Pete Cornish - Bypass buffer  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
User avatar
collingtech
Breadboard Brother
Information
Posts: 73
Joined: 08 Feb 2010, 18:52
Has thanked: 19 times
Been thanked: 25 times

Post by collingtech »

captain? anyone ?

User avatar
soulsonic
Old Solderhand
Information
Posts: 3880
Joined: 27 Jun 2007, 03:38
my favorite amplifier: Traynor YVM-1
Completed builds: too many!
Location: Morgantown, WV
Has thanked: 183 times
Been thanked: 468 times
Contact:

Post by soulsonic »

collingtech wrote:so you must put one buffer in every effect input ? if i have a 12 diy pedal this means 12 buffers? and about impedance correction? someone could explain better the placing ?
thanks
I really think it depends on the individual pedals used. If they have good hi-z inputs (not like a big muff), they probably don't need it. Really is a case by case basis thing
"Analog electronics in music is dead. Analog effects pedal design is a dead art." - Fran

User avatar
collingtech
Breadboard Brother
Information
Posts: 73
Joined: 08 Feb 2010, 18:52
Has thanked: 19 times
Been thanked: 25 times

Post by collingtech »

thanks , so its more a fix to mismatch circuit design then , thanks again soulsonic

User avatar
soulsonic
Old Solderhand
Information
Posts: 3880
Joined: 27 Jun 2007, 03:38
my favorite amplifier: Traynor YVM-1
Completed builds: too many!
Location: Morgantown, WV
Has thanked: 183 times
Been thanked: 468 times
Contact:

Post by soulsonic »

I suppose I would say it is to keep the overall effects chain from loading down the buffer too much. With a couple boxes, this wouldn't even be an issue really, but if you have, say, a dozen or more, I can see how maybe it might start to be a problem if many of them had a really low impedance input (though that seems unlikely, the majority of guitar effects don't have that problem). But then, it could also be to isolate the different effects from each other - I could imagine something where possibly the input circuit of one might effect the response of one of the others...maybe?

In any case, as far as your guitar is concerned, it's only going to be interacting with the first buffer that's driving the chain, so the characteristic of that one is what's going to affect the guitar's tone. Since that first buffer is the only one that's going to have any bearing on their guitar's tone, you can use the Cornish one for that, so you would have its characteristic "sound" (whatever that is), and then you could use simple opamp buffers to isolate all the effect inputs, and I don't think anyone would be the wiser.
Since you said these were all DIY pedals that you're chaining together, it shouldn't be difficult to determine the input impedance of each one to see if it even needs buffering. You could even potentially modify them if necessary.
"Analog electronics in music is dead. Analog effects pedal design is a dead art." - Fran

User avatar
digitalzombie
Breadboard Brother
Information
Posts: 77
Joined: 06 Feb 2012, 18:20
Has thanked: 7 times
Been thanked: 13 times

Post by digitalzombie »

So I built one of these using IvIark's layout. I didn't have a 50K for the output so I put a 47K and a 1K in series (not sure if this matters or not)

I stuffed the board in a 5 channel true-bypass loop switcher right after the input and before the first loop connected through a DPDT toggle. I'm used to using these two ten foot cables between my guitar & amp so I was shocked at how bright it was, in fact I felt like it was boosting the highs at first and then it started to settle in that this is probably what my guitar's supposed to sound like.

I tested it before and after 3 fuzz boxes I've built; a Fingerprint, a Meathead, and a Space Fuzz. Before the Space Fuzz and Fingerprint it sounds like complete shit. Just takes all the balls away and turns them into tame, characterless dirt boxes. The Meathead seemed to take little to no effect.

After it sounded fine on all 3. It actually made the Meathead sound more "Hi-Fi", which I liked because before I thought it sounded a little too brown.

So should I move this thing to the back of my loop switcher or am I going to wish I still had it on the front once I start adding in all my other pedals?

User avatar
modman
a d m i n
Information
Posts: 4890
Joined: 19 Jun 2007, 16:57
Has thanked: 4394 times
Been thanked: 2131 times

Post by modman »

digitalzombie wrote:So I built one of these using IvIark's layout. I didn't have a 50K for the output so I put a 47K and a 1K in series (not sure if this matters or not)
I stuffed the board in a 5 channel true-bypass loop switcher right after the input and before the first loop connected through a DPDT toggle. I'm used to using these two ten foot cables between my guitar & amp so I was shocked at how bright it was, in fact I felt like it was boosting the highs at first and then it started to settle in that this is probably what my guitar's supposed to sound like.
Right on, but remember: it's just a buffer, no Cornish magic. That's what a buffer needs to do...
digitalzombie wrote:I tested it before and after 3 fuzz boxes I've built; a Fingerprint, a Meathead, and a Space Fuzz. Before the Space Fuzz and Fingerprint it sounds like complete shit. Just takes all the balls away and turns them into tame, characterless dirt boxes. The Meathead seemed to take little to no effect.
After it sounded fine on all 3. It actually made the Meathead sound more "Hi-Fi", which I liked because before I thought it sounded a little too brown.
All this is the nature of the beast. Now check what makes the Meathead different from the other fuzzes and you know why it behaves like it does.

Just really nice to see somebody have the mojo-tone experience by DIY soldering...
Please, support freestompboxes.org on Patreon for just 1 pcb per year! Or donate directly through PayPal

User avatar
shredgd
Information
Posts: 16
Joined: 14 Oct 2013, 12:02
Has thanked: 2 times
Been thanked: 5 times

Post by shredgd »

I read on the Cornish LD-1 webpage that his buffer has a 1Mohm input impedance, but from what I see on the schematic it should be no more than 320k... are we sure the schematic is correct?

EDIT: I correct myself, it's even less! It should be 240k (120k + 120k) parallel to the 200k to ground... only 109k input impedance!

User avatar
shredgd
Information
Posts: 16
Joined: 14 Oct 2013, 12:02
Has thanked: 2 times
Been thanked: 5 times

Post by shredgd »

EDIT 2: I've just learnt that this buffer I "bootstrapped" from emitter to base so the real input impedance is raised... very clever! I must give this buffer a try!

User avatar
bmxguitarsbmx
Cap Cooler
Information
Posts: 510
Joined: 25 Mar 2010, 21:15
Has thanked: 704 times
Been thanked: 216 times

Post by bmxguitarsbmx »

shredgd wrote:EDIT 2: I've just learnt that this buffer I "bootstrapped" from emitter to base so the real input impedance is raised... very clever! I must give this buffer a try!
Input impedance is also determined by the hfe of the transistor and the emitter resistor and the load it is driving.

Zin =(hfe +1)Zload. With a hfe of 100, input impedance is basically 100x the 7.5K emitter resistor, or 750kOhm.

This may make very little difference, but it is worth it to keep in mind that BJT's require some amount of input current. Hope that's helpful :)

Post Reply