Passive HP Filter

[billzoe]

Hi All,

I need a way (for Home Tube Audio) to filter out typical Tube Hum - this is AC -Tx proximity hum (not due to old filters, etc - amps are rebuilt).

I dont want an active (IC based) EQ in line, i do want something passive.

Anyone had any experience with "In Line" FMOD (by Harrison Labs) HP Filters?

Whats in these? Cap-Resistor ckt?

I'm also open to any other passive (LC type, etc) HP-Low Cut filters, if anyopne knows of others - the higher the quality, the better - but i dont want to by a full Inductor EQ , just to get the HP feature.

Thanks all,
BZ

[billzoe]

Also meant to ask, if anyone has had experience with Hum filter, what is best Freq point as a comprimise of ridding hum, but leaving as much (wanted) bass in signal ?

I have a sub that can make up the rest (20-60 =/- hz).

[tINY]

The front end of the amp is a great place for a coupling capacitor, or an L or T or pi filter...

You have to know the input impedance of the amp.




-tINY

[JohnRoberts]

Audible mains hum is rarely just fundamental (50-60Hz), but commonly contains several higher harmonics overtones. To remove all of this with a simple passive filter will mean throwing away a lot of possible music too.

Perhaps better to address specific sources of hum. Is the amplifier clean with no input, or input shorted?

Perhaps better shielding or equipment grounding might help?

JR

[billzoe]

Originally Posted by JohnRoberts

Audible mains hum is rarely just fundamental (50-60Hz), but commonly contains several higher harmonics overtones. To remove all of this with a simple passive filter will mean throwing away a lot of possible music too.

Perhaps better to address specific sources of hum. Is the amplifier clean with no input, or input shorted?

Perhaps better shielding or equipment grounding might help?

JR

...thanks, however, the tech that rebuilt them , grounded-shielded as much as poss (new star ground, etc) and certified that this is permanent hum (low level) due to Power Supply proximity.

Also, it has a line out option off the Output tranny - if i record into a DAW , and use a Software (Waves REQ) HiPass @ 60hz, it takes out a good amount (although, yes the upper octaves stay).

I am not too worried with bass loss below cutoff freq - i will make do with hum octaves.

Thanks
BZ

[Matt Syson]

Hi
Perhaps the amp needs rebuilding properly. There is more to 'star earthing' than simply wiring it up as a 'star earth', you have to think in 3d.
There were many cunning dodges to keep hum out of valve amps, although the 'permissable level' was possibly greater than nowadays.
While it would not be sensible for a 'manufactured' design, how about introducing hum in antiphase?
Fitting a filter which is steep enough to get rid of 60Hz is likely to do some awful things to the audio.
Matt S

[billzoe]

Thanks,

I am pretty confident in the rebuild and the tech , he is super a__l about sheilding-ground .

... he said to get rid of the hum (again, this is normal LOW level hum) all it need is to Convert to DC Filament AND Relocate the Power Supply to an external unit, but thats another exorbitant cost i dont want to pay.

It is a Proximity of AC Tranny spray, that causes it - he even beefed up filter values and added a few more fiter stages and a choke.

From my experience most Vintage units have this hum, stock.

I understand the arguments - and i agree, however, i am really only looking into passive filter options at this point.

For the most part i too feel eq's - filters are "SFU's" (Sound F*cker Uppers) - unless its a Pultec-type !

Cheers,
BZ

[Matt Syson]

Hi
Your tech told you the answer.
Assuming it is a power amp, driving a speaker the only place you could add an effective filter is in the speaker wiring (or disconnect the LF driver).
Adding a filter on the input will not help you because there is no hum on the input to get rid of.
If you want to be rid of the hum spend the cash and get it built as your tech suggests.
Matt S

[billzoe]

Originally Posted by Matt Syson

Hi
Your tech told you the answer.
Assuming it is a power amp, driving a speaker the only place you could add an effective filter is in the speaker wiring (or disconnect the LF driver).
Adding a filter on the input will not help you because there is no hum on the input to get rid of.
If you want to be rid of the hum spend the cash and get it built as your tech suggests.
Matt S

Thanks - you are correct, i agree.

However, this is to go on the Line Out i mentioned above that feeds elsewhere (post amp/out put tranny) .

I know this is unorthodox , i cop to that, just wondering about user's experiences with these filters - i have seen them used in Audiophile systems - way better than mine.

6moons audio reviews: Anthony Gallo Acoustics Reference III


Cheers,
BZ

[Matt Syson]

Hi
If you have a flat tyre on your car it will 'pull' to one side.
The cure is not to let the air out of the tyre on the other side but to fix the first.
Assuming it is the power transformer creating the problem, move it!
Put it in a seperate box (properly earthed) and wire it to the existing chassis with an umbilical cable. The hum would go and the sound may be decent. Messing around with 'snake oil' filters is no substitute for proper design and assembly.
If you want to improve maters further you could go to DC powered heaters. This can be relatively cheap by using a rectifier (35 Amp, bolted to chassis / heatsink), some large caps (22,000uF or larger) and perhaps a power resistor to ensure you are feeding the correct voltage to the heaters.
Matt S

[Minion]

Maybe it would be easier to Just shield the power supply section from the Rest of the amp, Maybe Mu-metal shielding or even some other type of metal shielding...??