The Passinwind Open Source Preamp

[dcoyle]

Jim, that's a tasteful bit of kit. Looks like something Klaatu left behind in the '50's.

Great thing about the knobs on Charlie's middle pic and on this one, is that you can still read them in dim light when some fool 1/16" wide strip of paint is invisible. Chicken heads , too.

I wish I had glow in the dark fret stripes on my fretless bass.

 

[Passinwind]

Jim, that's a tasteful bit of kit. Looks like something Klaatu left behind in the '50's.

Great thing about the knobs on Charlie's middle pic and on this one, is that you can still read them in dim light when some fool 1/16" wide strip of paint is invisible. Chicken heads , too.

I wish I had glow in the dark fret stripes on my fretless bass.

I really like the Pick Knobs I've posted here previously:

You can feel knob postion without even looking down at your bass, and the ones he does with MOP inlays make settings easily visible even from a reasonable distance. He's also doing full blown wooden chicken heads now too.

 

[bowthing]

It’s off the current topic, but there’s a sidebar I wanted to share with you guys that might help with making Sallen-Key filters.

Early on in the development of my preamps, I really wanted to use high quality dual pots like the TE P260D series. They are 1M cycle conductive plastic, super nice feel, super low noise, mechanically solid, etc. They had pretty good track matching, and I would sort them out for the best match, and I even had two different boards so I could ensure that R1 (first series resistor) was always less than R2 in the filter, which meant that errors would reduce the resonant peak instead of increasing it to dangerous levels. (NOTE, this is for a variable gain (variable Q) filter; the unity gain style isn’t very sensitive to resistor errors, and any pot will work fine)

BUT, no matter how hard I tried (I ended up throwing away 80% of the pots) I would still occasionally get the dreaded “chirp” from filter oscillation as I was slowly sweeping through the higher frequencies. It happens when R1 >> R2 and the peak goes up over 16dB or so. I ended up having to hand test every system in a bass with careful and repeated knob twisting to make sure it was okay. When I finally got time to really dig into the problem and analyze the pots, I found that with VERY SLOW rotation, there were huge jumps in the resistance of the tracks. Once rotation stopped, the resistance usually (but not always) came back to normal. We’re talking near order of magnitude jumps; one track would be 700Ω, and the other would be 3000Ω, albeit super briefly.

Being the over-thinker and over-tester I am, I started drilling out rivets and taking the pots apart. I figured it might have something to do with lubrication, and I tried everything from squeaky clean tracks to various pot lubes, but to no avail. So then I started looking at other pots, and I found that ALL conductive plastic pots have this quirk to some degree. In normal use, the resistance almost always settles down to normal values when you make an adjustment and let go of the knob, but slow rotation occasionally makes it happen, and every once in a while it can get stuck there. Not acceptable, because the oscillation is screamingly loud.

What I found is that carbon pots, while generally a lot less durable and less accurate, do not have this problem. I’ve had badly mismatched pots that get near oscillation, but surprisingly, after 20 or 30 rotations they seem to be free of any big jumps—they get “worn in”. So I changed to carbon pots. I still sort them carefully and only use about 40% of the orders with variable filters, but I haven’t had any of the problems I did with conductive plastic.

Go figure…. Any of you guys have some of your own experience to share? I hope this helps if you’ve been building variable Q filters and can’t seem to keep them stable.

 

[Passinwind]

What I found is that carbon pots, while generally a lot less durable and less accurate, do not have this problem. I’ve had badly mismatched pots that get near oscillation, but surprisingly, after 20 or 30 rotations they seem to be free of any big jumps—they get “worn in”. So I changed to carbon pots. I still sort them carefully and only use about 40% of the orders with variable filters, but I haven’t had any of the problems I did with conductive plastic.

Go figure…. Any of you guys have some of your own experience to share? I hope this helps if you’ve been building variable Q filters and can’t seem to keep them stable.

Very interesting and not off topic at all AFAIC. My initial builds did in fact use Bourns conductive plastic pots and did suffer instability problems. I ended up settling for around 12dB of max resonant gain, a more limited sweep range via lower value pots, and also developed that little circuit tweak you and I have discussed, and that worked out fine. More recently I've been using mostly cheap carbon track pots and have been methodically bumping max gain and sweep range back up, all to good effect so far. I've defintely seen rogue hot spots at seemingly random places along the sweep when testing new builds, and now I guess I know why!

 

[dcoyle]

For pedal use, where there is room, would a shunt resistor sized for the max stable Q value, shunted by a 10, or whatever, position rotary switch work? The switch would have to have two sets of 1P10T switches on it to do both pots. Probably a little pricier than a nice two gang plastic pot but maybe more stable and accurate.

 

[bowthing]

For pedal use, where there is room, would a shunt resistor sized for the max stable Q value, shunted by a 10, or whatever, position rotary switch work? The switch would have to have two sets of 1P10T switches on it to do both pots. Probably a little pricier than a nice two gang plastic pot but maybe more stable and accurate.

By shunt do you mean adding resistors in parallel with a highest resistance "starting" resistor (that would be the lowest frequency). Seems like it might work, but you would need an awfully good switch to add them at exactly the same time. Might be a little noisy?

 

[dcoyle]

"By shunt do you mean adding resistors in parallel with a highest resistance "starting" resistor (that would be the lowest frequency)."

Yes.

 

[dcoyle]

"Seems like it might work, but you would need an awfully good switch to add them at exactly the same time. Might be a little noisy?"

You're right, bowthing, I didn't consider the glitch in changing positions.

 

[Dadum]

27301237[/URL], member: 166898"]For pedal use, where there is room, would a shunt resistor sized for the max stable Q value, shunted by a 10, or whatever, position rotary switch work? The switch would have to have two sets of 1P10T switches on it to do both pots. Probably a little pricier than a nice two gang plastic pot but maybe more stable and accurate.

Would a stacked pot that is wired with the stack in parallel also achieve some stability? Since total resistance would be product over sum, when they are the same, the resistance is one half and when one jumped high the combined number doesn’t jump too much.

 

[dcoyle]

"Would a stacked pot that is wired with the stack in parallel also achieve some stability? Since total resistance would be product over sum, when they are the same, the resistance is one half and when one jumped high the combined number doesn’t jump too much."

I expect a dual pot would even it out, but the circuit needs to change two R's at once here, and therefore already uses a dual pot, as it is.

 

[bowthing]

"Would a stacked pot that is wired with the stack in parallel also achieve some stability? Since total resistance would be product over sum, when they are the same, the resistance is one half and when one jumped high the combined number doesn’t jump too much."

I expect a dual pot would even it out, but the circuit needs to change two R's at once here, and therefore already uses a dual pot, as it is.

Quad Pot!

 

[dcoyle]

How about taking Dadum's idea of a second, varying resistor to average against, only half way? For a dual 50K variable resistor function, one could have a dual 100K pot, each section shunted by a 100k, fixed resistor and get Dadum's benefit of an averaging resistor for the varying resistor to work against, with only a dual pot needed. This pot wants to be a reverse audio, so the shunt resistor would change the taper. Someone else will have to say how much that matters. Maybe less glitches, though? Is it even needed?

Just to be stupid, I'll ask: bowthing, I'm assuming you're not reporting the settling time of your digital meter in post 1143, right?

 

[bowthing]

How about taking Dadum's idea of a second, varying resistor to average against, only half way? For a dual 50K variable resistor function, one could have a dual 100K pot, each section shunted by a 100k, fixed resistor and get Dadum's benefit of an averaging resistor for the varying resistor to work against, with only a dual pot needed. This pot wants to be a reverse audio, so the shunt resistor would change the taper. Someone else will have to say how much that matters. Maybe less glitches, though? Is it even needed?

Just to be stupid, I'll ask: bowthing, I'm assuming you're not reporting the settling time of your digital meter in post 1143, right?

That's actually what I do in my commercial preamp. It also allows me to tune the frequency range a little. The parallel resistor also helps the taper law (though I would like even less slope at the top).

As for settling time, I use a two channel scope for looking at the tracks. I can quickly turn the pot and watch for the lines to separate. No settling time. I want to build an automated fixture to do that job one of these days!

 

[Jeff Siddall]

Digipot?

 

[Passinwind]

How about taking Dadum's idea of a second, varying resistor to average against, only half way? For a dual 50K variable resistor function, one could have a dual 100K pot, each section shunted by a 100k, fixed resistor and get Dadum's benefit of an averaging resistor for the varying resistor to work against, with only a dual pot needed. This pot wants to be a reverse audio, so the shunt resistor would change the taper. Someone else will have to say how much that matters. Maybe less glitches, though? Is it even needed?

In the context of my designs, I've never observed this behavior. One pedal build someone else did ended up in my shop due to oscillation problems, but it was a simple build issue (soldering related, mostly) that I was able to fix very quickly. I'll just remove the Bourns conductive plastic pots from the original BOM and make the Tayda carbon ones the default to be safe though. The Tayda ones work better anyway and they're much cheaper.

 

[bowthing]

In the context of my designs, I've never observed this behavior. One pedal build someone else did ended up in my shop due to oscillation problems, but it was a simple build issue (soldering related, mostly) that I was able to fix very quickly. I'll just remove the Bourns conductive plastic pots from the original BOM and make the Tayda carbon ones the default to be safe though. The Tayda ones work better anyway and they're much cheaper.

The shunt resistor doesn't make much of a difference with the accuracy problem, but it's nice to be able to change from 100k to 80k or 63k or whatever you want. Yeah the Tayda pots work great.

 

[Passinwind]

The shunt resistor doesn't make much of a difference with the accuracy problem, but it's nice to be able to change from 100k to 80k or 63k or whatever you want. Yeah the Tayda pots work great.

I've tried shunting 100KC pots, partly for that reason. But I can get a lot of useful tunings in my newer boards already due to the parallel cap I added in one leg of the S-K filter. However, the cap value tweaking thing is not very DIY friendly, and I consider LTspice modeling and/or careful scope testing to be mandatory to avoid potential disasters.

The dual 50K angle pin pots I just picked up from Small Bear seem to have pretty bad section matching and I expect to have to "throw away" half or more of them. I just bought another half dozen, hopefully this next batch will be better.

 

[Passinwind]

Update:

While @Tassieviking has been working through his second gen build this week I decided to take a stab at doing a 125B pedal layout myself, what a rabbit hole! I stuck with the idea of using as many Tayda sourced parts as possible, including pre-drilled cases in 42 finishes: Invalid Link Removed

I'd actually never done a proper pedal layout with pots and switches mounted to the board, so having a template to dissect and modify has been a huge help. This is where I'm at so far:

Once I wrap this part up I'll get to doing a simple graphics treatment for UV printing at Tayda, and try to come up with an adapter baord to allow using straight pin pots in an angle pin application. But at least angle pins dual pots do exist, albeit not via Tayda sourcing at this time. It's always something!

BTW, a battery power option still looks feasible, yay!

 

[Tassieviking]

I found making angle pins fairly easy, I got a bit of 6mm building wire and used the copper wires to make the pins. I think 4mm might have been better as the 6mm wire was a bit heavy, I used a pair of long nose pliers to to bend the wire to a nice shape.

 

[Tassieviking]