Re: Non-functional tone control


"bg" <bg@xxxxxxxxxx> wrote in message news:fdu5mc$j3o$1@xxxxxxxxxxxxxxxxxxxx

Tony Done wrote in message ...

"bg" <bg@xxxxxxxxxx> wrote in message
news:fdsjll$nfc$1@xxxxxxxxxxxxxxxxxxxx

Tony Done wrote in message ...

"bg" <bg@xxxxxxxxxx> wrote in message
news:fdro0g$el0$1@xxxxxxxxxxxxxxxxxxxx

Tony Done wrote in message ...
This morning I tried jury-rigging a tone control to soften the tone of
my
Baggs M1 active soundhole pickup. I used a 0.047 orange drop cap, with
250K
pot, effectively wired across the guitar lead (Project box with in and
out
sockets wired in parallel, tone control across one of them). It work
fine
with a passive pickup, having about the effect I would expect from
that
size
cap. However the effect was barely discernable with the active
pickup -
I
don't think I would have heard the effect if I hadn't been playing the
guitar myself.

Can anyone explain why the active pickup isn't working with a standard
passive tone control?

Thanks,

Tony D

A big advantage to using an active pickup beside more signal level, is
that
it can drive a long guitar cable without loosing the high notes. This
is
due
to the pickup amplifier having a low output impedance. The way I see
it,
is
that you have two options of connecting the pot and capacitor.
#1 - If a 250K pot is inserted in series with the output, and the cap
is
shunted across the output, the output Z goes way up and you loose the
advantage of having an active pickup. This tone control arrangement
would
work best if it were at the very end of the cable right at the
amplifier
input.
#2 - If the pot is in series with the cap and the two are hung across
the
output, and you take the output from the high side of the pot, the pot
will
not add it's resistance to the output Z of the preamp so you still
have
the
advantage of a low z preamp driving the cable but -----chances are
that
the
tone will be either full on or full off. You won't get a smooth
transistion
with the tone knob.
You will be better off by using a lower value pot. A 1k pot in series
with
the pickup would still be considered as a low Z output for any
practicle
guitar cord and amplifier input, so you won't loose signal level. Then
you
can hang a capacitor across the output. Chances are that the 1k pot
will
be
the bulk of your output Z.
If the pot is set to 1K, a .15 microfarad capacitor across the
output,
will cause the high notes to roll off at 1000 cycles.
If the pot is set to 500 ohms the roll off will start at 2000 cycles
If the pot is set to 250 ohms, the roll off starts at 4000 cycles
See how it works? In other words less resistance causes more treble.
As you cut the resistance in half, you double the roll off frequency.
You can also scale the value of the capacitor, for example a .08
microfarad
cap will double the roll off frequencies. Setting the pot to 1k with
a
.08mfd cap causes the roll off to start at 2000 cycles. As you cut the
value
of the cap in half you double the roll off frequency.
Eventually, when the pot is set to zero ohms, the pickups output
impedance
will determine the roll off frequency. My guess is that it would be
beyond
human hearing, so you effectively have the tone control out of the
circuit.


Thanks, I think you answered another question as well, - Does the
rolloff
start at a different frequency if you increase the pot value, or does it
merely take out less of the same roll-off frequency/

Just to make sure I understand you, are you are saying put a low value
pot
**in series** with the pickup, then put the cap across the combined
output
from the pickup and pot? Won't this reduce the pickup output when the
pot
is
set to a high resistance?

My plan was your #2 option, put the pot and cap in series, and put them
in
parallel with the pickup output, but your idea sounds better if I
understand
you correctly, and it doesn't have too much effect on the output.

Tony D

With this arrangement more pot resistance or a larger cap, lowers the
treble notes. With the treble turned all the way up, the capacitor
should
have no noticeable effect on tone, and when the pot is set to zero ohms
it
really isn't in the circuit anyway. I'll leave it up to you to pick a
cap
that sounds good.
Like I said, a .15 mfd cap should drop 1000 cycles by about 3 db, and
the
treble notes will drop off by 6db per octave there after. If you change
the
cap to 1/2 that value then at 2000 cycles, the output will be down 3db
and
drop off 6db per octave there after. The short answere is that a larger
cap
knocks off more treble. Experiment with caps to get the sound you want,
but
I think somewhere around .1mfd to about .02 mfd is probably where you
need
to be.
The value of the pot changes the frequency also. But seeing as the pot
is
variable, so is the frequency where it starts to roll off the high
notes.
Turning the pot has the same effect as changing the cap value.
I picked a 1k ohm pot because it should be easy to find and also because
it
won't drop your level. The reasoning is that a guitar amp input is
usually
very Hi Z, and 1K in series with it will probably drop less than 1 per
cent
of the signal. Mixer consoles usually have a 10k input Z and even there
you
only loose 10 per cent. Another issue, is that your guitar cable acts
like
a
capacitor across the output signal and this will also reduce the treble.
But
if the output Z of the entire guitar stays low, then cable capacitance
will
not be an issue. You can get away with using any garbage that makes
contact.
As far as wiring this up - - - normally the tone pot is wired so that
when
the pot is fully clockwise you get full treble.
Position the pot so that the shaft is facing you.
Connect the pickup hot wire to the far left pot terminal.
Connect the pot wiper to the output cable center conductor.
Connect the capacitor to the pot wiper.
Connect the shield or low side of the pickup, the other side of the cap,
and
the shield of your output cable together.
The low side and shields can be used as a ground for any metal parts on
the
guitar.
BTW if this is for an acoustic guitar, I've noticed that most pickups,
especially the piezo ones have incredibly harsh midrange. A simple tone
control like this one doesn't solve that problem.
bg


Thanks, I'll get some parts soon and report back.

Yes, it is an acoustic, but it isn't a piezo. The Baggs M1 active is
stacked
humbucker, but the lower coil floats and responds to top vibration,
something like a moving coil microphone. It is very sensitive to guitar
contact noises, like a piezo, and it sounds like a combined magnetic and
piezo without the quack. I'm trying to get a warm jazz-like sound without
having to put an outboard EQ in the circuit. I'm not wild about piezo
systems, and this seems a good compromise.

FWIW I've found that simply wiring a magnetic and piezo in parallel gives
a
good "hybrid" sound, without the piezo quack.

Tony D


I downloaded the manual for your pickup and see that the output impedance
is
800 ohms. I was expecting something around 50 ohms. Here is a revised list
using some standard capacitor values.

With a .1 mfd capacitor -
And the tone control is set to 1000 ohms then F = 884 cycles
And the tone control is set to 0 ohms then F = 1989 cycles

With a .068 mfd cap
And the tone control is set to 1000 ohms then F = 1300 cycles
And the tone control is set to 0 ohms then F = 2926 cycles

With a .05 mfd capacitor
And the tone control is set to 1000 ohms then F = 1768 cycles
And the tone control is set to 0 ohms then F = 3978 cycles

With a .022 mfd capacitor
And the tone control is set to 1000 ohms then F = 4019 cycles
And the tone control is set to 0 ohms then F =9042 cycles


This range of values will cover from about 1000 cycles to 10,000. The main
difference is that for any value of cap that you choose, the tone pot will
not have as large a range of control as it would have if the output Z was
lower. For any value cap, notice that the tone pot will shift the cut off
frequency just about two times. That is because the pot just about
doubles
the 800 ohm output Z. If you need more range from the tone control, your
only option is to use a larger pot. A 2K pot will expand the tone range
about 3 to 1 and a 5K pot will expand the range to 7 to one. But as the
pot
value gets larger, you loose the advantage of using an active pickup.
Also, because of the 800 ohm output Z, there will always be some treble
loss
with any pot you choose.


Thanks for all the very helpful advice in this thread. What I finally
settled on, given the limitations of space and the 800 ohm impedance of the
pickup is a DP3T on-off-on switch with two different caps in the "on"
positions (bridging the pickup output) and no cap in the "off" position. The
caps I'm using are polyester film 0.12 and 0.22 microF, but I expect to
change these when I've tried it through my gig amp in a band context. They
are quite subtle, and in the right ballpark, but fairly close to each other
in the effect I can hear through my home amps.

I have mounted the switch on a bracket stuck to the neck side of the pickup
with double side adhesive. The knob sticks up just under the 4th string
where it can easily be flicked to either side with a finger. It is smaller
and neater than using a pot, but unfortunately if makes quite a loud click
when I switch it. - I'll try it and change it for a pot and a single cap if
it is too much of an annoyance.

Tony D


.

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