General electronic theory questions - Davisson Simple Drive

[fuzzy645]

Hi -

Newbie here with some general electronic theory questions about this schematic.  My goal is to understand the role of each component in the circuit, and how it might (or might not) effect the tone.   My questions below might sound a bit "anal" and I would assume might come off rather annoying to an experienced person, but that is not my intention. My goal is understanding.  Any help you can provide in understanding this simple circuit would be genuinely appreciated.

.1 cap on input/output
-----------------------------
1). what is the purpose of the .1 uf cap on the input and output?  
2). what would the effect be of using a larger value cap (such a .2) vs. a lower one lets say?
3). would the "brand" / "type" of cap matter such as the fancy paper in oil caps / audiophile type expensive caps etc...
4). what would happen to the circuit if these parts were left out?

220K resistors & the .01 between them to ground
----------------------------------------------------------
5). I noticed they are in series, I guess resulting in 440K.  What is the purpose of this?
6).  what is the purpose of the .01 cap to ground?
7). what would happen to the circuit if these parts were left out?

Clipping Diodes
--------------------
8.  What is the purpose of these?
9). what would happen to the circuit if these parts were left out?

680 uf and 22 uf caps to ground?
------------------------------------------
10). what is the purpose / what would happen if left out??
11).  what would the effect of raising or lowering the values be?

47 pf cap
----------
12). what is the purpose / what would happen if left out?
13). what would the effect of raising or lowering the values be?
14). would the "brand" / "type" of cap matter such as the fancy paper in oil caps etc...

Impedance
-------------
15).  And finally, based on these components, how could we calculate the INPUT impedance and OUTPUT impedance?  Is there some formula?

Thanks in advance,

Paul

[iccaros]

A lot of these questions could be answered here http://www.geofex.com/  bookmarking good 

I had a long answer before I realized your asking someone to do your homework for you..  as These questions would only come from a text book as most who do not know these answers would not know how to separate the circuity out in to its base circuits.

[fuzzy645]

Thanks for your reply, and thanks for the link.  I'm definitely willing to put in the hours of homework...don't get me wrong, but still would appreciate any partial answers in layman's terms, as that is why I posted.

Best regards!

[allesz]

Hallo fuzzy, and welcome.
I will try to answer to your questions, but I am a noob too, so... have mercy.

First an important advice: buy the components, a BREADBOARD, some solid core wire, a couple of jacks and a 9v battery plug.

The (some actually) answers:

.1 cap on input/output
-----------------------------
1). what is the purpose of the .1 uf cap on the input and output?  They isolate input and output from dc current.
2). what would the effect be of using a larger value cap (such a .2) vs. a lower one lets say? They affect frequency response of the circuit: bigger gives more bass, smaller gives less bass; a bigger imput also gives often bigger gain/distortion (have fun on the breadboard)
3). would the "brand" / "type" of cap matter such as the fancy paper in oil caps / audiophile type expensive caps etc... A BIG NO
4). what would happen to the circuit if these parts were left out? It won't work.

220K resistors & the .01 between them to ground
----------------------------------------------------------
5). I noticed they are in series, I guess resulting in 440K.  What is the purpose of this? They bias the base of the transistor.
6).  what is the purpose of the .01 cap to ground? I guess it is for tone shaping.
7). what would happen to the circuit if these parts were left out? You can leave out the cap but You need the resistors (at least one) to bias the base.

Clipping Diodes
--------------------
8.  What is the purpose of these? They are the distortion generator... dirty and lazy answer
9). what would happen to the circuit if these parts were left out? You will end up with a sort of booster.

680 uf and 22 uf caps to ground?
------------------------------------------
10). what is the purpose / what would happen if left out?? The cap gives some more higs to the circuit. The resistor help bias the transistor. They both can be left out, the circuit will work....
11).  what would the effect of raising or lowering the values be? here you have to experiment and find out what you like or not: go breadboard.

47 pf cap
----------
12). what is the purpose / what would happen if left out? It filters some excessive hiss, you can leave it out or you can try different values (don't go too big)
13). what would the effect of raising or lowering the values be?
14). would the "brand" / "type" of cap matter such as the fancy paper in oil caps etc... ANOTHER BIG NO

Impedance
-------------
15).  And finally, based on these components, how could we calculate the INPUT impedance and OUTPUT impedance?  Is there some formula? I don't know it.

Go for it: you will have a lot of fun.

[fuzzy645]

Hallo fuzzy, and welcome.
I will try to answer to your questions, but I am a noob too, so... have mercy.
Go for it: you will have a lot of fun.

Wow...thanks for your answers Allesz!!  I really appreciate you taking the time.   We are coming from the same place so your answers are very easy to understand.

...one follow up question (or two or three  ):  It would seem to me then the choice of the diodes to create the distortion is critical.  I would then deduce experimenting by trying different diodes might have a huge effect on the tone, correct?  If so, can I choose any diodes, or must they be rated the same as the 1N4001? Furthermore, if I swapped these for some other diode, would anything else in the circuit need to change as a result of this?

Thanks again!!

[R.G.]

You're walking down a well-traveled path. You have a series of questions that you don't yet know that you'll have.

The suggestion to go read http://www.geofex.com - all of it - was a good one.

In particular, read the Guitar Effects FAQ and the "Technology of..." articles first. It will skip you a long ways forward.

[therecordingart]

I'll take a crack....I may be wrong...

.1 cap on input/output
-----------------------------
1). what is the purpose of the .1 uf cap on the input and output? Keeps DC off of the input and output. Capacitors pass AC/block DC.
2). what would the effect be of using a larger value cap (such a .2) vs. a lower one lets say? In this case a larger cap won't do much
3). would the "brand" / "type" of cap matter such as the fancy paper in oil caps / audiophile type expensive caps etc? In this application you won't notice a difference.
4). what would happen to the circuit if these parts were left out? You'd have DC on the input and output

220K resistors & the .01 between them to ground
----------------------------------------------------------
5). I noticed they are in series, I guess resulting in 440K.  What is the purpose of this? Looks like a voltage divider that shunts part of the AC to ground through the cap. Tone circuit?
6).  what is the purpose of the .01 cap to ground? Passes AC to ground
7). what would happen to the circuit if these parts were left out? I don't know

Clipping Diodes
--------------------
8.  What is the purpose of these? Distortion
9). what would happen to the circuit if these parts were left out? No distortion

680 uf and 22 uf caps to ground?
------------------------------------------
10). what is the purpose / what would happen if left out??The 680R resistor is a feedback resistor used for temperature stabilization. As the temp increases so does the leakage current of the transistor. This resistor's voltage drop increases as the leakage current increases which stabilizes the circuit. The cap passes AC to ground so it isn't attenuated by the feedback resistor.
11).  what would the effect of raising or lowering the values be? See above

47 pf cap
----------
12). what is the purpose / what would happen if left out? No clue
13). what would the effect of raising or lowering the values be? No clue
14). would the "brand" / "type" of cap matter such as the fancy paper in oil caps etc... No clue

Impedance
-------------
15).  And finally, based on these components, how could we calculate the INPUT impedance and OUTPUT impedance?  Is there some formula? No clue

[nexekho]

Google Cook Your Own Distortion - it uses the same kind of clipping with a different preamp and explains it.

[LucifersTrip]

I'll tell you the best way to answer these questions (minus #15):

breadboard + 30-60 minutes

[defaced]

There were a series of threads, I think by Beavis Audio, that walked through every part in typical circuits and had tons of input from the members here.  I bet those threads would be a big help here because I remember some dirt circuits being in there.  Anyone remember the threads I'm talking about? I'm not having any luck finding them. 

[iccaros]

680 uf and 22 uf caps to ground?
------------------------------------------
10). what is the purpose / what would happen if left out??The 680R resistor is a feedback resistor used for temperature stabilization. As the temp increases so does the leakage current of the transistor. This resistor's voltage drop increases as the leakage current increases which stabilizes the circuit. The cap passes AC to ground so it isn't attenuated by the feedback resistor.
11).  what would the effect of raising or lowering the values be? See above

Sorry
I am not trying to be an ass, so please take this how I mean it, and not how my poor communication skills may write it..

This is a negative feedback loop with attenuation (frequency control) this will control gain of the transistor based on, coming from the output to the first resistor you encounters value and the cap. The frequencies this shunts to ground are boosted as they are not canceled out on the input of the transistor.  Frequencies that are passed are reduced by the combination of both resistor value.

Longer answer
negative feedback is a way of stabilizing an amplifier by not allowing it to runaway, if you will. The transistor wants to flow as much as it can when turned all the way on. when you put the output signal which is now opposite in polarity of the input signal they cross cancel, not unlike noise cancelling headphones.  Thus controlling the amount of gain the transistor will have.  We only care about controlling specific frequencies so we shunt the rest to ground (the cap). since this is using fixed resistors, the designer calculated the amount of NFB they wanted. you could put a pot in its place and adjust, removing it would give max gain, while adjusting frequency may also adjust or lower gain.

I have never read, or heard about feedback to control temperature of a transistor, but that may be a side effect, but I do not think is the primary goal of this design.  see http://en.wikipedia.org/wiki/Negative_feedback_amplifier

[therecordingart]

680 uf and 22 uf caps to ground?
------------------------------------------
10). what is the purpose / what would happen if left out??The 680R resistor is a feedback resistor used for temperature stabilization. As the temp increases so does the leakage current of the transistor. This resistor's voltage drop increases as the leakage current increases which stabilizes the circuit. The cap passes AC to ground so it isn't attenuated by the feedback resistor.
11).  what would the effect of raising or lowering the values be? See above

Sorry
I am not trying to be an ass, so please take this how I mean it, and not how my poor communication skills may write it..

This is a negative feedback loop with attenuation (frequency control) this will control gain of the transistor based on, coming from the output to the first resistor you encounters value and the cap. The frequencies this shunts to ground are boosted as they are not canceled out on the input of the transistor.  Frequencies that are passed are reduced by the combination of both resistor value.

Longer answer
negative feedback is a way of stabilizing an amplifier by not allowing it to runaway, if you will. The transistor wants to flow as much as it can when turned all the way on. when you put the output signal which is now opposite in polarity of the input signal they cross cancel, not unlike noise cancelling headphones.  Thus controlling the amount of gain the transistor will have.  We only care about controlling specific frequencies so we shunt the rest to ground (the cap). since this is using fixed resistors, the designer calculated the amount of NFB they wanted. you could put a pot in its place and adjust, removing it would give max gain, while adjusting frequency may also adjust or lower gain.

I have never read, or heard about feedback to control temperature of a transistor, but that may be a side effect, but I do not think is the primary goal of this design.  see http://en.wikipedia.org/wiki/Negative_feedback_amplifier

I didn't say the resistor controlled the temperature of the transistor. It does reduce thermal runaway. Putting a resistor on the emitter will cause the voltage drop across that resistor to increase as the current increases. This increase voltage drop across the emitter resistor keeps you from reaching thermal runaway.

Again...I could be wrong. R.G.?

[therecordingart]

It appears that this a common emitter amplifier that is using two types of feedback. Both current and negative feedback.

Here is a good link. Page 5.

http://www.sarts.org.sg/rae/LESSON006.pdf

[iccaros]

I didn't say the resistor controlled the temperature of the transistor. It does reduce thermal runaway.

NOTE: I COULD AND MOST LIKELY AM WRONG.. Wash and repeat....

sorry, it may be how I read it,
This is the only fun in answering text book questions is I get to rehash things I have not used in 20 years, as there is not a lot of current control in digital designs.

as for leaking:
Leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in Collector-to-Emitter leakage current. This is frequently seen in a push–pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.
One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that Vce ≤ 1/2Vcc
Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.
If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur. Special measures must be taken to control this characteristic vulnerability of BJTs.

Note above from wiki, but matches some of my old EE text books.

Since the resistors are not temperature compensated,and they not controlling input voltage to ensure it is at least half Vcc; I do not believe that is the goal.

It appears that this a common emitter amplifier that is using two types of feedback. Both current and negative feedback.

Here is a good link. Page 5.

http://www.sarts.org.sg/rae/LESSON006.pdf

Good link thanks
to wrap up, the resistors and cap we were talking about (2x220 and the cap) are NFB while the cathode resistor, while biasing the amp, also ..
as pointed out in the link ".This thermal runaway can be avoided by having a resistor in the Emitter lead."                                       

[therecordingart]

to wrap up, the resistors and cap we were talking about (2x220 and the cap) are NFB while the cathode resistor, while biasing the amp, also ..
as pointed out in the link ".This thermal runaway can be avoided by having a resistor in the Emitter lead."                                        

We're talking about two different things here. You initially quoted my response about the 680R resistor and 22uf cap that are between the emitter and ground, but you're talking about the 2 x 220k resistors that connect to ground? I'm lost.

[iccaros]

Oops... your right, I was really confused on why you was aguing aginst nfb...but all the time I had miss read the post.. so as I said I can be wrong.. and I was....

So I am sorry to confuse this. I was talking about the divider of 220k resistors from output with the small cap to ground. So I agree 100% that the part you was talking about is a current limiter and can help with thermial runaway......



I will now go drink to hide my shame


I typed this from my old phone so please excuse the typo's

[therecordingart]

Oops... your right, I was really confused on why you was aguing aginst nfb...but all the time I had miss read the post.. so as I said I can be wrong.. and I was....

So I am sorry to confuse this. I was talking about the divider of 220k resistors from output with the small cap to ground. So I agree 100% that the part you was talking about is a current limiter and can help with thermial runaway......



I will now go drink to hide my shame


I typed this from my old phone so please excuse the typo's

Haha. Everything is cool. I thought I was going nuts for a second!

[PRR]

> The 680R resistor is a feedback resistor used for temperature stabilization.

Emitter resistors are often used to "set" transistor current and thereby control temperature effects.

For that to work, the base voltage must be fixed with low value resistance.

Here the base voltage is set only via 220K+220K= 440K, not a low value.

That is not the function of the 680 here.

Also: the transistor will never "get hot". It's 9V in series with 68,680 ohms. The worst-case heat is half of available voltage, 4.5V, in 68,680 ohms, or 0.000,3 Watts. The transistor is a 0.3W part. At worst it can run 1/1000 of its rating, 0.15 deg "hot".

Write-out the bias equation. For any practical purpose you may approximate as

Ie = 8V/((440K/hFE)+68,680)

For hFe from 110 to 440, the resistances will be 4K+68K=72K to 1K+68K= 69K, and the current will be 0.11mA to 0.12mA.

Current is defined mostly by the 68K. With or without 680, same thing for any practical purpose.

The 680 does affect AC/audio gain and impedance.

[dano12]

There were a series of threads, I think by Beavis Audio

I was thinking the same thing

I didn't provide all the answers, only the questions. That was a really fun thread.

http://www.diystompboxes.com/smfforum/index.php?topic=47572.0

[allesz]

Hey fuzzy645, did you buy that breadboard then?

I am quite excited because beavisaudio.com in person came to help you.
He helped me a lot too actually, because i sucked his site for days, to breadboard my first working effect (a bazz fuss).
And I take this opportunity just to say thank you.

The link posted by him is just perfect.

But trust mine and LucifersTrip's advice: go breadboard! You will stop sleeping for a little while (it can be a problem, try to have regular sex though, at least for you wife/girlfriend health) and you will never look to a tubescreamer in the same way you did (it can be a problem too), probably you won't get better playing guitar, and your bandmates will hate you for the new shitty sounding boxes you will get to reharshalls (another problem, but you have to be proud of it instead).
Anyway, you will have loads of fun. And that's all that matter.