Surfy Bear Reverb: Mix mods

[j_flanders]

Hi, I just built the Surfy Bear reverb: http://www.surfyindustries.com/surfybear-pcb-diy-kit-faceplate
I like the fact that it's true to the original Fender 6G15 reverb unit but I'm not entirely happy with the way the mixing (ratio wet/dry) is done, so I'd like some advice about modding this aspect.

Main problem:
Muffled dry tone at higher mix settings (again true to the original). I don't mind this so much with my bright sounding jazzmaster but I do with my already very dark sounding Wildkat.

My solution:
Use my Boss LS2 to do the summing/mixing myself. This way I keep my original dry tone and I can mix in the wet part myself.
This works but the problem is that the Surfy bear and 6G15 still have a lot of the (muffled) dry tone in the output mix, even at the maximum reverb/minimum dry setting.
So with my LS2 mix I end up with: original dry tone + reverb + mufled dry tone.

Questions:
1) Why can this circuit do 100% dry but not 100% wet?
Dwell maxed out and mix at minium gives me 100% dry with no audible reverb
Dwell maxed out and mix at maxnium does not give me 100% wet, there is still a fairly loud (altough muffled) dry tone in the mix.
With dwell at minimum and mix at maximum I would expect no sound at all, but this is not the case. Why not?

2)What is the best/easiest way to get a 100% wet/reverb-only signal as output?
Is it as simple as adding an on/off toggle switch between c17 and lug 3 of the mixing pot work? Just preventing the dry from getting in the mix? Or would this 'upset' the circuit one way or another?

[Transmogrifox]

To answer your questions:
#1) The reason you can't get 100% wet on the SurfyBear is because the tone control output is relatively high impedance compared to the mix pot.  Even worse, because it's a tone control, it filters the dry sound so you hear the muffled part that you talk about because it's rolling off the highs more than it does the mid-band.  The mids and lows get a lot more in the mix because the output capacitor from the tone control is relatively small, so impedance is particularly high for low frequencies.

#2) The easiest way would be connect an SPST switch between the mix pot dry side and ground, so you either switch the mix pot to ground or to dry output.  Then the mix pot works like a volume pot on the wet signal.

Another somewhat simple way to do this is to add a buffer to the wet side output so the mix pot is going between JFET source connections directly, which should have output impedances closer to 300 to 1k ohms depending on your source resistor.

Alternatively, here's a more complicated (but more flexible) way to do it:

Check out this thread:
http://www.diystompboxes.com/smfforum/index.php?topic=113957.80
Page 5 has the final solution, reposted here for ease of finding which it is:


You connect the input of this in parallel with the surfybear input and it will have minimal loading effect on the guitar.  You can change SurfyBear R21 to 2.2Meg or something larger to get input impedance up higher, or tap this circuit off R18, other variations on the theme ok.  Main point in that thread was to remove the SurfyBear dry path and allow mixing in reverb without messing with the dry path tone.

In your case if you want 100% wet then you need a way to also adjust the dry level.

IMHO you would have the best flexibility to use the circuit I posted and put a second pot (10k) after the first op amp buffer stage on the dry input to independently adjust dry and wet levels.  Then it behaves more like you're using a mixer board where you send the dry channel pre-fader level output to the SurfyBear and return reverb to a separate channel.

Pot connection:
A -> U1, pin 1 (buffer output)
Wiper -> C1
B -> Vb

VB is not shown on my schematic above, but it is all connected like the AMZ simple mixer.  "VB" would be the point between the 10k resistors at the 47 uF capacitor which is shown as connected into IC1 pin 3.

[j_flanders]

Thanks for the detailed response.

To answer your questions:
#1) The reason you can't get 100% wet on the SurfyBear is because the tone control output is relatively high impedance compared to the mix pot.

Ok, that's a clear answer. I don't fully understand it yet but at least I have something to study/google for.

Even worse, because it's a tone control, it filters the dry sound so you hear the muffled part that you talk about because it's rolling off the highs more than it does the mid-band.  The mids and lows get a lot more in the mix because the output capacitor from the tone control is relatively small, so impedance is particularly high for low frequencies.

Do you mean the tone control affects the dry signal? I thought it only attenuated the highs of the reverb signal.
As far as the muffled dry tone goes, all of my googling lead me to the same explanation: dry signal + series resistance of the mix pot forms a LP filter with the capacitance of the cable to the amp. (Similar effect to turning down the volume on your guiter) But using a short patch cable into a buffered pedal and then 10ft cable to the amp didn't solve the muffled tone, so there must be more to it.

#2) The easiest way would be connect an SPST switch between the mix pot dry side and ground, so you either switch the mix pot to ground or to dry output.  Then the mix pot works like a volume pot on the wet signal.

Thanks for this solution.
I assume you mean a SPDT switch instead of a SPST ? Switching lug 3 of the mixer to either c17 or ground.

Alternatively, here's a more complicated (but more flexible) way to do it:
Check out this thread:
http://www.diystompboxes.com/smfforum/index.php?topic=113957.80

I read your thread last year, long before I bought the Surfy but revisited it again the past few days. But your problem is a bit the opposite of mine: too much fidelity of your dry signal, although with mix at minimum. I added a 3PDT switch + led to my surfy bear for true bypass so I don't really have this problem.

[samhay]

The other option is to use a dual-gang pot and wire it like a blend (between 2 pickups) pot in a guitar.

[Transmogrifox]

Even worse, because it's a tone control, it filters the dry sound so you hear the muffled part that you talk about because it's rolling off the highs more than it does the mid-band.  The mids and lows get a lot more in the mix because the output capacitor from the tone control is relatively small, so impedance is particularly high for low frequencies.

Do you mean the tone control affects the dry signal? I thought it only attenuated the highs of the reverb signal.
As far as the muffled dry tone goes, all of my googling lead me to the same explanation: dry signal + series resistance of the mix pot forms a LP filter with the capacitance of the cable to the amp. (Similar effect to turning down the volume on your guiter) But using a short patch cable into a buffered pedal and then 10ft cable to the amp didn't solve the muffled tone, so there must be more to it.

Yes, the tone control effects the dry signal.   Cable capacitance adds to this effect a little bit, but the tone control itself is an interactive network that effects everything coming out of the mix pot.  This is the reason many similar circuits would have a buffer on the tone control output, but this circuit came from a tube circuit originally.  It was probably considered too expensive to add another tube to buffer the tone control.   This interactive behavior is part of the original effect so it may not be considered a bad thing to some people.

The "rule of thumb" idea would be the 250k mix pot is so much larger than the output of the tone control that the tone control output impedance is small compared to the mix pot.  In reality, the tone control has significant output impedance compared to the 250k pot, especially at low frequencies -- in fact you don't roll off the lows much below 1.5 kHz so this mixer only really "mixes" frequencies > 1.5 kHz and it all changes depending on the input impedance of the next pedal or amp in the signal chain.  As a consequence the mix pot works like a tone control on the dry signal, rolling off the highs more and more as you go toward all-wet mix.

Whether this is a good thing or a bad thing, this is the reason you can't get 100% wet (0% dry). 

#2) The easiest way would be connect an SPST switch between the mix pot dry side and ground, so you either switch the mix pot to ground or to dry output.  Then the mix pot works like a volume pot on the wet signal.

Thanks for this solution.
I assume you mean a SPDT switch instead of a SPST ? Switching lug 3 of the mixer to either c17 or ground.

Correct, SPDT is what I had in mind.

Since we're talking about it I'll mention an SPST would work (it would be a bit hackish but it would work ok).  The consequence is the dry side output buffer would be driving the capacitor to ground -- most likely no harm would come of it.

I read your thread last year, long before I bought the Surfy but revisited it again the past few days. But your problem is a bit the opposite of mine: too much fidelity of your dry signal, although with mix at minimum. I added a 3PDT switch + led to my surfy bear for true bypass so I don't really have this problem.

I think you misunderstand the problem the OP was having.  The OP didn't like that the tone of the dry mix became more and more dull as you mixed in the wet, even when it was a small amount wet the dry tone was bad.  It took 2 or 3 pages in the thread for me to understand what the OP was asking for, but that's the summary of it.

If you don't mind, or even like that the dry tone gets rolled off as you mix in more wet then all you need to do is put the SPDT on the mix pot to ground the dry side when you want all-wet mix. 

As I mentioned above, this interactive behavior is part of the effect, but for the OP from the other thread it was a problem because he wanted reverb mixed in without the mix pot messing with the tone of the dry mix.

[j_flanders]

The other option is to use a dual-gang pot and wire it like a blend (between 2 pickups) pot in a guitar.

Thanks for your suggestion. This would be a too definitive mod for me.
I really like the fact that the creator of the Surfy Bear has tried to reproduce a solid state version of the Fender reverb unit as close to the original as possible, even if that includes the 'flaws'.
I guess it's about the philosophy behind pedals/modellers/amps. I like my Tech21 character pedals for example but they clearly went for tone instead of topology. Sometimes I wish they hadn't included the active EQ and opted for the original (flawed?) FMV tone stack. Same goes for the EQ in all the amp models in my Zoom G3. My Roland Cube on the other hand correctly emulates the original tone stack for each amp type setting.
Anyway, the Surfy Bear sounds great and authentic with my bright Jazzmaster but my super dark sounding Wildkat (even after all the mods I have done to it) doesn't like to loose even the tiniest amount of highs.
So, having the choice between stock and modded is kinda important for me.

Yes, the tone control effects the dry signal.   Cable capacitance adds to this effect a little bit, but the tone control itself is an interactive network that effects everything coming out of the mix pot. 

Ok, I see. Turning the tone pot with mix on max and dwell on minimum doesn't seem to affect my dry tone, which probably gave the wrong impression that the tone control (the entire circuit, caps + resistors) only affects the reverb part.

This is the reason many similar circuits would have a buffer on the tone control output, but this circuit came from a tube circuit originally.  It was probably considered too expensive to add another tube to buffer the tone control.

Apparently, for a very short time (1976–1978), there was a Fender Reverb unit with an extra tube for this:
http://forums.fender.com/viewtopic.php?f=12&t=91986

This interactive behavior is part of the original effect so it may not be considered a bad thing to some people.
The "rule of thumb" idea would be the 250k mix pot is so much larger than the output of the tone control that the tone control output impedance is small compared to the mix pot.  In reality, the tone control has significant output impedance compared to the 250k pot, especially at low frequencies -- in fact you don't roll off the lows much below 1.5 kHz so this mixer only really "mixes" frequencies > 1.5 kHz and it all changes depending on the input impedance of the next pedal or amp in the signal chain.  As a consequence the mix pot works like a tone control on the dry signal, rolling off the highs more and more as you go toward all-wet mix.

Whether this is a good thing or a bad thing, this is the reason you can't get 100% wet (0% dry). 

Like you said, it depends on the rest of the setup (pedals/guitar/amp) and the tone you're after, which is why I prefer an extra option instead of a definitive mod.

[Transmogrifox]

My solution:
Use my Boss LS2 to do the summing/mixing myself. This way I keep my original dry tone and I can mix in the wet part myself.

And this is a good solution now that you have a dry-kill switch on the surfy bear.

[PRR]

> Why can this circuit do 100% dry but not 100% wet?

What you said: "it's true to the original Fender 6G15".

Why would you want wet-only?? (Remember it is 1961, guitarists do not have mixers.)

If you want to be all-wet, break the connection on the dry side of the mix pot. Where C17 goes to R28. One switch. (Just remember that if you want dry, you need to flip the switch back, or play air-guitar.)

[j_flanders]

> Why can this circuit do 100% dry but not 100% wet?
What you said: "it's true to the original Fender 6G15".

It was merely a technical question. As far as I could tell this wasn't on purpose or by design in the original, but more of a side effect. I wondered what caused it. Transmogrifox explained the cause.

Why would you want wet-only?? (Remember it is 1961, guitarists do not have mixers.)

Because the dry signal mixed in gets dulled severely if you turn up the mix.
And it also gives the option to add effects to only the reverb part, like adding a pre-delay, or modulation like chorus.

Remember it is 1961, guitarists do not have mixers.

The Vox standalone reverb unit (1962) had a 100% wet option...
http://www.voxshowroom.com/uk/misc/reverb.html

If you want to be all-wet, break the connection on the dry side of the mix pot. Where C17 goes to R28. One switch.

So, yes, this was my actual question: do I simply break the dry side connection or do I connect it to ground, and what effect would both options have (the load for the wet side for example and the interaction with the preceeding caps of the tone control) .

Anyway, I did the mod a few days later and also did the c10 mod on a switch and have been playing and experimenting with it for a couple of weeks.
It definitely sounds more 'hifi' and maybe less authentic. I like both though, depending on the guitar and the sound or tone I'm after.

Correct, SPDT is what I had in mind.
Since we're talking about it I'll mention an SPST would work (it would be a bit hackish but it would work ok).  The consequence is the dry side output buffer would be driving the capacitor to ground -- most likely no harm would come of it.

Oddly enough I ended up buying the 'wrong' switch: SPST instead of SPDT.
And it wasn't until I was wiring it up that I understood what you meant. Like you said, I can simply connect the bottom lug of the mix pot to ground. This turns the mix pot into a volume control for the reverb side and at the same time connects/mutes the dry side to ground.

One last question:
Are all the pot/lug numberings wrong (reversed) on the schematic or why are they numbered like this?