The Passinwind Open Source Preamp

[Passinwind]

Desktop is nice. The PW3B seems like a top-of-the-amp device for voicing, rather than an effect to be stomped for the chorus.

Me; "I sure think the resonance and frequency controls are as useful for a HPF as for the LPF."

Charlie: "I ran into a lot of problems the last time I tried this, but never say never!"

I hadn't picked up on the problems implementing a variable frequency HPF. As a theory student, can I ask what you found?

When I think about it a little less superficially, I realize my notion was "Oh, it would be nice if this acted like a parametric equalizer". The bass Bax, which shelves at a higher boost or cut, followed by the steepish HPF to reject and protect, achieves a similar result.


Thanks

I was attempting to use the Sallen-Key format, which sounded fantastic until it didn't. All my test builds would eventually go into uncontrollable resonance at sub-fundamental frequencies, putting my test speaker in pretty grave danger. I later found a little blurb on Rod Elliot's website describing that very thing as a known drawback with at least some opamps, and recommending not to bother with messing with it. I've seen similar problems a very long time ago with some old school synth VCFs too. But I have done resonant HPFs successfully in the past with a state variable filter, a la Craig Anderton's and Alembic's iterations. I didn't/don't want to go there for onboard installs, but for a pedal it should be no problem at all. I also may have just gotten too cute with my S-K circuit tweaks, so I'[ll probably revisit that a well. In any case, as you mentioned, pairing a variable HPF with a Bax bass control can get to some of the same places, and that format seems more intuitive for everyday users to me, judging from passing a few of these boxes around at my friend's weekly jam.

And since you asked about it, here is @Tassieviking's teaser circuit board render that he sent me yesterday:

He was very patient in rolling with various tweaks I threw at him along the way, and in turn I had to do a bit or retuning to allow using all Tayda-available parts. He's already ordered test boards, so hopefully with a few weeks his design fork will go live for sharing. I had hoped that a few more people would brave teh "easy SMD" format that I'm so fond of for my own builds, but hey...open source! This is based on v2, so no HPF or gain controls.


If one were determined to change the frequency of the HPF, would using a switch to change cap values by, say, +/- 25%, likely be stable?

Using switched closely matched/spec'ed caps or resistors is always going to give the best results IMO, but using inexpensive standard issue pots is just so much less hassle and can still work quite well in many cases IME.

 

[dcoyle]

Looking forward to being able to build V2 PW3B with through hole, for a pedal.

Also looking forward to understanding the HPF.

I asked "If one were determined to change the frequency of the HPF, would using a switch to change cap values by, say, +/- 25%, likely be stable?"

I've lost my way. Over the time between your post 1111, where you mentioned the new iteration of the HPF as "(non-adjustable resonance)" and the post 1113, that showed an image with variable resonance (Q), and now, I started imagining the "frequency" pot was detuning one of two stages of second order HP filters, resulting in changing the Q as well as changing the -3 dB frequency, as depicted in post 1113.

Hence my question above, about changing the frequency where the detuning changes Q. Clear as mud, I'm sure.

Does the family of curves depicted in post 1113 occur as one changes the frequency pot?

If so, then it seems one might, by trail and error, find a center frequency for the HPF that worked well for one's particular rig.

If the ratio of cap values between the two 2nd order HP sections were kept constant, could one use a two position 4PDT switch on the cap values, reiteratively, to identify a sweet spot?

Thanks.

 

[dcoyle]

Thanks also, Charlie, for explaining what you experienced with the Sallen-Key filter.

 

[Passinwind]

Does the family of curves depicted in post 1113 occur as one changes the frequency pot?

If so, then it seems one might, by trail and error, find a center frequency for the HPF that worked well for one's particular rig.

If the ratio of cap values between the two 2nd order HP sections were kept constant, could one use a two position 4PDT switch on the cap values, reiteratively, to identify a sweet spot?

Thanks.

I could have expelained this better, let's try again.

1) Yes, so you can leverage interactive resonance changes but not adjust resonance with the sort of granularity a separate control would give. You could also hack in a dual pot so this thing behaves more like the various commercial ones, and I've been contemplating adding a couple of circuit board pads to accommodate that more readily. For standalone HPF duty this is probably the way to go, really. But in my amps I tend to take it even further the other way, with 4 individual cap values that give something more like an elliptical response familial with the Cauer alignment. That might also be known as high risk, high reward, it's very easy to get wrong. LTSpice really helps, but one has to have some working experience with reading those curves, it kind of borders on dark art at times.

2) Also maybe yes (I think, but I'll have to model that), and if you use Spice modeling or careful RTA measurement you can tune any (one) frequency in the pot sweep range to have a classical filter alignment, so with a switch you could have two or three such frequencies, but you could probably just switch resistors instead too, which would give many more tuning options. I'm working through that process of moving the classical alignment point around right now for a couple of different instruments, and IME the sweet spot is not necessarily at the same place the classic filter aligment is happening. Otherwise I would just build a normal HPF like everybody else!

Great questions again. thanks for that.

 

[Passinwind]

A quick heads up:

OPA2145 are back in stock at Mouser now, so if anyone is planning on doing a v2 build with my hybrid SMD/through-hole format for an onbaord install you can now revert to OPA2145/OPA145 instead of the higher curent OPA16412/OPA1641 parts.

And then I found some right angle dual pots for the new 125B build at SmallBear, but in 10k and 50k ohms rather than 20k/20k as we originally specified. So no jumper wires and/or daughtercard needed, and a wide frequency sweep range comes with the deal to boot. I'm still playing with new filter alignmments to best leverage these parts, which is going to delay getting things into the Wiki by a few days, but I feel it's best to just get all the ducks in a row right off the bat as much as possible. My baseline alignment is 275-6.8kHz, which seems pretty good, but there are at least a few others that might suit some of us better.

 

[bowthing]

I get OPA2145s directly from TI. They are usually in stock, and generally cheaper than distributors. Now if they would just make up some OPA145s !

 

[Passinwind]

This new one is optimized for a friend's upright bass with two piezo pickups, testing results have been quite good so far.

 

[dcoyle]

This is a sharp looking package and the idea that you have all these functions integrated into a box knocks me out.

When you mention "optimized for a friend's upright bass with two piezo pickups" I wonder if you mean as to voicing the EQ, or electrically?

I ask about the electrical wondering what beyond a higher input impedance makes piezos happy? A particular input sensitivity, or a capability in the bass boost or cut? Certainly the HPF has its work cut out with a standup.

Thanks

 

[Passinwind]

This is a sharp looking package and the idea that you have all these functions integrated into a box knocks me out.

When you mention "optimized for a friend's upright bass with two piezo pickups" I wonder if you mean as to voicing the EQ, or electrically?

I ask about the electrical wondering what beyond a higher input impedance makes piezos happy? A particular input sensitivity, or a capability in the bass boost or cut? Certainly the HPF has its work cut out with a standup.

Thanks

Thank you! More good questions, and BTW this new gain leveler format came about partly because of your last queries.

I see this as a long term optimization process, which is relatively easy since it's for someone I play with weekly and I typically take a turn or two on her bass at every jam. The pickups are Fishman and 10meg ohms was the number that came up most often over on the DB side when I inquired about preferred input impedance. But just as with magnetic pickups and this preamp format, I am actually finding lower to be better for this particular use case. In testing with anything from 10 Meg to 220K, the 249K larger format blue WTHPF pedal a bit upthread was the clear winner for everyone in the room, same results observed two weeks in a row. Next best was 475K, FWIW. But this reflected various other random factors, not a true apples to apples test.

So I took a leap of faith and put the new one at 700K, but replicated the lower boost and cut ranges for bass and mids that the blue box features. And I stuck with the same HPF tuning, so now we are pretty close to single variable testing. And as it turns out, 700K is a little better for this application, and the bass sounded great in a packed 100 seat room when we took it over there this weekend as well. I suspect that some midrange voicing tweaks could bring more improvement, and I will inevitably try bumping input impedance a notch or two higher again eventually. In any case, the bigger takeaway is that all my test boxes sound at least pretty good with this particular bass, and they all work fine with BGs with mag pickups too.

 

[Jim C]

I've been one of Charlie's guinea pigs and the blue box mentioned above is the latest of 3 of his pre-amps that I've tried. These are not basic EQ boxes but some pretty sophisticated tone circuits. I've had extensive studio experience with Neve, MCI, and Sony consoles as well as outboard graphic and fully parametric EQ's from White, Orban, Urei, and Pultec (both tube and SS). If anything this box is closest to a Pultec.

While the bass, middle, gain, and HPF are conventional, the freq. and resonance controls are very unique to guitar / bass EQ boxes.
I've been using only passive basses with this rig to include my Marco J bass and many amps & cabinets to include SVT, Ampeg tube pre's / big iron, GK MBF800, Traynor SB500, Bergantino, GB, and GR cabinets. Also Fender and Peavey combo amps.
It alters tone unlike anything I've ever used and is a masterpiece of audio engineering for bass players.
Even just a tiny bit of EQ goes a long way. Build one if you have the time as you will not be disappointed.
This setting was a 2 second adjust before rehearsal last night. I've been using the same rehearsal rig with the same band for about 5 years and never have sounded better!

 

[dcoyle]

Oh boy, oh boy, oh boy!

 

[dcoyle]

Hey Jim C,

Thanks for the report. Just to be clear, are you using a passive pickup directly into the box, no intermediate device? I note the "gain" on post 1130 is only at 1 o'clock. I ask cause my stuff is all passive.

And now into subjective world:

You mention "Even just a tiny bit of EQ goes a long way." Does this mean the sound of your passive pickup comes through intact when things are set neutral-ish? How you would distinguish that from everything else that is being done, I haven't a clue, but I ask.

 

[Jim C]

Hey Jim C,

Thanks for the report. Just to be clear, are you using a passive pickup directly into the box, no intermediate device? I note the "gain" on post 1130 is only at 1 o'clock. I ask cause my stuff is all passive.

And now into subjective world:

You mention "Even just a tiny bit of EQ goes a long way." Does this mean the sound of your passive pickup comes through intact when things are set neutral-ish? How you would distinguish that from everything else that is being done, I haven't a clue, but I ask.

I was using this in front of an SWR amp last and a little bit of boost allows a passive J to get closer to lighting the input LED. In other words, just a tiny bit of gain boost. The other two pre's did not have this feature and it works great for me. Especially if I'm pulling a lot of EQ like with a GK and need more make up gain that is dead quiet.

The mid and resonance / freq controls have a lot of range. They previous version had an even greater range which was cool from an engineering point of view, but nothing I would ever use with a bass guitar. I find the res./freq controls to be non-intuitive and a completely different EQ tool.
It will allow your bass to shine through, and you have to work to make it sound bad.

We just had this chat the other day about if the pedal was flat with no EQ. I thought I heard something but it was only because I forgot that flat = Res full left / Freq full right.
I had the two at 12:00 which sounds incredible for most of my basses and amps.
It might not be 100% flat with all controls flat, but there is no magic unless you want it!
Mid control is awesome BTW; my rigs don't need much. Cool to use as a big tone shaper for certain pieces or to help with bad room EQ.

 

[dcoyle]

Jim C., your two posts are very informative, thanks.

It's great that this iteration is passive pickup friendly and that it is able to supply " make up gain that is dead quiet."

 

[Passinwind]

Jim C., your two posts are very informative, thanks.

It's great that this iteration is passive pickup friendly and that it is able to supply " make up gain that is dead quiet."

All the versions are pretty much passive friendly and have easy mods available for tailored gain staging. I haven't really discussed those mods up to now because no one had asked until you and Jim did. But adding some form of volume or gain control started to get my attention once I realized that nearly everyone was just doing pedal builds rather than onboards.

I'm starting one more WTHPF build today and then when I get that done I hope to start playing around with the 125B board and see what if anything needs to be done to optimize it. I have a few other pressing obligations to meet first though.

 

[Passinwind]

Update:

Before I've even had a chance to dive into the through-hole 125B board in depth, @Tassieviking is well into revisions that he felt were needed during his first test build. It will now use a standard pattern Tayda pre-drilled Tayda enclosure that will only require a single very minimal modification, basically just enlarging one hole by a couple of mm. I really like where this design fork is going! We will come up with a UV print file for Tayda as well, so the builder can get at least very close to a single source buying experience. I'll be testing some different reverse log dual pots from Small Bear in the next WTHPF build, they are 50K ohms and can give a considerable wider frequency sweep range if desired. Might build up an LPF board like that today, even.

I also sent out a beta version variable HPF to @Jim C, who has some great ears and gave me some interesting feedback. He seems to like it pretty well in any case. I'm going to buy a couple of spiffy 1590B enclosures and see what if any feature creep might be in the cards.


And then I'm contemplating going off the rails a bit with the third WTHPF build, maybe with side jacks since it's for myself andf I prefer desktop mode. But I'll try whatever looks to bring the least sonic compromise as far as layout, and actually measure vs. the URB bass one, which is staying with a new owner who lives nearby. Someone shot a phone video of me playing through her rig last week at a jam and it sounded realy nice. She has now requested making the footswitch do gain switching for bowing vs. plucking, which could be useful for BG soloing as well.

I have a lot of knob choices available for my new build, currently leaning toward my old standby ones that are on most of my DIY amps:

But I also dig the oxblood and cream look:

Or maybe the bling:

Cheap thrills...yay!

 

[Bemo9]

Wow amazing how this is taking off. Charlie the knobs on the top picture are the clear winner

 

[Jim C]

Wow amazing how this is taking off. Charlie the knobs on the top picture are the clear winner

Not so fast mister...

 

[bowthing]

I like the cream. But I like old British sports cars too!

 

[Passinwind]

I like the cream. But I like old British sports cars too!

Yeah, I think any of the cream ones would look especially killer with the chameleon finish Gorva boxes, they kind of have a BRG thing going on in green mode. I'm still trying to avoid duplicate cosmetics as much as possible though.