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Author Topic: Big Muff Pi passive tone control and small amps TL072CP, JRC386D.  (Read 349 times)

Oooscar

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Hello,

I am starting to experiment with opamps as preamps, and/or headphone amps.
I am trying to add a tone control, and the big muff pi seems a simple one. I put it between the two op amps of a TL072CP but the sound I get is very bad. When there is not tone control, I get at least a clean sound when I listen with my headphones, but when I introduce the big muff pi tone control, the sound is noisy/distorted (in a unpleasant way). What I am looking for is a clean and filtered sound. If I want musical distortion I will add it later. But just to go step by step.

I have tried also to add the big muff pi passive tone control to the output of a JRC386D simple circuit, with just a capacitor at the output, and I get no sound at all. I am thinking if the output current of the opamp is not enough for the tone control circuit. When I remove it, I get sound in my headphones. I know that there is a circuit called Smash Drive that has a tone stack in the output of a LM386, but it is used as a pedal, and I have no tried and don't know if it has enough power the signal to drive headphones.

I am trying things with ingenuity, just step by step. Maybe some of you here have tried the same circuits or similar ones, or have more knowledge, and can give me some clues to help me get those circuits work.

Cheers,

Oscar.
« Last Edit: April 13, 2020, 08:49:56 AM by Oooscar »

Jazz P Bass

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Re: Big Muff Pi passive tone control and small amps TL072CP, JRC386D.
« Reply #1 on: April 13, 2020, 11:57:10 AM »
Some type of schematic of what you have tried would be a help.
The tone control is only 5 components.

You can even 'build' it on to a control pot.
http://beavisaudio.com/techpages/BigMuffToneControl/

Here is the schematic of the actual Big Muff TC:

Oooscar

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Re: Big Muff Pi passive tone control and small amps TL072CP, JRC386D.
« Reply #2 on: April 13, 2020, 01:35:48 PM »
Here it is.

The first issue is solved. I added a 100n capacitor to isolate the filter from the dc of the second op amp as well, and it seems to work.

The second issue with the JRC386D remains. The picture shows both schematics.


Jazz P Bass

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Re: Big Muff Pi passive tone control and small amps TL072CP, JRC386D.
« Reply #3 on: April 13, 2020, 02:49:21 PM »
I am no expert on the LM386D amplifier IC but it seems to me that adding a 'tone control' circuit on the output of the device is not a good idea.
Have you tried the circuit on the input side?

phatt

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Re: Big Muff Pi passive tone control and small amps TL072CP, JRC386D.
« Reply #4 on: April 14, 2020, 04:35:29 AM »
Looks like you are using a single supply.
If so you need to create a bias supply.
Look at any old TS9 schematic or similar circuit and you will see how it is done.
if your supply is  9 volt then you need 4.5Volt bias.
The LM386 is not ideal to drive the tone and likely there are better designs around if you want to drive head phones.
As Mr Jazz siad, "Tone before the LM386" better idea 8|
Phil.

Oh forgot to mention the opamps need bias,, while (IIRC) the LM386 is designed to use Single supply,, no need for bias.
« Last Edit: April 14, 2020, 04:37:10 AM by phatt »

joecool85

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Re: Big Muff Pi passive tone control and small amps TL072CP, JRC386D.
« Reply #5 on: April 15, 2020, 09:40:08 AM »
Looks like you are using a single supply.
If so you need to create a bias supply.
Look at any old TS9 schematic or similar circuit and you will see how it is done.
if your supply is  9 volt then you need 4.5Volt bias.
The LM386 is not ideal to drive the tone and likely there are better designs around if you want to drive head phones.
As Mr Jazz siad, "Tone before the LM386" better idea 8|
Phil.

Oh forgot to mention the opamps need bias,, while (IIRC) the LM386 is designed to use Single supply,, no need for bias.

Phil is correct on all accounts here.  You need some sort of booster before the tone circuit.  This could be as simple as an LPB-1 or MXR Microamp style.  You never put a tone stack in the speaker level section of the circuit, always in the preamp side.

LPB-1: http://tonepad.com/getFile.asp?id=68

Microamp: http://tonepad.com/getFile.asp?id=6

My suggestion is to use one of those two boosters (or similar) -> BMP tone stack -> LM386 amp.

It's also worth playing with the BMP tone stack and adjusting values as, to me, it isn't terribly useful in it's stock form.  If using it as a preamp EQ like you are, I suggest giving a larger more sweeping mid scoop, kind of like a cranked up VOX AC-30.

Looking at https://www.guitarscience.net/tsc/bigmuff.htm you can adjust R1 and R2 to be 220k, C1 to be 1,000pf and C2 to be 20,000pf and it gives you a fantastic sweep across the knob.
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joecool85

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Re: Big Muff Pi passive tone control and small amps TL072CP, JRC386D.
« Reply #6 on: April 15, 2020, 10:42:19 AM »
For what it's worth, you could do a BMP tone stack directly at the front of the LM386 circuit and not even do a boost in front.  It would work, but depending on what you plug into it (different guitar pickups, an effects pedal etc), it will change the frequencies of adjustment.
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