I have a led vu meter based on the KA2284 IC. If I connect it to the preamp input it works fine but as soon as I connect it to the output of the preamp the signal level of the output drops drastically. The preamp is based on NE5532 opamp. Im a total novice, I'm guessing it's to do with impedance matching but can any experts help me diagnose the issue? Preamp board front/rear and vu meter images attached....
Attachments
Connected to the input of the pre-amp you are measuring the full signal (say 2V line level) so you are measuring the level of the music. Whereas connected to the output of the pre-amp you are measuring the volume adjusted level. Thus if adjusted to indicate at 'wake the neighbourhood' volume, the LEDs will barely flicker at normal listening levels.
Is this the symptom you are describing?
Is this the symptom you are describing?
I have adjusted all levels to match so the input is 1v and the output is 1v. The vu meter works perfectly on the input but as soon as it is connected to the output, the output voltage drops to 0.1v
No, you have us at a loss here.
The LED unit has a 10K adjuster pot. If the LED unit is set to minimum sensitivity this is the only load across the input.
Does connecting a 10K resistor across the pre amp output have the same effect on volume?
The LED unit has a 10K adjuster pot. If the LED unit is set to minimum sensitivity this is the only load across the input.
Does connecting a 10K resistor across the pre amp output have the same effect on volume?
A 10k resistor across the output reduces the voltage by about half. So does this mean the output impedance of the preamp is around 10k? Combine that with a possible low input impedance of the led vu meter and I guess we have an answer. I'll try and measure the input impedance of the vu meter (still a total novice but trying). I've ordered a different vu meter (velleman kit k4305) with a 741 ic at its input so I'm hoping this will act as a buffer and provide a high input impedance. I've also got another preamp kit I could try using different opamps. Hopefully I'm talking some sense... six months ago I wouldn't have understood a word of this thread, lol I still may not, time will tell. Really appreciate your input guys, its a massive help in getting this project off the ground but also massive help in my learning. FYI I'm building a mobile battery powered sound system and this preamp is acting as a crossover and signal splitter to drive various amps.
I'm going to monitor 3 channels eventually. The input from a mixer, the preamp straight output and the preamp crossover output to the sub. Im just prototyping at the moment. The whole reason for the preamp is that originally we were using multiple y cables to split the mixer signal out to a sub, a minirig and the main amp. We were getting a volume drop so I thought, let's add an active preamp with crossover. Seems I have exactly the same issue. I'm wondering if my mates mixer has a high output impedance which was causing the volume drop or maybe the active sub had a low input impedance. I'm causing new problems every day but Im learning.
What do the numbers read on the preset pot on the VU board ? because the resistor should have given the same drop in voltage .
The KA2284 will measure both DC and AC voltages . For DC you short circuit the input capacitor and vice versa for AC is that what the removable link on the board is for ? If that is what the link is doing and you still have it inserted then you may be measuring DC instead of AC .
No markings but reference circuit shows 10k
If I measure resistance across the pot it's 100 ohms but not sure if that's valid if its still connected to the board and other components
Unfortunately the data sheet I found does not give any internal details of the KA2284 . If the resistance you measure across the preset does not change when you reverse the meter leads then that would instill a bit more confidence , but the results so far suggest that it is lower than 10K and 100 ohms would fit the results quite nicely .
The outputs of the preamplifier are fed directly from the volume pots so the output impedances will vary from very low to 1/2 of the value of the individual pots.
So I had incorrectly assumed that the volume pots controlled the input to the op amp. Now I have measured the output impedance with the pots fully open I get 8K Ohms. I received the new preamp kit and its output impedance is around 50 Ohms. The led vu meter does attenuate the voltage down on the new preamp but only a little but the leds light up. So am I right in thinking that the high output impedance of the preamp combined with the low input impedance of the vu meter create bit of a problem. This situation will also be a problem for the 10k Ohm input impedance of my amp even without the vu meter, the voltage/volume will be pretty much halved. I will report back with the results of the new preamp and new vu meter kits. Thanks to all your input, ive learned some new things about diagnosing problems and impedance issues (and also "dont buy cheap crappy kits off ebay with no spec, support etc.")
You are on the right track. Ideally, you should use jfet buffer before the VU meter. It should be only 'sniffing' the signal on the preamp output, not going through or affecting it.
I recently introduced two spectrum analyzers to my system, just for visual fun. They are green fluorescent type, nice, fast, and you can select any type of curve imaginable. I did not want to affect main audio signal in the least way possible. I separated the spectrum analyzers by simple jfet buffers, worked great.
ps: if you use jfet buffer before the VU meter, you can use any preamp, even the one with high output impedance 🙂
I recently introduced two spectrum analyzers to my system, just for visual fun. They are green fluorescent type, nice, fast, and you can select any type of curve imaginable. I did not want to affect main audio signal in the least way possible. I separated the spectrum analyzers by simple jfet buffers, worked great.
ps: if you use jfet buffer before the VU meter, you can use any preamp, even the one with high output impedance 🙂