Reply With Quote
Old 5th February 2015, 05:45 PM   #8
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by iinself View Post
I am interested in learning, so a "convenient stereo build" is not what I want
Might as well do a high end regulator. It only takes one more reg to do it (nested regs). Look up "Tracking Pre Regulator" which can be found in some ST reg datasheets. They'll probably use two of the same. However, a high-spec regulator can be used to provide some finer guidance to a more sturdy regulator that actually drives the load.
A better regulator gets rid of more power noise without either getting rid of some audio signal or interposing its own signal on the audio.
Quote:
Originally Posted by Arty View Post
. . .we choose a higher than 4 A regulator. . .
LM1875's protection scheme is incomplete and mostly ineffective. I would have the different preference that the amp should survive the experimenting and therefore it would be good to have something in the chain weaker than the amp. So, a smaller regulator that happens to have a *working protection scheme* may be a little more satisfying. The typical "everything stronger than the amp" approach doesn't work long-term for LM1875 applications.
__________________
Tools, Models & Software for DIYClipNipper boostLM1875 TurboPowerful TDA7293 kitTDA7294 pt2pt ♦ My post has opinion.
  Reply With Quote
Old 5th February 2015, 05:55 PM   #9
diyAudio Member
 
Join Date: Dec 2012
Ok, 4A per LM1875, so for 2 we are looking at 8A. That looks like a lot of amps. There is a fuse on my old receiver psu for 5A near the AC input and it was handling 280W of power. I am only looking at about 10W/LM1875 why are we in need of so much amps, am I missing something?
  Reply With Quote
Old 5th February 2015, 06:00 PM   #10
diyAudio Member
 
Join Date: Dec 2012
Also when looking at the spec sheet graphs (THD vs Power Output , THD vs Frequency, Open Loop Gain and Phase vs Frequency graphs) the reference suppl voltage they show is +/- 25V. I am guessing from this that 50V single supply has been tested, so why all this worry about keeping the supply voltage down.
I think there is a confusion regarding single and dual power supply - since most of the posts on the forum is for dual supply, 25V is fine but the equivalent single supply voltage is 50V ! Again am I missing something?
  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
Single supply LM1875 on a double supply pcb iinself Chip Amps 32 8th February 2015 06:15 PM
LM1875 power supply Chricket Chip Amps 30 21st February 2014 02:28 PM
Power Supply for LM1875 Sh00pus Power Supplies 2 7th February 2013 12:24 AM
Dual supply for LM1875 split supply dovganj Power Supplies 30 6th November 2009 03:19 AM


New To Site? Need Help?

All times are GMT. The time now is 05:43 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2