Ad VU meter api 312
- Thread starter mahoule
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Help Support GroupDIY Audio Forum:
djmiggymigz
Well-known member
That's what I'm asking, just wanted to confirm that it wouldnt cause any issues to run a track from the 500 card +-16v over to the buffer circuit.
Brian Roth
Well-known member
Nope! Just be sure to add PSU bypass caps from each rail of the buffer opamp, located close to the power pins of that chip. That's assuming a "pre-made" buffer doesn't already have the caps.
Bri
MidnightArrakis
Well-known member
>> Some time ago I had designed this "1U Rack-Mount 8-Channel VU-Meter Monitoring Chassis" purely as a means to monitor audio-levels either from equipment that may have lacked any type of level-monitoring indicator or to simply place within the audio-signal path just to keep tabs on the levels somewhere that you felt doing so was needed:
>> I had designed this chassis so the chassis and its connectors are in a "Loop-Thru" configuration, meaning.....the audio-signal simply "loops-thru" the chassis (Input/Output) and the circuitry merely is monitoring the audio-level. The audio-input connectors are NEUTRIK "Combo's", as they accept both 1/4-inch and XLR connectors, and the output connectors are "dual-stacked balanced/stereo 1/4-inch" connectors configured as a "dual-mono" or "stereo" connection.
>> As a result of working on this little project of mine, I have also collected a plethora of VU-meter driving circuits and schematics of which I could share with you should you need some additional circuits to review.
>> (This response has been certified as being "Graphically Text-based Expression-free).
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>> I had designed this chassis so the chassis and its connectors are in a "Loop-Thru" configuration, meaning.....the audio-signal simply "loops-thru" the chassis (Input/Output) and the circuitry merely is monitoring the audio-level. The audio-input connectors are NEUTRIK "Combo's", as they accept both 1/4-inch and XLR connectors, and the output connectors are "dual-stacked balanced/stereo 1/4-inch" connectors configured as a "dual-mono" or "stereo" connection.
>> As a result of working on this little project of mine, I have also collected a plethora of VU-meter driving circuits and schematics of which I could share with you should you need some additional circuits to review.
>> (This response has been certified as being "Graphically Text-based Expression-free).
/
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djmiggymigz
Well-known member
Oay awesome thank you!! I think I'll probably just run traces to an empty pad so I can use wire to send the pwer and audio signal to the buffer circuit! And yeah, I assume the JLM has the bypass caps right? Looks to me like they might be C4 and C6 on the schematic?
JLM Schematic
That would be greatly appreciated thank you!! I'm at the point in my electronics learning that I'm reading and comparing all the schematics I can! Feel free to PM me when you can! I can also share my email if it's easier for you to share that way.
One more question,, could I just build and layout the buffer circuit on my main 500 series card? I have a pretty large empty space as the 312 is relatively simple and think I could squeeze it on there pretty easily. In which case I wouldn't have to buy a buffer it'll just be self contained. Just not sure if the buffer being a completely independent PCB serves a purpose in isolating the circuit. I guess maybe the gnd connections on the buffer would need to be a separate gnd somehow?
Brian Roth
Well-known member
Yes, since you have room, a variable gain buffer would be easy to add. The JLM has extra stuff to handle the meters without diodes (unless you do select one of those) and make it functional if you don't have a bipolar (+/-) supply.
Do you want/need the peak blinkie LED?
What 312 schemo are you working from?
Bri
A vu meter adds distortion to the signal connected to it because of the diode bridge in the meter.. Some meters have a resistor (usually 3.6kohm) inside the meter body, in front of the diode bridge. 3.6kohm gives enough isolation that few will hear the distortion and it seems to enhance the sound a little bit but of course as I mentioned some will object. All vintage consoles, tape recorders, some compressors, other stuff had real vu meters that added the tiny bit of distortion.
I'm not an authority on cheap vu meters...I rarely use them. In the few cases I have done so I added a diode bridge (use germanium diodes, not silicon) and whatever resistor was needed to get 0vu with +4dbu signal level. So zero vu was accurate with 1khz tone but probably not at other frequencies I'm not sure but the ballistics aren't right...needle moves too fast and meter reading accuracy varies as level moves away from +4dbu. The last time I had to use a cheap vu meter (VU printed on meter face but it was dc voltmeter) I added diode bridge, resistor and a buffer amp cuz meter sensitivity was too low...needed something like 4 volts to get zero reading.
One of the less expensive Sifams is probably less money in the end cuz you won't have to mess around.
Cheers
Brian Roth
Well-known member
You have to dig through some of those VU articles I posted in the tech docs (and linked above), but the 3.6K series resistor for a REAL/classic VU meter was for use in true 600 Ohm balanced systems with a 600 Ohm source feeding a 600 Ohm load. With a buffer (essentially a 0 Ohm source) driving a bridging load the series R is supposed to be 3900 Ohms.
Bri
Bri
djmiggymigz
Well-known member
Variable gain buffer? Is that just referring to using a trimmer to dial in the gain of the opamp feeding the meter?
Are you referring to the actual JLM meter? (Here) Do I interpret that correctly where the only buffer I would need to add to my board then would be the op-amp to make up the gain needed? And not the circuit that's converting the AC to DC current to actually power the meter movement.
Do you have a link to what you're thinking of here? As i've linked above, I found a Sifam that fits my 500 series build but it's $67ish USD. Would love to find something that's maybe a bit cheaper that's still gonna perform well!
Brian Roth
Well-known member
Yes a variable gain buffer is the primary circuit on the JLM board. The other half of the opamp makes the LED blink for peak signals. I ASSume the inexpensive JLM meter has no internal diodes...like the alliexpress, etc. meters. The price is the clue. However, it might behave somewhat like a VU meter in terms of ballistics. I've never messed with any of the JLM meters.
The Sifams appear to have internal diodes which are part of the "elements" which make a "true" VU meter meet those published specs. Hence, the meter needs only a variable gain buffer to tweak the signal level into the meter. A buffer can be adjusted so that the VU meters reads "0" to match the system oprtating signal level.....be it -10 dBV for a hifi level circuit to +8 dBu for an ancient broadcast radio station signal level...or anything else. Welllllll....within reason.....lol.
Bri
The Sifams appear to have internal diodes which are part of the "elements" which make a "true" VU meter meet those published specs. Hence, the meter needs only a variable gain buffer to tweak the signal level into the meter. A buffer can be adjusted so that the VU meters reads "0" to match the system oprtating signal level.....be it -10 dBV for a hifi level circuit to +8 dBu for an ancient broadcast radio station signal level...or anything else. Welllllll....within reason.....lol.
Bri
djmiggymigz
Well-known member
Yeah I was thinking the same thing re: the JLM meter and it's price point! I just asked them directly on the forum so we'll see what the response is. Looking like the Sifam is gonna be my buy once cry once meter! Also realizing, I may need to go with the external/off board buffer circuit. Looks like I may actually have to get rid of my current empty space to make room for the VU meter. That's okay though. It'll probably be easier to mount the buffer directly to the meter anyway.
Brian Roth
Well-known member
My guess is the JLM meter and JLM buffer are intended as a combo. Somewhere I seem to have read the buffer may be available in different physical sizes to bolt onto the rear of various meters.
Again, the JLM combo may behave fairly closely to a true VU meter vs. being a twitchy (or sluggish) ballistic mess. Never saw or messed with one.
Indeed, you need space for a meter to "intrude" into the area behind the front panel of a 500 module. The 312 circuit will eat up available space with the two xfmrs and the 2520. You might be able to shoehorn in an opamp and a few associated components....dunno!
Bri
Matt Syson
Well-known member
The 'expensive' Sifam meters, (along with the other OLD makers of VU meters) are expensive because the magnetit pattern and the 'springs' define the ballistics and are difficult to make (or at least expensive to do so). A 'real' VU meter when hung across a 600 Ohm output introduces around 0.1% distortion of a 1khz signal when reading 0VU and worse as the level increases. Thus the need for the buffer amplifier. The AL19 (?) Sifam meters read accurately for a sinewave at 0VU for frequencies 20Hz up to 100KHz, actually below 20Hz but you have to average the needle position by eye as it is wiggling so much. Not bad for a magnet and springs!
