Author Topic: A simple passive logarithmic VU-meter  (Read 677 times)

Offline El Sid

A simple passive logarithmic VU-meter
« on: July 22, 2018, 12:09:57 AM »
I was looking for a way to drive some VU meters kindly given to me by fdlsys. I was going to just copy the circuit of the Scott amp it came from, but realised i couldn't use it because that amp has a split supply, so I wouldn't have the midpoint ground reference. I found all sorts of ideas, mostly using op amps. But then I found this - A simple passive logarithmic VU-meter. It works similarly to the Scott circuit but just needs input and ground. I'll probably have to fiddle a bit with the resistor values because my meter is 1200 ohm. (interestingly it is so sensitive that the tiny current the multimeter uses for measuring deflects the needle almost to 0dB). I thought may be useful to share in case others have VU/Power meters lying around.

Pretty straightforward:

Offline Steerpike

Re: A simple passive logarithmic VU-meter
« Reply #1 on: July 22, 2018, 12:54:46 PM »
Depends very much on your application & requirement.
That will be a "sort of general level meter", but not a "VU" meter. "VU" means it has a very specific electrical and mechanical response, including timing, defined by the ASA.
Diodes used for tailoring amplitude response are very temperature sensitive, so it will behave differently depending on ambient conditions if no temperature compensation is added.

Offline El Sid

Re: A simple passive logarithmic VU-meter
« Reply #2 on: July 22, 2018, 06:55:24 PM »
Point taken Steerpike - in my case it is really just to have a cool-looking meter that bounces when you play music, not for any meaningful measurements or setting recording levels. In fact I misnamed it for my purposes because I'm actually going to apply it to a power meter

Offline Murray

Re: A simple passive logarithmic VU-meter
« Reply #3 on: August 22, 2018, 07:32:20 PM »
Not sure why R2 does not just join between D3 and D4, Leave D4 out entirely.

Offline El Sid

Re: A simple passive logarithmic VU-meter
« Reply #4 on: August 22, 2018, 11:23:19 PM »
Not sure why R2 does not just join between D3 and D4, Leave D4 out entirely.

I have no experience of using diodes at low forward bias, so can't answer definitively. But with 2 diodes in the second leg, isn't there a wider range of non-linear (logarithmic) behaviour once that pair starts conducting than if D3 was connected to D2, as D2 would already be conducting fully once D3 starts conducting?

But it is a smart idea. I will play around when I build the circuit - at a whopping 21c apiece from Mantech I can afford to play around with 1N4148s...