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
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
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.
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.
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.
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.
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
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
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.
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.
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
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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?
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?
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.
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.
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!
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!
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
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
Car amp
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
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
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
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
Car amplifier
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
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
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.