Sunday 19 May 2013

Runoffgroove Matchbox

Here's one more from the ROG archives. Apparently amazing sounding OD/dist. Should be doable in 1590B, if you feel brave enough with the rotary :)




From ROG: Back in the mid-90s, the Matchless DC/30 became one of the first boutique amps to hit the market. It excelled at clean tones, as well as thick Vox-like overdriven lead sounds.
There is good reason for the similarity in sound to a Vox amp. The lead channel of the DC/30 appears to be a modified Vox AC30/4. The AC30/4 amp featured the EF86 pentode preamp tube, just as the Matchless design.
runoffgroove.com set out to capture the lead sound of the Matchless DC/30 amp, using the technique developed by Doug Hammond for his excellent Meteor circuit.
We opted for a MOSFET to replace the EF86 tube. The MOSFET has better gain and fidelity attributes than the JFETs we normally use. We used J201 JFETs for the remaining two stages.
We've named this circuit the Matchbox.
An item of interest in this circuit is the odd-looking Tone control. This is in some ways similar to the FAC control on old Orange amplifiers. The sound is "thinnest" on setting A, and gets thicker as you progress through the settings. The Cut control acts as a simple Low Pass Filter. As you turn it counter clockwise, it will roll off some of the treble frequencies. We departed from the schematic a bit here. We used a 10n cap where the amp used only a 2n2. The reason for our deviation is to achieve a more dramatic effect.
NOTES:
-Try using a different MOSFET for the first stage. A 2N7000 sounded a bit "tighter" than a BS170. BS170 had a nice "loose" sound. 
-Any N-channel MOSFET can be used, but as always, pay close attention to the pinout. 
-Try to use all metal film caps in this circuit. It really seems to add to the smoothness of the sound. Ceramic caps or "greenies" will work fine, but you will probably notice a bit more "grainy" sound. 
-A 12v Zener diode can be used in place of the LED used for static protection of the MOSFET. 
-Philip Miller Tate (a.k.a. Ge_Whiz) found the Cut control to be quite subtle when using the stock values. He reports that substituting a 47nF capacitor and 50k-B pot provided a much more effective control. 
-You can use other JFETs, but be warned! The middle stage may not bias correctly or have enough gain to distort. Try other transistors at your own risk. PLEASE do not post on Aron's Stompbox Forum with a complaint about the circuit does not working or sounding good when you do not use the specified components. You can always purchase J201 FETs from our friend, Steve Daniels at www.smallbearelec.com, who will ship to anywhere in the world.


14 comments:

  1. Nice one! This is actually a circuit I've been experimenting with a lot; I built a standalone booster with just the first MOSFET gain stage, and I've got a schematic drawn up of a Matchbox/English Channel hybrid that should emulate the EF86-equipped Vox AC30/4.

    ReplyDelete
    Replies
    1. can you share this interesting hybrid?

      Delete
    2. Sure. It's completely untested/unbuilt.
      http://i.imgur.com/tC4nB2p.png
      Let me know if you build it, I don't have any JFETs on hand right now.

      Delete
    3. Sorry, I forgot to mention: The toggle switch is a 3-way, it adds caps in parallel to the 10n. I also am unsure if that 1u cap after the common drain buffer is required.

      Delete
  2. This is one of the only ROG designs I haven't built. After I was so happy with the English Channel I didn't think I needed another AC30 style pedal.
    Can anyone who built both share some notes on the differences between the two?

    ReplyDelete
  3. ROG recommends the Lorlin 2p6t switch. Unfortunately, it is currently backordered, at Mouser.

    ReplyDelete
    Replies
    1. They have them in stock at Small Bear.

      http://www.smallbearelec.com/servlet/Detail?category=ALL&keyword=2p6t&no=117&param_referer=http%3A%2F%2Fwww.smallbearelec.com%2Fservlet%2FCategories&searchpath=277397

      Delete
  4. Does anyone know if a 1p12 switch could be used in place of the 2p6 seeing as only 1 pole is being used?

    ReplyDelete
    Replies
    1. Yes, use it with no problem, just configure the switch for 6 positions and forget about the remaining 6.

      BR

      Delete
    2. Cheers, that's what I figured but I thought it was best to check. They've only got the 1p12s at my local store and seeing as I've got a day off I thought I'd make good use of it!

      Delete
  5. Well after a long couple of hours with this it's definitely verified. Fired up first time, the layout is perfect but the amount of weird values of caps and resistors this thing uses had me searching through my stash a bunch of times and even then I had to sub a couple of parts. I've only been able to test it through a headphone amp because my girlfriend's taking a nap so I'll let you know what it sounds like once I've been able to crank it a bit.

    ReplyDelete
    Replies
    1. Right, after being able to play with this through my amp I can say it sounds pretty nice and exactly like the demo video. One thing I have noticed is that currently the tone knob seems to work backwards to normal, as you click it on it opens up more bass, this does seem to add a bit more gain though so it's not necessarily a bad thing.

      Delete
    2. Cheers Madferret!

      The tone switch numbering is per the schematic, so i do think that's a feature. If anyone wants to, they can just reverse the wire order :)

      I'll tag it!
      +m

      Delete
    3. Yeah, it feels quite nice as it is, I just thought I'd let you know just in case. I've not boxed it yet, that's this afternoon's job so I'll let you know if I was man enough to try a 1590b!

      Delete