Here's a fun little project IMHO. I was looking for a Tweed sounding pedal. I built a few on breadboard and I liked the Red Llama. It sounded pretty good.
A few things I didn't like, though:
- audible tone suck (old manual says 100k input impedance)
- it sounded a bit too tame
- scratchy noises tailing the higher notes (maybe not immediately audible, but when you pay attention you hear it and then you cannot unhear it)
So I played around with it on the breadboard:
I added a dual opamp. I used an OPA1678, but any FET-input dual is fine. The first stage I added in the front as a buffer/pre-gain. Turn the pre-gain pot all the way down and it's just a buffer. Turn it all the way up and you get about 15 dB of clean, flat boost, hitting the Red Llama. Now it goes from tame to pretty excited.
The next thing I added was a Tight control a-la Amptweaker. For that I split up the 68nF input cap of the RL. When not using the preamp I don't feel like it's really needed. But once you turn the pre-amp up you're glad it's there. Putting it to one or two o'clock sorts out the bottom end.
The scratchiness could only come from the inverter, so I thought I'd try a low pass filter after it. I knew it doesn't have a low output impedance. Google suggested about four or five k Ohms. So I tried this without an extra resistor, starting with a 3n3 to ground (C8). That didn't do anything, neither did 3n9. But 4n7 was the ticket. It removed the noise but didn't change the sound. I ended up socketing this in case I got my hand on another inverter. Maybe it wouldn't need it.
And behind the RL's volume pot I used the remaining opamp as an output buffer. I figured that'd be nice here, considering the mentioned output impedance of the inverter, so long cables won't be a problem.
Here's the schematic:
Once I was happy I built this on vero and boxed it up:
I actually built this on the bread board using a TI CD4049UBE. I had read some stories about this part before, apparently quite noisy. So I also ordered some NOS from RCA. Not sure how old they are, but they do look a bit ancient.
I thought that maybe the scratchy noise would only happen with the TI chip, but actually the RCA chip had the same issue. And using the same value for C8 (4n7) it was gone as well.
So with the vero pedal in hand I built the standard RL again on BB, to compare the inverters.. I had the RCA in the pedal and the TI on the BB. Holding the strings the pedal was less noisy, I'm talking white noise in the background. I switched to another RCA on the BB and then pedal and BB had about the same noise level. But I don't think the difference was actually worth the 10€ I had to pay for a single NOS RCA inverter ;-) Sound-wise I can't tell a difference, really.
Anyway, with pre-gain and tight off, the pedal sounds like an original RL (without the tone suck), so that's nice. And with with the two extra knobs you can spice it up a bit.
Oh, one last thing. The RL is LOUD. With 1kHz at 200mV p2p at the input I measured about 5.2V p2p at the output. Plugging the RL into the top input of my amp was crushing, it pretty much immediately and completely overloaded the amp's input stage. So I added a series resistor in front of the volume pot (R8). I ended up with 62k. I played with the resistance until I had a little bit more "clean" boost than my OD-1 clone (vol up, gain down) and pretty much the same output when dirty boosting with the OD-1 (vol up, gain up). Like this the volume range makes more sense, me thinks.
OK, one more thing to mention. The RL schematic lists a 330uF. I tried different values on the BB, but couldn't tell a difference. I ended up using the standard 100uF.
Here's the vero stuff:
Anyway, hope you like it!
A few things I didn't like, though:
- audible tone suck (old manual says 100k input impedance)
- it sounded a bit too tame
- scratchy noises tailing the higher notes (maybe not immediately audible, but when you pay attention you hear it and then you cannot unhear it)
So I played around with it on the breadboard:
I added a dual opamp. I used an OPA1678, but any FET-input dual is fine. The first stage I added in the front as a buffer/pre-gain. Turn the pre-gain pot all the way down and it's just a buffer. Turn it all the way up and you get about 15 dB of clean, flat boost, hitting the Red Llama. Now it goes from tame to pretty excited.
The next thing I added was a Tight control a-la Amptweaker. For that I split up the 68nF input cap of the RL. When not using the preamp I don't feel like it's really needed. But once you turn the pre-amp up you're glad it's there. Putting it to one or two o'clock sorts out the bottom end.
The scratchiness could only come from the inverter, so I thought I'd try a low pass filter after it. I knew it doesn't have a low output impedance. Google suggested about four or five k Ohms. So I tried this without an extra resistor, starting with a 3n3 to ground (C8). That didn't do anything, neither did 3n9. But 4n7 was the ticket. It removed the noise but didn't change the sound. I ended up socketing this in case I got my hand on another inverter. Maybe it wouldn't need it.
And behind the RL's volume pot I used the remaining opamp as an output buffer. I figured that'd be nice here, considering the mentioned output impedance of the inverter, so long cables won't be a problem.
Here's the schematic:
Once I was happy I built this on vero and boxed it up:
I actually built this on the bread board using a TI CD4049UBE. I had read some stories about this part before, apparently quite noisy. So I also ordered some NOS from RCA. Not sure how old they are, but they do look a bit ancient.
I thought that maybe the scratchy noise would only happen with the TI chip, but actually the RCA chip had the same issue. And using the same value for C8 (4n7) it was gone as well.
So with the vero pedal in hand I built the standard RL again on BB, to compare the inverters.. I had the RCA in the pedal and the TI on the BB. Holding the strings the pedal was less noisy, I'm talking white noise in the background. I switched to another RCA on the BB and then pedal and BB had about the same noise level. But I don't think the difference was actually worth the 10€ I had to pay for a single NOS RCA inverter ;-) Sound-wise I can't tell a difference, really.
Anyway, with pre-gain and tight off, the pedal sounds like an original RL (without the tone suck), so that's nice. And with with the two extra knobs you can spice it up a bit.
Oh, one last thing. The RL is LOUD. With 1kHz at 200mV p2p at the input I measured about 5.2V p2p at the output. Plugging the RL into the top input of my amp was crushing, it pretty much immediately and completely overloaded the amp's input stage. So I added a series resistor in front of the volume pot (R8). I ended up with 62k. I played with the resistance until I had a little bit more "clean" boost than my OD-1 clone (vol up, gain down) and pretty much the same output when dirty boosting with the OD-1 (vol up, gain up). Like this the volume range makes more sense, me thinks.
OK, one more thing to mention. The RL schematic lists a 330uF. I tried different values on the BB, but couldn't tell a difference. I ended up using the standard 100uF.
Here's the vero stuff:
Anyway, hope you like it!
