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Old 07-17-2018, 07:32 AM
lkingston lkingston is offline
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Default “Straight sound” on a guitar effects pedal somehow altered.

Last night I ran into a strange tech issue that I don’t understand. I brought a Zcat reverb/delay pedal to an open mic, but didn’t use it during my set. I was using a Martin with a Schertler AG-6/S-Mic pickup. I dialed in lots of mic of mic and the sound was terrific.

At the end of the night, I was going to show a friend what the delay reverb pedal sounded like, so I plugged it into the PA and turned it on. This is where I ran into this issue.

So here I am with the guitar plugged in just like it was during the set except that now I have a reverb/delay pedal between the pickup and the board. The mic level is pushed to the point where it is just below the threshold where it will feed back.

The pedal is made by Zcat and it is a “true bypass” pedal. When the effect is turned off with the switch, it passes the input audio to the output with no processing.

As soon as I turned the effect on, the mic started feeding back like crazy. I turned the reverb and delay effects all the way down to zero. Now the sound and the volume should be exactly the same when I turn the effect on and off. There is theoretically no processing going on and the input should be the same as the output. If I was to do this with the PA level turned down or with less mic dialed in, you would hear very little (if any) difference between the effect on or off with the effect amount set to zero like this.

The thing is that THEY ARE NOT THE SAME! Last night, with the effect pedal turned on but no effect dialed in, there was feedback like crazy. The pedal bypass switch became like an instant feedback switch! On: feedback. Bypass: no feedback.

This immediately brought back memories of myself fighting feedback with pedals like my Boss AD-2 and AD-10 which I had with an M-80 pickup. In those cases I remember trying to notch out the feedback when I pushed the levels.

So my question is, what the heck is happening to the supposedly unprocessed sound in a digital effects pedal? It sounds like the input but it isn’t the same. Is the overall phase different? Are the phase of different harmonics altered? Whatever it is, it seems to significantly affect the headroom before feedback on my pickup.

Here’s another way to look at it: If I am plugged into a mixing console or my acoustic guitar amp and I dial in a little reverb, the straight sound isn’t altered. If I set up the same amount of reverb on a reverb pedal, the straight sound IS altered. And whatever is going on with the straight sound is making the pickup far more likely to feed back, even if I turn the effect all the way off!

Is it a phase shift issue? I don’t know. Does anyone have any ideas?
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Old 07-17-2018, 07:48 AM
MikeBmusic MikeBmusic is offline
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When you turn the effect on (even with the dials zeroed) the signal is still being processed through the pedal's circuits, these are obviously boosting the signal (or some frequency band), causing the feedback.
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Old 07-17-2018, 08:21 AM
lkingston lkingston is offline
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Quote:
Originally Posted by MikeBmusic View Post
When you turn the effect on (even with the dials zeroed) the signal is still being processed through the pedal's circuits, these are obviously boosting the signal (or some frequency band), causing the feedback.


The sound doesn’t seem to be altered. Obviously it is different (because it is feeding back) but aside from the fact that it is way more prone to feed back, the difference isn’t readily audible. I think it has something to do with the overall phase or the phase of different harmonics. I’m guessing though because I really don’t know.
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Old 07-17-2018, 08:36 AM
Vancebo Vancebo is offline
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Purely a guess here. You are probably right. Boosted frequency, or a volume change that is not so noticeable. Reversing phase could possibly help.

True bypass should not effect the sound. Turn your mic down. It’s the compromise we all make. Effects like Reverb on internal guitar mics are tough to pull off anyway.
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Old 07-17-2018, 09:58 AM
lkingston lkingston is offline
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Default “Straight sound” on a guitar effects pedal somehow altered.

On this pedal, there is “true bypass” when you turn the foot switch off. It’s when you turn the pedal on that it feeds back, even with the reverb and delay effects turned off. The sound quality otherwise is fine.

I’ve noticed something similar with Boss effects. With the Boss AD-2, VE-8 and AD-10 for instance, right as you begin to dial the body resonance in, at that first bit of turn in the dial you can hear the straight sound make a sort of shift which then stays the same throughout the rest of the dial turn (as more effect is added in). I haven’t noticed that with the reverb (ambience).

What I think is happening is that the sound is going into a DAC, then both the straight and effects sounds (including the mix between them) is being calculated by the pedal processor. There is probably some sort of phase shift introduced at this point that is causing the problem.
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Old 07-17-2018, 02:06 PM
Vancebo Vancebo is offline
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I believe phase issues can only happen between two or more sound sources. Like a guitar top and speaker. Phase button is a tool to possibly help when feedback is present. It is not caused by a piece of gear.
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Old 07-17-2018, 04:35 PM
lkingston lkingston is offline
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That makes sense. Maybe it is the difference in phase between the guitar itself and what is hitting it from the speakers.
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Old 07-17-2018, 04:51 PM
zhunter zhunter is offline
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Quote:
Originally Posted by Vancebo View Post
I believe phase issues can only happen between two or more sound sources. Like a guitar top and speaker. Phase button is a tool to possibly help when feedback is present. It is not caused by a piece of gear.
Many pedals do reverse phase. Just like the phase button you mention does. So if the levels are set based on a certain reaction between top and speaker and that relationship is reversed, feedback could be the result. Also time based pedals often change levels between the bypassed and non-bypassed positions even with controls at zero. It is certainly possible that even a small boost from the pedal is enough to push you over the edge.

hunter
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Old 07-17-2018, 07:26 PM
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noledog noledog is offline
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Quote:
Originally Posted by zhunter View Post
Many pedals do reverse phase. Just like the phase button you mention does. So if the levels are set based on a certain reaction between top and speaker and that relationship is reversed, feedback could be the result. Also time based pedals often change levels between the bypassed and non-bypassed positions even with controls at zero. It is certainly possible that even a small boost from the pedal is enough to push you over the edge.

hunter
+1...very true! This happens when I engage my small effects board that i sometimes use at shows. Most times I plug my 000-18 Retro straight into my Loudbox Performer and use the amp's built in delay and set the F1 Aura Plus on the guitar to - which usually sounds best in most venues...when I use my pedal board (that houses my Line 6 Wireless, AD-2, Carbon Copy Delay, and Neunaber Seraphim), the phase reverses and so I switch the F1AP on the guitar to + and all is good.

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Old 07-19-2018, 11:38 AM
lkingston lkingston is offline
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Default “Straight sound” on a guitar effects pedal somehow altered.

I just realized it might also have something to do with the input impedance of the pedal being a better match to my pickup and producing more level than a direct connection to the board, especially in lower frequencies.
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Old 07-20-2018, 06:29 AM
atticus1019 atticus1019 is offline
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Most reverb pedals add a slight bump in volume to make up for a perceived drop in volume introduced by the reverb. Generally the more reverb you add to something the quieter our ears perceive the sound, so many pedal builders compensate for this by adding a slight boost. If you were right at the point of feedback it probably bumped you over the edge. The way to overcome this would be to put your effects like reverb and delay into an effects loop and then blend the volume of the loop into your direct signal to taste. Hey
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Old 07-20-2018, 01:00 PM
lkingston lkingston is offline
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...except that I have this feedback problem with the effects entirely dialed out.
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Old 07-20-2018, 01:25 PM
atticus1019 atticus1019 is offline
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Quote:
Originally Posted by lkingston View Post
...except that I have this feedback problem with the effects entirely dialed out.
..... you do turn the pedal on right? The boost wouldn’t be controlled by the reverb/delay knob. It would be the first thing the signal hits in the circuit. I get that you are saying your pedal is true bypass, but by definition, the true BYPASS only matters when the pedal is off or, bypassed. Once the pedal is engaged the input buffer or boost or whatever the analog signal starts with is in the signal path, even if the wet effects are dialed out.
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Old 07-20-2018, 04:14 PM
lkingston lkingston is offline
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Originally Posted by atticus1019 View Post
..... you do turn the pedal on right? The boost wouldn’t be controlled by the reverb/delay knob. It would be the first thing the signal hits in the circuit. I get that you are saying your pedal is true bypass, but by definition, the true BYPASS only matters when the pedal is off or, bypassed. Once the pedal is engaged the input buffer or boost or whatever the analog signal starts with is in the signal path, even if the wet effects are dialed out.
Yes on everything you said. If I dial out the effects completely at home and turn on and off the pedal at home through my amp, it sounds the same on or off. In the off position it is true bypass so it is the same as not having the pedal plugged in. In the on position, at home at low volumes through my amp it sounds the same as bypass with the effects dialed out. At the gig the other night plugged into the house mixer, that switch (with the effects dialed out) became an "engage masses of uncontollable feedback switch". In bypass, I could dial in plenty of mic and the sound was great. This is the issue I am trying to solve.
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Old 07-20-2018, 08:29 PM
atticus1019 atticus1019 is offline
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Quote:
Originally Posted by lkingston View Post
Yes on everything you said. If I dial out the effects completely at home and turn on and off the pedal at home through my amp, it sounds the same on or off. In the off position it is true bypass so it is the same as not having the pedal plugged in. In the on position, at home at low volumes through my amp it sounds the same as bypass with the effects dialed out. At the gig the other night plugged into the house mixer, that switch (with the effects dialed out) became an "engage masses of uncontollable feedback switch". In bypass, I could dial in plenty of mic and the sound was great. This is the issue I am trying to solve.
Looking at the specs of the pedal on the website, the input impedance is 1M which doesn’t 100% mean there is a buffer at the front of the pedal but I would bet there is. This is generally to make sure the DAC has a solid input level to drive the effect and also to make sure there is no perceived volume loss when dialing in the effect. It’s not going to be a huge volume increase, but your initial post stated that you had the mic level just below the feedback threshold and it would be enough to push it over that threshold. Like I said in my previous post, the reverb/delay knobs are not going to control this level increase caused by an input buffer, it’s just something that automatically happens when you turn the pedal on. I don’t really know what other solutions to suggest other than turning it down or like I said in the last post, put the pedal in an effect loop and blend it.
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