need help getting rid of clicks, pops, and hum in design

[tempus fugit]

Hmmm...

I think this transistor idea may have been ill-conceived. I still can't get
rid of the ground loop hum, and there is still a pop everytime I switch.
However, it is only in one direction, if that makes a difference. Switching
from float to ground is quiet, but the other way round gives a loud and
clear pop.

May have to just switch this one back to relays...

 

[Jamie]

Hmmm...

I think this transistor idea may have been ill-conceived. I still can't get
rid of the ground loop hum, and there is still a pop everytime I switch.
However, it is only in one direction, if that makes a difference. Switching
from float to ground is quiet, but the other way round gives a loud and
clear pop.

May have to just switch this one back to relays...

to properly switch audio with out clicks, you should be
past through at line level via an analog type switch.
something like the below link.
http://www.fairchildsemi.com/ds/CD/CD4066BC.pdf
that is a 4 channel analog switch that you can
softly control the gate signal with an RC timing
switch. This will fade in/out your audio..
have fun..

 

[niftydog]

Tempus, if you can, take some voltage readings of the transistors. Take
readings for both collector and base with respect to ground; One for the
off state and one for the on state, that might give us a clue.

One thing I just thought of;
When you're going from grounded back to floating, the discharge path for
the cap (if you've got it in circuit) is via the base emitter junction,
which is quite a low resistance. This would make the cap discharge very
quickly and that might be why it's popping in that direction. Perhaps
try something like this;

|
___ ___ |/
-|___|--o--o--o-|___|--|
| |>
--- |
--- |
| |
o-------------o
|
===
GND

Experiement with that second resistor to slow down the discharge. Keep
in mind that there's a second discharge path (back via the logic chips)
that might become more attractive to the current from the cap if you
make the resistor much bigger. This isn't a problem, but there's a bit
of a balancing act going on there.

to properly switch audio with out clicks, you should be
past through at line level via an analog type switch.
something like the below link.
http://www.fairchildsemi.com/ds/CD/CD4066BC.pdf

Thanks Jamie, but in this case it's not audio that's being switched.
Even if it were, making everything line level and converting it back
down is a lot of added circuitry!

 

[tempus fugit]

Thanks for the reply Jamie.

Actually, I was planning to use an analog switch like this one, but it was a
MAX4661 (much lower on resistance). The only problem with this was that it
would have a problem with voltage sitting on the inputs before the supply
voltage was connected. This wouldn't be a problem as long as the pedalboard
was switched on before the amp, but you know that me or someone would switch
things on in the wrong order and then..... So this got me into thinking
about using a transistor to buffer the IC, sitting the amp's 15v on the
transistor, which of course would block the amp's voltage before the unit
was turned on. Then I figured I might as well forget about the IC and just
use the transistor to do the switching. Which leads us to our current
problem. Bear in mind that for this part of the switching, I'm not switching
audio, but 15v to ground from the amp for channel switching. Also, it stands
to reason that I should be able to soft switch the thing with an RC timing
circuit on the transistor. Does that make sense?

Thanks

 

[tempus fugit]

Thanks again Nifty.

So a resistor around 1K or so for the 2nd one? There's been quite a bit of
solderin/desoldering going on in this area the past week or so, and I;m sure
it won't be long before that section of the board is rendered useless. BTW I
tried a 100 ohm resistor to separate the pedalboard ground from the amp
ground, but the channel doesn't switch completely, and there is a fair
amount of added noise, so the ground loop issue is still there.

I've got a jam tomorrow, so I probably won't be able to mess with it for a
day or 2. It's working now, even if it is noisy, so I better leave it till
Tuesday.

I'm going to show you how I've got my various voltages from the pedalboard's
PS hooked up, and you can see if they might be a problem also. Perhaps the
popping issue lies here.

 

[tempus fugit]

OK, I think it's time to abandon the transistor as a switch idea for this
portion of the pedalboard. I think the ground loop hum problem is
unsolveable anyway.

I'm thinking about trying an optoisolator like a 4N25 or something instead -
should be pop free (of course the transistor was supposed to be too) and
there shouldn't be a ground loop problem.

That being said, I measured the voltages at the transistor:

Collector => ground on: 55.8mV off: 12.3V
Base => ground on: 800mV off: 0V

Do those look about right?

Thanks

 

[niftydog]

Collector => ground on: 55.8mV off: 12.3V
Base => ground on: 800mV off: 0V

Do those look about right?

Yup. No joy there.

Do you have a link to the amps schematic or do you have it in hard copy?

nifty

 

[tempus fugit]

Sorry Nifty;

I only have a hard copy. I have several different schems on my computer,
none of which are for my actual version. I remember they actually sent me a
paper copy when I requested it.

I guess I better take good care of it. If you want I could get it scanned
and send it to you. What do you think of the optoisolator idea? It seems
like it should work, I just have to make sure I've got enough drive current
from my logic to turn on the LED.

 

[niftydog]

Sorry Nifty;

I only have a hard copy. I have several different schems on my computer,
none of which are for my actual version. I remember they actually sent me a
paper copy when I requested it.

I guess I better take good care of it. If you want I could get it scanned
and send it to you. What do you think of the optoisolator idea? It seems
like it should work, I just have to make sure I've got enough drive current
from my logic to turn on the LED.

Sounds like a good thing to try, whether it works or not I don't know.
It was only a theory I had that there's no isolation between the 5VDC
and the 15VDC supplies with a transistor. The opto thing might prove it
one way or the other!

Re the schem, I'm just interested in the circuitry at the footswitch
sockets. Like, how does the 15VDC come to be at the socket etc. I'm just
wondering if, given that I found that other version that was
significantly different, maybe we're assuming something about it that
isn't correct.

nifty

 

[tempus fugit]

I tried the opto idea, but it didn't switch the channels fully (both stayed
on). It must not have a low enough on resistance. I'm looking into some
solid state relays with very low on resistance as an alternative. I was
thinking more along the lines of eliminating the ground loop than the 5/15V
isolation thing.

As for the circuitry at the sockets, the schem shows 15V going through a
100K resistor to the socket, like this:


15VDC

 

[niftydog]

I tried the opto idea, but it didn't switch the channels fully (both
stayed on).

Try driving more current into the LED side of the opto... or are you
already at maximum? You should be able to get it down about as much as a
regular transistor.

How was the hum when the optos were in circuit?

I'm looking into some
solid state relays with very low on resistance as an alternative.

Kinda expensive alternative isn't it?!

As for the circuitry at the sockets, the schem shows 15V going through
a 100K resistor to the socket, like this:

So the amps switching circuitry is connected at the same point as the
amp socket?


nifty

 

[tempus fugit]

Hi Nifty;

I got it to work with about 45-50mA of current going into it, so I should be
able to make this work, even if I do need a transistor to drive it with.
ATM, there is a latch driving a 4049 inverter to work the transistor
switching method. Hum seems to be gone, although I've only breadboarded it
so far, and I don't recall any hum in the breadboarded circuit last time. It
also seems to be switching fairly quietly, although again, it's only on a
breadboard and I'm touching a wire from the PS to the resistor to make it
turn on and off. I'll have to actually try it in circuit to see if it
switches quietly. I'm pretty sure the hum will be gone though, since it was
most certainly the result of a ground loop before.

So the amps switching circuitry is connected at the same point as the
amp socket?

I'm not sure what you're asking here.

 

[niftydog]

I'm not sure what you're asking here.

Sounds promising!

Ok, in the circuit you posted yesterday there must be a point where the
circuitry in the amp that switches the channels (or reverb) is
connected. Is it like this? Seems obvious, but just need to be sure
we're on the same path!

15VDC

 

[tempus fugit]

The amp circuitry is actually connected between the 1M resistor and the cap.
So the 1M resistor feeds a JFET at the gate.


I've been looking at my opto circuit, and realized that I absentmindedly
used a 1/4 200 ohm resistor (with 12v through it to the LED side of the
optocoupler). I would think that I would need at least a half watter here,
but the resistor isn't even warm to the touch. Am I missing something about
the current here? If I can get away with 1/4 watt, I can install the whole
thing tonite, if not, I have to order parts to see what the outcome will be
in the actual circuit.

 

[niftydog]

I absentmindedly
used a 1/4 200 ohm resistor (with 12v through it to the LED side of the
optocoupler). I would think that I would need at least a half watter here,
but the resistor isn't even warm to the touch. Am I missing something about
the current here?

The LED is gonna have some voltage drop across it, so the full 12V isn't
totally across the resistor. But, it does seem odd. Measure the voltages
around the resistor and opto to find out what's happening. I'd say you
want something in the vicinity of 10-20mA, just like a normal LED, and
it should drop around 1.5V.

12V now? I thought you'd be sticking with the 5V logic, no?

nifty

 

[tempus fugit]

Well, there's 12v to power the relays, and I plan on installing this on the
relay board. This wasn't the original plan but it suddenly dawned on me that
I am using a ULN2003 to drive the relays, so I might as well use it to drive
the opto as well.

To get the amp to fully switch, it looks like I need more like 60-70 mA
(ouch). The channels have LED on indicators on the amp, and with much less
than that 1 channel's LED is still partially lit. It kind of sucks that I'm
using an opto component that requires more current than a mechanical relay.
I think in the end I may still end up having to go to more expensive low Ron
SS relays, or just revert back to the mechanical ones. Fairchild makes one
that's only a couple bucks at Mouser, but I need to look at the datasheet a
little closer to make sure it'll work.

 

[niftydog]

Good luck with it, I'm gonna be out of the loop for a week, hope someone
else around here takes an interest in the project! I'll be keen to see
what results you get so I can start on my project.

nifty

 

[tempus fugit]

Ya thanks for all the help nifty.

What kind of design are you going to use for your project? I'd be glad to
show you how mine's set up; aside from the pop and click issue, I couldn't
be more happy with it. It works flawlessly and sounds great.

 

[niftydog]

I'll be building essentially a three channel looper (bypasser) on
steroids. The whole idea was to make something that acheived four
commonly requested tasks whilst being flexible and simple.

It'll be able to bypass the individual loops as in a normal passive
looper. Using extra relays I'll be able to select the series order of
loops 1 and 2 (ie; 1>2 or 2>1.) Loop three will be switchable between
series or parallel, where the output of loop 1/2 and the output of loop
3 will feed a 2 channel mixer stage. Also, because I'm a bass player, it
will also include a low pass filter which will assist when using
bass-sucking geetar pedals!

The whole thing is controlled using relays, a microcontroller and 5
momentary switches. This gives the flexibility to use practically any
momentary switch you want. Personally, I'll be using three footswitch
actuators in a triangular arrangment for the basic loop bypassing. This
should allow me to use different parts of my foot to control all three
loops by hitting 1, 2 or all 3 swtiches at the same time.

I'm going to use a split rail supply and offset adjustable op amps to
minimise pops (hopefully!) It's early days, although I have already
written and tested the code. The analogue design is happening at the moment.

nifty