Explain scheme

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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black03
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Post by black03 »

Who can explain me in detail, the parts of an overdrive scheme?

I am learning and I can not understand some things.

For example: The transistors that are in the beginning and at the end of a circuit, those of the end many times are post pot volume and other times behind the pot, why and that they grant, more gain? or are they buffers?

because some use a max 044 or tl7660 at the power input What benefits this?

thanks to who explain me

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Manfred
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Post by Manfred »

For example: The transistors that are in the beginning and at the end of a circuit, those of the end many times are post pot volume and other times behind the pot, why and that they grant, more gain? or are they buffers?
There are transistor stages with gain and with unity gain (buffers),
please give us schematics, which you are interested in.
because some use a max 044 or tl7660 at the power input What benefits this?
Parts mainly used to generate the double supply voltage on the output or a split power supply with the same voltage as the supply voltage is.
A benefit of both applications is, for example, a large amount of headroom.

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Nocentelli
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Post by Nocentelli »

black03 wrote: For example: The transistors that are in the beginning and at the end of a circuit, those of the end many times are post pot volume and other times behind the pot, why and that they grant, more gain? or are they buffers?
If the volume pot is first, the output impedance will vary as the volume varies. If you have a 100k volume pot, the output impedance is only 100k when the volume is at maximum, it decreases as the volume is turned down. An overdrive circuit with the volume pot before the buffer will have a fixed 100k resistor on the output capacitor and this value cannot vary. Changes in impedance may be audible in the tonality of the output.
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by Nocentelli »

(can't edit post :roll:) PLEASE NOTE my first sentence should read "If the volume pot is SECOND (after the transistor/opamp), the output impedance will vary as the volume varies"
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by black03 »

Hi, thanks for answering, I am trying to learn and this helps me a lot.

1-Here I leave schemes with different ways of adding a transistor, and I do not understand why the variant, which is actually achieved.

this transistor gives more gain there, before the pot?
Image

How do they work here?
Image

There are no transistors and it sounds great
Image

lovepedal cool sound, there are no transistors and it sounds great
Image

How do you get so much gain?
Image

2- Most lovepedal schemes do not use transistors only the op, and they sound the same or better than those that do use transistors.

3-As you can add gain to an over, when increase the pot of gain or modify the resistance that goes with the capacitor 1k and 220nf to vb for example, it starts to oscillate, then increase the capacitor of the ic that is from 100pf to 220pf or 330pf the oscillation goes away or decreases but the sound changes and loses treble, I do not understand how to solve this problem of oscillation at higher gain.

4-The circuit of the crunch box is very simple, it has a lot of gain, it does not oscillate, it does not have transistors, how does that happen?

Thanks again to anyone who can answer.

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Post by Nocentelli »

1)

First schematic - Yes, the FET has a lot of gain and is probably being overdriven by the previous two stages, altering the sound compared to if it was a FET buffer (with the 15k drain resistor shorted to +9v and the output taken from the source).

2nd schematic - You don't say where this schematic comes from, but the transistor buffers might be there because the pedal might use FET/BJT+flipflop bypass switching instead of true bypass, or the designer may have wanted to buffer the ins and outs for a number of different reasons including maintaining a constant output and input impedance as i mentioned before, or to protect the opamp input from voltage transient spikes etc etc etc

3rd schem - No transistors, but input and output buffers exactly like in number 2. above

4th schem - Maybe lovepedal don't use FET/BJT+flipflop bypass, doesn't care about constant output impedance or the risk of frying the input opamp, wants to save a few cents on a handful of parts to squeeze even more profit out of his ripped-off and overhyped "designs"

5th schem - x100 gain on the first opamp then x100 on the second = x1000 gain. The opamp have no clippers in the loop so must be clipping themselves (maybe 833 opamps sound better than TL0xx etc) and the LEDs probably only shear off the massive peaks due to a relative high vf. There is a LOT of treble being rolled off at the end of the circuit, so i assume the sound direct out of the second opamp is very ragged and harsh, but sounds good with all that extra boosted high end taken off

2. It's often thought as "good design practice" to buffer in/out/both, but it doesn't sound good in every pedal (e.g. many transistor fuzzes) and is always strictly necessary - Love pedal maybe decided this

3. If you want to +increase the loop resistance or decrease the vb resistor in these opamp overdrives, you can maybe reduce the value of the vb capacitor - this actually reduces the gain applied to lower frequencies and has the effect of reducing the mushyness but it can also help with oscillation - Check out the paul cochrane timmy where the cap to vb is a fixed 39n but it is in parallel with a much larger 1u cap. The bass pot varies the resistance between the big cap and the vb resistor, adjusting the low end gain.

4. Maybe to do with splitting the large amount of gain between two stages and no feedback loop clippers and masses of treble roll-off at the end of the circuit?
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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Post by Nocentelli »

Nocentelli wrote:
2. It's often thought as "good design practice" to buffer in/out/both, but it doesn't sound good in every pedal (e.g. many transistor fuzzes) and ISN'T always strictly necessary - Love pedal maybe also decided this
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

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drewl
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Post by drewl »

here is a very informative site which explains the different functions of various stages for a few very common effects
like the Tubescreamer:

https://www.electrosmash.com/tube-screamer-analysis

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Post by roseblood11 »

www.geofex.com
"the technology of..." in the upper left corner

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