Vemuram - Jan Ray  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
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ShortScaleMike
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Post by ShortScaleMike »

You'd think after 3 years of R & D, they'd route the PCB in an intelligent enough way as to negate the need for that blue jumper wire.

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Post by Philippe_CGC »

The dead giveaways for me are the exact same cap values in the bass control and the 3k3 resistors in the feedback loop and going to Vbias in the buffer. It's a *buffer*, performing an unbelievably simple function, just walling off the tone and volume controls. It's a one-to-one ratio, and these guys didn't even bother trying (a) an alternate solution or (b) even different values in that same 1-1 ratio.

Every builder borrows and uses some chunks of circuit, but I think this goes beyond emulating somebody's approach, or hell, even copying their answers on the test - this looks more like copying their typos, or the extra words and articles they needed to get to a word limit.

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Post by coldcraft »

Philippe_CGC wrote:this looks more like copying their typos, or the extra words and articles they needed to get to a word limit.

unfortunately, these clowns turn in PaulC's term paper as their own, without paying him a dime.
Black Dynamite wrote:you need to shut the fuck up when grown folks is talkin.

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Post by alexradium »

mmolteratx wrote:Did up a single sided layout with board mounted pots, jacks and switch. Enjoy.

[ Image ]
this layout works,but the bypass switch needs rewiring,when off the effect output and jack output need to be swapped.
how does this sound?
same as an OD11 or a Timmy,no big news being the same circuit,its just a matter of turning knobs in the right spot.

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Post by Mbas974 »

Old post.... but full of infos :-)

wondering about capacitor types ( mojo mojo... ) ref to post pag1, I never seen/don't know the type of
44.9pF capacitor (C8)
20.9pF capacitor (C3)


Also, into the schematic C7 and C2 are 47nF while the images state 50nF. Is it just a tolerance matter right ?

Thanks for helping in understanding.
Regards.

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Post by fakcior »

Yes, it's a tolerance.

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Post by Mbas974 »

Concerning C8 and C3 types ?

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Post by mmolteratx »

Mbas974 wrote:Old post.... but full of infos :-)

wondering about capacitor types ( mojo mojo... ) ref to post pag1, I never seen/don't know the type of
44.9pF capacitor (C8)
20.9pF capacitor (C3)


Also, into the schematic C7 and C2 are 47nF while the images state 50nF. Is it just a tolerance matter right ?

Thanks for helping in understanding.
Regards.
Those values are as measured. The nominal values are in the schematic.

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Post by Intripped »

i've found an error in your schematic, mmolteratx:

the 2 resistors used to get the Vref have swapped values; 9.1k goes to ground and 7.5k to V+
[i've also checked with the pics and accordingly to your markings of ground connections (white dots), the 9.1k res goes to ground]

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Post by mmolteratx »

Intripped wrote:i've found an error in your schematic, mmolteratx:

the 2 resistors used to get the Vref have swapped values; 9.1k goes to ground and 7.5k to V+
[i've also checked with the pics and accordingly to your markings of ground connections (white dots), the 9.1k res goes to ground]
That was a screwup in the pic on my part. The 7k5 does go to ground. Both the top and bottom layers are ground planes, and you can clearly see the 7k5 going to ground in this picture.

Image

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Post by Intripped »

Yes, your right.
... Isn't it strange anyway? I was comparing this circuit with the Timmy's and I noticed this difference.
In the Timmy the 2 resistors have different values, but the one with higher value goes to ground, so that the Vref is higher than half V+.

Here Vref is lower than half V+.
I know that this is for ...something concerning the dc offset of the opamp, in order to get a better headroom, if I'm not wrong.
But if you decrease Vref under half V+, aren't you worsening the headroom?

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Post by Frabbio »

I think it depends on the type of opamp that you are using!
I remember PaulC wrote he used a scope to choose the right resistor values to maximize the headroom.

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Post by Intripped »

I've read again the topic about the Timmy pedal and found nothing more than what you said: Paul C. used a scope to find max headroom for the opamp.

I've also looked for some more info on the net and discovered that, if I've understood correctly, this dc offset could be positive or negative, so that you have to adjust Vref in the correct way (making it higher or lower) in order to compensate it. So it could actually depend on the opamp.

... It would be nice if someone chime in and explain this dc offset thing

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Post by grrrunge »

Take a look at http://www.ti.com/lit/ds/symlink/rc4558.pdf
On page 6 in the lower left figure it becomes apparent, that at a supply of +/- 6V will yield an available voltage swing at the output from +5V to -4.5V. Thus it cannot swing as far down as it does up.
Moving the bias point to sit in the middle of the available output swing instead of in the middle of the supply rails will then yield the greatest possible symmetric headroom. No need for scoping there ;)
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Post by mictester »

Intripped wrote:... It would be nice if someone chime in and explain this dc offset thing
It's quite simple: op-amps were originally powered from dual rail supplies - often +/- 15V. It was realised quite early on that if you supplied the device from a single supply, biased the non-inverting input to about ½Vsupply and you had DC feedback from the output to the inverting input (as you would in almost any amplifier circuit), the output DC voltage would also sit at about ½Vsupply. If you then used DC blocking capacitors on the way in and the way out, you have an amplifier stage whose DC characteristics are determined purely by the component values around the IC.

Paul C discovered that the op-amps he was using were less than ideal, and their output offset could differ (a bit) from their DC bias voltage on the non-inverting input. To get the maximum headroom, you want to get the output of the op-amp exactly centred on ½Vsupply. he fiddled with the bias resistors to centre the DC on the output pin. This is not terribly elegant and necessitates time with a multimeter, tweaking tool and a handful of resistors. It would be much better done with a preset pot!
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Post by Intripped »

so, if i measure the voltage at the output pin(s) of the opamp i want to have exactly ½Vsupply in order to get max headroom?

...and if the measured value is higher or lower than ½Vsupply then i can adjust the bias voltage (Vref) at the non-inverting input pin(s) in order to compensate the DC offset of the opamp - is it correct?

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Post by grrrunge »

Intripped wrote:so, if i measure the voltage at the output pin(s) of the opamp i want to have exactly ½Vsupply in order to get max headroom?
No. Look at the figure again. The op-amp can get approximately 0.5V closer to the positive supply rail than the negative.
Biasing the op-amp at half the supply voltage will then still get you asymmetric headroom. You need to bias the op-amp to the halfway point of available output voltage.
This point will of course vary with the supply voltage.
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Post by mictester »

Intripped wrote:so, if i measure the voltage at the output pin(s) of the opamp i want to have exactly ½Vsupply in order to get max headroom?

...and if the measured value is higher or lower than ½Vsupply then i can adjust the bias voltage (Vref) at the non-inverting input pin(s) in order to compensate the DC offset of the opamp - is it correct?
Theoretically, you're right. In practice, most op-amps will go closer to one rail than the other at their output because of their design (look at the output stage of an op-amp circuit, and you'll see why). The TL072 (for example) can get down to about 0.65V above the voltage on pin 4 and to about 0.95V below the voltage on pin 8, so its output is very slightly asymmetric. Fiddling the bias resistors feeding the non-inverting input could get as much as 0.15V extra headroom! Wow!

Some op-amps are much more lop-sided than others - the "low noise" types (5532, 5534, LM833 and so on) seem to be slightly worse than their noisier counterparts. Perhaps Paul C thought that squeezing every last bit of headroom out of the second op-amp in his pedal would improve its performance.... Well it doesn't - at least not to any noticeable extent: after all, it's meant to be a dirt pedal!

For all practical purposes, equal bias resistors for op-amps are perfectly good enough.
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Post by rwdavis2 »

Although I don't know how accurate it is but rullywow is offering a PCB for sale that sounds pretty good to me. Just finished it a couple weeks ago.

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Post by roknroll »

Would some one kindly explain to me (being an ic nube), how this works considering the discrepancies that I have penned in above.
Thanks for your answer.
JK
Image
mmolteratx wrote:Actually, screwed up typing one of the values in. 12k in the tone control should be 1k2. Mods, can you swap these?

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