General response by Rainwulf (3/2009) I did electronics training in the army, and this PSU circuit has some inconsistencies.

The primary one being that these snubber circuits that consist of a cap and a resistor are designed for one thing only, that is, the suppression of high frequency ringing that is caused between the capacitance of a switched off , and the leakage inductance of the secondary.

The resistor part of the snubber circuit is designed to dampen the ringing caused at switch off, with the chosen only to reduce power dissipation in the resistor during the rest of the sine wave.

The capacitor passes the high frequency ringing through the resistor, which then dampens the ringing to a single pulse (ideally), thus limiting the RF noise going down the line to the main filter which at this frequency, have too high an inductance to effectively filter it. This high frequency noise is will cause intermodulation and will affect SN ratio, as well as increase the chance the chip amp will go into oscillation, especially as a lot of "audiophile" circuits either reduce or eliminate completely the at the output.

This snubber network is at once and completely separate to the issue of higher capacitance main filter caps reducing the high end response, and causing "slow" supplies which dampen high end.

The only reason this occurs is that as you go up in capacitance value, you go up in esr and inductance of the cap, which limits the peak current capability of the supply to power the amp during high current peaks, and will also lessen the damping ability of the amplifier, as the inductance of the main caps will limit their ability to sink and source current.

So far, there are two completely separate effects in place. 1. the damping of the high frequency ringing of the transformer secondary and the capacitor that is momentarily caused in a standard silicon diode during switch off. This is an LC network, with damping resistor R, where L is the secondary, and C is the capacitance of the .

2. the inductance of the main filter caps creating a current shaping network of C and L and R, with all of these originating primarily in the capacitor itself.

Using the principle of GIGO, that is, Garbage In, Garbage Out, there are multiple steps here to go through.

One. Reduce to as high as degree as possible any RFI/EMI interference going into the circuit. This includes using an rfi filter on the mains input and shielding the transformer. Twisting the leads in and out of the transformer will also help. As well, the leads into and out of the transformer (especially in a toroid) should be perpendicular to the core and straight for at least a few inches, this helps reduce stray magnetic field pickup from the core.

Two. Type of rectifier, two schools of thought here, standard block silicon , or high speed schottky . The cons and pros are as follows. Standard Bridge: Slow speed, higher and current ratings then high speed diodes. Cause HF noise on switch off. Can handle brief current surges far in excess of the rating. Schottky: High speed, lower current and voltage ratings then standard diodes. More expensive. Not to capable of high current surges. I personally prefer bridge. Cheaper, easier to mount onto a heatsink, and the four diodes in the bridge are matched because they are one silicon assembly. Its up to you here. If you use standard bridges, you need to read the next section on snubbers.

Snubbers: Only needed with standard "slow" silicon rectifiers. On switch off, when the diode is reversed biased, there is a momentary conduction in the opposite direction as the "holes" in the silicon are filled by electrons. Once filled, the diode is then an open circuit. During this filling, the diode creates HF noise as the electrons are shunted around. Once filled, there is a barrier region between the two P and N elements of a diode. This non conductive barrier region is effectively a capacitor. Its a capacitor, directly connected to an inductor. ( the transformer) This LC circuit has a resonant frequency that is excited by the noise of the collapsing diode barrier. This noise is carried along the DC line to the rest of the circuit, and its often a very high frequency, 400khz or above that quite easily radiates into space as much as propagates along the conductors.

So, you must SNUB this oscillation. Putting a capacitance in parallel with the transformer secondary is BAD!!! DONT DO IT. It effectively adds to the capacitance of the diodes, increasing the C of the LC circuit. This brings down the frequency of the oscillation, possibly into the high range of the sound band. I say again, do not put a capacitor of any value across the secondary of a transformer.

Its a LC Tank circuit begging to be energized by any noise.

The only component that needs to be put after the secondary is the diode bridge. This is where the snubbers come in. They are simply a resistor of a low value, 10-20 ohms, in series with a capacitor, 100nf is common. The cap is NOT part of the tank circuit of the diode/transformer due to the resistor. Its only job is to allow any HF noise created by the diode its attached to pass through the resistor, where the resistor will then dissipate it as heat. Not much, a few microwatts, you can safely use a 1/2 watt resistor.

As you can see, the R and C combination serve as the brakes on any possible oscillation caused by the diode reverse capacitance, and the inductance of the transformer. This finishes the First part of this post, being the role of a snubber. Now, to the aforementioned lack of the main filter caps to provide high current pulses to the amplifier.

The bigger the cap, the more inductance. The higher ESR. The less the amp has the ability to reproduce High Frequencies, which leads to the fake assumption that a smaller cap is better. Which is why some gainclone amps suggest 1000uf caps for main filter caps, or even less for the "audiophile". Right assumption, wrong solution as most have found out that a cap of 1000uf is barely enough to provide bass, and can reduce dynamics and can also increase hum. A small cap has lower ESR, and Lower self inductance.

The solution as shown in this thread is to NOT put a snubber network on the rear end of the PSU. All it will do is snub any HF noise that’s made it down the circuit from the diode bridge. Its a good solution, to the wrong problem, in the wrong place.

Put the snubber where it belongs. One per diode.

To solve the problem of higher inductance in bigger caps, you don’t add an RC network. An rc network by definition has a specific rise and fall time. You want the quickest RISE and the quickest FALL to help provide sink and source to the rather messy load of a loudspeaker. The simplest solution to this is to provide a low esr/low inductance cap close as possible to the chip amp power leads.

The caps that are in switch mode power supply secondaries are an ideal choice if you can find the right . The closer the cap is to the amp, the less inductance there is, which means a quicker rise and fall time. Inductors RESIST current change. Capacitors RESIST voltage change. You want your amp to be supplied with current from a power supply that can easily supply 1 amp to 10 amps without a delay, and vice versa. The less inductance you have, the better. The less resistance you have the better.

This leads to the best solution, find a set of caps with the lowest esr and self inductance possible, and mount them as close to your chip amp as possible. Make the wires between the amp and your low esr cap as thick as possible, and as short as possible.

A good compromise is to add a set of smaller caps to your amp rails. Lets say you have 2 5600uf caps per rail. Add two 1000uf caps per rail. Maybe even a 470uf. The smaller the cap, the lower its inductance.

Think of it backwards.

When a chip amp looks at its power supply, it wants the power supply to be a very low impedance. When the chip wants current, it wants it NOW. not in a few microseconds, as you wait for your inductances to rise in magnetic flux to allow the current flow. YOU WANT IT NOW.

This is the definition of a "FAST" power supply. It can go from 100 ma to 10 amps in a flash. This is quite simply achieved by following the rules of high frequency/high current design.

Thick Wires. Short Wires. Low inductance and low ESR capacitors.

To sum up.

Snubber networks, being the R and C combination, should go on your diodes. Close as possible, short as wires as possible. Shield your psu. GIGO. Low inductance. Use a set of high value capacitors to give your amp your bass, but have a set of lower value, low esr caps to provide that initial current burst while the main caps ramp up their current output. Short Thick Wires.

Cheers.

Response to above by AndrewT thank you!

A simple, but accurate, explanation of what a PSU should be capable of doing and how to help prevent it doing what it shouldn't.