Dunlop Rockman ACE  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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darthoverdrive
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Post by darthoverdrive »

I don't know if there is much interest for this as it is not really a pedal but I find the circuit interesting. Also if I posted this in the wrong section please move it to where it should belong.

I've always liked the Boston guitar sound although I find these headphone amps to be a little weak in accurately producing that sound. I haven't had the chance to play one of the original headphone amps but I have read that these sound nothing like them, minus the reverb and chorus. I mainly use these for quiet practicing in the wee hours.

I believe the schematic to be accurate but it is always possible that a mistake may be in there. I have not shown all the details like input/output/power jacks and connectors but the majority of the amp is shown. I spent a couple of weeks tracing out the circuit with a meter and magnifying glass and then another week redrawing schematics. I have shown the different values for the Guitar ACE and the Metal ACE for the differences that I readily saw. I haven't verified all the resistors between the two but the caps are all the same. The PCB is the same for both models.

Originally I was going to mount it in a stomp box with foot switches for switching between sounds. However this is a double sided board with plated through holes and I really don't have the tools for successfully removing the solder and parts without destroying the board in the process. Besides these don't always sound good going through a guitar amp since they have a cab sim built into them. Still they work well for playing through PC speakers or recording directly to a PC. Anyway, enjoy!

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Post by Greg »

Cheers for the schematic.. but I can't open it.
When I click it I get a blank screen.
culturejam wrote: We are equal opportunity exposure artists.

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Post by darthoverdrive »

I'm not sure what I'm doing wrong, it attaches but is blank.
First time uploading a file :scratch:

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Chugs
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Post by Chugs »

I can see the schematic fine.

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Post by darthoverdrive »

I guess it doesn't like my gif, here is a jpeg.

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Post by Greg »

darthoverdrive wrote:I guess it doesn't like my gif, here is a jpeg.
That one works for me.. thank you.
:thumbsup
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Post by jonasx26 »

Thanks for the schematic!
Are you sure U4 is drawn correctly?
Would make more sense if the inverting and non-inverting inputs were flipped.
That is, D5 would be connected to U4 pin 3.

BUT.. I might be wrong.

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Post by darthoverdrive »

jonasx26, you are correct. After looking at my schematics apperantly I got the pins reversed. Pins 2 and 6 are connected in the fb loop and pin 3 connects to D5. I'll make the changes to the schematic and repost. Thanks for finding that.

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Post by darthoverdrive »

Ok, lets try this again. Here is the corrected schematic.
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Rockman ACE schematic
Rockman ACE schematic
Rockman ACE.jpg (656.25 KiB) Viewed 6094 times

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Post by Crumbchildz »

The gent who designed the original Rockman works at the college I work at. Neil Miller work with Shultz on a bunch of products and then moved on to design stuff for Fishman. I'm sure he'll get a kick out of it being reversed here! He's also a hell of a organist/key player, does a bunch of great gospel stuff.
Loud's a tone, right?

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Post by darthoverdrive »

Crumbchildz wrote:The gent who designed the original Rockman works at the college I work at. Neil Miller work with Shultz on a bunch of products and then moved on to design stuff for Fishman. I'm sure he'll get a kick out of it being reversed here! He's also a hell of a organist/key player, does a bunch of great gospel stuff.
Well how cool is that! I wonder if he ever thought that 20 years later people would still be buying up Rockman gear. I have a sustainor, eq and distortion generator and peaked inside of them and they would be very hard to trace out. This little one was a big pain as it was.

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Post by blackbunny »

Thanks darthoverdrive!
How does it produce the characteristic "Rockman" sound? There is a TS-style symetrical diode clipping circuit in the feedback loop of U3d, but the circuit around U4 looks pretty unusual. The 2N4340 Q1 seems to be in the feedback loop in a shunt config. I can't remember seeing that before in a dirt box.

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Post by RnFR »

yes, u4 is an odd one. am I right saying that it's in the fb loop of u2a? never seen anything quite like that before. anyone know exactly what's happening here?
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Post by DSV »

U2A + the JFET is a compressor. This is typical of rockman units.

How compression works:
U2a, at the negative input, has 2 capacitors (big ones for flat low frequency response of the stage), and then
(R35 in series with Q1) in parallel with R34
- when Q1 has a low negative control voltage at the gate, its resistance is low (500Ohm - 2k), and the gain of U2a is high.
- when Q1 has a high negative voltage at the gate, its resistance is high (> 1Mohms), leading to less gain of U2a.
R35 limits the compression action (minimum gain), as well as R34 (maximum gain).

The control signal for the JFET is provided by the output ou U2a (feedback compressor). The signal is rectified by D5 and charges capacitor C29 with a negative voltage (negative because of the JFET). R23 controls the attack, R39 the release.
U4 provides buffering + gain control for this control signal. TP1 (trimpot?) is used to set the right control signal for different JFETs (which vary in terms of Vgs and Idss).

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Post by DSV »

A little mistake:
R35 limits the compression action (minimum gain), as well as R34 (maximum gain).
is actually:
R34 sets the minimum gain of the U2a stage, at 1+R22/R34 = 1+160k/56k = 4
and
R35 sets the maximum gain, at 1+R22/(R_Q1+R35)

Oh, and D4+the LED limit the negative voltage going to Q1: they are usually introduced in Rockman stuff to avoid pumping effects.

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Post by RnFR »

in the back of my mind I thought it might be some sort of compression technique. the gain stage in the negative feedback loop was ringing a bell, but I couldn't place it. thanks for the great explanation!
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Post by darthoverdrive »

blackbunny wrote:Thanks darthoverdrive!
How does it produce the characteristic "Rockman" sound? There is a TS-style symetrical diode clipping circuit in the feedback loop of U3d, but the circuit around U4 looks pretty unusual. The 2N4340 Q1 seems to be in the feedback loop in a shunt config. I can't remember seeing that before in a dirt box.

As DSV nicely details, U4 and Q1 are part of a compressor. TP1 is a trim pot, mine was set to 11K5. I have to turn the volume on my guitar down when playing one of these otherwise I get a kind of 'thunk' sound after a string is plucked. I'm sure this is the compressor doing that and maybe my guitar is overloading it.

What creates the Rockman sound is compression followed by distortion, U3d, and then the notch filters, U3b and U3a, and some of the filters below the compressor circuit, depending how the switches are set. These emulate the response of a speaker cab which is why Rockman gear won't sound too good going through a guitar amp. Patent 4,584,700 describes the original Rockman unit and details the different frequency responses he was after for each of the sounds.

I believe the concept of the Rockman gear was to do away with guitar amps altogether. Plug your guitar into a Rockman unit and then run that into a mixer or PA and play through full range speakers.

DSV, thanks for detailing the compressor circuit. I'd been trying to figure it out to try and fix the thunk problem. :thumbsup

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Post by darthoverdrive »

Well I compared the Guitar and Metal ACE to find the differences and changed the schematic to show them. I also added the illusive R25 which for some reason kept disappearing from my drawings. And here is a photo of the Metal ACE pcb with and without components. Hopefully this will be the last schematic revision too.
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Rockman Metal ACE pcb
Rockman Metal ACE pcb
Rockman ACE PCB.jpg (232.77 KiB) Viewed 5930 times
Rockman ACE schematic Rev 1.1
Rockman ACE schematic Rev 1.1
Rockman ACE Rev11.jpg (697.05 KiB) Viewed 5930 times

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Post by atreidesheir »

How does this compare to the sound-alike Mockman from ROG?

I have an original Sustainor/DIstortion. It is a thing of singular beauty. Instant "Roughboy" And sometimes that is good.

Does the ACE circuit sound nearly as good as the larger Rockmans ?
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Post by darthoverdrive »

I think the Guitar ACE sounds thinner espically with the distortions. There is a big drop in volume when using the HVY mode and the SEMI mode doesn't have the bite you get with the sustainor's EDGE mode. The Metal ACE has more distortion in both settings but it doesn't sound scooped, at least not to me. It too has the drop in volume. The CLN setting is so so and the same for both units.

I don't know about the sound differences with Mockman but the Rockman stuff used some sort of diode for the clipping, some hard some soft, sometimes LEDs. Then followed by filtering for the amp sim.

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