Educational OverDrive

[antonis]

It started as a "block to block" design/explain for a friend who has likes to understand "WHY" togheter with "what" & "how".. 
Initial project was a mild "bluesy" distortion pedal using the enclosure of a fried MXR Carbon Copy Delay..
Of course  , the use of all "educational necessary" blocks (e.g. input buffer, 2 stage CE amp gain, tone pot, recovery amp, output buffer, etc) resulted in a board of double size of that fitting in the above enclosure..
So, we started to delete items, resulting in the below circuit..



It might appears some kind of "funny" design but it actually sounds GOOD..!!! 

Despite its messy design, it's a more or less simple circuit (a first stage bootstraped CE amp of about 11dB Gain followed by a second stage direct coupled CE amp of about 22 dB Gain resulting in overall midband gain of 33dB..)



Tone pot isn't actually what its name declares..
It's a High Boost/Cut configuration leaving setting margins between 20% & 80% almost flat..
(-/+ 1dB or so..)


Of course, T1/T2 stage could be a single JFET CS amp but we liked to stay bipolar..
(it can safely be substituted by a singe Darlington BJT, any pair of medium to high gain transistors or even by a super beta single one.. - in case of single BJT, voltage divider bias point should need a little tweak..)
47nF caps value on "tone" pot may be set according to taste (togheter or independently..)
Same for 1k2 & 4k7 resistors (R6 & R8) - their value is arbitray set to - roughly - halve respective Emitter & Collector load at extreme Tone pot settings..

Plz feel free for any sarcastic/ironic comments..!!
(after all, that's the main point for posting it..)

[duck_arse]

it looks like good thing, Mr. D. one things, they'll be wanting a layout. you planning on showing, perhaps, a breadboard layout? going to mention capacitor voltage ratings? pot tapers? and will it work better on 18V?

[antonis]

Breadboard layout as soon as picture captured..

Pot tapers, Linear..
(although a Log taper for Tone pot should be more effective..)

Capacitor voltage ratings..?? 

[ElectricDruid]

Capacitor voltage ratings..?? 

Lol. Anything over a thousand volts will be fine.

[PRR]

27uFd seems an odd value-- might say what common values are "close enough". 22? 33?

Both pots will scratch when turned. Need a "scratch OK!" sticker.

C6 does not have to be NP.

[Prehistoricman]

What software did you use for the schematic and graphs? I'm a TINA-TI user and it looks extremely similar.

[antonis]

Yeappp.. It indeed is a Tina-TI simulator..!!! 
(I use it 'cause I like too much to get voltage readings of 600V say on a CE amp Collector fed from single 9V power supply..  )

[antonis]

Instead of answering different PM queries (with also the risk of uncorrected inaccuracies..) plz allow me a brief (and boring to many of you.. ) description of the above circuit..

First stage is a typical CE amp of voltage gain slightly less than 4 (about 12db). The reason for doing that (other than preventing unwanted distortion & high pitch oscillation caused by a single high gain stage) is dual:

1. For using 1st stage as a clean(*) booster when 2nd stage gain set to unity..
(*) Well, not so "clean" due to low Collector bias point resulting in greater than 600mV signal positive waveform clipping where negative waveform of 1.35V (or less) is amplified without clipping..
(for a design correct practice, Collector should be biased slightly above 5V but that would result in the need of extra cap & resistors for 2nd stage proper bias of ridiculously low equivalent impedance – about 16k for GAIN pot set to max..)

2. Due to fixed gain and consequently "fixed" Emitter resistor value in conjunction with its relatively high value it could also be used as an input buffer..
Considering 1st stage as a single transistor of ultra high gain, 4000 - 5000 say, Base impedance is effectively raised to h_FE times R_E+r_e ( about 5M ) in parallel with bootstrapped (via C2 & R3 ) equivalent impedance of 390/150 voltage divider..
(bootstrap gain A_B = A_V/(1 – A_V) where A_V is voltage gain = R_E / (R_E +r_e), neglecting source internal resistance, here set at about 53 so R3 value is reflected as about 2M5 – those two set input impedance at about 1M5 (depended on Tone pot setting)..

Second stage is also a CE amp of voltage gain slightly lower than 13 (about 22db max) resulting in overall gain of 34db.
For space/item save, both Collector & Emitter pots are used as Volume & Gain respectively..
(prone to crackle due to DC, as Paul noticed..)

Tone initially aimed to be comprised of 2 Collector - Emitter pots with wipers connected to GND via a cap (for Bass) and an inductor (for Treble)..
Of course, there wasn't enough space for that (especially for a simulated inductor) so we resulted in that ^&%%$* Tone which only works effectively on both extreme settings..(partially due to high pot body resistance..)
(no, it isn't a configuration inside NFB loop 'cause that specific loop isn't negative and is also "broken" via grounded pot wiper..)


P.S.1
Sorry for any possible terminology inaccuracies together with scientifically incorrect analysis..

P.S.2
It's rather peculiar the fact that, during the whole project, speakers repeatedly sound  "You can't always get what you want"..