Alternative to JLM VU buffer Boards

[ruffrecords]

Attached is an updated schematic using your transformer circuit. Please check it out to see if I have everything OK, OK???

It looks OK to me

I do have a couple of questions though. 1) Your schematic did not show any resistor value for what is shown as R7/R14 in my version. Do you remember what this value is?

They are just to provide a dc path to 0V for the output caps. 47K will do fine.

2) I have shown the types of the diodes connected to the VU meters as "BAT85" only because you used that type in your voltage reference circuit. But, isn't the type of diode typically used in this application a type 1N4148? Or, it doesn't matter what is used?

The diodes inside a true VU meter used to be germanium types IRRC. The BAT85 is a Schottky type with a similarly low forward voltage. Here is a link to the original JLM schematic which also uses them. You only need the diodes if you use a cheap VU meter which does not have them built in.

3) The 10K potentiometer is just a single-turn trimmer pot, correct? And.....what exactly is the function of this pot? Level control?

I use a 25 turn trimpot. It is used to calibrate the input level. You feed in whatever level you want to read 0VU and tweak the pot until the meter reads correctly

4) Does the capacitor that is connected to the wiper of the pot have its ( + ) side connected to the ( + ) pin of the TL072 (pins 3 & 5) and your R3/4? I am unfamiliar with your polarized capacitor symbol. THANKS!!!

Because of the dc bias, all the op amp signal pins are at half the rail voltage so the +ve of all the caps connected to the op amp need to go to the op amp pin.

Cheers

Ian

 

[lestatdog]

Only if your Classic Solos have been built with 12V dc heater, If not then you will need a separate 12V dc supply for the meters. You might be able to create s simple dc supply from the heater winding if you are using ac heaters.

Cheers

Ian

I'm sure I can find 12 v from somewhere? Do you have pcbs for sale? Thanks

 

[ruffrecords]

I just noticed a couple of errors in the schematic, C2 should b 10nF and C5 should be 10uF.

Cheers

Ian

 

[MidnightArrakis]

I just noticed a couple of errors in rge schematic, C2 should b 10nF and C5 should be 10uF.

Cheers

Ian

[I just noticed a couple of errors in rge schematic] -- Where or who's -- rge schematic -- are you referring to?

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[ruffrecords]

[I just noticed a couple of errors in rge schematic] -- Where or who's -- rge schematic -- are you referring to?

/

Sorry, should be the schematic. See post #14

Cheers

Ian

 

[MidnightArrakis]

If I am understanding you correctly.....your C2 (10uF) is in the "Reference Voltage" section of the schematic and is now shown as "C3" (.01uF) in my attached version. Then, what you are showing as C5/6 in your schematic (3K3uF) in your schematic is now equivalent to "C4 & C7" (10uF) in my schematic, am I correct?

THANK YOU!! for all of your assistance!!!

/

 

[MidnightArrakis]

Sorry, should be the schematic. See post #14

Cheers

Ian

I apologize for having to bother you, but I just wish to make sure that I am understanding your corrections properly.

If I am understanding you correctly.....your C2 (10uF) is in the "Reference Voltage" section of the schematic and is now shown as "C3" (.01uF) in my attached version per your recent correction. Then, what you are showing as C5/6 in your schematic (3K3uF) in your schematic is now equivalent to "C4 & C7" (10uF) in my schematic, am I correct?

THANK YOU!! for all of your assistance!!!

/

 

[ruffrecords]

I apologize for having to bother you, but I just wish to make sure that I am understanding your corrections properly.

If I am understanding you correctly.....your C2 (10uF) is in the "Reference Voltage" section of the schematic and is now shown as "C3" (.01uF) in my attached version per your recent correction. Then, what you are showing as C5/6 in your schematic (3K3uF) in your schematic is now equivalent to "C4 & C7" (10uF) in my schematic, am I correct?

THANK YOU!! for all of your assistance!!!

/

I was referring to the component designations in your schematic. So your C3 was correct at 10uF. It is your C2 that needs to change from 10pF to 100nF. Your C4 and C5 should revert back to 3.3uF (although 10uF will do if you want to reduce the number of different cap values) and it is your C5 that needs to change from 10pF to 10uF.

Cheers

Ian

 

[MidnightArrakis]

I was referring to the component designations in your schematic. So your C3 was correct at 10uF. It is your C2 that needs to change from 10pF to 100nF. Your C4 and C5 should revert back to 3.3uF (although 10uF will do if you want to reduce the number of different cap values) and it is your C5 that needs to change from 10pF to 10uF.

Cheers

Ian

I apologize!!! Since you are the designer/originator of the VU-meter circuit, I incorrectly assumed that you were referring to - your - Reference Designators. THANK YOU!! for getting me straightened out. These threads, at times, can become rather confusing.

Meanwhile and forging ahead.....I have attached an updated schematic with your suggested corrections. Please give this a quick review and let me know if "all is well" with the entire schematic. I believe that I now have everything correct (I hope).

I do have one quick question, though. You had mentioned that the 3.3uF capacitor could be changed to a 10uF capacitor, which I have done. But, I was wondering if that value change wouldn't affect the low-end frequency response and thus, the general meter needle movement? Or, it just doesn't matter?

Again.....I greatly and sincerely appreciate all of your kind assistance and knowledge (and, "patience") with getting this VU-meter driver circuit functional!!!

THANKS!!!

JBW

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[ruffrecords]

Looks to me like we have a good schematic now. The low frequency response is set by the 10uF capacitor and the 2K2 resistor R5/11. This means the response will be 3dB down at 7.2Hz. With the 3.3uF capacitor it would have been 3dB down at 22Hz. I do not think anyone would be able to detect the difference from the meter motion alone.

Cheers

Ian

 

[MidnightArrakis]

[Looks to me like we have a good schematic now] -- "THANK YOU!!" so very much!!! And, even though this is essentially - your - schematic, only updated and re-drawn by me, please feel free to share it with anyone who needs a VU-meter buffer-amplifier circuit such as this. There are never enough VU-meter buffer-amplifiers, ya know???

JBW

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[PermO]

I have been using some of these;

https://nl.aliexpress.com/item/4001013648120.html?spm=a2g0s.9042311.0.0.2eb84c4dbh7K1q
Price has come up though, by quite a lot, I got 3 boards for 19,- euros 6 months ago, shipping included (VAT rules have changed in the meantime)

It takes 12-18V AC, can be callibrated and you can set the voltage for the backlights.

I should have gotten more of these...

 

[MidnightArrakis]

I have been using some of these;

https://nl.aliexpress.com/item/4001013648120.html?spm=a2g0s.9042311.0.0.2eb84c4dbh7K1q
Price has come up though, by quite a lot, I got 3 boards for 19,- euros 6 months ago, shipping included (VAT rules have changed in the meantime)

It takes 12-18V AC, can be callibrated and you can set the voltage for the backlights.

I should have gotten more of these...

Should you have one of these VU PCB's that you can spare for awhile, I could "Reverse-Engineer" the PCB, develop a BOM, create a schematic from the PCB and then by inputting all of this information into my CAD schematic program, I could then design a new PCB of this little board!!! This is what I do!!! In fact, I just finished doing a "Reverse-Engineering" project for a guy who had some API 2520 modules. Whaddya say??? Interested???

JBW

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[PermO]

They're all in use...

It would be cheaper for you to buy one from China then it would be for me to ship mine to you from the Netherlands.

I could unmount 1 and post some close up pictures top and bottomside later.

 

[MisterCMRR]

I'm always a bit amused at the way "VU meter" is casually used to mean simply a level indicator whose needle moves in response to a signal. To quote "A real VU meter has a very specific "ballistic characteristic". This means that it responds to changing audio signals at a very precise speed, rising from no signal to 99% of "0 VU" when a 1 kHz sine wave tone is applied for 300 milliseconds." (see https://en.wikipedia.org/wiki/VU_meter). Meters that claim to be "VU" meters, never seem to even mention ballistics ... and yes, it's a big deal if you're doing serious work. Interesting to also note that, by today's definition, the level difference between reference level (+4 dBu) and the clipping point (16 dB above reference level) becomes +20 dBu or 7.75 Vrms or ±11 V peak or 22 V peak-to-peak). This would define a system with 16 dB of "headroom."

 

[Gold]

As a resident VU meter liker/defender I will just say that as a level meter for digital, a VU meter is inferior to a PPM. A PPM which displays level accurately is misleading when it comes to loudness. A Volume Unit meter displays both level and loudness. It does neither one perfectly but with experience gives you a good indication of both.

Lets take that snare drum which because of the integration time misses the peak level. If you look at a PPM you would probably back off the level until you don't see an over. Technically correct. If you are using a VU meter you know it should sit somewhere around -10VU because the peak level could be as much as 30dB above that. Since clipping a snare drum by a few dB is practically inaudible the VU meter would lead you to use your ears to balance the drum set and not worry about the inaudible clipping on the snare. I think the VU meter leads you to produce better results.

Since we want to know both level and loudness the PPM needs interpretation also. If the snare drum is hitting 0dBfs and a vocal is sitting around -8dBfs you might think the snare drum sounds louder from looking at the meter. It wouldn’t.

 

[MisterCMRR]

It's for exactly this reason that I've always thought the best arrangement for a low-cost (and compact of panel space) solution was a VU meter (real one) plus a red LED (either integral to the meter or very near it) to flash (using a 50 ms one-shot) at clip. Best of both worlds! Many Quad-Eight boards during my era there (early seventies) had this feature standard.

 

[JohnRoberts]

It's for exactly this reason that I've always thought the best arrangement for a low-cost (and compact of panel space) solution was a VU meter (real one) plus a red LED (either integral to the meter or very near it) to flash (using a 50 ms one-shot) at clip. Best of both worlds! Many Quad-Eight boards during my era there (early seventies) had this feature standard.

The customer is always right, but I like being right too... When customers ordered mechanical VU meters on my big consoles last century, I provided red LED peak indicators by each mechanical.

1 VU (Volume unit) = 1 dB. Indeed "true VU" meters have a very specific attack/release ballistic when driven from a 600 ohm termination.

I am not a fan of ineffective eye candy, but understand that the customer is always right, even when horribly wrong.

JR

 

[MidnightArrakis]

It's for exactly this reason that I've always thought the best arrangement for a low-cost (and compact of panel space) solution was a VU meter (real one) plus a red LED (either integral to the meter or very near it) to flash (using a 50 ms one-shot) at clip. Best of both worlds! Many Quad-Eight boards during my era there (early seventies) had this feature standard.

SOUNDS GREAT!!! Do you have any schematics of a combined "VU-meter/LED-clipper" circuit of which you can share? I would be highly interested in one!!!

I do also understand the importance of the meter ballistics, but such VU-meters then become exorbitantly expensive!!! Sometimes there is a need to monitor an audio signal level in a more -- general -- sense than in a precise scientifically accurate manner. I have had more than enough experiences where I was just thrilled to see a VU-meter wiggle at all than I was in knowing how accurately and quickly the needle was reacting to the incoming signal.

I can remember receiving catalogs of the Quad-Eight consoles in the mail long ago and always thinking that they were interestingly designed. I feel honored to be writing to someone who once worked there and designed their circuitry!!! THANK YOU!!!

/

 

[ruffrecords]

SOUNDS GREAT!!! Do you have any schematics of a combined "VU-meter/LED-clipper" circuit of which you can share? I would be highly interested in one!!!

Well, strictly speaking ,a VU meter needs no extra circuitry for it to work; it is totally passive. Peak LED schematics abound - search this site and you will find discussions of them

JLM's latest VU buffer includes a peak LED. They publish the schematic on their site. Now normally I would not suggest copying a design currently in manufacture but in this case, the original circuit the JLM is based on was first published on the internet many years ago.

Cheers

Ian