GSSL pedal mods

[earthsled]

Hello DIY-ers!

I have decided to build a compressor pedal using the GSSL pcb set. I'm excited to try this variation, I think the project will be fun, educational, and great for bass! It looks like the GSSL components will fit a Hammond 1550G box with room for a small internal power transformer (Hammond 187C36). It's a big chassis for a pedal, but good enough for my experiment!

My first question is about how to modify the input and output sections to be unbalanced and proper impedance/level for instrument input and amp output. It would be great if this "mode" could be switchable so I could select line level mode vs. instrument/amp level mode.

The input section of the GSSL uses 5534 op-amps. Is there a way to adjust the resistance values around these ICs for unbalanced instrument level? Do I need to use different op-amps for this kind of input? Similarly, could I adjust resistance values around the 5532 op-amps in the output section to make the signal unbalanced and proper impedance/level for a bass/guitar amp? Or, maybe some kind of pad can be added to the output circuit for this?

My second question is about the GR meter. Ideally, I'd like to use an LED array or bar-graph for this pedal as the LEDs will be more durable than a traditional moving-coil meter.  Can I use a VU-type LED meter with the GSSL pcb? Maybe something like this CEBEK VM-20 kit http://www.mcmelectronics.com/content/ProductData/Spec%20Sheets/28-5185.pdf ? Or, is it possible to design a simple 12-segment 1mA meter with LEDs?

Thanks in advance for your help!
Cheers!

 

[Di_Sousa]

This can be a great idea.
Probably it would be better for you to redesign a pcb for it with modified input and output.
You could have a much smaller pcb for that since the GSSL is a Stereo unit, and for a bass pedal you will only need a mono channel.
So the components and space needed would dramatically decrease.

 

[earthsled]

Whoops, thank you for your post!

I apologize that it's taken me so long to reply. Other projects have taken priority lately, so the SSL comp pedal has been put on the back burner.

You have some excellent points about the project! Indeed, re-working the PCB into a smaller size is an eventual plan. There are plenty of components that can be discarded (especially if I don't plan to use the DBX VCAs). Converting the circuit to mono is also a great idea!

For now, I'm going ahead with building a working prototype in pedal form. Once this is up and running, I'll post pics and continue the discussion(s) about changes to the inputs/outputs and using an LED meter.

I appreciate your encouragement!

 

[Di_Sousa]

Use the THAT VCA

unbalanced input and output

Mono

Probably you can simplify the controls also

 

[Di_Sousa]

Check out this project, maybe you can take some ideas from here.

It's a transformerless 1176, it's small enough to fit in a pedal box.

You can make it even smaller if you have an external PSU.

http://groupdiy.com/index.php?topic=61719.msg788537#msg788537

 

[john12ax7]

For the input change the configuration from a differential receiver to a non-inverting unity gain buffer with 1M input impedance, this is what you would typically see in a guitar pedal.

Also using a jfet opamp would be better with the higher impedances.

 

[earthsled]

Thanks john12ax7 and Whoops for your posts!

Whoops — the tranformer-less 1176 PCB thread is cool! Have you seen the Cali76 pedal? It is certainly an inspiration for this SSL comp pedal idea. Thankfully the SSL doesn't have any transformers to begin with!

john12ax7 — thanks for your suggestions regarding the input! I am certainly no expert when it comes to op-amp circuits, but I do understand that a higher input impedance would be best for guitar/bass. JFETs are typically more responsive / sound better too.

I'd love to find a way to keep the pedal dual-purpose so that the input would have a switch for "line" vs. "instrument" and the output would have "line" vs. "amp." This way, users could use the pedal in the studio as well as live performance. The Eventide Modfactor and Timefactor pedals have an i/o setup like this (image attached). I'd love to know how there're accomplishing this switching in the circuit. ???

Here are some helpful threads regarding the LED GR meter...
This one: http://groupdiy.com/index.php?topic=1996.0
mentions that the Gyraf SSL requires a meter that has a "DC movement."
This one: http://groupdiy.com/index.php?topic=9634.0
mentions that the LM3914 is an appropriate IC for the LED GR meter.

Some inexpensive PCBs for the LM3914 can be found here:
http://www.ebay.com/itm/10-LED-Bargraph-display-Meter-PCB-LM3914-LM3915-VU-op-amp-music-audio-sound-volt-/331156369622?hash=item4d1a7530d6:g:I1QAAOSw3ydVhwfy

I've just ordered a few of these PCBs to experiment with! More posts coming soon...

 

[Di_Sousa]

I'd love to find a way to keep the pedal dual-purpose so that the input would have a switch for "line" vs. "instrument" and the output would have "line" vs. "amp." This way, users could use the pedal in the studio as well as live performance. The Eventide Modfactor and Timefactor pedals have an i/o setup like this (image attached). I'd love to know how there're accomplishing this switching in the circuit. ???

For most situations that would probably not be a problem or make a diference.
Instrument inputs can take unbalanced line level, impedance will not be a problem, so it's a matter of turning the level knob down a bit on the line level device in case your distorting the unit input. (volume on a keyboard, or Akai MPC for example)
As for the output guitar amps can take the line level out of the GSSL pedal, then you use the makeup gain of the gssl knob as the volume going into the amp, if it's to high for the amps input  use the makeup in the minimum setting.

to be honest, for this reasons I wouldn't worry about it.
Remeber that Boss pedals, and other pedals that are buffered turn your guitar signal into something that it's not the same instrument level anymore.

 

[Kid Squid]

Have you been reading my mind ? !!!

I started doing exactly this, but other jobs, work, family blah etc got right in the way. It's deffo something that i'm gonna get back into !

Most excellent stuff