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True peak programme meter

GK.

Member
Joined
Nov 22, 2024
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Here is my initial prototype build of a LED level meter design, with a few fudges made to the PCB which have since been included into the second board iteration.

I wanted to make a meter that is not in anyway average or quasi-peak responding to audio-frequency signals and that has a dynamic range sufficient to indicate the noise floor of a signal source.

As far as audio frequency signals are concerned, the unit has practically zero attack or integration time. It will accurately indicate the peak amplitude of one single half-cycle of a 20kHz sine wave of either polarity.

As for the dynamic range, there are 48 LEDs at 2dB intervals. The most significant yellow LED is the 0dB indicator, calibrated to +4dBu (professional line level - 1.736V peak). The least significant LED is activated by a few tens of uV.

The decay rate is adjustable via the potentiometer from very fast to very slow, so you can have either a dynamic display visually dancing to the music or one capturing and holding the peaks.

The audio input is balanced with a high CMRR for ground loop hum rejection.


The achieved dynamic range and true peak reading response couldn't have been done along the lines of traditional techniques using active rectifier and peak detection circuitry, so there is none of that whatsoever in the design.

The input signal, split into three separate gain groups, is fed directly to level comparators and the LED driving and decay is handled after the comparators digitally.

The PCB is designed with a low profile for stacking. I am currently designing a 1U rack panel in CAD (Front Panel Designer) for inclusion into my audio system which will sport a stereo pair.

Most of the parts are dirt-cheap garden variety sorts, except for the Analog Devices op-amps in the low-noise/drift signal section which are quite pricey.

I'll eventually publish the design and post the Gerber files when I've finished writing it up.


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The power supply specifically intended to power the stereo pair of level meter boards in my next aforementioned build.

At 20mA per LED there is a current of nearly 1A peak being digitally switched per board. Given the input sensitivity of the design, if the power supply distribution and routing wasn't given adequate care things wouldn't work out too well.

The LED bank of each board has its own isolated power source to prevent ground loops. The most practical and cost-effective way to achieve this is with a pair of isolated-output DC-DC converters. The main filter capacitors are axial-leaded to keep the profile low enough for a 1U case.

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this looks like a really cool project . I'd love to have something like this in my system .
if when your finished it looks achievable I might have a go . I guess I'd have to get the boards built and populated by pcbway or similar then what would be involved in putting it altogether? make a case source the power supply and other bits? how would you rate the difficulty of this project for a beginner with some soldering experience
kind regards James
 
That's a difficult question to answer because beginners aptitude for this kind of thing varies an order of magnitude between individuals, but in any case I wouldn't call this a beginners project.

-------------------------

Just about to send off the final Gerber files for manufacture after fixing an error upon review (R270 connected to GND instead of -15V) and making some minor component value revisions. Fingers crossed.......


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Excellent DIY project. For what is it intended?
 
I installed a stereo Type 1 PPM in my home system for monitoring the peak levels in both LPs and my digital sources when I became interested in home many CDs were pegged at 0dBFS, or if they cheated, and still pegged the levels, but normalised to -1dBFS. Horrible just how many CDs have flat-top levels.

For LPs, I was interested in peak levels, as a way of knowing what the overload margins need to be on decent phono stages. Most of my LPs peak at no more than +13dB above 5cm/sec but the occasional click goes higher, so the minimum of 20dB overload margin that we accepted as a 'rule of thumb' in the 1960s and '70s is still very much valid.

I would like a true-peak PPM, as a Type 1 still will under read somewhat, but it's still faster than the Type 2 PPMs in common use in the UK.

S
 
Whats the typical frequency range and duration of a vinyl tick and pop? I remember reading an article in a 70's electronics magazine about a noise blanker developed to audibly eliminate ticks and pops. They were banging on like it was the most amazing new invention in audio, oblivious to the fact that this had already been done in radio since before WWII.

They were talking about durations in the order of a mS or so.

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As I wait for my boards to arrive, I am laying out the PCB for another true PPM for another purpose. This one only has 33dB range and 16 LEDs. 33dB is rather undemanding from the signal processing standpoint, so the design is a lot simpler and cheaper. Just run-of-the-mill TL74XX op-amps do here. I've also included an averaging detector, so it can be run in either true PPM or quasi PPM modes. I am going to apply this design to a 1/3 octave spectrum analyser build.

The 16 LEDs indicate as follows:

+3
+2
+1
0dB
-1
-2
-3
-6
-9
-12
-15
-18
-21
-24
-27
-30
 
New boards are loaded and verified to be error free. Now I just have to send off the 1U rack front panel design for manufacture. When that arrives I'll fabricate the enclosure.

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That's a nice build.

I did briefly contemplate the sampling option using a high-end audio ADC, but I am not sure that I could have as easily achieved the same combination of dynamic range and true peak reading response to a single cycle at 20kHz.

The total RMS system noise needs to be substantially less than the DC threshold at which the lowest level indicating LED triggers.

I aimed for 20dB less and achieved ~23dB in a bandwidth substantially greater than 100 kHz. Improving upon this much would have increased cost and complexity excessively for little reward. For real world usage the absolute indicating accuracy at the end of the scale below -90dB isn't of much concern.

The datasheets for the best audio ADCs specify signal to noise ratios or dynamic range in this order, at around 120dB, but often the test conditions are unstated or they specify in a 20kHz bandwidth or with audio A-weighting only, regardless of the sampling rate. That is fine as far as audio goes, but is a bit lacking when it comes to evaluating the part for instrumentation purposes in the full bandwidth at higher sampling rates.

So -120dB is probably not going to happen and that is for the ADC only. The signal amplifier preceding the ADC which absolutely has to be balanced is going to add to the total system noise substantially.
 
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Well this project has come to a depressing halt. I've just finished after putting an inordinate amount of time into designing the front panel for this project and one other, then I got to ordering.......


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Prior and mounting experience with increasingly insular suppliers in America demanding ridiculous shipping surcharges so as to be adequately compensated for the indignity of having to deal with a foreigner leaves me not very optimistic that I'm not just going to get fobbed off here.

Any suggestions for a viable alternative to Front Panel Express?
 
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So I got a not particularly useful bot response from FPE more or less confirming the above shipping quote and offering zero explanation for the ridiculous markup above standard shipping rates.

Quoting from their "about" page where they expound upon their awesomeness (my bold):

Across the world, thousands of technicians, engineers, hobbyists and companies of all sizes place their trust in our long-standing production expertise and delivery dependability. We still have a lot of plans for the future and will work passionately to keep impressing you with our quality, service, and innovation

Yar! Very friendly shipping rates for international hobbyists (sarcasm). Pity. I last used them in 2019 and three or four times before then and the results was always worthwhile. Now they are just unviable turkeys from my geographic location.

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This level meter project is the first of multiple planned components for a complete rack-mount Hi-Fi system which has to end up professional looking. So I need an alternative.

There are plenty laser/CNC/water jet places which I can get to cut/drill/route my basic front panels. There's a local anodizing firm which I've used in the past who I can then deliver the panels to.

After that there is only the engraving and wax filling to sort out. I did the wax filling stuff on pantograph-engraved panels in a workshop 25 years ago when I was an apprentice.
I'm not keen on pantographing anymore, but have been pondering getting a half decent CNC engraver/router for a while now.

The engraver/router doesn't necessarily have to be large and expensive enough to fit an entire 5U-high 19" rack panel on the bed. The bed only needs to be large enough for a 5U (222mm) panel to slot into and I could engrave it in halves or in thirds.

So, any CNC router enthusiasts here?
Model recommendations?

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This is amazingly cool and impressive (professional) DIY project, indeed!:)

If possible, I believe it will never be possible though, I really would like to have six (6) of your True Peak Programme Meter in addition to my DIY-built IEC_16028-17_ballistic-behavior-compatible large-glass-face 12-VU Meter Array monitoring SP-high-level output of multiple amplifiers in my PC-DSP-based multichannel multi-SP-driver multi-amplifier fully active audio setup (ref. my post here #535 and #931on my project thread).
Please visit post #535 on my project thread for the details...
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Because of my personal nostalgia, I still love/like large-glass-face conventional-needle-type peak log-compression program meter plus IEC_16028-17_ballistic-behavior-compatible VU-Meter; my dream post here, therefore, would be also of your interest and reference, I assume.
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Schematics

Really interesting old school design.

Is there a reason you chose this approach rather than a microcontroller + ADC system? (Other than just the for fun "because it's there" :) )
 
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Well this project has come to a depressing halt. I've just finished after putting an inordinate amount of time into designing the front panel for this project and one other, then I got to ordering.......


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Prior and mounting experience with increasingly insular suppliers in America demanding ridiculous shipping surcharges so as to be adequately compensated for the indignity of having to deal with a foreigner leaves me not very optimistic that I'm not just going to get fobbed off here.

Any suggestions for a viable alternative to Front Panel Express?

I have also recently been surprised by very high shipping charges (USA to UK).

I don't want to get into a political discussion but I think the reason may be that shipping capacity is currently in very high demand as importers/exporters try to stockpile goods in warehouses inside/outside of the USA in an attempt to avoid the proposed tariffs.
 
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