Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

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Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Sun Jul 26, 2020 10:40 am

On my module with the Xvive VTL5C3, the brick reverb is miserable (I tried R110=4.7k and R110 = 47k but still not ok).
On another module with another kind of vactrols, it's fine (R104/R105=10k - R108=68k - R110=47k).
I highlighted the brick section in the schematics and I wonder what resistance you chosed on your module with Xvive VTL5C3?


EDITED with the hope that it's more efficient.

Original post:
I tested with 2 kinds of VTL5C3 vactrols and with the bottom one (Xvive VTL5C3 vactrol fron Electric Druid) the brick reverb does not work properly (BTDR-2H - Accutronics Digital reverb brick).
With the Xvive there’s always an unchanging minimum reverb, no matter how the BLEND knob is set (full CCW, min or max, same).
I used values as mentioned in the schematics on Github (except for R110 where I tried 47k and also 4.7k), but it seems that different resistances are needed in the brick circuit for these Xvive ones.
On the schematics different resitor values are mentionned for the SPRING circuit, but not for the BRICK part.

I don't have the specs from the OK ones (I'm showing a VTL5C3/2, but I used a VTL5C3. The VTL5C3/2 when omitting one of the 3 legs also works fine of course), so I can't even try to make deductions by comparing the OK with the NOT OK specs.

I wonder what you use for resistance values with your Xvive vactrols in the red area of the screenshot I'm posting.
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Last edited by batchas on Mon Jul 27, 2020 1:52 pm, edited 1 time in total.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Mon Jul 27, 2020 1:51 pm

I edited the first post.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Tue Jul 28, 2020 10:53 am

I was asking here because I didn't want to start unsoldering, soldering, unsoldering etc etc, swapping values. Makes the PCB looks shitty. I use SMD, so if I need to lower the value no prob, I can quickly put a TH resistor on top, easy to solder/unsolder, but if I want to increase, then I have to take the SMDs off.
Anyway. I tried with changing values.

As mentionned with these Xvive vactrols which are causing me real trouble as I spent hours/days on my builds, when the BLEND is at 0, there's some reverb going through. And with BLEND at max, the reverb is pretty weak.
If I put 2k5 at R110, then still a very little bit of reverb when BLEND is at 0, and at 100% the reverb is not worse then before.
If I put 2k at R110, BLEND at 0 has no reverb anymore, but at 100% the reverb is not usable.

So I started to change R108 too, lower, higher values... Not better.
I also changed R104/R105 and cannot come to any satisfying result. With 5k instad of 10k the signal seems a bit more noisy, a little bit more reverb at the output maybe, but not satisfying either.

Knowing that the Xvive vactrol lets some WET signal through even when BLEND is full CCW and does not let enough WET signal when BLEND is full CW, I believe there's no need anyway to look around the brick signal and change values like I did, but more around the vactrol, IC103A, IC103B, U2A, R24, the VAC_AN_F and VAC_CA_F area. Though all works fine with the PRC vactrols. Tomorrow I'll look at R21, R22, R23, starting with R24 (schematics).


EDIT: they are not PRC vactrol but Vactec vactrols working really well. I feel stupid I ordered these Xvive vactrols.
Last edited by batchas on Wed Jul 29, 2020 4:29 am, edited 1 time in total.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Wed Jul 29, 2020 3:57 am

With 2k5 instead of 47k at R110, and 680r instead of 2.2k at R24 makes the reverb better. I can't get any better.

Still a bit of reverb when BLEND is full CCW and also loosing a bit clarity on the feedback, but the feedback is extreme anyway.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by EATyourGUITAR » Wed Jul 29, 2020 8:20 am

I started with 4K7 then had to switch to 47K. the xvive vactrol I purchased was probably fake. synthrotek sent me something that says "ST". no xvive logo at all. so I got screwed and had to take apart this module 5 times. I had to change every resistor from the vactrol driver, the mixer, and the LED current limiting resistsor. one by one. with improvements at every step. it works perfect now. there should be instructions to tune the vactrol on the breadboard. the instructions only have 1 resistor being suspect when in fact all 3 of them are customized to the vactrol. or maybe my resistor values should be in the BOM instead if it is even possible to standardize that.

I swapped 2.2K LED current limiting resistor to 4K7. the mixer needs a smaller resistor also.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by Bartimaeus » Wed Jul 29, 2020 4:35 pm

just rub some isopropyl on the boards after you swap the components, it'll look nice and clean

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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Thu Jul 30, 2020 7:00 am

@EATyourGUITAR: thanks for sharing. I really appreciate.

Today I decided to do what I should have done from day one after installing the Xvive vactrol on the proto and discovering it was not working like the Vactec did without changing resistor values listed in the original DIY BOM.
I took R110 and R24 away and installed instead trimmers, to be able to tweak values while hearing the result, not keeping on forever to swap values, as I was never satisfied by the results compared to what I knew was good, having one board with Vactrec and knowing what the circuit is capable of.
It means that I'll make new boards one more time with these 2 trimmers in place, allowing fine tuning.

This way I was able to first tweak R110 until the BLEND full CCW does not let the reverb signal through and then tweak R24 to get the best reverb effect. Meaning finding a good compromise to get a reverb as good as possible as the one at the WET output.
Most satisfying results: R110 is 2K and R24 is 60R. Drastically different then with the Vactrec vactrol!
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by EATyourGUITAR » Thu Jul 30, 2020 1:56 pm

bathas, I think the best way to tune the vactrol is to operate in the linear region. it is possible to have many good working resistor combinations that work ok at the %100 dry and %100 wet knob position but have not a very usable range when sweeping the knob. the knob is also sending too much current into the mixer. I doubled the resistor value from the pot so that knob CW %100 no CV input = knob CCW + CV 5v. if you don't increase this resistor, you are sending a max CV of 12v from the knob but you never use +12v at the CV input. they should match. that should be step 1 before any other mods. then the 47K for the vactrol. then the 2.2K for the LED gets changed to your preference. I used 4K7. it depends on what vactrol you use both the 47K and the 2.2K are customized. after all that. you can change the audio crossfader that the vactrol is mixing. increasing this audio mixer resistor makes it more wet in the middle of the knob. decreasing it makes it more dry on the dry. if you hear reverb in the dry then you must change this resistor to a smaller value start 680R go down to 470R.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Fri Jul 31, 2020 12:46 am

I never thought about the 2 resistors you are mentioning. Thanx a lot for the suggestion!!!
It's worth the try at this stage.

Could you please confirm:
EATyourGUITAR wrote:
Thu Jul 30, 2020 1:56 pm
The knob is also sending too much current into the mixer. I doubled the resistor value from the pot so that knob CW %100 no CV input = knob CCW + CV 5v.
the resistor from the pot is R21, is that correct?
EATyourGUITAR wrote:
Thu Jul 30, 2020 1:56 pm
after all that. you can change the audio crossfader that the vactrol is mixing. increasing this audio mixer resistor makes it more wet in the middle of the knob. decreasing it makes it more dry on the dry. if you hear reverb in the dry then you must change this resistor to a smaller value start 680R go down to 470R.
When you refer to the audio mixer resistor, you mean R13, am I right?
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by EATyourGUITAR » Fri Jul 31, 2020 8:12 am

Change R21 to 220K and R22 100K.

Or you can leave R21 at 100K and change R22 to 47K. As long as the ratio is 2 to 1.

Turn it on to test. Then change R24 to 4K7.

Then change the mixer R110. I think maybe it was the 4K7 I used for R110. But I went to a smaller value to make the dry %100 dry.

If you start with 47K R110 then maybe you only need 33K or 22K. The rule here is that this resistor must be 100 to 1 in off mode and 1 to 10 in on mode.

If the vactrol has an On resistance of 2K then 22K R110 is 1 to 10. Then the off resistance of 12M that is 50 to 1 good enough. R110 is the audio mixer under CV control and knob control.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Fri Jul 31, 2020 9:03 am

Thank you very much. I'll try and report.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Fri Jul 31, 2020 11:58 am

So... About 3 hours later of testing, retesting, soldering, swapping trimmers and so on (starting with large values), the XVIVE vactrols are definitely different that what you used as I arrive to different results. Actually nearly to same results I mentionned in my previous post.


I changed R21 to 200K.
Turn it on to test. Then change R24 to 4K7.
And change R110 to 47K.

Then only way to have dry signal when BLEND is full CCW is to have R110 around 2.5k, no matter what R24 value is. I tried everything. Impossible when the value is higher.
Once I get BLEND ok with R110 being around 2.5k, then I change R24 until the reverb is okay, because the reverb is too weak when BLEND is full CW (I see it anyway even without listening as the blend LED is not fully lighten).
When R24 is around 100R, then the reverb is decent.
Then I fine tune values (as I installed trimmers in place of R110 and R24, allowing me to tweak and listen directly) and the results are:
R110 = 2K2.
R24 = 107R.

Also not bad:
R110 = 2K5.
R24 = 100R.

I thank you again for your time EATyourGUITAR and your previous posts.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by papz » Fri Jul 31, 2020 1:09 pm

Vactrols have a huge tolerance in resistance, tail, behaviour...
In an ideal world, you'd rather try various vactrols untill you find the good one, but this might end up expensive since you may need to test a lot of ones untill the good one comes.
The NSL-32 range could be a cheaper alternative https://be.farnell.com/fr-BE/search?st=nsl-32
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Sat Aug 01, 2020 12:42 am

papz wrote:
Fri Jul 31, 2020 1:09 pm
Vactrols have a huge tolerance in resistance, tail, behaviour...
In an ideal world, you'd rather try various vactrols untill you find the good one, but this might end up expensive since you may need to test a lot of ones untill the good one comes.
The NSL-32 range could be a cheaper alternative https://be.farnell.com/fr-BE/search?st=nsl-32
Thanx papz. I was not aware of these NS-32.
As I replaced in the circuit the 2 critical resistors with trimmers, I decided this morning I was going to unsolder the Xvive, put headers instead on the proto to swap easely the Xvive I got from Electric Druid (I should have done this from day one, like I did in the past for oscillators with JFET), and check how they behave, hoping they do not vary too much, so the trimmers can have an optimal range (for more precision).
I could also swap each vactrol one by one, set the optimal resistance for each one via trimmers, measure both trimmers and place resistors accordingly to each one instead of having trimmers on the production boards, but I'll see first if and how they differe.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by BugBrand » Sat Aug 01, 2020 3:10 am

I got 200 VTL5C3/2s from Xvive - they absolutely did not match the behaviour of vintage ones, far far faster in their decay. I'm used to checking all before soldering & know the typical response times (some are too long) - perhaps with some digging I could change the regular circuit to work with the Xvives but haven't pursued that yet, just finding it frustrating that a repro is not an actual repro..
Back when I used the NSL-32 range I found they had *really* wide variations - very hard to work with. Perhaps fine for one offs but bad for production.

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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by batchas » Sat Aug 01, 2020 4:57 am

BugBrand wrote:
Sat Aug 01, 2020 3:10 am
I got 200 VTL5C3/2s from Xvive - they absolutely did not match the behaviour of vintage ones, far far faster in their decay. I'm used to checking all before soldering & know the typical response times (some are too long) - perhaps with some digging I could change the regular circuit to work with the Xvives but haven't pursued that yet, just finding it frustrating that a repro is not an actual repro.
The difference with the vintage ones I used on diverse Buchla DIY builds is indeed huge!

I finally found a satisfying solution with the trimmers and the lower resistance values.
I installed headers and could test each of the 10 x Xvive quite fast. To my surprise they all match pretty well. I could not hear any difference actually! The attack on two of them vary a little bit, but i'ts barely noticeable and easy to trim anyway.
I'm glad all work well with the settings I mentionned earlier so I can move forward.
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Re: Anyone built a Music Thing Reverb MK2 using Xvive VTL5C3 vactrols?

Post by EATyourGUITAR » Sat Aug 01, 2020 8:39 am

you should be focused on getting the knob to go through the LED brightness in a linear way. after you get the range of brightness on the front panel LED, then go back to change R110. listening for dry wet is not the best method to determine R110 if you did not yet set R21 R24 correctly and you did not adjust the trimmer for reverb send level. if the reverb send is too low, you will think it is too dry when actually there is no problem. adjust the trimmer to get some distortion then bring it back down. check the volume at fully CW and fully CCW. then change R110 if you want to change R110.
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