| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 2251.1 | need schems | MILKWY::JACQUES | Vintage taste, reissue budget | Wed Jun 26 1991 23:44 | 13 |
| It sounds like the LED is in series with the preamp. I believe LED's
are usually connected in parralel, and there is usually a resistor
used with the LED to limit the current going through it. I'm
surpised you didn't blow the LED.
It's possible that when you switch from one side of the AB to the
other, there is a momentary latch-up condition happening that
stresses the preamp chip.
It would help if you could show a simple schematic diagram of your
circuit.
Mark
|
| 2251.2 | Custom design by one of our own | GOES11::G_HOUSE | and I might be too far down | Thu Jun 27 1991 02:08 | 14 |
| I know this doesn't really relate to the question, but Stevie J.
designed me a custom A/B switch awhile back. I never made it (I
changed equipment and lost the immediacy of my need for it, but may
still make it someday), but it looked like a good idea. Pretty cheap
on the parts as well.
FWIW, this design had separate variable gain preamps in it that you
could use on each branch.
I'm sure he'd share his schematic with anyone that was interested in
it. (or you can come copy it off the white board in my office...)
Greg
|
| 2251.3 | | RGB::ROST | I believe she's a dope fiend | Thu Jun 27 1991 10:58 | 5 |
| Re: .1
I think Mark is right, you want to current limit the LED.
Brian
|
| 2251.4 | 10mA ? Check me on this. Anyone? | LEDS::BURATI | Fender Bender | Thu Jun 27 1991 11:58 | 21 |
| > I think Mark is right, you want to current limit the LED.
...with a resistor, typically like so
led 820 ohm
+ o------|>|---/\/\/\/---+
|
9v _|_
///
- o--+
_|_
///
this should draw a nominal 10mA from the battery, which if memory serves
me well, is about what an LED needs.
Q? Do you want to power the preamp off when the signal is not directed
through it? I ask because it may produce a nasty pop that could make
your amp and speakers very unhappy :(
--rjb
|
| 2251.5 | here's more | TOOK::SUDAMA | Living is easy with eyes closed... | Thu Jun 27 1991 15:44 | 56 |
| Q? Do you want to power the preamp off when the signal is not directed
through it? I ask because it may produce a nasty pop that could make
your amp and speakers very unhappy :(
A: Yes, I want to power it off so I'm not draining the battery. i don't
think the noise will be a problem, because this is actually a "preamp
to a preamp". Maybe I should be more precise about what the
configuration is: I've got two guitars, an electric and an acoustic,
that I want to run through one amp. I've already got two preamps in the
chain - one in the amp itself, and one in an effects unit I use in the
input stage. I used to just have a soundhole pickup in the acoustic
with an output level similar to my electric, and I could just switch
between them with the A/B switch. But I recently installed a thinline
pickup inthe acoustic, which requires an additional preamp to bring the
signal level up to that of my electric. I don't want to install this
inside the acoustic, so I decided to put it in the A/B switch.
Now that I've explained how I got here, what I want to have is fairly
simple. When the A/B switch is on one side I want the electric to pass
signal, and the preamp shut off. When the A/B switch is on the other
side I want the acoustic to pass signal through the preamp, the preamp
turned on, and the LED turned on. This is how it used to be wired:
ground
o
\
input A ----o o---- input B
\
o
output
In other words, with the switch in the A position input is signal out,
and input B is tied to ground. And vice versa for B position.
This is the way I re-wired it:
preamp- <--- LED- <--- ground <--- -batt+ ---> LED+ ---> preamp+
o
\
input A ----o o<---- preamp in/out <--- input B
\
o
output
That doesn't completely capture it, but I don't have time to enter a
full schematic. Basically I'm using one side of the A/B switch to pass
the - on the battery to the LED and - on the preamp. Even when I look
at this I can see why there might be problems with the way things are
grounded. If that's not enough information i'll try to put together a
full schematic.
- Ram
|
| 2251.6 | Schematic | FOO::BHAVNANI | SYS$UNWIND - laid back VMS | Fri Jun 28 1991 13:19 | 36 |
| This should do it. Note the "X" means the connections don't
meet; they overlap. R1 is an 820ohm resistor. D1 is the LED.
Remember to observe the LED polarity. The band on the LED is
connected to 0V.
Basically, S1, S2 and S3 make up a 3PDT (3 pole double throw)
switch. In the "A" position,
S1 powers the preamp and the LED
S2 selects the acoustic input
S3 selects the pre-amp output
In the "B" position,
S1 disconnects the battery
S2 selects the electric input
S3 selects the pre-amp input
S1 A
+---------------------+-----------------O====o---------------o 9V
| | 9V o B
[R] ____+____
[1] | | IN S2 A
| OUT o--+----+ Pre +----------+--O====o---------------o Acoustic I/P
V | | Amp | | o---------------o Electric I/P
- D1 | ----+---- | B
| | | 0V |
| | | |
| | | S3 A |
| +--------X----O====o |
| | o----+
| | B
| |
+---------------------+--------------------------------------o 0V
/ravi
|
| 2251.7 | almost got it | TOOK::SUDAMA | Living is easy with eyes closed... | Fri Jun 28 1991 14:06 | 27 |
| Thanks, Ravi, I can follow this. I was beginning to wonder if my
problem was that you couldn't actually tie - on the battery to ground,
but that doesn't seem to be the problem. However, what I incorrectly
described as a 3-pole switch before, I've come to realize is actually a
2-pole switch. Would it be possible to adapt your schematic as follows,
or do I really have to get a 3-pole switch? And if I do need a 3-pole
switch, any ideas on where I can find a heavy duty footswitch of that
nature that would stand up to stomp box-type treatment?
S1 A
+---------------------+-----------------O====o---------------o 9V
| | 9V o B
[R] ____+____
[1] | | IN S2 A
| OUT o--+----+ Pre +-------------O====o---------------o Acoustic I/P
V | | Amp | o---------------o Electric I/P
- D1 | ----+---- | B
| | | 0V |
| | | |
| | | |
| +--------X----------------------+
| |
| |
| |
+---------------------+--------------------------------------o 0V
- Ram
|
| 2251.8 | CMOS switch | FOO::BHAVNANI | SYS$UNWIND - laid back VMS | Tue Jul 02 1991 14:22 | 18 |
| Ram,
There may be a problem with your config - the O/P of your box
always sees the elctric guitar I/P. If you short the electric
I/P (to kill it), you'd also be shorting the O/P of your pre-amp.
I agree it's hard to get a 3PDT footswitch (Active Electronics
in Westboro may stock them, at a price). I would instead build a
simple electronic switch using CMOS chips freely available from
Radio Shack. In fact, you can build an nPDT switch pretty easily
(n = 2, 3, 4, ...).
The CMOS chips eat very little juice. The switch would also be
completely noiseless. I think I have a circuit somewhere; I'll
try to dig it up. Radio Shack carries an "Engineer's Notebook"
that describes how to build them.
/ravi
|
| 2251.9 | Alternative to CMOS | FOO::BHAVNANI | SYS$UNWIND - laid back VMS | Tue Jul 02 1991 14:25 | 9 |
| Use a regular DPDT footswitch for S2 and S3. For S1, use the
"free" switch you get with some phone sockets - when you plug
in a jack (acoustic I/P), the switch closes, thereby powering
your pre-amp.
Bummer: the preamp will always be powered if the acoustic I/P
is plugged in, even if you're not actually using it.
/ravi
|
| 2251.10 | my last shot | TOOK::SUDAMA | Living is easy with eyes closed... | Mon Jul 08 1991 11:29 | 16 |
| Ok, I thought this over some more, so let's see if I can't get this to
work with one DPDT switch. If this doesn't hack it, I'll give up and go
for a new switch. Any problems with this?
S1 A
+--------------------------------------------+------O====o 9V
| | 9V o B
[R] ____+____
[1] S2 A | | IN
| OUT o----O====o--------------------+ Pre +-------o Acoustic I/P
V o B | Amp |
- D1 | ----+----
| | | 0V
| +---o Electric I/P |
| |
+--------------------------------------------+------------o 0V
|
| 2251.11 | Alternative wiring | AKOV05::DERRICO | Stand and deliver! | Tue Jul 09 1991 13:26 | 61 |
| It's possible to get an isolated input jack to turn the battery
on for you. The power to the preamp is only on when you have the
acoustic plugged in that particular jack. You can get these jacks
from U-Do-it Electronics in Needham. (Side note: Active is no longer
in Westboro - They've moved to Woburn.)
The jacks can have "isolated" switches in them. You can get a
S.P.D.T. or D.P.D.T. options.
I think it's better to have the op-amp powered up before you switch.
You can have an A/B Switch "Post-preamp". The power drain is only when
you have the acoustic plugged in to it. If you use a D.P.D.T. switch
to do the A/B function, and the other side just powers the LED on to
indicate that you have the preamp switched in.
Consider this:
---------------------------------------------------------------------------
+-----------------o +9v (switched)
|
o--------9V-----O===O O
(+) ^
9V batt. |_______jack-switch (on when acoustic plugged in)
(-)
o--------GND
------------------------------------------------------------------------
o +9v (switched)
Acoustic |
input jack | (1/2)
_ |-----+-------| A/B SWITCH
| | V---------------| pre amp |----------------O Output jack
| | in |-----+-------| out \ _
-| | O---------V | |
| | | |
o GND o GND +--------O |-
| |
| o GND
|
- |
| | V-------------------------------------+
| |
-| INPUT from electric.
|
o GND
-------------------------------------------------------------------------
O--------/\/\/\/------+
\ |
o-------(+)9v Switched-------O |
|
O LED
|
(1/2)A/B SWITCH |
o GND
---------------------------------------------------------------------
|