• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Rotary SUT Load Switch

HalfApt said:
The 2 end wires plug in to the main board in place of the on-board load resistor. Each switch deck is wired the same.

View attachment 2719432

Can't tell ya what length of wire is acceptable as I'm not an EE. I don't think added resistance is an issue, as wires would only add a small fraction of an ohm, but radiated hum/noise pickup might be an issue. I would at least use twisted pair or coax.

Ideally (but not necessarily), it would be mounted as close as possible to the inputs, with a shaft drive to the front panel. But that would only be possible with the PCB mounted version of the switch (or you'd have to jury-rig a bracket to mount the solder lug switch).
Click to expand...

That layout sketch is very helpful. Thank you! As you note and @Nashou66 shows on his build the ideal is that the loads are close to the board and I'm looking at a variety of layouts inside the enclosure to accomplish that. Turning the board 90 degrees from what @Nashou66 did and if the enclosure is wide enough then I could put the switch off to the side and avoid having a long shaft over the board. Regardless, twisted pairs will be used.

@Nashou66 : How did the shaft connect to the back of the front knob? I will probably have to make some brackets should I put the load switches towards the back of the enclosure.

FYI, Mouser has a 12 position 30 degree throw switch faceplate that matches the Grayhill switch I linked in post #17. I like the faceplate numbered so I can change out resistors or capacitors if needed. I'll just have to tape on the enclosure a list of what each number represents.
 
You might look also for someone who has a Decware MC SUT as it also has a resistance knob. I was wondering how that worked...says to rotate knob until volume no longer increases? at that point it's matched?
Anyways, maybe someone can give you a look inside.
 
AvFan said:
That layout sketch is very helpful. Thank you! As you note and @Nashou66 shows on his build the ideal is that the loads are close to the board and I'm looking at a variety of layouts inside the enclosure to accomplish that. Turning the board 90 degrees from what @Nashou66 did and if the enclosure is wide enough then I could put the switch off to the side and avoid having a long shaft over the board. Regardless, twisted pairs will be used.
Click to expand...
If you'd consider the PCB mount version of that switch, here's my take.

Board is same width as the main preamp board, mount it upside down on standoffs using the main board mounting holes, 2 short wires per channel connect to main board resistor pads. Shaft drive to front panel, straight down main board centerline.

This is quick-'n-dirty just to see if it could be done:

Loads-204.jpg

But...Mouser switch plate would also be upside down
 
HalfApt said:
If you'd consider the PCB mount version of that switch, here's my take.

Board is same width as the main preamp board, mount it upside down on standoffs using the main board mounting holes, 2 short wires per channel connect to main board resistor pads. Shaft drive to front panel, straight down main board centerline.

This is quick-'n-dirty just to see if it could be done:

View attachment 2719579

But...Mouser switch plate would also be upside down
Click to expand...

I like that a lot! It would be perfect for MC or MM resistance load plus my original idea for SUT loading. I could see stacking it on top of the end of preamp board and using an extension shaft to get the knob on the front of the enclosure or just run it out the back if that is preferred. For a pure MM preamp is the board wide enough to include both capacitance and resistance switches?
 
AvFan said:
I like that a lot! It would be perfect for MC or MM resistance load plus my original idea for SUT loading. I could see stacking it on top of the end of preamp board and using an extension shaft to get the knob on the front of the enclosure or just run it out the back if that is preferred. For a pure MM preamp is the board wide enough to include both capacitance and resistance switches?
Click to expand...
I’m interested if this becomes reality.
 
AvFan said:
I like that a lot! It would be perfect for MC or MM resistance load plus my original idea for SUT loading. I could see stacking it on top of the end of preamp board and using an extension shaft to get the knob on the front of the enclosure or just run it out the back if that is preferred. For a pure MM preamp is the board wide enough to include both capacitance and resistance switches?
Click to expand...

I think it be tight, and another set of 4 pins would be needed, 4 for the caps and 4 for the resistors .

Now if that horizontal board could have header pins lined up right over the cap load and resistor load sockets it be awesome, !! but then the layout would be limited to be able to make sure the rotary switches can fit. , now as another member posted his build on the B build thread, he used relays to
switch in the loads. very neat design. I bet Sean could come up with a nicer layout . this user just used project boards.

index.php


I would have laid it horizontal, in his build he would need a tall case .

Athanasios
 
Nashou66 said:
I think it be tight, and another set of 4 pins would be needed, 4 for the caps and 4 for the resistors .

Now if that horizontal board could have header pins lined up right over the cap load and resistor load sockets it be awesome, !! but then the layout would be limited to be able to make sure the rotary switches can fit. , now as another member posted his build on the B build thread, he used relays to
switch in the loads. very neat design. I bet Sean could come up with a nicer layout . this user just used project boards.

index.php


I would have laid it horizontal, in his build he would need a tall case .

Athanasios
Click to expand...

Maybe this should be moved to the build thread but we might be overthinking this a bit. I've got an idea that could allow two load switches with a MM preamp, on boards or not, by rotating the preamp board 90 degrees. The enclosure might need to be 17" wide but it wouldn't need to be more than 2U high.

But not now, I'm going to watch the rest of Elton's last show...
 
AvFan said:
.... I could see stacking it on top of the end of preamp board and using an extension shaft to get the knob on the front of the enclosure or just run it out the back if that is preferred.
Click to expand...
"Preferred" is up to the user.

For switch knobs on the back panel.

1) without a PCB: Solder lug switches mount directly to back panel, resistors and/or caps soldered directly on switch. 2 wires per channel to main board.

2) with PCB switches: no mounting holes needed, PCB fully supported by switch to panel mount. Easier to solder resistors and/or caps. 2 wires per channel to main board.

For a pure MM preamp is the board wide enough to include both capacitance and resistance switches?
Click to expand...
Still with 12 R loads & 12 C loads? Seems excessive to me, and might be tight, I'd have to try it to be sure.

Depends heavily on what you want to use for caps: type & values. Polypropylene are huge, board would have to be much larger overall, if possible at all. Quality ceramics ( MLCC / C0G) would be best fit.

eg. the example board above uses 1/8w size resistors, 1/4w size increases board size by 40%

I already mocked up such w/6 loads each, using 2P6T grayhills:

Combo-RC-plan.jpg
 
@HalfApt : I really appreciate you taking the time to indulge my rambling ideas about how to incorporate loading options to either a SUT or phono preamp.

I never quite understood the logic of putting an adjustment knob on the back of an enclosure except to minimize the internal distance to where it connects to the preamp board. My preference is to have the switch knob conveniently located on the front of the enclosure which then requires an extension shaft. Soldering up all those resistors to the actual switch would certainly be more of a chore than soldering them to a board so I really like the board option. After thinking about it I am concerned that stacking the load boards would put the knob off-center on the face of the enclosure (unless it was very tall) so setting them off to the side might be better. I will literally sketch something up a little later to see what folks think.

For a MM preamp I don't think more than six caps (50pf to 500pf) would be needed but it would be nice to have 12 load positions. For example I use a Grado that requires 10kohm and I want a 1Mohm resistor for when I use a SUT that is loaded on the primary side. The balance of positions could be between 20kohm and 100kohm that would provide lots of loading flexibility when using a variety of SUT ratios and accommodate cartridges, such as some Shures that perform better with other than the standard 47kohm.

For a MC preamp I didn't think it was necessary to have adjustable capacitance. Correct? If so, then only adjustable resistance loading is needed and I want to get fairly granular with loading options so it would be easy to fill up 12 positions.
 
AvFan said:
@HalfApt : ...I never quite understood the logic of putting an adjustment knob on the back of an enclosure except to minimize the internal distance to where it connects to the preamp board.
Click to expand...
How often do you swap cartridges? Unless you do it quite often (daily? weekly?), I don't see the need for front panel. Most will set the load for the cartridge in use, then play it for a long time (until the next cart swap).

I keep a diagram of my back panel handy so I know which switch to turn which way. Haven't touched it since initial tune.

rear-panel-2.jpg

But whatever floats your boat ;)

... After thinking about it I am concerned that stacking the load boards would put the knob off-center on the face of the enclosure (unless it was very tall) so setting them off to the side might be better. I will literally sketch something up a little later to see what folks think.
Click to expand...
Switch shaft minimum height above main board is 12-13 mm (just enough to clear the electrolytic caps). If that puts the knob too high on the front panel, well you're out of luck. If it's low on the front panel, you just need taller standoffs to bring it up to center.

For a MM preamp I don't think more than six caps (50pf to 500pf) would be needed but it would be nice to have 12 load positions. For example I use a Grado that requires 10kohm and I want a 1Mohm resistor for when I use a SUT that is loaded on the primary side. The balance of positions could be between 20kohm and 100kohm that would provide lots of loading flexibility when using a variety of SUT ratios and accommodate cartridges, such as some Shures that perform better with other than the standard 47kohm.
Click to expand...
The board won't care what values you use (as long as the resistors/caps you want will fit the footprints). It's up to you to pick the values you want.

For a MC preamp I didn't think it was necessary to have adjustable capacitance. Correct?
Click to expand...
Depends who you ask. MOST MC carts don't care what the cap load is (within reason), but some more esoteric carts have atypical recommendations.

eg. Wyn needed large value for his cart, thus the LOMC board was originally made to accommodate physically large caps. Still accommodates multiple smaller sizes.

Anyway, 2 12-way switches will fit. Again, this is quick-n-dirty, still need to juggle parts around if a 2nd mounting hole is needed. This 2-switch board would work better if the resistors were mounted vertically, could use 1/4w size then too. Switch shafts are 50mm apart (@2").

RC-loads.jpg

Just for grins, this is my preference. R/C loads & RCAs all on-board (requires a specific RCA jack). RCAs are threaded from outside of chassis. 3-way switches provide 8 values each R & C. 4-way switches would provide 16 values.

inner-obl.jpg

outer-obl.jpg
 
Last edited:
HalfApt said:
Anyway, 2 12-way switches will fit. Again, this is quick-n-dirty, still need to juggle parts around if a 2nd mounting hole is needed. This 2-switch board would work better if the resistors were mounted vertically, could use 1/4w size then too. Switch shafts are 50mm apart (@2").
Click to expand...

That looks great and good to know the distance between the centerlines of the switches along with the clearance from the board for the switch extension rod. I can now figure out the right enclosure height. I think a second mounting hole is necessary to secure it whether to the preamp board or within the enclosure. That board would certainly take care of my preferred way to adjust loading. Only two sets of pins are needed because capacitance and resistance are parallel? Some of the Grayhill switches have stops so even if it is a 12 position switch it can be configured to less, handy.

I don't change carts all that often but when I do it comes in bunches as I move carts between tables and try out MM, MC or MM/SUT combos. The enclosures I chose for my V1 and V2 builds aren't really designed to be opened regularly and I don't like reaching around behind my gear so I prefer front panel adjustments. The trick is choosing loads that will work for a variety cartridges. Interestingly, most of the MC carts I'm looking at are below 8ohm internal impedance so loading probably falls within a narrow range.

Again, I appreciate your thoughts and efforts figuring out how to make this work.
 
I get the reason why you want to use a rotary but with the small signals incoming to the SUT, the fewer contact surfaces/interfaces/places for oxidation the better, IMHO.
 
edwyun said:
I get the reason why you want to use a rotary but with the small signals incoming to the SUT, the fewer contact surfaces/interfaces/places for oxidation the better, IMHO.
Click to expand...

Agreed but at least the Grayhill switch I'm looking at has gold plated contact points and a matching $32 price.
 
AvFan said:
That looks great and good to know the distance between the centerlines of the switches along with the clearance from the board for the switch extension rod. I can now figure out the right enclosure height. I think a second mounting hole is necessary to secure it whether to the preamp board or within the enclosure. That board would certainly take care of my preferred way to adjust loading.
Click to expand...
This about as good as can be with thru-hole parts. More minor tweaks before finalizing, but I've got a couple of busy days ahead, so further work will have to wait.

What version of main board will you use it on?

RC-loads-2.jpg

Some of the Grayhill switches have stops so even if it is a 12 position switch it can be configured to less, handy..
Click to expand...
Might be hard to find an adjustable stop version, Mouser doesn't stock them. 10-position switch will work on the same PCB, just omit 4 resistors and/or 4 caps.

Use a shorting switch (make-before-break) for the resistors. Caps won't matter.
 
HalfApt said:
This about as good as can be with thru-hole parts. More minor tweaks before finalizing, but I've got a couple of busy days ahead, so further work will have to wait.

What version of main board will you use it on?

View attachment 2720706


Might be hard to find an adjustable stop version, Mouser doesn't stock them. 10-position switch will work on the same PCB, just omit 4 resistors and/or 4 caps.

Use a shorting switch (make-before-break) for the resistors. Caps won't matter.
Click to expand...

That is terrific! I plan on using this board with the V3.2 main board for both MM and MC preamps as well as the original intent as part of a SUT. I'll have to look at the PCB switch versions but the standalone switches have holes for pins that act as stops. Regardless, I'd find a way to fill up the locations. Were you able to fit in 1/4 watt resistors or are those 1/8 watt? What caps did you assume? Last question: How are the positions paired? For example, 1 and 13, 2 and 14, etc. Thanks again!
 
You must log in or register to reply here.
Back
Top Bottom