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Old 12th September 2005, 06:39 AM   #1
Bosium is offline Bosium  South Africa
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Default Car Amp Design

Hi all

I have designed a car amp, 2 channel 2-ohm stable using the TDA7293 IC.

I would like anyone who may have some input to have a look and tell me what you think?

I have used the protection circuit from Rob Elliot's pages in it, thanks to him for that.

Output power is adjustable by varying the rail voltages. I think that 30-0-30 should be fine but the design is quite flexible.

Thanks!

Gareth
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Old 12th September 2005, 08:08 AM   #2
Bosium is offline Bosium  South Africa
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I guess I should have stated power outputs. This amp should be capable of at least 150Wrms per channel @ 2ohm, ie 300W into a 4 ohm sub bridged. If you pump up the rail voltages some, 200W per channel is possible but it is running the chips very hot and out of the SOA in my opinion.

If you wish to drive higher impedance loads just crank up the voltages and you can still acheive full power.

One point of concern is the 7815 and 7915 regulators, which have a Vin max of 40V.
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Old 12th September 2005, 08:33 AM   #3
kpero is offline kpero  Croatia
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I don't see any temperature sensitiv element (diode, NTC) in your temperature protection.

And, why is there two pot in your preamp circuit. One is for sensitvity and second ??

Anyway ,god job
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Old 12th September 2005, 08:33 AM   #4
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Please don't be offended by any of the following suggestions. They are simply suggestions. Building a usable car amplifier is not easy. It takes experience to know what works and what doesn't.

The audio input will have to have some sort of balanced input or isolator. A transformer will likely be the easiest solution.

If you change to a simple unregulated power supply, you can float the secondary ground then the input ground can be connected to the secondary ground and no other isolation should be needed (RCA shield directly to the secondary ground). Connecting the secondary ground to the chassis ground with a 220 ohm resistors will keep the secondary ground close to ground but shouldn't cause engine noise to be a problem.

If you want to use a regulated power supply AND a floated secondary, you will need to use opto-couplers to get feedback to the PWM IC.

The ICL7667 is a cmos IC and will be damaged if the input voltage goes above 15 volts. If you read the application notes, there are several suggestions to prevent problems associated with the power supply input. You would likely have fewer problems simply driving the FETs with an emitter-follower pair. On the datasheet, I could find no mention of output protection. When the FETs fail, the gates will short to +B. The IC will have to try to drive that to ground. The only resistance will be the gate resistors (in parallel). If the chip can't handle the current, it will also fail. If you decide to use this IC, you may want to install it in a socket. To help protect the IC, you could increase the gate resistors to 47 or even 68 ohms. Increasing the gate capacitance may also have the effect of reducing switching transients.

The delay on (pin 4 of the 494) is going to be too short. Increasing the 560 ohm to 10 k and removing the 6k8 will be more effective. The cap will be drained when the chip is powered down.

You may want to tie pin 15 to a voltage divider at ~2.5v. Pin 16 doesn't need the 47k resistor because the LM324 has emitter follower outputs (totem pole outputs).

I suggest that you wind the transformer with a ratio of about 2.5:1 to start. This should cause the power supply to run at full duty cycle (effectively eliminating the regulation). This will allow you to work out bugs outside of the power supply. THEN, when you have a stable amplifier, you can rewind the secondary (it takes less than 10 minutes to rewind the transformer). Instability in the power supply regulation can cause all sorts of strange symptoms that will lead you to chasing problems that don't really exist.

You'll likely need some capacitive feedback between pin 3 and pin 2. The error amps are diode isolated to pin 3 and you'll likely have to use a bit of trial and error to get a quiet regulator. Sometimes, it's easier to use an op-amp like an LM358 for the regulator input then you can drive pin 3 directly with its output. The diode isolation is the only thing I don't like about the 494/594.

I'd suggest changing the high voltage diodes to something like a MUR820. The 1000volt diodes have a Vf of almost 2 volts. The MUR820s have lower Vf and are much faster.
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Old 12th September 2005, 09:11 AM   #5
Bosium is offline Bosium  South Africa
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hi, thanks for the replies!

kpero - the 10K R54 is a NTC, i forgot to note that. The first pot in the signal line is a stereo one for volume control. R16 and R27 are trimmers meant for compensation in the output of the low pass filters.


Perry - Thanks for the suggestions. I dont take offense to anything, I am always happy to have input and I am grateful that you have taken the time to reply with suggestions.

I do have some experience in building SMPS's and amplifiers, but this will be my first car amp designed from scratch.

I have experimented with opto-isolation and have found that it introduces a non-linearity that is unfavourable. Direct coupling seems to work best.

The +12V line in my car is very clean and I have had no problems with engine-noise or alternator whine whatsoever. The amp is connected directly to the battery as it is in the boot (trunk) of my car (BMW 3 series).

I realise that the 7667 is CMOS, there is a 15V transorb clamping the +12V line on the input, and there is a diode and a FET inline with the power rail feeding the 7667 so it should always be below +15V, given a +13.8V supply, it will see about 13.2V.

I have considered just using a push-pull discrete driver stage, but I like the chip route as it takes up less space and I like to experiment with new things. I will try increase the base resistances to 56Ohm at your suggestion.

Are you suggesting that my DT is too short? I am prototyping this weekend, I will try change it to see what difference it makes.

What benefit will I see by tieing pin 15 to +2.5 rather than 5? I assume the circuit will kick in quicker?

I have built this SMPS before with PWM without any problems - with and without opto-isolation and separate grounds, but I accept it could forseeably be improved. In the past I left out the prot circuit and did not use pins 15 and 16, also used the DTC as shown and no capactive feedback from 2 to 3 - buit I like that idea and will add it to the design, even if I dont populate it in the end.

Also I assume you mean my BYT08's by high-voltage diodes, I will use the MUR's rather, it sounds like a good idea as the BYTs will prob dissipate too much heat for my liking as it is.


Thanks again for all feedback
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Old 12th September 2005, 10:05 AM   #6
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In the schematic it looks like 15 is tied to pin 3 which is an unknown quantity. Even though it would probably work OK there, I would prefer to see it tied to a known voltage. If you tie it to 5 volts, that would be fine since the 324 will swing well over/under 5 volts. Tying it to 2.5 or 5 volts won't make a difference because the 324 will swing from low to high in a few microseconds.

It would be less confusing if you changed the way the 5volt line is routed. If it's connected to pins 15 and 13, it looks as if it's also connected to pins 3, 4, 5 and 6.

For the dead time, I've used a 20k and a 22uf and it comes on relatively quickly. Of course, you'll have to see what works best for your amplifier.

I don't know if you noticed but the protection circuit will cause the amp to have no delay when it comes out of protection. In the past, I had one amp that would run at ~36volts. When it would come out of thermal protection, the regulators would often fail. In that amp I was using 100uf output caps on the regs. I found that reducing the caps to 47uf or keeping the rails below 36 volts would prevent the regs from failing when the amp snapped back to full rail voltage. Be very careful not to exceed the 35 volts for the 78/7915. Even though the regs have protection circuits, the combination of high temps, large capacitances and slightly high rail voltage caused failures.
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Old 12th September 2005, 01:01 PM   #7
Bosium is offline Bosium  South Africa
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Ja i know space is at a minimum so i had to do it like that - but there are no junctions where the wires dont intersect. I'll work it a bit tonight, maybe I can make a more visually pleasing solution.

Pin 15 is tied to it, but I will change that. I did not think about the stupid diodes in the 494, you're quite right.

I'll give it some more thought later, thanks again for the feedback.

Gareth
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Old 18th September 2005, 03:50 AM   #8
djQUAN is offline djQUAN  Philippines
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Quote:
I don't know if you noticed but the protection circuit will cause the amp to have no delay when it comes out of protection.
I contributed the protection circuit in the ESP pages and have installed it in two of my DIY amps. they work very well and have a delay and soft start if any of the protections have been tripped and resets.

it is the resistor and capacitor combination R38 and C45 which lets the output voltage slowly swing from gnd to +12V which then controls the TL494 PWM output from 0 to maximum via the input at pin 16.

Bosium,

I think you have exchanged the positions for the trimmer and the thermistor. (unless you're using a different type of PTC thermistor.) if you're using the NTC type, the trimmer should be on the ground side and the thermistor on the +5V side.

for the dead time, I think I'm using 1k and 33uF. it comes on on the quick side but it still limits inrush current very well.
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Old 18th September 2005, 04:39 AM   #9
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Hi djQUAN,

You're right about the resistor and capacitor allowing pin 14 to swing slowly but error amp 2 of the TL494 is operating without negative feedback so the output of EA2 inside the TL494 will snap from fully positive to fully negative as the output of pin 14 passes through whatever voltage is on pin 15 of the 494 (5 volts as it's currently drawn).

If the output of the protection circuit is left connected as is, he may want to tie pin of the 494 to a lower voltage reference (via a voltage divider off of the 5v reg) and use positive feedback from pin 3 of the 494 to pin 16 of the 494 to provide some hysteresis (as was done on the thermal protection circuit). Unless the output of the 324 is absolutely immune to power supply voltage fluctuations, the output voltage may fluctuate a tiny bit as the amp comes out of protect causing a less than clean turn on of the 494.

To get a soft turn on, he would have to drive either pin 3 or 4 of the 494.
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Old 18th September 2005, 06:15 AM   #10
djQUAN is offline djQUAN  Philippines
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you are right about not having NFB in the error amp but the one without NFB is used for the PWM regulation. the one used for the protection circuit has NFB (wired as unity gain) pin16 is the input coming from the protection circuit, feedback into pin15 from the error amp outputs at pin3. pin1 is used for the PWM regulation and pin2 is connected to Vref.

even if the error amp for the regulator is set to max duty cycle, the other error amp could still control the output from 0% to the max duty cycle limited by the regulator error amp.

since this (regulator EA) is running at open loop and the output acts as a feedback, I would recommend connecting pin2 to pin3 via 1-10nF cap and removing C74 to slow down the EA response.
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