attenuator inductor emf trouble

[Chuck H]

I designed an attenuator that works great except...

There are two inductors in my design. One of them is speaking to my single coil pickups (constant whistle). The one that sets the high frequency impedance rise. The other inductor sets the low mid rise and is bypassed by a cap so this could be part of why it's not giving me trouble. This one is a higher H value so it has an iron core. I also have a copper band around it. The high Q inductor is an air core unit. I tried the copper band on this one with no luck. I've also tried shielding both inductors in steel and grounding to the main chassis ground with no luck. I haven't tried the full copper/steel Faraday shield yet.

Do air core inductors have more stray EMF than iron core units? If I used an iron core inductor for the high Q and used the copper band would that push some of the stray EMF back into the core? could that help?

I know most attenuator circuits only use one inductor. This is probably why. But I'd like the keep the design as it is because it sounds so transperant.

I'm trying not to use C core or toroid units because this design may go to market so I need to keep the costs down if possible. So any way to make a simple bar core inductor work would be best. But I'll be happy for any answers.

Any ideas appreciated.

Chuck

[Enzo]

SO if you disconnect that choke, the guitar no longer feeds back?

[cbarrow7625]

How close do you have to be to the inductor for it to "speak" to your pickup? The fields generated by inductors in that type of application are not all that large. I design speaker crossovers regularly as part of my profession and have done a good bit of study on how inductors couple to each other. They do couple, but only when they are VERY close together (at audio frequencies).

I think Enzo is on the right track of thinking; you may be assuming something that is not true. You may have other issues with your design that is showing a symptom that somehow interacts with your pickup.

One inductor or two should make no difference, in theory, to cause the problem as you stated it.

[Chuck H]

Thank you for the replies.

Yes it is the inductor interacting with the single coil pickups. Doesn't happen with humbuckers because they're humbuckers. The EMF gets cancelled. It does not happen when I disconnect or bypass that inductor. I can't get closer than about four feet from the inductor (when it's in the circuit) when the whistling starts. Further away, to about seven feet, I still get a little "ping" on the end of the notes.

I appreciate your experience cbarrow. But keep in mind that this is not home audio. The reason a guitar player uses an attenuator is because the amp is cranked. This is a high gainer too so the pickups are going to be even more sensitive. And FWIW I don't believe Enzo was "assuming" I was wrong. He just wanted to see if I was. Only people who know me better assume I'm wrong

Any idea how to control the EMF? Do air fore inductors have more stray EMF than cored inductors? Would a cored inductor with a shield help? I notice that the Marshall Power Brake has a similar circuit and it uses a cored inductor.

Thanks

Chuck

[cbarrow7625]

Chuck, sorry if you took anything I said the wrong way. I'm trying to help, not call you out on anything. Sounds like you think I'm being hard on you in some way.

Thanks for the additional information regarding the humbuckers vs. single coils and shunting the inductor. Have you measured the impedance of the attenuator with & without the extra coil? I would be interested in seeing it if you can post it, it could help. I am assuming that, right now, you have the attenuator very close to the amp. Have you tried moving the attenuator 10-15 feet away from the amp and approaching the amp alone with your single coil guitar? What I am thinking, and I haven't seen your circuit so I am again making assumptions, is that it is possible that the impedance rise in the attenuator is very sharp at the highest frequencies. At higher frequencies it is possible that the amp iself is becoming unstable and wanting to oscillate becasue it may be working into an almost "open" load, if the impedance is high enough at the upper frequency ranges.

I am a high gain amp fanatic and have tried a bunch of designs over the years and had many that worked great with humbuckers but started getting all squeally with single coils. I'm sure we've all had that experience. Single coils just pick up more junk from outside & excite the amp at frequencies that humbuckers do not.

If the entire attenuator assembly is in a steel box you should not need to individually shield the inductors unless you are tring to keep them from interating with one another. THe chassis will shield both of them from the outside world. is this in an enclosure now? Can you shield the whole thing if it is not already?

I would suggest trying to isolate the effect of the amp from the effect of the attenuator at this point and seeing if you get the same or different results. If something still squeals with a shielded attenuator box placed 10-15 feet away you might have a different problem. I would also suggest trying each step one at a time to isolate which thing makes the issue go away (if it does at all).

FWIW, I don't design home audio equipment. I design large professional loudspeakers; the kinds that are found in theaters, stadiums, major rock events, etc. Most of these are in the range of 200-1000 watts a piece running off of anywhere from 500-5000 watt amplifers. The crossovers designed for them have to handle larger voltages and currents than most of our guitar amp speakers / gear. I only mention this because you seem to be discounting my experience based on assumed "home audio" credentials. It is not the case. I alsoam very familiar with why a guitar player uses an attenuator; I have been building & repairing guitar amplifers for over 18 years. Not as many years as some others here but I am no noob by any means.

I hope eveyone here can help you get this sorted out & tracked down. It is a really intersting problem to me. I'll stay tuned.

[octal]

I believe that toroidal core inductors have low radiated fields. I could be completely wrong about that though!

Have you tried a Mu metal shield can? I have a small one (I'm sure it's too small for your application) which purports to attenuate fields by 20dB if memory serves. Might be just the ticket if you can find/fab something for your inductor.


Nathan

[Enzo]

I am a bit puzzled as to why this happens, but other speakers with crossovers and the coils therein don;t speak to your guitar.

[Chuck H]

In a previous post Dai mentioned that his Power Brake will do it even with humbuckers. But he had to get right up on the unit to make it happen. The Power brake uses two inductors. One is for the top end inductance rise that speakers present. This is similar to my design. I have noticed that most attenuators don't use this inductor and I suspect this oscillation is why. Steve Conner said that he sometimes uses a big tubular type rheostat as an attenuator and it does the same thing as mine with single coils. It's just the EMF in this high gain application I guess. I'm thinking a different inductor may not give me as much trouble. I'm hoping, that is.

I'm intrigued by cbarrow's idea that the attenuator may be causing an instability in the amp itself that makes it sensitive to the brighter single coil pickups. But since this inductor is bypassed by a resistor that sets the impedance (and therefore it can't be TOO high) I don't have much hope that it will be the answer. But I'm going to try his test and see.

Cbarrow, You and I seem to be a little touchy here. I was honestly just poking when I mentioned the "assume" thing. Note the wink icon and humble humor at the end of that paragraph. The mention of "home audio" was just a generic reference. You said you build crossovers and I know that isn't typically a "distorted tube guitar amp" kind of application. I meant no disrespect. And I do regard your input and plan to implement your suggested test tonight.

Peace

Chuck

[Enzo]

One thing occurred to me, and I hope I can articulate it.

Sometimes I encounter amps that have SOOO much gain the pickups will feed back even at really low volume. I can turn the master down to bedroom levels - easy to talk over - and maxing the gain will have the pickups squealing even though the amp is not remotely loud. Seriously, I am talking about the volume of a table radio in an office. And the shop guitar has humbuckers.

I am thinking that somehow the gain of your amp turned up so high into this load is just itching to go unstable.

Does your feedback have a characteristic note? Is ther a cap involved with this inductor? If so, slap a parallel cap of some value across the other cap and see if the note changes.

Or...

Can you replace this inductor with two of half value? Possibly orient them to cause destructive coupling?

[Steve Conner]

Hi Chuck, Enzo, all,

I thought about this, and I think maybe the problem is just that guitar players use so much gain nowadays! With so much gain in the loop, it only takes a tiny sniff of feedback, be it acoustic or EM, to get it screaming. My solution would be to make it an offence to possess anything with more gain than a 5E3 :-)

Now, the problem with magnetic cored inductors is that they need an air gap in their magnetic circuit. This is where they store the magnetic energy that makes them "induct". The inductors Chuck has been using are all air cored, or have an iron bar as a core, and this means that they have an open magnetic circuit, hence a big magnetic field extending all around them. Essentially the rest of the universe is the "air gap" and that's not good for feedback.

The only type of inductor I can think of that would be immune from this is an iron powder toroid, because toroids have a closed magnetic circuit. The flux just goes round in a donut shape, and theoretically none leaks, even if the core was air, as long as everything is symmetrical. A laminated iron toroid core needs an air gap sawn into it to make it usable as a power inductor, which might break the symmetry and cause it to leak flux again. Iron powder has a "distributed air gap". However, you mostly have to buy cores and wind them yourself, although you can get ready-made ones for suppressing noise from lamp dimmers.

Or maybe instead of a single inductor of the type you have just now, you could use 2 or 4 smaller ones, connect them in series, and arrange them so that they cancel each other's fields, at least in the spot that the guitar player usually stands.

[Enzo]

"Or maybe instead of a single inductor of the type you have just now, you could use 2 or 4 smaller ones, connect them in series, and arrange them so that they cancel each other's fields"

Hey, there's an idea...

[cbarrow7625]

Chuck, it's all good. I just wanted to clarify since nobody here really knows what I do in my day job.

[drewl]

What Enzo noted about high gain amps was the first thing that popped in my head.
I've often run into this problem with Mesa's and other multi stage amps while trying to get them to stop feeding back or amplifying unwanted noise.

[Chuck H]

Great info and ideas. I actually thought of using two inductors in series out of phase, but not really knowing what I'm doing it seemed too simple an answer to work. I'll try it.

By the way, Enzo gets the droll award. Nicely done

Steve, what can you tell me about those lamp noise supressors. Sounds interesting, and "affordable". I've looked at every inductor Mouser has but I don't think I saw those.

Also, let me clarify one thing. The amp works perfectly without the attenuator. The problem presents gradually as I increase attenuation.

Great info on the iron powder cores Steve. Since I hope to mount these in the amp cabinet (not the chassis) I may use toroids. I'll have to wind my own and have them copied. I just need to keep cost down as much as possible since this design will be considered for MFG.

Thanks heaps. Lots of avenues to work now. I'll let you all know how it goes.

Chuck

[Steve Conner]

Hi Chuck, all,

The out-of-phase thing was just a thought, if it works for pickups, I don't see why it shouldn't for attenuator chokes.

The dimmer chokes I'm thinking of are known in the industry, I believe, as "storage chokes", "suppression chokes", or just "dimmer chokes". Either way they're just inductors with toroidal iron powder cores. Though I've seen some with ferrite rod cores, which would get you back where you started.

Almag light dimmer rise-time and emc chokes
DJ - Liniar Storage Chokes - www.nordtek.se

Note, don't confuse these with the common-mode chokes you asked about a while ago. Those look similar but are on ferrite cores that can hardly store any energy.