Cardinal Tremolo Probz - paging midwayfair

[eldanko]

Howdy friends -

Very excited to finally have my Cardinal put together. However... having trouble getting much depth out of the effect. With the switch up, I can barely get some effect with the depth maxed and the vol/gain fairly high. In the other two modes (center and down) I get little to no effect at all. In those two modes, the pedal squeals like a banshee with the gain much higher than 50%.

The LFO appears to be functioning properly, although the LED never turns fully off, even at max depth.

Here's some voltages taken with switch up and all controls at roughly 50%.

Supply: 8.8V

IC1
1- 2.5V-6V
2- 3.5V-4.5V
3- 3.5V-4.5V
4- 0
5- 3.5V-4.5V
6- 3.5V-4.5V
7- 3.5V-4.5V
8- 8.0V

Q1
D- 7.9V
S- 0.3V
G- 0

Q2
D- 7.2V
S- 0.3V
G- 0

Q3
D- 5.5V
S- 0
G- 0

[midwayfair]

When you say gain, are you talking about the volume trimpot? It's a voltage divider at the end of the circuit; it shouldn't affect anything to do with the tremoloing (it's always a 50M/100K voltage divider with vactrol 1 regardless of where it's set).

When you say "effect" you do mean tremolo in general, right?

Since there's squealing in center and down, I'm going to assume that a big part of the problem lies in Q3 (Q3 is out of the circuit when your switch is up). I've experienced some very, very high pitched squealing with the volume maxed (it's why I used FETs instead of BJTs), but it's VERY high pitched and IME only when it's literally 0 impedance on the volume pot. This sounds like something different.

Going forward, it'll be best to pull Q2 and/or Q3 to work on one band at a time. Also, audio probe through to make sure nothing's going wrong at Q1.

Voltage look okay though the voltage swing at pin 7 seems a little low (mine is closer to 2-7 volts).

It's possible that the indicator LED won't go off completely. It would be more likely to do so in square wave, but it might always be dim. It sounds a little (and from the voltage looks) like yours might be kind of bright all the time, though?

Triple check your LEDs in the LFO section, too. The Madbean library has the square pad as the anode on the vactrols but it's the cathode on the indicator LED. So your cathode should be to the right.

I'd like to fix and track down the squealing from Q3 first, so pull Q2 and set the depth at max while working on that. See where the squealing is introduced.

[eldanko]

Sorry about that. Yes, Gain = Volume trimpot. Effect = tremolo in general. Bingo.

- Yep, indicator LED is on the bright side most of the time. I'm using a 3mm blue waterclear if that matters.

- LEDs in the LFO section: I have the anode in the square pad for the indicator as well as the vactrols (thus the vactrols are oriented with the text downward). There's a plus sign next to the square pad for the indicator LED, so I arranged it as such (as I normally would). Cathode is to the right, as you've prescribed.

- I actually get an intermittent squeal from the pedal with the vol trimpot maxed and the switch up.

- Unless my ears are playing tricks on me, the vol trimpot is almost certainly having an effect on whether or not I'm getting tremolo at all.

- Double checked pin 7 on the IC. Definitely getting a narrower voltage swing than you. 3.2-4.7 at min rate... at max rate, it sits at a steady 4.0 and doesn't swing at all. In fact, all of the pins seem to be behaving this way.

So what's my next move? Swap out Q2/Q3? I *believe* my trannies are genuine Fairchilds, if that's pertinent. Thanks for the help so far.

[eldanko]

Hmmm. Just realized that you specified a diffused LED for D2. Should I swap it?

[midwayfair]

Yeah swap the led. It's an unfortunate limitation of letting the led be a depth indicator that I had to push the forward voltage to the limit. Try that first, then let's see if we can track down the squeal.

[eldanko]

Ok, swapped the LED for a 3mm green diffused. Definite improvement. I'm now getting a definite trem effect in all three switch positions. It's still not as deep as I'd prefer, but we're on the right track. Some notes:

- In the up position, I can run the volume *almost* to max - much higher than in the other two positions - before it starts to squeal. I can get just under unity volume before the problem occurs. The squeal here is intermittent and throbby.

- In the center position, the squeal seems to come on quickest, barely letting me get past 50%. The squeal is high pitched, piercing, and constant. I do hear the "phasey" sort of effect happening though, and I like it.

- In the down position, I can get the volume to about 60% and then it squeals. Again, high pitched and constant. The trem effect seems to be least pronounced/deep in this mode.

What else can I tell you that will help?

[eldanko]

One more thing: the squealing only happens with the guitar volume rolled up. If I turn it down, it doesn't squeal.

[eldanko]

CRAP.

Did you know...

...that if you bridge the pedal input with the output tip, your pedal will squeal and do all sorts of crazy stuff?

One stray piece of metal/solder/whatever that was...

In short, we're in good shape. This thing sounds lovely. Really, really nice. I'd take it with me on the gig tonight, but I'd like to photograph it before I beat the crap out of it.

Last question: switch down is harmonic mode, correct? Is it normal to lose low frequencies at higher depth settings in this mode? Will modifying C6 help preserve them at all?

Finally... for a guy who doesn't actually sell anything, you've got the best customer service in the biz, Jon 

[midwayfair]

One more thing: the squealing only happens with the guitar volume rolled up. If I turn it down, it doesn't squeal.

Does it not squeal regardless of how high the volume is set if you roll off your volume a hair?

That is REALLY weird. Until you said that, I was going to assume it was LFO bleed and suggest a larger limiting resistor to IC1, but that's sounding almost like positive feedback ... [EDIT: HOLY CRAP I WAS RIGHT!]

Try audio probing at the output to Q1. Is the squeal present there?

I'll try to think of some more things to help, but I need to know the exact point in the circuit that the squeal is happening. I'd rather you didn't have to put a floating resistor in there to the input wire, but that possibility exists.

On the LFO side, you can try a chip with some more gain if you socketted it. Like a 4558 (the original chip for this LFO) or maybe even something rail to rail like a LM358 if you have one (in fact, that might have enough gain to clip the LFO wave). Also, is your waveform control working properly? The square wave setting is the one that should do a real "on-off" sound. And the phasing will be most evident on a more sine wave setting. Just trying to cover all the bases to make sure nothing else is wrong in the LFO. Also, R15 and R18 are the final arbiters of how much effect the LFO output has on the LEDs (they are the power rail references), so maaaaybe fiddling with them (especially R18) just a hair will improve things, but we're getting into desoldering land.

at max rate, it sits at a steady 4.0 and doesn't swing at all. In fact, all of the pins seem to be behaving this way.

I'd chalk this up to your multimeter not reading it fast enough. It's always best to read LFOs with the rate at minimum. But we've established that it's working, we just want it to work a little harder now.

If you get through all this stuff and are still dissatisfied with the total depth, try temporarily jumpering the rate LED (just do it with an alligator clip). If that "fixes" it, I can tell you how to kind of hack the board to move the indicator LED. I'd really rather you didn't have to do that, though, obviously.

[jimilee]

yaay! Nice job, i love mine, it's a great lush sound on the harmonic side. I don't use it on bass, but my guitar player uses it.

[midwayfair]

Last question: switch down is harmonic mode, correct? Is it normal to lose low frequencies at higher depth settings in this mode? Will modifying C6 help preserve them at all?

On your build, yes, down is harmonic mode (if you're not sure, it's the mode where the gate of Q3 is fully grounded).

The pedal in general boosts the upper midrange and treble frequencies to counteract some cancellations created by the harmonic mode and some general treble loss at R5 in full-range mode. If you used a 1uF (older value) for C7, then definitely swap it out for the 4.7uF in the current BOM. Morgan over on BYOC preferred a 22uF, but I actually had some issues getting that to work (this is going to sound weird, but the pedal had less output when I tried that and I had to crank the volume, even though that makes no sense to me ... it could have been some internal resistance in the electro I used, so maybe a tantalum is the better call for larger values).

The other thing that can happen is that if Q3 is a REALLY high output JFET, it could simply overwhelm the signal. But since yours is biased a little cold, I don't think that's what's going on here, so I'd consider a swap at C7 first.

However, I will suggest playing with it in a mix before desoldering if you can; what sounds like too much sparkle or too much bass loss now might sound a little better with context. I know that can sound like a cop-out or adspeak, but I actually remove C2 in mine to get it a little brighter and create a stronger harmonic effect, and that's going through a bright amp with a single coil guitar.

Finally... for a guy who doesn't actually sell anything, you've got the best customer service in the biz, Jon 

Aww, thanks. I'm just trying to give back to the community. I've learned so much from this. Plus your pedals (specifically the Pixar series) were one of the things that inspired me to start building and do the cute handpainted stuff.

[eldanko]

I feel like such a freaking idiot. The giveaway was when, on a whim, I plugged the pedal in backwards and realized that nothing changed.

Nevertheless....

1) The indicator LED problem that we fixed earlier was legitimate, so this wasn't a completely pointless exercise.

2) I learned a good bit more about biasing JFETS, which was educational.

3) I was indeed using an older BOM and will be changing C7 first thing in the morning.

I did some searching of your other posts and saw the part about how the perceived bass cut is more likely because of a mid boost in the EQ of the effect. I was testing this on my Matchless, which has pretty strong upper mids anyway, so I'll have to give this a shot with one of my Fenders and see if I still feel the same way.

Either way, this thing's going on the road with me for the next few weeks 

[eldanko]

Ok.... I've done a good bit of socketing/experimentation, and have got this thing dialed just about perfect now. One more question....

Should the value of C6 affect the sound at all when switched to normal trem mode? I have increased that cap to 3n3 for a more intense sweep in harmonic mode, and I swear that I'm losing high end at higher depth settings in the normal mode compared to the 2n2 that was there. Please tell me my ears aren't playing tricks on me.

[midwayfair]

Should the value of C6 affect the sound at all when switched to normal trem mode?

No, it shouldn't. It's not connected to anything in normal trem mode or the center mode.

It's possible you didn't notice some treble loss before you switched the cap.

Take a look at how the tremolo is accomplished: It's a big resistor in series with (essentially) the volume pot. It's like turning a volume control up and down. What happens when you normally do that? You get a little perceived treble loss. It's not a ton, but it's might be enough to notice when listening closely. (There's also a tiny bit of treble loss at R5 due to some "hidden" capacitance in FETs.)

When the depth is low, the resistance at the output is always less; as the depth increases, it will be more resistance on the "down" swing, so it might sound a touch less trebly. This happens in other optical tremolos like the Tremulus Lune, too. Actually, it happens with EVERY type of amplitude modulation to some extent, because treble is attenuated "faster" than bass frequencies. (This is why a lot of guitarists put treble bypasses on their guitar pots.)

But let's say that's not the part that's bothering you, or you also notice some treble loss (that bothers you) when the depth is at 0. Remember you can always ditch or reduce C2 to increase the treble. It'll also enhance the harmonic mode. But dirt will sound better with it in. One of those tradoff things ... :/

[eldanko]

Derp. You are right. I actually realized that before you posted your explanation (as I bothered to go read the schematic and realized that the cap was no longer in circuit when switched). Nevertheless, I had never considered that treble loss is perceived "faster" than bass loss.

Either way, I left C2 alone and ended up boosting the output volume a bit, which seemed to nicely preserve the highs without adding a perceivable boost in gain.

Build report's up - YAY!  http://www.madbeanpedals.com/forum/index.php?topic=12446.0