Behringer VM1 Vintage Time Machine

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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 05 Dec 2018, 23:50

Some more frequency responses.

The dry signal as it arrives after the compressor, just before the anti alias filter and the BBD section for the Deluxe memory man:
(Level and mix is set for unity gain, feedback is at 50%)
DMM_INPUT_TO_AAFILTER.jpg


Same thing but for the VM1:
The VM1 delivers a much more scooped dry signal to the bbd section. The main 'culprit' is that small 22k bottom resistor (compared to 100k in the DMM) in the voltage divider after the first opamp.
VM1_INPUT_TO_AAFILTER_FB50.jpg


Another thing to note is that, for the feedback pot, because of its large size (50k vs 10k in the DMM), its position affects the frequency response quite a bit.
In the DMM the feedback pot is 10k, which is 'insignificant' compared to the 100k voltage divider resistors, so the postion of the feedback pot has little to no effect on the frequency response.
Here's the frequency response for the VM1 with the feedback pot at 0% or 100%¨, giving an even more scooped dry signal:
VM1_INPUT_TOAAFILTER_FB0.jpg


In the Boss DM2 or 3, from which the feedback path was copied, the small 20k resistor made sense because they use a different way of summing dry and delay signal for the feedback.
The DMM uses the same passive mixer for the feedback as it does for the output mix, but with two 100k resistors instead of the 10k pot at the output.
The DM2/3 uses an active inverting, summing opamp mixer and it actually uses the internal opamp of the compressor for this. It has a 20k input resistor (R3 at pin 6) and by connecting the feedback signal through a 20k resistor to pin 6 you get a 50/50 suming.
But combining the 100k resistor of the DMM with the 20k resistor of the DM2/3 makes little sense.
Compressor internals (pin 3 is 20k, Boss connects feedback signal through 20k to pin 5):
COMPRESSOR_VM1.png


Because the 3205 bbd chips have less headroom and worse frequency response for higher frequencies than the 3005, this 'mistake' kinda works out in the end. The VM1, because of the scoop, sends less signal to the BDD and has a pretty big pre-emphasis on the highs.

But for now we've only looked at the 'mistake' from the dry signal perspective. Next post is about how it affects the delay signal.
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Re: Behringer VM1 Vintage Time Machine

Postby j_flanders » 06 Dec 2018, 01:40

Frequency reponse of the delay / feedback signal. Level and mix set for unity gain, feedback at 50%.
In real life the signal coming out of the bbd section has been filtered by the anti-alias filter and the reconstruction filter, but to see the effect of the differences between the DMM and VM1 feedback loop I'm using a FRFR input signal.

DMM: ( - 3dB at 1kHz)
DMM_FEEDBACK_TO_AAFILTER.jpg


VM1: ( + 3db at 1kHz)
VM1_FEEDBACK_TO_AAFILTER_FB50.jpg


The DMM has an extra LP filter, right before the feedback pot.
The VM1 is flatter and has stronger repeats in the feedback. It should oscillate earlier on the dial than the DMM.
The VM1 has a softer first repeat but relatively stronger later repeats.
From the plot you'd expect much brighter repeats on the VM1, but in a later post you'll see that in the VM1 the signal looses more highs along the way(in the filters, bbd's, make up gain etc) than the DMM and will eventually have less highs than the DMM.

Once again, you could say that the 'incorrect' 22k and 100n in the feedback loop kinda works out in the end.

Replacing the 22k with a 100k pot and the 100n with a switch to various caps, really lets you shape the repeats both decay-wise as frequency wise.
22k and 100n is better for faux reverb
100k and 22n is better for distinct repeats.
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