BJFE - Emerald Green Distortion Machine  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
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Manfred
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Post by Manfred »

jalmonsalmon wrote:So ...
:scratch:
MP38A's work for this and sound more or less the same as the CV7351's?
Like blind fold comparison test? :secret:
I am not sure, since the frequency response is not the same on the low end.
Theres is a diffrence from about 10db @ 10 Hz, that means there is three times less low frequency level using the MP38A below 30Hz,
but I think it is finally not audible.

Next, will omitted more specifically change the capacitor between base and collector, to see the influence on the frequency response.

The next task is the measurement of each FET-stage seperatly.

To the end with, I will build the device, but that is a winter work.

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Post by Manfred »

As announced, here the frequency responses for differten Caps placed between base and collector.
CV7351_FrequencyResponseDifferentCaps.jpg
MP38A_FrequencyResponseDifferentCaps.jpg

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Post by Manfred »

Output curves and frequency responses for different "Voice" potentiometer settings.
On the measurement frequency of 1 kHz, the potentiometer setting has had no influence on the shape of the clipped signal curves.
MeasurementCircuitStage1.jpg
On the annotation in last columns the "k" is missing, 50 means 50k.
2N5952_Stage1_Outputs_R=0_to_R=50k.jpg
2N5952_Stage1_FreqResp_R=0-R=50k.jpg

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Post by Manfred »

I mad further frequency response measurements because nothing has changed significantly for potentiometer resistances greater than 5 Kiloohms.
That corresponds to the half rotation of the "Voice" button, due to the logarithmic potentiometer.
Those values are 1 Kiloohm, 2.2 Kiloohms, 3.3 Kiloohms and 3.9 Kiloohms.
In summary, on Stage 1 the "Voice" button affects the low and low mid frequencies on its lower half rotation,
the mid and high cut frequencies remained largely unaffected.
2N5952_Stage1_FreqResp_R=0-R=3.9k.jpg

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Post by Manfred »

Output curves and frequency responses for different "Voice" potentiometer settings of stage 3.
MeasurementCircuit Stage3.png
OutputSignalsStage3.png
OutputSignalsStage3.png (9.32 KiB) Viewed 1109 times
The value of R_pot means the resistance between terminal 2 and terminal 3 of the "Voice" potentiometer, so the right stop corrospondes with 0 Ohms.
Using the 4.7 nF capacitor, the potentiometer setting has not a noticeable influence on the frequency response,
so I using a 47nF capacitor, that gave a marked change, but only up to a resistance of 5.1 Kiloohms.
This gives only a small rotation left away from the left stop.
Maybe this capacitor have been read incorrectly.
Later on, the build circuit make the truth come
2N5952_Stage3_FreqResp_C=4,7n_R_pot = 0 - 5,1k.png
2N5952_Stage3_FreqResp_C=4,7n_R_pot =  5,1k -15k.png
2N5952_Stage3_FreqResp__47n_R_pot = 0-6.8k.png

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Post by Manfred »

The final chapter discusses the clipping circuit.
For comparison purposes I also measured the circuit using 1N4148 diodes.
The spectrum shows amplitude peaks on the odd harmonic multiples of the input frequency.
The amplitude amounts of the harmonics are significantly greater using the BY458 diodes.
MeasurementCircuitClipping.jpg
ClippingCircuit BY458 1N4148 Rtone = 50k Input 500mV- 800mV.jpg
Spectrum BY458 & 1N4148 clipping diodes f_in = 1kHz.png
Spectrum BY458 & 1N4148 clipping diodes f_in = 1kHz.png (6.1 KiB) Viewed 1082 times

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Post by sopapo1 »

Hello, Im am interested in doing this build as I am built another two bjfe designs (drd and model h) and I am very addicted to his designs, sound great and the responsiveness are huge, I think this pedal live to the hype¡¡
The original version use a bjt in q2, I dont have any germanium to try, so I would like to try a silicon version, which one would be a good candidate? bc549? bc108? somehting with low gain I think. Obviously I will have to play with several resistors to bias it...which biasing?' vcc/2? or lower......
Thanks for all
Best regards

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Post by Manfred »

sopapo1 wrote: 03 Aug 2022, 00:13 Hello, Im am interested in doing this build as I am built another two bjfe designs (drd and model h) and I am very addicted to his designs, sound great and the responsiveness are huge, I think this pedal live to the hype¡¡
The original version use a bjt in q2, I dont have any germanium to try, so I would like to try a silicon version, which one would be a good candidate? bc549? bc108? somehting with low gain I think. Obviously I will have to play with several resistors to bias it...which biasing?' vcc/2? or lower......
Thanks for all
Best regards
Try a BC547A, BC548A or BC549A these are the successors to the BC107A, BC108A or BC109A, the gain group A is important.
Change the values of the resistors from 100k and 22k to 150k and 56k.

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