RDV Overdrive

[RDV]

I've made some modifications to the old HMP and have changed the name as it doesn't really fit this pedal. I like it, I hope you will too.

I added a simple JFET buffer in front.
I changed a # of cap values throughout.
I narrowed the bandwidth of the parametric section. It now goes from around 450hz up to about where the treble rolloff knob starts.

Sound samples to come later.

Schematic

RDV

[MartyMart]

Looks good Ricky, glad that you kept this one "alive"  as I said i like the
HMP lite a LOT !! 

MM.

[Toney]

Thats one I've been meaning to look a for quite a while.
Now that it's an overdrive.....sweet.
Will have a crack at it this week.
Thanks, Toney.

[MartyMart]

I dont know what's going on here these day's, this is typical :
147 views, a ton of downloads and only two replies !   

....  MM.

[RDV]

I felt the HMP was a failure as a defacto "Heavy Metal Pedal" with the problem probably being not enough gain stages. I think it would take another 2 at least with a gradually increasing gain structure to accomplish it.

I did think however that I had the beginnings of a good versatile overdrive/distortion if I could just make it more responsive to guitar volume changes and if I could make the EQ make more sense in the range of guitar frequencies.

I felt the guitar volume change problem was due to the relatively low input impedence which I addressed with a simple JFET buffer. With the buffer in place the sound cleans up well with guitar volume changes giving a wide variety of sounds available(as opposed to one sound that's OK with the rest being crap).

The EQ problem was more complicated as the problem was multifold. Nothing is more important than which frequencies that you are trying to distort. I took a lesson from the tubescreamer and used their .047µF bpass cap for both gain stages. This certainly helps retain focus in the sound even with a whole lot of gain going on.

When I decided to rework the circuit, my initial prototype had been modded to death(quite literally). In trying to get it to a point where it even worked I had to trace out the entire circuit from beginning to end(not easy with a perf prototype with no layout). In doing so I discovered I had never even wired the parametric section correctly. It worked, but I had the final buffer stage bypassed, which was big in that it seemed to make the sound very unpredictable. After the complete going through it turned out that the 1st opamp had taken a dump on me, which has never really happened to me before. I replaced it with a fresh RC4558 and pressed on.

I next decided to make the range of the parametric section much more narrow, staying away from frequencies that guitar doesn't even produce. I got this section "lock, stock, and barrel" from GEOFEX's Simple EQ article. I at first had simply copied it(guessing which cap values to use) without trying to understand the text. After a bit more study I decided to use the values that R.G. suggested for the 2nd section of his simple parametric which would go from about 450hz up to about the point where the rat-style-treble-rolloff- EQ begins. What I now seem to have is a parametric section which allows for mid-bass to mid-high cut or boost which allows me match the pedal to various amps. I've been testing it with my DIY SS Thomas/Vox preamp LM3886 creation which by nature has a ton of high-mid and treble with the typical vox tonestack mid scoop. I set the RDV OD's EQ to the high-mid cut setting(both knobs full CC) and I'm achieving a nice round smooth OD. I've yet to test it with my Marshall, but those results will be forthcoming.

The sound overall seems to have that "cranked bassman" tone reminding me a lot of the tone of 'Dear Mister Fantasy' by Traffic which I'm enjoying a great deal. I just start with the guitar volume rolled down a bit for the rhythm sound and just dime it for lead. Of course YMMV. 

Bye 4 now.

RDV

[petemoore]

  RDV Overdrive
  The name has a real nice 'ring' to it.

[ragtime8922]

RDV, love it. I always loved your HMP and my favorite part was the adjustable EQ section. Modding it for guitar tone efficiency was an awesome idea!

I've been away from the forum for some time so a great place to start is going to be busting out the hmp (sorry, the RDV Overdrive) on the breadboard rig.

[brett]

Hi.
Looks cool.
1 suggestion for simplifying it.  If using a jfet op-amp (eg TL071) at the input, you can do away with the jfet input buffer.  But you'd need to change that 100k resistor (linked to Vbias and AC ground) to 1M ohms, so that you get the full benefit of the op-amp's high input impedance.

Can't wait for a sound clip.
cheers

[RDV]

Hi.
Looks cool.
1 suggestion for simplifying it.  If using a jfet op-amp (eg TL071) at the input, you can do away with the jfet input buffer.  But you'd need to change that 100k resistor (linked to Vbias and AC ground) to 1M ohms, so that you get the full benefit of the op-amp's high input impedance.

I know what you mean(that would be a good mod for anyone who has built the HMP), but I like the idea of being able to plug almost any dual oa in there as well. I even debated using a JFET gain-stage in front for a bit more drive, but had a pre-wired JFET buffer on a tiny piece of perf, so it was easy to add in there. I think for my purposes it has enough drive as is. Who knows what it'll look like in another year of my slow thought process?

RDV

[lenwood]

Hi Ricky,
this looks very nice. For any and all with the burning question,
can the RDV overdrive do SRV?   

[RDV]

Hi Ricky,
this looks very nice. For any and all with the burning question,
can the RDV overdrive do SRV?   

It does RDV.
It will get a lot dirtier than a TS, but has some things in common.

RDV