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cctsim
Silver Member
United Kingdom
418 Posts |
 Posted - 03/13/2010 : 13:08:19
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I am currently working on digital implementations of modulation pedals and I stumbled across the pre-emphasis and de-emphasis circuits used in most of analogue modulation boss pedals.
The list includes:
BF-2,
CE-2,
CE-3,
CH-1 (analogue),
CE-5 (analogue),
DC-2,
DD-2,
DD-3 (some versions),
DM-2,
DM-3,
DSD-3
GE-7,
HF-2,
RV-3
Although the functionality of pre/de-emphasis is useful for the pedal operation when the signal is on, I was surprised to see it is connected on the signal path when the pedal is bypassed. This is similar to the transistor-based input / output buffers found in most of boss pedals but the circuit has much more significant frequency shaping properties.
This is an example found on BF-2. First the pre-emphasis part
Followed by the de-emphasis part
Here is the frequency response of the pre/de-emphasis circuits:
So be aware, if you connect several of these pedals in series, you effectively load the guitar signal unnecessarily.
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Edited by - cctsim on 03/13/2010 18:31:11 |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/13/2010 : 15:16:07
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quote:
Originally posted by cctsim
THanks for the info!
Do you have a plot of the combined effect of the pre-de emphasis? |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/13/2010 : 18:34:12
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quote:
Originally posted by Laurie
THanks for the info!
Do you have a plot of the combined effect of the pre-de emphasis?
No, but I can imagine that given the component tolerances it will not be a flat spectrum.
It should be easy to plot though. I will do it later on. |
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Scarymonsta
Bronze Member
United Kingdom
90 Posts |
Posted - 03/13/2010 : 18:45:06
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Hi cctsim
Out of interest, what package do you use for your simulations?
Is it TINA?
Chris |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/13/2010 : 18:56:40
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quote:
Originally posted by Scarymonsta
Hi cctsim
Out of interest, what package do you use for your simulations?
Is it TINA?
Chris
I use LTspice or Matlab depending on the complexity of the circuit. |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/13/2010 : 19:04:38
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quote:
Originally posted by cctsim
quote:
Originally posted by Laurie
THanks for the info!
Do you have a plot of the combined effect of the pre-de emphasis?
No, but I can imagine that given the component tolerances it will not be a flat spectrum.
It should be easy to plot though. I will do it later on.
Do you hav the ability to do a phase plot? That would tell a tale too. |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/13/2010 : 19:09:17
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Here is the magnitude plot between 10 Hz and 20 kHz:
This plot assumes components have ideal values with 0% tolerances. |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/13/2010 : 19:14:22
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quote:
Originally posted by Laurie
Do you hav the ability to do a phase plot? That would tell a tale too.
There you go !
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/13/2010 : 19:16:02
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quote:
Originally posted by cctsim
quote:
Originally posted by Laurie
Do you hav the ability to do a phase plot? That would tell a tale too.
There you go !
Oh... that's ugly!! |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/13/2010 : 19:24:38
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Exactly my thought.
I am not sure why Boss decided to leave this part in the signal path when the pedal is bypassed. It looks probably ok-ish between 100 and 2kHz but all harmonic content is really effected.
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 03/14/2010 : 02:38:55
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Hi cctsim
Great work & always informative.
I would love to see the plots for the CE-2.
Regards Dr. Bob  |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/14/2010 : 05:32:12
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Hi Dr. Bob,
The pre/de-emphasis circuit is identical in most cases. In CE-2 the only difference is C2 being 0.047uF instead of 1uF. The rest of the
components are identical.
C2 is more of a coupling capacitor that effects the lower frequency range a bit. So I expect the responses to be almost identical.
Best Regards
cctsim |
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nathanscribe
Silver Member
United Kingdom
376 Posts |
Posted - 03/14/2010 : 11:45:24
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I'd like to see the GE-7.
I'm not surprised to see this circuit in the BBD/digital pedals, but didn't expect it in the GE.
Looking at the schematic it's different though, there's a 470R and 15nF in series to bias from the inverting input of the first op-amp and a 15nF (22nF on the 7B) & 470R at the output side. |
Edited by - nathanscribe on 03/14/2010 11:46:18 |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 03/14/2010 : 12:12:08
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quote:
Originally posted by nathanscribe
I'd like to see the GE-7.
I'm not surprised to see this circuit in the BBD/digital pedals, but didn't expect it in the GE.
Looking at the schematic it's different though, there's a 470R and 15nF in series to bias from the inverting input of the first op-amp and a 15nF (22nF on the 7B) & 470R at the output side.
GE-7 uses a different circuit topology and values but it's still the same principle. |
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Topic |
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Pre-emphasis and de-emphasis in Boss pedals
