Emphasis knob on "Edward the Compressor"?

[transient]

Does anybody know how the emphasis knob on Marshall's Edward the Compressor works?

I'm guessing that it blends some of the dry signal with the wet signal, and that the emphasis knob is just a simple tone knob to adjust which frequencies of the dry signal to blend with the wet signal.

Is this true, because i'm thinking about adding a similar feature to my Orange Squeezer

...
emre

[Mark Hammer]

Nope, that's not it at all.

The ED-1 is essentially the same basic circuit as a Dynacomp, but souped up.  The Emphasis control is part of an op-amp stage in between the 3080 and the rectifier circuit.  The op-amp stage is noninverting.  The Emphasis pot provides a variable common path to ground for both the ground leg of the op-amp AND the output of the op-amp.  Roll it one way and it increases the gain of the op-amp, at the same time as it rolls off the bass output of the op-amp.  Roll it the other way and it drops the gain a bit, at the same time as it brings in the bass and rolls off the highs.

Note that this stage is NOT part of the audio path.  The control signal passes through it but not the audio signal.  So, what it does is make the compressor more sensitive in the low end or more sensitive in the high end.  Roll it one way and it compresses more for low notes than for high ones.  Roll it the other way and it compresses more for high notes than for low ones.  This allows it to be more compatible with more instruments, but also to achieve different feels.

The reason why you couldn't simply tack on a much simpler Big Muff tone control is because the BMP tone control comes AFTER all those mids and highs are added by the fuzz.  This results in relatively balanced volume levels as you rotate the tone control from bass to treble.  In the case of the ED-1, when you want greater treble sensitivity in the compression you need to goose the signal a bit to achieve equivalent sensitivity in general.

Make sense?

[transient]

Thanks Mark I should've guessed that it wouldn't be so simple.

So, the knob does not make any difference (EQ-wise) in what you hear, it merely adjusts which frequencies the compressor will be sensitive to. And even though i do understand how it works, i cannot visualize how that opamp stage is constructed. Do you have a schematic of that stage? I tried to draw one by following your explanation, but got confused where/how to place the pot.

.
e

[Mark Hammer]

I have no immediate way of showing you so we'll let ourimaginations work a little.

Imagine your favourite one op-amp distortion box, say an MXR Dist+, or DOD 250, or DOD YGM308, yaddah, yadah.  You'll see a cap and a resistor going to ground from the "-" input pin on the IC. On all those pedals, there is a fixed resistor in series with a pot, used to set the gain.  As you change the gain by altering the resistance, there is an interaction between the resistance and cap that changes where the low-end rolloff happens.  This is calculated using our old favouirite formula: F = 1/[2*pi*R*C], where R is megohms and C is microfarads.

Okay, now imagine that instead of a pair of diodes to ground after the op-amp, we have a 47nf cap and a resistance going to ground.  That combo works just like the tone control on your guitar, bleeding off high end.

Okay, now imagine the vaiable resistance that sets the gain is ONE half of a single pot, and the variable resistance setting treble bleed after the op-amp is the OTHER half of the pot, and the wiper of said pot goes to ground.  So, now as one variable resistance to ground goes up, the other goes down.

[transient]

Thanks for the explanation Mark.

Is this correct?

.
e

[Mark Hammer]

Hmmmm....not quite.

I think I'll wait until later tonight (and I mean MUCH later - it's a very busy schedule this evening) and post the circuit fragment, unless someone else can post the link the the PDF.

[transient]

No problem Mark, whenever you can. I'm not in a hurry, just curious

I've been thinking, and i understand why the previous schematic won't work. It breaks the feedback path of the opamp, connecting it to ground.

I think i came real close to figuring it out, but there's one thing i don't understand. There will be a lowpass filter after the opamp stage, right? For a lowpass filter, you place a resistor first, then a capacitor to ground. But in order to share the pot with the gain/highpass stage, there will have to be another resistor (which is the pot itself) after the cap, and you'll end up with a highpass filter.

I'm obviously missing something here

By the way, i searched the forum (with both the old and the new search engines), but i couldn't find a schematic, nor the PDF file you mentioned.

.
e

[Mark Hammer]

Okay, the circuit fragment is found here: http://img.photobucket.com/albums/v474/mhammer/ED-1.gif

Rec-In comes from the output of the 3080, and the transistor you see on the right has the identical function to Q2 on the Tonepad schematic for the Dynacomp/Ross.

In the ED-1, the gain of IC1:B is set by the combined resistance of R18 (5.6k) and the resistance between points 1 and 2 on the pot.  Point 2 is the wiper of the pot.

Can't remember where I got this, but someone posted a zipfile of several Marshall pedal schematics last year and this was in there.

[transient]

Thanks a lot Mark

I'm guessing that it's the only piece you have from the schematic, right? 'Coz it would be interesting to compare it to a Dynacomp to see what else is different.

...
emre