HPF sounds super compressed / sputtering

[steveyraff]

Hey all,

I was building a HPF for someone, using this layout: http://tagboardeffects.blogspot.com/2015/01/hpf-technology-hpf-pre-2.html

I've built this before with no problems. This time however, it seems to work in terms of the filter and volume controls etc, but I notice that when I pick a note aggressively with a heavy attack, theres a big volume dip and a bit of a crackle. Its almost like when I strum the strings hard, there is really strong compressor pulling the volume down almost completely for a second. Oddly it happens with some notes more than others. I am not sure if this has something to do with the fact that I only have a guitar to test it and its designed for bass instruments. I don't recall this happening on the previous build I did of it.

I have triple checked all component values. I mean, the only thing I can think of is perhaps replacing the two 2n4416 transistors or the 2n4403.

Any ideas? Thanks,
Steve. 

[steveyraff]

I swapped the 2N4416's and the 2N4403.

Its no longer doing the volume dips / compression upon strumming.

However, now the output is much lower, lower than unity gain - and also, everything sounds dark, like a clean tone with the tone pot of the guitar rolled off. Uuuuuuuugh. WHAT IS HAPPENING!? 

[steveyraff]

I replaced the entire circuit. It sounds the exact same. I am about to bounce this thing off the walls. I did use the same 2n4416 transistors. But this is the second set that I tried on the previous circuit. What the hell is going on... AAAAARGGGHH

[ElectricDruid]

Replacing the entire circuit when you haven't identified the fault is just making work. You've only been looking it a half-day - barely long enough to forget what you think you know (an important part of the process). You need to slow down and stop *doing* stuff, just 'cos. Now you have to debug the whole *new* circuit, so you're back to exactly where you were before you started. Sorry, but it's true. Don't change things unless you've got a reason to do so, or you risk introducing new faults that weren't there before and making the debugging *more* difficult. It's like trying to shoot a moving target!

Ok, so let's start a debug process on what you've got in front of you. Something that responds to heavy playing with compression sounds a bit like "power supply sag" in the old fender amps, so I'd be looking at the bias levels and the power supply circuit.

I'm guessing your circuit is the "series 2" schematic from here?:

https://github.com/bassistTech/HPF-Technology-LLC/tree/master/HPF-Pre-Schematics

Have you got the battery indicator and the zener? That little bit is where I'd be looking.

[steveyraff]

Sorry, yes, I have been extremely over worked lately and grew a bit frustrated. I have been at this and other circuits for 14 hrs today thus far.

I think I have just discovered the issue. So, since I replaced the entire circuit, but used a second set of 2n4416's that I'd used on the previous, I decided to socket those transistors. I stuck two J201's in to the sockets out of curiosity. MUCH better. Volume back up, no sputtering issues, and no dark tone issues. I didn't think the transisters alone could affect all of the above.

So I guess both sets of 2n4416's which I bought from the same supplier a few days apart are both faulty!?

Steve.

[duck_arse]

what markings are on those 4416's? a photo, maybe?

[Rob Strand]

The front end can be replaced with a buffer.  I don't see the value of a phase switch for a low-cut pedal.
As is, the first stage make the biasing harder to get right and has an unnecessary loss in headroom.

If you have a digital multimeter measure the voltages on the transistors and JFETs.
The voltages will tell you what's going on and how to fix it.  Bad biasing for sure.

If the second stage is the problem you can tweak the 2k resistor on the base of the transistor (to a degree).