Simple Attenuators - Design And Testing

[lwgramith]

@Gene Ballzz ,
Thanks for taking a look! You’re right that the wire termination and soldering is rough. It was my first soldering since the pandemic. I should have practiced a bit to knock off the rust before jumping into a real project, but my impatience won out. The Bassbreaker I have is the 15W head, purchased, I believe, in 2017 or 2018. (Tried to link, but I don't have the 5 required posts to do so. It's model #2263000000 on the Fender website.) I’ve attached some photos of the attenuation box without the tape, special focus on the input jack. Thank you again for any time you can/do spend on this.

Thank you, @JohnH ! I ran the tests, and I’m getting measurements that differ significantly from the expected results. Both tests come back at 6.5/6.6 Ohms.

Out of curiosity, I also re-ran the pre-startup resistance test and discovered some interesting things. My measurements no longer come in at the 6.9-8.8 range I had before my initial startup. Instead, I’m getting 6.4/6.5 on all switch combinations that have stage 4 disengaged and 8.2/8.3 on all combinations that have stage 4 engaged. Stage 4 is the switch on the left-hand side in the photos. I'm not sure if any of this is related to my problem. It just seemed odd that the readings were no longer showing a minimum of 6.9 as when I originally tested and that the stage 4 switch is the only one that impacts the measurements. I used the same speaker and speaker cable for all of tests (when applicable).

I plan to reexamine the connections very closely when I can to see if a connection failed. Clearly something is wrong with the reactive stage, but it's not immediately apparent to me what is happening. I’ll be thankful if you can make sense of these measurements.

Photos without tape:

 

[JohnH]

I can't figure out how if might occur, but those two additional tests which are both giving 6.5 or 6.6, that's as if your R1 and R2 are being connected in parallel somehow. eg, a short to ground at the right side of R2, hence meeting the lower side of R1.

 

[gbarker7815]

This is a great resource. Looking forward to putting together a build soon. In sourcing the parts, for the resistors, does it make a difference if the tolerance is 1% or 5%? I've been trying to find all matching 1% Arcol ones, but for a few I can only find in 5%.

 

[Gert-Jan van der Heiden]

This is a great resource. Looking forward to putting together a build soon. In sourcing the parts, for the resistors, does it make a difference if the tolerance is 1% or 5%? I've been trying to find all matching 1% Arcol ones, but for a few I can only find in 5%.

The alternative values are way out of 1%. Based on that alone I would see 5% is perfectly fine. Don't think speakers are that precise impedance either.

 

[Gert-Jan van der Heiden]

I think it would be safe to connect a headphone (left and right in parallel) with all stages attenuating in design M2 or am I missing something? Without those stages my ears and headphones will be gone haha

 

[Gert-Jan van der Heiden]

at lwgramith :
Not the problem your facing right now, but your speakers are connected so that only 1 of them is connected if you're using Cliff's. Not sure that is your intention.

 

[JohnH]

Hi @gbarker7815 , welcome to our thread. 5% tolerance is fine for any of the resistors.

For the base set of values, I selected from the most common ranges of resistors that have steps of about 20%, eg 10, 12, 15 etc. That shows how 5% is accurate enough if that is what you can get.

The most important values are in stage 1, which is what the amp sees and also the ratio of R2A and R2B sets up the tonal balance at all settings. Elsewhere, the ratio within each stage sets the attenuation level, and so the consistency of steps as you move across switch settings.

 

[JohnH]

hi @Gert-Jan van der Heiden , thanks for joining in.

Yes It would be safe to plug in headphones at max attenuation. But it may sound bad since there's no treble roll off or other tone shaping due to a guitar speaker. It really needs at least a cab-sim circuit, or better, an IR device, before headphones.

 

[Gene Ballzz]

@lwgramith ,
I may be able to help, but you need to help me, help you!
First> Where are you located on this planet? If in North America, may we could talk on the phone?
Next> Communicating via email may make the whole process easier. It will be much easier to send pics and edits of pics, back & forth. If you PM/Conversation me your email, I'll share my email and phone with you.
Then> If you can snap a duplicate pic of the first one in Post # 2,741, but with wires moved around/aside so that I can see the component labeling for identification and which wires go where, that would aid my assistance. I have a couple sneaking suspicions of a couple of improper connections, but don't want to send you off chasing your tail, if I'm wrong. If I'm correct, you may only need to move and/or replace a couple wires.
Finally, For Now> If my suspicions are incorrect, you may need to de-wire most of it and mostly do it over again.
Let Me Know?
Gene

 

[lwgramith]

Thank you @JohnH and @Gert-Jan van der Heiden for the insights. Gert, I’m glad you caught that. I probably won’t ever use it with more than one speaker, but it’s good to fix the wiring now just in case.

@Gene Ballzz, I am in the Eastern time zone of the US. I’ll PM you my contact info and the requested pictures sometime today. Thanks!

 

[lwgramith]

@Gene Ballzz , I have the pictures ready to send, but I can't find how to PM you. I figured there would be a button for it if I visited your profile, but nothing. Perhaps I haven't unlocked this feature yet given my low post count?

 

[Gene Ballzz]

The button is labeled as "Start conversation" after you single click my user name in any post, instead of PM/Private Message. The problem is that I've not found a way to attach pics in a "conversation." That's another reason that doing emails may be easier. I'm gonna send you a "conversation" and the alert will show up near the upper right corner of the page!
HTH,
Gene

 

[Gene Ballzz]

@lwgramith
I believe I have all components identified, via comparison between your diagram and pics. Of special concern is the jumble of wires between R3, R4, R5 & R6 and where they go to & from?
Gene

 

[JohnH]

hi @lwgramith , you're in good hands with Gene. If a problem does not quickly become apparent, I'd suggest to do a little temporary rewiring and one more set of tests.

Test 3
check that everything that should be 'grounded' (ie connected to the sleeve lugs of the jacks, though it's not a real ground) is so. So resistance checks between input sleeve and output sleeve, and from each of them to the 'ground' side of R1, R3, R5 and R7. All of these should be very low resistance, determined only by lead and connection resistance and the meter All less than about a half Ohm in practice, and zero in theory.

Then, if still hunting for issues, I'd suggest to unwire the tip connection at the output jack, disconnect R2 from R3 and the first switch, and wire directly from R2 to the output this should bypass any effect from the switched stages and give just stage 1 at -7db. Check input resistance before testing, which should be a full sound, pretty loud, but a step down from max amp volume

 

[Gene Ballzz]

@JohnH ,
That's a pretty slick and fairly simple, yet logical testing/toubleshootong procedure. I can see how one could add and test each stage in the same manner, consequtively, after first wiring back in the stage 2 switch and R4 and then the disconnected other end of R4 to the output. Then simply rinse and repeat for the next two stages, at R6 and finally at R8.
Thanks John,
Gene

 

[lwgramith]

Quick update on my attenuator. I spoke with Gene yesterday to do some troubleshooting. He had me shift some wires around so I could get better photos for him. We talked for a bit but weren’t able to identify a problem that would cause my issues.

Then something weird happened. Gene left me with a few suggestions for better jack wiring, as well as some tests to run as he continued to look over the photos I had sent. Well, while I was running those tests, I just happened to measure for resistance from sleeve to tip of the input and it came in at 16.1 -- the number John said I should expect for a properly wired attenuator. I then reran the other tests John described, and everything came out right. After that, I plugged the attenuator in for a test run and it worked!

This thing is awesome! Such a great range of attenuation! I’ve never been able to drive the power sections of my amps hard due to my living arrangements, but now I can. It’s like I now have a whole new set of amps!

I guess my task is to figure out what changed between then and now. My suspicion is that I had either a bad joint or a stray wire strand that was “resolved” when I moved the wires around to take new pictures for Gene. I’ll be rewiring my jacks in line with Gene’s suggestions, so I will probably reflow the solder joints to ensure proper connections. I'll also do a very, very close examination for stray wire strands. Hopefully that makes it durable in the long run.

Anyways, just want to say thanks to @Gert-Jan van der Heiden , @JohnH , and especially @Gene Ballzz for giving their time to my project. This is such a great community.

 

[Gene Ballzz]

@lwgramith
While you could certainly connect your meter as described by @JohnH and jostle the wires around to "pinpoint" any errant connection(s) and/or short(s), I highly recommend simply "shaotgunning" and closely inspecting all solder joints! Really happy that you don't need to completely dismantle and start over. Also happy that you have generally found the anomaly!
Happy Attenuatin'
Gene

 

[JohnH]

Hi @lwgramith , Im very happy that you got it going! My hunch from the various readings is it was some kind of short to ground, maybe between stages 1 and 2.

 

[Gene Ballzz]

Hey @JohnH ,
What do think of this enclosure?

Pedal enclosure box- 6 1/2 X 6 X1 7/8 inches- .08 aluminum | eBay

Would be excellent for any line/pass through type recording or sound projects.

www.ebay.com

I would think that six, 3/8" - 10mm in the top and six more, in between components, on the bottom would provide sufficient cooling? Looks quite easy to work with and can be easily painted or powder coated to taste. Seems easier than working down inside the Hammond style boxes? Might provide a cosmetically pleasing end product!
Watcha Think?
Gene

 

[Gene Ballzz]

And then one other set of P.I.T.A. questions. On an 8 ohm M2, if the only choice for the 0.9mh coil is 0.8mh or 1.0mh, what would be the better choice and what are the downsides of going in either direction from the specced 0.9mh?
Thanks @JohnH ,
Gene