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TB11 Preamp Project

#103
Stealth said:
Much obliged.

Suppose someone prefers the EQ to work solely in two-band mode (bass and treble), would it be enough to use two fixed 50K resistors and solder them in place of the pot? From what I figure, (and the earlier post with pot values) that would act as the midrange control centered.
Click to expand...

Yes that would work. Leave out C3, short C4. Short JP5 pins 1, 2, and 3. Then make R8 and R9 50K. It would convert the stage to an inverting unity gain buffer.
 
#104
Ok, combining the designs helped immensely. I removed all but two mounting holes which also saved me the restrict areas around them.

As a result, I could get everything onto a 0.75" x 3.05" piece of PCB real estate using only plated-through-hole components, which means no squinting through a magnifying glass soldering 1206 SMD resistors. ;)

Here is a screen shot of the layout:

tb11-combined.jpg


I did some re-arrangement of my panel to fit it without going over my budget so when I order this panel I should get 6 copies of the preamp as tag-alongs.
 
#105
I put together a preliminary BOM for this combined project (part numbers are Mouser part numbers):

TB11-BOM.jpg


As is typical, sourcing the pots is the hardest part. What I would have liked to do is something like this (for fitting to a standard three-hole jazz neck plate):

Hole #1 - Neck Vol / Bridge Vol (50K linear concentric)
Hole #2 - Bass / Mid (100K linear concentric)
Hole #3 - Treble (500K)

Concentric pots are very difficult to find for a reasonable cost in small quantities (Potentiometer.com was the closest I've found). So I changed the BOM to use the tiny 1/2" width Alpha pots. I think I can fit all 5 on a standard jazz plate. My frankenbass (a Peavey Foundation from the 80's) has ample room to mount dozens of pots and switches so no worries for the first experiments!

If anyone knows a better source for concentric pots I'm all ears!
 
#106
BestBassGear seems to have them, they have a 250K stacked pot with center detents (which is pretty much a must for tone controls) so you could ask the owner (Mike, I think) if he could source 100K stacked center-detented ones. He's been very helpful with any questions thus far.

Edit: he also stocks stacked 50Ks. There's just no getting the right value here, it seems. :p
 
#110
The more I think about it (and from suggestions from other members) having a modular design seems like the best approach.

For example, a two channel buffer circuit can be used before or after pots, and can have the various switching topologies for bypass and/or the various low-z or cap loading switches that are popular. Having a single buffer for each pickup is really all that I need for most of my basses...however having two bands of parametric EQ (rather than fixed bass/mid/treble points) seems appealing to me (especially if there were one band per pickup!).

A modular design also allow for Tillman type circuits in the same layout. While I wait for boards and parts to arrive I might standardize a layout template for the modular sections so that different designs can be dropped in. I'm thinking a square design (since that is cheaper to do in a multi-project panel for the PCB house) of a size about 1" square.

So for my example above, I could combine a buffer board, and send each buffered pickup output to it's own parametric EQ board, then passively blend together. That would be just 1" x 3" for three boards which is pretty compact.

A few thoughts on modular sections:

1) A state variable topology for filtering
2) Parametric EQ sections
3) Active blend/mix
4) Two band Baxandall
5) Three band Baxandall
6) Tillman buffer stage (discrete JFET buffers)

1" square is pretty small, so I'm not sure how much the designs will have to be SMD based but I'll start some experimenting.
 
#112
First boards are in!

Here's a (crappy cellphone) pic of the boards. I placed a USB connector next to it so you can get a sense of the overall size: I would say it's about as wide as a jazz pickup and about 3/4" shorter:

TB11_First.jpg


On closer inspection I realized I accidentally re-generated the part designations on my panel so the part names are not consistent with the schematic...no big deal, I'll just have to fix the schematic to match later.
 
#113
MglMatador said:
First boards are in!

Here's a (crappy cellphone) pic of the boards. I placed a USB connector next to it so you can get a sense of the overall size: I would say it's about as wide as a jazz pickup and about 3/4" shorter:

TB11_First.jpg


On closer inspection I realized I accidentally re-generated the part designations on my panel so the part names are not consistent with the schematic...no big deal, I'll just have to fix the schematic to match later.
Click to expand...

for sale? :)
 
#114
BassLife77 said:
for sale? :)
Click to expand...

Before I've even found out if they work properly??? You have much more faith in new layouts than I do. ;)

My plan is to solder one up and attach it to my beater Peavey Foundation bass. I'll make a few 'stock' recordings, then wire up the buffer sections to make sure the sound isn't damaged. I'll also check out the 'low Z' mode and 'high cap' modes to see if they offer anything interesting.

Then I'll move on to the EQ sections, starting with the shelving bass and treble, then lastly mids. What will be extra interesting is to get a look to see if DC offset is a problem, as I didn't plan on stuffing any DC blocking caps (although there are provisions for them).
 
#115
Ok, finally got out from non-stop overtime at work to warm up the soldering iron.

Here's a shot of the bare board with just the 9V battery clip attached:

IMG_8838.jpg


I started by soldering in everything to get the dual-rail supply going, as well as the posts for the input section and first buffer stage for both pickups. Here's a shot of the test setup in the bass itself:

IMG_8840.jpg


Lastly, here's a shot with the two volume controls attached:

IMG_8842.jpg


Aside from two minor layout mistakes which require two short jumper wires (I found two breaks in my ground plane that left two pins that were supposed to be ground floating), everything through the passive summation stage appears to be working properly.

I took sound clips of all of the front end options (high-loading for both bridge and neck, low-Z mode, volume control/blend, etc) direct from the jack, so I'll post those shortly after I can mix them down and convert them to MP3's.
 
#116
Ok, here are some MP3's:

1) Original Pickups straight to the output jack

Bridge Pickup - Passive

Neck Pickup - Passive

2) Output of buffer sections with 500K load resistors

Bridge Pickup - Buffered, with 500K load

Neck Pickup - Buffered, with 500K load

3) Bridge Pickup - Buffered, with 1000pF load cap engaged

Bridge Pickup - Buffered, 1000pF load cap

4) Bridge Pickup - Buffered, with Low-Z resistor (changes load to about 65K)

Bridge Pickup - Low-Z

5) Neck Pickup - Buffered, with 1000pF load cap engaged

Neck Pickup - Buffered, 1000pF load cap

6) Neck Pickup - Buffered, with Low-Z resistor (changes load to about 65K)

Neck Pickup - Low-Z

7) With Volume Controls - starts with bridge coming up and down, then neck coming up and down, then finally bridge + neck together

Volumes - Both pickups

I apologize for the unimaginative sound clips - my entire rig was precarious, so I had to hold the bass upright with my fretting hand and pluck with my right hand...so I really could only do open strings. ;)
 
#119
Stealth said:
I'd say the "passivizing section" does pretty much what it says on the tin, then.
Click to expand...

Aye, I wasn't expecting much change in sound with only buffers, and it's good that it sounds pretty much the same. ;)

A few notes (and feel free to add if anyone notices anything):

1) The 1000pF loading cap doesn't change the sound much in any way that is immediately apparent: 1000pF simulates a 30+ foot cable, but perhaps larger caps would make more of a difference.

2) These Peavey Super-Ferrites must be wound to a fairly low impedance, as even at 65K loading they don't change drastically in sound: they are definitely darker, but I was expecting much more pronounced cut to the highs at 65K. Perhaps a lower value load resistor is another option.

3) I might also build a second one with 250K load resistors, as these are bright as hell with new SS round would strings. Or I might leave 500K on the neck, and switch the bridge to 250K.

The header posts I used are the same 0.1" headers used as jumpers on PC motherboards, so one can pop the cover and engage the load cap or low-Z mode for either pickup with just a small jumper added by hand.

Now, on to the EQ sections!
 
#120
I'd have to say that the buffered tone was as I expected, with a brighter top end. At first I thought it was lacking a little lows, but since you aren't using input coupling caps, I think it was just the added highs and the clip sounded slightly lower in volume.

The 1000pF load sounded closer to the passive pickups, but still a little brighter. Like you said, probably due to the pickup's impedance.

I tried this trick back when I was making low impedance pickups, and because of that it didn't do all that much.

I was surprised how much I liked the low Z mode! Nice cushiony lows.

Can't wait to hear the EQ section.

Nice work!
 

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