A "Transparent" Buffer?

[Loose]

Hey guys,

I've built the Surfy Bear reverb (transistor based 6G15) and I really like it.
The only issue is that it really alters the sound, even with the mix control all the way to the dry side. I suspect the buffer is the culprit.

The circuit is 12v and the buffer is a simple jfet one.
Any suggestions on how to modify it so it won't alter the sound?



Thanks!

[FiveseveN]

it really alters the sound

In what way?

[Loose]

it really alters the sound

In what way?

There's much more highs\presence.

[Mark Hammer]

Um, that's what buffers do, and usually why people use them.  It's not adding anything.  It's simply not losing what's already there as much as might happen otherwise.

[Loose]

Um, that's what buffers do, and usually why people use them.  It's not adding anything.  It's simply not losing what's already there as much as might happen otherwise.

Thanks Mark. Yes I know that's what buffers do, but I prefer my sound to be "colored" with impedance\capacitance like it sounds straight into the amp without a buffer. I think the buffer is essential for this circuit in order to not lose any gain, correct? Because if it's not, I would just remove it.
But if it is essential, I would like to know of a workaround if there is one.

[FiveseveN]

Just strap a few pF across R21 to simulate (more) cable loading. Adjust the value to taste.

[slacker]

You could add a capacitor from the gate of the first fet (Q4?) to ground, something between 100pF to a few nF would roll off some top end.

[Loose]

Just strap a few pF across R21 to simulate (more) cable loading. Adjust the value to taste.

Correct me if i'm wrong, but with a buffer you could have 100 feet of cable before and it wouldn't make a difference since it changes the impedance?

[anotherjim]

Try taking C12 input straight from the input jack instead of the gate of Q4. I think that could either fix it or prove coloration is down to the buffer effect?

Assume you have it feeding a high impedance input? Low impedance line inputs would cause it to be a bit bass light due to value of C17. Ideally, if you wanted to drive line inputs, it would want another buffer after the mix pot.

I suppose it's copying the old tube scheme, but if I were doing it from scratch, I think I'd try to put the buffer before Q4, so feeding both the reverb input pre-amp and the mix pot from the buffers source.

[induction]

Just strap a few pF across R21 to simulate (more) cable loading. Adjust the value to taste.

Correct me if i'm wrong, but with a buffer you could have 100 feet of cable before and it wouldn't make a difference since it changes the impedance?

Buffers prevent treble-loss in the part of the signal chain that comes directly after the buffer. FiveseveN is suggesting you dump the treble before the signal reaches the buffer.

[karbomusic]

Can't we just lower the input impedance setting resistor, if there is one, or does that screw something else up? Isn't the higher input impedance the reason more highs are getting through to begin with? If so, seems like if possible, it would be better to just lower it. I had an opamp buffer I did that with using a 5 position switch as an impedance selector for similar reasons which is what makes me want to ask the quesiton.

[dbp512]

Instead of messing with the impedance, couldn't you just add a passive low-pass to bleed off any unwanted treble? I know its not the same as playing the guitar straight into the amp, but wouldn't it effectively have the same effect?

[thermionix]

Any suggestions on how to modify it so it won't alter the sound?

What is the point of an effect that doesn't alter the sound?

The real 6G15s alter the sound quite a bit (with fewer components too) it's part of their charm.  Don't know how this transistor thing compares, but maybe you want a different type of circuit.  For unaltered sound on full dry...true bypass.

"Hey Ghandi...do you surf???"

[Mark Hammer]

Now that I can actually see the posted circuit (which I couldn't do when I originally responded, I think the problem lies in what the buffer does to the clean sound.  Obviously, there is precious little that a buffer could do to alter the tone of a spring pan.  Hence the problem would seem to be that Loose wants the reverb unit to simply add reverb to his existing clean sound, and not change what the clean sound sounds like when he uses the reverb unit. 

I am assuming that Loose bypasses the entire unit when he turns the reverb "off", such that a no-reverb signal to the amp is unbuffered, and reverb IS buffered.

I think there are two solutions to make use of here.  One is to modify the clean path within the 6G16 so that it sounds very close to the bypassed tone, permitting Loose to blend in as much of the reverb with his original clean tone as he wants.

The other solution is to do the switching differently.  On most amps with built in reverb, cancelling the reverb sound generally involves simply grounding the reverb-pan output signal.  IN this circuit, you simply ground the output of the pan.  Essentially, you have a shielded wire running to a footswitch (or toggle), and closing the switch grounds the input to R26.  You can leave your balance and Tone controls wherever you want them, and your clean tone will not change between reverb and bypass settings.  That's how the "pros" do it.

[blackieNYC]

Mark- the dry will still have to pass thru a buffer, no?
Loose- have you tried an MF102 in there, instead of the 5457? just for the halibut.
Then again, you could build the pickup simulator at AMZ. Put it after the dry buffer, but it will possibly change the nature of your mix control.  Worth a try, and if you don't like it you can use it elsewhere.

[Gus]

find a 6G15 schematic
Note that the circuit you built does not have the cut off pedal.  The cut off grounds the signal at the recovery stage and also keeps the plate voltage from "moving" so the reverb side of the mixer control does not have a signal.
You circuit always has a signal at the mixer control side connected to the reverb recovery amp

[Mark Hammer]

Mark- the dry will still have to pass thru a buffer, no?

Yes, but my sense is that the problem to be solved is the change in tone resulting from the change between buffer and no buffer.  If the same buffer is always in-circuit, then a simple tweak of the amp tone control gets the same dry tone at all times.

[Loose]

I should point out that the bypass signal is irrelevant here.
I was emphasizing the all dry side of the pot to show that the buffer is probably what I want to change, without taking into consideration the effect the reverb side of the pot has on tone.

So a lot of ideas came up here, a bit hard to narrow it down to what would work best for me. Would sending capacitance to ground before the buffer really give me the same tone as with the guitar straight to the amp?
I've also read on a different thread that for the same tone as straight into the amp the buffer's input impedance should be the same as the amp's input impedance. But if that's correct, how would it work with different amps?

[FiveseveN]

I've also read on a different thread that for the same tone as straight into the amp the buffer's input impedance should be the same as the amp's input impedance.

Unless you're going into a line or microphone input it's probably already the same. But you use a longer cable when plugging into the amp than into the pedal, right? Does it sound the same with the same cable?
I was suggesting adding capacitance at the input because it has a different effect (shifting the pickup resonant frequency) than a lowpass filter after the buffer.
It takes a few minutes to try out the basic options (input cap, lower input impedance, tone control) and see what works for you.

[Mark Hammer]

I should point out that the bypass signal is irrelevant here.
I was emphasizing the all dry side of the pot to show that the buffer is probably what I want to change, without taking into consideration the effect the reverb side of the pot has on tone.

So a lot of ideas came up here, a bit hard to narrow it down to what would work best for me. Would sending capacitance to ground before the buffer really give me the same tone as with the guitar straight to the amp?
I've also read on a different thread that for the same tone as straight into the amp the buffer's input impedance should be the same as the amp's input impedance. But if that's correct, how would it work with different amps?

In theory, if the input impedance of the amp or effect are equally high, then yes, you are correct.  However, don't forget you have a cable going to the effect, and another going from the effect to the amp.  I don't know how much cable that involves in your case, but hypothetically that sequence could involve twice as much cable.  If the guitar's output impedance was low enough, and the amp's input impedance high enough, then you could run oodles of decent cable between them without much audible impact on tone.

What's not defined here is how much loss via cable is occurring in your circumstance.