Bearhug Compressor - schematic, demo, layouts, & forthcoming 1776 PCB!

[chuckd666]

Jon, what is the purpose of the 10M resistor between the input cap and 10uf cap going directly to ground? I seem to get 4.1V or so, then when it reaches the Q1 gate, I only get 0.63V. I tried replacing it, but I have the same results.

[midwayfair]

Jon, what is the purpose of the 10M resistor between the input cap and 10uf cap going directly to ground? I seem to get 4.1V or so, then when it reaches the Q1 gate, I only get 0.63V. I tried replacing it, but I have the same results.

It's the gate bias resistor. The 10uF to ground is a filter cap for the bias voltage (Vb), which is formed by two 100K resistors. (The newest version uses a trimpot instead of those two 100Ks.) Something's pulling your gate down, though I'm not sure exactly what. You could try trimming the 100K between the Vb and 9V connection, or just making it smaller like it is in the Amz MOSFET Booster, and see if that brings the gate up. Otherwise check for solder bridges and all your components that connect to Q1. It's not that many.

[duck_arse]

if you try and measure the gate voltage, your meter will load the 10M resistor, and out-whack yr voltages. if youse gets good volts drain and source, you should be alright. I usually trim the bias resistors on the bb for each mosfet build.

can we see pics of your build?

[chuckd666]

I have returned with pics of my pcb.





Etch was ok. Messy soldering in parts because I wanted to cover up my patchy etch. Only other anomaly is the 22k and one of the 1n4148s are going into the same hole because I broke my 1mm drill bit because of my crappy drill-fu.

[duck_arse]

those "cuts" around "things" are all good/ok, are they? and pray tell, what is the pinout of your fet adaptor board (it took me a while to figure out what that was ....)?

[chuckd666]

Heh yeah there's a few correction cuts. I'm definitely still green with PCB making, but I had correct continuity when I tested it.

The FET adaptor is one of Paul Puntere's - http://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=6&products_id=375

I suppose I'll go back and quadruple check my traces. Dang it Bearhug. I will not give up on this!

[midwayfair]

Heh yeah there's a few correction cuts. I'm definitely still green with PCB making, but I had correct continuity when I tested it.

The FET adaptor is one of Paul Puntere's - http://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=6&products_id=375

I suppose I'll go back and quadruple check my traces. Dang it Bearhug. I will not give up on this!

Pull the FET out of the socket. What's your gate voltage now, the same? Is it still distorted? If you replace the FET with a 2.7K resistor, is it clean sounding and about unity volume?

Did you try reducing the 100K resistor between Vb and +9V?

There are only a couple places that matter for the drain voltage, and only a few components. Check them all. There may be problems elsewhere in the circuit, but get the dry path working first, then move on to the envelope detector.

[chuckd666]

I pulled the FET and am getting -0.05 on the gate. It is pretty much unity volume with or without a 2.7K resistor across it. There was probably no difference. Though it was clean!

I will try changing the 100K resistor.

EDIT: I put a 27K instead of 2.7K initially - with 2.7K it sounds very clean and slightly above unity/very slightly 'fuller' sound. Seems to be issue with the FET I suppose.

[duck_arse]

can we see a close-up of your fet board-build?

[chuckd666]

Will do, but I also have a bunch of regular Vishay 2N5457s that gave me the same results. I will double check both again and report back.

[midwayfair]

Fix the gate voltage before you bother worrying about the FET. It's irrelevant at the moment when your gate voltage is ~4V below what it should be. The FET's just a variable resistor.

[chuckd666]

Btw I worked out my problem - one of the diodes on the layout I used was oriented incorrectly.

http://effectslayouts.blogspot.com.au/2015/07/bearhug-fet-compressor.html

Please note that the topmost 1n4148 is the wrong way around! Was wondering why I was getting a large boost instead of compression from the FET (at least that's what I think it was doing). Hm.

Working really well now Jon! I have noticed there's 47pf on one schematic, and 100pf on another schematic, between the drain and the gate. I assume this is just a "taste" thing?

[Craggles]

Hey Jon,

Another debug question... sorry in advance.

The compressor works great, but I've noticed when running it with high-treble or comparatively full-range signal chains, it's attenuating the highest frequencies. With a strat through a Deluxe Reverb or clean Marshall, for example, it seems to be cutting a lot of the jangle out.

Should I look at changing some cap values to alter how it's filtering? You mentioned early on that it should be totally transparent... I've built it stock (including the long-short switch) so have no idea what might be wrong.

Any pointers you might be able to offer would be a huge help

[midwayfair]

Hey Jon,

Another debug question... sorry in advance.

The compressor works great, but I've noticed when running it with high-treble or comparatively full-range signal chains, it's attenuating the highest frequencies. With a strat through a Deluxe Reverb or clean Marshall, for example, it seems to be cutting a lot of the jangle out.

Should I look at changing some cap values to alter how it's filtering? You mentioned early on that it should be totally transparent... I've built it stock (including the long-short switch) so have no idea what might be wrong.

Any pointers you might be able to offer would be a huge help

Which version did you build?

There are a couple things happening. I'm pretty sure it's not broken.

Truth be told, the audio path does cut some high frequencies when the compression is only moderate. There's a pF cap between the drain and base of the MOSFET, which produces an increasing low pass filter (at some very high frequencies) as the gain goes up (i.e., when the compression isn't triggered). I put it there because (a) in general, when compression kicks in, the gain goes down, which can slightly reduce the treble (and also reduce distortion, which gives the impression of treble); (b) the attack is very fast, which tames high frequencies (this part can't be helped -- it's what compressors do).

I reduced the capacitor's value between versions and I now use a 22pF, but I also tend to keep it on a lower compression setting than I used to. I used to use 47pF, but Josh (at 1776) preferred 100pF so that was the value that was in the 1776 schematic for version 1.

When I say that the compressor is "tonally transparent" I'm referring to the difference between the tone when the compression is not kicking in and when it's running full tilt. When the compression is strong enough to kick the FET up to over 2.7K (and it can go higher), though, the feedback cap really is "out of the circuit" because gain = 0x (or close enough to it). I think the later versions did this better than the original version, but the principle was the same.

I suggest either playing with that capacitor or, if you're feeling adventurous, considering increasing the attack, which does involve adding a resistor (after the Q2 collector) and changing a few components.

Chuck D: I hope this answers your question, too. I'm sorry I missed your post.

[Craggles]

Thanks for the reply Jon. Well explained as always

I built the vero version on lVlark's blog, with your v2.0 mods. Not sure if we're ok to link there from here, but the URI after the domain is /2013/01/bearhug-fet-compressor.html?showComment=1379340195357#c8802540165897254457

I think I might have boobed the build, as it was only my 2nd or 3rd. But then again I do remember it working perfectly back when I first built it, but then it sat off my pedalboard for a few months as the power supply was grounding out on the chassis. Maybe I fried something...

Now though, I'm noticing that the comp knob doesn't even do anything, and the pedal is *very* lightly distorting at unity volume. Something's definitely amiss - and detiorating.

I'll have another dig around, with the guts outside the enclosure, with my DMM.

For reference, I live near the ocean in Queensland, Australia, so corrosion could also be a factor.

[chuckd666]

Ahh Craggles, a fellow QLD-er.

[rankot]

What's the final design - the first image, drawn by Josh McClarren, or the schematic that is found in PDF link? 

[midwayfair]

What's the final design - the first image, drawn by Josh McClarren, or the schematic that is found in PDF link? 

The one in the build document when you got to 1776. This forum doesn't allow me to update posts after a few days, which means old wrong links stay

[rankot]

Well, Jon, first of all, thanks for sharing this design. I built it according to your first schematic in this thread and it works just fine. Now, I tried to remove R12, R13 and C10 to make it like v2.1 schematic, and I get overdriven sound (some light clipping - could be nice to have ability to adjust it from no clipping to clipping - but how?). I have measured voltage at Q1 drain and it is 6.1V. I don't have a trimmer, just two 100k resistors as a voltage divider. Could this distortion be due to that voltage, or there is something else? Also, on the first schematic, R14 is 1M, while on v2.1 it is labeled R7 and value is 1K.

[midwayfair]

Well, Jon, first of all, thanks for sharing this design. I built it according to your first schematic in this thread and it works just fine. Now, I tried to remove R12, R13 and C10 to make it like v2.1 schematic, and I get overdriven sound (some light clipping - could be nice to have ability to adjust it from no clipping to clipping - but how?). I have measured voltage at Q1 drain and it is 6.1V. I don't have a trimmer, just two 100k resistors as a voltage divider. Could this distortion be due to that voltage, or there is something else? Also, on the first schematic, R14 is 1M, while on v2.1 it is labeled R7 and value is 1K.

You have to connect the comp pot to ground, not just remove C10.

R14 blocks signal to the FET after the rectifier; in the original schematic, it cut the signal by about half, while in the new schematic that's achieved by increasing the Fv of the diodes (because the diodes used are cheaper and easier to find). 1K is better all around but you'll want to make sure you're using 1N4148 for D3&D4 because otherwise it'll compress into oblivion on everything.