[Beginner]My first pedal ... not for a guitar

[antonis]

The key is to get a basic thing that works and try out the options.

Nothing more - nothing less..!!

[DaPloup]

Thank you all for your input 

Looking closer to the specs of my recorder and sampler, not only should I send "line level" signal but also ... balanced

I guess we are no longer talking about a (not so) simple mixing preamp, but a mixing DI, now ...
I wonder if a JDI from Radial Engineering would be a better choice ... but less fun as far as DIY is concerned :icon_lol and I have other plans in the future based on the DIY design (more than 2 inputs), which the JDI cannot handle.

I guess one other advantage of the DIY mixing-preamp is the opportunity - with preamp set to unity - to still be able to use the pickup with standard guitar-like pedal rigs (effects in-line with instrument, before mixer and PA), right ?

Then sticking to mixing preamp and adding a DI afterwards should do the job ; I was just hoping I could clear a bit of the mess in cables and boxes...

Regarding the pickups, still no info on the impedance but a user told me the pickup signal is hot ; the signal does not need as much preamp as a SM57 ... which is not really a surprise I think. He's put his pickup through all sort of multi-effet guitar pedal with various PA systems behind and has never had a weak signal.

[antonis]

So, do you wish for an active or passive balanced mixer..??

[DaPloup]

So, do you wish for an active or passive balanced mixer..??

Not sure I understand why you ask this question 

But I guess that as in the end I need line level (which means the DIs don't match by the way ... stupid me), I will need an active balanced mixer. Right ?

What is sure : I need a mean to combine 2 mono unbalanced signals (maybe more in the future, but we'll keep that for later) and raise them to line level delivered on a balanced cable. This should be the prime function of the "box".

This means however that I would have no other option than use effects at line level.
In my current setup, not an issue.
Not an issue if I use guitar pedals than can be switched to line ... but not all pedal allow that. I guess life is a matter of compromise

[anotherjim]

You could add a pair of jack sockets to make a send-return FX loop between the mixer stage and line amp driver stage. Switched sockets can automatically route in or out depending on whether a plug is inserted.
Use the mixer pots to set the level to suit any FX in the loop and then have a master volume pot for the line amp to boost as needed to line level.

Some of the design ideas you need can be found already done in existing designs. Plagiarize!

At the top is a good example of something very necessary in a single power supply circuit. It divides the supply voltage in half and in this case is called Vcc/2 AKA Vb, or Vref or Ub. Note the parts around the opamps that connect to Vcc/2 and not ground as you find in most general opamp tutorials as they assume a second negative voltage supply - which we don't normally have.

IC1a is an example of a guitar input buffer. R1 provides voltage bias from Vcc/2 and at 1M sets the input impedance to 1M which is guitar gear standard.

IC2b is an inverting amplifier. If you add extra resistors feeding the -input pin6 it's also a mixer.
Note the direct connection to Vcc/2 on pin5. Tutorials show a resistor making the connection - you do not need to.

On the lower right there are 3 opamps. One is a buffer that drives a normal to guitar amp output jack while 2 form a balanced line XLR output. If you want this facility copy it.

[DaPloup]

You could add a pair of jack sockets to make a send-return FX loop between the mixer stage and line amp driver stage. Switched sockets can automatically route in or out depending on whether a plug is inserted.
Use the mixer pots to set the level to suit any FX in the loop and then have a master volume pot for the line amp to boost as needed to line level.

Indeed, that would be uselfull. I was just unsure that such a setup was easy enough for a beginner. But the way you describe it makes it easy
Now I need to find an existing schematic including such switched sockets to see how I can add them to my design.

Some of the design ideas you need can be found already done in existing designs. Plagiarize!

That's exactly my thinking. Why re-invent the wheel ?
The things that make it difficult for me are, first to find existing schematics (but this forum is a form of answer), then be sure that the circuits drawings I find are applicable to the specifics of a guitar pedal design (9v power ...) and last but not least I need to understand which section of the circuit does the function I need.

For example you mention IC1a as an example of a guitar input buffer. I assume that the buffer circuit starts from the input connector, but where does it end ? Is it right after the op-amp including the loop from pin 1 to 2 ? or should I also include part of the circuit afterwards ?
Maybe I need to spend more time looking at existing circuit designs on this forum to understand what is required and what is not. 

Anyway, thanks a lot for this circuit example, it's really helpfull 

As my music project evolves, so is the stompbox list of requirements ... I think we're now talkin of 4 inputs on the original design

Current LoR :
- 4 instrument inputs with input level adjustment and if possible one mute switch per input
- an instrument level FX loop with send and return
- a preamp section with level adjustment to line balanced output
Sounds complicated enough for a start 

[antonis]

For example you mention IC1a as an example of a guitar input buffer. I assume that the buffer circuit starts from the input connector, but where does it end ? Is it right after the op-amp including the loop from pin 1 to 2 ? or should I also include part of the circuit afterwards ?

You have to study about basic op-amps (or BJTs or FETs) configurations served as "building blocks"..
(e.g. IC1A as a bare buffer (voltage follower) is compromised only by pins 1, 2 & 3 but in practice it also needs R1 and C3 - R2 is optional for some particular designs..)

After some degree of consolidation, you'll be able to recognize, understand and settle on any particular requirement..
(e.g. is C4/R4 configuration essential for reasons other than forming a HPF..??)

P.S.
IMHO, you can't plagiarize so easily as you might suppose, even for theoretically "identical" instances..

[DaPloup]

Thanks once again for your help and comments, it's motivating

I first needed to know exactly what I want to achieve and I think the last LoR is the right one.
Based on this, I decided to try to work my way following the signal path.

Here's where I am now :


I understand that as we are working with 9V DC we need a "power section" (in blue) to create the VCC/2 "bias" required by some of the op-amps later on.

In red is the "input buffer section" as I understood it. Not sure about the supply of the op-amp, though.

In green is the "input gain" section to manage the mix between input signals.
As mentioned earlier, ideally I would like to mute each individual circuit. I could achieve that with the gain pots but I guess a mute button for each track would make it easier. I'm not sure what is the best way : a simple switch on input section (before C3) or something more elaborate in the green section ?

Inst. Buff 2 to 4 are just replicates of the red+green sections for other inputs.

Then comes the "summing section" in black. At the end of this stage, I have a summed inverted signal of the inputs.

Next section is supposed to be the "FX loop section" but I need to find examples first.

Any comment on this first part is more than welcome 

[DaPloup]

You could add a pair of jack sockets to make a send-return FX loop between the mixer stage and line amp driver stage. Switched sockets can automatically route in or out depending on whether a plug is inserted.

Seems that the online tool I use to draw circuits does not have audio specific symbols so I can't design the solution you describe. Would it be as simple as 2 SPST switching jacks in series (like 2 conductors closed tip jacks) ?
When nothing is plugged, signal should just go through the "tip loops" (if that's their name  ) heading to next section of the pedal.
When a jack is plugged in the Send connector, link is broken between the "tip loops" so that the signal goes in the FX loop and back through the Return connector to go to the next section of the pedal.

But I guess the send and return connectors should also be buffered in some way, right ? 
Or it is that simple ?

Edit : rough sketch in the orange section. Can't think of a better way to draw it now

[DaPloup]

Gave a try to the Balanced output section (black section at the bottom) but I guess there are mistakes ...



PS : if you have recommendation for free online tools to draw such circuits ... https://www.circuit-diagram.org/ is rather limited

[ElectricDruid]

Current LoR :
- 4 instrument inputs with input level adjustment and if possible one mute switch per input
- an instrument level FX loop with send and return
- a preamp section with level adjustment to line balanced output
Sounds complicated enough for a start 

Uh-Oh! Feature creep! The death of many projects!

[DaPloup]

Uh-Oh! Feature creep! The death of many projects!

Indeed, that's a lot of features for a single box ... and above all a first one

That being said, the basic LoR didn't change : I need to mix various inputs together (min. 2) and convert them to line level.
If adding the mute / gain per input creates too much complexity, I can live without. I'll just hope that when I'll use only part of the inputs, I won't get noise or whatever coming from the other inputs. As for the FX Loop, it's indeed an add-on that may need to be removed if too complex to implement.

I could also separate each section of the project in one specific pedal if easier :
- a summing pedal with input management (mute / gain)
- a preamp pedal for signal conversion to line level
With these two I could insert the optional FX loop in-between if/when required...

Might be a better way to proceed ...

[DaPloup]

I tried to combine everything in a single circuit as best as I could ... 
Couldn't find the mono jacks in the software's library so please ignore the "sleeve" pins on each jack.
I used the values from other schematics for some of the components but most of them are just labelled as I can't guess the correct values



I guess it's far from being realistic
Any help more than welcome.

Have great weekend.

[DaPloup]

Uh-Oh! Feature creep! The death of many projects!

I've been thinking about this and ... finally found a kind of workaround 

Looks like part of my LoR can be addressed by using one of-the-shelf pedal.
I discovered today the JHS Clover pedal which takes an unbalanced instrument input and can convert it to line balanced
It may not be the only pedal capable of doing so but I didn't know there were such pedals on the market 

This means that points 2 and 3 of my LoR are no longer required.

Current LoR :
- 4 instrument inputs with input level adjustment and if possible one mute switch per input
- an instrument level FX loop with send and return
- a preamp section with level adjustment to line balanced output

We're then back to a simpler project I think (but already way more complicated than I thought  ) with a summing pedal for 4 inputs (maybe 6 ) with mute and level adjustment per channel. I guess it will also require input / output buffers...

I've seen some existing designs of splitter/blender pedals with 3 inputs (like this one). I'll try to design something based on these with only the blending part, I guess it will make more sense.

Thanks for challenging my complicated project, it made me find a different route

[PRR]

...discovered today the JHS Clover pedal which takes an unbalanced instrument input and can convert it to line balanced It may not be the only pedal capable of doing so but I didn't know there were such pedals on the market  ..........

"DI", "Direct Interface box".

The Clover is a $50 tone control plus a $50 (at most) DI with a $100 box and marketing.

You can get functionally fine DIs for $150 VERY fancy, $80 fancy, $40 generic, $23 on sale, $27 dual.
https://www.amazon.com/Premium-Direct-Injection-Audio-Box/dp/B0027V760M
https://www.amazon.com/Premium-Direct-Injection-Audio-Box/dp/B00356J8KE

Dr Wolfrum invented a DI in Detroit in the late 1950s. www.youtube.com/watch?v=fp2i7ag3tFk

[DaPloup]

"DI", "Direct Interface box".

Most of the DIs I could find (like Radial Engineering ones) only convert the intrument level input down to mic level which is not what I need for my setup (balanced Line in only)

To be honest, I found a second hand Clover not far from home and the extra tone control could bring something to the setup. But indeed, there might be cheaper options on the market ... I'll keep on digging

[DaPloup]

Up with my project, it's been a while

I finally got my piezo pickups and took a bit of time to test them with my current equipment to try and see what could be the impact on the blender / mixer I need.

I was surprised that I couldn't have them work properly with my soundcard, my recorder or my looper/sampler : I had a terrible hum as soon as the pickups were plugged in (directly or through the JHS Clover) and the signal was a bit weak, some of the notes couldn't go through correctly. Hum was amplified as I raised gain.
The only way I could get a clean signal was by connecting them to a guitar amp. Any other destination would generate a hum.

Took me quite some experimentation to find that it may be an impedance issue.
When I connected them to a mixer with super hi-z input (10MOhms, not 1, 10 !) I could hear a clean sound and send it to my looper/sampler without any issue.

I thought the JHS Clover and its 1MOhm input would be sufficient ... but sadly it's not. Piezos are indeed another beast.
Not sure why it works well with my "noisebox" (just another piezo within a case with various bits and bobs on it) and not with the pan pickups ... but that's how it is.

When I looked at the specs of the guitar amp I had access to, seems that the input impedance was 2.2MOhms.
Don't know if amps are any different than other equipment but it could mean that 2.2 would be enough for my signal chain to remain clean ... or not.

I guess my blender/mixer pedal now needs to include super hi-z input like 5M or 10MOhms but I don't know how I could confirm that easily.

[antonis]

I guess my blender/mixer pedal now needs to include super hi-z input like 5M or 10MOhms but I don't know how I could confirm that easily.

Figure 6..

[DaPloup]

Seems that I was wrong 
Impedance may not be the culprit behind the disturbing hum.

After my (only) success with my piezos using a small mixer equipped with 10MOhms inputs I looked at possible alternatives which could fit a more compact / pedal board type setup.
I discovered the Radial Engineering PZ-Pre, a pedal with 2 piezo compatible inputs, possibility to blend the signals, EQ/tone correction and DI. Seemed like a good idea to give it a try before I could go further with my built.

Had the opportunity to test it...but result is a complete fail : terrible hum is back 

Don't know what could explain the mixer gives proper results and not this pedal. I guess the mixer's circuit incorporates an element that gets rid of the hum.