L5 Preamp (Full Version) Overloading

[jutanious]

Hello everyone, first time posting here! Thanks in advance for the help. I pulled the L5 Preamp off the Pile 'o Shame and almost have it working, but I'm stuck. The pedal is fully functional, but it easily overloads and distorts with normal guitar input. If I roll down the guitar volume control to the extreme, the pedal sounds incredible. I tried swapping the input pulldown resistor (RPD on schematic) to 1M as suggested, and I even got ridiculous and tried as low as 100R. This didn't really change anything. I verified that RPD is actually connected to ground. I also checked that R1 is rated properly. I've also checked that all ICs are seated correctly and are in the right spots. I'm using the 13700 and the chips on the current BOM.
I've never heard overloading like this in SS applications - it isn't "walling out", it's screaming unpleasant distortion with a noticeable volume spike when the input source crosses its threshold. This happens without the compressor engaged. When the compressor is on, it clamps down hard and drops the signal with a faint pop until the compressor is off again.
It's obvious that this pedal is capable of some ridiculous volume output when functioning properly... even in this state, the master volume is super touchy. My instinct says input gain issue, but perhaps its further in? Any ideas? I can tell that this circuit is amazing, but it's currently unusable.

Here's the schematic (begins on page 14): https://aionfx.com/project/l5-preamp/

P.S.: This build has been haunted since the beginning. It was dead, then I scoped the entire audio path and it magically worked. So it goes.

 

[comradehoser]

This is for both channels?

In mine, master volume is master volume, and the character and level of distortion is controlled by the channel volume (I think it's really a gain control). The left side channel should stay fairly clean/edge of breakup through the channel volume sweep, the right side is the one more eager to be dirty, especially with compression turned more CCW. So turning down the channel volumes has no effect?

Also, be aware that the multifilter doesn't toggle on and off, it seems to toggle between the left channel and the right, IIRC.

 

[jutanious]

Hello! Thanks for your response! Yes, this is for both channels. I double checked what you mentioned about the volume/gain, but something is certainly wrong. Check out this video that I took:

Thanks in advance for any help. If I'm going back in with the scope, how can I reliably tell where the overload/distortion is happening? Feed it a loud/constant signal and look for where the read out gets wild?

 

[aion]

Just to be clear, you restored the 1M or 2.2M RPD resistor, right? Because 100R is going to almost completely kill the signal before it gets to the first op-amp... we definitely do not want that!

How did it bias? Were you able to get the target voltages from the build doc?

 

[comradehoser]

Hello! Thanks for your response! Yes, this is for both channels. I double checked what you mentioned about the volume/gain, but something is certainly wrong. Check out this video that I took:

Thanks in advance for any help. If I'm going back in with the scope, how can I reliably tell where the overload/distortion is happening? Feed it a loud/constant signal and look for where the read out gets wild?

Hahaha.... oh yeah, that's not right, and definitely not anything a pulldown resistor will fix.

Start with finding the answers to the aion man himself's questions--it's always nice when the builders chime in-- thanks @aion ! So see if you can get the correct bias--sounds like that could be it, actually.

If that doesn't work, visual inspection! Post pics of your board and offboard wiring component side and solderside. Verify all components (yes, including resistors) and their orientation when orientation is relevant--to the build doc.

Next step is reading the voltages on the transistors and the ICs.

Then the audio probe, although you could do it after you bias before the rest because they are super fun. Probe the components in the audio path and you will actually hear what each does. When you hear your funky noise, then you know the issue is right there. You can use any kind of audio source, so a recorded passage in a looper, a coerced significant other with a guitar, or even the headphone line out of my stereo or phone and a good record on the platter. Plug that into the input jack, and follow the schematic in with the probe.

 

[jutanious]

Well yikes, I went back to test this again, and there's a new problem. It's passing signal in bypass, but it won't pass any guitar signal at all when the circuit is engaged. A step backward. HOWEVER, when I hook up my audio probe (sine wave, 3amps, 100hz-4k) it passes everything, plus the pedal is working properly! When I unplug the probe and get back in there with the guitar, nothing.

I swear this one is haunted. I did replace the RPD resistor. I'll post some photos in case anyone sees something that I missed.

I've never had a circuit do something like this, so when we figure it out, this will be a good one for the knowledge bank!

 

[jutanious]

Photos attached - RPD is missing due to previous tomfoolery.

 

[Nic]

Are your pots covered so they do not make contact with your board ?

Not relevant, but remove the tab on your pots or they will not sit flush.

 

[jutanious]

Yes, that's the reason. I didn't have extra dust covers, so electrical tape it is (temporarily). I've bent the pots away from the board just in case.

 

[comradehoser]

Are your pots covered so they do not make contact with your board ?

Not relevant, but remove the tab on your pots or they will not sit flush.

Ooo--I always forget that one as a good possibility since pots can ground out components and such if the back of the pots contact legs.

I'll also add it's standard operating practice to clean the flux off the board with isopropyl alcohol even though I've never actually had this resolve a troubleshooting issue.

If your audio probe is getting through, it probably means there's an issue with your offboard wiring between the board forward through the 3pdt and the out jack. Reflow solders. That's also good standard operating practice in troubleshooting--reflow all solder points.

 

[jutanious]

Thanks, I'll try this tonight! I was connecting the probe to the actual input jack - why do you think that the probe would get through while the guitar signal won't? Just as issue of volume/amps at the source? Thanks!

 

[comradehoser]

Well, I don't know what you are doing for an audio probe, "hooking up" and "getting through" sounds a little bit too easy--not how I usually think about the process unless you mean that you traced every component in the audio path and they registered as expected. Here is what I do:

First, plug in your continual audio source to the in jack. From the point of view of the back of the pedal, make sure it's the in on the left and not the out on the right. I make that mistake testing all the time and would explain why you have bypass but no effect.

Second, make your probe: take a TS jack, solder a wire to the sleeve terminal, and a small cap to the tip terminal, leave the lead long or solder another wire to the open lead.

Plug a cable into the jack, and then into an audible amp device.

Connect the Sleeve wire to pedal ground in some way (I use an alligator clip cable). With the Tip wire, you can probe directly, but I find it easier to alligator clip to a multimeter probe. Then, you touch each circuit point in the audio path [edit: in order following the schematic and build doc] including jack tip and 3pdt connections, and it should render your audio source in the amp.

The signal should definitely vary in tone and amplitude and you should get some popping and hum when you break contact with the probe, and silence when you probe ground [edit: don't worry about the power part of the circuit--it doesn't sound very good and I've temporarily disabled ICs that way].

I would definitely start with biasing the transistors with the help of a multimeter as a likely first source of that distortion before you embark on this wonderful sonic journey, because the circuit, it's a big one, and it will take you quite a while to get through everything.

 

[Nic]

reflow all solder points

This, there is very little solder showing through the holes on the component side, not that there should be a ton, be we should see some.

 

[aion]

The calibration process has you insert the test signal into pin 13 of IC10, which is the same as the pads marked "TEST". If you're inserting it there instead of the input jack, then the issue is somewhere between the input jack and IC10. That doesn't narrow it down much, but it does rule out the output and compressor at least. I would say it's most likely with the jacks or on the bypass PCB since both channels have the same issue and they don't split into two paths until you reach the main board.

On the other hand, if you're inserting the test signal into the input jack instead of the test pads then you are definitely calibrating it wrong and that could be the issue.

 

[comradehoser]

Ooohhhh.... Well, hush my mouth for not reading through the build doc calibration routine. Now I'll have to go do mine!