Archive of the former Yahoo!Groups mailing list: MOTM

----- Original Message -----
From: <keithw@...>
Hi all,
I measured the voltage coming out of the 820's bypass socket and got roughly
+12V, I know this is designed for a footswitch
but it would be quite nice to use say a pulse to switch the bypass off and
on, maybe for some nice glissando effects. I only
remembered the voltage coming out of the bypass socket when I plugged in an
LFO which promptly locked up. One or two LFOs I have dont lock up but I dont
like the thought of sticking +12V in to any output.

Does anyone have a mod?

--LH--
Disclaimer: I am not an engineer. I did do some of the beta testing on the
820. I do not think any mod is necessary to use a control voltage into the
bypass jack. When the pulse is high, there will be no bypass. When the
pulse is at zero volts or negative, the bypass function is enabled.

As with all MOTM stuff, Paul does an excellent job of making them "plug
proof" so that plugging the wrong thing into the wrong place does not cause
problems. Understanding the circuit will help. Here is how I see it (some
more detail than the theory of operation in the MOTM manual):

With no bypass input, Q1 is turned on. Emitter / base current flows from
ground through R6 through R3. R3 and R6 form a voltage divider network.
Ignoring the voltage drop across the junction: I = 15/ (10K+51K) = 0.25ma.
Since Q1 is turned on, LED2 is shorted and therefore off. The voltage drop
across the resistors R3 and R6 can be found (10K∗0.25mA) and (51K∗0.25mA)
You will find the mid point near 12 volts (as you measured). 12 volts is
applied to DG213 sw3 and sw4 and keeps lag active (logic 1).

Shorting this mid point voltage to ground turns off Q1 since there is no
longer any voltage across the BE junction of Q1. Shorting the voltage to
ground is no problem as the current is limited by R3 to 1.5mA (15V/10K).
When this is shorted by the bypass switch, the Vbe at Q1 is actually zero.
When shorted by the bypass input, Q1 Vbe is actually ~ 0.15 volts due to the
voltage drop across R4 when shorted to ground. However, that voltage is low
enough to keep Q1 turned off and the DG 213 sw3 and 4 at "logic 0"
According toe the data sheet, the voltage must be pulled below 0.8V for
logic zero and be raised above 2.4 volts for "logic 1" (to switch to bypass
off). What we can see here is that any voltage more negative than ~0.6
volts will both turn Q1 off and switch the DG213 to bypass on. However, we
also see that a variable voltage will not turn both on at off at the exact
same time. However, with a pulse that should not be an issue. With a ramp
up input voltage, the LED would turn off before the DG213 actually switched
to logic 1 (so the LED would disagree with the actual state).

So, using a pulse input from ~ 0 volts to + 10 volts should be just fine.
Sure, current will flow to +15 through R3. However, that current is limited
by R3 and will be a non-issue. Voltage going negative presents some
additional aspects to examine:

D1 protects the input voltage from going below -0.6 volts by providing a
short to ground. Current is limited by R4 and the output impedance of the
voltage source. If you hooked -15 volts directly from your power supply to
the bypass input, you would have a problem. Current through R4 and D1 would
be around 150 mA. If I am seeing that right, that is about 2W of power
dissipation by R4. That's too much. So, don't do that! However, if we
look at a real world example, we see there is not normally a problem.
Typically, the output current of the voltage source is limited by a resistor
to protect the output amp. Let's say that resistor is 1K. So, current
through R4 and D1 with at -10volts is 10V/ (1K+100) = 9mA. R4 power
dissipation is now very low.

But, the output impedance of the voltage source does present a control
issue. If the bypass voltage delivered to the MOTM-820 is supplied by an
amp with a 1K output resistor, that resistor becomes part of the voltage
divider from +15 through R3 through R4 through the 1K output impedance
resistor (in the source module). At zero volts output, current is 15 /
10K+100+1K) = 1.35mA. voltage at the mod point is now 1.5 volts. That is
not low enough to switch DG213 to logic zero. So, you may find that to turn
bypass ON (and deactivate lag), you may have to deliver a voltage from the
source module slightly negative to actually turn the bypass on. But, that
output impedance is needed to protect R4. To stay below the power rating of
R4, the output impedance of the voltage source module should probably be at
least 150-200 ohms. Most certainly are.

So, using a +/- 5 volt control voltage to turn the 820 lag on and off could
be a good strategy. Certainly the pulse output of a MOTM-320 does it quite
well. That's how I see it. I could be wrong. I often am (at least that is
what my wife tells me) <snicker>

Larry