Choke question

[diynewbie]

I've been enjoying my crack w/ speedball for several months and now considering some of the upgrades.

After looking at lots of pictures of the things this group has stuffed into this device ("their cracks" seemed liked a bad idea) I noticed that some mounted a choke behind the transformer and others traversed the transformer.

From a practical standpoint (i.e. this amp) is one better than the other?  From a design standpoint what could potentially be a problem with either location?

What would be the theoretical benefit of replacing one resistor with an inductor and how would/could this translate to the sound signature (if f I did this, what should I be listening for)?

[ALL212]

Not sure if everyone agrees but this article helped me a bit...

http://www.aikenamps.com/index.php/chokes-explained

[Doc B.]

You want the inductor cores to be perpendicular to minimize coupling. You can base this upon faith and just mount it in the theoretical null spot, or you can connect it and move it around to find the actual best position by looking at the 60Hz output of the PS or the headphone output with a scope.

[diynewbie]

ALL212 for the link.  If my understanding is correct, an inductor will reduce the magnitude of the ripple current more than a resistor (not clear to me whether a resistor reduces the magnitude of the ripple).   In any case, if the ripple was causing an audible affect, I would expect to hear improvement in the base region and perhaps what some might call more “air” between the musicians.

@DocB:  I’m not sure I understand.  The core of the mounted Crack transformer runs front to back i.e. horizontally.  A Triad C-7X has the core running vertically.  Assuming I have this correct, Mounting the choke anywhere on the plate will meet the perpendicular requirement, correct?  At this point further minimization will be based on distance between the Choke and Transformer.  Is this correct?

I was able to borrow a triad choke and wondered what would happen if I attached the choke only to a multi-meter and looked at the AC frequency as well as AC voltage at different locations with 1” spaces attached to the choke.  It seemed to me if I was looking for coupling, this might give me at least a crude insurance to location.

When I straddled the transformer on the underside of the plate (like in the image of the Green Crack), I was able to detect a steady 60hz signal and it measured ~ 40mV AC.  If I raised the choke to about 2 1/4“ above the plate the 60hz signal dropped to 0.  When I placed the choke behind the transformer I had an unsteady 300hz signal (small variation) and ~ 110mV AC.  As the choke was raised, the 300hz signal changed to 60hz.  (Placing the choke to the side seemed to be the best location, but I doubt it would fit inside the box if mounted there).

So what was wrong with my idea and what was I measuring behind the transformer?  If my idea was good, my best guess would be coupling with the transformer and capacitors 1 and 2 with different phases for each?

Based on what I did, I’m beginning to wonder if adding a choke straddling the transformer at a height of less than 2 ¼” might do more harm than good.  I also wonder if placing it behind the transformer is just plain bad.

[Doc B.]

Alright, now you're getting it! Good work! Doing this kind of thing right is just not as easy as asking somebody where to stick something and assuming their theories are correct. You can start to see why over 20 years I have gotten pretty tired of people asking for simple answers to "fix" everything we supposedly do wrong based upon one guy throwing a new part in a proclaiming it life changing without ever taking a measurement.

300Hz is unusual. We do often see 120 and 180 in smaller proportions to the 60. Maybe it's really different, lower frequencies stacking up to look like 300Hz to the meter. I can't guess much more than that without seeing an actual scope trace. There would be vastly more info available in a trace than from a DMM. If you see varying amplitudes across the wave peaks you know you have multiple harmonics stacking up.

[Bacci]

Interesting. With Mouser selling these C-7X puppies at 12$ I assumed the choke mod would be a no brainer and have one arriving tomorrow as I needed a bunch of components anyway.
Seems like finding a good location for the inductor will be a challenge, thereby not just looking at pictures of what others have done.

Did you ground the metal case of the C-7X at every location i.e. no nylon standoffs or anything like that?

[diynewbie]

I honestly can’t say if the there was ever any grounding of the metal case.  Parts of the case have a non-conductive coating including the surface with the mounting holes (made me more confident to try what I did, although I also put on neoprene gloves – sure would have like to have had a GFI – yes, I am a chicken at heart).

While I did 1” use aluminum standoffs, I can’t swear that at least one of the screws was in contact with the inner surface of the mounting holes on the case (I am assuming the hole sides would be without coating).  But a good deal of time there was no grounding because I was slowly and carefully lifting the choke off the base to see when the 60hz signal dropped to 0 (sure would have liked to have had some nylon standoffs - okay, maybe I am a little bit of a brave chicken).

Keep in mind my electronics knowledge is next to nothing.  At the risk of dazzling readers with my ignorance, I should probably explain what I at least thought I was attempting.  I wasn’t sure that DocB’s answer on how to check for proper location could be done with just a DMM.  But it seemed that while I might be able not test for coupling, I could use the choke as an awkward and unwieldy probe to detect the magnetic field of the transformer.

If my “probe” detected the transformer’s magnetic field, then I should see a 60hz signal with a DMM connected to the “probe”.  The stronger the field, the larger the AC voltage.  In this way I could at least see if I could detect the magnetic field and perhaps compare field strengths at different locations.  Hopefully this is correct.

The idea of magnetic fields coupling forces me to further demonstrate my ignorance with additional questions.  When the choke is hooked into the Crack circuit it will generate it’s own magnetic field.  To me this implies that a safe distance with improper orientation would be further than the null distance when using the choke as a probe.  If orientation affects coupling, how does it do so?  Would proper orientation place the safe distance between my “probe” null point and the improper orientation?  Can the magnetic field of a properly oriented part shield it from the magnetic field of another part?  In other words, could the parts safely be closer than the “probe” null point?  And finally, is there a cheap and easy way to shield one magnetic field from another to prevent coupling?

So, keep in mind that what I did may well be wrong, stupid and dangerous;  and that the results may be of little practical significance.

[Paul Birkeland]

If my understanding is correct, an inductor will reduce the magnitude of the ripple current more than a resistor (not clear to me whether a resistor reduces the magnitude of the ripple).

The combination of each 270 Ohm resistor and each 220uF capacitor reduce power supply ripple.  This filter is not unlike a loudspeaker filter, but the goal is to pass DC and have low impedance to ground for frequencies above DC.  If the resistors did not help to reduce the power supply ripple, they wouldn't be in the circuit.

To generalize, the 270 Ohm resistor has about 270 Ohms of impedance at DC all the way up to tens of thousands of Hertz (depending on the inductance of the resistor).  The 220uF capacitor is reactive, and its impedance at 120Hz (power supply buzz) is about 6 Ohms, and its impedance at DC is very high.  This is what creates the attenuation of power supply noise.  The 10H/250 Ohm choke will show about 7800 Ohms of impedance at 120Hz, hence more attenuation of noise in the power supply.

[diynewbie]

Thanks for the explanation.  I had gotten somewhat confused on the properties of various components in AC vs. DC from reading on the web.  Beginning info separating DC/resistors/resistance and AC/inductors/impedance doesn't really give one a good grasp of how resistors behave in AC or inductors in DC.  Throw in reactance and I got things jumbled up.

[Bacci]

Where did you end up installing the choke?
I used 20mm alu risers and a perpendicular orientation (see pic), no coupling issues afaik.

The choke makes a very noticeable improvement, in more ways than I expected. Definitely worth it imo.
The big film cap I put in 12-13 on the other hand (not boutique), can't really say I heard a difference.

[diynewbie]

I haven’t purchased one yet.  My experiment was done with a borrowed choke.  Your orientation is the one that gave me the ~300hz/~110mV DMM readings with a 1”/25mm standoff.  (Choke is not in the Crack circuit, it is just attached to a DMM to probe for magnetic field – I hope.)  As I raised the choke, the frequency reading dropped to 60hz then to 0.  The height at 0hz was greater than that needed when the choke was rotated 90° from your orientation - going across the transformer.

I have also played around with the power supply simulator mentioned in this forum (PSUD2).  And while I did not try to mimic the Crack power supply it really shouldn’t be necessary to see the affect of replacing the first or second resistor with a choke.  The simulations indicated that replacing the first resistor (21 to 15) rather than the second (15 to 12 - which looks to be what you did) will have greater ripple reduction.

Did you try my experiment, use some other method or just install the choke?

Did you check choke to plate grounding and is grounding necessary?

How would you describe the change?

[JamieMcC]

Adding the choke is popular mod it’s also a relatively straight forward and inexpensive modification the effect it had was also easily audible I would definitely fit one again if building another Crack for myself.

This is how I mounted it in my old Crack. It would be interesting to test if mounting the choke on a metal platform offers any worthwhile shielding.

(https://www.head-fi.org/image/id/5980078/width/900/height/900/flags/LL)

[Bacci]

@DIYnewbie I was thinking about getting one of those cheap USB scopes but then decided to fit the choke where a lot of others have fitted it without issues.
@Jamie, awesome build! Adding another "floor" in your crack was a clever solution to fit everything! I might replace the output caps too at some point.

I spoke too soon about the film cap. It seems the Epcos 75uF PP film cap took a few hours to break in but now I feel it also added an audible improvement: After these 2 mods, it's as if the reverb that the speedball added to the Crack changed into a more natural decay of instruments and voices (if that makes sense), and a bit more impact of highs and lows. Really nice, and it only cost me 35$ worth of parts.

[diynewbie]

Thanks for the replies.  I think a free oscilloscope should come with every beginner kit to further fuel interest in DIY audio.

I’m planning on looking for a small metal plate at a hobby shop (to see if it affects the choke as a field probe) as well as a safer more stable set-up for trying the choke in different positions while in the power supply circuit.  A potential problem with the plate is that I might be tempted to get two chokes since it will create a higher platform for mounting stuff.

[diynewbie]

I purchased a choke picked up a tin-coated plate and cut some pieces (five 2” x 4”) to see if it affected the choke used as a probe.  When the choke plus plates were place as in Bacci’s image, it had little to no effect.  When the choke plus plates were rotated 90 degrees and placed over the transformer, the DMM no longer detected an AC frequency, but it was still detecting around 10 mV AC.

I also tried the choke as a probe in Bacci’s position with the plates extending over the transformer rather than under the choke (choke + plates make a “T” shape).  Without plates I saw 102 – 102.9 mV and 297.5 – 301.9 Hz (my DMM as a min/max function).  With the plates passing over the transformer, the voltage dropped to 67.9 – 68.2 mV and 179.4 – 233.4 Hz.

I removed both resistors and add lead wires to test the choke in the circuit replacing either resistor and out of the circuit..  This I did by listening to music.  To be honest, it was difficult to discern much, if any, of a difference in a Crack with Speedball.  At times I felt there was no difference with the choke in either position or with just the resistors.  At times I felt that the choke replacing the second resistor was slightly better.  This was unexpected based on ripple reduction in PSUD2 simulations.

I also tried the choke over the transformer with and without plates and could tell no difference while listening to music.

In the end, I mounted the choke traversing the transformer – perpendicular to Bacci’s image.  I left the plates on and replaced the second resistor with the choke.