Special Edition Build Off: The Thumfinizenvette

[dwizum]

I'm using a push/pull volume pot with the switch acting as a ground for the coil tap on these pickups, so I might be constrained in terms of what pots are available in push/pull... But I'm off to mouser and ebay to take a look at what's available! Thanks yet again for sharing your design and thanks to you and @Jeff Siddall for helping troubleshoot my dumb error.

I have one other question actually - how does the bright switch function? By "leaving it out" like I did, am I essentially in bright mode, or non-bright mode? I'm wondering because if this is "bright" the way I have it without the cap, I might hard wire the capacitor in to get a permanent "not-bright" setting. My only complaint right now besides the volume is that it's a touch brittle sounding on the top end, and the treble pot doesn't seem to be configured quite right to roll the brittleness off, in terms of it's frequency response. I like the function of the treble pot as is, I don't want to mess with that, but I feel like it also might need a gentle low pass filter to take the edge off, maybe at a higher frequency than the treble pot? Maybe I can swap the treble pot for a push-pull pot and wire that as the bright switch.

And then I could also wire the mid pot push-pull and have it switch capacitor values for C5 to shift the mid frequency, right?

I do really like the sound of this preamp overall though. After finishing my Air Jazz winter build off bass with the barts, I was really in love with the way it sounded. But now that I'm hearing this in comparison, I think I was in love with those pickups more than that preamp. I have parts to build several more of these preamps, I might try one in that bass some day. Heck, if I can get this tweaked just the way I want it, I will probably start using it in everything, and also go back and retrofit it in my current collection of basses too. I still have my very first bass, an Ibanez ATK stingray clone. I love that bass but the factory preamp is wimpy and gutless. I feel like this preamp would really wake it up.

I would really encourage anyone who has thought about DIY'ing their own preamp to just go ahead and dive in. It's a fun project and a good learning experience. And cheap! pots aside, I probably have less than $10-15 in this preamp and it sounds every bit as good or better than any commercial preamp I've played. Sure beats paying $150+ for a boutique brand preamp...

 

[Ripthorn]

The bright switch is on when the capacitor is in place, as it lets more high frequencies through when in place (meaning the highs as defined by the value of the cap bypass the volume pot). If you want to roll off the brittleness some, there are a couple things you could do. You could change the output capacitor to a larger value to allow more of the lows through, which will help balance any brittleness. The bypass caps could be changed (the 47 uF caps), but those are already pretty large, or you could place a small cap from Volume 3 to ground (volume 1). The cap from V3 to V1 will shunt the high frequencies to ground. I would start with a very small value, say, 47-150 pF. This will really depend on taste as well as the pickups and amp.

As for the push pull pot, you could probably go with a 250k linear and then put tapering resistors to create a quasi-log 100k pot. Of course, you could just try it with 250k and see what happens. Also, a 50k used for active electronics might work, but I don't know if they have those as push/pulls, but I would assume so.

 

[Jeff Siddall]

The volume pot is just a simple voltage divider. If you want to limit the range just put resistance between the output of the tone controls and the input to the volume pot. The simplest option would be to keep the 1M volume pot and just add resistance (1M, 2.2M etc. until you find a max. volume you like) between T2 and V3.

If you wanted to keep the load on the tone circuit the same you could replaced the volume pot with a smaller resistance (ex: 500K version) and put the difference as a resistor between T2 and V3 (i.e.: 500K). Either way those are simple changes.

Regarding the bright switch, the simple principle to keep in mind here is that capacitors pass high frequencies. So when the switch is closed, C6 is effectively adding extra high frequencies directly to the output transistor, meaning adding it to the circuit would make it even more bright than what you have currently. However, you can achieve the opposite effect by connecting C6 to ground instead of T2V3, taking away brightness.

Bottom line, yes, as you point out, there is no harm in experimenting. Try different size caps or try putting them, in different configurations.

 

[TerribleTim68]

I just wanna say thanks to Rip for jumping in on this circuit board. The amazing amount of help that happens on this forum blows me away every time. In the middle of a build-off, everybody stops to help the other guys get done too. You guys seriously are the best of humanity, which is really good to see right now. It probably needs to be said more often. Props to every one of you!

 

[dwizum]

I ran downstairs at lunch time (the joys of working from home) and checked my spare parts. I don't really have much to experiment with. I did find some 1M resistors (0805 surface mount!) so I put one between the tone controls and the volume pot (between T2 and V3). It did knock the volume down some, but not really enough. But it also increased the noise coming from the bass. Is this because I've changed the load on the tone controls? I'm assuming that a "better" approach at this point, rather than adding a ton more resistance at that part of the circuit, would be to drop to a lower value pot. I took a quick look earlier and there isn't that much out there in knurled shaft push/pull audio taper pots other than 250k, 500k, and 1M. Bourns makes 10K, 25k, 50k, and 100k according to the data sheet, but it seems like no one stocks them. I may order a 250k just to try. I want to get a handful of different cap values to play with anyways.

This is really fun stuff. And to echo what @TerribleTim68 said, thanks again to everyone who has been so eager to answer my basic questions as I work through this.

 

[Ripthorn]

I ran downstairs at lunch time (the joys of working from home) and checked my spare parts. I don't really have much to experiment with. I did find some 1M resistors (0805 surface mount!) so I put one between the tone controls and the volume pot (between T2 and V3). It did knock the volume down some, but not really enough. But it also increased the noise coming from the bass. Is this because I've changed the load on the tone controls? I'm assuming that a "better" approach at this point, rather than adding a ton more resistance at that part of the circuit, would be to drop to a lower value pot. I took a quick look earlier and there isn't that much out there in knurled shaft push/pull audio taper pots other than 250k, 500k, and 1M. Bourns makes 10K, 25k, 50k, and 100k according to the data sheet, but it seems like no one stocks them. I may order a 250k just to try. I want to get a handful of different cap values to play with anyways.

This is really fun stuff. And to echo what @TerribleTim68 said, thanks again to everyone who has been so eager to answer my basic questions as I work through this.

Ah, you found my newbie mistake. Because this was my first real design, there were some things I wasn't aware of. In particular, the tone stack as it is is noise prone, but true to original. It can be scaled to reduce noise, but I've never tried it.

Your 1M resistor in series reduced your max output by about 6 dB. Changing to a 250k pot ought to reduce max output by close to 12 dB. If you need more than that,a series resistor can be used and should be less noise than the 1M.

 

[dwizum]

The noise wasn't super bad prior to adding the resistor. It wasn't really even THAT bad after. It just got noticably louder. And I ran out of shielding foil so the control cavity cover isn't even shielded right now. So I don't know if I'd really call it a mistake on your part at any rate!

I've found 250k and 50k push/pull audio taper pots on eBay, I'm going to order both to try for the volume pot.

In terms of scaling the tone controls. I did some googling and found this reference, which handily includes an example that starts with the exact same values in your circuit:

Tone Control Scaling

But what I don't know is, do you want to scale up or down, in order to reduce noise? In the example given there, it scales the bass and treble pot up to 500k and adjusts the other components as required. Would that make for less noise? Or should the aim be to scale down to smaller pots?

 

[Ripthorn]

The noise wasn't super bad prior to adding the resistor. It wasn't really even THAT bad after. It just got noticably louder. And I ran out of shielding foil so the control cavity cover isn't even shielded right now. So I don't know if I'd really call it a mistake on your part at any rate!

I've found 250k and 50k push/pull audio taper pots on eBay, I'm going to order both to try for the volume pot.

In terms of scaling the tone controls. I did some googling and found this reference, which handily includes an example that starts with the exact same values in your circuit:

Tone Control Scaling

But what I don't know is, do you want to scale up or down, in order to reduce noise? In the example given there, it scales the bass and treble pot up to 500k and adjusts the other components as required. Would that make for less noise? Or should the aim be to scale down to smaller pots?

In circuits like this you want top go down. The cause of the noise is the thermal noise through the resistive components.

 

[dwizum]

I guess that makes sense, in terms of why most active onboard preamps use really tiny pot values. I could drop the bass and treble to 100k or even 50k or 25k but I don't know if there will be pots available smaller than 10k for the mids. I'm assuming I can scale each pot and cap separately, so I can go ahead and scale down the treble and bass, and leave the mids at 10k?

 

[dwizum]

Just to play that out. If I went to 25k pots for bass and treble, then I'm scaling by a factor of 1/10. So I would use caps that were ten times bigger for C3 and C5. So they would be:

C3: 10 x 250pF = 2500pF
C5: 10 x .047uF = .47uF

 

[Ripthorn]

I I'm assuming I can scale each pot and cap separately, so I can go ahead and scale down the treble and bass, and leave the mids at 10k?

Tone stacks like this rely on the interplay of the components. Changing only one or two will change the balance of the whole system. You should be able to find pits down to about 1k, though I don't know what form factor you are using.

 

[dwizum]

Ok good to know - So far I'm sticking with the common Bourns 17mm knurled shaft pots, their GTR01 product. I'll look around and see what I can find.

 

[JayGunn]

If you check this in LTspice you can see that the circuit is very sensitive to the value of R3 and R6, the source resistors for each of the JFETs. If you were to increase them by even 50% your gain would drop considerably. A trim pot in place of R3 and R6 would let you dial in the over all gain pretty easily. But without a scope or good DMM you're guessing about what value gives you the amount of gain you like. Since JFETs don't necessarily match well, even from the same batch, it is guesswork whether its R3, R6 or both that need to be subdued without measurements.

Another common circuit trick for a JFET buffer like yours is to divide R3 (and same with R6) into a pair of resistors in series. So R3 instead of being 1.8K would be a pair of resistors, call them R3a and R3b, maybe both are 900 ohms, or one is 1k and one is 800 ohms. Then connect the cap (C1, C7) one end to the junction of the two resistors and the other end of the cap to ground. This reduces the gain without leaving out the cap altogether (which will really reduce the gain, possibly below unity).

But to start with if you just swap in say 2.4K for R3 and R6 I think you'll find the effect pretty close to what you want!

 

[dwizum]

@JayGunn thanks for the input! I was wondering how those resistors affected the gain. I just so happened to have some 2.2k resistors on hand in the right size so I swapped them in. It definitely cut things down - It's still louder than my other basses but it's in the right ballpark. Ironically I had the 2.2k resistors on hand because that's the value used for the source resistor in the piezo buffer I've built around the J201.

I am thinking I need to order some more parts and just try different combos until I end up with something that works for this bass. Probably will scale down the tone stack as planned and try a smaller volume pot and/or slightly higher values for R3 and R6. I like the idea of splitting the R3 and R6 with the cap at the junction, but given that this is a pretty tight PCB I don't really have room to reroute the circuit like that so I'll try these other options first.

 

[dwizum]

Well I think I've done 8 or 9 combinations of resistors and pots in the last two days with respect to R3, R6, and the volume pot. I'm just using what I have on hand and playing around to see how it reacts to different combinations. This has been a great learning experience and it feels good to get some real hands on experience playing with a preamp like this, changing one variable at a time and seeing what the result is. I think I may leave these three components where they are at now. I have a 250k pot with a 1M resistor in series for volume. And I upped R3 and R6 to 6.8k. It's still a hair louder than my other basses but it's within maybe 10%, subjectively. So that's a good spot to end up at. And this is the lowest-noise configuration that I've tried, as well. I did go ahead and order parts to scale down the tone stack so once those are here I will try playing around with those sections of the circuit once the parts are here. And I do still want to put some caps in parallel with the volume pot to roll off the highs a little, since it is still a little brittle.

Once I'm done playing with the preamp I think I need to decide if I really want to keep these pickups in it or not. It's not that I don't like them - they're great. I just don't know if this is the exact tone I want from this bass. My real intention for this bass was for it to become my "main" 4 string. There's really nothing wrong with the tone I'm getting, but it's just not really "my" tone. I think the real problem is, I want the tone I'm getting from the Bartolini J's I put in my Air Jazz bass. Those are pretty much my favorite pickups of all time at this point. Those sound like "me" more than any other pickup I've tried in any bass. I'm sure that Bartolini make the same configuration in a soapbar shape, so maybe I just need to figure out what it's called and keep a lookout on the classifieds for a set of them.

 

[ctmullins]

I am laughing my butt off at your expense right now!!!!



I’m sure Madame Karma will make me pay for that later......

.... and that day was yesterday. One small moment of inattention at the drill press and I have a hole where there shouldn’t be a hole....

 

[dwizum]

Well shoot. I'm sorry to hear that.

Also,

 

[dwizum]

I reached out to the friendly folks at Bartolini and they thought the BC42J would be the best fit since it can give the classic 9j jazz pickup tone I have in my 5 string, but can also be switched as serial or parallel humbuckers. Unfortunately, none of the online Bartolini retailers have them in stock and Bartolini is quoting a "we can't say for sure but it will be at least many, many months" lead time on getting a set made.

But then I reached out to best bass gear, even though they showed them as out of stock, and they called their distributor who happened to have one set of them on hand. Apparently this is the only set available in Bartolini's whole retail network, and they're mine now!

 

[dwizum]

I'm starting to fret over wiring choice for those pickups. Right now, given the holes/pots/switches I have in the bass, I only have one DPDT switch to use, which is currently a pickup selector (bridge-both-neck) and one push/pull pot, which is currently set up as coil tap (serial humbucker-single coil). Those were really the only options that made sense with the Nordstrands. These Bartolinis with their 4 wire setup will give me access to a lot more options (north single coil, south single coil, serial, parallel) but I don't have enough switching available, so I've got to either pick specific options, or add switches. And there's really no room to add new switches...

 

[Dark Horse]

I'm starting to fret over wiring choice for those pickups. Right now, given the holes/pots/switches I have in the bass, I only have one DPDT switch to use, which is currently a pickup selector (bridge-both-neck) and one push/pull pot, which is currently set up as coil tap (serial humbucker-single coil). Those were really the only options that made sense with the Nordstrands. These Bartolinis with their 4 wire setup will give me access to a lot more options (north single coil, south single coil, serial, parallel) but I don't have enough switching available, so I've got to either pick specific options, or add switches. And there's really no room to add new switches...

Push-pull and dual concentric knobs are your friends.