Defeating ripple voltage in pre-amps

[akis]

Here, I am only talking about hum as a result of ripple voltage entering the pre-amplifiers.

This ripple is present from the moment you attach *anything* to a simple bridge/filter PSU.

The filter capacitors discharge into the load continuously, but 100 times a second they also charge from the bridge - the result is a sawtooth, very sharp rise and a gentle fall.

Therefore, in my opinion, calling this voltage "DC" is plain wrong, it is modulated DC, it has a base and a variable component. The base component is the smallest voltage as seen on the oscilloscope, the ripple component is at the top of that.

This formula is a very good approximation if you do not have a scope: Ripple voltage peak to peak = I (current drawn) / 2 * f * C, where f is 50Hz for EU.

The more current you draw the worse ripple voltage gets.

This ripple voltage enters your amplifiers and generates hum at the speakers, even with pots turned to 0 sometimes!

This ripple voltage is vile. Assume a typical 12 V DC suppy and a current of say 150 mA to power all your pre-amps, LEDs, tone controls, mixers etc. Your 50VA transformer is man enough for the job and your 4000uF capacitors feel like an overkill for just 150 mA.

However, using the above formula, or a scope, will show a ripple voltage of 380mv peak to peak!!!!

This competes easily with the signal that my guitar puts out, and it completely obliterates other sources, eg a microphone. It will enter the base and the collector paths of my pre-amps and arrive at the power amp amplified dozens of times resulting in a huge hum coming out of the speakers.

There are 4 ways to beat it that come to mind.

1. Include RC filters everywhere you can. Or, if you know from the start how much current you will be drawing then you could have just a single R/C to power all pre-amps in your system. For more flexibility it is better to have local R/C filters at every amplifier / buffer stage, it means you can keep adding/modifying components without affecting the existing ones.

In order to eliminate hum, so that ear or scope cannot see it, I have found you need the single pole R/C to have a frequency of 1 Hz or better. For example if R is 100 Ohm then C needs to be 1600uF. Capacitors like this are bulky and cost quite a lot. You can increase R (which costs nothing) and decrease C accordingly - but increasing R means a greater voltage
drop, and if R exceeds 10% of the load you are driving then you will substantially modify the available voltage and you will have to sit down and recalculate all bias points for every transistor (more or less). Ouch.


2. Take a single, passive R/C, and add a zener diode then buffered by a bulky
transistor. You first calculate how many volts will be dropped on the R (the rest will be on the zener). You then choose an R to allow you to drive the zener so that it does not get hot (5mA is very typical for little, 0.5Watt zeners). Based on that R you choose a C to achieve the single-pole low pass of 1Hz (or better). You then bootstrap a heavy transistor
capable of withstanding shorts (eg a 50W/8A transistor can definitely survive temporary shorts from a smallish, pre-amp PSU). I used the audio transistor MJE15030 which has quite high hFE, and very linear as a bonus, unlike some other rubbish eg TIP31C/BD911 etc. When you connect all your pre-amps to this semi-regulated supply, there will be like 2 dozen 100uF capacitors all eager to be charged, and they will in general blow the likes of BC639/BC337 in an instant. Additionally it is more than likely that you will inadvertently short the supply yourself, and every time you do that it means a blown "lesser" transistor. Hence the need for an 8A transistor capable of withstanding even prolonged shorts. A 100uF capacitor at the emitter of the transistor is also needed to stop oscillations that might arise if you, for example, decide to power a fan from this regulated voltage. Of course, if you can, it is 1000 times better to power fans, relays, LEDs etc from the unregulated voltage and keep the "clean" voltage just for the amps where the audio signal will flow.

3. A combination of the above: a regulated supply and local R/C filters on every stage! Assuming that you can afford a 20-30% voltage drop on the R, then you can use smaller Cs and it should not cost that much. But you will have to design the circuits from the outset - you cannot easily do it retrospectively, eg if you have planned a 12V supply, you cannot feed it 9 volts and expect it to work just as well. Well, unless your circuits are somehow immune to voltage variations, like op-amps.

4. Use monster caps at the PSU. Remember we are talking pre-amps here, and for my typical example of 12V and 150mA, I would need something like 100,000 uF to achieve a 20mV ripple voltage! 20mV is still huge it terms of pre-amps...

I hope the above info will be of some use to someone :-)

[keantoken]

I like the idea of the pass transistor best. Try using a CFP instead of Darlington, as it will have lower output impedance (don't know about ripple isolation though, I think it's the same).

Look at the attached schematic for a simple ripple suppression technique that works well.

- keantoken

[454Casull]

That looks like a simple Pi (CLC) filter.

[keantoken]

Oops, I posted something obvious. Haven't messed with supplies that much.

- keantoken

[iko]

I tend to prefer a good, discrete, mosfet shunt regulator. The latest incarnation I got measures around 10uV ripple with a 200mA load, and less than 100uV ripple with a sine load of 40mA p-p over the 200mA DC. That to me is acceptable even for phono stages.

[Jan Dupont]

Use shunt regulation

[wwenze]

Well these are somethings not so obvious to newbies like me though.

[Jan Dupont]

Do a search here for Jung regulator (Jung super regulator)

[akis]

Originally Posted by wwenze

Well these are somethings not so obvious to newbies like me though.

This is the essense of my approach: to design (and understand) as much as possible from first principles. My zener regulator plus R/C filters comprise of 4-5 components in total, and I have experimented and seen on the scope and heard on the speakers the effects of ripple, filters, different capacitors, oscillations, blown transistors due to accidental shorts, current limiting etc.

It would feel like cheating to use someone else's design with two dozen components and techniques I do not (yet) understand - I might as well go buy a ready made guitar amp :-)

I attach the final circuit I came up with - nothing clever but hum has almost disappeared. I will try (to understand) the shunt regulator suggested here on my next project. Assuming I ever finish this one :-)

[peranders]

akis, it all depends of the circuits you want to feed. It's a big difference between some opamp circuits and a MC preamp built with discrete parts.