Lt filter idea

Would making a filter from common and diferential mode chokes, as used in mains filters, combined with large value electrolytic caps be usefull for filtering 50/100hz , as well as harmonics of, in a regulated LT supply output?
Theres a number of times Ive gotten noise down to mostly resistor noise ,but a small residual low frequency component is nagging in the backround and it comes from my heater supply ,a small  ,maybe 30db additional reduction would render this noise totally masked by circuit noise .

So lets say, two common mode chokes in series ,large caps 1000uf or more ,across the sections , then two diferential mode chokes in series with each output followed by another large cap to ground off each leg .These chokes are normally used with small caps ,will they work as effective filters at low frequencies with large electrolytics ?

Would there be a benefit to applying an extra filter stage post regulator ?

In most of my preamps resistance noise already dominates hum by a large factor  , but its just to remove that final remenant of the 50 hz thats very difficult . In normal usage this small hum is buried anyway , but in the case of very low input level from the mic it is there in the backround .

Just in case someone mentions grounding ,thats all as good as can be ,power and signal are on different chassis also and Ht hum is in the uV range.

Have you tried 12V DC for heaters? An SMPS w/ choke would be a very quiet heater supply. You could easily elevate it too since the whole thing would be floating.

Otherwise, what you describe seems unnecessary, expansive and bulky. Depending on power, you might need a huge choke to get the 60Hz. And it's not even crystal clear to me that it will even work. What is the choke filtering precisely? Obviously not AC heater supply!

It would be a standard regulated dc output of 6.3 volts I need ,
So transformer ,rectifier, filtercaps ,regulator into heater
I did the calculations for a 15mH choke with a 10,000 uf cap ,18hz cutoff, ideally you might want lower to get a good reduction at 50hz. I have used a common mode choke on Lt outputs before although with much smaller foil caps ,it certainly did reduce higher frequency garbage quite a bit ,but no effect on the low frequency noise.

I agree Square they way Im doing things is bulky and old fashioned  , but simple passive solutions are my favourite even if there is some extra work in construction and layout . I often use chokes on Ht smoothing and get superb results with modern compact electrolytics , I know a transistor regulator could stabilise and quieten a ht supply more cheaply than a choke ,but its not all about the money either .

Tubetec said:

..... I have used a common mode choke on Lt outputs before although with much smaller foil caps ,it certainly did reduce higher frequency garbage quite a bit ,but no effect on the low frequency noise...

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You can't reduce the harmonics residues related to primary frequency using common mode chokes because that signals are not common mode signals. You should use LC or RC filters there.  Or a capacitance multiplier  (which will provide you a soft start for heaters as bonus).

Tubetec said:

It would be a standard regulated dc output of 6.3 volts I need ,
So transformer ,rectifier, filtercaps ,regulator into heater

Click to expand...

What power are you looking at?

I did the calculations for a 15mH choke with a 10,000 uf cap ,18hz cutoff, ideally you might want lower to get a good reduction at 50hz.

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Simulation indicates cut-off at 13Hz, 22dB attenuation at 50 Hz. With just a single 10000uF cap after the rectifier, hum voltage would be about 12mVpp for one 150mA load, with the additional 15mH and 10mF cap, it drops down to 1.6mV. The difference is not 22dB, because the former has higher harmonics (sharp edges).

I have used a common mode choke on Lt outputs before although with much smaller foil caps ,it certainly did reduce higher frequency garbage quite a bit ,but no effect on the low frequency noise.

Click to expand...

I would think common mode is irrelevant. Let's say one of the legs is grounded, you just need to make sure the other leg is clean. Works also if not strictly grounded, e.g. pseudo-balanced via virtual center-tap.

Hi and thanks for the replies,

Most often I need a few hundred ma ,no more.

I used a common mode choke after the regulator , with two smaller foil caps to chassis ground and it did filter some out some mush. I wasnt sure if common mode is better with two caps to ground or simply one right across the supply .I almost always use a centre tapped resistor pair of 50 ohms to ground on my designs.

So Abbey, is it the actual choke creates these harmonics ,or do you mean the mains transformer resonances mean the full 22db noise reduction isnt realised?

Tubetec said:

So Abbey, is it the actual choke creates these harmonics ,or do you mean the mains transformer resonances mean the full 22db noise reduction isnt realised?

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No, it's a matter of waveform. Peak-to-peak voltage is a single figure that does not describe the whole picture. A comparison of spectra would be more significant.