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TB11 Preamp Project

#43
Been researching op-amps for other drop-in alternatives that might work in this application (for prototyping). You can see the Burr Brown part (OPA2277) completes dominates the specs, but at $4 per part (as compared to about $0.50 for the others) it should be!

#1 - MC33178

Offset: 0.15mV - 3mV
Supply Current (Dual): 1.4mA
Slew Rate: 1.2 - 2V/uS
Input Bias Current: 100 - 500nA
Input Offset Current: 5 - 50nA
CMRR: 80 - 110dB
GBP: 2.5 - 5 MHz
Input Impedance: 200kOhm
Output Impedance: 150 Ohm

#2 - OPA2277 (I really like this one!)

Offset: 20 - 50 uV
Supply Current (Dual): 1.6mA
Slew Rate: 0.8V/uS
Input Bias Current: 2.8nA
Input Offset Current: 2.8nA
CMRR: 130-140dB
GBP: 1MHz
Input Impedance: 250 GOhm
Output Impedance: unspecified

#3 - LM358

Offset: 2 - 7 mV
Supply Current (Dual): 1 - 1.2mA
Slew Rate: unspecified
Input Bias Current: 45 - 250nA
Input Offset Current: 5 - 50nA
CMRR: 65-85dB
GBP: 1 MHz
Input Impedance: unspecified
Output Impedance: unspecified
 
#45
Here is a link to the schematics in PDF format:

Invalid Link Removed

A few notes:
1) Schematic lists TL072 op-amp, but it has way too high a power draw for an on-board preamp. It can be substituted for any of the others mentioned above
2) The headers are not necessary, they were just for making connections to the board.
3) Any resistors will work, however I listed 1% values.
4) The pots are not listed, but the design needs:

Two volume controls - pots in the 25k to 50k range will work fine (audio or linear tapers are fine).
Bass control - 100k linear
Mid control - 100k linear
Treble control - 500k linear

These can be stacked to work with a standard three-hole jazz setup:
Volume/Volume (25k - 50k stacked)
Bass/Mid (100k stacked)
Treble

5) C3 and C4 control the capacitance added to the pickups. Size to taste (bigger values will shift the pickups resonant peak down more).
6) R4 and R6 set the pickup loading. I stuffed a 500k, but you can use 250k if you like that sound better.
7) C1 and C2 can be lower: probably 10uF would suffice.
8) Gain of the preamp can be increased by increasing R9. A 200k resistor will increase the gain 6db, 400k, 12db, etc. Gains above 6db should be achieved by reducing R7 and R8 (which sets the input impedance of the blend section). Given there is 12-20db of boost in the tone controls (and no insertion loss) increasing the gain should not be needed.
9) Wire battery negative to the sleeve connection of a stereo jack to get auto-shutoff when the cable is unplugged (not shown in schematic).

I have most of everything needed to prototype (except the op-amps), so I'll order some and try to prototype this in a few weeks.
 
#47
51nf is a pain to find in stock :( (could make one with a couple of caps, but thats messy..)

Could you run a sim and see what the penalty would be for running 56nf is?

So far in parts for everything its $27.03 at mouser. To put that in perspective, just over 50% of that cost is the opamps alone. Subtract $11.68 if you go with MC33178 or subtract roughly $4 if you go with the lower spec OPA2277.
 
#50
12bass said:
Other op amps to consider:

OPA1642: http://focus.ti.com/docs/prod/folders/print/opa1642.html

AD8512: Invalid Link Removed

My own experience suggests that the quality of the op amps can noticeably affect the tone of the instrument, particularly with very low power devices.
Click to expand...

Those are both very nice, but can you run them on batteries? The quiescent current (Iq) of the OPA1642 is 2.3mA. The AD8512 is 2.5mA. The OPA2277 is 800µA. Big difference.

For those not familiar with the term, quiescent current is the amount of current consumed when the device is idle. It will use even more when you are putting a signal through it.
 
#51
Those are maximum figures, per channel. OPA1642 is nominally 3.6 mA (1.8 mA x 2) quiescent, which would give roughly 170 hours from a 9V alkaline. There's a trade-off between sound quality and battery life. For me, if it means better tone, I'd rather change the battery a little more often, or, even better, use rechargeables.
 
#52
SGD Lutherie said:
Why? It's .051µF, so just use .05µF (50nF). That's a common value. It's used in plenty of passive tone controls.

Part of designing circuits is to find common part values.
Click to expand...

Even that's non stocked at mouser, already thought of that or else I wouldn't be asking, you have to consider stock levels too (thats also part of design ;))... I'll check a few other places to see if there is anywhere that has all the parts in-stock but mouser is a quality company with reasonable shipping on small qty stuff that is accessible for the hobbyist.

Its going to be very little fun for anyone doing this as a hobby-style project if they have to source stuff from 2 or more suppliers... Not everyone enjoys the chase man.

What good is any of this if 1 person with limited experience couldn't do the whole thing front to back with little to no hassle, including sourcing the parts in small qty?
 
#53
Part of designing circuits is to find common part values.
Click to expand...

I picked 1% tolerance standard cap values for metalized film polypropylene caps (standard according to my layout program that is).

What good is any of this if 1 person with limited experience couldn't do the whole thing front to back with little to no hassle, including sourcing the parts in small qty?
Click to expand...

You are exactly correct...I should have checked for stock at Digikey and Mouser sorry about that.

I'll check for substitutable values tomorrow.
 
#54
12bass said:
Those are maximum figures, per channel. OPA1642 is nominally 3.6 mA (1.8 mA x 2) quiescent, which would give roughly 170 hours from a 9V alkaline. There's a trade-off between sound quality and battery life. For me, if it means better tone, I'd rather change the battery a little more often, or, even better, use rechargeables.
Click to expand...

Thats the wonderful thing about what MglMatador is doing here, feel free to swap them in and make an A/B comparison recording with the reference design he's come up with.. well when it becomes the reference design ;)

I'd be quite excited if there is a difference I can hear with my ears.
 
#55
MglMatador said:
You are exactly correct...I should have checked for stock at Digikey and Mouser sorry about that.

I'll check for substitutable values tomorrow.
Click to expand...

What I said might have come out the wrong way, wasn't intended as anything harsh. I just know that when I started doing projects out of magazines I always got frustrated tracking down that one component that was a make/break part... and that was before the internet made it so easy!

I went with 5% MLCC's instead of polyfilm when I did my list at mouser, and they had plenty of stock. I trust that ceramics can 'take the heat' from a hobbyist behind the iron ;)
 
#56
Substitutions:

C3, C4, and C8 can be 1000pF / 1nF / 0.001uF
C5 can be 0.047uF or 0.056uF

The C5 and C8 substitutions move the stop and turnaround frequencies less than 10 Hz.

Is anything else hard to find? I'll amend the BOM with Mouser and Digikey part numbers if anyone helps to supply them.

The Wima Metalized Polypropylene Film Caps are great quality caps that fit the layout I used (0.2", or 5mm lead spacing), and have all of the required values. The Xicon Film caps are good too. Ceramic caps are good so long as you order a handful and measure them as they tend to be much higher tolerance.

In terms of battery life, a 9V lithium battery can have in excess of 1200+ mAh which might be used if the high(er) current op-amps sound better to some listeners (twice the capacity of alkaline based batteries).
 
#57
MglMatador said:
I picked 1% tolerance standard cap values for metalized film polypropylene caps (standard according to my layout program that is).
Click to expand...

Yes, that's good, as is using the Wima film caps, but 0.051µF is not a common value. It seems even .05 is hard to find these days in that type of cap, but .047µF is easy.

But you are doing a great job. :)
 
#59
Interesting thread. Keep up the good work!

My brother-in-law is working on a similar project of his own. His is a three-pickup buffered preamp with three bands of EQ, of a design not too dissimilar from this one. We've been testing his prototype and it sounds really nice.
 
#60
MglMatador said:
Ceramic caps are good so long as you order a handful and measure them as they tend to be much higher tolerance.
Click to expand...

These ain't what many of us remember ceramic caps to be; wildly variant and seldom on spec. Kemet MLCC's actually come tighter spec'd than those wima's and probably outperform them in a majority of ways that don't apply to this application ;) But the main reason to use them is they can tolerate around 30% more time under the iron. I'm pretty sure most people will be going at this thing with 300-400C non-controlled irons and might need the extra space.

But I ordered a run of everything I didn't already have laying around, so I will have a proto ready next week :)
 

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