Argh! (Aka OCD impedance issue)

[Redvers]

So built an OCD clone from a kit, tested it straight into my valve amp. Heavenly tones. Next thing, I put it into my pedalboard. The volume pot does very little until 4 o clock, the tone pot is maxed out, and when I kick in my digidelay (in a TB loop) the volume jumps massively.

Why do we only find out about these problems after buying pedals? What can I do? I have a buffer circuit, maybe put that straight after? Or put the pedal first/last on the board?

 

[MilwMark]

So built an OCD clone from a kit, tested it straight into my valve amp. Heavenly tones. Next thing, I put it into my pedalboard. The volume pot does very little until 4 o clock, the tone pot is maxed out, and when I kick in my digidelay (in a TB loop) the volume jumps massively.

Why do we only find out about these problems after buying pedals? What can I do? I have a buffer circuit, maybe put that straight after? Or put the pedal first/last on the board?

I was never able to fix that with a separate buffer before or after the pedal. Not sure if you can "fix" that issue (at least without losing the magic of the pedal?

 

[Redvers]

True. I need some consistency though. I'll try one after.

 

[Lupo]

Looking at the scematics, I think the issue is the current flowing to the pedal you put after.
If you have a log pot in that position, try to use a linear one.
Otherwise I would scale down the impedances at the output stage.

 

[Lupo]

BTW: in you schematic, what pot is used fot the volume? I find both 500kohm and 100kohm...

 

[Redvers]

Thanks, mine is 500ka. Version 4 I guess.

 

[Lupo]

I would try to put a linear one...

 

[Redvers]

Good call. I did wonder if I'd wired it wrong but I'm pretty sure it's right. It was less noticeable straight into the amp.

 

[reddesert]

Looking at the OCD v4 schematic: If you have a 500K output volume pot in the OCD, it has a high output impedance (presuming that you have the volume somewhere below max). Putting that into something that has a low-ish input impedance will load the OCD's output stage and cause a volume change. The input impedance of the next pedal becomes part of the voltage divider formed by the volume pot.

It may also load the tone pot because the tone pot is right at the output, unlike some pedals where there is a tone stack and a recovery stage.

You could try following the OCD with a pedal or buffer that has a high input impedance, like 500K to 1 Megohm, and/or changing the value of the volume pot in the OCD to 100K. I don't know why it uses such a high value pot. This type of issue is a reason that many circuits use output buffers.

 

[Redvers]

Agree. On a basic level adding more to a circuit (400k in this case) over complicates things, especially if you're not going to use it. .

 

[codamedia]

I don't own an OCD for this very reason. The design is far too temperamental for me. I know pedals will sound different depending on placement, but that is not what I am talking about - it's much more than that. I've said it many times on these forums... if a pedal doesn't play nice with my other pedals, it needs to go! It isn't worth the extra attention with hundreds of great alternatives to choose from.

Just my 2 cents...

 

[Redvers]

I'm going to put a switchable klon style buffer in there, should sound good at 18v. Will report.

 

[Redvers]

image removed (link removed) Not sure this pic will work. Anyway I built a buffer and put it after the ocd. Works like a charm. I just need to decide whether to put the buffer in with the ocd or keep it separate. When you flick the buffer on the pedal jumps up to its intended volume and the volume dial is much smoother.

I might still try reducing the volume pot value.

 

[johnhe]

I used to be really irritated by the issue, but I love my OCD and I've come to think that the output impedance is an integral part of the magic of the sound (to my ears).

So for the past few years, I simply run my Boss tuner right after the OCD and since I started doing this, I haven't had a single issue. I specially use the
Boss over the alternatives for the buffer.

However, as soon as I remove the buffer from straight after the OCD, all sort of issues start cropping up again!

 

[Redvers]

Isn't that a contradiction though? As soon as you put a buffer after it any output impedance issue/magic is gone. Not that I hear a difference or am that bothered. I never run a single pedal into an amp anyway so I'm happy.

 

[Wampler Pedals]

I always wondered why Mike didn't just add a buffer onto the end of the original design...