ODS 124 - revisited for the 21C

[Stephen1966]

I think the operation of the relay is the source of my sleepless nights. So let me see if I have this straight...

The relay contacts are in the nc (normally closed) position when the coil is unpolarised. The relay coil is energised (polarised) when a current is passed through it. From +12V to -12V. Except -12V in this case is being defined as GND 5 in the layout which leads back to -12V on the relay power board. GND 5 is not an Earth Ground as such, but a potential ground in the 12V power rail. To break this circuit and de-polarise (de-energise) the coil, we can break the circuit either after the relay (switching to n.c. - no contacts - in the footswitch) or before the relay with a power off/on switch. Is there a less direct path back to -12V from GND 5 through the chassis, I am not taking into consideration?

"I'm not sure why you would want another switch."
Because at present, it seems only the footswitch can [directly] activate or deactivate the FET boost. [Edit: we can indirectly break the circuit and deactivate the FET boost by pulling out the pedal jack and switching the FET Manual/Pedal switch to Pedal. That also means the OD and PAB switches have to be in Manual to work.]

[Stephen1966]

I've just updated the Clean Preamp and Tone Stack schematic (1.4) in the first post on the first page, to show the updated thinking on the input grounds and with labels on the OD and PAB circuit to show how I think they work. Of course, looking at it from this point of view, the Manual/Pedal switches offer the same on/off (engage/disengage) function when the pedal jack is removed... insofar as they're only engaged in the Manual position. For that reason, Martin, it's fair to ask why I would need another switch. Your FET boost works in the same way. But going further, could the OD benefit from an additional front panel on/off switch as well for when the pedal jack is not plugged in? There is already a PAB engage/disengage switch baked into the DP3T Rock/PAB/Jazz switch.

[erwin_ve]

Stephen, there is no -12v on the fet power supply. Only +12v and ground.
The supply from the voltage regulator has 2 feeding points: 1 to the relays and 1 to the led part of the footswitch. Both relais and led are energized when their respective part is grounded through the footswitch. The optional backpanel toggle for the Fet has only the grounding for the relay, no led grounding.

Erwin

[Stephen1966]

Stephen, there is no -12v on the fet power supply. Only +12v and ground.
The supply from the voltage regulator has 2 feeding points: 1 to the relays and 1 to the led part of the footswitch. Both relais and led are energized when their respective part is grounded through the footswitch. The optional backpanel toggle for the Fet has only the grounding for the relay, no led grounding.

Erwin

Sure, sorry, my EE degree is in the post My vocabulary is a bit lacking here. It's the nature of this grounding that I am grasping at. -12V is wrong, it is rather, the negative side of the coil that is tracing back to the negative side of the power board, the secondary coil of the transformer, 0V perhaps, or circuit ground. The DC reference voltage rail, doesn't need to go back to mains Earth to work. It couples with the GND in the chassis at GND point 5 but that may not be enough to energise the coil on its own. It still needs to go back to the secondary coil of the 12V transformer. But it's reassuring to know that it needs this ground to energise the coil of the relays.

[didit]

Hello --

Just posted a variation on the FET+relay here in Martin's topic thread.

On a careful schematic reading one should see there are a few independent circuits with separate ground points.

• Relay has one feed, with nominal +12 and a return to connect with whatever switching is desired. A LED wired correctly in parallel with the relay feed will light when the switching is engaged.

• Signal ground (GNDS) is "boost" preamp and provides return for what's been called B+5. This is independent & should be carefully tied to main "input" grounding point.

• And there is poured "plane" over most of the board top-side with FET shield and wire points to attach shields for any coax feeding in and out.

Effectively three distinct circuits, although some might want the shield disqualified. Despite good humour regarding mail-order EE, perhaps some dipping into extensive reading resources on basics of circuits would help. A true "ground" is useful but unnecessary. Source of electrons in connection to your voltage potential is really key. Drawing a design out as an independent set of "batteries" is one robust way to model understanding.

Best .. Ian

[Stephen1966]

Hello --

Just posted a variation on the FET+relay here in Martin's topic thread.
...Despite good humour regarding mail-order EE, perhaps some dipping into extensive reading resources on basics of circuits would help. A true "ground" is useful but unnecessary. Source of electrons in connection to your voltage potential is really key. Drawing a design out as an independent set of "batteries" is one robust way to model understanding.

Best .. Ian

Affirmative

I've just started reading Blencowe's Designing Preamps... If there was one title that you could recommend though, what would it be? When I built the Tweedle-Dee I pretty much did it just by following the recipe... and it turned out pretty good but it does rather annoy me that I didn't fully understand why. So in this build, I'm doing things a little differently. They say experience is the best teacher and in my case, that's true, and in the case of Mr D's amps there is a long history in these pages of his genius. It's true there are gaps in my knowledge and that must be annoying for those who figured this stuff out years ago. I apologize if that's the case, as you can see though, I'm not afraid to lay it all out there. I'm just starting really and there are others who will follow, who will probably possess even less knowledge than I do now. Anyway, you're a good-natured crowd, the good guys, and I'll never tire of saying thank you. Where else could we go to understand so deeply and singularly, the work of Mr D?

Good health.

Stephen

[Stephen1966]

So I thought about Martin's FET boost, I will post the revised schematic in a moment. I'm going to go with a toggle Manual/Pedal switch on the front panel and an LED to indicate when the FET boost is engaged. The one niggling question I had about Martin's design was that the switch was on a push/pull pot. Speaking from experience I know these are hard to see, from the front anyway. They're good on a guitar because they are seen in profile. I was considering how of all the mods the FET boost probably has the biggest tonal impact and it isn't something I would like to engage or disengage accidentally. It is a quirky arrangement, to have a Manual/Pedal switch also acting as an engage/disengage switch as well, but it works nonetheless and not everything needs to be so neatly modularised... This new schematic is simpler than the first and could be made even simpler but this (version 1.2) is the one I am going for.

On a sidenote, I put the relay supply board together last night. This is the Tube Town kit.

SAM_7334.JPG

As you can see, this is pretty compact and it measures 30mm x 71mm x 35mm assembled.

SAM_7342.JPG

I am curious about how it will perform so this week I will be rigging it up and taking some measurements. If there is anything of interest I will note it here. If you would like me to check on anything in particular, let me know.

This is what I'm calling B+6 in the schematics I'm drawing. If it works it will be replacing the transformer, regulator, filter supply board in the original.

Good health,

Stephen

[martin manning]

It is a quirky arrangement, to have a Manual/Pedal switch also acting as an engage/disengage switch as well, but it works nonetheless and not everything needs to be so neatly modularised...

I'm going to disagree there, I think it's an efficient system. The panel switch either turns the effect on or gives control to the footswitch. If you wire it as shown in the link I provided above, the footswitch LED is illuminated when the effect is switched on at the amp, and it can't be turned off at the footswitch, alerting you to the panel switch position.

[Stephen1966]

It is a quirky arrangement, to have a Manual/Pedal switch also acting as an engage/disengage switch as well, but it works nonetheless and not everything needs to be so neatly modularised...

I'm going to disagree there, I think it's an efficient system. The panel switch either turns the effect on or gives control to the footswitch. If you wire it as shown in the link I provided above, the footswitch LED is illuminated when the effect is switched on at the amp, and it can't be turned off at the footswitch, alerting you to the panel switch position.

Thanks for the feedback Martin, I think I have it in your configuration now (version 1.2 in the first post). You were right - the extra switch is just not needed. The toggle is just a more visual configuration from my point of view and an LED on the front panel for added affect for when the pedal isn't hooked up. That, and I'm using DPDT switches for some added redundancy.

[didit]

One can readily & easily wire an LED that's on the faceplate into the circuit while still using SPST on the level pot.

Really struggling to parse --

... just not needed. The toggle is just a more visual configuration from my point of view and an LED on the front panel for added affect for when the pedal isn't hooked up.

Needless to say, in agreement with Martin's disagreement.

Best .. Ian

[Stephen1966]

One can readily & easily wire an LED that's on the faceplate into the circuit while still using SPST on the level pot.

Really struggling to parse --

... just not needed. The toggle is just a more visual configuration from my point of view and an LED on the front panel for added affect for when the pedal isn't hooked up.

Needless to say, in agreement with Martin's disagreement.

Best .. Ian

Hi Ian, I appreciate that you are probably very busy at the moment, but if you get time, have a look at the latest schematic I posted (version 1.2). The preference for a separate toggle switch and a regular pot over a switched pot, is really just a preference, for the reasons I posted earlier. And there is no intrinsic difference between SPDT switches over DPDT, my DPDT switches don't do anything different from the customary SPDT switches, they only add a factor of mechanical redundancy to the circuit. The addition of an LED, (in parallel) to the switch is also just a matter of preference. I have to watch the current demands on the transformer but with low current LEDs this shouldn't be a problem. Other than that, and unless I have made a mistake somewhere else, I think I have the same circuit as Martin's, in a very similar configuration.

[didit]

Hmm --

Upon quick scan of the updated schematic it appears there is mislabeled ground "5" reference for "B+6" and your LED is in series with as opposed to parallel with the relay coil. You might double-check and confirm?

Also noted: Do you really want to exclude FET drive when OD is engaged -- it appears that's intended design?

Best .. Ian

[Stephen1966]

Hmm --

Upon quick scan of the updated schematic it appears there is mislabeled ground "5" reference for "B+6" and your LED is in series with as opposed to parallel with the relay coil. You might double-check and confirm?

Also noted: Do you really want to exclude FET drive when OD is engaged -- it appears that's intended design?

Best .. Ian

Many thanks Ian. You noticed the LED... I did it earlier when I was short of time and only realised after it wouldn't work - dead short - pretty useless I've revised in version 1.3... would that work?

In Tony's layout, the ground wire goes to pin 3 on the 5 pin din, while in Martin's "Pedal Detail" it goes to pin 1 and pin 3 carries the B+ to the pedal's LEDs. All the GND goes back to GND 5 in the chassis - it's quite specific about that and GND 5 isn't used for anything outside the relay coils' circuit. I'm not sure it's mislabeled. There's no change to the PAB and OD pins

The switch can either include or exclude the FET boost. Yes, it's intentional. As Martin said: 'Here's a foot switch mod that will prevent FET boost in OD, which might result in too much gain. A switch can be added to enable/disable this feature if desired. In the end, what this does is save a step (literally) when going from clean with FET boost to OD without or vice versa.' [https://ampgarage.com/forum/viewtopic. ... 4&start=30]

[Stephen1966]

I'm having trouble viewing the latest update in the first page.

Here's the latest FET Boost/Pedal schematic:

ODS FET FS 1.3.pdf

[martin manning]

On a sidenote, I put the relay supply board together last night. This is the Tube Town kit... ...This is what I'm calling B+6 in the schematics I'm drawing.

I'd recommend that you not call the relay supply B+6. That designation should be assigned to the FET filter cap node, which is fed from B+5 on the main power rail.