Behringer VM1 Vintage Time Machine

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Re: Behringer VM1 Vintage Time Machine

Postby JiM » 08 Jul 2011, 23:58

el-folie wrote:I´d like to contribute an idea about the gain trim stage that´s preventing the original DMM from collecting too much noise between the two bbds. Attached is a first idea about how to implement that on the VM-1. I know it won´t be correct yet or maybe even totally wrong but maybe with your help we can do it. Please let us know what you think about the circuit additions within the red line.

First of all, welcome on board !
And thanks for trying to design things yourself, instead of asking for a "solder-by-numbers" layout. That's the way to go if you wanna learn !

As you said, there is a problem with your schematic : the opamp has no negative feedback, and an adjustable positive feedback. This will lead to oscillations ! Just swapping the + and - inputs of the opamp would look much better.
Also 43k is not a very common resistor value, 47k is.
A standard inverting opamp circuit would have the feedback resistor(s) connected directly to the inverting input, not before the input resistor. http://en.wikipedia.org/wiki/Operational_amplifier_applications#Inverting_amplifier
Inverting the signal may pose some problems, especially when used in stereo : this will change the "width" of the chorus/vibrato. A non-inverting opamp stage would be better.

Keep going, you're close !
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Re: Behringer VM1 Vintage Time Machine

Postby el-folie » 09 Jul 2011, 20:15

JiM wrote:First of all, welcome on board !
And thanks for trying to design things yourself, instead of asking for a "solder-by-numbers" layout. That's the way to go if you wanna learn !

As you said, there is a problem with your schematic : the opamp has no negative feedback, and an adjustable positive feedback. This will lead to oscillations ! Just swapping the + and - inputs of the opamp would look much better.
Also 43k is not a very common resistor value, 47k is.
A standard inverting opamp circuit would have the feedback resistor(s) connected directly to the inverting input, not before the input resistor. http://en.wikipedia.org/wiki/Operational_amplifier_applications#Inverting_amplifier
Inverting the signal may pose some problems, especially when used in stereo : this will change the "width" of the chorus/vibrato. A non-inverting opamp stage would be better.

Keep going, you're close !



Sounds nice - thanks for the kind words and your help! Yes, I just reversed the op inputs because of the negative/positive ground circuit design difference. I thought that it would have to be reversed as the VM1 is negative ground. Obviously this seems to be wrong thanks to my naive thinking ;-)

Ok, a non inverting op amp stage with gain implemented like on DMM - I will think about it for the next days. Looking at the DMM schematics there is also just an inverting op amp - well it looks like that to me but maybe it´s implemented so, that the phase is turned a second time at another stage of the circuit. Maybe at the gain stage after the second bbd as it´s negative input, too. Any hint about that?

Apart from that my VM1 came with the post today. I did test it briefly and must admit this device is begging for serious mods as it sounds horrible as it is. Volume drop, muddy, the delay sounds very very muffled (too much high freq filtering somewhere) and I would call it a vintage NOISE machine. I don´t know but maybe my unit is very poorly calibrated - I will have a go at it and wreck my head about the make up gain.

Any tips are always welcomed. Happy to be on this forum! BTW - I also found Dirks page and some info about his AD9 memory extension design where no op amp stage is used between the bbds (maybe not needed due to good design?), also very interesting... He definitely knows his stuff very well...
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Re: Behringer VM1 Vintage Time Machine

Postby el-folie » 09 Jul 2011, 22:10

Ok, I thought I could do a quick and dirty 2nd draft, so here it is. Please let me know what you think - I´m really curious... On one hand it seems to be right now due to the insight on the non inverting op info from wikipedia (thanks for the link!), on the other hand the DMM gain circuit seems to be much more complicated and is proven to work for 30 yrs now :) Also the R1,R2 values won´t be good at 100k, too much, maybe 47k for both would be better, then the circuit would double the input level at maximum gain trim and and retain almost the same input level at minimum gain trim like without the modification - of course if only I have it correctly ;-)


Behringer VM-1 gain trim draft2.PNG
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Re: Behringer VM1 Vintage Time Machine

Postby el-folie » 10 Jul 2011, 15:53

A few observations so far about VM1:

- 50:50 mix level seems to be at 7.3 mix pot position due to log pot. What I don´t like is that the pure delay has very very low output on my unit. The pure pre amp level can be much louder depending on input level. Is my unit faulty or is that normal for the VM1?

- I tried all the different cap values suggested by rep_techy on page 3 of this thread to get clearer sounding delays. The only one that really made some difference is C6 (from 47nf to 100nf). The pre amp now has a bit more treble, nice. But unfortunately that doesn´t help much on the poor muffled delay sound which implies that the real problem of VM1 sound will be at the different filter stages before and maybe also after the bbds.

- Changing the caps C7, C9 to different values didn´t change anything apart from a much louder foot switch clicking noise - so I will leave those caps at the default 47µf for now.

- I need to investigate the pre/post bbd filter stages. Any ideas more than welcome...
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Re: Behringer VM1 Vintage Time Machine

Postby rasta_maleek » 11 Jul 2011, 08:02

el-folie wrote:A few observations so far about VM1:

- 50:50 mix level seems to be at 7.3 mix pot position due to log pot. What I don´t like is that the pure delay has very very low output on my unit. The pure pre amp level can be much louder depending on input level. Is my unit faulty or is that normal for the VM1?

- I tried all the different cap values suggested by rep_techy on page 3 of this thread to get clearer sounding delays. The only one that really made some difference is C6 (from 47nf to 100nf). The pre amp now has a bit more treble, nice. But unfortunately that doesn´t help much on the poor muffled delay sound which implies that the real problem of VM1 sound will be at the different filter stages before and maybe also after the bbds.

- Changing the caps C7, C9 to different values didn´t change anything apart from a much louder foot switch clicking noise - so I will leave those caps at the default 47µf for now.

- I need to investigate the pre/post bbd filter stages. Any ideas more than welcome...


try it with other amp, i did the c7 and c9 mods and really improves the sound, maybe depends of the sound of the amp.
check this video, it,s my vm-1 unit modded.
http://www.youtube.com/watch?v=y6cux3U0Ek0

maybe needs more changes in the pre and post BBD´s filtering, but sound nice.
I think that we will work on filtering before install the new bbd´s gain section
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Re: Behringer VM1 Vintage Time Machine

Postby el-folie » 11 Jul 2011, 14:18

I just made a simulation of the pre bbd1 filter stages. As it seems the main filter capacitor is C8 (10n) which forms a first order lowpass filter with R19. For AC sine output (1.5V) the result of the complete circuit is a curve with -3dB loss at about 800Hz and an eq cutoff freq at 1592Hz for R19,C8 -> fc = 1/(2*Pi*R*C), which is quite low and will be one main reason why the VM-1 sounds so dark. Experimenting with smaller C8 values results in higher fc, which may also lead to more overload/feedback/oscillation in the bbds. We need to have some input from people with knowledge, as I´m just guessing here.

Also the bbd output filter stages do look more complicated by a good measure, I don´t know if i can do a sim for that, too. :scratch:

VM-1 bbd input filter sim.PNG
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Re: Behringer VM1 Vintage Time Machine

Postby Dirk_Hendrik » 11 Jul 2011, 16:57

late response. Sorry.
Behringer%20VM-1%20gain%20trim.PNG
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Re: Behringer VM1 Vintage Time Machine

Postby el-folie » 11 Jul 2011, 18:49

Dirk_Hendrik wrote:late response. Sorry.


Hi and thanks for your input! Are R28 (connected somewhere?) and C38 still necessary or can they be omitted?

As an experienced electronics engineer do you think it´s worth the hassle of converting the VM1 to DMM specs? There are so many differences in filtering, gain staging and compressor/expander stages I think I feel a bit uncertain about all the needed work. So many variables and definitely not an easy task.
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Re: Behringer VM1 Vintage Time Machine

Postby Dirk_Hendrik » 11 Jul 2011, 19:45

el-folie wrote:
Dirk_Hendrik wrote:late response. Sorry.


Hi and thanks for your input! Are R28 (connected somewhere?) and C38 still necessary or can they be omitted?

As an experienced electronics engineer do you think it´s worth the hassle of converting the VM1 to DMM specs? There are so many differences in filtering, gain staging and compressor/expander stages I think I feel a bit uncertain about all the needed work. So many variables and definitely not an easy task.


Sorry. R28 can be ommitted. So is C38.
In this config the biasing of the 2nd BBD is done by the DC offset of the opamp. This will change a little by altering the gain of that opamp. This configuration is theerfore the most "correct" from an original DMM design perspective. The C38 cap should have a filtering effect but since it's driven actively from a low impedance source (the opamps output) driving a high impedance source (BBD Input) it's effect is nil.

the less correct from a DMM design perspective, but far more correct from a headroom perspective is this one:
Behringer%20VM-1%20gain%20trim.PNG


In the original DMM circuit the gain of the opamp will also alter the opamps DC offset. This means that when the following BBD is biased optimal by that offset altering the gain will change that bias voltage as well.
This way you can set the gain of the opamp while it's DC level stays at exactly half the power supply, giving optimal headroom for both positive and negative signal sides. Because it's DC decoupled from the BBD that you can set the BBD bias independent of the opamp gain.

* ignore the double component numbering. I copy-pasted in MS paint.
* Use a FET opamp like a TL072 instead on a 4558
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Re: Behringer VM1 Vintage Time Machine

Postby Dirk_Hendrik » 11 Jul 2011, 20:15

el-folie wrote:
Dirk_Hendrik wrote:late response. Sorry.


Hi and thanks for your input! Are R28 (connected somewhere?) and C38 still necessary or can they be omitted?

As an experienced electronics engineer do you think it´s worth the hassle of converting the VM1 to DMM specs? There are so many differences in filtering, gain staging and compressor/expander stages I think I feel a bit uncertain about all the needed work. So many variables and definitely not an easy task.



yes.
The DMM is a great circuit with serious flaws, most likely in an attempt to cut costs . Many of these flaws are directy incorporated in the Behringer which in turn, had flaws added in turn, again, to cut costs. The smart part in the DMM is the gain makeup after the first BBD. One should not omit that. The second, but harder, part is tracing through the circuit and ensuring that at all stages the DC offset is at exactly half the supply voltage. Not such a big concern in a real,15V DMM, but a lot more in a 9 volts DMM where only 4.5 volts is available for each positive or negative signal halve.

(as an offtopic example, a Maxon AD999 delay has a 3-stage filter after the BBD delay line. The first stage is biased by the last BBD at around 3.5 volts. Each stage loses 0.6 volts (base emitter junction drop), leaving the output of the last stage at 1.7 volts in a non-linear operation. Result: more distortion than necessary). So, bottom line, when working with a supply voltage of 9 volts only keeping that signal at a DC offset of 4.5 volts is critically important.

Other than that, for your initial question,
A behringer like this one gives a good platform where screwing up might make you sore for a day but no more than that. Therefore it's an excellent practice platform to find out which tweaks have wat result. And, considering board quality, they'll teach you how to (de)solder without ripping off PCB tracks :thumbsup :blackeye
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Re: Behringer VM1 Vintage Time Machine

Postby el-folie » 11 Jul 2011, 21:23

Dirk_Hendrik wrote:
Sorry. R28 can be ommitted. So is C38.
In this config the biasing of the 2nd BBD is done by the DC offset of the opamp. This will change a little by altering the gain of that opamp. This configuration is theerfore the most "correct" from an original DMM design perspective. The C38 cap should have a filtering effect but since it's driven actively from a low impedance source (the opamps output) driving a high impedance source (BBD Input) it's effect is nil.

the less correct from a DMM design perspective, but far more correct from a headroom perspective is this one:
Behringer%20VM-1%20gain%20trim.PNG


In the original DMM circuit the gain of the opamp will also alter the opamps DC offset. This means that when the following BBD is biased optimal by that offset altering the gain will change that bias voltage as well.
This way you can set the gain of the opamp while it's DC level stays at exactly half the power supply, giving optimal headroom for both positive and negative signal sides. Because it's DC decoupled from the BBD that you can set the BBD bias independent of the opamp gain.

* ignore the double component numbering. I copy-pasted in MS paint.
* Use a FET opamp like a TL072 instead on a 4558



Thank you very much for all the circuit/dc/voltage loss tips and hints about the DMM flaws. I think I just understood a lot better how the circuit is intended to be. In fact your bbd gain circuit makes much more sense in that regard than the original DMM circuit. It was exactly that - DC coupling of the gain op - in the DMM that made me think, why is it implemented like that. As for tracing, I have an old analogue oscilloscope from the 70ies but I think it will do.

About your circuit: Jim said it would be better to use a non inverting op like in my second draft (if that´s correctly implemented), otherwise phase problems with the modulation could occur. What do you think?
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Re: Behringer VM1 Vintage Time Machine

Postby Dirk_Hendrik » 12 Jul 2011, 16:49

el-folie wrote:
Dirk_Hendrik wrote:about your circuit: Jim said it would be better to use a non inverting op like in my second draft (if that´s correctly implemented), otherwise phase problems with the modulation could occur. What do you think?


That's no issue. Essentially a delay line is a phase shifting (many cycles) device already. That means that there is always a frequency that will be in antiphase with the dry signal, Adjust the delay time and that frequency is "good" and another is in antiphase.
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Re: Behringer VM1 Vintage Time Machine

Postby rasta_maleek » 14 Jul 2011, 13:06

Amazing.
I would like a lot understand and speak inglish much better than i can now.
Thanks for all the info, i try to translate slowly.
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Re: Behringer VM1 Vintage Time Machine

Postby colm » 14 Mar 2014, 22:52

this is a great thread and Ive been referencing it alot, things become clearer the more I learn about electronics

I have some questions, I know this is an old thread but Im sure theres still plenty of interest and contributing active members still here


if an op amp is used between both BBD's how much gain is needed? what kind of adjustment would be needed after BBD2 or is there enough headroom next in line?

what exactly do VR4 and VR5 do, I know its some kind of biasing and its a set and forget

3205 bbd's have 2 outputs with 1 bucket stage difference between them so they are slightly out of phase? why are 2 outputs needed if its supposed to feed another bbd or was the initial IC design/tech back in the 70s(?) based on implementing 1bbd per circuit?

bbd1 outputs are summed together using symmetrical values, why is there a balancing trimmer after bbd2?

is it possible to add another pair of bbd's to the circuit without using a buffer for the clock signal?
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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 07 Nov 2018, 03:14

Sorry to resurrect this thread, but this seems to be the definitive VM1 place on the web.
What a great thread. It helped a lot. Many thanks for the excellent schematic!

Here are my mods and fixes to the circuit:

1) Increase input impedance to 1M (see image below around UC3B)
Added additional series Rin and Rf, to increase input impedance and compensate a little for the gain loss.
This also pretty much solves the massive tone suck in bypass.

2) Remove ripple from compander (see image below around IC5A and IC5B, compressor + expander)
Added additional parallel capacitors to clean up the dirty white noise and buzzing trails of the repeats.

3) Fixed muddy and distorted repeats (see image below between feedback pot and IC3A)
Added additional series resistor and cap to the feedback loop to make the thing useful and behave.
Gain reduction through Rf of IC3A is optional.

Other mods I'm considering:
-Modulation rate pot.
-More delay time (but probably also more clock whine)

Image

No idea why the image isn't fully viewable. Right click and choose 'view image', or direct link: http://i67.tinypic.com/1zwjcxk.png

The full explanation is in the next post:
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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 07 Nov 2018, 03:18

A few months ago I scored a used VM1 for 20 euros, looked like new.
The 'playing a strat into an AC30' youtube video comparing it with a DMM sounded so promising.
Mine sounded nothing like it, but I have no strat nor ac30 and all my guitars have rather hot and high output pickups.
I greatly prefer my Memory Toy, which a lot of people think is rather dark, but compared to the VM1 it is bright!
The VM1 I have is a murky, muddy, boomy, distorted mess of repeats.
So after playing it a couple of hours it ended up in a drawer until a few days ago, when I decided to 'fix' this thing.

I knew very little about bbd delay circuits but after spending countless hours comparing the circuits of different DMM's and of those that are based on it and reading what every part does, I finally managed to get some awesome tones out of it.
Too bad there's so much smt in there which makes extensive modding pretty much impossible for a diy person.
So, most of my mods were chosen because they did not require removing or replacing smd components.

So here it goes:


1) Input impedance:

100k input impedance? And on top of that, it's left hanging there in bypass?

I unsoldered one leg of C6 ( a greenie, non smd), so I could place a 1M Ohm resistor in series with the 100k (R2) input resistor.
This brings the input impedance up to 1,1M Ohm.

While you're in there you could also add a 47nF cap in parallel to the greenie, or replace it with a 100nF cap to raise the input capacitance. This will let some more bass in, but eventually I removed it and kept it at 47nF. Lower notes contain lots of energy and since this thing already distorts so quickly, I'm keeping them out for now.

The gain of the preamp has now dropped to less than 1: 22k (R5) + 1M (VR1) / 100k (R2) + 1 MOhm (new) = 1,022/ 1,1 = 0,93.
With the dry/wet mix between 60:40 and 50:50 I found, I needed a gain between 2 and 3 to compensate for the volume loss.
The best solution would be a 3M pot instead of the 1M (VR1) but I don't think that exists so I choose for the easy way: solder a 2,2M resistor in series with the 1M pot (VR1)

Not wanting to unsolder where not needed I simply clipped one outer lug of the pot in the middle and bent the two parts (one sticking in the pcb, the other in the pot) a little outwards and soldered the 2,2MOhm resistor in between. I know, it's a hack, but this thing cost 20 euros and it's an easy fix if you want to revert to stock.

The only downside of this fix/mod is that the minimum gain (level pot at 0) is now 2 instead of 0,2: 22k(R5) + 2,2 Ohm (new) / 100k + 1,1MOhm (new) = 2,222/1,1 = 2.
The max gain is 3 with the level pot at 10.

It's a compromise and you might want to experiment with slightly lower input resistors in series with the 100k instead of my 1MOhm. That would reduce the input impedance and increase tone suck in bypass though. Or go for a new pot to have more gain range.
I find the gain range perfect for my needs. I'm not putting a delay pedal on my board for the 10db boost anyway...

Result:
1)Bright guitar tone going in and consequently, brighter repeats coming out. I might have stopped there.
2)True bypass is no longer absolutely necessary. Having that 1,1M Ohm input resistor 'hanging' there in bypass is not perfect but it's like plugging your guitar into a stompbox or amp with a 500kOhm input impedance. Acceptable.

2)Pre and post filtering and make up gain in between bdd stages:
This is where (according to the experts) the VM1 fails compared to the DMM.
I plotted/simmed the simpler filters and couldn't find anything out of the ordinary compared to the DMM.
Yeah, some filters are passive instead of active through an opamp, but the corner freqs seem the same.
The 3 pole post filtering is done with transistors instead of opamps, but that shouldn't matter if done correctly.
Since most of the filter components are smd there was little to try out there.

Then there's the 'oh so important' make-up gain in between bbd stages. I fail to see the importance or why it's called that way. It looks more like a gain trimmer than a gain make upper to me...
It took a lot of hours of googling to find out its purpose or why it's so important.
Eventually I found a very old EHX service note and also some calibration tips for a DMM based delay on the GGG site.
They both said the same thing: It's there to match the output of bbd1 to the input of bbd1. So it seems bbd can add gain which could need to be trimmed back.
I checked with an audio probe right before bbd1 (C52) and directly after bbd1 (TP1).
If there had been a trimmer I probably couldn't have set it more equally.
If there's any other purpose (bias?) I'd like to know but afaic it's not needed here. And if it were, I guess the Behringer dude wouldn't have left the other half of IC4 unused. It's not as if it cost them another IC.
One thing I did notice however, when probing the input at C52, was that there was already a distorted signal coming in. No wonder that turning the bbd bias trimpots didn't give me any cleaner repeats than the factory setting.

Result:
I'm no longer worrying that the Behringer dude messed up the filtering and overlooked the importance of the make up gain. As it stands, afaic, nothing to do here.

PS: The example Dirk_Hendrik posted above for adding the makeup gain is a cut and paste copy from the 'official'/factory EHX DMM schematic.
Unfortunately Mr Dang from EHX made a mistake, so the copied example above contains the same mistake. Rin should be outside the NFB loop.
No disrespect for Dirk_Hendrik, as he's probably the only person on the planet that has ever crammed a DMM in a 1590.

3)Compander:
With the raised input impedance and a bright guitar signal going in and brighter repeats coming out most people would have been happy about it.
But when investigating and debugging, one tends to listen for every tiny nuance and it's at those times your ears tune into a little thingy and then you're doomed. You can never again 'unhear' it.
The white noise was 'dirty'. It didn't go ssssshhhh but ssRRhhhsRRRhhhSS as if the white noise itself was distorted. From that point on it was the only thing I could hear with every repeat.
After a lot of Googling I finally stumbled upon a forum post by Mark Hammer, on the 'other' forum', and a blogpost about 'ripple' in companders that could explain what I was hearing.
Here's the blogpost or article: https://electricdruid.net/noise-reducti ... ompanders/

And more specifically:
The C1 and C7 1μF values are a compromise between fast response and low ripple. If you want a faster response, try 470nF. If you want less ripple, try 2.2μF.


Checking all the dmm (and pedal derivatives) schematics I had and comparing them to the VM1 schematic, I notice that the VM1 is the only one using such low values: 200nF for C32 and C33. Others used at least 470nF and the DMM uses 1uF.
So, VM1 opts for fast response at the expensive of more ripple. Others go for slow(er) response and less ripple.
I soldered extra caps parallel to C32 and C33. I tried several values: 470nF, 1uF, 2,2uF, 3,3uF.
I ended up using what the DMM uses: 1 uF.

Result:
Dirty white noise is completely gone. It does a smooth sssshhhh. The trails of the repeats are much cleaner now.
Higher values removed even more white noise, besides eliminating the ripple, but it also seemed to slightly dull the repeats. 1uF was perfect.
I won't deny it's a subtle thing, but as said before, once you know it's there, it's the only thing you can hear.
As for the slower response, that's a subtle thing as well. Similar to dialing in a compressor. You want to make it do something, but not so much that it becomes obvious or overbearing.
It's a 'you'll only notice it was there, when you turn it off' kinda thing.
Big improvement afaic as this really bugged me.

I got the slight impression that the max delay time became a tiny bit longer as well, but it's probably an illusion. I'll have to measure it again.
Before all the modding, I measured it to be 499ms. My Memory Toy does 650, but is only completely clock whine free up to 500ms.

4)Distorted repeats, boomy, murky mess and overeager self oscillation.
Going back to investigate the already distorted signal entering the first bbd and the way over the top oscillation I decided to temporarily turn the gain of IC3A down from 2 to 1. Calculation: 1+ 200k (R10) / 200k (R9) = 2
I soldered two wires at each end of that tiny smd R10 resistor. The ends of the wires were then soldered together, actually jumpering R10 and turning IC3A into a unity gain buffer (gain = 1+0/200k)
Afterwards I'll maybe solder a resistor between the wire ends to have it parallel to R10, or maybe a 1 MOhm pot, to reduce its gain but not all the way down to 1.
Having a unity gain buffer there does impact the gain of the repeats, with a good chance of loosing the possibility to make it oscillate.

However..., while doing that I stumble upon the feedback loop and more importantly R26 and C36.

While every DMM schematic I have, uses:
100k + 22n

VM1 uses:
100n + 22k

Coincidence? Or did the Behringer dude mess up and accidentally swapped values?
If EHX had used a value for the resistor that wasn't available as cap or vice versa, he might have noticed.
Is that the reason for trying a 50k FB pot instead of 10k?

Anyway, time for another snip and tuck as I'm still not unsoldering smd parts.
The good thing is that for both resistor and cap we need additional series components: 78k series for that 22k resistor to obtain 100k and some cap between 22nF and 30nF to obtain 22nF (100nF || 28nF = 22nF).
I did the same thing as with the level pot: cut the middle lug of V3 in two, bent the two lug parts a bit outwards and soldered wires to lug parts. The wire ends are soldered to a 100k pot (instead of a 78k fixed resistor) and that in series with a 22nF cap.

Result:
Gone is the boomy murky mess of repeats, welcome bright repeats.
Gone is oscillation as well but that's probably because I reduced the gain of IC3A and turned it into a buffer. I will need to up the gain a bit again there.

Coming out of the expander, the repeat goes to both the output IC4B (and eventually to the mix/blend pot) but the same repeat also goes back to the start of the circuit, so that the repeat itself can be repeated.

With the faulty(?) 22k resistor the repeat-signal going out was (too) soft and going back in it was too strong, resulting in runaway feedback.
On top of that, due to the faulty(?) 100nF cap the feedback repeat retains a lot of the low frequencies, turning it into a muddy, farty mess.
With the correct(?) 100k resistor the repeat going to the output is much stronger and the same repeat going back in is softer.
With the correct(?) 22nF cap a lot of the lows of the feedback repeats are removed resulting in brighter repeats and less oscillation.

In stead of the fixed cap I installed a switch with a couple of caps to choose from.

Together with the 100k pot instead of a fixed 100k resistor it gives great control over the way the feedback loop sounds and behaves.

At the moment IC3A is still at unity gain and with the feedback knob at 10 I have infinite repeats. It does turn into some kind of droning, but because IC3A is at unity gain it never gets out of hand like every other delay does. Once it's saturated it just keeps going but at the same loudness level.

Maybe hard to believe, but I'm wondering if the repeats have become too bright now...
DMM does roll off some highs in the feedback loop. LP filter with R35(1k) and C24 (47nF) in the factory schematic. 3db down around 3,5kHz. That could easily be added to the resistor and cap we added in the FB.


Additional notes:
I hated the chorus on this thing but now with the brighter repeats, for some odd reason, it suddenly sounds great.
I never understood how people could use delay as (faux) reverb. On my delay units it was either a rockabilly slapback which to me does not sound like reverb or I had the flying saucer sounds because when I wanted to dial more repeats to simulate longer reverberation the feedback just ran away. But now that I'm able to precisely control how long it takes for the oscillation to come down and not runaway at the slightest strum, this pedal makes for a great reverb. Especially with a hint of chorus.
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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 08 Nov 2018, 23:03

I came across a schematic for the Boss DM2 and several sections looked very familiar. I started comparing those sections to the VM1 schematic and they're pretty much identical, part for part.
So, the VM1 is a combination of the Deluxe Memory Man and the Boss DM2, we could call it the 'Memory Boss'. :D
In the image below:
highlighted in blue: sections copied from EHX DMM
highlighted in red: sections copied from Boss DM2
highlighted in grey: section added by Behringer
VM1_verifiedmemoryboss1.png


In the VM1 schematic I didn't really understand C54 and r59 (around the transistor just before the first bbd chip) but now that I see them in the Boss DM2 schematic, they make more sense.
Maybe we need to retrace the vm1 in that location.
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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 09 Nov 2018, 02:33

j_flanders wrote:Maybe we need to retrace the vm1 in that location.

I traced that part of the circuit and it confirmed that the VM1 schematic contains a mistake for c54 there.
Instead of this: (eventually connecting to the base)
ScreenShot006.jpg


C54 should be connected to the emitter:
ScreenShot005.jpg


It's the Boss DM3 by the way, not the DM2. (The DM2 is very similar but uses slightly different cap values in those filters.)
Here's the same section but from the DM3 schematic:
ScreenShot001.jpg
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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 11 Nov 2018, 18:32

I've found another error in the VM1 schematic, around c12 :
Original:
ScreenShot004.jpg

Correction:
C12-correction.jpg


I also compared the anti-aliasing and reconstruction filters:
I did this in the online Falstad web app: http://www.falstad.com/afilter/circuitj ... hipass.txt
Since it only lets me to use opamps and no transistors in the frequency plot mode, I converted the VM1 filters, which use transistors, to opamps.

Anti-aliasing filter (pre delay):
ScreenShot001.jpg


Reconstruction filter (post delay):
ScreenShot003.jpg

No idea what's up with the little bumps/ resonance peaks around the two poles in the VM1/DM3 filter.

Some revisions of my previous remarks:
j_flanders wrote:Then there's the 'oh so important' make-up gain in between bbd stages. I fail to see the importance

I measured the output vs input level again, and contrary to what I found/said before, there's actually quite a bit of gain.
BBD output level is a few db's higher than the input level.
DMM offers a trim pot to trim the bbd's output back, to prevent overdriving the second bbd.
Last time I must have probed right at the input of bbd2, and overlooked the voltage divider (r38/R28, 43k/100k) before it, which does the gain trim:
So VM1 does a fixed trim vs the DMM which offers a variable trim pot:
ScreenShot005.jpg



j_flanders wrote:Feedback loop:
While every DMM schematic I have, uses:
100k + 22n

VM1 uses:
100n + 22k

Coincidence? Or did the Behringer dude mess up and accidentally swapped values?

So they simply copied the values from the Boss DM3, and didn't accidentally swap values, but how this fits in with the 100k mixing resistor coming from IC3B, which they took from the DMM, is another matter.
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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 12 Nov 2018, 21:25

Another correction for the schematic:
The overload circuit should come after the antialias filter, connected to the emitter instead of the base. Same goes for C54, but I posted that before.

Currently:
vm1-antialiaswrong.png


Corrected:
vm1-antialiascorrection.png
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