Seeking Advice on building a stepped potentiometer

[NE WAY]

Hi

I just got hold of a 15 steps Plessey make before break rotary switch and intend to use it as a stepped potentiometer for volume control of my passive pre-amp.

I am seeking advice on the resistor value required.

Using the HiFi Collective 50K Shunt 46 steps as a guide, I decided to skip every 2 steps on their 46 steps, and used the remaining for mine. Is this the correct way?

(Number rounded)

1 - 0R
2 - 18R
3 - 50R
4 - 100R
5 - 200R
6 - 400R
7 - 800R
8 - 1K5
9 - 3K
10 - 7K
11 - 11K
12 - 17K
13 - 27K
14 - 50K
15 - 121K

Thanks!

 

[rayma]

What is the series resistance? And what circuitry and components are before the volume control?
Is this a make-before-break switch?

 

[NE WAY]

1) What is the series resistance?
Do not understand your question, I am trying to build a 50K (or there about) shunt stepped potentiometer if that was your question. It starts at about 9 dB step attenuation and gradually reduced to 3 dB attenuation per step.

2) what circuitry and components are before the volume control?
CD/DAC output, it connects to power amp. Intend to use it as passive preamp initially and may use it for volume control of active preamp later.

3) Is this a make-before-break switch? Yes

 

[Ray Waters]

You might find this helpful.

https://goldpt.com/r_series.html

 

[rayma]

If this is a 50k shunt volume control, that means there is a fixed value series resistor just before the control.
Probably that resistor is 50k in value.

In fact, the HiFiCollective info in post #3 does indeed say that there is a 50k input series resistor.

 

[NE WAY]

You might find this helpful.

https://goldpt.com/r_series.html

Thank you!

Are there any advantages on series or shunt style?

 

[Ray Waters]

Here is a good explanation of the various attenuator types.

https://www.goldpt.com/attenuator_types.html

The series attenuator behaves pretty much like a regular potentiometer with detents, while a shunt type presents a variable impedance to the circuit preceding it. My preference would be the series attenuator, especially when used as a volume control in a passive preamp.

Note that Goldpoint no longer offers ladder and shunt attenuators, but you can of course build one yourself if that is what you want.

 

[1audio]

You can't just delete steps that way. You can select the steps you want and add all the resistors between them. That is a very substantial switch. It looks like its for high current. Hopefully those are silver contacts.

 

[Ixnay]

Another possible resource is the John Broskie dual stereo control where ladder and shunt are mixed together producing an incredible amount of steps. In that design, you have 'coarse' volume control in concert with a 'fine' volume control. You can still use your type of switch instead of his if yours is better quality.

 

[Ray Waters]

You can't just delete steps that way.

Actually, you can do this with a shunt type attenuator, which was described in the original post. That type has one large series resistor (50k in this case), and an arbitrary number of additional resistors that form the other resistor of the voltage divider. So the OP could take the design he has and use every other resistor value if he wants, and that will work as I think he intends.

See the Goldpoint description of the shunt type attenuator in the link below.

https://www.goldpt.com/attenuator_types.html

 

[Mark Tillotson]

Note that the shunt attenuator is the worst performer from a noise perspective as it has a fixed large series resistor, the enemy of quiet.

 

[NE WAY]

Here is a good explanation of the various attenuator types.

https://www.goldpt.com/attenuator_types.html

The series attenuator behaves pretty much like a regular potentiometer with detents, while a shunt type presents a variable impedance to the circuit preceding it. My preference would be the series attenuator, especially when used as a volume control in a passive preamp.

Note that Goldpoint no longer offers ladder and shunt attenuators, but you can of course build one yourself if that is what you want.

After reading the article I decided to use series instead of shunt. Thank you

 

[NE WAY]

You can't just delete steps that way. You can select the steps you want and add all the resistors between them. That is a very substantial switch. It looks like its for high current. Hopefully those are silver contacts.

Thank you, I was going with a shunt initially but decided to change to a series…

My understandings is that with the shunt I could just jump and omit those between, but with series I will have to add the value of the omitted resistors, as confirmed by another contributor.

The switch is a NOS Plessey Type 72

 

[NE WAY]

Actually, you can do this with a shunt type attenuator, which was described in the original post. That type has one large series resistor (50k in this case), and an arbitrary number of additional resistors that form the other resistor of the voltage divider. So the OP could take the design he has and use every other resistor value if he wants, and that will work as I think he intends.

See the Goldpoint description of the shunt type attenuator in the link below.

https://www.goldpt.com/attenuator_types.html

Thank you for the clarification.

 

[Michael C]

Nice switch.

 

[Ixnay]

Gold Point/Elna switches are great to work with. I use them anytime that I can afford to.

 

[steveu]

An R/nR Resistor ladder like a R/2R ladder allows you to use only a couple resistor values. In any case, with only 15 steps, you need to choose the steps carefully and the last step to "off" is a big one. For a 50K "pot" with about 2dB steps, you need 10K series and 200K shunt resistors, terminated with 40K. (Somebody check my calculations? -2dB~=40K/50K and 1/40K-1/50K=1/200K)

 

[steveu]

Note that the "Goldpoint" article does not actually cover a ladder circuit??? Not Invented Here? What they call a ladder circuit is not. See R/2R ladders:
https://en.wikipedia.org/wiki/Resistor_ladder
Also, a more convenient resistor values simply gives slightly different attenuation per step, which may be a better solution.
Personally, I'd go for a digital solution. 4 relays is 16 steps, 5 relays is 32 steps...
https://en.wikipedia.org/wiki/Logarithmic_resistor_ladder

 

[Hearinspace]

I just got hold of a 15 steps Plessey make before break rotary switch and intend to use it as a stepped potentiometer for volume control of my passive pre-amp.

I think one of the questions you have to answer will be how to define the curve you want.

 

[Ixnay]

I agree that if you have 15 steps, a particular curve will suit you best. An averaged curve is not what would give you the most control. In fact, I wonder if things were set up to actually use more of the curve by starting with zero volume actually being low volume, thereby allowing 1 or more resistors to be used at a higher attenuation.