Suitable mains filter for amp PSU

[tonga]

Hi all,

I've read through various other posts on mains filtering but cant seem to find any definitive answers to my questions, those being:

Is it worth using an integrated IECsocket/filter at all on my power amp supply (an active PSU)...yes or no. I may build one to the designs I see floating about on the web but for now I need to get my amp in a box pronto to appease the missus..so to filter or not to filter that is the question.

If it is a good idea would this one be up to task, its a two stage hi-performance jobbie from belling lee:

FILTER, 2 STAGE HI-PERF 3A; RoHS Compliant:NO; Voltage rating, AC:250V ac; Current rating:3A; Phases, number of:1; Length / Height, external:40mm; Width, external:53mm; Depth, external:75mm; Capacitance, X type capacitor:0.15µF; Capacitance, Y type capacitor:2.2pF; Capacitors, X type number of:2; Capacitors, Y type number of:2; Centres, fixing:75mm; Current, leakage:0.2mA; Diameter, fixing hole:4.7mm; Frequency, supply max:400Hz; Frequency, supply min:0Hz; Inductance, L1:2.2mH; Inductance, L2:2.2mH; Inductance, L3:0.3mH; Resistance, bleed resistor:1MR; Temperature, operating (a) max:85°C; Temperature, operating (a) min:-25°C; Terminal type:6.3 x 0.8mm fast-on

I notice that the filters rated "medical" have a much lower current leakage, would they be a better choice?

thank in advance

tonga

[richie00boy]

TBH unless you have a real problem with interference, they are not really all that necessary. If you are still wanting one, try to get one with a surge supressor in as well.

Medical filters have lower leakage current at the expense of attenuation.

[real]

Hi Tonga,

I construct a simple mains filter which I have attached; sorry about the bad drawing. This filter simply takes active and neutral in (no earth) and the output connects to the transformer. I am not sure what values of L and C are ideal, but I used about 100uH and 0.47uF. Please note that the capacitor must be an X rated type (for connection between live conductors).

I actually got this circuit from the Morgan Jones' book 'Valve Amplifiers' and the reason why there is no earth involved here is because any filtering to earth would introduce mains frequencies on the earth, making it noisy; not ideal for audio.

I am not sure whether this filter actually makes a difference to anything at all or not (sound wise) - I can't hear any difference. I suspect that having good regulation would have more effect. If you have time, try with it and try without it. If you can't hear it, why bother? Who cares if someone else thinks they can hear it.

Regards,

Chris.

[jackinnj]

this will work, but it is more important to know what kind of interference you are receiving -- (and transmitting) so that you surpress the right harmonics and birdies on the power line: the small caps can be 1kV disc ceramics, the 100nF cap is line rated polypropylene. the choke is wound bifilar.

not that all (well almost all) switch mode power supplies have nice filters at the AC input -- these are one of the few things worth removing and implementing in our designs.

[tonga]

Thanks for the replies chaps....I think I'll probably leave out the filter for the moment.

I had the amp wired up on a bit of wood using an mp3 player as a source as I dont have an attenuator (its on the way as we speak) and the player allowed me to control the volume, anyway the amp sounded sweet as a nut even with crap hookup wire/RCA's dangling all over the place and plugged into the same surge protected 4 gang with my PC. I guess I'm not experiencing enough mains noise to warrant the filter, but I'll reasses the sonics once its boxed up with decent wiring and then use one of your solutions if needed.

Thanks again for the replies

tonga

[kilowattski]

A simple Corcom IEC AC input connector/RFI filter would be my choice. The are fairly inexpensive, real estate frugal (don't take up alot of room) and available in many current values. Lastly, they seem to work very well.