Tech21 - Character Series.  [schematic]

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mxrmxr
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Post by mxrmxr »

Just been messing around with the British circuit, trying to add a couple of 1N4148 clipping diodes to GND at various parts of the circuit to see what effect they have. To my surprise they have no effect, no clipping what so ever !

Am I missing something here ? Apart from a brain :scratch:

Should I be connecting them like the 2 protection diodes at the input & output, i.e. one connected to GND and the other to +V ? :scratch:

Is it something to do with the TLC2264 opamps ? :scratch:

Can anyone plese help me before I go crazy :hmmm:

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Post by JiM »

mxrmxr wrote:Just been messing around with the British circuit, trying to add a couple of 1N4148 clipping diodes to GND at various parts of the circuit to see what effect they have. To my surprise they have no effect, no clipping what so ever !

Am I missing something here ? Apart from a brain :scratch:
If the signal amplitude is not larger than the threshold of the diodes (about 0.7V for Si diode), then the diodes have no effect.
Try a pair of diodes with lower threshold, such as Ge or Schottky.
http://nerdbastards.com/wp-content/uplo ... 00x260.jpg
mxrmxr wrote:Should I be connecting them like the 2 protection diodes at the input & output, i.e. one connected to GND and the other to +V ?
This only has the effect of clamping the signal to the supply rails in order to protect the CMOS inputs of the opamps.
http://www.edn.com/design/analog/437545 ... ion?page=2
A passive guitar should not be able to output that much power.
I only give negative feedback.

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mxrmxr
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Post by mxrmxr »

JiM wrote:
If the signal amplitude is not larger than the threshold of the diodes (about 0.7V for Si diode), then the diodes have no effect.
[/quote]

Yep I know that, and I've tried Ge & Schottky diodes but get no clipping :hmmm: Just a slight decrease in level. I haven't scoped the circuit, but I would have thought there would be enough signal in the circuit somewhere to get clipping :scratch: Seems to be something weird going on in this Tech21 circuit that I can't put my finger on :hmmm:

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sezare56
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Post by sezare56 »

Hi.

I want to bypass the speaker simulation on the SansAmp PSA-1. The pictures of the PCB are here :
http://www.groupdiy.com/index.php?topic=40781.0

Is it possible ? How can I identify or test to find that speaker sim and bypass it ?
That would be great for a lot of users, instead of using an uncab IR.

Thanks.

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Post by Stephanovitch »

Speaker Simulator circuit is "encapsulated" in the black square, it's not possible to remove it

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Post by kcstratslinger »

I know this post is really old...sorry. I see that someone has an effects loop mod, but I can't find any schematics for it. Does anyone know how to do this? My MXR Carbon Copy sounds horrible going into the Blonde. I don't want to put it after the Blonde in fear of clipping the delay pedal at loud volumes. If this has been mentioned already, I apologize.

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Post by 12Bass »

Unhappy with the way the DIY speaker simulation defeat switch was working on my (real) VT Bass, I spent some time trying out some alternatives. Originally, I was taking the feed for the sim bypass from the input of C11 on the schematic. However, that resulted in a huge level and treble boost when the sim was bypassed.

Now, the pre-sim signal is taken from the junction of R14, R15, R16, and C12. The signal here is significantly reduced in level and has some of the nasty treble rolled off. After reading some posts by Tech21 on TalkBass about a 6db/octave low-pass filter used with the official bypass used on version 2 of the VT Bass, in order to to roll off the highs I added a 0.01 μF capacitor to the pre-sim feed which goes to ground when the sim bypass is engaged. This requires a DPDT switch.

The result from this modification is noticeably smoother frequency response in sim bypass mode. Gone is the huge level and treble boost which occurred with the previous bypass arrangement, and gone is the harsh ~3 kHz peakiness added by the stock VT Bass speaker sim circuit (though that clankiness can easily be put back in by disengaging the bypass switch). Bypassing the sim no longer results in such a big change in output. As it is now, the signal is just slightly hotter with the speaker sim engaged, mostly due to the bump in the 3 kHz zone.

Finally, because the pedal is so bright sounding, most VT Bass users end up keeping the treble well below 12:00, often closer to around 9:00. To mitigate this, I inserted a 22 k resistor between legs 1 and 2 of the treble potentiometer; this tames the overall brightness and makes the range of the treble control more usable, IMO. The end result is that the pedal has a closer to flat frequency response when the tone controls are at noon and the speaker sim circuit is bypassed.

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Post by artaommahe »

12Bass, thanks
you can try to check bybass from this fotos
viewtopic.php?f=1&t=14600
some where in this thread was a pics with elements selected on them, for compare

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Post by theV »

Outstanding job, 12bass!
:applause:

Honestly I don't know if I'll try to fully implement the new version of the mod (I already did the "standard" as explained by you time ago):
the R14/R15/R16/C12 track has no holes, we can attach only to the component leads, and I'm afraid of mistakenly unsoldering/displacing one of those tiny bastards...

And even if I dare to do it, if - as you say - with this first step it already looses just a bit of treble (together with volume), I will definitely not install the extra cap.
I recognize this might be matter of taste/other equipment in the rig.

I fancy more the treble pot mod you explained last, but I have to kindly ask you:
apart from the more gradual, manageable response, how does it change the pot at the extremes of its setting?
Does it provide less treble boost when turned fully clockwise to MAX (and the MIN setting remains unchanged compared to stock), or just the opposite?

Thank you in advance.

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12Bass
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Post by 12Bass »

Regarding the photos of the V2 Character pedal, I had already taken a look at the layout a while ago. However, I haven't looked closely enough to see if I can determine the component changes. My guess is that Tech 21 has done something similar, though I doubt that I have replicated their sim bypass circuit.

Regarding the treble pot modification, I think that it probably reduces the maximum amount of treble available somewhat. Regardless, when set to maximum, the modded VT Bass still has way more treble than I would ever want. Not sure how it would be in a guitar rig. Also, after a bit of closer listening, I'd say that the volume drop when using the new speaker sim bypass is a bit more than I had suggested in my previous post, but still much less drastic than the big volume increase with the previous bypass mod. Haven't found that the slight sim-bypass volume drop is an issue in terms of the overall output; there is still lots of level available.

If anyone is interested in doing something similar, I would suggest trying different values for the treble roll off capacitor on the pre-sim feed, in order to find a tonal balance that suits your preferences. BTW, I took the pre-sim signal from one of the SMT parts, just ran a small tinned wire parallel to the edge; it isn't that tricky if you're careful. The new sim bypass sounds much less "in your face". Oh... and I swapped out the stock super-dim status LED for a nice bright one. I'll never have any doubt whether the VT Bass is engaged or not, that is if my eyes ever recover from the glare.... :wink:

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Post by 12Bass »

Out of curiosity, I spent a little time trying some different values for the low pass filter capacitor used on the bypass pre-sim signal. The value was 0.01 μF, which I thought made the bypassed tone just a little bit dark. To increase the treble, I lowered the value of the capacitor slightly to 0.0092 μF, using a 0.0082 μF polyester film and a 1000 pF polypropylene in parallel. With just the polyester capacitor there was still a little bit of high frequency "fizz" coming through when the VT Bass was set for distortion. Adding the small polypropylene to ground seemed to tame those "fizzy" highs, making for a smoother tone. I've found that polypropylenes seem to do a little better job passing the high frequencies, in this case passing them to ground.

The result is that the pedal sounds fairly neutral with the speaker sim bypassed and the tone controls and character control at 12:00. It still adds some interesting "character", but is not as "in your face" as when the speaker sim is in the circuit.

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Post by 12Bass »

For anyone who might be interested in trying the revised speaker simulation bypass, here are a few thoughts. I think that taking the pre-sim signal after C11 is a definite improvement from the previous bypass arrangement; the level discrepancy is far more manageable. The bypassed signal is very bright using the R14, R15, R16, C12 junction as a feed, so a low pass filter seems advisable (Tech 21 uses one in their bypass circuit). The question then becomes how much. A 0.0033 μF cap to ground might be a good choice for those who want a brighter bypassed tone relative to the sim, while 0.01 μF makes the tone somewhat darker than the signal going through the sim. At the moment, I'm using around 0.0056 μF, which is sort of a happy medium.

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Post by sssteeve »

12Bass wrote:Finally, because the pedal is so bright sounding, most VT Bass users end up keeping the treble well below 12:00, often closer to around 9:00. To mitigate this, I inserted a 22 k resistor between legs 1 and 2 of the treble potentiometer; this tames the overall brightness and makes the range of the treble control more usable, IMO.
Perhaps someone here can tell me which terminals of a potentiometer are legs 1 and 2? (As far as I know there is no standard numbering for pot terminals.) I would assume that terminal 2 is the wiper so my question is whether terminal 1 would be the "hot" terminal or the "cold terminal" (the one connected to ground when the pot is wired up as volume control.)

Steve Ahola

P.S. A big thanks to 12Bass for all of his tips on modding the VT Bass!
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Post by sssteeve »

12Bass wrote:I think that taking the pre-sim signal after C11 is a definite improvement from the previous bypass arrangement; the level discrepancy is far more manageable.
Looking at the follow drawing which terminal of C11 are you referring to? (I haven't located the schematic in this thread yet- I was waiting for the Reader's Digest version. :? )

http://www.freestompboxes.org/download/file.php?id=4133

Thanks for sharing all of this information with us!

Steve Ahola
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Post by sssteeve »

I found the answers to those questions:
C11 would be tapped at its junction with R13 and R14
It looks like Terminal 1 of the pots in the schematic would be the "cold" terminal (the grounded terminal on a typical volume control.)

Now I just need to find where to connect the pre-sim signal...
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theV
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Post by theV »

Link to a link to the schematic:
http://www.freestompboxes.org/viewtopic ... 80#p134003

Link to stylized drawing of latest version of speaker sim bypass mod (basic, not showing capacitor for reducing treble, pot mod etc.)
Component J2 (pointed by purple arrow with a cross) must be unsoldered and removed.
The "jumpy" red line at the top right means you can connect to any one of the upper terminals of R16, R15, C12 or R14, not to every one.
CharacterBypass.jpg
All credits to 12Bass.
If the picture or anything else is inappropriate, I apologize, please remove it.

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Post by sssteeve »

theV wrote:Link to stylized drawing of latest version of speaker sim bypass mod (basic, not showing capacitor for reducing treble, pot mod etc.)
Component J2 (pointed by purple arrow with a cross) must be unsoldered and removed.
Thanks for the color-coded drawing! A slightly different approach occurred to me last night. Instead of dealing with the traces going to the SMD components why not just connect to the empty copper pad at the input of C11 and add another 47nF capacitor for the bypass line. By doing that you would eliminate most of the effects of the other components like R13-16 and C13-14. I think I'd run leads from the added capacitor and to the input of volume control and try jumpering in components until the signal level and tone sounded right.

I am wondering where people are mounting the cabsim switch.

Steve Ahola

P.S. I am going to put together a ZIP file with all of the schematics, drawings and photos in this thread (at least the ones that I can track down). Along a PDF file with the posts that I found to be most useful. (My interest is in adding cabsim switches to the Version1 Character series and hopefully for the Joyo clones as well. And also in converting one Character to another.)

P.S.S. Another big thanks to all of the great contributors to this thread- I just read through all of the posts and it is an amazing compilation of facts and observations on these pedals.
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Post by sssteeve »

Yikes- the PCB on my VT Bass does not look the same as the other boards posted here. :? The markings on the back of the board are:
UV_21
1020 HS3
. . . 94V-0

The "1020" appears to be a date code punched into a light block on the pcb
The 8 pin module is CH34-6

It looks like I may have to reverse engineer this board to be able to add the cabsim switch.

Request for a hint: how are the terminals on the 8 pin module numbered? I'm looking for #6 so would that be closer to the top of the board (as shown) or closer to the middle of the board?

Thanks!

Steve Ahola
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Post by theV »

sssteeve wrote:I am wondering where people are mounting the cabsim switch.
This is mine, it was the closest to the new wire solders and as out of the way of the pots as possible (between 2 of them):
IMG_0779.JPG
sssteeve wrote:Request for a hint: how are the terminals on the 8 pin module numbered? I'm looking for #6 so would that be closer to the top of the board (as shown) or closer to the middle of the board?
Unfortunately my PCB was identical to the one in the picture,
but based on both on the picture and the schematic, pin 1 of CH34 should have a square solder place (instead of rounded) and be the only one directly connected to ground, so a continuity check with a multimeter should reveal it.

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Post by sssteeve »

circuit board! :P
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