Brüel & Kjær - gilding the lilly

[unseenninja]

Recently, I found a potentiometer from Brüel & Kjær for sale on a local auction site. Thinking that it would be nice to have a top quality logarithmic potentiometer for when such things are needed, I managed to buy it at a sensible price.

After a while, a parcel arrived which contained this wooden box:



"Made in Denmark", at the bottom of the rear side, is the only text on the entire box.

Inside the box, the foam which held the pot in place had turned to dust. My first thought was that the knob in the middle was for adjusting the pot...



But no. The knob is just for the rod which goes through the middle of the unit which, with the help of the three alignment pins, screws it into the piece of equipment which is designed to be used with.



So where's the pot?

I looked closer at the part which looks suspiciously like a wire-wound linear potentiometer track and thought, "Really? For this much money?"



So then I took it apart. The contents of the box pretty much blew my mind...

(continued in the next post)

[unseenninja]

After removing the bottom plate, a second shield presented itself.



After unscrewing the three locating pins (one of which had a pure copper washer underneath it), the contents of the magical box fell out into my hands.



The "linear potentiometer track" is actually a large number of thin metal plates with an insulator between each plate.



Each plate is made so that there are six different versions, each one with a solder tag which sticks out the back of the stack at six different heights. The result is this:



The resulting matrix of 216 connection points (36 by 6) is then routed to eight PCBs, each one containing 27 resistors and the occasional capacitor or extra resistor. It's all hand wired and contains what appears to be a mixture of custom and standard value resistors. One for each wire presented as a linear potentiometer track on the outside of the unit. Undoubtedly selected by grey bearded Danish engineers fuelled by Gammal Dansk and smørrebrød who are now long dead and gone.

More pictures (all clickable for full size):









Truly an insane way to build a potentiometer! Of course, it's probably the only way to build a logarithmic potentiometer, unless someone has invented resistance wire with a logarithmic resistance per millimetre.

When I find some way to reliably traverse the track with something repeatable, I'll give it a measure.

[Roehrenonkel]

Hi unseenninja,
 
if you collect them i got the 25dB-Version (without box).

Best regards

[unseenninja]

Whew. Hope I swapped those posts before anyone noticed!

[unseenninja]

Hi unseenninja,
 
if you collect them i got the 25dB-Version (without box).

Best regards

I hadn't actually thought of collecting them. But.... you never know!

[TimFox]

Just a nit-pick:
That line about lilies is almost always misquoted from Shakespeare:  "To gild refined gold, to paint the lily,"  King John, Act IV, Scene 2
(To gild is to coat with gold, which is unnecessary for gold and hard to do with a flower, and lilies do not need painting.)

[Roehrenonkel]

Hi again,
 
the pots are for the 2135 (chart-plotter).
Long long gone.....

Best regards

[unseenninja]

Just a nit-pick:
That line about lilies is almost always misquoted from Shakespeare:  "To gild refined gold, to paint the lily,"  King John, Act IV, Scene 2
(To gild is to coat with gold, which is unnecessary for gold and hard to do with a flower, and lilies do not need painting.)

Indeed, the lilly needs no gilding to be beautiful. However, I think the general connotation, as used by our host, which implies going over and above what is needed to make something exceptional is an equally apt interpretation of the bard's words.

And you probably can electroplate gold onto a lilly. It might not last long afterwards, but it would seem possible.

[TimFox]

Just a nit-pick:
That line about lilies is almost always misquoted from Shakespeare:  "To gild refined gold, to paint the lily,"  King John, Act IV, Scene 2
(To gild is to coat with gold, which is unnecessary for gold and hard to do with a flower, and lilies do not need painting.)

Indeed, the lilly needs no guilding to be beautiful. However, I think the general connotation, as used by our host, which implies going over and above what is needed to make something exceptional is an equally apt interpretation of the bard's words.

And you probably can electroplate gold onto a lilly. It might not last long afterwards, but it would seem possible.

I always interpreted the original as involving the redundancy of adding gold to gold.

[manganin]

Back in the days there were no good enough components (first tubes and then semiconductors) to create the logarithmic scale of the level recorder electronically.

[ivo]

Truly an insane way to build a potentiometer! Of course, it's probably the only way to build a logarithmic potentiometer, unless someone has invented resistance wire with a logarithmic resistance per millimetre.

But they are a common item? Probably 90% of audio gear has a log pot

https://au.mouser.com/ProductDetail/CUI-Devices/PT01-D120D-A502?qs=Rp5uXu7WBW%252BzkfdiW5exQQ%3D%3D

https://www.vishay.com/docs/51031/p11sp11a.pdf see all the curves for the taper spec

[TimFox]

The original meaning of "potentiometer" is a device to measure voltage ("potential" + "meter").
One applies a known voltage across the potentiometer and measures the voltage (or current) from the moving contact to an unknown voltage connected to the "low" end of the potentiometer.
Some potentiometers have two voltage dividers:  one to measure the applied voltage with respect to a standard cell, or other reference device, and the other to measure the unknown voltage.
The potentiometers used as gain controls, etc. have the same set of connections, but measurement potentiometers have accurate voltage-division characteristics;  most are "linear", so the device shown is interesting.
Logarithmic pots used for audio are not particularly accurate, since that is not necessary for their application;  it is difficult to make an accurate variable-width resistive trace on the substrate.
When accuracy is required for audio, use a stepped attenuator.

[1audio]

General Radio also made similar pots for their 1521 graphic recorders. They did have a way to make a log wirewound pot with as close as .1 dB accuracy. I believe they used a tapered core and several sections with trim resistors to get the accuracy. They also used versions in some of their other instruments, e.g. 1559, 544 etc.

[dietert1]

I think inside the chart plotter the potentiometer gets "adjusted" by the movement of the chart plotter, so the plotter could resolve 216 level steps. Compare that to a modern 12 bit scope with its 4096 vertical steps.

[ArdWar]

But they are a common item? Probably 90% of audio gear has a log pot

Practically none of the mass produced "log taper" pots are actually logarithmic. Most are dual linear slope, slightly fancy one might be n-slope piecewise approximation with some special behavior near the ends. For short span one (~25dB) it might even simply linear pot with output load.

Well, this one is still a 215-slope 216-step approximation.