New Guy, New Circuit

[baldrich]

Hi,

I’m new to the forum and have drawn up a simple circuit today I’m hoping I can get a critique or three. But I thought I’d introduce myself first as the new guy.
I had a few analog circuit courses in college 30 years ago but never used it in my professional career so most of that knowledge was lost with time. Several years ago I attended a 2 day amp building class given by Bruce Egnater and it sparked my interest in tube guitar amp design. I’ve been a full-time RVer for a few years now so I don’t have the tools and equipment with me to work on anything anymore but want to get back into circuit design as it is great mental exercise now that I’m retired and slowly loosing brain cells! I recently started studying guitar effects hoping to build a few stomp boxes which is more feasible for me than working on big amps; given my space limitation.
I’ve started out pretty simple with a circuit I’m eventually going to use. I will have a need for a simple signal splitter that I can send one guitar signal to my amp and a second to my USB audio interface. The interface has a hi-z instrument input and I’m hoping to be able to record a raw guitar track, while also playing through a small amp, without the need to purchase or build a DI box. The recorded track will ultimately be used to reamp as part of my effect circuit breadboard tweaking.
So, I started with a Klon like buffer circuit and essentially duplicated it twice. I was certain the unity gain design would actually give 2 unity output signals so I built in a gain of 1 in each half of the op amp. I’m not totally confident that I chose appropriate resistor and capacitor values where I modified the Klon circuit.
I attached the Klon schematic I used and my final schematic. I’d love some feedback on whether this is going to work. I’m looking forward to picking up my tools, breadbord and goodies when I’m back home for Christmas so I can try it out.
It’s good to be on here and I really look forward to any feedback.

Thanks so much!

 

[Khron]

Welcome (back) to the field!

First of all, you'll want to stick with a JFET-input opamp, like the TL072. The NE5532 has a very low input impedance, which won't play all that well with passive guitar pickups.

Second, instead of having two such buffers in parallel, you'd be much better off having a single buffer (like the Klon's), and have the output of that, on one hand, go to the guitar amp, and on the other hand, drive a second buffer (plain buffer is enough, not sure there's that much need for that 2x gain in your schematic). This second one could have an impedance-balanced output to go into the line input of your interface (and thus, be quite cable-"agnostic").

https://sound-au.com/project87.htm#cheat

 

[gyraf]

Welcome!

 

[Newmarket]

As Khron said - no need for parallel buffers that means your input impedance is halved. And yes - use a jfet input opamp. TL072 is the 'goto' classic here but you can do better wrt noise / distortion with eg OP2134 ort loads of other alternatives (note many newer devices available in SMT only in case that is an issue).
And I'd say make both outputs impedance balanced. Nothing to lose for the sake of one resistor and an extra cap and resistor if "going the whole way".
Or whatever flavour of balanced output you prefer - a well worn topic round these parts. But impedance balanced keeps it super simple.
Also - you may want to think about "ground lift". The parallel R//C circuit works well ime. See Bo Hansen DI for example.

On a technical note - the input impedance of a NE553x isn't really a problem - the datasheet input impedance gets multiplied by the op amp gain margin so it' still - to use a technical term - "very big" - and the actual impedance remains defined by your external resistor value. BUT it's not a good choice for a passive guitar / bass pickup source. With a high Z source current noise becomes more important than voltage noise. A look at the respective figures for the opamps should tell the story.

 

[abbey road d enfer]

TL072 is the 'goto' classic here but you can do better wrt noise / distortion with eg OP2134

+1

the actual impedance remains defined by your external resistor value. BUT it's not a good choice for a passive guitar / bass pickup source. With a high Z source current noise becomes more important than voltage noise.

+1

 

[mhelin]

With TL072, high level input signal and low rails you might encounter problems with the phase reversal. TI's current recommendation for musical instruments and pro audio are probably the OPA165x and OPA164x (FET-input) op amps, so you might need a SOIC-8 to PDIP adapter PCB depending on the footprints you have chosen to use in your design. Single stage op amp is enough, it can easily drive the rather high impedance amp input and the sound card input.
How is this going to be powered? If your audio interface comes with P48 phantom power that would be handy to use as power source even though you were not planning to build a DI.

 

[Ricardus]

Welcome!

 

[baldrich]

Thanks so much for the greetings and help. Especially Khron For the very helpful article link.
mhelin, I was going to power it with a wall wart but, yes, the interface has a xlr balanced input with phantom power. I never thought of using that as my power source.
Thanks again!