OK, I like your circuit. The only slight disadvantage is that it will
double its distortion level when running in mono, but thats very small
point if you use a reasonable quality opamp. The other thing is if you
have a patchy connection on the shorted ring this would feed into your
mono sound channel.
Ok. When you say distortion you are talking about the overall signal
level and clipping? I don't think that is a problem. I intend to
drop the signal down a bit anyway so that it is more like a guitar
level signal, and so it can handle the battery dropping in voltage
somewhat. (That is how it will behave, right? As the 9V battery dies
it will just reduce the possible output swing of the op-amp (to a
point, of course)?) The shoddy ring connection is a valid concern,
but wouldn't it do the same thing with the FET circuit?
|\
left --|-\ ___
| >---|___|----+------- Vtip
--|+/ 5k |
|/ |_|
-------|_
|\ | |
right -|-\ ___ |
| >---|___|----+------- Vring
-|+/ 5k
|/
There we go. With the fet off it works in stereo, with the fet on it
outputs mono. To switch the fet youd need another opamp which detects
the dc level on the jack ring, plus an RZR etc to put a small dc
offset onto the ring. When a mono plug is inserted, this dc offset is
shorted and the opamp switches the fet on, resulting in mono ouput.
That's a valid way of mixing? Whatever happened to "use a summing amp
to sum voltages! never connect two outputs together!"? clearly
something else I need to learn...
So there are some different possibilities as I see it:
1. Build my type of circuit with the preamp included so it is just two
opamps total. But the current preamp circuit requires high input
impedances (see below), to match the piezo, and my above switching
circuit has 100k. If it is possible to use 10M or so resistors for
the switching circuit, I guess that would work. (but I always see
op-amp circuits with resistors from 1k to 100k and never more or less,
so it must be bad to use those values, right?)
2. Use the preamps and switching circuit separately, which requires 4
op-amps and either two duals or one quad chip.
3. Use the FET switching circuit, which requires 3 op-amps, and hence,
two duals or one quad chip, same as 2.
4. Can the FET be triggered with the dc voltage directly, without the
op-amp? It seems like it could be, with the right voltages in the
right places and the right FET.
Here is my preamp circuit, exactly. I intend to drive either guitar
amps, around 50k inputs, i believe, or sound cards, about 600ohm
inputs (and certainly feel free to tell me if I did something wrong or
need something else to make it better.):
___ |\
.---|___|----o------------|+\ #| ___
| 6.6M | | >---o-----#|----|___|----o
| | .--|-/ | #| 1k
--- Piezo .-. | |/ |
[===]6 Vpp | |2.2M | | 4.7 uF
--- at max | | '---------' Vout
| '-'
| |
| |
'------------o----------------------------------------o
|
| ---(Top section x2)---
|
9V |
+ |
| |
o------------|-------o-------.
| | | |
.-. | | |
| |4.7k | | |
| | | | |
'-' | |\| |\|
| | |-\ |-\
o---------o--' | > | >
| | |+/ |+/
.-. | |/| |/|
| |4.7k --- | |
| | --- | |
'-' | 103 | |
| | | |
o---------o----------o-------'
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
John Woodgate said:
No, it's the way, or at least a good way, to do what you want. It's
virtually the same as the solution proposed by Tony Roe Whether 1 k to
ground is OK or not depends on the op-amp. If you use NE5534 (or any one
of several more modern types) it's OK, but 741 and 324 aren't so keen on
1 k (for different reasons). I wouldn't use a 324 for this job anyway.
I am using a tl062 right now because that is what i had in my parts
box. I need to order an unrelated chip (and probably this circuit's
output jack) from a catalog, so i should get some other things while
I'm at it. ne5532s are pretty good quality? I see them used in audio
a lot. Are the more expensive opa134 burr brown type chips really
worth it?
Oh wait... *Looks through box* I guess i do have some 5532s, and
072s, 741s, and another 062, and one each lmc60 and lf412. I remember
having problems with the lf412 and lmc60 in the older version of the
circuit. Which would you use?