Modifying a L-pad for rheostat

[Kay Pirinha]

Hi,

for my 4430 project I need a pair of 30 Ω rheostats for the treble controls. At first I've found and ordered these. Unfortunately their threaded necks turned out to be very short, they wouldn't reach through the 3/4" baffle board. At the same time I ordered this pair of L-pads for the midrange controls. Their threads are 25 mm long, hence long enough:



These L-pads consist of a 8 Ω rheostat that is in series with the driver and works cw, and another 42 Ω one that is in parallel with the xover's high pass output. The latter one works ccw, which is undesirable, and completely shuts off when fully cw, which also doesn't help.

Fortunately my L-pads are very easy to dismantle. Just remove the two screws and they fall apart:



Note the C clip in the 2nd pic that holds the wiper assembly together. After removing it, the wipers also fall apart:



To get cw action for the 42 Ω rheo, both rheo's need to be swapped. Both are soldered at their left handed end to the corrsponding lugs #1 and #3. As the case is made from bakelite, desoldering ain't no problem at all:



Next picture in the next posting!

[Kay Pirinha]

Here's the desoldered 42 Ω rheo:



The 42 Ω rheo comes into the 8 Ω position and vice versa. It turned out that the 8 Ω needed to be cut at both ends with a file to fit. Both were soldered to the left handed lugs again:



Finally both rheostats were reassembled and marked accordingly:



Yes, the actual rheostats' resistances of 42 and 43 Ω are too big. Paralleling fix resistors of somewhat more than 100 Ω will fix this minor issue. I measured a bunch of 220 Ω % W resistors to find two pairs that I can put in parallel with the new rheo's to match the desired value within close tolerances.

Best regards!

[Earl K]

Nice Work!

Thanks for the pictorial ( always very useful )

E

[Kay Pirinha]

Hi,

I feel the need to show some measurements on my new rheostats with paralleled resistors. I've written each actual resistance value on the components.

First I wanted to know the resistance of my probes and test leads:



That's about 2 Ω.

Now let's have a look at the rheos' resistances when turned fully cw:



And now they're turned fully ccw:



Fits, I'd say, after we've subtracted the probes' and leads' resistance .

BUT: There's no free lunch! There's some drawback instead as the characteristics ain't linear no more. E.g. let's calculate for the 43 Ω rheo and the paralleled 214 Ω resistors' actual values when turned halfway between cw and ccw:

R = 1/(2/43 + 2/214) = 17.9 Ω

and for the 42 Ω rheo:

R = 1/(2/42 + 2/215) = 17.6 Ω

As we expect a resistance of 15 Ω from a linear 30 Ω rheo being turned halfway, the differences are +19 % and +17.6 % (wrt to 15 Ω). I think the biggest difference actually can be expected at halfway, but for now I'm too lazy to calculate other possibilities .

Being a hearing aid junkie I'm surely not the right one to tell how important these deviations are, but those who feel more catholic than the pope most certainly will complain . For me it fits, especially as most of us don't have any other possibility than to judge by ear and adjust the HF controls accordingly.

Best regards and have a nice listening weekend!

[Ian Mackenzie]

Hi,

for my 4430 project I need a pair of 30 Ω rheostats for the treble controls. At first I've found and ordered these. Unfortunately their threaded necks turned out to be very short, they wouldn't reach through the 3/4" baffle board. At the same time I ordered this pair of L-pads for the midrange controls. Their threads are 25 mm long, hence long enough:



These L-pads consist of a 8 Ω rheostat that is in series with the driver and works cw, and another 42 Ω one that is in parallel with the xover's high pass output. The latter one works ccw, which is undesirable, and completely shuts off when fully cw, which also doesn't help.

Fortunately my L-pads are very easy to dismantle. Just remove the two screws and they fall apart:





Note the C clip in the 2nd pic that holds the wiper assembly together. After removing it, the wipers also fall apart:



To get cw action for the 42 Ω rheo, both rheo's need to be swapped. Both are soldered at their left handed end to the corrsponding lugs #1 and #3. As the case is made from bakelite, desoldering ain't no problem at all:



Next picture in the next posting!

I am just reading your post above.

For clarity the L pad works to attenuate the signal in an anti clockwise direction. So when turned fully clockwise there will be zero signal attenuation.

In this condition the 8 ohm series wire wound rheostat will have its wiper blade at zero ohms.
The other 30-39 ohm wire wound rheostat will be open circuit. This rheostat is deliberately designed so the track goes open circuit so there is no shunt across the load in the fully clockwise connection.

With a nominal 8 ohm load the load to the source will be 8 ohms.

When the L pad is turned counter clockwise the wiper blade of the 8 ohm series rheostat will move to create a small value of series resistance and increase as the the L pad is turned counter clockwise.

At the same time the other rheostat blade will engage and measure a high value of resistance around 30-39 ohms which is shunted across the load. As the L pad is turned counter clockwise the value of resistance will fall. This avoids burn out of the rheostat.

With an 8 ohm load attached to the L pad the source will see a relatively constant load impedance and the L pad attenuation is increased.

I am not entirely sure what you have done above. I am just pointing out the normal operation of an L pad.

[Kay Pirinha]

Hi Ian,

in short words I swapped both resistor tracks to get the desired action (higher level when turned cw) for the 42 ohms (per my measurements) one. JBL's 30 ohms statement for the rheo that is in series with the resonant circuit drove me to do this.

As said in my 4430 project thread, an 8 ohms rheo that won't open when fully ccw also would make more sense to me.

Best regards!