A "Transparent" Buffer?

[Transmogrifox]

I would vote for getting rid of the buffer on the guitar side.  You can quickly evaluate the result just by connecting the dry side of the pot to the input jack.  If it sounds good with the 250k impedance to ground loading your guitar then you're all done.  I don't see the buffer as being absolutely necessary and in your case might be less change in your tone with the parallel 250k pot to ground than what the buffer does by decoupling your guitar output impedance from your specific amp.

Ideas for rolling off highs going into the buffer (making it look like an amp's input) may have some merit if you're not very picky. 

My thought is it's easier to get rid of the buffer and let your guitar interact with the amp and cable if that gives you the sound you like.

but...if what you really want is a "Transparent buffer":
http://www.geofex.com/FX_images/Onboard_Preamp.pdf

[Loose]

I followed Transmogrifox's advice and got rid of the buffer. I did this by removing C12 and connecting terminal 3 of the mixer pot to c12, thus bypassing the buffer.

Now I have a slight drop of output, the wet signal bleeds into the dry signal when the mixer is at zero, and there's feedback when increasing the dwell or tone controls.

Transmogrifox: what did you mean by having the 250k pot to ground? Wouldn't I need an opamp mixer section for that?

Thanks

[Transmogrifox]

Now that you mentioned feedback I can see my folly.  I failed to consider the guitar output impedance.  However this helped me realize the problem you are having with the buffer.

I downloaded the PDF so I could actually read the part values in the schematic around the buffer and now I see why it's a problem.

Here's what I expect will greatly improve how the full-dry mix sounds:
1)  Change R31 to 10k or less (less is better and you should go down to maybe 3.3k before I start to get concerned about the JFET being overstressed).
2)  Change C17 to 1 uF or greater to get its AC impedance down.  Basically the 100 nF capacitor is an impedance you can't ignore for dividing lower frequency content.  Granted the output capacitors are small so lower frequency content is cancelled there, a rough calculation shows low frequency feed-through at -47 dB, which will be audible and give you perception of the buffer not sounding transparent even if the buffer output impedance was zero.


If that doesn't greatly improve Dry sound, then you will need to replace the buffer with an op amp and make C17 more like 22 uF.  The op amp will give a lower output impedance and is certain to make the FX side inaudible.

A way to test the mod is to remove C12 and play with the mix knob.  If the mix knob acts like a volume control, and you can't hear any bleed-through coming out of the reverb side, then you will know the buffer output impedance is low enough.

Then when you're convinced wet signal is totally inaudible at full-dry then you can pop C12 back in and confirm the dry signal sounds normal when mixed all dry.

[amptramp]

Your buffer output impedance is the impedance of C17, the output capacitor of the buffer in series with the parallel combination of the buffer source resistor, R31 and the output impedance of the FET itself which is close to 1/transconductance.  You might be better off with an op amp buffer set for unity gain (where the output impedance would be on the order of a few ohms) and a larger capacitor for C17.  What you are trying to get is a very low output impedance so when the mixer pot is at the buffer side, the buffer swamps all signal coming from the reverb side.

[Loose]

Thanks for the replies.

I was thinking about how transparent my Mad Professor Deep Blue Delay's buffer sounds, and I also like that there is no mix ratio control, but a wet level control (the dry stays unity).
So I want to incorporate the opamp buffer and mixer circuit from the Deep Blue into the Surfy Bear circuit and I have a few questions:

1. Do I need to modify any of the Deep Blue buffer and\or mixer component values considering the Surfy Bear is a 12v circuit?
2. The input capacitor and resistor in the Surfy Bear are 100n\10k and on the Deep Blue they are 22n\180k. Taking that into consideration, do I need to modify any of the Deep Blue buffer component values, Or should I change the Surfy Bear input capacitor and resistor values to match those on the Deep Blue?
3. Should the buffer output go to Q4 like it is on the Deep Blue?
4. Can I actually skip the buffer all together if i'm using the mixer opamp section?


Thanks!

[Transmogrifox]

R2=R3=470k
C16=15pF

R4=R5=R17=100k
C20=82pf

Mix Pot = 250K per surfy bear schematic
C14 = 2x220pF per surfy bear schematic

Surfy Bear input connects to IC1A pin 1 instead of C3.

You can leave C1 as 22nF

Keep in mind this will change the sound of the Surfy Bear because the Mix control as designed on the Surfy Bear has the small-valued capacitor which acts as a high impedance to low frequencies.  So at the guitar's lower frequency range it is getting close to unity gain.  Cut-off is around 1 kHz, so guitar mix ratio is pretty much unity with full wet mix even at A440 fundamental.

The Surfy Bear mix output works as a low-pass filter on the dry signal and high-pass filter on the wet, so it's like a crossover between wet and dry. 

When you apply the op amp mix where the guitar is unity gain across the frequency range then you will lose this effect.

This is where a breadboard can help you.  You can try this stuff and see what you like better. 

Your root problem as amptramp more concisely pointed out is the output cap impedance + buffer output impedance being too high to let you get a 100% dry mix.

My prior post gave you a solution to get the wet mix down to -60 dB, which to me is just as good as inaudible (1000:1 Dry to wet ratio).

If you want to do better than -60 dB on the wet then you need to use an op amp buffer as amptramp suggested.

Either way I don't think the buffer is the problem, it's that even at full dry crank on the pot you're not 100% dry, and that comes along with a little bit of high frequency roll-off along with an audible amount of reverb which all sounds wrong when you hear it.

To determine whether there is anything bad sounding about the JFET buffer you could lift the Surfy Bear side of the mix pot and you should be able to hear an audible difference in the sound.

In some ways I wonder if a stomp switch for true bypass wouldn't be an easier way to take care of this problem.  Really when you're using reverb, you want reverb so it won't matter if you can't get 100% dry mix, and the easiest way to turn off reverb when you don't want it is to stomp a switch.

[Loose]

R2=R3=470k: Do you mean that both R2 and R3 in the deep blue schematic should be 470k?

C14 = 2x220pF per surfy bear schematic: I don't understand this one, in the surfy bear schematic these connect to one side of the mix pot, but in the deep blue the cap goes at the mix pot output.

The bypass signal isn't the problem. I also agree that it doesn't matter if I can't get 100% dry mix, I was just mentioning it as one of the side effects of removing the buffer. What matters to me is that the dry signal doesn't change when the effect is on, and that the dry signal will stay at unity when the wet signal volume goes up.

[Transmogrifox]

R2=R3=470k: Do you mean that both R2 and R3 in the deep blue schematic should be 470k?

That is correct, both R2 and R3 are 470k. From that you can follow the same logic on the other references made this way.

C14 = 2x220pF per surfy bear schematic: I don't understand this one, in the surfy bear schematic these connect to one side of the mix pot, but in the deep blue the cap goes at the mix pot output.

Yeah, this was a confusing way to say what I was thinking.  Try this:

Eliminate the deep blue cap C14 and mix pot.  Connect surfy bear mix pot output to R17.  The grounded side of surfy bear mix pot needs to be connected to "VB" instead of ground.  I hope that makes more sense.

Also you can make R17 smaller if you find max wet doesn't give you as much effect in the mix as you need.  Conversely you may find it allows you to mix in way more 'verb than you will ever use and you might make R17 larger to extend the useful range of the pot.

The bypass signal isn't the problem. I also agree that it doesn't matter if I can't get 100% dry mix, I was just mentioning it as one of the side effects of removing the buffer. What matters to me is that the dry signal doesn't change when the effect is on, and that the dry signal will stay at unity when the wet signal volume goes up.

Ok, makes sense now.  What is bothering you is how the dry signal changes when reverb is being mixed in.  This interaction between the surfy bear output impedance and what it does to the dry signal as you mix in more reverb is part of the emulation of the original amplifier circuit, so it's working as-designed...but you tried it and you don't like it.

You are on the right track with the deep blue mix circuit to keep the dry path unaffected while you adjust the mix.

The mods as I suggested will get you close to where you want to be.  Maybe some tweaking to set your maximum wet mix if you find a lot of the range of pot motion is not useful.

[Loose]

Thanks for all of the tips man!

Could you please explain the reasoning behind all of these changes? I would like to have a basic understanding of what's going on if possible.

And if you could also explain why the pot should go to VB instead of ground.

[rumbletone]

If you are ok with the tone of a more neutral OP amp buffer circuit, perhaps you could just add a switch to kill the dry path and run it 100% wet in a parallel fx loop - which, if you don't already have one, you could easily build using a simple op amp buffer/splitter/summer circuit? Might be easier than re-engineering the surf bear clean path....

[Transmogrifox]

R2=R3=470k
C16=15pF

R2 is the input impedance.  This mod allows you to increase input impedance so you aren't loading down the guitar pickups quite as much. 

If R2 is 470k, then R3 needs to be the same value to keep the unity gain relationship.  The deep blue boosts the signal to the input of the PT2399, probably to reduce signal/noise ratio.  You don't need to boost input to the Surfy Bear so it makes more sense to keep it all at unity.

C16 sets the cut-off on the high end.  When you increase R3 then that roll-off moves down in frequency.  I changed it to a 3 dB cut-off of 22.6 kHz so it doesn't have any audible effect on the guitar sound but still works to suppress incoming high frequency (like Radio or EMI from fluorescent lights that can couple onto your cable).  This high frequency stuff tends to get demodulated into the audio band so the more places you suppress it the better.

R4=R5=R17=100k
C20=82pf

R4 and R5 need to be the same value to maintain unity gain on the dry side.  From above, the deep blue boosts dry then this stage cuts it to restore it to unity.  Again, since you don't need any boost, then you just keep dry at unity all the way through.

I selected 100k for R17 so the surfy bear output doesn't see an excessively low input impedance.  The value of 100k makes the surfy bear wet channel approximately unity gain between input and pot wiper output when the pot is set near center.  Then setting R5 to 100k makes it approximately unity gain through to the output of the effect.  Then setting R4 to 100k make the dry side unity.

So in the end the goal was that the surfy bear gives unity gain on the wet and dry channels when mix is set to center.  Turning mix to the right gives a little boost on the reverb, and turning to the left gives some cut.

Now if you change R17 to 470k, then max wet is about unity, and turning mix down is all cut.  From there it's up to you where you think it sounds best to max out the wet mix but you can now do it independently of the dry mix.

Mix Pot = 250K per surfy bear schematic
C14 = 2x220pF per surfy bear schematic

As explained above this is just a less clear way of stating that the output of surfy bear plugs in at R17.

The 250k pot and capacitors are all interactive with the output of the tone control.  Not a good idea to feed the tone control into a 50k pot and expect it to sound the same as this whole output network interacts with the tone control frequency response.

[Loose]

rumbletone: Thanks, that's a good idea which I considered before, but since I don't have a parallel looper I would rather have it built into the pedal than add another pedal.

Transmogrifox: Thanks for the explanations. Everything sounds pretty clear besides the 250k pot going to VB instead of ground, i'm not sure I get that one.

Also, does it make a difference that it's gonna run on 12v?

[Transmogrifox]

Oops, forgot to explain the pot going to Vb.

This is a single-supply circuit and the op amp inputs need to operate at something referenced somewhere between ground and Vcc.  Vb divides Vcc/2 so you end up with a "virtual ground" that's used to reference all op amp inputs to 1/2 the supply.  With 12V in, Vb looks like ground and the supply to the op amp is like +/-6V.

If you connect the 250k pot to ground then that is like pulling that input to -6V.  Then the op amp would respond by driving the output to +6V.  Since these usually limit about Vcc-1V, then the output will be pegged at about 11V (or +5V with reference to Vb).  In that state there's no space for the AC audio signal to wiggle.

If you tie the 250k pot to Vb then it will be at a voltage equal to where the op amp "+" input is referenced (VB).  Then the output of the op amp will be stuck at around 6V and there will be 5V headroom on either side for the AC audio signal to wiggle.

And no, it won't make a difference that it will run on 12V.  It would probably be ok on up to 20V with most op amps.

[Loose]

I'm breadbording the Deep Blue buffer with your suggested modifications right now.
I noticed that everything in the Surfy Bear schematic goes to +12v or VDD. Should I still connect the IC pins and the mix pot to VB as per the Deep Blue schematic? And if so, is VB on the Surfy Bear between R4 and R9 (where it says 75mV)? Or should I connect two 10k resistors in series from VDD to ground and the point in between them would be VB?

Thanks

[Transmogrifox]

There is nothing on the Surfy Bear that you can use as a Vb.  You will need to make a VB as per the Deep Blue schematic.

And yes, it looks like VDD is the right place to tap power to the opamps as well as the VDD resistor divider. 

The +12V higher up the power supply chain is the "dirty" power that gets chopped up by the power amp driving the reverb pan.  VDD filters this so the small-signal (preamp) stuff doesn't become a noise and feedback loop.

[Loose]

I've drawn a schematic, does it look right to you? Especially the VB part, do I need to add caps or should the two resistors be enough?

https://drive.google.com/file/d/0B4lW4vfWZ87wRHJhQUNHanpKZGs/view

BTW is it not possible to upload attachments here? I had to upload the file to google drive.

[Transmogrifox]

You will want to include the 47 uF capacitor on VB from the Deep Blue circuit.  Your R8 and R9 should be 10k just so op amp input offset currents don't move VB from where you expect it to be (100k is probably ok, but 10k would be according to a best practice).

Where you have "C18" marked on on Mix pot should be going to C1 and C10 from the Surfy Bear schematic.
http://cackleberrypines.net/transmogrifox/Misc/surfybear-jfet-reverb-r3.pdf

Otherwise looks good to me.

Can't upload attachments here.  Better to use a service like imgur or something.

I get an error from Firefox on your google drive link

Code Select Expand

Your connection is not secure

The owner of drive.google.com has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.

This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate.

So I cannot view the schematic with Firefox.  I did some hacking in the script and managed to upload it to my server for benefit of all.

Do you have the ability to copy a direct link?  Then you can post it directly into your post in the img /img brackets and it can be viewed as I have done above.   Just do "quote" to see the BBCodes. 

[Loose]

C18 was a mistake, looking at the schematic without zooming in makes C10 look like C18 Thanks for spotting it!
100k was also an accident, should have been 10k.

I will test the breadboard and if it sounds good i'll find a better way to post the updated schematic.

[Loose]

BTW, does any of the opamp pins need to be grounded?

[Transmogrifox]

If you're using a dual op amp then you would be using all the pins and the only pin grounded would be pin 4, ground.  I would guess if it sounds good on the breadboard then you have power and ground connected correctly.

Pin 4 ground, Pin 8 VDD on a typical dual op amp package. 

If you're using a quad or 2 duals for some reason then the best way to handle unused op amps is to tie Vout to IN-, then IN+ to VB.  Basically makes it a unity gain buffer on VB.  Keeps the unused op amps stable.

The recommended way would be a dual op amp (like a TL072), one op amp is the input, the other is the mixer/output.  VDD to pin 8, ground to pin 4.