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TB11 Preamp Project

#121
SGD Lutherie said:
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.
Click to expand...

Another thing I realized: in deriving the virtual ground from the 9V supply, I added a significant resistance into the ground path. Given there isn't much bass content up above 2KHz or so, I wasn't expecting this to be a problem since the input C to the op-amp was so low....

...however I might try and reduce the series resistance in the power supply down from 33.1K to 10K, as all dynamic current drawn from the positive and negative rails will cause bias shifts to the ground terminal across this resistance. This will increase the static current draw up to 500uA (from about 150uA), but that shouldn't hit battery life much (it's about the same as a single op-amp).

The other option is to use two 9V batteries and derive the ground from the inner junction of the series connection. Then I don't need resistors to derive ground and the impedance of the power supply will be only a few hundred ohms. That should help with bass extension.

I think I'll try that on preamp #2. ;)
 
#122
MglMatador said:
Another thing I realized: in deriving the virtual ground from the 9V supply, I added a significant resistance into the ground path. Given there isn't much bass content up above 2KHz or so, I wasn't expecting this to be a problem since the input C to the op-amp was so low....

...however I might try and reduce the series resistance in the power supply down from 33.1K to 10K, as all dynamic current drawn from the positive and negative rails will cause bias shifts to the ground terminal across this resistance. This will increase the static current draw up to 500uA (from about 150uA), but that shouldn't hit battery life much (it's about the same as a single op-amp).

The other option is to use two 9V batteries and derive the ground from the inner junction of the series connection. Then I don't need resistors to derive ground and the impedance of the power supply will be only a few hundred ohms. That should help with bass extension.

I think I'll try that on preamp #2. ;)
Click to expand...

TI makes a chip to provide a virtual ground that works better in most applications than resistors. Have you considered that? The big problem is that its limited to about 20 or 40 mamps IIRC.
 
#123
2 sets of inputs and 2 sets of tone/level controls with an A-B/Y switch.

Could be used as 2 separate preamps for people with 2 basses or upright players using magnetic plus mic signals

or

Could be used as a 2-channel pre for someone who wants to switch between 2 separate tones (slap/fingerstyle etc...)
 
#124
Rob22315 said:
TI makes a chip to provide a virtual ground that works better in most applications than resistors. Have you considered that? The big problem is that its limited to about 20 or 40 mamps IIRC.
Click to expand...

Excellent call! Last time I checked, I only found these in PDIP packages, for which I had no room on the board.

I ended up finding a TI TLE2426 rail splitter in a TO-92 package, which takes the same space as the two current resistors....and it has less than an ohm output impedance!

Already incorporated this into Rev 2 of the board....totally worth the $2!
 
#125
MglMatador said:
Excellent call! Last time I checked, I only found these in PDIP packages, for which I had no room on the board.

I ended up finding a TI TLE2426 rail splitter in a TO-92 package, which takes the same space as the two current resistors....and it has less than an ohm output impedance!

Already incorporated this into Rev 2 of the board....totally worth the $2!
Click to expand...

Yes, that's the one!
 
#126
Since I'm a newbie, and all 3 basses I have owned are passive, I'm trying to figure out what EQ parameters I'd want in an active bass, without any previous experience of owning one.

I'm starting a build in the next month or so. I already have the wood, and I'm building an active 33" scale 4-string semi-hollow body with a homemade preamp, with this TB11 circuit as the inspiration.

I'm drawn to the sound of P-bass (currently have a Jazz) and I'm willing to sacrifice some mwaahh for that beefy solid fundamental. But, some P-basses I hear just don't have enough attack for me. So I THINK I want a P-bass pickup that's voiced more modern, i.e. on the bright side. I'll probably be using Cirkle-K strings, not flats, which is a start. I note from sound samples and comments of TB reviewers that some P pickups, like Delano, and DiMarzio Split-P may be somewhat brighter, so I'll go for one of them.

All I THINK I need is a little sculpting ability--the sweepable mid boost/cut of this project. This is my attempt at cutting down the TB11 pre you guys worked so hard to design so it uses one OPA2277 and loses (a) summing of 2 pickups and (b) the bass and treble IC. If I find that this is a workable approach, I'll build it on perfboard and leave room for revisions in case I miss the extra treble/bass EQ section. The sweepable mids appeal to me because you can do a low mid (scoop out) cut or a high mid boost.

At first I just pulled out the 2-pickup summing and the bass and treble stage. But I saw that it makes the output inverted, so I inverted the first stage and came up with this:

Caution to browsers (like me), DON'T build from this schematic, it's just amateur guesswork!

SimplifiedTB-11BassPreampDraft1.jpg


Any suggestions about whether it will work and whether I need to adjust component values would be very helpful. If this is too far off-topic I'll repost, but I figured this could stand a bump and to me the mid-range control is what makes the TB11 unique.

John
 

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#127
JayGunn said:
SimplifiedTB-11BassPreampDraft1.jpg


Any suggestions about whether it will work and whether I need to adjust component values would be very helpful. If this is too far off-topic I'll repost, but I figured this could stand a bump and to me the mid-range control is what makes the TB11 unique.

John
Click to expand...

I've been so swamped with other projects I haven't had much time to look at this since last summer.

Unfortunately the midrange control will not work properly as drawn: this was a mistake in my earlier schematic due to the way I separated the controls. The second op-amp needs to have a DC bias point. This can be simply done by adding a feedback resistor from the output to the negative input.

I have also found that the 250K "drift" resistor is not needed if you use a OPA2277, as the bias current is in the nA range. The 100 ohm output resistor and cap are also not needed (again, only with the OPA2277) as the DC error is almost immeasurable at these gains.

Lastly, this control is not "sweepable". The midrange frequency is fixed and is set by the combinations of the 5K, 100K, and 10nF resistors. The gain is adjustable from +- 12-15dB however. If you want a sweepable control that was provided in post #64.
 
#128
Looks like the link in post #64 is broken.

Here is the sweepable midrange schematic:

parametric_mid_eq.gif


The ratio of C5 to C6 controls the 'Q' (or width of the filter), and these with R12, R13, and R10 define the 'bottom' and 'top' frequencies of the sweep.
 
#130
Glad to see you reappear, MglMatador, and just as I was thinking about the preamp and how to finally assemble it. :D

If my tally is correct, the total number of modules the TB11 now supports is five:
  • power supply section
  • low-Z, load cap and buffer section
  • two-band bass/treble tone stack
  • fixed-frequency mid-range control
  • parametric mid-range control

Were you planning on redrawing the PDF with all those modules? The old PDF had the first four, although the bass/treble and mid were conjoined in the last one, so if you drew a new one it'd clarify a bit which is which for those who don't know.

Also, in post #102 you had JP4 and JP6 (two-pin jumper headers) in there, but I couldn't find their function - what do they do?
 
#132
Stealth said:
Were you planning on redrawing the PDF with all those modules? The old PDF had the first four, although the bass/treble and mid were conjoined in the last one, so if you drew a new one it'd clarify a bit which is which for those who don't know.

Also, in post #102 you had JP4 and JP6 (two-pin jumper headers) in there, but I couldn't find their function - what do they do?
Click to expand...

Sure I can redraw them, included the fix for the missing resistor in the midrange section!

JP3 and JP6 are for the neck and bridge "load cap's" (respectively). They simulate a long cable load on the pickups and change where the resonant bump happens (higher C moves this bump down in frequency).

JP4 and JP7 are for the neck and bridge "low-z" modes (respectively). They simulate a lower load on the pickups which darkens the tone by damping the magnitude of the resonant peak.

All of these can be moved to pickguard or plate SPST switches if you want to fiddle with them in real time.
 
#133
See, that's what confused me. I know those can be either jumpers for "offline" or SPSTs for "online" changes, and that's clear in post #102, file tb11-frontend - there's the volume-control-full-on loading resistor, the simulated cable capacitor (JP3, JP6) and low-Z resistor in parallel (JP4, JP7), just as you said.

However, in the tb11-filter file the JP4 and JP6 are there again - those are the headers that confuse me. What do those two headers, in the filter section, do? Is JP4 an output from the bass/treble stack and JP6 from the entire three-band EQ?
 
#134
Stealth said:
However, in the tb11-filter file the JP4 and JP6 are there again - those are the headers that confuse me. What do those two headers, in the filter section, do? Is JP4 an output from the bass/treble stack and JP6 from the entire three-band EQ?
Click to expand...

Oh....I see what you are talking about!

On the filter page, JP4 and JP6 are layout provisions for RF bypass caps.

I wanted to reserve space in the layout in case the circuit oscillated in the MHz range. If it did, one can add a small film cap across those terminals to make the gain 0 in the MHz range. They also make a convenient place to attach a scope.

When I redo the schematic I'll label all of that stuff.
 
#139
Nobody ever built these because I had the only two prototypes. :smug:

I found three layout errors on the first boards: the last of which was not fixable as it was an incorrect connection of a pin to a ground plane. There no way to dremel it up to fix it. Hence the first boards only worked as buffers: there was no way to make the parametric midrange work or EQ sections work.

I did fix the layout and added the provision for the virtual earth chip. But I never made a second round of prototypes as I got sucked in to other projects.
 
#140
In case anyone is interested, here was the fixed layout I came up with some time ago.

I haven't had any space on my last few panels but if someone is interested I can see if I can have a few made and send them out.

Keep in mind: I think this should work properly but I haven't built/tested it yet!

tb11_layout_v2a.jpg
 

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