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HELP needed! Bass Preamp Pedal
HELP needed! Bass Preamp Pedal
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Old 26th January 2018, 04:31 PM   #1
Baabi74 is offline Baabi74  Finland
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Default HELP needed! Bass Preamp Pedal

Hello there to each and every one.

I've been fiddiling around with analog audio circuits for some time now, and have made a few onboard preamps for my basses that I build.
Now, as my pedalboard seems to be growing bigger and bigger every year (no surprice there...) I'm having trouble to find a preamp pedal that suits my specific needs.
What I need is a high impedance input, a gain stage, 3-band eq and three seperate outputs. One output for the "dry" signal chain and two outputs with a high-pass filter, to feed the "wet" portion of the board (stereo chorus, stereo reverb, etc.).

So I've been searching the net for various circuits and have learn a lot about analog audio circuits, BUT some components I do not understand what they do and why they are in the circuit at the specific locations they are. Not to mention why that specific component has the value it has and how the value will effect the signal IF the value is changed.

Anyway, what I've done is put together a schematic using various circuits and I would appreciate if someone would care to take a look at it and comment on it.

A jpg file of the schematic is attached.

What's wrong with it? Will it go up in smoke if I build it and turn on the power?

Cheers,
"Bobby"
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File Type: jpg The-Vanguard-Bass-Preamp-2.0.jpg (194.9 KB, 111 views)
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Old 26th January 2018, 10:52 PM   #2
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by Baabi74 View Post
What's wrong with it? Will it go up in smoke if I build it and turn on the power?
I can't anything wrong, for what that's worth.

Why not breadboard it, or at least the individual modules, and test them to see if they work (and sound) the way you want?

Is a maximum gain of 7 times (i.e. 60k/10k + 1) okay with your bass guitar and +/- 9V supplies? Still have some headroom left before clipping?

-Gnobuddy
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Old 27th January 2018, 12:19 AM   #3
PRR is offline PRR  United States
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The OPA chips are better inputs, and better outputs, than simple JFETs. Lose the FETs.

The OPA parts' DC error is low enough that I would skip output blocking caps. However your gain stage may multiply DC error, and I would stick a blocking cap after it.

Outputs to cables should have a few hundred Ohms series resistance.

This is configured for bipolar +/- supplies. That's often awkward on a pedalboard.
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File Type: gif bass-preamp-pedal-vanguard-bass-preamp-2-0-1.gif (15.1 KB, 104 views)
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Old 27th January 2018, 11:11 AM   #4
Baabi74 is offline Baabi74  Finland
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Originally Posted by Gnobuddy View Post
Why not breadboard it, or at least the individual modules, and test them to see if they work (and sound) the way you want?

Is a maximum gain of 7 times (i.e. 60k/10k + 1) okay with your bass guitar and +/- 9V supplies? Still have some headroom left before clipping?
-Gnobuddy
Hi "Buddy"
My thought exactly. I know that the input buffer and the eq section works. The high-pass section is another question.
Speaking bout the high-pass section... Should there be a input buffer in front of the high-pass section?
The original circuit had a input buffer, but I removed it so I could use that op-amp for the output buffer for Output A.

Well, these days most electric basses have some sort of active electronics built into them (onboard preamps). They do boost the signal a bit, but not much. So therefore a LOT of gain is not neceserilly needed, I mean this is not a overdrive pedal. Besides, if you boost the signal too high in this pedal, it's possible that the effect pedal(s) that is connected after this pedal will clip.
However, you've got a good point there, this issue remains to be tested out in the future...
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Old 27th January 2018, 11:44 AM   #5
Baabi74 is offline Baabi74  Finland
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Quote:
Originally Posted by PRR View Post
The OPA chips are better inputs, and better outputs, than simple JFETs. Lose the FETs.

The OPA parts' DC error is low enough that I would skip output blocking caps. However your gain stage may multiply DC error, and I would stick a blocking cap after it.

Outputs to cables should have a few hundred Ohms series resistance.

This is configured for bipolar +/- supplies. That's often awkward on a pedalboard.
Hi "PRR"
Youre right, I too would like to use only op-amps. If you think the JFET output buffers are not really needed, I will probably remove them.
However, the JFET input buffer is there to get that valve/tube-like sound that I'm after. What's YOUR experiences with JFET's and the sound of them?

Aaaa, ok. Didnt know that a gain stage can be "unstable" in that sence. Thanks for that info! However, the reason why I put a cap on the outputs is to protect the circuit(s) from static discharging that could occur in the long cables that are connected to the outputs. Is this a good idea or not?

Ok, so WHY should there be a few hundred Ohms series resistance on the outputs? There's one in the high-pass circuit too, but I dont know why...

Correct me if I'm wrong but it seems that this OPA2134 is designed to run on +/- supply rails only.
So I'm going to use a switched-capasitor voltage converter (the ICL7660) to get that -9V.
I've done this before and it seems to work, check it out on my website here; The Seven Seas

Cheers
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Old 28th January 2018, 01:47 AM   #6
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by Baabi74 View Post
Should there be a input buffer in front of the high-pass section?
It has to be fed from a low impedance output, yes - but your Output A is already low impedance. That means you don't need any additional buffer.

Quote:
Originally Posted by Baabi74 View Post
Well, these days most electric basses have some sort of active electronics built into them (onboard preamps).
Agreed. I have three bass guitars. None of them are expensive models. And all three have active electronics.

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Originally Posted by Baabi74 View Post
..If you boost the signal too high in this pedal, it's possible that the effect pedal(s) that is connected after this pedal will clip.
Exactly, and you could also clip this DIY bass preamp itself. Particularly if you like to slap and/or pop. Those sorts of playing techniques tend to put out a very "hot" signal, so you either have to turn the volume control on the bass down a lot, or have a lot of input headroom in your bass preamp.

Do you use a compressor with your bass? If so, will it go between bass and this preamp, or after the preamp? Compressors tend to be fussy about signal level.

-Gnobuddy
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Old 28th January 2018, 02:02 AM   #7
Gnobuddy is online now Gnobuddy  Canada
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Originally Posted by Baabi74 View Post
However, the JFET input buffer is there to get that valve/tube-like sound that I'm after. What's YOUR experiences with JFET's and the sound of them?
This is the sort of topic people argue about for days.

I am currently building a small solid-state guitar (not bass) amp. For the guitar, my ears definitely prefer having a JFET at the input, preferably a common-source gain stage (rather than a source follower). I can't actually hear any "tube-like sound" with the JFET, but it does sound a little less harsh than an op-amp gain stage.

Here's the thing: modern op-amps are absolute engineering marvels. They are precise, accurate, and fast. So I have no doubt that a properly designed and powered (i.e., keep it out of clipping!) op-amp circuit will do a perfect, accurate, precise job of reproducing a guitar signal.

Meantime, a JFET common-source stage is not precise or accurate at all. Gain varies with signal. Big transients are probably "squashed" by this action.

So the fact that I prefer the sound of the JFET is not a criticism of op-amps. Rather, I think the truth is that electric guitar does not sound good if you reproduce its signal accurately. I think squashing the signal a little makes it sound more attractive and less harsh.

I have never built a bass amp or preamp, so I don't know if my ears feel the same way about bass. And of course, I have absolutely no idea how your ears feel about any of this!

So I would say this is another reason to breadboard your circuit and try it out before you build the final version. Maybe you'll want the JFET inputs, maybe you won't. Only you and your ears can tell.

Quote:
Originally Posted by Baabi74 View Post
Ok, so WHY should there be a few hundred Ohms series resistance on the outputs?
Op-amps (and other high-gain, high feedback amplifiers) don't like having a capacitor connected directly to their outputs. But if you connect a length of guitar cable to the output, it has quite a lot of capacitance. This can drive the op-amp into instability, worst case.

Adding a few hundred ohms of series resistance, right at the op-amp output pin, removes the problem. The resistor buffers the cable capacitance from the op-amp, so it can remain stable.

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Originally Posted by Baabi74 View Post
Correct me if I'm wrong but it seems that this OPA2134 is designed to run on +/- supply rails only.
It is possible to get around this: use two equal-value resistors across the power supply to create a 4.5V point, which then acts as the "0V" for the op-amp. In effect, you get a fake 0V ground, along with +/- 4.5V supply rails. This is done in a lot of guitar FX pedals.

However: 9V is really a small amount of voltage to run a guitar preamp on. I like your way (with 0, +9V, - 9V rails) better.

-Gnobuddy
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Old 28th January 2018, 12:33 PM   #8
Baabi74 is offline Baabi74  Finland
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Quote:
Originally Posted by Gnobuddy View Post
It has to be fed from a low impedance output, yes - but your Output A is already low impedance. That means you don't need any additional buffer.

Do you use a compressor with your bass? If so, will it go between bass and this preamp, or after the preamp? Compressors tend to be fussy about signal level.
-Gnobuddy
Allright, I'll take your word for it...

Yes, at the moment I'm using the TC Electronic Spectra Comp. Finally I've found a small bass pedal compressor that you can adjust the threshold on! AND it is a 3-band compressor too. Happy days
Before the preamp, as most people do.
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Old 28th January 2018, 01:11 PM   #9
Baabi74 is offline Baabi74  Finland
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Quote:
Originally Posted by Gnobuddy View Post
This is the sort of topic people argue about for days.

I am currently building a small solid-state guitar (not bass) amp. For the guitar, my ears definitely prefer having a JFET at the input, preferably a common-source gain stage (rather than a source follower). I can't actually hear any "tube-like sound" with the JFET, but it does sound a little less harsh than an op-amp gain stage.-Gnobuddy
So what are you saying here? Are you saying that a simple JFET buffer (a stage that NOT amplify the signal) does not sound "tube-like"? I think you are right about that. Been building the Tillman JFET for various projects and it seems it does sound "warm","round" and a bit compressed. Maybe its possible that to get that "tube-like" sound out of a JFET stage it HAS to boost the signal?

Quote:
Originally Posted by Gnobuddy View Post
Here's the thing: modern op-amps are absolute engineering marvels. They are precise, accurate, and fast. So I have no doubt that a properly designed and powered (i.e., keep it out of clipping!) op-amp circuit will do a perfect, accurate, precise job of reproducing a guitar signal.

Meantime, a JFET common-source stage is not precise or accurate at all. Gain varies with signal. Big transients are probably "squashed" by this action.

So the fact that I prefer the sound of the JFET is not a criticism of op-amps. Rather, I think the truth is that electric guitar does not sound good if you reproduce its signal accurately. I think squashing the signal a little makes it sound more attractive and less harsh.

I have never built a bass amp or preamp, so I don't know if my ears feel the same way about bass. And of course, I have absolutely no idea how your ears feel about any of this! -Gnobuddy
I agree. But the question is "Are we trying to make the instrument (guitar, bass or whatever) connected to this preamp as perfect, accurate, and a precise job of reproducing instruments signal?"
Don't know about you, but I'm not. I'm not trying to build a high-end "hifi-like" preamp, I'm simply trying to build a preamp that sounds "good". "Good" in this case would be a "warm", "round" bass sound that still cuts trough the entire mix of the band. Easier said than done...

Quote:
Originally Posted by Gnobuddy View Post
So I would say this is another reason to breadboard your circuit and try it out before you build the final version. Maybe you'll want the JFET inputs, maybe you won't. Only you and your ears can tell.-Gnobuddy
Yeah, gonna breadboard the high-pass circuit first to see how that performs.

Quote:
Originally Posted by Gnobuddy View Post
Adding a few hundred ohms of series resistance, right at the op-amp output pin, removes the problem. The resistor buffers the cable capacitance from the op-amp, so it can remain stable.-Gnobuddy
Allright, got it. Thanks. That's one of the "wierd" things I've seen in schematics over the years and often wondered "What the *ell is that resistor there for?"

Quote:
Originally Posted by Gnobuddy View Post
However: 9V is really a small amount of voltage to run a guitar preamp on. I like your way (with 0, +9V, - 9V rails) better.
-Gnobuddy
And speaking of that, I think I've run into a problem with the charge pump IC. The OPA2134 draws about 4mA per amp (according to the spec sheet). Using both amps in the chip will draw a total of 8mA:s. The max output current of the ICL7220 is 10mA:s. Adding another OPA2134 chip will raise the current drawn to about 12mA:s (IF using JUST one amp in the chip). Will the ICL7220 charge pump "blow up"? It might.
So I think I'd better find a charge pump chip that is capable of higher output currents, like the MAX634CPA.
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Old 28th January 2018, 07:27 PM   #10
Gnobuddy is online now Gnobuddy  Canada
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Originally Posted by Baabi74 View Post
Allright, I'll take your word for it...
My word is good on this one, because it's based on engineering fact: the reason you need a buffer before your active high-pass filter is because it (the high-pass) is sensitive to source resistance of whatever is feeding it. Any resistance there will affect your intended cutoff frequency, and also affect the "Q" of the filter.

So, in order to work as designed, the high-pass needs to be fed from a low impedance source. And that's what you have at your output A.

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Originally Posted by Baabi74 View Post
Finally I've found a small bass pedal compressor that you can adjust the threshold on! AND it is a 3-band compressor too. Happy days
Sounds good! I am pretty happy with this: ART Tube MP/C | Sweetwater

It is not specifically designed as a bass guitar compressor, but it works very well for me in that role. Gain and threshold are adjustable, as is output level.

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Before the preamp, as most people do.
Okay, then the preamp gain won't cause problems for the compressor. One less thing to worry about.

-Gnobuddy
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