Author Topic: Mark my home work please  (Read 1123 times)

Mr Plums

Mark my home work please
« on: June 29, 2020, 10:48:16 AM »
Hi Guys so over the weekend i built the EQD Chrysalis. I have to say it's the best overdrive I have heard so far and when I plugged it into my old Fender Champ Clone, I just went ARRRRRRRR, that's the sound I have been looking for.

I have ordered the parts to build another one this time I am hoping to tweek. As I am quite new to the hobby I was hoping for some help.

I have found the layout and schematic on tagboard effects...





From what I have learnt so far:

The circuit looks like an electra drive going into an LPB 1 style boost stage with a tone shaping in the middle.

So looking at the schematic I think C1 is a decoupling cap and is there to remove any DC from the guitar signal. Is this right?

If C1 is a decoupling cap then I do not know the function of C2. Guessing I would say it is some sort of tone shaping and would shave off some bass.

P1 is the drive pot but I donít understand how drive can be set here. I thought the guitar signal would be too weak to really have any effect but obviously I am wrong. Any help explaining how it works would be appreciated.

Again I am at a loss as to how R1 affects the circuit, help or pointing me in the direction of a good resource to help would be appreciated.

I believe R2, R3, form a voltage divider and that is Vout= 4.5. This is the bias for the first Transistor?? And goes to the base.

I get the Transistor it amplifies the signal??

R4 is to bias the Transistor at this stage if you decrease the value it will add more gain and decrease less gain?? This goes to the collector. R5 sort of does the same job and the value changes will have the same effect but joins the emitter to ground.

I have no clue what C4 does. I may be thinking feedback but i donít know.. Please can you help.

C3 is again a coupling cap and is there to remove any dc?

The diodes clip the signal and placing different diodes will clip the signal in different ways. Like the switch in my Rat Clone. Asymmetric, Symmetric, and so on.

The next stage is the tone control. I am not sure of the function of R6.  The capacitors C5, C6 affect the cut of the tone pot. Changing the cap values will alter the tone like upping the values will roll off more highs.

Does anybody have a suggestion for a good mid boost  value in this circuit???

C7 is a coupling cap and is there to remove any DC picked up from the diodes and tone control. But does this also work by shaving off some highs.

I am not sure why R5 is connected to R7 my guess would be they share the 9v supply as R7 forms a voltage divider with R8 form a voltage divider to Bias the second transistor.

The voltage works out at Vout = 0.818. I am thinking that the second transistor turns on a 4.5 volts as it's the same MPSA 18 as the first Tranny,  so the remaining voltage is coming from the first transistor?

R10 and R9 do the same job as R4 and R5..

The Last cap C8 is there again to remove DC and slightly shape the overall tone.

The Last pot is just volume control.

Thanks guys in advance for the help










« Last Edit: June 29, 2020, 10:51:44 AM by Mr Plums »

r080

Re: Mark my home work please
« Reply #1 on: June 29, 2020, 11:37:09 AM »
If you do some reading on Common Emitter amplifiers, you will get a lot of your questions answered, especially on biasing and the operation of C4.

C1 looks like a treble bleed cap, changing the amount of treble while you change the gain pot, whereas C2 is the decoupling cap.

P1 might be better called an input volume, rather than gain. However, explaining the function to an average guitarist, you would probably want to call it gain. R1 sets the lowest input volume.

I would agree that R7 and R5 must be connected to power.
Rob

antonis

Re: Mark my home work please
« Reply #2 on: June 29, 2020, 11:38:56 AM »
C1 & pot wiper-lower lug form HPF..
C2 is coupling cap (DC block) and also forms HPF togheter with Q1 Base impedance (we'll see it later..)
Drive pot actually sets input signal level..
R1 sets minimum signal level when Drive pot set all the way down..
R2/R3 are part of biasing configuration (Collector shunt feedback..)
R4 is for setting DC Q point (Emitter series feedback) and is bypassed (decoupled) by C4 for maximun open-loop gain..
(the above sets Q1 close-loop gain as close as R3/Drive upper resistance - ignoring signal impedance)
C3: Correct..
Diodes: Correct.. (C5 is for smoothing diodes clipping corners - nothing to do with following tone control)
Tone control: R6+P2 form LPF with C6..
C7: Correct (see also HPF as for C2)..
R5 should be connected to R7 and R10 and the three of them to Power supply.. :icon_wink:
Voltage divider Q2 bias: Correct..
There isn't any "remainig voltage comming from "somewhere"..!! :icon_wink:
0.818V on Q2 Base result into about 200mV on Q2 Emitter, hence about 200μA Emitter current - same current flows from +9V through R10 and Collector.. That sets Collector quiescent voltage at about 4V (for a 5V drop on R10)
R10/R9 set Q2 voltage gain..
Level pot: Correct..
« Last Edit: June 29, 2020, 11:43:17 AM by antonis »
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

antonis

Re: Mark my home work please
« Reply #3 on: June 29, 2020, 11:40:40 AM »
to continue with input caps / input impedances HPFs..

Q2 input impedance is the parallel combination of voltage divider resistors (R7//R8) and Emitter resistor Base reflected value [ (hFE + 1) X R9 - ignoring intrinsic emitter resistor]..
So, Q2 stage input HPF corner frequency is set from 0.159/(C7XRin), where 0.159=1/2π & Rin = R7//R8//[(hFE+1)XR9]

Q1 input impedance is a bit trickier, due to R3 apparent value and C4 bypass cap..
A brute calculation calls for replacing R3 value with R3'= R3/(stage voltage gain + 1) and considering C4 as AC short, so input impedance is R2//R3'//[(hFE+1) X re], where re=0.026/Collector current..
Input HPF corner frequency should be calculated as above..

There is also a HPF formed by C4/R4 (2nd pole of Q1 stage HPF) but let it be for the moment..
(or consider Q1 stage Gain as frequency depended with its precise calculation out of present interest..) :icon_wink:

P.S.
Of course, C8 & Level pot form output HPF..
« Last Edit: June 29, 2020, 03:20:30 PM by antonis »
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

idy

Re: Mark my home work please
« Reply #4 on: June 29, 2020, 12:12:05 PM »
C4: emitter resistor bypass. The resistor means the emitter can never get closer to ground than 470 ohms, but the bypass cap means ac (audio) can swing all the way down. Many fuzz circuits use a pot in series with this cap as the "fuzz" knob.

PRR

Re: Mark my home work please
« Reply #5 on: June 29, 2020, 03:45:08 PM »
> I believe R2, R3, form a voltage divider and that is Vout= 4.5.

How do you get 4.5V? Do you think that collector is surely at 9V? (It surely isn't.)

My advice: that's a pretty fancy circuit. It has some odd details (like R1!). Back up to something simpler.
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Mr Plums

Re: Mark my home work please
« Reply #6 on: June 29, 2020, 03:46:10 PM »
Thanks guys that's really helped.

I am of too do some reading on the topic now again.

I am sorry I don't post much but I am on quite a bit reading and researching.

Mr Plums

Re: Mark my home work please
« Reply #7 on: June 29, 2020, 05:44:43 PM »
> I believe R2, R3, form a voltage divider and that is Vout= 4.5.

How do you get 4.5V? Do you think that collector is surely at 9V? (It surely isn't.)

My advice: that's a pretty fancy circuit. It has some odd details (like R1!). Back up to something simpler.

What like LPB 1???

antonis

Re: Mark my home work please
« Reply #8 on: June 30, 2020, 05:21:34 AM »
What like LPB 1???

Why not..??
It's a fair enough plain booster (easily transformed into distortion effect..) utilizing most of CE amps design analysis principles (like bias, gain, in-out filters & impedance, etc..)

P.S.
In case you've gone awry with Paul's suggestion, I can ensure you that there wasn't any hidden irony or relegation in it.. :icon_wink:
(just the result of your queries appraising depth of knowledge..)
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Mr Plums

Re: Mark my home work please
« Reply #9 on: June 30, 2020, 05:37:00 AM »
Ohh no I didn't think that Pauls comment was off...I thought "yeah he maybe has a point...."

I like the LPB 1 boost..Out of all the boost I have built that's the one that stayed on my pedal board.

Again thanks to all for taking the time to reply, believe it or not I was struggling finding detailed info on the guitar pedals online with respect to there parts and function, however, I now know there are called "common emitter amplifiers"   :icon_redface:and at last I have found a ton of information.
« Last Edit: June 30, 2020, 05:43:04 AM by Mr Plums »