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

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.

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!

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

get

Switching

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.

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

Yup. No joy there.

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

nifty

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

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 . | | | .-. | | 100K | | '-' | o-----------------o amp socket | | | .-. | | 1M | | '-' | o | | --- .01 --- | | === GND

me a

scanned

current

tempus fugit wrote: > 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?

Kinda expensive alternative isn't it?!

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

nifty

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.

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 . | | | .-. | | 100K | | '-' | to amp circuitry o---o---o amp socket | | | .-. | | 1M | | '-' | o | | --- .01 --- | | === GND

nifty

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.

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

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.

here,

about

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

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.

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