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Old 18th September 2005, 06:44 AM   #11
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I'm not sure I completely understand what you posted so correct me if I misunderstood something you wrote.

You can't have pin 15 tied to pin 3 for feedback. If pin 3 is tied to 5v, the IC will never produce output. If pin 3 sees anything over ~3.3 volts, there will be no output.

EA1 does see a feedback from the rail voltage but that feedback doesn't effect EA2. When the amp comes out of protection, the amp will snap back on (no delay/soft start). Since the rail voltage will be well below the target voltage, the input to EA1 will tell it to run at full duty cycle until the rail voltage reaches its target voltage. The feedback cap that you recommended will help to quiet the regulator.

One way to assure that the amp has a soft start is to use a latch to shut down the supply. Then the user has to turn the head unit off, then back on to reset the latch and restart the amp. Since a properly installed amp should never go into protection, the cycling of the remote input should not be an inconvenience. It's a good thing if it DOES become an inconvenience because that would cause the user to correct the reason for the amp going into protection.
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Old 18th September 2005, 07:14 AM   #12
djQUAN is offline djQUAN  Philippines
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Quote:
EA1 does see a feedback from the rail voltage but that feedback doesn't effect EA2. When the amp comes out of protection, the amp will snap back on (no delay/soft start). Since the rail voltage will be well below the target voltage, the input to EA1 will tell it to run at full duty cycle until the rail voltage reaches its target voltage. The feedback cap that you recommended will help to quiet the regulator.
OK, so to make things simpler, EA1 is for the regulator feedback. EA2 os for the protection.

now, in the TL494, the two error amps' output are combined into one. it is not summing but sort of an open collector type so whichever has an output which will result to a lower PWM output, that will be followed. so if EA1 is at max, EA2 is at min. then output will be at minimum. and if EA1 is at min, EA2 is at max, then output will still be minimum. if both are at max, then output is at max.

if what you said is true, then my SMPS should not have the soft start function when the protection resets since my SMPS's are the unregulated types which have EA1 at max all the time and EA2 as the protection input. but my amps are working as it is designed.


Quote:
You can't have pin 15 tied to pin 3 for feedback. If pin 3 is tied to 5v, the IC will never produce output. If pin 3 sees anything over ~3.3 volts, there will be no output.
pin3 is both an input for the comparator (which turns triangle waves into PWM) and the output of both error amps all tied together with a CCS (from what I recall). in my case, I used it as an output where EA2 got its feedback from.
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Old 18th September 2005, 09:35 AM   #13
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I haven't seen your design but it doesn't sound the same as the one posted here.

It sounds like yours has pin 3 connected directly to 15 and that's OK but he has pin 15 connected to the 5v reg. If 15 is connected to 5v and pin 3 is connected to pin 15, then pin 3 will be at 5 volts which will completely kill the output.

The voltages on pins 3 and 4 are compared to the 3.3Vp sawtooth wave of the oscillator. When either is above the instantaneous voltage of the sawtooth, the output is inibited. Tying pin 3 or 4 to 5 volts will prevent the chip from producing output.

The output of the two error amps are connected to pin 3 via diodes (cathodes on pin 3).

Are you using the protection circuit for soft start on your amp?
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Old 19th September 2005, 10:34 AM   #14
Bosium is offline Bosium  South Africa
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Hi again

I spent the weekend prototyping, and I have tried a few variations, but I must agree that the protection circuit works exactly as stated.

I have wired EA2 as a unity gain buffer, i realise my schematic is somewhat unclear but pin 15 is NOT connected to 5V.

The amp turns off and on sweetly, and slowly. I will post an updated schematic soon.

Thanks for all the suggestions, I really appreciate the feedback.
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Old 20th September 2005, 05:52 PM   #15
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Hi Bosium

Great to finally find a place where people from South Africa can share their electronic stuff!!!

Can you maybe tell us what tipe of core you are using, where you got it and the winding details? Please??? If I can get all of the parts, then this is going to be my first homemade car amp.

Thanx alot,

Werner

Ps. My class D amp is almost finished, and I'll be posting it soon!
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Old 21st September 2005, 09:52 AM   #16
djQUAN is offline djQUAN  Philippines
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Quote:
It sounds like yours has pin 3 connected directly to 15 and that's OK but he has pin 15 connected to the 5v reg. If 15 is connected to 5v and pin 3 is connected to pin 15, then pin 3 will be at 5 volts which will completely kill the output.

Quote:
I have wired EA2 as a unity gain buffer, i realise my schematic is somewhat unclear but pin 15 is NOT connected to 5V.

Quote:
I spent the weekend prototyping, and I have tried a few variations, but I must agree that the protection circuit works exactly as stated.
I rest my case.

the schematic I used is at http://djquan.angelcities.com/smps.html
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Old 21st September 2005, 01:52 PM   #17
Bosium is offline Bosium  South Africa
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Hey Werner

Nice to see another south african!

PM me and ill send you my email / msn address and we can chat more - i am in the process of refining the design and sourcing the parts, but almost done and then i will lay out the PCB.

If you are interested, I can probably send you a board for the cost and i can tell u where to get the heatsink extrusion too.

Time i update my profile methinks, it still says im at tech i didnt realise

Class D you say? Only amps iv ever made are BJT power amps and I love them (Class AB), next project will be a 600W super leach with redesigned board and TO3P output transistors. Leach amps sound fantastic - I have built the original 250W ones.

Anyway, hopefully chat soon

Gareth
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Old 27th September 2005, 06:06 AM   #18
MOER is offline MOER  United States
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Hello there, from SA

Our company builds car amps in the USA. www.zedaudio.com

Why is your turn on cct for the TL so complex? A simple 2 tranny cct works fine without all that extra mosfet stuff

Yoyr cct shows the secondary xfr gnd as the same as the primary gnd - this = engine noise unless you float your preamplifier or it's inputs.

Pin2 of the TL494 should go to say 2.5v to keep it well within the common mode range.

Why are you using such a weird drive chip to drive the Z44s?

A simple totem with active turn off from the emitter outs of the TL does just fine and will turn off the Z44s quickly. Be carefull of leakage inductance. With such low value of gate resistors you may hurt the FETs.


Your 22 ohm secondary snubber will run HOT!

What is your turns ratio as this affects the final regulation of the PSU. We use a full bridge feedback in all our amps and this allows us to maintain 100% regulation. No need for optocouplers.

In your preamplifier try to gain up as early as possible and put your gain pot downstream just in front of the power amplifier. This way the signal is lifted high ASAP and thus lives way above the noise floor. The gain pot near the power amp input will give the best subjective S/N ratio at all positions of the pot.

Best way is too use a 4 gang pot, the first two in the feedback loop of the first gain opamp and the second pair as simple passive pots just in front of the power amplifier.

Make the gain structure of the power amplifier as low as possible and rather drive heavy signal into it. It's noise is lowered by doing this. We typicallly make our power amplifiers about 4 volt sensistive for rated power.

Hope things "back home" are OK. Have not been back since 1989.

Vat hom flaffie

Steve Mantz

Zed Audio Corporation
Los Angeles CA
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Old 30th September 2005, 09:27 AM   #19
Bosium is offline Bosium  South Africa
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Hi Guys

Herewith updated schematic.

Steve (MOER) heheh nice nickname I wonder if anyone there knows what it means. Can u still call SA home if u havent been here in 16 years? just kidding - im sure u miss it here.

Initially I used the 7667 as it is a nice solution but I agree that the totem will be better as it is somewhat more robust.

The turnon is simple, it IS just a 2-transistor cct, altho one is a FET. I like the FETS, very efficient anyway and cheap too.

I neglected to show that my snubber(s) are not fitted, and will only be fitted if need be (hopefully not). I also gave the wrong value resistor (56R not 22R).

How do you provide feedback to your PWM IC if you do not have a common ground and do not use optocoupling? How do you mean I should float my signal inputs? I can easily switch to optocoupling to circumvent this problem altho in my car I have never had problems with engine noise at all - very clean power supply I guess.

I have changed the opamp circuitry as you suggested, I feel I have neglected this aspect of the amplifier as I placed too much emphasis on the PSU. Do you think the gain structure is sufficient? I am considering changing the gain of the amplifier modules down to 26dB from 30dB as it is.

Any additional comments would be greatly appreciated - perhaps PM me if you have the time or email me?

Thanks to everyone who took the time to post.

Gareth
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Old 30th September 2005, 03:43 PM   #20
MOER is offline MOER  United States
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Dear Gareth,

On further examination there are some errors in your TL494 cct.

Pin3 cannot go to pin 15

Pin 3 must have aseries R-C to pin 2 for high freq stability. We use a 2K2 amd a 0.1mfd in series.

Use the 15-16 internal opamp for protection. We set pin 15 at 5v and when our protection circuits are activated (V-I, DC offset) we push the error signal into a latch (The amp is kept off) and shove the output of the latch into pin 16. This of course turns off the TL494.

The input impedance of your turn on cct is low and may load down some heads. Again why the complex cct with zeners, an extra diode and a mOsfet. I use a similar configured cct with an NPN and a PNP, 4 resistors and a small lytic cap. Cost = $0.079! hard to beat.

Why a 15v zener across the 12v? If you are using a regulated TL494 cct there is no reason for this zener, it does niks nie.

Please put 1N4002 diodes across the IN-OUT ports of your LM7815/7915 reguslators. These prevent latch up on turn on. These diodes must of course be in the reverse direction. These reg chips have this common problem. Ie the 7815 has anode to the output port and cathode to the input port.

On the secondary of your main xfr, use larger vale choke and use a common mode choke at that. We always use a composite core using about 20% ferrite and 80% iron powder. This gives the choke it's swinging value from many mH to microhenries from low to high current. This of course prevents crossconduction of you main rectifiers from blowing up your mosfets.

Also increase your supply capacitance from the 1000mfd values.

To obtain good regulation you should "overwind" your transformer secondary. Here are some numbers. Primary 4+4 turns, secondary 14+14 turns. This gives a 20% "overwind" and allows rated power at 12v battery supply.

We have all our transformers both bifilar wound and we interleave the primaries with the seondaries to reduce leakage inductance.

Your 0.1mfds primary snubbers will make a braivleis of your 56R resistors.

NE5532's have some offset on their inputs and if a pot is wired directly to these pins it can become scratchy over time.

Again your ground symbols are common throughout your schematic and this is bad as alternator whine will be a big problem.

By the way, Cape Town is my favourite city in the world. My Mom was born in Hermanus.

More to come

Regards

Steve
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