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Old 28th August 2009, 02:55 AM   #11
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Interesting! would you mind posting the circuit diagram of what you did? I'm sure I'm not the only person interested in ways of making a zero ripple supply

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Old 28th August 2009, 03:30 AM   #12
iko is offline iko  Canada
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It was probably a grounding problem in the first place, which went away as you modified the circuit. In any case, it's good that you found a solution. If you want an order of magnitude improvement you may want to have a look here
The simplistic Salas low voltage shunt regulator
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Old 28th August 2009, 05:49 AM   #13
fotios is offline fotios  Greece
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Quote:
Originally Posted by akis View Post
I wonder why the LM317 regulator I tried earlier did not achieve the same (or better) result.
Maybe because you did not used the proper bypass electrolytic caps connected accross input/output of LM317 and GND node.
In the pic is the Bryston ripple eliminating circuit that i told you. I tried it and except that is very simple, it is also very effective. The only mistake, it is the zener (1N4760 - 68V) connected between the bases of transistors in schematic, because does not offers enough bias current to make the transistors conducted, consequently there is not output voltage. By replacing the zener with a resistor between 15 to 22KΩ (22K it is prefferable because is not warmed so much like a 15K) the problem of biasing is resolved and we can get a very clean output voltage free of ripple by 90%.

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Old 28th August 2009, 07:29 AM   #14
fotios is offline fotios  Greece
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Quote:
Originally Posted by ikoflexer View Post
It was probably a grounding problem in the first place, which went away as you modified the circuit. In any case, it's good that you found a solution. If you want an order of magnitude improvement you may want to have a look here
The simplistic Salas low voltage shunt regulator
Indeed, it is a very inspired circuit but with a wrong title like "simplistic". With 6 BJTs and FETs and arround 8 to 10 caps-resistors-zeners included, this circuit is not so much simplistic. Instead, it is some complex. It has not to offer any practical solution to ripple noise of akis preamplifier which is oriented for musical instrument use. Instead, it is well suited for other circuits of high gain or big sensitivity which demands the zeroing of supply ripple remaining and other noises, like RIAA stages.
Please, don't missunderstand me but i wish for everyone to be very cautious in his suggestions and propositions according to the circumstance of a concrete circuit for which is asked help from any member here. I have suffered from non-related replies to my agonizing questions, enough times in the past here. Fortunatelly, i am 35 years in the round, and by 99% i find solutions in the lot of the service manuals which i have in my stock. Schematics from the service manuals of well known companies, are a priceless treasure!

Fotios
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Old 28th August 2009, 09:28 AM   #15
akis is offline akis  United Kingdom
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I think this is the categoric answer of what went wrong. The simple PSU intended to supply all the pre-amps in my guitar amp produces normal ripple, nothing out of the ordinary, for example as I said in the original post, ripple of 80mV p2p with a current drawn of 36 mA. The transformer / PSU can provide up to 4 A at 12 V (but is probably AC at that stage).

This normal ripple entered some of my pre-amps (not all), and created this huge hum. I tried modifying some of the pre-amps to no avail, it only made things worse and it is hard modifying on a printed PCB, makes such mess.

I did the following measurements. I tested drawing various levels of currents, up to 1.3 A, which is 25% of the rated total (of my PSU). I measured the ripple voltage right at the PSU filter. I also measured the voltage after the regulator. I also tried different regulator types, including a LM317, at various loads, from 36mA to 1.3A.

I must say here that on the scope, even at the 5mV division, it is very hard to see "trace" ripple, my ears are usually better at detecting "no ripple" than the scope. I had a pair of headphones on for that reason.

The simplest way to reduce ripple, since we are talking a few hundred mA of current at most, is a fat resistor and a capacitor after it (RC). I did some experiments and the ripple can be halved, or better, but is still there, and my pre-amp faithfully amplifies what is there it. :-)


I also tried the common base transistor at the rail line suggestion, it did not achieve much.

I then tried two separate LM317s on different boards and the result was the same, very bad regulation and half the ripple through (with another shape, very assymetrical, but ripple never the less). I have no idea what's wrong with the LM317, perhaps that is how it works, I tried adding various capacitors at its adjustment pin, before, and after, no luck, the ripple remains.

I finally tried the a combination of the RC circuit and zener/diode regulation amplification. That seemed to work wonders. The final diagram is below. The sries voltage sense and associated circuitry is for shorts protection and may add some modicum of ripple reduction too.

I also added a 12V 12 cm fan and its noise is also completely eliminated , even though it draws a hefty 80mA and not only increases ripple dramatically at the PSU, it also introduces its own noise while spinning. The fan was added at the PSU, BEFORE the regulator.

I cannot remember what current I tested it for, the combination of R2/C5 is used to eliminate ripple at the chosen current. I used my ears for that measurement because everything was a line at the scope at that stage.
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Old 28th August 2009, 10:01 AM   #16
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Thanks Akis
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Old 28th August 2009, 09:22 PM   #17
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Quote:
Originally Posted by akis View Post
thanks for all comments.

The NPN transistor in front of the PNP did not fix it but resulted in a rather messier PCB.

However I eliminated ripple completely by using a typical common collector regulator comprising of a single NPN transistor in line with the positive rail (there is no negative rail), and a zener for voltage reference at its base bypased by a capacitor. Taking the voltage reference straight from the zener into the base of the transistor halves the ripple, but this is not good enough. So I used two RC networks in series before feeding the base and there is absolutely no ripple at all any more and the hum is gone!! I wonder why the LM317 regulator I tried earlier did not achieve the same (or better) result.

The problem with that design is feedback through the power supply from output stages into the input stage.

I always decouple the first stage with a resistor and a large capacitor.
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Old 4th September 2009, 01:24 PM   #18
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OK, so ripple did not go completely due to gain and loading. I did some more experiments and will post a new thread will all my observations about ripple/humming noise and how to defeat it.
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Old 4th September 2009, 09:19 PM   #19
fotios is offline fotios  Greece
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Akis, because i am faced 1 month ago with a similar problem like your (the only difference it is my project which is a power amplifier with +/-60V supply rails and a a gain of 41) i try to transfer in you my experience. The main problem of my project it is the noisy core of the 500VA toroid xformer. This amplifier it is no so simple, because includes balanced and single inputs and potentiometers for volume control. By some way is like an integrated amplifier. The drive boards (which are implemented with discrettes and not ICs) in which included the balanced to single converters and the volume pots, are seperated from the main amplifier boarbs. Although each one circuit it is very noiseless, when joined between them it starts the hum and buzz caused from the crackling noise of the core to heared enough loud in the output. I have redraw for second time the whole amplifier but the noise reduced only in half. After enough research i found that the pot which is connected between the output of drive board and the input of amplifier must have a value of 1,5KΩ as much. This eliminates almost completelly the noise. I use a pair of very sensitive AKG headphones to hear the noise exists. Ths moment, the buzz noise has the same level with the hiss noise (between 0,8 to 1,2 mVrms measured with the DSO), but it still exists! Of course, a so small noise is inaudible from the most senseless Hi-Fi speakers of 90dB/1W. But this issue becames an obsession for me. I have discovered as well, that the alu plates which used to join the case and those used for covers, increases the noise when are mounted on the case of amplifier. I don't know if plates (3mm thick) behaves like mirrors or conductors of electromagnetic flux. I think that are like mirrors. For this, these two days i am in drilling proccess of thousands ventilation holes in the top and bottom covers, under and over the toroid. If my assumption is correct, then the holes instead to reflect the magnetic flux, they may cause successive refractions of flux to eliminate it (like STEALTH ). I will inform you in Sunday for the result, when the drilling (with milling machine) will completted.

Fotios
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Last edited by fotios; 4th September 2009 at 09:21 PM.
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