Maestro Boomerang Hum

[Shepherd]

I built two Boomerangs from this layout: http://www.diyguitarist.com/PDF_Files/BoomerangPCB.pdf

They both hum substantially.

I used 2n5089.

First build:

Q1: 
c  2.3 v
b 0.6 v
e 84.3 mv

Q2
c  9.37 v
b  7.49 v
e  7.36 v

SECOND BUILD

Q1
c  2.6 v
b  0.3 v
e  78.4 mv

Q2
c  9.2 v
b  7.8 v
e  7.5 v

I can't really 'trust' my DMM, but since the readings came up so close I figured it was at least pretending to work.  The wah works.  It just hums.  I want to eliminate the hum.  I checked the grounds (there are only a few), any suggestions are welcome.  I was extra careful the second time building it.

Also, it seems that the input resistor is 68K on the schem, but 48K on the layout...

[Paul Marossy]

Try orienting the the PCB differently in the enclosure, or try laying the inductor down flat on the PCB. The input resistor is a 68K, verified from an actual Boomerang.

Just for the record, the first one I built used that layout, and mine never hummed...

[Shepherd]

How often do you use that one? 

I didn't notice the hum til I played it through my halfstack.  Prior to that it passed audition through my practice amp and a friend's bass amp.  I played it quite a while because it does sound good.  Once I knew what I was looking for, I could hear it very clearly on the practice amp.  Build two was the same.

I'll try one more time with a recently scavenged crybaby inductor, since the first two times I used three 150mh inductors in a row (like R.G. suggested).

[Paul Marossy]

I'll try one more time with a recently scavenged crybaby inductor, since the first two times I used three 150mh inductors in a row (like R.G. suggested).

That right there might make a difference, IMO.

[jonathan perez]

you bet!

to 150mH in a "row" will make for 300mH...

[Paul Marossy]

Someone correct me if I'm wrong, but I think it matters just how you connect the two inductors in series, meaning that it could actually excerbate the problem if you don't pay attention to the polarities of the inductors.

The fact that it's only a sum of 300mH should still give you a wah sound, it will just be a lot more mid-rangey. Think Jimmy Page, he had that Vox "grey wah" with a 300mH inductor in it. 

[R.G.]

You can't create hum by misorienting multiple inductors. You may REDUCE hum by correct physical and electrical orientation of multiple inductors.

Hum in inductors comes from the external power-line fields picked up by these coils around magnetic cores. The cores suck in the external fields and the coils convert that changing magnetic field to a signal voltage. In this, the coils are just like guitar pickups. Remember HUM buckers? Humbuckers are two coils arranged so that the signal voltages are electrically in phase, but the coils are magnetically out of phase, so the hum tends to cancel. It is possible to do this with multiple inductors. Two inductors could do it relatively well, three inductors is no worse than one if the physical orientation is correct. It's also possible to set up three inductors so you get three times the hum of one by orienting them so the signals and the picked-up hum both add.

It's always good to be sure that your hum is not coming from somewhere else. Get the grounds wired correctly, put it in a metal box. Still hum? Does the hum change when you move the whole unit nearer or further from:
guitar amp
computer monitor
fluorescent lights
refrigerators (electrical motors)

[Paul Marossy]

You can't create hum by misorienting multiple inductors. You may REDUCE hum by correct physical and electrical orientation of multiple inductors.

Well, I didn't mean to say that you would create hum, only that if you got the electrical orientation wrong it might be more sensitive to hum...  I do know from experience that physical orientation does make a difference!

It's always good to be sure that your hum is not coming from somewhere else. Get the grounds wired correctly, put it in a metal box. Still hum? Does the hum change when you move the whole unit nearer or further from:
guitar amp
computer monitor
fluorescent lights
refrigerators (electrical motors)

Yep, a wah inductor loves to pick up EMI from all of those sources.

[Shepherd]

The three (3) inductors add up to 450mh...fat, but not big enough to get muddy.

It is quite probable their orientation contributed to the hum.  I had successfully used the three L trick on the more spacious crybaby layout.  The Maestro layout is tighter.  However, by no means cramped.  It also explains why the problem wasn't apparent on the bread board.

The third build with the crybaby inductor has passed critical scrutiny on the practice amp, but it will be a few days before I play it through the half stack of judgment.

The hum was in a pretty low frequency that didn't seem too bad as long as it was amplified through speakers smaller than 12".

What is the best way to orient three inductors?

P.S.  Don't worry too much about the 25K pot if that's keeping anyone from building this.  Even the standard 100K provides pleasing results, perhaps moreso than the 25K.

[Paul Marossy]

The hum is probably 60Hz (or 50Hz if you're in that part of the world).

So... what's the "three L" trick? I've never heard of that one before....

[Shepherd]

I was too lazy to write out "inductor" and chose "L" instead. 

In "The Technology of Wahs" R.G. suggests using three 150mh inductors as a cheaper alternative.  I figured there must be a good reason.  Perhaps the sum value is more accurate than an off the shelf 500mh inductor.

[Paul Marossy]

Oh,  the three L trick. L = inductance, not some kind of physical orientation thing. Me feel dumb now. 

[R.G.]

It was three inductors instead of four because the biggest inductors Mouser had was 0.1H and 0.15H. Three 0.15's is .45 and it takes four 0.1's to get to 0.4.

However, four 0.1's could make an almost humbucking inductor by orienting them in opposing-parallel physically and series electrically.

[Shepherd]

It works really well with the crybaby inductor.  No more hum.

I chose the 100k crybaby pot over values of 25K,35K, and 75K.  It reminded me of the sonic blender remark Geoffrey Teese made in this http://www.analogman.com/kraft.htm

Used a 4.7uf instead of the 6uf, and 1M2 instead of 1M5 because that's what I had available.

I used 2n5089s and there is a volume boost when engaged.  Swapping these in or out would probably be easier than messing with the input resistor and stuff like that.

[Paul Marossy]

It works really well with the crybaby inductor.  No more hum.

Cool. That's more like it. I think there must have been something going on with those three inductors. I've settled on using BC109s in my Boomerang clones. I use 10uF caps in mine, too.