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Simplfying the SLO

12/27/2004 9:11 PM
BWilliamson	Simplfying the SLO Having trouble wrapping my head around this, but S1(clean/crunch) switch got me a bit confused. Not much for channel switching amps so what is exactly going on there. From what I can figure out, when the OD channel is engaged. The signal coming off the 1st stage heads into the switch. The signal then proceeds thru the 470K on the switch to the next 470K/470pF network and at the end of that the 39K goes to ground and proceeds thru the rest of the clean circuit. When the clean is engaged, the 470K on the switch and the 39K to ground isn't in the circuit. When clean, the vactrols at this point: VR1 is grounding out the signal to the 3rd stage VR2 has broken the signal chain am I reading this correctly? Other thing, plan on eliminating the Effects loops as I'm stuffing this into a Bassman chassis. I'm assuming the first CF is the one to eliminate. Any changes or suggestions for the second stage or leave it as shown? thanks bw

12/27/2004 9:11 PM

BWilliamson

Simplfying the SLO

Having trouble wrapping my head around this, but S1(clean/crunch) switch got me a bit confused. Not much for channel switching amps so what is exactly going on there.

From what I can figure out, when the OD channel is engaged. The signal coming off the 1st stage heads into the switch. The signal then proceeds thru the 470K on the switch to the next 470K/470pF network and at the end of that the 39K goes to ground and proceeds thru the rest of the clean circuit.

When the clean is engaged, the 470K on the switch and the 39K to ground isn't in the circuit.

When clean, the vactrols at this point:

VR1 is grounding out the signal to the 3rd stage
VR2 has broken the signal chain

am I reading this correctly?

Other thing, plan on eliminating the Effects loops as I'm stuffing this into a Bassman chassis. I'm assuming the first CF is the one to eliminate. Any changes or suggestions for the second stage or leave it as shown?

thanks
bw

12/28/2004 8:18 AM
MKB	Been staring at this lately myself. Your description look correct to me. However, I think the schem may have the Crunch/Clean switch marked incorrectly, the setting on the schem is actually clean, as there is a 470K+470K/39K voltage divider before the normal vol pot in that position. The other position removes one 470K and the 39K to ground. Another good resource to understand this circuit is the schem for the X88R preamp, as it has the crunch and OD circuits separated a bit more. I have read that the two CF stages (V3 and V4) are important to the tone, remove one and the tone isn't as good. Haven't experienced this myself though.

12/28/2004 8:18 AM

MKB

Been staring at this lately myself. Your description look correct to me. However, I think the schem may have the Crunch/Clean switch marked incorrectly, the setting on the schem is actually clean, as there is a 470K+470K/39K voltage divider before the normal vol pot in that position. The other position removes one 470K and the 39K to ground.

Another good resource to understand this circuit is the schem for the X88R preamp, as it has the crunch and OD circuits separated a bit more.

I have read that the two CF stages (V3 and V4) are important to the tone, remove one and the tone isn't as good. Haven't experienced this myself though.

12/28/2004 3:51 PM
Ray Ivers	BW, As MKB mentioned, if the 39K is in-circuit you have the Clean mode of the Normal channel engaged. The LDR's work just as you said. If you're willing to go a little outside the box... you can eliminate V4 and derive the FX loop output from the unbypassed cathode of V3b, then use V3a as an FX recovery/tone stack driver (a 12BZ7 would be a good candidate here). Of course, using a JFET (2N4393, 2N3955, etc.) for loop recovery would give more post-loop gain, drive the stack better, and allow you to keep V3a as a CF to enhance V3b's overdrive tone... I just like having the FX-loop option available if at all possible. Ray

12/28/2004 3:51 PM

Ray Ivers

BW,

As MKB mentioned, if the 39K is in-circuit you have the Clean mode of the Normal channel engaged. The LDR's work just as you said.

If you're willing to go a little outside the box... you can eliminate V4 and derive the FX loop output from the unbypassed cathode of V3b, then use V3a as an FX recovery/tone stack driver (a 12BZ7 would be a good candidate here). Of course, using a JFET (2N4393, 2N3955, etc.) for loop recovery would give more post-loop gain, drive the stack better, and allow you to keep V3a as a CF to enhance V3b's overdrive tone...

I just like having the FX-loop option available if at all possible.

Ray

12/29/2004 8:34 AM
Terry Shipman	Ray, I'm interested in what you are saying about finding ways to retain the FX loop. I am in the process of designing a PTP based SLO board and am looking at FX loop options at the moment. Have you used the JFET option you mentioned? If so, was there sufficient gain to go from the typical -20dB from pedals to the +4dB needed for the SLO loop return. Do you have a schematic or component values etc. An option I have been considering is to keep the FX loop to the SLO design, but simply add a opamp based booster with fixed gain, to the end of the pedal chain to raise the level to 4dB before feeding it back to the return on the amp. Attenuation at the input of the pedal chain would be a simple resistive attenuator to lose the 24dB to match normal effects pedals to the SLO send level. I would be interested to hear of your experiences around this area of the SLO. Any advice appreciated. TIA, Terry

12/29/2004 8:34 AM

Terry Shipman

Ray,

I'm interested in what you are saying about finding ways to retain the FX loop. I am in the process of designing a PTP based SLO board and am looking at FX loop options at the moment.

Have you used the JFET option you mentioned? If so, was there sufficient gain to go from the typical -20dB from pedals to the +4dB needed for the SLO loop return. Do you have a schematic or component values etc.

An option I have been considering is to keep the FX loop to the SLO design, but simply add a opamp based booster with fixed gain, to the end of the pedal chain to raise the level to 4dB before feeding it back to the return on the amp.

Attenuation at the input of the pedal chain would be a simple resistive attenuator to lose the 24dB to match normal effects pedals to the SLO send level.

I would be interested to hear of your experiences around this area of the SLO. Any advice appreciated.

TIA,

Terry

12/29/2004 1:15 PM
Ray Ivers	Terry, In my prototype preamp I've got a 2N4393 common-source gain stage direct-coupled to another 2N4393 source follower (5k Rs), and it effortlessly drives a modified Fender-type tone stack with all values 1/10 of the Fender ones, with a gain of either 25 or 60 depending on whether or not the 350R gain-stage source resistor is bypassed (12K drain resistor, Vdd = 50V). The lower gain figure represents @ +28dB, so this would be more than enough for your stated -20 to +4 loop level situation. You could just Zener-derive 50V @ 10-15mA from B+, filter it, and you're ready to go. I recently recommended an MXR 10-band AC-powered graphic EQ to a customer with a very hot loop return, mainly for the +12db of loop gain it would provide. This seems similar to your op-amp idea. I've also got a DOD 250 Overdrive Preamp from which I've removed the clipping diodes, and I've used it quite a bit to increase loop return levels; even running on a single 9V battery it's always had enough signal swing to get the job done, although not always with a lot of extra level to spare (the MXR uses 18V). I would prefer to have this built in to the amp's circtuiry rather than as another item in the loop, but that's just me. I've had very good luck with a fixed -15dBm loop level, which will work with every effect unit I've ever seen. I'd much prefer the risk of a slightly higher noise level to a hard-clipped digital processor, but at this loop level I've had neither. Ray

12/29/2004 1:15 PM

Ray Ivers

Terry,

In my prototype preamp I've got a 2N4393 common-source gain stage direct-coupled to another 2N4393 source follower (5k Rs), and it effortlessly drives a modified Fender-type tone stack with all values 1/10 of the Fender ones, with a gain of either 25 or 60 depending on whether or not the 350R gain-stage source resistor is bypassed (12K drain resistor, Vdd = 50V). The lower gain figure represents @ +28dB, so this would be more than enough for your stated -20 to +4 loop level situation. You could just Zener-derive 50V @ 10-15mA from B+, filter it, and you're ready to go.

I recently recommended an MXR 10-band AC-powered graphic EQ to a customer with a very hot loop return, mainly for the +12db of loop gain it would provide. This seems similar to your op-amp idea. I've also got a DOD 250 Overdrive Preamp from which I've removed the clipping diodes, and I've used it quite a bit to increase loop return levels; even running on a single 9V battery it's always had enough signal swing to get the job done, although not always with a lot of extra level to spare (the MXR uses 18V). I would prefer to have this built in to the amp's circtuiry rather than as another item in the loop, but that's just me.

I've had very good luck with a fixed -15dBm loop level, which will work with every effect unit I've ever seen. I'd much prefer the risk of a slightly higher noise level to a hard-clipped digital processor, but at this loop level I've had neither.

Ray

12/30/2004 4:00 PM
Terry Shipman	Ray, This Common source/Source follower arrangement sounds ideal. If I may, I have a couple of questions to ask about it: Is this circuit and the resistor values device specific for the 2N4393, or could other JFETs be substituted? Is the Vdd level of 50 volts critical? Since I don't need the full headroom available, could it be a bit lower, say 36 volts? TIA, Terry

12/30/2004 6:06 PM
Ray Ivers	Terry, I wanted the largest linear signal swing I could get without frying the SF, so for that purpose those values would work best with the 2N4393 and other similar units. For JFET's with a much higher or lower Rds and/or transconductance you would probably want to substitute different values, maybe quite different depending on your Vdd and device dissipation(s). The 50V Vdd was the highest I could run without losing sleep (the 2N4393's I'm using are rated for 60 Vds which is pretty high for a JFET, and NTE's replacements are rated for 40V) but your 36V would probably be more sensible, actually. There are also high-voltage small-signal MOSFET's like the LND150 which might work well, I haven't tried them out. Ray

12/30/2004 6:06 PM

Ray Ivers

Terry,

I wanted the largest linear signal swing I could get without frying the SF, so for that purpose those values would work best with the 2N4393 and other similar units. For JFET's with a much higher or lower Rds and/or transconductance you would probably want to substitute different values, maybe quite different depending on your Vdd and device dissipation(s).

The 50V Vdd was the highest I could run without losing sleep (the 2N4393's I'm using are rated for 60 Vds which is pretty high for a JFET, and NTE's replacements are rated for 40V) but your 36V would probably be more sensible, actually. There are also high-voltage small-signal MOSFET's like the LND150 which might work well, I haven't tried them out.

Ray

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