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Old 30th July 2005, 10:34 AM   #1
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Default change the SMPS circuit in car amplifiers. + picture

hii all...

i have amplifier in my car that stated (14.4V)

200Wx2 ~ 4 ohm
300Wx2 ~ 2 ohm
600Wx1 ~ 4 ohm bridge.

Class AB
SMPS ~ TL494
2 Channels.
6 transistors per channel. (3-2sc5198 3-2sa1941)
6 transistors per transformer. (IRFZ44)

now... i check the volt inside. if i put a : (in\out)

11.3V the SMPS out 67.3V (33.5V +/-)
12.8V the SMPS out 77V (38.5V +/-)
14.2V the SMPS out 84.8V (42.5V +/-)
14.5V the SMPS out 88V (44V +/-)

what i need to change in the circuit to get 88V all the time?
or to get 88V in 12V. ?? the frequency?

many thanks.
tamir.

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Old 30th July 2005, 11:57 AM   #2
edl is offline edl  Hungary
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Hi!

You need to change a resistor near the PWM IC (TL494).
To the non-inverting "leg" goes the voltage reference. To the inverting "leg" goes the output voltage, divided by to resistor. Let's change one of the divider resistor to get 88V output voltage when the input is 12V.
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Old 30th July 2005, 01:45 PM   #3
mzzj is offline mzzj  Finland
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I f i am not mistaken you need complete rebuild. Your amp doesnt have secondary side inductors wich are needed for PWM controlled output voltage on push-pull power supply.

Simple modification around 494 are not enough in this case.
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Old 30th July 2005, 05:52 PM   #4
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Quote:
Originally posted by edl
Hi!

You need to change a resistor near the PWM IC (TL494).
To the non-inverting "leg" goes the voltage reference. To the inverting "leg" goes the output voltage, divided by to resistor. Let's change one of the divider resistor to get 88V output voltage when the input is 12V.
u can show me how to do that in circuit.....?

thanks.
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Old 30th July 2005, 06:17 PM   #5
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just my 2 cents in order to complete confusion:

it looks like the SMPS is non regulated and the output voltage
is defined by the transformer ratio only.
...looks like 1:1:3:3 (pushull:sec1:sec2)
....or 1:1:6 center tapped....

BTW:
Your plan to increase the supply voltage of the amp around 20%
will bring just less than 2db for your ears.
....not worth any effort...
If it does not play loud enough, you need bigger steps.
By far bigger steps.
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Old 30th July 2005, 06:31 PM   #6
mzzj is offline mzzj  Finland
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Quote:
Originally posted by ChocoHolic
just my 2 cents in order to complete confusion:

it looks like the SMPS is non regulated and the output voltage
is defined by the transformer ratio only.
...looks like 1:1:3:3 (pushull:sec1:sec2)
....or 1:1:6 center tapped....

BTW:
Your plan to increase the supply voltage of the amp around 20%
will bring just less than 2db for your ears.
....not worth any effort...
If it does not play loud enough, you need bigger steps.
By far bigger steps.
Yes, looks like unregulated indeed. And there is no easy way to regulate as i told. secondary side inductors are needed for regulation, this kind of "crappy" power supply without inductors is quite popular in car amps.
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Old 30th July 2005, 07:39 PM   #7
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I am not convinced that you need additional output inductors to allow a regulation. The leakage inductance of the transformer often is
higher than one might think......
...and offers the possibility to regulate the output voltage by the duty cycle of the PWM...
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Old 30th July 2005, 08:48 PM   #8
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Quote:
Originally posted by ChocoHolic
just my 2 cents in order to complete confusion:

it looks like the SMPS is non regulated and the output voltage
is defined by the transformer ratio only.
...looks like 1:1:3:3 (pushull:sec1:sec2)
....or 1:1:6 center tapped....

BTW:
Your plan to increase the supply voltage of the amp around 20%
will bring just less than 2db for your ears.
....not worth any effort...
If it does not play loud enough, you need bigger steps.
By far bigger steps.
it is 1:1:3:3

2db it is what i need to get to win in the 1_th plase is SPL (CAR AUDIO competition )

thanks....
tamir
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Old 31st July 2005, 12:09 PM   #9
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Unfortunately the output voltage is already at the upper that you
can reach with a 1:1:3:3 transformer.
But be careful a 1:1:4:4 ratio could already blow your amp !


...if you would have needed 2db during the last contest, then you
will need at least 5 db more in the next contest.
The other guys are not sleeping!
So you have several choices:
a)... to loose your girl friend, because you loose the
SPL competition.
b)... to loose her, because you buy a 1kW amp & speaker system
but don't have left any money to invite here for dinner.....
c) ...to loose her because you spend all your time with the
modification of your amp and blow your overloaded system during the next contest..
d) ..forget about SPL competition and become lucky with
a girl friend that does not judge you by your amplifier rating

From my perception c) would be worst..
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Old 31st July 2005, 12:42 PM   #10
anatech is offline anatech  Canada
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Location: Georgetown, On
Hi,
ChocoHolic is right, you need way more power. Don't mess with that one or you may blow it up. You can gain 2dB by starting the contest with your voicecoils cold in most cases.

Most car amps I used to work on did have output inductors (small) and they were regulated by a TL494, SG3524 or SG3525. But you should do the math. Assume your amp is, say 85% efficient in the power supply and 65% in the amplifier section - I'm being nice here. How much current do you need to run this thing at full power? What are the IR losses in the wire. Don't forget the little wire that goes from the chassis to the battery return.

-Chris
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