MI Audio - Super Blues Pro

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MI Audio - Super Blues Pro

Postby roseblood11 » 05 Dec 2015, 00:07

The old Blues Pro was great, but this is even better:

Link

This version should really be retraced!
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Re: MI Audio Super Blues Pro

Postby modman » 06 Dec 2015, 20:54

you mean: degooped?

Go right ahead!
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MI Audio Blues Pro - what does this section do?

Postby tonymcbony » 27 Jan 2016, 08:35

Hi folks, I was trying to figure out what the second opamp section here does:

Image

So from R4, I assume that's a tone control, R4 setting the minimum and the tone pot + c7 acting as a low pass filter.

From there R7, which I'm not sure what it does, but in particular the R5 R6 C6 C8 part in the IC loop, what does that do?

I've got one of these of vero but I'm trying to get more presense/high treble out of it, and I'm trying to figure out how to get that, and I wondered if that section is the place to tinker?
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Re: MI Audio Blues Pro - what does this section do?

Postby tonymcbony » 28 Jan 2016, 10:53

Ok, so after reading all sorts of stuff, I think this section is an active low pass filter.
I would understand if it was as simple as a cap in parallel with a resistor in the ic loop but the two resistors and caps together are throwing me a bit. Not sure what it does?
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Re: MI Audio Blues Pro - what does this section do?

Postby grrrunge » 28 Jan 2016, 15:54

The tone pot and C7 form a passive variable 1'st order lowpass, while the second op-amp is just an inverting amplifier with frequency dependent gain.
It's rather simple, it's just been drawn funny.
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Re: MI Audio Blues Pro - what does this section do?

Postby grrrunge » 28 Jan 2016, 16:16

Also:
If you look at the capacitors in the feedback loop as frequency dependent resistors, whose resistance drops gradually towards 0 above a certain frequency, it becomes obvious that for very low frequencies the gain of this stage is -6.6v/v. Above some frequency the gain drops to -3.3 v/v and at some rather high frequency, gain drops to -1v/v.
I'll throw it in the simulator, when i get home, to illustrate ;)
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Re: MI Audio Blues Pro - what does this section do?

Postby grrrunge » 28 Jan 2016, 22:07

Here's a frequency response plot of the amplifier consisting of IC1B, R5-7, C6 and C8.
http://pics.knucklehead.dk/pedals/MIA_blues_pro_freq_response.png

If you want more treble from this, change the ratio between R5 and C6. As long as the sum of the two is still close to 660KΩ, you'll stay close to the output level of the original, but with a changed frequency response.
Let's say you change R5 to 220KΩ and R6 to 440KΩ, you would get a gain of 6.6 V/V below 1 KHz and 4.4 V/V below 10 KHz. Experiment with those two values, and see what suits you best ;)

On a sidenote R4, the tone pot and C7 form a variable low pass filter ranging from 690 Hz to 15.4 KHz
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Re: MI Audio Blues Pro - what does this section do?

Postby tonymcbony » 29 Jan 2016, 00:18

Thank you so much!

This is above and beyond, I really appreciate it.

This is bit of a noob question, but why are there two filters linked together, and how/why does that work?
In other words, how do r5, r6, c6,,and c7 interact together to filter?

To explain, it would've make a lot of sense to me if it were just r5 + c8 OR r6 + c6, or the resistors in series and caps in series (although that would be pretty redundant. What does the link create? I'm guessing two interactive filters?
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Re: MI Audio Blues Pro - what does this section do?

Postby grrrunge » 29 Jan 2016, 07:37

First consider this:
A capacitor acts like an open circuit (infinite resistance) for low frequencies, and as a short circuit (no resistance) for high frequencies.
The amount of low frequencies that are allowed to pass, is determined by the capacitance. If you increase the capacitance, you'll let more low frequencies to pass through, and vice versa.

Next:
If you look at the frequency response for the filter, there's two slopes going downwards: One that starts around 100 Hz, and one that starts around 10 KHz.
The first slope is caused when the capacitor letting most low frequencies through starts to pass frequencies, effectively shorting out the resistor it's sitting across.
The second slope is caused when the second capacitor also starts shorting out the the resistor it's sitting across.

I drew you a little drawing, explaining what happens in circuit equivalents:
Image

Summing up: The link between C1, C2,R1 and C2 in the above drawing is what makes the frequency dependent gain possible.
Does it make better sense now? ;)
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Re: MI Audio Blues Pro - what does this section do?

Postby tonymcbony » 29 Jan 2016, 20:17

Wow, that's magnificent, yep I totally get it now.

Thank you so much for taking the time!
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Re: MI Audio Blues Pro - what does this section do?

Postby Shango » 07 Mar 2018, 21:00

grrrunge wrote:Here's a frequency response plot of the amplifier consisting of IC1B, R5-7, C6 and C8.
http://pics.knucklehead.dk/pedals/MIA_blues_pro_freq_response.png

If you want more treble from this, change the ratio between R5 and C6. As long as the sum of the two is still close to 660KΩ, you'll stay close to the output level of the original, but with a changed frequency response.
Let's say you change R5 to 220KΩ and R6 to 440KΩ, you would get a gain of 6.6 V/V below 1 KHz and 4.4 V/V below 10 KHz. Experiment with those two values, and see what suits you best ;)

On a sidenote R4, the tone pot and C7 form a variable low pass filter ranging from 690 Hz to 15.4 KHz



Could you also change the capacitor values to alter the frequency response?

When I looked at this initially I saw the resistor/capacitor pairs as high pass filters (thus active low pass filters in a feedback loop) with corner frequencies of 321 hz and 48 khz. Altering the cap values would change the corner frequencies, but would it be the same effect as changing the resistor values (as long as you get the corner frequency you want)?
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