AC LINE FILTER [Take 2] -- HOW TO???

[jjasilli]

Since I've had no takers on this thread. http://www.el34world.com/Forum/index.php?topic=12250.0
Here's another take on the issue.  Attached is a schematic of the existing 2-prong circuit & a proposed 3-prong solution.  Clearly Fisher thought a line filtering circuit was needed.  So I might get hum & noise if I eliminate it.  

With a change to a 3-prong power chord, the chassis is no longer floating, but is more safely at earth ground.  If I get hum & noise, I'm guessing a proper X-rated crossline cap will suppress it.  The crossline cap is not connected to ground, and will not bleed away AC unless the power SW is ON.  

Will this work?

[DummyLoad]

yes. are the courtesy outlets polarized?

edit: you could use a MOV across the line after the switch.

[FYL]

Why don't you get an inline filter from Schurter or one of their competitors?

http://www.schurterinc.com/en/Components/EMC-Products/Single-Phase-Block-Filters

[stingray_65]

yes. are the courtesy outlets polarized?

edit: you could use a MOV across the line after the switch.

Also, the courtesy outlets should go before the fuse or be fused separately.

[kagliostro]

You can find it for free in broken appliances

Kagliostro

[HotBluePlates]

Attached is a schematic of the existing 2-prong circuit & a proposed 3-prong solution.  Clearly Fisher thought a line filtering circuit was needed.

Fisher made that product when you were certain to have 2-prong outlet in your house. And your outlets were not "polarized," with one prong larger than the other. Therefore, you had a 50/50 shot of plugging it in so that the cold wire would be attached to the chassis.

If you plug it in the wrong way, you got hum, and maybe a shock under certain circumstances. PRR explained those circumstances in the other thread. Fisher, Fender and everyone else who might have worried about hum gave you a polarity switch to flip to connect the cold wire through a cap to the chassis, which killed the hum.

If you want to use a 3-prong cord, modify the Fisher the same way you would a vintage Fender amp. If you want to eliminate noise riding in on the a.c. wiring in the wall (as from a fridge compressor, etc), then use an a.c. filter (as FYL linked) at the point the power wiring enters the chassis.

[Fresh_Start]

Why don't you get an inline filter from Schurter or one of their competitors?

http://www.schurterinc.com/en/Components/EMC-Products/Single-Phase-Block-Filters

What is a "single phase block filter" and what specs would we need?

Thanks,

Chip

[jjasilli]

go to mouser.com.  Search for "emc filter".  Filter your search to 125V 2.5A or 4A units.  Look at the data sheet; scroll down for schematics showing inductors and caps.  The Just Radio's site displays a similar schematic.  The incoming ground wire is not shown.  Also not shown is the value of the components inside the "black box" you are buying.  I too find this all very confusing.

I'm thinking that with a floating chassis (not connected to earth ground), the AC line filter circuit draws no current with the power chord plugged in BUT the power SW OFF.  But with an earth grounded chassis, the line filter circuit will be a vampire & draw current if the power chord is plugged in, even with the power SW OFF; UNLESS a STDP power SW is used to simultaneously switch both the hot & cold AC power lines before the filter circuit.

Another alternative from Dave Funk's "Tube Amp Workbook" is to use a 3-prong power chord, and run a cap top ground off each leg of the AC supply.  But I think this is another power vampire, without a properly located SPDT ON/OFF SW.

I think HotBlue is supposing that the neutral side of the AC supply is truly cold.  But in practice I think it may show voltage.  If so, a cap hanging off the cold side > chassis > earth ground will draw some current, unless the power SW comes before the cap.  

Due to my confusion, I'm reluctantly inclined to leave the Fisher circuit as is.  Not to mention that the customer wants as cheap a rebuild as possible.  What would you guys do in this situation?

[FYL]

What is a "single phase block filter" and what specs would we need?

Most consumer mains connections are single phase, consumer high power connections (electric stoves, clothes dryers) and industrial supplies uses three phases for a more efficient power delivery.

A single phase block filter has a few components: chokes in series on live and neutral, X caps between live and neutral (differential mode) and Y between live / neutral and ground (common mode). X and Y designate open circuit failure modes and self-healing properties: if the cap dies the chassis won't be at mains potential.

Specs aren't critical for consumer-grade applications : get a filter rated for at least twice the max current drawn - say if the receiver draws 300 W max, get a filter rated at least for 600 W (say 5A or 6A on US mains). The Schurter FMLB-41 or similar would be a good choice here.



 

[jjasilli]

Am I right about my vampire concerns (2 posts up)?

[Fresh_Start]

Personally, I would not leave the 2-prong cord as is.  But that's after being shocked repeatedly by an old Princeton Reverb.

Don't have time to draw it right now, but

Hot & Neutral to convenience outlets in series (getting polarity correct)

Neutral to one PT primary

Hot to fuse, fuse to other PT primary

Safety ground to chassis!  Chassis is never floating TTBOMK, although the circuit may be if there's a "ground lift".

Hopefully the KOOKs will step in with their knowledge 

Chip

[FYL]

Also not shown is the value of the components inside the "black box" you are buying.  I too find this all very confusing.

Typically two small chokes rated app. 2 to 10 mH/10A, 22n to 100n X cap across lines, 1 to 10n Y caps from lines to ground.

But with an earth grounded chassis, the line filter circuit will be a vampire & draw current if the power chord is plugged in, even with the power SW OFF;

It will because of the small leakage of std. models, say 0.25 mA or so for a good quality filter - medical versions show much smaller leakage. With 120V mains, 0.25 mA leakage translates into 0.03 W, a very acceptable value.

[jjasilli]

Fyl -- thanks for that good inside info.  It's probably easier to buy a module than source & squeeze in the individual components!

Fresh start -- per prr's response in my other thread, unlike guitar amps, hi-fi amps are less likely to cause shocks.  Also polarity is not reliable, especially in my old brownstone neighborhood, because the house wiring at any AC outlet is open to question.

[PRR]

> Am I right about my vampire concerns

No.

It IS a vampire (with or without 3-pin) but so tiny that so-frikkin-what?

The current must be limited to around 1mA for human safety. 1mA at 120V is 0.12W. That much 24/7 for a month is 86 Watt-Hours, 0.086KWH. At my electric rate it is a penny a month.

For comparison, after checking a washing-machine circuit and unplugging the washer, I found a 13 Watt mystery load still sucking. That turned out to be my network hub. This is 100 times more than the sneakage in a line filter, and I don't mind the buck a month. Hopefully the hi-fi is as valuable to the owner, or at least not 100 times less valuable than my blinky-box.

However I would just cut and toss the line-to-chassis cap, refresh the 2-wire line cord (Fisher 500 cords were cracking in 1975, I hate to think what it looks like now), and let it be. Often an ungrounded hifi in a dry room is not a shock hazard and does not hum. If the system does hum, it is usually the phonograph which should be grounded. And many hifi hums are not about absolute grounding (to utility and dirt), but about poor contact in the crappy RCA plugs and jacks.

[topbrent]

This is one click up on the page.  I have seen this type of filter assembly used in a few boutique amps:

http://www.schurterinc.com/Components/EMC-Products/Power-Entry-Modules-with-Filter#

http://www.schurterinc.com/en/Components/EMC-Products/Power-Entry-Modules-with-Filter/5120

[FYL]

The IEC-sized inlets/filters are pretty much standard on modern equipment. Retrofitting them to vintage gear is a royal pain, an inline module is much easier to mount.

[jojokeo]

Do these filters really work well when in a bar gig setting and getting out the noises from blenders, refridgerators, neon signs, fans, flourescent lighting, etc.?

[FYL]

Do these filters really work well when in a bar gig setting and getting out the noises from blenders, refridgerators, neon signs, fans, flourescent lighting, etc.?

They can be somewhat effective, but most of the noise is common mode, induced thru the gear used on and around stage.

[Willabe]

KOC in TUT 5-20, has a "Symmetric Source" build for stage or studio.

The heart of it is a toroidal step-down PT, 120v to 60v+60v, with a CT. He shows it with an "optional" line filter like you guys are talking about. Or he shows how to make your own discrete line filter. Pretty simple build, with up to a 30dB noise reduction.    

I belive a number of companys have them for sale and I think there are plans on other forum's to build them.


           Thanks,      Brad  

[FYL]

KOC in TUT 5-20, has a "Symmetric Source" build for stage or studio.

Balanced power can reduce by up to 80 dB common mode mains-induced noise. Pretty easy to build - going DIY is IMO the best solution, as retail solutions are very expensive - around $2,500 for a Furman 2400.



From the Equitech web site.

[jjasilli]

I posted this on my related thread, but it probably belongs here: 

"Thanks for all the info in this thread.      I scoured the web before posting and good info was not forthcoming. 

Actually the power chord on this amp is in excellent condition.  Perhaps the thick coating of grease & tobacco residue protected it from oxidation.     I removed it; soaked in cleanser; scrubbed it clean; then treated it with rubber conditioner.  And, the plug proudly displays the Fisher logo.  So I guess I'll re-install it for old time's sake.

I'm definitely intrigued by the in line power conditioner modules.  Will consider them on future builds (and maybe this Fisher re-build, if necessary).  On another note, those modules may be just the ticket for another project -- in conjunction with a big iso tranny I've had laying around."