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HELP needed! Bass Preamp Pedal
HELP needed! Bass Preamp Pedal
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Old 28th January 2018, 07:52 PM   #11
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Baabi74 View Post
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.
To my ears, a JFET sounds better than an op-amp, but not as good as a real valve. And a JFET common-source gain stage sounds better (i.e. a wee bit warmer) than a source follower. But still not as good as a real valve (and I've tried to make it as good!)

I think the trouble is the 100% negative feedback in a source follower. The feedback makes the JFET pretty linear. And linear is not what sounds best.

Quote:
Originally Posted by Baabi74 View Post
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?
The Tillman circuit is set up for a voltage gain of somewhere between 2x an 3x. It has a lot of negative feedback, too, from the unbypassed 2.2k source resistor.

I have used something very similar in my current project amp. Mine has a 47k drain resistor and 10k source resistor, which happen to be the right values to bias this particular MPF102 properly using a 24V supply. (JFETs have huge manufacturing variations, another MPF102 may need something other than 10k at the source to bias to the proper operating point.)

The trouble, I find, is the (relatively) low supply voltage that JFETs can tolerate. If you bypass that source resistor with a cap, the JFET immediately becomes less linear, more "tubey". But the gain also goes up, and so it will also overload and clip easily, and may not handle the full output from your bass.

A valve stage doesn't have the same trouble, mainly because you can run it from +300V rather than +/- 9V. There is a lot more output headroom before clipping.

So far, I cannot find a way to make a JFET sound as good as a real vacuum triode. I haven't yet explored using a small MOSFET along with a high voltage supply rail (+300 V if necessary.)

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Originally Posted by Baabi74 View Post
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.
We are on the same page...imperfect amplification sounds better when it comes to guitars!

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Originally Posted by Baabi74 View Post
And speaking of that, I think I've run into a problem with the charge pump IC.
You might try Amazon.com to see if you can find a DC-DC converter that does what you want. There are a tonne of positive voltage ones, but fewer ones that take a positive input and generate a negative DC rail from it. But you may get lucky and find what you need, a DC-DC converter that runs on +9V and generates -9V.

-Gnobuddy
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Old 29th January 2018, 10:34 PM   #12
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Gnobuddy View Post
You might try Amazon.com to see if you can find a DC-DC converter that does what you want.
I took a quick look for you, and found these two:

1) Amazon.com: Icstation DC Voltage Regulator Step Up Boost Converter Power Supply Module 3-10V to +-12V Postive Negative Dual Voltage Output: Industrial & Scientific

2) Amazon.com: DROK Mini Boost Voltage Converter DC 3.2~13V to DC +-15V Positive Negative Dual Volt Output Non-isolated Module 100mA Power Regulator Step Up Volts Transformer Board: Automotive

-Gnobuddy
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Old 30th January 2018, 04:40 PM   #13
Baabi74 is offline Baabi74  Finland
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Quote:
Originally Posted by Gnobuddy View Post
To my ears, a JFET sounds better than an op-amp, but not as good as a real valve. And a JFET common-source gain stage sounds better (i.e. a wee bit warmer) than a source follower. But still not as good as a real valve (and I've tried to make it as good!)-Gnobuddy
Nope, there's nothing like real tube-amp. Had the pleasure to test out a Ampeg SVT II Pro (even the power amp is tube only) once. OMG! THATS what a bass amp should sound like! Too bad it weighs 70 Pounds!

Anyway...

I saw this interesting looking schematic of a op-amp driven clip LED. See attached file. I do have one spare op-amp to use, so maybe it would be cool to use that op-amp to drive a clip LED? Don't know, we'll see...

Maybe I'll make a JFET input stage in front of the op-amp gain stage, and make it bypassable (is that even a word?) with a DPDT mini toggle switch...

Desicions desicions...
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File Type: jpg Op amp clip LED schematic.jpg (43.5 KB, 53 views)
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Old 30th January 2018, 09:25 PM   #14
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Baabi74 View Post
Too bad it weighs 70 Pounds!
Yeah! I don't know how musicians coped in the days when every guitar amp weighed 50 lbs, and the bass amps weighed even more. I guess you either had to be young and strong, or have roadies.

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Originally Posted by Baabi74 View Post
I saw this interesting looking schematic of a op-amp driven clip LED.
I wonder if it would be worth adding one more diode, and a capacitor across the LED (to make it stay on a bit longer after each clipping event)?

I think this should work (attached). Some experimentation with the capacitor value might be needed to keep the LED on long enough to be an improvement, without slowing down the clip indicator so much that brief clipping might be missed altogether.

-Gnobuddy
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File Type: png Clip_Indicator_001.png (59.9 KB, 52 views)
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Old 31st January 2018, 02:39 PM   #15
Baabi74 is offline Baabi74  Finland
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Quote:
Originally Posted by Gnobuddy View Post
I wonder if it would be worth adding one more diode, and a capacitor across the LED (to make it stay on a bit longer after each clipping event)?

I think this should work (attached). Some experimentation with the capacitor value might be needed to keep the LED on long enough to be an improvement, without slowing down the clip indicator so much that brief clipping might be missed altogether.
-Gnobuddy
Did some breadboarding today.
The high-pass circuit works. Who-hoo! Lovered the value of C13 and C14 to 100nF. That resulted in a bit higher max. cut-off freq, around 300Hz.

Also breadboarded the op-amp clip circuit. Both works, but the 1uF cap across the LED (and its resistor) dont have any visual effect. Only about 100uf and 220uF makes it light up longer, BUT the intensity of the LED is not as bright.
Good idea, but not very practical.

Something way more practical would be to somehow integrate a green "signal" LED to the circuit. I have no idea how to experiment with something like that, so...

One perhaps a very insignificant thing I've been wondering about is the values of R6 and R7 in the gain stage. Are those value critical somehow? They're now set to 10k, but could they be like 1k or even 100k? Does it matter?

Cheers, "Bobby"
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Old 31st January 2018, 03:22 PM   #16
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Baabi74 View Post
Good idea, but not very practical.
Thanks for testing it out. The trouble is that (relatively speaking) it takes a lot of current to charge up a 100 uF capacitor in a millisecond or two, and the TL072 cannot deliver that much current.

I think it would work better to have the capacitor working at a smaller signal (current) level, and buffer it to drive the LED, but that would require two opamps.

There's a red/green LED indicator circuit here that might be of interest (I haven't tried it): MUFF WIGGLER :: View topic - [Project] 1 LED signal level meter (sch+PCB)

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Originally Posted by Baabi74 View Post
...the values of R6 and R7 in the gain stage. Are those value critical somehow? They're now set to 10k, but could they be like 1k or even 100k? Does it matter?
They're not critical, over a reasonably wide range. Lower values reduce thermal noise a little, but demand more current from the opamp. Extremely high values will misbehave because stray capacitances in the circuit will start to affect impedance, and/or may increase DC offset due to input bias current of the opamp.

10k is a very commonly used value here. Big enough to not load the op-amp too much, low enough not to add much thermal noise for most applications.

-Gnobuddy
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Old 1st February 2018, 10:45 AM   #17
Baabi74 is offline Baabi74  Finland
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Quote:
Originally Posted by Gnobuddy View Post
There's a red/green LED indicator circuit here that might be of interest (I haven't tried it): MUFF WIGGLER :: View topic - [Project] 1 LED signal level meter (sch+PCB)
-Gnobuddy
3 op-amps to feed 2 LEDs? That seems a bit overkill to me, but if that is what it takes...

Gonna stick to this original clip circuit. Made a few changes to the overall schematic.
See anything wierd or questionable?
Attached Images
File Type: jpg The-Vanguard-Bass-Preamp-2.1.jpg (255.2 KB, 47 views)
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Old 1st February 2018, 07:32 PM   #18
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Baabi74 View Post
3 op-amps to feed 2 LEDs? That seems a bit overkill to me, but if that is what it takes...
For commercial manufacturers, op-amps are dirt-cheap, and you can get four of them in one cheap chip.

For us hobby builders, op-amps are still cheap, but now you have all those joints to solder, resistors to install, wires to connect...it gets tedious!

For this reason, one project I will never DIY is an audio mixer. I could use a six-input one, but building six identical channel strips is just too tedious for me. It would probably cost me more than a commercial product by the time I was done, too.

Quote:
Originally Posted by Baabi74 View Post
Made a few changes to the overall schematic.
My only suggestion is to breadboard and test that JFET input stage. With the reduced supply voltages that JFETs can handle (compared to old-fashioned vacuum triodes), there is a much finer line between "too clean" and "too distorted".

-Gnobuddy
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Old 9th February 2018, 04:12 PM   #19
Baabi74 is offline Baabi74  Finland
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Quote:
Originally Posted by Gnobuddy View Post
My only suggestion is to breadboard and test that JFET input stage. With the reduced supply voltages that JFETs can handle (compared to old-fashioned vacuum triodes), there is a much finer line between "too clean" and "too distorted".
-Gnobuddy
Allright...
Did some breadboarding on the JFET stage AND some testing at the same time. Had to test out what the minimum (and/or maximum) values for all those caps.

So I decided to remove the cap from source to Gnd. Don't need that much gain. Actually I did test how much gain you can get out of a circuit like this and it seems to be about 10. Fed it with sinuswave 70mV and out came approx. 700mV. Did test out at what point this circuit starts to clip and 800 - 850mV on the output is pretty close, depending on the JFET of course.

So I guess this is what I'm gonna build, working on some layouts.

Cheers, "Bobby"
Attached Images
File Type: jpg The-Vanguard-Bass-Preamp-2.1.jpg (255.2 KB, 35 views)
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Old 5th March 2018, 03:18 PM   #20
Baabi74 is offline Baabi74  Finland
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Default Build finished

Hi guys (and gals).

Thought I just let you know how this project ended up. Just finished the making of the web-page on the project. If you're interested, check it out here:

The Vanguard

Thanks "Gnobuddy", your help was/is much appreciated

Cheers,

"Bobby"
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