I’ve just had a VU meter experiment myself for a diy preamp build. I had one Radio Shack VU meter from the 80’s, and thought to finally give it some life. The meter measured to be 200uV to reach full scale and had a 600-ohm coil resistance value. I only have one meter, so I was going to connect to both channels to give a sense of peak scale. I wanted to minimize any impact to the preamp by the meter circuit, and since there is +- 12v power in the preamp, I used a LM324 quad op amp chip. Each and all op amp was configured as non- inverting buffer, one each for the left and the right channel with 1M-ohm input impedance, and combined signals to a peak and hold circuit with a buffered output to the meter. The design was to aim for a 2V p-p input signal for the 100% mark on the meter - around 18% of the input peak, after rectification and diode voltage drops. I’m happy with the results. I’m surprised to see a passive design for a VU meter. If you liked the front look of this meter and don’t mind a “peg board” look on the back, I will be happy to build an active circuit for it for you, free of charge. Just let me know. -Peter
On the FWB, many times these are included when the module is meant as a retro-fit to an existing system whereby there is not a known power-supply configuration. Also, I've seen this done as a defensive move that makes sure the correct polarity is always applied, especially if the module is subject to a warranty option, whereby it prevents destruction of expensive (relatively speaking) components (say the meters in a repair scenario), and also eliminates the hassle of arbitrating fault between the customer and the manufacture--pretty cheap insurance...
That’s a great way to provide the voltage, but the current should only be less than 10 mA so that’s why I used a zener in series just to drop the aux voltage down to the range needed.
Great question! You can see from the board that I got the polarity correct as the caps negative end is the black part of the cap, but as far as if it is correct or not - let me explain. The positive pin gets the signal, but when it goes negative, the diode D8 keeps it 0,7V from ground (negative), so it will get a small negative charge but a diode drop is generally OK. A poly cap would have been better:)
@@KissAnalog Is the input connected to line or speaker level? I guess speaker because it would load a line output to much. So the right hand side of the cap can't go lower than let's say -0,6V but the left hand side (marked +) can go close to -24V rail, if the pot R2 is turned all the way to input (R) - max sensitivity.
@@KissAnalog Yeah, it looks like you got the schematic right based on the board, as far as I can tell from the video. But I think it's still wrong, the cap has a low-res path through D8 to get charged in reverse of its polarity and about a 1K res path via D6 to get charged correctly, so it will average out in reverse. Another way of looking at it, the input amplitude is gonna average around ground (or the DC offset on the output of the amp), but the output of this circuit is positive compared to that same ground. That can only be if the right hand side of the cap is positive compared to the left hand side.
Yes I think you are correct John! I put DC voltage in and it worked, but then the aux supply I was using died:( It should have had plenty of current handling for these LEDs, but maybe I shorted it - which it should have handled as well. OH well - you win some and some you don't;)
Thanks! I appreciate your time, watching your vids save me a lot of time and I learn a lot, so thank you for that.
Thanks so much! I appreciate you!
I’ve just had a VU meter experiment myself for a diy preamp build. I had one Radio Shack VU meter from the 80’s, and thought to finally give it some life. The meter measured to be 200uV to reach full scale and had a 600-ohm coil resistance value. I only have one meter, so I was going to connect to both channels to give a sense of peak scale. I wanted to minimize any impact to the preamp by the meter circuit, and since there is +- 12v power in the preamp, I used a LM324 quad op amp chip. Each and all op amp was configured as non- inverting buffer, one each for the left and the right channel with 1M-ohm input impedance, and combined signals to a peak and hold circuit with a buffered output to the meter. The design was to aim for a 2V p-p input signal for the 100% mark on the meter - around 18% of the input peak, after rectification and diode voltage drops. I’m happy with the results.
I’m surprised to see a passive design for a VU meter. If you liked the front look of this meter and don’t mind a “peg board” look on the back, I will be happy to build an active circuit for it for you, free of charge. Just let me know.
-Peter
Thanks Peter for your awesome feedback! I’d love to test anything that you think is worth a look:)
On the FWB, many times these are included when the module is meant as a retro-fit to an existing system whereby there is not a known power-supply configuration. Also, I've seen this done as a defensive move that makes sure the correct polarity is always applied, especially if the module is subject to a warranty option, whereby it prevents destruction of expensive (relatively speaking) components (say the meters in a repair scenario), and also eliminates the hassle of arbitrating fault between the customer and the manufacture--pretty cheap insurance...
Thanks for your feedback!
I Eddie, thanks for doing this Video. I really liked it. 🎃
Thanks so much!
A 7809 or 7812 fixed voltage regulator would supply a nice low impedance dc supply.
What is the current draw of the VU board?
That’s a great way to provide the voltage, but the current should only be less than 10 mA so that’s why I used a zener in series just to drop the aux voltage down to the range needed.
😀Maybe the resistance is too low in the meters.
4:57 Shouldn't the polarity of C3 be the other way around? Now it looks like it's gonna get reverse charged when the input signal goes negative.
Great question! You can see from the board that I got the polarity correct as the caps negative end is the black part of the cap, but as far as if it is correct or not - let me explain. The positive pin gets the signal, but when it goes negative, the diode D8 keeps it 0,7V from ground (negative), so it will get a small negative charge but a diode drop is generally OK. A poly cap would have been better:)
@@KissAnalog Is the input connected to line or speaker level? I guess speaker because it would load a line output to much.
So the right hand side of the cap can't go lower than let's say -0,6V but the left hand side (marked +) can go close to -24V rail, if the pot R2 is turned all the way to input (R) - max sensitivity.
@@KissAnalog Yeah, it looks like you got the schematic right based on the board, as far as I can tell from the video. But I think it's still wrong, the cap has a low-res path through D8 to get charged in reverse of its polarity and about a 1K res path via D6 to get charged correctly, so it will average out in reverse.
Another way of looking at it, the input amplitude is gonna average around ground (or the DC offset on the output of the amp), but the output of this circuit is positive compared to that same ground. That can only be if the right hand side of the cap is positive compared to the left hand side.
It's cool that it's Analog but even the build quality looks disappointing for it being new
It would have been great to have screw down connector blocks or something like that.
@@KissAnalog for sure. Be cool to diy the gauges from old parts on devices
It's AC or DC powered I think, and the DC has no polarity due to the bridge. I thought the meter says 5-12v.
Yes I think you are correct John! I put DC voltage in and it worked, but then the aux supply I was using died:( It should have had plenty of current handling for these LEDs, but maybe I shorted it - which it should have handled as well. OH well - you win some and some you don't;)
@@KissAnalog Maybe it needs a floating supply.
i'm not sure he has enough multimeters...
LOL I might need to get just one more;)
Coffeeeeeee
Yahoo!! ;)