Turning a pair of Afterlifes into a tone control...

[btml]

Hey there-
First post, probably dumb question.

What I'd like to do is elegantly modify two Afterlifes to give myself dynamic control over Bass and Treble.
So- one AL just doing highs, another just doing lows, and running in parallel.

I figure there must be some sneaky places to insert a couple caps so they wont interact across the units-

It seems like Highpass would be easy, just change the value of C1 in one unit, or add a cap coming off of pin1 of IC1a, filtering it's lows without affecting the other compressor..
But for Lowpass- Can I get away just shunting the highs to ground on the positive side of C2? Or will I have to use the negative feedback loop of IC1a somehow?

Also, if that works- I should be able to make a mid control.. yes?

I have an AL, and I'll poke around with that this week, but any thoughts would be welcome-
Thanks!

[midwayfair]

Put a FET or BJT unity volume gain stage in front of both, with no source/emitter bypass cap. Run your Afterlifes directly into the source and collector. They've already got the decoupling caps. The emitter and collector will both send a unity volume signal into the Afterlife.

Solder a 2.2nF cap across R3 in the "bass" Afterlife. Make the input cap 10nF.

Change the output cap in the "treble" Afterlife to 22nF. You're changing the output cap on this one because it gives the compressor full signal and

Remove the volume pot from either one and simply connect the pad for lug 3 from that one to the lug 3 pad of the other. The sustain pot will now be your gain control for both, and the volume is a master volume.

These are theoretical changes, but the calculator says you should shelve the frequencies pretty high up. There should be enough crossover to keep things from sounding weird or scooped in the mids. I'd need a scope or sim to do more than make an educated guess.

There is one problem that MIGHT exist, and that's that the first stage puts the signals out of phase, and consequently I'm not sure if you need a buffer inverting the emitter follower back to "normal" to avoid cancellations.

Before you get too excited, though, a lot of people have tried parallel compression and often come to the conclusion that it's simply not worth the headache. Usually people just want more treble back in their compressed tone and the easiest ways to handle that are parallel compression (i.e., a blend circuit) or treble bypasses of various stripes -- the Engineer's Thumb has an excellent treble bypass mod that boosts the treble by leaving it untouched, and the Bearhug has a treble bypass that also functions as a treble cut (depending on whether the compressor is working very little or working very hard). The treble bypass in a Ross is less satisfying, since it just adds a little presence at the output, after the compression has already happened.

[btml]

Hey Jon, thanks for the thoughtful response- Your demo videos are what got me on the madbean site in the first place-

You may have answered my question-

I should probably explain more what I'm going for, apologies if I sound redundant.
I'm a bass player, and I want a 2band dynamic eq. It would be my compressor, and my eq. So I'm not just looking to regain my highs, I want to compress and control each band separately. I need both Compression knobs, and both Volume knobs, and I would probably add your sensitivity control to each side as well.

There's quite a few units that do something like this, and I have a couple, but they're mostly designed more for general PA use than an individual instrument(I don't need a lot of range of controls, I'm never going to put a vocal through it). I know of two bass specific pedals, one with a million knobs and another with none. For a bass compressor(or eq), I just want a few knobs that are musical and to fiddle with a few component values until I'm happy with the way it works- which is why I like the AL.

From what you're saying, I need an extra stage in front of each compressor, perhaps there's a better compressor build for this to start with? I really like the opto, though- I'll have to research what you're talking about with emitters and followers before I attempt to ask a real question about that-

Thanks!
Ben

[midwayfair]

From what you're saying, I need an extra stage in front of each compressor, perhaps there's a better compressor build for this to start with? I really like the opto, though- I'll have to research what you're talking about with emitters and followers before I attempt to ask a real question about that-

OKay ... a source (FET) or emitter (BJT) follower means that you take the output from the emitter or source instead of the drain (FET) or collector (BJT). A simple buffer is an emitter follower:
http://www.muzique.com/lab/buffers.htm


This will be unity gain as long as the transistor is biased correctly.

A basic transistor amplification stage will come from the collector or drain. Take a look at, say, the Big Muff schematic.

We can actually take the output from BOTH at the same time. They will not interfere with each other this way. It's the simplest possible method of getting a split output that I've found, since it just involves three resistors, a capacitor, and a transistor. Here are a bunch of things that use this trick in-circuit:

AMZ Mosfet booster with second output:
http://www.muzique.com/schem/mosfet5.gif

My Hamlet delay -- the delay line comes from the emitter while the dry path comes from the collector:
https://docs.google.com/document/d/1P7uOYFRH6ZW5eGoquhAiMQMyUU76bod_gNKkiSlPNJM/edit

My Blue Warbler -- imagine an Afterlife in place of the envelope section:
https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20layouts/Nonmini%20builds/Blue%20Warbler%20Vibe%20schematic%201.2.png

You can, of course, make a TL072 input buffer and connect both circuits to pin1. In my experience this loses a little bit of input gain, which is a critical consideration for reducing noise in a compressor design, and you might get some sort of parasitic crossover. It's probably worth testing -- it's really easy to set up on a breadboard, since it's just a chip and a voltage divider network.

I don't think the question is whether there's a better compressor to "start" with. I'm sure there are some DIY dual compressor designs, though I'm not aware of any off the top of my head that are known to be good ... perhaps you might be the guy to get them working? At any rate, since I'm a compressorholic and I can't think of one off the top of my head (and haven't built it), it's safe to say it's going to take some digging ...

I'll try to explain further why you might want to use only a master volume instead of individual volume controls. You can use the sensitivity (threshold) control and "compression" control to mimic the two stock controls on the Afterlife: The "compression" control is actually just the gain of IC1A, so if you turn down the sensitivity, you will get less compression at higher gain settings. Turn up the sensitivity and you'll get more compression at higher gain settings. This lets you use the gain control as a volume for each band.

The alternative is much more complicated. You'll need two more transistor or op amp stages, because you can't put the two volume controls in parallel with each other. So you have to run each volume control into a new buffer, then connect the outputs of the buffer. There is a benefit to doing this: You'll get super low output impedance, and the afterlife doesn't have the best possible output impedance at 100K.

If it were me designing this, I'd simply build two Afterlife boards with Comp and Sense controls (and otherwise as I described it above), then make the MOSFET booster as shown without C5 or R6. Put the Treble Afterlife on the Drain side of thing and it will be slightly boosted (~3 dB) -- that should help even things out compression-wise. You can also omit the decoupling capacitors (C4 and C6) because they'll be part of the Afterlife PCBs already.

[ch1naski]

That's why Jon rocks: explanations that are education.


Sent from my Nexus 7 using Tapatalk HD

[Vallhagen]

I should probably explain more what I'm going for, apologies if I sound redundant.
I'm a bass player, and I want a 2band dynamic eq. It would be my compressor, and my eq. So I'm not just looking to regain my highs, I want to compress and control each band separately. I need both Compression knobs, and both Volume knobs, and I would probably add your sensitivity control to each side as well.
[...]
Thanks!
Ben

Hi Ben.

So; you want to split your signal in two bands - then run the two bands independently from eachother through some compressor(s) - and then mix them together again.

Now i havent read through Jons complete posts indepth (i will, just that its breakfast here), but i would start to play with a Linkwitz Riley phase compensated filter (google it). Because you have to take care of the phase shift artefacts in some way, and the best thing must be to compensate directly in the splitting filter stages. 

Take a look at my Blue Monster Project: http://www.madbeanpedals.com/forum/index.php?topic=6968.0 , where i do something similar, just that i run the separated bands through overdrive Circuits, not compressors. You may borrow the complete idea from there, i guess you wanna re-calculate Component choices for "your" split frequency though, as you aim for bass.

Cheers
/Bengt

[btml]

Cool- I understand that-

I happen to have a karate shop laying around, which has a buffer in it- maybe not the right transistor for this application?
I was trying to use that effect before compression as a semi-parametric low/mid booster.. but of course that combo gets super noisy.
Now I'm wondering if I can (boxing them together) use the KS buffer in front of the AL's (in the way you described) and feed the compressor output into the KS post buffer...
KS transistor is a 2N5457, but I suppose I could change it out with a more appropriate part if necessary.

Seems like the buffer for the KS would be superfluous given the output of the AL, and the fact that I don't intend on switching parts of this contraption on and off. But I could be wrong?

[kothoma]

You really should take a look at http://diy.thcustom.com/shop/pro-cessor-pcb/

[btml]

And here's a rough picture of that idea- sort of- just wanted to make sure I understand.
Forgive me for just chopping up those schems, I don't have a drafting app.

Buffer from KS split into 2ALs, with Jons cap suggestions, sans side chain, one master volume, into the rest of the KS. I left out all the power sections-

Jon, your cap locations, I'll have to filter the sidechain audio separately if I don't want them to track full range, yes?

Bengt- I just read yours but will check it out as well as I'm heading to bed over here-

Cheers- Ben

[kothoma]

Don't think this will work as expected. Even if you'd get that FET stage right (at least one resistor missing) then treble and bass will be out of phase? Mixing them at the VOL pot should create a nasty mid notch. Just my intuition, haven't tried it.

[kothoma]

Here's a completely different approach:

Take an SVF (like in the Nautilus/Mutron or the Bloviator or the Boss Enhancer or Anderton Super Tone Control) with a fixed center frequency (and fixed low Q). Use vactrols to control the mix levels of the HP and LP outputs.

I confess I'm a big fan of SVFs.

[midwayfair]

Even if you'd get that FET stage right (at least one resistor missing) then treble and bass will be out of phase? Mixing them at the VOL pot should create a nasty mid notch.

That's what I was worried about ... this is tough on pre-existing PCBs.

btml, I think no matter how you go about it, this is going to involve some breadboarding.

[btml]

Yeah, if it were an effect I would probably try it with a nasty notch, but I want to be able to leave it on- Then again, the notch filter on an acoustic amp is apparently how Albert Kind got that weird sound-

Found this on one of the above links-
The Mad Box http://diy.thcustom.com/shop/madbox-v3-1-pcb/
It seems like it would be the simplest approach for me if it works. It's just a two channel effect send, and a two channel effect return with a blend knob. I would put the Afterlifes(with mods to bandpass them as above) one in each loop.

Does that sound feasible? And does Mad Bean have anything like this?

[Vallhagen]

Does that sound feasible? And does Mad Bean have anything like this?

Sorry if i keep repeating myself, but you have to deal with / solve the phase shifting dilemma caused by filtering, and thats maybe not trivial. Or accept it as it is without phasecompensation, but then you will have artefacts at the crossover frequency, which maybe will be cool, but possibly sound like ****.

For the splitting and summing part of the circuit, THcustoms pcb (as in your link) looks clever though:)


Cheers

[btml]

Vallhagen-
So I (sort of) understand now how the Linkwitz-Riley corrects the phase, with two lines of two stages which turn 90 degrees in opposite directions, and then 90 degrees again. I don't understand why there is a phase shift of this type in the first place, but there's probably some math involved?

That makes the Pro-cessor (also linked to above) a better place to start to see how this idea works-
I was starting to wonder if I will like the notch, but I can always put one in later.