Catalinbread SFT with too much volume

[guidoilieff]

Hi! Its been a while


I made an SFT pedal and I can barely move the volume control because it has too much of it. The first like 5% is all I need for unity. The transistors are not matched so I have no idea whats going on. Can I simply put a resistor on the volume control to make it more manageable or should I swap it for a logarithmic pot or with more resistance?

Heres the project https://effectslayouts.blogspot.com/2016/06/catalinbread-sft.html


Thanks and sorry for my english!!

[drdn0]

Hi! Its been a while


I made an SFT pedal and I can barely move the volume control because it has too much of it. The first like 5% is all I need for unity. The transistors are not matched so I have no idea whats going on. Can I simply put a resistor on the volume control to make it more manageable or should I swap it for a logarithmic pot or with more resistance?

Heres the project https://effectslayouts.blogspot.com/2016/06/catalinbread-sft.html


Thanks and sorry for my english!!

The schematic calls for a 250k logarithmic pot for volume

[guidoilieff]

Hi! Its been a while


I made an SFT pedal and I can barely move the volume control because it has too much of it. The first like 5% is all I need for unity. The transistors are not matched so I have no idea whats going on. Can I simply put a resistor on the volume control to make it more manageable or should I swap it for a logarithmic pot or with more resistance?

Heres the project https://effectslayouts.blogspot.com/2016/06/catalinbread-sft.html


Thanks and sorry for my english!!

The schematic calls for a 250k logarithmic pot for volume

And thats what it has, a 250k log pot.

I solder a 500 ohm resistor from the volume pot to ground and that kinda fixed it but I think I didn't solve the problem of too much volume inside the circuit

[drdn0]

Hi! Its been a while


I made an SFT pedal and I can barely move the volume control because it has too much of it. The first like 5% is all I need for unity. The transistors are not matched so I have no idea whats going on. Can I simply put a resistor on the volume control to make it more manageable or should I swap it for a logarithmic pot or with more resistance?

Heres the project https://effectslayouts.blogspot.com/2016/06/catalinbread-sft.html


Thanks and sorry for my english!!

The schematic calls for a 250k logarithmic pot for volume

And thats what it has, a 250k log pot.

I solder a 500 ohm resistor from the volume pot to ground and that kinda fixed it but I think I didn't solve the problem of too much volume inside the circuit

Ok, just a language thing then - your original post made it seem like you hadn't used a log pot.

Adding a 500 ohm resistor to ground would make absolutely zero difference whatsoever to the volume controls if they were working how they're meant to. It sounds to me like you've got some sort of wiring issue going on because a 250kA pot should be a smooth wipe from min to max volume regardless of what's going on inside the circuit

[guidoilieff]

Hi! Its been a while


I made an SFT pedal and I can barely move the volume control because it has too much of it. The first like 5% is all I need for unity. The transistors are not matched so I have no idea whats going on. Can I simply put a resistor on the volume control to make it more manageable or should I swap it for a logarithmic pot or with more resistance?

Heres the project https://effectslayouts.blogspot.com/2016/06/catalinbread-sft.html


Thanks and sorry for my english!!

The schematic calls for a 250k logarithmic pot for volume

And thats what it has, a 250k log pot.

I solder a 500 ohm resistor from the volume pot to ground and that kinda fixed it but I think I didn't solve the problem of too much volume inside the circuit

Ok, just a language thing then - your original post made it seem like you hadn't used a log pot.

Adding a 500 ohm resistor to ground would make absolutely zero difference whatsoever to the volume controls if they were working how they're meant to. It sounds to me like you've got some sort of wiring issue going on because a 250kA pot should be a smooth wipe from min to max volume regardless of what's going on inside the circuit

Thanks! Yeah, thats what I think is happening. Wiring is ok, pretty straight forward. The same is happening to my Sabbra Cadabra clone https://effectslayouts.blogspot.com/2015/03/catalinbread-sabbra-cadabra.html

In the sabbra cadabra comments someone said to rise the value of the 47ohm resistor on the top left to 1k or 5k. I think I need to do the same to the SFT but I dont know what resistor should be. I'm guessing a resistor that controls the source of a transistor maybe?

[antonis]

It's annoying to trace boards without schenatics, but.. 

In "original" Catalinbread there are two output resistors forming a voltage divider hence dividing output volume (before VOL pot) 50%..


Those resistors are absent from SFT board so the only amplitude attenuation between output buffer and VOL pot relies on 2k2 + 3k9 resistors..
(practically negligible..)

If you don't wish to add those 2 extra voltage divider resistors, you might try to raise 2k2 + 3k9 values roughly X40 (e.g 82k + 150k ) while simultaneously lower respective caps values (e.g. 120pF for 4.7nF)..

P.S.
X40 raise is set by 250k Volume pot value..
(like R22/R23/VOL pot above)

[guidoilieff]

It's annoying to trace boards without schenatics, but.. 

In "original" Catalinbread there are two output resistors forming a voltage divider hence dividing output volume (before VOL pot) 50%..


Those resistors are absent from SFT board so the only amplitude attenuation between output buffer and VOL pot relies on 2k2 + 3k9 resistors..
(practically negligible..)

If you don't wish to add those 2 extra voltage divider resistors, you might try to raise 2k2 + 3k9 values roughly X40 (e.g 82k + 150k ) while simultaneously lower respective caps values (e.g. 120pF for 4.7nF)..

P.S.
X40 raise is set by 250k Volume pot value..
(like R22/R23/VOL pot above)

Thanks, thats great. So in the schematic those are 100k, but in my design are 2k2 and 3k9 right?

[GibsonGM]

Schematic of the actual build (from the vero site linked in first post):

[ElectricDruid]

The R19 and R21 values are a side-track really.

If the volume pot is wired correctly and working correctly, it goes from nothing to maximum. That "maximum" might be louder than you need, but currently it doesn't sound like it's doing anything like going from nothing to maximum.

I'd suspect bad pot wiring, followed by bad pot.

[antonis]

The only wiring issue I can imagine is VOL pot lug1 left open (floating)..
If so, the role of its wiper-lug1 part is replaced by succeding effect input impedance..

[amptramp]

There is a major difference in the schematics posted by antonis and GibsonGM.

The version posted by antonis has no source resistor in the upper transistors, making this a µ-amp design.  The upper transistors operate as current sources with a very high impedance that allows a very high gain from the lower transistor, but the upper transistor does not do any of the amplifying.

The version posted by GibsonGM uses SRPP stages (shunt-regulated push-pull) where the upper and lower transistors both amplify the signal.  As the input to the stage goes positive, the current increases through the lower transistor but the source resistor in the upper stage develops more negative bias on the upper transistor, tending to turn it off.  The difference in current between the upper and lower transistors flows through the load and an SRPP stage must have a load or there is nowhere for this current to go and the output signal gets slammed either to the negative rail or the positive rail.  It would be quite simple for an SRPP stage to be driven from linear to distortion by changing the load on the stage.  All of the current difference has to go through the load and the resistance of the load has to be low enough to absorb the current difference without going beyond the linear range (which is within the rail voltage).

If the value of the source resistor in the upper stage is the reciprocal of the transconductance of the upper transistor, the push-pull action will be balanced with the same swing in both directions.  In the design posted by GibsonGM, the upper stage has most of the gain as a typical 2N5457 has a transconductance of 1000 to 5000 µmhos.  This would mean that the ideal source resistor for matched clipping in the positive and negative direction would be 200 to 1000 ohms, not the 10 Kohms shown.  As shown, the upper transistor is doing most of the amplifying.

The point is, the two designs function in a totally different way due to the addition of three resistors.

[antonis]

Well said, Ron..

Additionally, in the original (I presume so) circuit I've posted, there is an additional transistor (Q8) which "isolates" the load from the last μ-amp (in the degree of a Source follower can do) so the existence of R22/R23 made me think the designer gestimated very loud output..