M.I. Audio - Blue Boy Deluxe  [traced]

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asbestosaurus
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Post by asbestosaurus »

Please Note! The first image in the thread (below this post) shows the bright control drawn incorrectly. The corrected schematic is attached with my response to Manfred, thanks again.

Also worth pointing out the stock Dual Op Amp was a JRC4558 but is socketted and can very easily be swapped.

Wiring convention:
I really should have mentioned this earlier, thanks snk. Lug 3 of each potentiometers is the side shown above or to the right of the wiper.

Was curious about the circuit but couldn't find a schematic for it online so I got a hold of one and traced it. I've heard it called a Tube Screamer derivative and that's about true if the only notable thing about the Tube Screamer was the soft clipping stage and then some EQ. Honestly it's more like a scaled down version of the Tube Zone, very similar EQ shaping controls, mostly just different values. It's a nice sounding pedal, very versatile, the only thing that bugged me while playing it was noise, which became audible at about 1/4 of the Gain pot sweep. Looking at the signal path it becomes clear why, almost 2.5 Mohms of resistance in the signal path, depending on Gain/Tone/Volume settings.
I really want to modify that inverting output buffer to a non-inverting stage with much lower value resistors, perhaps with some gain too. I'll let you all know what I work out.
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MI Audio Blue Boy Deluxe.PNG
Last edited by asbestosaurus on 23 Sep 2021, 03:36, edited 3 times in total.

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snk
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Post by snk »

Interesting, thank you.
So, the "character" knob makes the distortion circuit less bass sensitive, that's it?
Is it suitable for bass, or does the overall sound cut bass range too much?

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Post by Manfred »

Please recheck the bright circuit.
As it is the circuit drawn now the 22nF capacitor and the 470 Ohm resistor form a low pass with a cut-off frequency of about 15kHz.
The bright potentiometer has no effect on the cut-off frequency,
it forms with the 100k resistor a voltage divider which has only a very small change of the output voltage.
BlueboyBrightControl.jpg

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Post by asbestosaurus »

snk wrote: 19 Sep 2021, 22:39 Interesting, thank you.
So, the "character" knob makes the distortion circuit less bass sensitive, that's it?
Is it suitable for bass, or does the overall sound cut bass range too much?
Yes, the character knob does exactly that, it's the same setup as in the Timmy (except not wired as cut). As the pot is progressively shorted out (with a nice sweep thanks to the C taper) the gain at the lower cutoff (~ 1 to 10 Hz depending on pot setting) created by the pot, 150 ohm resistor and the 100 uF cap rises until it meets that of the (~ 700 Hz) cutoff created by the 150 ohm and the 1.5 uF cap.

I'd say there's plenty of bass. The mid control is cut (at ~1k) only and at max mid, the tone knob acts like a flat big muff tonestack, so there's more room there to boost low end, and still plenty of control over high end from the bright control.

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Post by snk »

Thank you. It seems to be an interesting design. I think I will build it soon :)

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Post by asbestosaurus »

Manfred wrote: 20 Sep 2021, 06:50 Please recheck the bright circuit.
As it is the circuit drawn now the 22nF capacitor and the 470 Ohm resistor form a low pass with a cut-off frequency of about 15kHz.
The bright potentiometer has no effect on the cut-off frequency,
it forms with the 100k resistor a voltage divider which has only a very small change of the output voltage.
BlueboyBrightControl.jpg
Ah! Thank You for catching this.

That's what I get for posting late (from where I live) after staring at it for a few hours on end...

The schem I've attached here is the correct one and I'll edit my first post to mention as much.
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MI Audio Blue Boy Deluxe Schem.PNG

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Post by asbestosaurus »

snk wrote: 20 Sep 2021, 07:30 Thank you. It seems to be an interesting design. I think I will build it soon :)
If you do, I recommend a version like I've attached below.

converting IC2 to a non-inverting topology has a few nice benefits.
-The load on the Volume pot, and thus the Tone section, goes from being mostly negligible to being infinitesimal.
-If configured for some gain, you get a wider range from the Volume control and some clean boost settings available too.
-Thermal noise drops significantly, depending on values of R1 & R2. I suggest 1s to 10s of Kohms for a good balance of noise and output swing/loading
-The entire circuit remains nice and non-inverting (this one is just a pet peeve of mine but I feel strongly enough about it that I think it should be mentioned)

As an aside
Gain calculation, if you (or other possible readers) aren't aware, is (R1/R2)+1.
For an answer in dB, take the log in base 10 of the above result and multiply that by 20.
To make the stage a unity gain buffer, short R1 and remove R2 completely.
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MI Bboy D MOD.PNG

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Post by snk »

Thank you for the suggestion, Asbestosaurus!

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Post by snk »

asbestosaurus wrote: 20 Sep 2021, 08:18 As an aside
Gain calculation, if you (or other possible readers) aren't aware, is (R1/R2)+1.
For an answer in dB, take the log in base 10 of the above result and multiply that by 20.
To make the stage a unity gain buffer, short R1 and remove R2 completely.
So, am I right assuming that with R1 = 47k and R2 = 6.8k, I would stay in the suggested values ballpark (so keeping thermal noise low), and reach +18dB gain?
What is the gain of the original pedal (as there is no "R1 & R2")?

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Post by asbestosaurus »

snk wrote: 20 Sep 2021, 19:30
asbestosaurus wrote: 20 Sep 2021, 08:18 As an aside
Gain calculation, if you (or other possible readers) aren't aware, is (R1/R2)+1.
For an answer in dB, take the log in base 10 of the above result and multiply that by 20.
To make the stage a unity gain buffer, short R1 and remove R2 completely.
So, am I right assuming that with R1 = 47k and R2 = 6.8k, I would stay in the suggested values ballpark (so keeping thermal noise low), and reach +18dB gain?
What is the gain of the original pedal (as there is no "R1 & R2")?
Yep, your gain calculations check out!
The second opamp section in the original was inverting, for which Gain = Feedback resistance/Input resistance or specifically 1M / [1M + 220k].

This gives a gain of about -0.8 (since the stage is inverting) or -1.7 dB so even a unity gain buffer stage will give you a slight boost over the original.

Keeping this in mind, the +18dB boost you propose is maybe a little excessive, personally I wouldn't give it more gain than about 2 or 3, to preserve headroom and noise performance. The stock pedal has plenty of output as is.

I'd probably shoot for R1 = 10k and R2 = 4.7k leaving you with a gain of 3.1 or 10dB, mostly negligible thermal noise (most of it will come from the first stage anyway), and an output that can happily drive loads down to about 10k, or even lower depending on the op amp you choose.

These are just humble suggestions, of course. Let your ears be the judge!

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Post by snk »

asbestosaurus wrote: 21 Sep 2021, 04:51
Thank you Asbestosaurus, your "humble suggestions" are really helpful :thumbsup

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Post by snk »

I am building it, and realized i don't have any more 22nF capacitor left.
Am I right assuming that :
- by using 33nF caps at the input & output, I will let more bass pass through,
- and by using 15 or 18nF caps for the 2 others, I will change the cutoff point of the tilt eq and the low pass filter, making it a bit more "trebly"?

Also, I don't have any 1.5µF cap. My plan is to use a 2.2µF instead. Am I right assuming that it will let more bass pass through in the side-chain circuit triggering the distortion?

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Post by snk »

Hello,
I built it today : it's a very nice overdrive, with a very interesting EQ section.
Thank you for sharing the schematic!

I used a 5532 opamp, because i was aiming for a more hi-fi sound.
Some knob action seems backwards (namely the CHARACTER and TILT pots), but I may have to check my wiring :)
I noticed that the mid, tilt (tone) and output pots were very interactive. In fact, a bit like the MXR Dist+ (which change its spectral content according to the gain), the more you turn the output down, the less treble you have (and vice versa, with more volume you get a much brighter sound).
I also noticed that the MID eq is very broad, so there can have some overlap with the BRIGHT pot. I thought the MID eq might have been a bit narrower (even if I enjoy it as it is, it's just different than the kind of eq you can find on a console). Out of curiosity, is there any easy way to make the Q of the mid eq a bit narrower?

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Post by asbestosaurus »

snk wrote: 22 Sep 2021, 13:35 I am building it, and realized i don't have any more 22nF capacitor left.
Am I right assuming that :
- by using 33nF caps at the input & output, I will let more bass pass through,
- and by using 15 or 18nF caps for the 2 others, I will change the cutoff point of the tilt eq and the low pass filter, making it a bit more "trebly"?

Also, I don't have any 1.5µF cap. My plan is to use a 2.2µF instead. Am I right assuming that it will let more bass pass through in the side-chain circuit triggering the distortion?
Yes you're correct in those assumptions
using a 2.2 uF will drop the default cutoff from ~700 to ~500. The tradeoff is reciprocal, the frequency will decrease by the same factor as the capacitor is increased by.

The tone and mids control is a fairly clever expansion of the Big Muff tonestack and can very easily be simulated using the Duncan Tonestack Calculator if you haven't yet, just to see what your changes do.
snk wrote: 22 Sep 2021, 20:38 Hello,
I built it today : it's a very nice overdrive, with a very interesting EQ section.
Thank you for sharing the schematic!

I used a 5532 opamp, because i was aiming for a more hi-fi sound.
Some knob action seems backwards (namely the CHARACTER and TILT pots), but I may have to check my wiring :)
I noticed that the mid, tilt (tone) and output pots were very interactive. In fact, a bit like the MXR Dist+ (which change its spectral content according to the gain), the more you turn the output down, the less treble you have (and vice versa, with more volume you get a much brighter sound).
I also noticed that the MID eq is very broad, so there can have some overlap with the BRIGHT pot. I thought the MID eq might have been a bit narrower (even if I enjoy it as it is, it's just different than the kind of eq you can find on a console). Out of curiosity, is there any easy way to make the Q of the mid eq a bit narrower?
It's so obvious in hindsight, I should have specified pot rotations in the schematic.
Damn I really had some tunnel vision in getting this done.
The convention I use is that the wiper of the pot moves upward or to the right as it is turning clockwise.
This point is important enough I'll edit it into the first post.

The mid scoop EQ is a byproduct of the hi-pass to low-pass blend of the BM/tilt control pretty closely resembling a Bridged-T Notch filter, The Q of which isn't particularly sharp and hard to make sharper without making the circuit a good deal more complicated.

also with fast(er) opamps like the 5532 it does pay to bypass the supply pins with a ceramic or poly 100n cap.

Glad you've found the schematic to be useful!

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Post by snk »

Thank you, Asbestosaurus!
Duncan Tonestack calculator was very useful, and I'm sharing my findings :
- The BigMuff tonestack is setup with values such as it is flat when not used (without the usual mid dip). You can get a nice tilt eq if you don't use the MID pot.
- The MID pot is only cutting values, not boosting.
- The corner frequency changes when you turn turn the TILT pot, so along with the MID action it can be pretty bright or dark.
Blueboy tonestack.gif
blueboy tonestack MID 22k.jpg
blueboy tonestack MID 2k.jpg
Blueboy tonestack midcut.gif

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Post by snk »

also with fast(er) opamps like the 5532 it does pay to bypass the supply pins with a ceramic or poly 100n cap.
Newb question : Out of curiosity, what does it do?
Am I right assuming that it aims at getting a cleaner signal by removing DC noise?
A bypass capacitor is a capacitor that shorts AC signals to ground, so that any AC noise that may be present on a DC signal is removed, producing a much cleaner and pure DC signal.
If so, why isn't it here by default, even with 4558 opamps? Why is it needed only for faster opamps?

[edit : I took the time to learn about it, now I understand :) ]
Last edited by snk on 25 Sep 2021, 20:48, edited 1 time in total.

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Post by snk »

(by the way, I wired the pots correctly, and I am enjoying the pedal a lot : thank you for the schematic!)

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Post by Manfred »

I assume that Asbestosaurus means by "faster" the so-called slew rate.
This is the maximum slope rise up to which frequency a sinusoidal signal remains undistorted at a given amplitude Vp.
For the 4458 S=1.6V/uS for the 5532 S=6V/us.
The formula is f_max = S/2*Pi*Vp.
From this follows f~56khz for the 4458 and f~212kHz for Vp=4.5V.
Then there is the so-called unity gain, which is 3 MHz for the 4558 and 10 MHz for the 5532.
This is determined by the internal low pass behavior of the OP-amp, and specifies at which frequency the diffuse gain reaches the value 1.
There are usually three lowpasses within the opamp with different corner frequencies, the last two at higher frequencies. This ensures that the phase shift increases with increasing frequency and this eventually becomes 180°.
At this point the inverting and non-inverting inputs are swapped and the opamp starts to oscillate.
Don't worry, most opamps are inter compensated, but under certain external conditions, such as a capacitive load, this can still happen.
To prevent this we shift the cutoff frequency with the capacitor between the output and the inverting input.
The 5532 has a lower noise figure.
It is noted that the 5532 sounds different in the same circuit than the 4558, but this can be compensated by the external circuit.
Last edited by Manfred on 26 Sep 2021, 15:54, edited 1 time in total.

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Post by snk »

Thank you for your comprehensive answer, Manfred.

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Post by asbestosaurus »

Big apologies to all.

This is awkward. While going back into the pedal to try some mods, I discovered - much to my horror - that the circuit wasn't quite what I thought it was.

My mistake was in the tracing of the components around IC2. I had thought the junction of the two 1M resistors was connected to the -ve input, instead it is isolated and the output of the 220k from the volume control is connected directly there, rather than just to the input side 1M. I was running on rather little sleep and somehow managed to botch both a visual inspection and a multimeter continuity test. [smilie=a_doh.gif]

I've double-checked and triple-checked and I'm sure the schem is correct now.
MI Audio Blue Boy Deluxe Schem Final.PNG
The frequency response of the IC2 stage, calculated in MultiSim, is attached below.
MI BBDOD-Grapher.png
As can be seen, the output stage has a boost of around 19 dB until a -6dB shelf starting at 800-900Hz which just starts to flatten out at about 6 kHz before another low pass somewhere above 20 kHz.

Sorry once again. I feel quite foolish, and I've learned my lesson about trading sleep for the joy of tracing.
asbestosaurus wrote: 18 Sep 2021, 03:44 Please Note! The first image in the thread (below this post) shows the bright control drawn incorrectly. The corrected schematic is attached with my response to Manfred, thanks again.

Also worth pointing out the stock Dual Op Amp was a JRC4558 but is socketted and can very easily be swapped.

Wiring convention:
I really should have mentioned this earlier, thanks snk. Lug 3 of each potentiometers is the side shown above or to the right of the wiper.

Was curious about the circuit but couldn't find a schematic for it online so I got a hold of one and traced it. I've heard it called a Tube Screamer derivative and that's about true if the only notable thing about the Tube Screamer was the soft clipping stage and then some EQ. Honestly it's more like a scaled down version of the Tube Zone, very similar EQ shaping controls, mostly just different values. It's a nice sounding pedal, very versatile, the only thing that bugged me while playing it was noise, which became audible at about 1/4 of the Gain pot sweep. Looking at the signal path it becomes clear why, almost 2.5 Mohms of resistance in the signal path, depending on Gain/Tone/Volume settings.
I really want to modify that inverting output buffer to a non-inverting stage with much lower value resistors, perhaps with some gain too. I'll let you all know what I work out.

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