Imput impedance on volumecontrol? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: November 16, 2021, 06:04:49 am
content: Hi all you great people, I am wondering on how and if impedance is important on a circuit where input +/- is wired to a volume control without grounding.Below have I tried to make two similar circuits where the resistive value is min 120 Ohm, the left uses one pot and do thereby change the impedance, while the one at the right is the value of both pots, no matter position should be the same value, due to one pot raises in value while the other one lowers in value at the same time.

author: fourfathom
author_rank: Super Contributor
timestamp: November 16, 2021, 06:44:18 am
content: If you ground pin 3 of the left-hand circuit it will be functionally identical to the right-hand one. As you have it now, the left-hand circuit will not be a particularly good volume control, as it will only work if the potentiometer value is *much* higher than the input impedance of whatever is connected to the output jacks. Even then, it won't give you "from maximum to zero" control.

author: FriedMule
author_rank: Frequent Contributor
timestamp: November 16, 2021, 09:19:54 am
content: The pot(s) has only two pins, the ground is not connected to the case or to anything other than the negative wire.Does that change anything? :-)I hope to make it a zero to max volume control, I am not able to change the pot value, but its lowest resistance is 120 Ohm and a max of 300M Ohm.What if I set the pot go from, i.e. 100K instead?

author: tszaboo
author_rank: Super Contributor
timestamp: November 16, 2021, 11:39:14 am
content: A dedicated volume control should have 10KOhm input impedance, and low output impedance of 100 Ohm (for a line output) or close to 0 for a headphone output.The best implementation will probably have an OPAMP before and after the pot, just like on the O2 headphone amp.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 16, 2021, 12:04:37 pm
content: This is the usual rule of thumb is- Zo =10x pot- its good enough.A buffer amp or an emitter follower solves most impedance matching problems. You should use a buffer to drive cables. Thier low output impedance swallows low level hum from the line.

author: CaptDon
author_rank: Super Contributor
timestamp: November 16, 2021, 03:22:43 pm
content: I guess it depends on your 'load' impedance also. A 10k pot would be a bad choice to drive headphones directly, and a 500 ohm pot would be a bad choice as the Input Level control for something like a guitar. Look also at L-Pads where the impedance shown to an amplifier's output remains constant but the volume of the speaker can go 0 to 100%.

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2021, 05:50:02 pm
content: The variation in impedance can be a problem with some circuits. There is a configuration using a dual pot to make a volume control with less impedance variation.

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2021, 06:02:45 pm
content: Google "L pad attenuator" for the two variable resistor solution.e.g.,https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.htmlThe original posting is ill-posed: a single variable resistor, as mentioned in replies, will not take the level down to zero, and the results depend directly on the source and load impedances that were not specified.The usual "volume control", in the middle of an amplifier, works between a low source impedance (preamplifier) and a high load impedance (power amplifier input). A three-terminal potentiometer is the normal device there.L pads are typically used between the output of a power amplifier (for example designed to drive an 8 ohm load) and a loud speaker that has an 8 ohm impedance.

author: FriedMule
author_rank: Frequent Contributor
timestamp: November 19, 2021, 09:16:18 am
content: I am sorry that I have been so slow to reply, had some illness.The output impedance is whatever is normal for a music player source, such as a CD-player, tape deck, tuner or what may be interesting, except an LP-player.Output shall be to a power amplifier of what ever brand and type I can get hands on.As it is now, does my "pre-amp" have an input selector that goes directly to where an attenuator should have been, I have measured the impedance to 20 Ohm for the input selector, then is there these chip-pots with 100 Ohm impedance as minimum.Each "pot" has one input and one output. It looks like some "dimmer" circuit for LEDs but for audio.David Hess wrote: "There is a configuration using a dual pot to make a volume control with less impedance variation." this is what I think is going on in my drawing on the right side.But how would you do it, if you had to, logarithmic dim two LEDs in the opposite way, one up while the other down, so the total impedance is the same?