DC on the mains

The parts I ordered at the end of last week have already been delivered.

Just waiting for the PCB now, which I will hope will arrive by the weekend, so I can build and test it, and hopefully find the space and time to install it.

I wanted to test the "DC blocker" in the real world, so I built what I drew earlier in this topic, and tested it on the 'scope using my current probe. I used a regular toroidal transformer rated 160VA. I also measured the AC current.

It does work, but not as good as claimed. The upper current waveform could be seen to sag ever so slightly. My test interference comes from a Candy auto washer.

However, without the "blocker" it sagged considerably more, so it's good?

Well, it has to have losses, obviously, and with the "blocker" the mag current read about 15mA, and without it 18mA.

These are rough measurements plus or minus a few fractions of a milliamp, but shows there is a loss.

My interest in this is for the '70s amp so as to prevent deterioration of sound quality during interference. The SQ is excellent in the dead of night, but not as good when the automatic washer or other daytime things are on.

Now, the '70s amp has a high mag current transformer for reasons of regulation, but it is still a toroid - a rather special one the transformer manufacturer has worked hard to develop. It has 80mA mag current, so should not be taken so "one sided" as a regular toroid, but as the SQ depends on this mag current, then a partial collapse, would, I think, worsen the SQ.

It will take the total mag current down slightly as demonstrated above, but perhaps only by a few mA, so should be OK. The important thing I think, is waveform symmetry.

Does it work? First impressions are it does, but I'm not so easily kidded, and will give it time.

I'll do the 'scope shots shortly.

Making a safer "DC blocker" (or transformer primary current centraliser)

Not much room in the amplifier case, so the blocker had to be made compact enough to fit in any spare space away from sensitive parts of the circuit.

Using 4,700uF capacitors instead of 10,000uF improves the blocker's response. I chose capacitors rated for 6,000 hours at 105C, with the hope they'll run cooler extending their lifespan to years.

I also chose the 25V rating as the ripple current is about 250% larger than the largest expected peak current, for this particular capacitor.

The proximity of the rectifiers to the capacitors was a worry in that the rectifiers can dissipate 1.5W each in this application. However, that is at full amplifier power which only ever happens on the test bench when testing for maximum output. Normally the rectifiers will dissipate less than one watt.

I decided to attach a thermocouple to monitor the temperature in actual use.

The insulation has to provide 100% coverage meaning encapsulating the capacitors in heat shrink tubing. I would not normally recommend this as the safety vent is bunged-up, and if the capacitors were ever to fail and liberate their contents, it would create a very loud bang. However, I am the "Guinee pig" and will test this over a long period.

If the temperature is kept well below 105C there should never be a liberation. As it would be contained inside the amplifier case I would venture to say it should be safe.

The best I could imagine to fit the minimum space

Connections soldered (using another short piece of stiff wire through the middle hole to solder the common capacitor ends together either side).

The capacitors are insulated with some 3-1 heat shrink tubing with as little heat applied as possible.

The thermocouple held temporarily in place with a rubber band

I made use of an IEC connector insulating boot to cover all the live connections, held in place with a cable tie.

This is the finished compact "blocker" ready to be located inside the '70s amp.

I am happy with the lack of humming traffos now. Does it make a difference to the sound quality?

I have no idea…