The Passinwind Open Source Preamp

[Passinwind]

Then I'll contribute.
<snipitty doo dah>

Cool, very interesting. Somehow OPA141 had eluded my potential sub parts list, but the current supply looks good and I may have to pick a few up the next time I do a Mouser order. I use OPA1641/1642 quite a bit and they generally work quite well for my purposes.

 

[taketwo]

Cool, very interesting. Somehow OPA141 had eluded my potential sub parts list, but the current supply looks good and I may have to pick a few up the next time I do a Mouser order. I use OPA1641/1642 quite a bit and they generally work quite well for my purposes.

I kinda stumbled upon it. Important things, besides being in stock, were:
- low noise
- jfet input
- rail to rail
- low voltage
- no phase reversal under overdrive (most important)

 

[Passinwind]

I kinda stumbled upon it. Important things, besides being in stock, were:
- low noise
- jfet input
- rail to rail
- low voltage
- no phase reversal under overdrive (most important)

Yeah, that old school latchup/reversal thing is a definite deal breaker in what passes for my world. We've had plenty of opamp discussion through the course of this thread BTW.

 

[taketwo]

Cool, very interesting. Somehow OPA141 had eluded my potential sub parts list, but the current supply looks good and I may have to pick a few up the next time I do a Mouser order. I use OPA1641/1642 quite a bit and they generally work quite well for my purposes.

My question is: what really distinguishes the 1641 from the 141? (I don't buy the marketing on the page). What parameter would make you choose one over the other? (I can't tell, I'm too new at this.)

 

[Passinwind]

My question is: what really distinguishes the 1641 from the 141? (I don't buy the marketing on the page). What parameter would make you choose one over the other? (I can't tell, I'm too new at this.)

The 141 has lower input bias current and probably offset voltage although I didn’t check that and can’t right now. Hence “precision” opamp. Quiescent current is the same. Current noise is the same for both, voltage noise is slightly better for 1641. 141 is spec’ed for single supply but 1641 works fine for that too IME. If you need to run at 5V I’d dig a little deeper. 1641 is a little cheaper, but for my purposes I don’t see much at first blush to make me call them anything but interchangeable. I’ll revisit this later tonight since 141 might be useful for some upcoming projects if I do find anything more to differentiate them. It’s easy to go cork sniffer on this stuff when circuit design is actually the most important aspect, IMO and IME.

 

[taketwo]

The 141 has lower input bias current and probably offset voltage although I didn’t check that and can’t right now. Hence “precision” opamp. Quiescent current is the same. Current noise is the same for both, voltage noise is slightly better for 1641. 141 is spec’ed for single supply but 1641 works fine for that too IME. If you need to run at 5V I’d dig a little deeper. 1641 is a little cheaper, but for my purposes I don’t see much at first blush to make me call them anything but interchangeable. I’ll revisit this later tonight since 141 might be useful for some upcoming projects if I do find anything more to differentiate them. It’s easy to go cork sniffer on this stuff when circuit design is actually the most important aspect, IMO and IME.

One thing I noticed, is that the 141 behaves very strangely at 5V single supply if there is significant offset (say from uA leakage to a MOhm input resistor), which skews it away from a 2.5V bias. I eventually figured it out and switched to the 9V supply, and everything then just worked.

 

[Passinwind]

Update:

Variable high pass filter board has been revised, so official release is probably still 2-3 weeks out. I'm ordering several today for testing in the new larger format WTHPF pedals. Using DigiKey's DK Red service and rolloing the dice on getting a few freebies.


Here's the OSHPark render:

As usual, that pricing is for three units, which is the OSH minimum order quantity.

I'll be trying some different enclosures from Cusack/StompBox Parts this time around, and adding a volume control. There's probably room for either a DI output or headphone amp in the next version, but first things first!

I tested the second WTHPF pedal (the orange one) with piezo pickups in both acosutic guitars and upright bass this weekend and even with a rather low-ish input impedance for that application it absolutely killed.

 

[Passinwind]

Here's the mockup of the new Cusack 1590BBs layout:

 

[dcoyle]

Recently came across this thread and wanted to thank you, Charlie, for your gift of time to share what you have learned and make it accessible with sharing boards and parts lists. I'm also glad you're still pursuing the HPF.

I was concerned to see V2 of the circuit required SMD resistors until I realized that the V1 and the V2 circuit are the same topology with different values and a blocking cap added before the Bax driver stage and a extra buffer opamp before the LPF. So I'd like to try building the extra opamp stage using a dual package on a DIP 8 socket on a postage stamp size bit of perfboard and use the V2 values on a V1 board. Comments?

I'm building this into a pedal and am going to use the second amp of the buffer opamp as a gain pre-pre stage for passive basses. You mentioned all your basses were active and you had no experience with developing passive front ends and you hoped that some other member of the community would take up the design of a pre-pre. The trouble is the member of the community turns out to be the village idiot, me. May I ask some questions?

You mentioned the native peak of the pickup may interact with the emphasized resonance, so I take it you might not be real concerned about a high input impedance, and of course the higher the resistances, the higher the noise. Given that, if one were to want a half meg or one meg input Z, and using the same topology as your first stage, what might be the best choice for the feed back resistor? Any clue what gain is enough, 10dB or more? A 470K R to Vref and a 47Kin the NFB loop would give 10 dB. How would you massage that for gain and noise and headroom?

Could one use a variable pot in the NFB of the pre-pre stage to effect a volume control for the pedal or would it be better to have a fixed gain stage working into a passive pot, say 25K and then on to your circuit? The pot would have more output impedance than an active bass, I would imagine. Is that a concern into the front end of the V2?

If I get it to work I'll share the part conversion values.

Thanks

 

[dcoyle]

Make that extra cap after the Bax driver stage.

 

[Passinwind]

@dcoyle :

Quite a few things have changed over the years and I have fairly substantial experience with doing preamps for passive pickups by now. You're asking the right questions, good job! I'm currently working on builds for upright bass and acoustic guitar that both use piezo pickups, so I should be a lot smarter in about two weeks or so. Both sound excellent even through a 220K input impedance, interestingly enough. And neither necessarily sounded better at 1 Meg, just different, so I'll be working through a full range of test values. In these cases I'm using my new HPF module as the front end, but the buffer/gain stage is exactly the same as in all the OS versions anyway.

First off, someone is currently doing a V2 layout using through-hole parts and a 125B case format. It can be its own design fork and go wherever it needs to as far as I'm concerned, but the guy doing it has already offered to share freely. PedalPartsPlus will probably be doing a similar one as well, and selling boards on their site, but I haven't handed off specs to them just yet. The first one and quite possibly the second as well will be spec'ed for all Tayda parts, which should be welcome news for those outside the US and Canada.

And then that new pedal version in post #1108 happens to be my test bed for various gain leveling topologies. I needed something like that for a prototype build for a luthier last year and I solicited advice from a few very experienced pro audio designers, none of whom thought it was all that easy to get right in a simple build. I actually have some new boards coming in tomorrow to try head to head against what I sent that luthier. Finding and addressing the most critical bottleneck in the signal chain is an interesting thought exercise, and so far my best results have come from doing gain leveling at the back end. By gain leveling, I mean something like -10, +6dB with (hopefully) no tonal variance, which the guy I sent the first one to thought it did well. I tend to avoid using the feedback loop for gain controls, but that is just what I've been doing here so far. I'll be trying some other approaches soon as well though. For pedals we don't have to sweat current consumption much at all, happily. So an added dual opamp for this function seems totally acceptable, but I'll get back to you in a few days on that.

Make that extra cap after the Bax driver stage.

The new one before the Bax stage in V2 is the money zone for bass peak tuning, IME. The latest boards use two caps in parallel so we can fine tune things with much better granularity now.

And yes, the regular flavor (non-adjustable resonance) HPF should be ready for release just as soon as I build up the latest board spec, the new boards for that just showed up a couple of days ago. Small enough to fit in a 1590A case:

 

[dcoyle]

The nice thing about the 1 meg input is that the passive magnetic pickup can sound as we're used to hearing it, going in. Useful for a pre EQ feed to a balanced out.

Glad to learn the front end the HPF of post #1108 can be configured for 1 Meg input Z. Since this is the same topology as the front end of this series, it would be very interesting to learn the R values you used for Z and gain, when you care to.

I look forward to learning more about the HPF, such as what Q is the nonadjustable resonance?

 

[Passinwind]

The nice thing about the 1 meg input is that the passive magnetic pickup can sound as we're used to hearing it, going in. Useful for a pre EQ feed to a balanced out.

Glad to learn the front end the HPF of post #1108 can be configured for 1 Meg input Z. Since this is the same topology as the front end of this series, it would be very interesting to learn the R values you used for Z and gain, when you care to.

I look forward to learning more about the HPF, such as what Q is the nonadjustable resonance?

Please PM me with an email address and I'll get you a few different relevant schematics. But in general: primary loop feedback at 2K-5K usually and the other leg at 6K-33K, with input shunt resistor at 47K-1Meg. Bear in mind that the overall preamp is inverting, so if adding another stage making it also inverting would flip you back to normal. V2 already has an added non inverting opamp stage between the Bax and LPF sections, so if you build to that spec you can do gain staging trades pre- and post- Bax stage. I should probably look at making that stage inverting instead, actually. Hmmm...

The HPF I'm currently working on for the pedals is not the synth filter-ish one I was doing a few years ago, it's more like the fdeck et al. Maybe 2-3dB of resonant boost at most, and that goes away by the time it gets to the most used frequency range:

 

[Passinwind]

Update: just received the first renders of the 125B pedal build. It's looking very good, so naturally I will be throwing another curveball or two at our friend who is doing this. Off to the shop to try something new in a minute...

And @Freekmagnet mentioned wanting to try a Bass/LPF onboard preamp format with one of his very cool new pickup builds, so I built up a Bass-only module using the V2 front end board yesterday:

 

[dcoyle]

So the #1108 HPF is a band limiting device, not a color, nor a corrector for over-damped bass speakers . As in the #1114 device, use the bass Bax for color.

Is that a third order filter @ 40 Hz?

 

[Passinwind]

So the #1108 HPF is a band limiting device, not a color, nor a corrector for over-damped bass speakers . As in the #1114 device, use the bass Bax for color.

Is that a third order filter @ 40 Hz?

The one in post 1108 is tuned differently than the one I showed the graph for, in fact each WTHPF version so far has its own tuning. Very much meant to interact with the bass control and inherent interstage rolloffs. The overall response can go from roughly 4th order+ to around third order depending on setting. Still tweaking the standalone ones, where I've found that people might actually be willing to turn the knob further than the 10-2 o'clock range.

 

[dcoyle]

"So the #1108 HPF is a band limiting device, not a color, nor a corrector for over-damped bass speakers . As in the #1114 device, use the bass Bax for color."

Sorry, that should have been a question.

"Still tweaking the standalone ones, where I've found that people might actually be willing to turn the knob further than the 10-2 o'clock range. "

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

 

[dcoyle]

Charlie said "First off, someone is currently doing a V2 layout using through-hole parts and a 125B case format. It can be its own design fork and go wherever it needs to as far as I'm concerned, but the guy doing it has already offered to share freely"

Who is that, please?

 

[Passinwind]

Charlie said "First off, someone is currently doing a V2 layout using through-hole parts and a 125B case format. It can be its own design fork and go wherever it needs to as far as I'm concerned, but the guy doing it has already offered to share freely"

Who is that, please?

That would be @Tassieviking. I'll let him post details when he's ready, he just sent me a BOM for review this morning and everything else looks about ready to go. He may want to do a test build before he posts the fabrication Gerbers (his call of course), and he can decide whether to put his design fork in the existing PW3B-LPF Wiki or elsewhere. All the parts will be available from Tayda Electronics, in one shot.

I'm very happy to see someone else taking this on and I've already learned quite a bit from it. And as far as my end, I finished up the first v4 build a few days ago, and tested it very successfully on bass guitar and upright bass:

This layout was done with desktop use in mind, rather than a dedicated floor pedal. In this build the HPF is always-on and the switch bypasses all the rest. On the next one the switch will be a global mute. Plenty of room to rearrange things to give better access to the switch, or maybe to go with side or front jacks as I personally prefer. Build something once, why build it again?

The gain function currently comes between the Bax board and the LPF, is at unity gain at noon, and has a range of -10dB to +6dB. I now think I can make this work without a separate daughtercard, have already ordered a test set of new LPF boards to try that, and am at probably a week out from testing that board.

"

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

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

 

[dcoyle]

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.

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?

Thanks