Fulltone - OCD  [traced]

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Postby DougH » 20 Sep 2007, 14:49

SIMETRIX for me.
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Postby DougH » 20 Sep 2007, 15:22

MKB wrote:IMO what you are paying Fulltone for is the ears and knowledge to combine such tiny mojo effects together to make a superior whole.


(I'm not picking on you MKB, I swear... :D )


...But I hear this stuff a lot and have to respond:

This sounds real good when you say it fast, but when I see stuff like a 4.7M shunt resistor hanging off of a 500k output volume pot wiper, it appears that whoever "designed" the circuit had absolutely no clue what he was doing...
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Postby jg » 20 Sep 2007, 15:46

DougH wrote:Here's a couple frequency plots of the "OCD" output stage in "HP" and "LP" mode to compare. I ran an AC simulation in SIMETRIX, swept from 10Hz to 20KHz, 3v peak input signal (with a 4.5v DC bias) into the output stage, fixed the tone control pot at 5k, left out the 56k to simplify, etc... R3 was changed from 33k for "HP" mode to 13.2k for "LP" mode to compare.

http://s14.photobucket.com/albums/a320/Doug_H/Analysis/



It does pretty much what I expected. The resistor reacts with the 47n tone control cap and in "HP" mode rolls off more highs. Yawn...

3V peak? meaning 6 V p-p? Into a circuit with a gain of ~3.8 powered by 9V? Hmmmmm.

I would have also expected shelving of the higher frequencies since there is no cap directly to ground but your sim doesn't show that......

Maybe grab another cup of coffee and stop yawning? ;)
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Postby cbriere » 20 Sep 2007, 15:49

"Electronics WorkBench " for me,

i simulated the output circuit only .
top SW is OFF
bottom SW is ON.

http://i77.photobucket.com/albums/j69/c ... ircuit.jpg



I am in doubt of the retracing of the OCD?
as the switch is not doing LPF/HPF
what would be better is to put the 0.1u+20K circuit
in series to do HPF.
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Postby jg » 20 Sep 2007, 15:55

cbriere wrote:"Electronics WorkBench " for me,

i simulated the output circuit only .
top SW is OFF
bottom SW is ON.

http://i77.photobucket.com/albums/j69/c ... ircuit.jpg



I am in doubt of the retracing of the OCD?
as the switch is not doing LPF/HPF
what would be better is to put the 0.1u+20K circuit
in series to do HPF.


120V/60Hz source?!?
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Postby DougH » 20 Sep 2007, 16:03

jg wrote:3V peak? meaning 6 V p-p? Into a circuit with a gain of ~3.8 powered by 9V? Hmmmmm.


Yeah John, I just pulled the voltage level out of my butt... I didn't take the 1st stg into acct just to keep it simple. I figured it *might* be able to do 6v p-p, and that might be a better representation of the signal (instead of 1v) if the gain was turned up.

I can do it again at 1v but I don't think the basic freq response will change.


I would have also expected shelving of the higher frequencies since there is no cap directly to ground but your sim doesn't show that......


There is somewhat of a shelf from 1k to 4k. I didn't duplicate all the fixed resistor details in the tone control and etc because it wasn't necessary to show the basic effect of the series resistance changes due to the switch.

I can sim the circuit from input to output if I get some time, if that would help. But I don't think the basic story of what the series resistance does will change.

I've had plenty of coffee, thanks... :D

I just get bored talking about the same ol' same ol'... Which means I probably should do something else.
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Postby DougH » 20 Sep 2007, 16:12

OK John.

Here are the new pictures with 1v input signal.

Not much difference...

http://s14.photobucket.com/albums/a320/Doug_H/Analysis/
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Postby jg » 20 Sep 2007, 16:52

DougH wrote:OK John.

Here are the new pictures with 1v input signal.

Not much difference...

http://s14.photobucket.com/albums/a320/Doug_H/Analysis/

Most pspice simulators don't care about levels when sweeping for gain, which to me already casts a shadow of doubt on their accuracy especically when you are driving things into the non-linear region. Pspice is a linear simulator. But using levels that are obviously too high for the circuit will always leave you questioning. IMO.

The shelving would probably show up if you run the sim up to 100K. The opamp starts rolling off around 4.8 kHz, which is probably the second rolloff you are seeing. The pot value you have set makes the max attenuation of around 17.8 dB LP(or 11.2 dB HP) and you see the effect the tone control has is around that before the rolloff of the opamp takes over. When the opamp is done rolling off it will have unity gain so the output would be 17.8 dB below that, your graph doesn't go that low for the one trace and doesn't go down to -11.2 dB for the other.

From your simulation result I would say the values look like they have been rather well chosen such that the opamp rolloff takes over just as the tone control would start to shelve. Between the gain setting R and C on the inverting input, the 220pF feedback cap, the tone control and the HP/LP switch, I'd say all these things have been very well 'centered' even though it gives the appearance of being very simple.
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Postby DougH » 20 Sep 2007, 17:38

http://s14.photobucket.com/albums/a320/Doug_H/Analysis/?action=view&current=ocd_output-graph2.gif

The upper set of curves is "LP" mode while the lower set is for "HP". Tone resistor settings are 1k, 5k, and 10k.

I realize Pspice has its limitations, but it can be useful for comparing before/after effects relative to ea other. I'm not trying to prove/disprove how "well-centered" or well voiced the circuit is. Just trying to show the effect of the LP/HP resistors.

After all these curves my point still stands. The "HP/LP" switch is just a variable low-pass filter. Nothing more- nothing less. Doesn't affect low freq response, etc, etc... In this case, pspice agrees with my original intuition anyway. :D
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Postby jg » 20 Sep 2007, 18:08

DougH wrote:http://s14.photobucket.com/albums/a320/Doug_H/Analysis/?action=view&current=ocd_output-graph2.gif

The upper set of curves is "LP" mode while the lower set is for "HP". Tone resistor settings are 1k, 5k, and 10k.

I realize Pspice has its limitations, but it can be useful for comparing before/after effects relative to ea other. I'm not trying to prove/disprove how "well-centered" or well voiced the circuit is. Just trying to show the effect of the LP/HP resistors.

After all these curves my point still stands. The "HP/LP" switch is just a variable low-pass filter. Nothing more- nothing less. Doesn't affect low freq response, etc, etc... In this case, pspice agrees with my original intuition anyway. :D

I thought the HP/LP stood for High Peak/Low Peak (of the tone control)? Did you think it stood for High Pass/Low Pass or something?
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Postby DougH » 20 Sep 2007, 18:31

jg wrote:I thought the HP/LP stood for High Peak/Low Peak (of the tone control)? Did you think it stood for High Pass/Low Pass or something?


Hey, I have no idea what it stood for. :D

Others in this thread mentioned a high pass filter effect.

But even if it does stand for high/low peak, it appears to be labeled backwards (according to the posted schem). I don't really care about the labeling though- wasn't addressing that.
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Postby jg » 20 Sep 2007, 18:53

DougH wrote:
jg wrote:I thought the HP/LP stood for High Peak/Low Peak (of the tone control)? Did you think it stood for High Pass/Low Pass or something?


Hey, I have no idea what it stood for. :D

Others in this thread mentioned a high pass filter effect.

But even if it does stand for high/low peak, it appears to be labeled backwards (according to the posted schem). I don't really care about the labeling though- wasn't addressing that.
On the website http://www.fulltone.com/ocd.asp it describes the switch settings as "Low Peak" and "High Peak" in the descriptions of the sound clips. So maybe there was some confusion about what they meant by a number of people.

Possibly the posted schematic has it wrong? Just a thought.
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Postby kagaxdx » 20 Sep 2007, 18:56

I think so, maybe the posted schematic isn't really accurate, even if it's very close may not be exactly the real circuit.

I just wonder about that.
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Postby JHS » 20 Sep 2007, 21:04

It'a a bit strange that Fuller labled the switch high peak and low peak. Soundwise it should be labled more bass/less bass or
normal/bass cut, 'cos the HP setting cuts lows, nothing more.
The switch settings define two different rollof frequencies for the high pass filter (output cap + vol pot).

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Postby cbriere » 20 Sep 2007, 21:15

if that"s the case, then the schematics is not good!!!!!!!
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Postby DougH » 21 Sep 2007, 12:37

It's probably a perception thing. More high freq response can make something sound "thinner" even if the bass response is the same. I've found that my hearing perception tends to respond to the shape of the curve moreso than the absolute amplitude of a particular frequency band.
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Postby jrc4558 » 03 Nov 2007, 00:24

Dear friends! I tried looking at the photobucket, but the link is already gone. Can someone kindly repost the schematic? Thanks!
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Postby briggs » 08 Nov 2007, 20:26

Did it ever get figured out how the HP/LP + Tone contro was working? It's still confusing me!
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Postby GeJohn » 15 Nov 2007, 03:39

I'd take the schematic with a large pinch of salt.

Looking at the gut shots of all three genuine variants there are clearly three plastic TO-92 devices on the board. The schematic previously seen in this thread shows only two.

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Postby JHS » 15 Nov 2007, 11:44

The 3rd trannie is for the noiseless LED switching (not shown on the schem).

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