Class D amp using TL494
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 Class D Switching Power Amplifiers and Power D/A conversion

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 8th November 2005, 08:15 AM #21 powerbecker   diyAudio Member   Join Date: Dec 2004 Location: Berlin snip Heinz : I think it will work BUT only if you have c o n s t a n t powersupplys. If not, you need a 2. divider from ps-midpoint to get a new reference! snip Hi ledmania, Now I see your supplyvoltage is not 2X 25V but it`s 2X50V. this is even worse without: "a 2. divider from ps-midpoint to get a new reference!" in your circuit (post1) you have without inputsignal constant 25V at the topside of the speaker, OK! Which voltage is on the bottom side? When I interpret the complete circuit (You don`t draw it!) is on the bottom side of the speaker the mid-point of the powersupply (- 50V / + 50V). -50V is also the ground for your 2V reference. In this case the speaker will see a offset of 25 V! Regards Heinz!
ledmania
diyAudio Member

Join Date: Feb 2005
Location: Manila.Ph.earth
Quote:
 Originally posted by powerbecker snip Heinz : I think it will work BUT only if you have c o n s t a n t powersupplys. If not, you need a 2. divider from ps-midpoint to get a new reference! snip Hi ledmania, Now I see your supplyvoltage is not 2X 25V but it`s 2X50V. this is even worse without: "a 2. divider from ps-midpoint to get a new reference!"
Hi Hienz,

For easy viewing of the power supply, just imagine it that you have a two pcs. of battery with 25V each connected in serries, then imagine that you put your voltmeter across the batts with red probe on the + side, and black probe on the - side...Of coarse you will get +50V right? But if you change the polarity of the probe across the batts, you'll get -50V....How about putting the black probe at the -50V then the red probe is put at the center points of the batts, here you'll get +25V..This is exactly what my power supply is.
But I can't figure out what is that mid point reference that you want me to include for

Quote:
 Originally posted by powerbecker in your circuit (post1) you have without inputsignal constant 25V at the topside of the speaker, OK! Which voltage is on the bottom side? When I interpret the complete circuit (You don`t draw it!) is on the bottom side of the speaker the mid-point of the powersupply (- 50V / + 50V). -50V is also the ground for your 2V reference. In this case the speaker will see a offset of 25 V! Regards Heinz!
Hienz, There will be no offset of 25V that will occur across the speaker because the speaker is referenced to the mid point of the supply that we call neutral.

Regards,
ledmania
__________________

 8th November 2005, 01:55 PM #23 Pierre   Banned   Join Date: Nov 2004 Location: Paris Ledmania, Now I think I understand you: you were using a wrong and confusing naming for the rails in the schematics: if you put -50V, everybody understand -50V referenced to GND, and +50 means +50V referenced to GND so we all assume you have +/-50V supplies. So following your explanation, you have +/-25V supplies. Then it is true that you have 2V offset in the NFB network as you wished, but the problem of the dependency on the supply rails voltage is still there, and believe me: supplies for amplifiers are not so estable at all. Even if you plug it in different homes, you will get different voltages and that will create an important offset at the speaker.
powerbecker
diyAudio Member

Join Date: Dec 2004
Location: Berlin
snip Heinz :
I think it will work BUT only if you have c o n s t a n t powersupplys.
If not, you need a 2. divider from ps-midpoint to get a new reference!

snip Pierre
but the problem of the dependency on the supply rails voltage is still there, and believe me: supplies for amplifiers are not so estable at all. Even if you plug it in different homes, you will get different voltages and that will create an important offset at the speaker.

Hi ledmania

a picture says more than 1000 words (when it is complete and correct).
I believe Pierre and I think right.
The problem (You see it?) was solved with a new reference.
For clarity I "draw" this :

Regards
Heinz!
Attached Images
 better1.jpg (66.9 KB, 2977 views)

 8th November 2005, 09:31 PM #25 Perry Babin   diyAudio Member   Join Date: Nov 2003 Location: Louisiana Assuming ±25 volt rails and the speaker ground and the chassis ground (center tap of the transformer) are at the mid-point (0 volts)... Why not run the 494 with its ground pin (7) at 0 volts? Then you could use a good quality op-amp (5532 driven from ±15 volts) to drive pin 3 of the 494 (with proper protection for the input). Then the audio input and feedback would operate essentially normally with no references other than ground (0v). The 494 could drive an opto-coupler to drive the 2010. I don't know if you've tried this circuit yet but you may need to take feedback from the IC side of the inductor. Also, the 5532 probably won't work for over-current protection because it can't swing it's output close enough to it's negative rail input. An LM358 would work, I believe. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links
ledmania
diyAudio Member

Join Date: Feb 2005
Location: Manila.Ph.earth
Quote:
 Originally posted by Pierre Ledmania, Now I think I understand you: you were using a wrong and confusing naming for the rails in the schematics: if you put -50V, everybody understand -50V referenced to GND, and +50 means +50V referenced to GND so we all assume you have +/-50V supplies.
LOL!!..At last, you understant it..Well, thats nice
But if you have experience in repairing TV/VHS using japanese or
American diagrams then my sch. would be "simple" to you.
I finally realized my mistake in over estimating the comprehention level of the viewers of the sch and should removed that anoying -50V, then should exaggerate the appearance of that pathetic arrow neutral that brought lots of controversy
Oh well..

.
Quote:
 Originally posted by Pierre So following your explanation, you have +/-25V supplies. Then it is true that you have 2V offset in the NFB network as you wished, but the problem of the dependency on the supply rails voltage is still there, and believe me: supplies for amplifiers are not so estable at all. Even if you plug it in different homes, you will get different voltages and that will create an important offset at the speaker.

Nice point pierre, In fact I anticipated that problem before I design the circuit because of the nature of the current involved on it.So, this beast will be same as to those class D amps that will surely drain your wallet in putting lots of filter caps on PS.

On the other hand, for those who want to try in building this amp with tight budget, then you can tweak the circuit very easy by substituting low power fets on the o/p then crankdown the PS to 24V. So you will get here now 12V at the neutral. Nothing to change on the circuit except recalculating the current sense resistor and R15 and R14 again with 2V Vdrop.

You will also notice that the current sense on the 4 parallel resistors at the source of the fet was calculated with a tripping point of 10amps which is maybe too high for the beloved fet to endure. Based on the data sheet it has 16amps of max current, so maybe I have to reduce it to something like 8? or maybe 5 if you love your fet very much.

I think the current sense is reliable enough to shutdown the fet driver in safe mode wether the short circuit is accidental or "intentional" on the o/p side.

The setting of the current sense resistors is an arbitrary choice for the builders of this amps depending on their target output power. the calculation is simply ohms law.

Regards
ledmania
__________________

ledmania
diyAudio Member

Join Date: Feb 2005
Location: Manila.Ph.earth
Quote:
 Originally posted by Perry Babin Assuming ±25 volt rails and the speaker ground and the chassis ground (center tap of the transformer) are at the mid-point (0 volts)... Why not run the 494 with its ground pin (7) at 0 volts? Then you could use a good quality op-amp (5532 driven from ±15 volts) to drive pin 3 of the 494 (with proper protection for the input). Then the audio input and feedback would operate essentially normally with no references other than ground (0v). The 494 could drive an opto-coupler to drive the 2010. I don't know if you've tried this circuit yet but you may need to take feedback from the IC side of the inductor. Also, the 5532 probably won't work for over-current protection because it can't swing it's output close enough to it's negative rail input. An LM358 would work, I believe.
Perry,
Your suggestion on paragraph 2 of your post will simply jeopardize the 700uA pulldown current at pin 3 of Tl494. Please go back to your PDF on that part..It says the opamp is "only active high" and rely only on that 700uA as pulldown.
Also the use of ne5532's o/p to drive pin 3 with +/-15V will surely hurt pin 3 b'coz the opamp was powered only by 5Vref(pin 14).
And the use of optocoupler, well its another story.

Your comment regarding the use of ne5532 as the current sense amp, and you are concern that it will not work because of the low sink capabilty? well, please look at the PDF of IR2011 and notice their i/p. You will see that it has "schmitt trigger" i/p with sharp hysterises response. this means that even ne5532 cant switch digitally, then schmitt trigger will take care of it..

Regards
ledmania
__________________

 9th November 2005, 02:22 AM #28 ledmania   diyAudio Member     Join Date: Feb 2005 Location: Manila.Ph.earth Hi guys, The sch that you have seen on my 1st post, then another correction was made with the 2nd and 3rd sch, regarding the NE5532. You will notice that the original supply was 12V. But then was change to 5Vref to match the supply of Tl494 error amp supply. I think ne5532 will not work on that supply level. So, that opamp must be substituted with another opamp qualified to run @ 5V. Regards, ledmania __________________
ledmania
diyAudio Member

Join Date: Feb 2005
Location: Manila.Ph.earth
Quote:
 Originally posted by powerbecker Hi ledmania a picture says more than 1000 words (when it is complete and correct). I believe Pierre and I think right. The problem (You see it?) was solved with a new reference. For clarity I "draw" this : Regards Heinz!
Hi Heinz,

LOL!!!. I think I have to congratulate you for this! Your proposed new reference makes a lot of sense to me!.......Lets say the PS varies,then of coarse the 2V on that node will also vary therefore the + side i/p of the opamp will copy it!!! Bingo!!

Good job Heinz!!!
I will add your proposal into the final schematic...
Regards
ledmania
__________________

 9th November 2005, 11:00 AM #30 ledmania   diyAudio Member     Join Date: Feb 2005 Location: Manila.Ph.earth Hi guys, Here is the complete sch of the circuit that was added with a new reference node as suggested by Heinz.(thanks Heinz) I have no time to look for qualified opamp that will run beter @ 5V to replace NE5532 as over current protector. So, you still see it on the sch. LO and behold..... An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this. Regards ledmania __________________

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