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Old 1st October 2005, 12:42 AM   #21
Bosium is offline Bosium  South Africa
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Hi Steve

Thanks for taking the time to reply.

I have prototyped that circuit with the 15/16 opamp configured as a unity gain buffer and it works very well infact, and it softstarts slowly when it comes put of protection. What exactly is the problem with it?

I have increased the resistance after the remote line very slighty while still maintaining 4 or 5ma or so through the zener and at least 1V across the base of Q8. This I feel should not stress the headunit.

As for my poor FET, i like using them
I agree in terms of cost engineering it isnt as good as two BJT's, but it works extremely well and I am not trying to get this amp through production - merely making it for my own pleasure. I have a whole bag of these FETs too
The 10uF is to keep it on for half a second or so and protects it from turning off under mild brownout conditions. The zener is to protect the FET from gate-source overvoltage. Not really necessary I guess cos it's 20V max and thats unlikely to be found in this circuit. Force of habit i guess - Poof! It's gone.
Again the S3G diode was there to provide a small drop to help keep the ICL7667 well within its SOA - but poof! its gone too


The 15V Zener is in fact a Transorb, or transient voltage suppressor and protects the circuitry from voltage spikes much like a zener would but far far quicker. It can absorb huge amounts of power for a very short period of time. It was initially there to protect my ICL7667 which is CMOS. I agree it is not completely necessary anymore, but it will not hurt and will protect the amp from reverse voltage connection until it or the fuse blows.
I work for a company that develops fleet manager on board computers for Siemens/VDO and we use just such a device on all our products. I have often come across a faulty unit where the transorb has blown but invariably it has blown into a dead short and in doing so protected the rest of the circuitry in front of it.

I have moved the diodes on the 7x15's as per your advice, and decreased the values of my primary- side snubber caps.

In this later revision of the design, there are no pots wired to the inputs of any 5532, only to the input of the amplifiers themselves.

As for the secondary inductors, I am a little hazy on this. Can I use a ferrite core like my main core or must it be iron? By common mode, do you mean both inductors wound in opposite directions on the same core? What value inductance would you recommend? I will be winding this(these) and my transformer myself on toroids.

As for isolating the grounds, should I just go the opto-isolated route with zeners? This works well and uses both rails for feedback. I thought isolating the grounds would not be necessary. I could also improvise a virtual earth with a voltage divider and an opamp, and then connect this as my signal ground and then connect it to power ground through a 100R or so to keep it near true ground potential.. what do you think?
I have implemented optocoupling with separate grounds in the latest revision.

Again, do you think the gain structure will be sufficient as is? I was worried about too much attenuation through the LPFs. I have a gain of two for the signals being fed through the LPFs, but no additional gain on the full range signal. The amp module has a pretty high gain configured as is tho.

Sorry to send u such a barrage of questions mate, perhaps a msn or skype chat would be quicker..

Anyways, thanks again.

Gareth
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Old 1st October 2005, 12:46 AM   #22
Bosium is offline Bosium  South Africa
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And the Amplifier Module circuit:
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File Type: pdf amp module rev2.pdf (19.0 KB, 187 views)
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Old 1st October 2005, 09:12 AM   #23
bowdown is offline bowdown  Australia
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Sorry if i have posted this in the wrong section but i guesses as the post says"car amp design" it might be ok to post my question here.

In Rods project89 he uses an SG3525 ic, can i directly substitute a TL494 ic for that one?

thanx

Regards

Bowdown
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Old 1st October 2005, 02:48 PM   #24
djQUAN is offline djQUAN  Philippines
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nope. you cannot use TL494 without modifications.

try the circuit that I have. http://www.djquan.angelcities.com/smps.html
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Old 1st October 2005, 03:16 PM   #25
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If the Transorb is anything like the transient absorbers I found, it won't work as you want it to. The reason is that it has too high of a forward voltage (~0.8v). The intrinsic diodes in the Z44s have a lower forward voltage and will pass virtually all of the current. If you want to help protect the amp from reverse polarity, use another MUR820 across the power supply.

For the optocoupler circuit, you may want to use a low pass filter on the LED side. Also, there's essentially no reason to go from rail to rail since there is no way to control the positive rail and negative rail outputs independently. If you were building a mono amp, then there may be some argument for it but a bridgeable stereo amp will essentially load both rails evenly.

The instability of the PWM control circuit (if there is any) may not be noticed until you do some critical low level listening on some good speakers in a quiet environment.

For the totem pole drivers, you could tie the emitters together and use a single resistor. The resistor is actually not needed but can serve as a sacrificial component when/if the FETs fail. A 1/8 watt 10 ohm resistor would be enough to do the job. This will help to protect the drivers and the gate resistors in the event of a failure.

In my opinion, the overvoltage lockout isn't really important. Even if your alternator's regulator failed and caused it to produce 17+ volts, the amp would likely be OK. If you freed-up that op-amp, you could use it for the soft-start. Then you could tie the protection circuit to pin 4 instead of pin 16. This would allow you to use feedback between pins 2 and 3 for stability. Pin 16 would be tied to ground. Pin 15 could be connected to 5v.
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Old 2nd October 2005, 05:26 AM   #26
MOER is offline MOER  United States
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Default Car amplifier design

Dear Gareth,

Here are some pointers.

TL494 - should not be operated with pin 3 joined to pin 15. You must have loop compensation between pins 2 and 3 - a series RC is fine. Simply join pin 15 to 5v ref.

Use a simple form of single ended feedback. Simply put pin 1 to a pair of resistors, one to ground and the other to +35v. The gnd end use 2K4 and the +35v end use a 33K series a 5K trimmer to set final HT.

Put a cap from pin 12 to gnd - 10-47mfd OK

Take out those 10 ohm resistors from your totems they only hurt performance.

If you wind your transformer with interleaved windings the leakage inductance will be low and then you can use lower gate resistors which will improve the switching of the Z44s

The common mode coil MUST be on a composite ferrite/powdered iron core of about the same size as the main xfr. The PI must be about 80-85% of the core and the ferrite (same material as your xfr) the balance.

Use #15 wire (or 2 x #18 in parallel) for each winding

Wind one winding around the complete circumference of the core with one layer. Start this winding at say 6 o'clock on the core.

Eind the second winding starting at 12 o'clock and place the same # of turns as the first winding.

From say your +ve rectifier come in on the first winding at one end (You choose this). Now the -ve rectifier connects to the other winding but on the end that is opposite to the first.

So if the first end came out of the top of the core, then use the other winding's end that comes from the bottom of the core.

If you mess this up you will find out quickly enough when you tuen on and look at you switching waveform on a scope. Always turn on through a series 2 to 4 ohm 50w resistor which is is series with your +12v supply and then bypass when power testing. We have done this forever and it sure saves amplifiers.

You MUST separate your power grounds (primary side of xfr) from your audio grounds (secondary side of xfr) To make the simple single ended feedback work put a 220 ohm 1 watt resistor between these gnd. if they are joined you WILL get alternator whine as there WILL be a ground loop once you connect your RCA inputs.

Pre-amplifier section:

Get rid of U! as it serves zero purpose. Your schem is wrong as U1 has no bias on the + inputs.

Change U5 to an inverting stage with a gain of say 2 (This will limit your maximum input to about 4.5 volts and I know of no head that puts out this kind of signal) I have tested too many heads with BS specs and NONE of them put out more than 1 to 1.5 volts at full volume. Configure U5 with say a 15K input resistor and then use a 30K feedback resistor with a 22pF in parallel.

Your LP section has 0dB gain in the pass band so there is no need for gain up. Simply split your signal from the inverting inout stage and feed the LP opamps together with the selector switch.

Now from the poles of your selector switch feed a variable gain inverting opamp stage which does two things. It brings the signal back to normal phase and it allows you to vary the gain over a wide range.

Let's assume we shall have a 200mV to 4v input sensitivity and that the main pwr amps can put out 22 volts.

Make the main amps have a gain of 11 x so they then need 2 volts to drive them to 22 volts.

The variable gain inverting stage is made from 2 resistors a dual pot and 33pF cap. it looks like this. Ground the + input of course.

Join the -ve input of the opamp to the slider of the pot. From the CCW end put a resistor to the output of the opamp and this is a 5K8 in value. Then from the CW end put a 1K6 which is fed from the poles of your selector switch.

Put the 33pF cap from the output to the -ve inputs. The second channel is identical of course.

So your full range signal path has only two opamps in it and your LP has the Linkwitz opamps as well.

This makes it simple and elegant.

Back to the power supply.

There is no need for over voltage protection as you have a regulated power supply.

Add more capacitance to your main 35v supplies, 1000mfd is not enough, go to at least 4700mfd and use 50v caps

There is no need for the 15v zener across the +12v supply

Put back diodes across your 7815/7915 regs to cure their latch up problems on turn on. 1N4002 are OK

Not knowing your power amp schems I cannot comment.

I am not a big fan of NE5532 there are many nicer opamps which sound way better.

By the way Cape Twon is my favourite city in the world. I lived in JHB all my life but went to the Cape many many times. I just love the scenic drives and Paarl was the best.

Is Chapman's Peak drive still open? That is a spectacular view which these guys in the USA can only dream about. Is the Hout Bay hotel still there? It used to have a restuarant called the Plaza Espania which served a Sunday evening buffet for R1.50 eat what you want --- crayfish tails till you burst and all the other good things we like to eat. This was back in the 60's and in the 70's when the price went up to R7.50 per head I thought we were being ripped off!! How times change.
Regards

Steve
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Old 2nd October 2005, 02:23 PM   #27
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Assuming that the inverting stage is left as is...

If U1 is removed, the amp (in full range) would not be able to be driven from a passive volume control (i.e. a pot inserted in the preamp line). It would have to be driven from an op-amp (used in virtually all commercially available head units and signal processors). The reason is that the resistance of the passive control changes the gain of the inverting section. This would cause the gain of the left and right channels to be different.
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Old 2nd October 2005, 05:27 PM   #28
MOER is offline MOER  United States
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Well NOBODY I know of runs their car amplifiers from a passive volume control. All car systems I have EVER seen have a head unit either CD/Cassette or Mini disc. To the best of my knowledge no head has a simple passive volume control. They all have electronics post the volume control and furthermore it would be suicide for a head manufacturer to supply a unit with a passive pot on it's output. The output impedance would be high and varibale over a wide range and any cable capacitance (5 metres or so) would hurt the high end.

Why one channel would unbalanced from the other (besides the natural inbalance between sections of ALL potentiometers) is beyond my knowledge of electronics.

"If U1 is removed, the amp (in full range) would not be able to be driven from a passive volume control (i.e. a pot inserted in the preamp line). WHO DOES THIS? It would have to be driven from an op-amp (used in virtually all commercially available head units and signal processors). The reason is that the resistance of the passive control changes the gain of the inverting section. YES WE KNOW THAT This would cause the gain of the left and right channels to be different." WHY?????

If the sliders of a dual passive potentiometer drive two identical invering opamp stages then the two channles WILL track equally within the tolerance of the potentiometer.So forgive me I do not understand the purpose of your post

Stephen Mantz

Zed Audio
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Old 2nd October 2005, 08:55 PM   #29
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I know lots of people who hate the digital volume controls whether they're rotary encoders or up/down switches. They often use a potentiometer mounted in their consoles to have a more convenient/user friendly control. Sometimes it's done for the signal feeding all of their amps and sometimes just for the bass. The Radio Shack 100K stereo volume control is the pot of choice in most cases.

The gain of the inverting op-amp is dependent on the input resistor. When the pot is near the middle of it's range, the resistance between the wiper and the signal source is added to the value of the input resistor. The higher input resistance lowers the gain in the middle of the potentiometer's range of travel. This will cause a gain imbalance between the left and right channels.

In his amplifier, if he removed the input buffer, in the full range setting the left channel signal drives a non-inverting input. On the right channel, the signal drives an inverting input. These are not two identical input stages (when you remove the buffer and run it in full range).

I noted that all commercially available units used op-amps on their outputs.
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Old 3rd October 2005, 12:17 AM   #30
MOER is offline MOER  United States
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Dear Perry,

I did not ask Gareth to make one channel inverting and the other non inverting. I asked him to make BOTH channels inverting.

Good luck to those who want to run the outputs of their head into a 100K control and then run this fragile high impedance signal down an RCA cable to the amplifiers in a the nastiest noise situation an audio signal finds itslef - car vehicle.

For those who believe that one more digital pot will make a big difference - good luck. Does anyone know what the small micro volt signal wentr through to finally get put onto a CD or Vinyl?
If they did they would not worry too much about one more pesky control especially in a car!

Steve
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