DC Meter buffer question

[mark2]

I built a buffer + rectifier for a DC meter, and although it works, it dramatically alters the volume and the EQ. I haven't yet taken the time to figure out exactly what it's doing to the audio, but I'm assuming it's severely attenuating some frequencies.

My questions:
1. Is this expected?
2. What if anything can I do to mitigate it? Does it need a higher impedance?

My goal is to put these meters on some boost pedals, as simple eye candy.

I used the following circuit:
(edit: probably doesn't matter, but I substituted 1N5817, and am using a tl072)

from this document: https://www.audiotechnology.com/PDF/TUTORIALS/AT51_DIY_VU.pdf

The "-IN" is connected to GND, the "+IN" is the guitar's signal simply split from the output to the amp.

Here's a video demonstrating with and without the circuit connected, though the audio quality doesn't quite do justice to the severe difference:

[blackieNYC]

Maybe the 47k to ground on the input? Increase to 1 meg. 

[mark2]

Unfortunately switching to 1M didn't make a difference.

One interesting thing I found is that it exhibits the exact same tone sucking behavior even when the circuit is powered off.

[blackieNYC]

Those 4 3.3k resistors- what if you up them to 100k?   They also provide a path to ground. And unaffected by lack of dc power.

[Rob Strand]

You don't need differential inputs, do you?

That design has both DC (2x47k's to gnd) and AC loading (2x47k's on the opamp side).

It's not the thing for the job.

Consider a simple non-inverting gain amp, as used on say the MXR distortion plus but maybe with bigger caps.   Use a 2M2 resistor to 10M resistor to Vref=Vcc/2, 47n input cap.  Use a TL071 or TL072.   The take the circuit driving the meter, ie. 10uF and ckt to the right, on the current circuit and connect it to the output of the opamp in the new circuit.

[mark2]

You don't need differential inputs, do you?

I don't, that's why I was running one side to ground.

What you're saying makes a lot of sense and is simpler so I'm inclined to try it since I'll be building at least a few of these.  That said... Do you think this could be related to the tone/volume problems I'm experiencing, or is this just a side note?

[mark2]

Those 4 3.3k resistors- what if you up them to 100k?   They also provide a path to ground. And unaffected by lack of dc power.

Thank you; I'll try this as well.

[PRR]

[Rob Strand]

What you're saying makes a lot of sense and is simpler so I'm inclined to try it since I'll be building at least a few of these.  That said... Do you think this could be related to the tone/volume problems I'm experiencing, or is this just a side note?

Highly likely.  All that loading isn't good for guitar circuits.

I couldn't find and exact schematic on-line but here a few close one.

These drive the current through the meter.   Note also you don't need the cap driving the meter.

So for this first one ignore everything to the left of the opamp -input.   Just copy the MXR Distortion+ with a 2M2 to 10M input resistor.



Here's another one, it also doesn't show the input biasing circuit, For single supply you will need a cap in series with R1 & R2.


In both cases, instead of using a cap + 2M2 to 10M resistors you can further simplify the circuit by simply connecting the opamp + input to the signal *provided* that part of the circuit is biased as Vcc/2.   So that means connecting to inputs, where it gets the biasing for free,  or to opamps outputs.

I hope you get the general idea.

IIRC a lot of VU meters have rectifiers inside the meter.    Some need voltage drive and rely on the diode drops in order to get the VU display correctly scaled to dB.  I think most produce the correct dB scaling when driven by a current source like the above two circuits.   I guess you will have to play around to work out the details of your meters.

[mark2]

Thanks Rob and PRR.

I'm going to try to remedy the first build, if for no other reason than learning and curiosity.

But once I fix or abandon that one, I'll try one of your two suggestions. Any major pros/cons of one over the other? I'm tempted to try PRR's first just because of its simpliciity, and try putting a pot for Vgain.

[mark2]

So I built this using the MXR Distortion, changing C2 from 10n to 47n, and R2 from 1M to 4.7M.
I believe this changes the impedance from 1.176M to 1.913M. Either way that sounds sufficiently high.

I experimented with RV to find 13k (~60 gain) put a hard strum into the 200-300mV range that makes the meter move.

Unfortunately I'm still getting the same tone suck that becomes apparent when I disconnect this circuit from the output and hear it brighten up.

Question 1 (my most pressing)
Are there other factors I'm not considering or aware of that can be causing this?

Question 2 (a minor curiosity)
Is it expected that the heavier strings should make the meter dance more than the lighter ones? Estimating (with a dso-138 scope'ish toy) the input signal of course I see a bigger p-p difference for heavier strings, but the output of this seems to be disproportionate, almost as if the higher frequencies are being filtered out, or failing to amplify.

[mark2]

And the 4.5 Vref is directly from the MXR as well, with a 1M voltage divider. Added here:

[Rob Strand]

So I built this using the MXR Distortion, changing C2 from 10n to 47n, and R2 from 1M to 4.7M.
I believe this changes the impedance from 1.176M to 1.913M. Either way that sounds sufficiently high.

I experimented with RV to find 13k (~60 gain) put a hard strum into the 200-300mV range that makes the meter move.

So one thing about separating the meter circuit via C4 & R5 is the diode D1 and D2  block small signal.  Whereas the other circuits I gave hide the voltages drops using feedback.

One source of low sensitivity could be R6 and R6.   Decreasing those can increase sensitivity.

Unfortunately I'm still getting the same tone suck that becomes apparent when I disconnect this circuit from the output and hear it brighten up.

Question 1 (my most pressing)
Are there other factors I'm not considering or aware of that can be causing this?

The loading is caused by the 1n.   You don't need that.

You could probably take out the 10k at the input as well.

If you existing circuit has a 4.5V rail you could remove R8, R9, C6 and use your circuit's 4.5V.

Question 2 (a minor curiosity)
Is it expected that the heavier strings should make the meter dance more than the lighter ones? Estimating (with a dso-138 scope'ish toy) the input signal of course I see a bigger p-p difference for heavier strings, but the output of this seems to be disproportionate, almost as if the higher frequencies are being filtered out, or failing to amplify.

You would expect that to some degree.   If you only want a visual representation then you can decrease either C2 or C3 to nudge the level of the lows down.

[PRR]

1nFd on a guitar is like 30 feet of cheap cable. On top of the cable you already use.

[mark2]

Thank you both again. I'm going to try removing the the 1n and (after testing it) the 10k at the input to see how it sounds.

The loading is caused by the 1n

My googling and vocabulary is failing me here... what's a term or topic I can search for regarding this cap to the ground on the input, or the effect it's having.  I don't understand why this was here or what its impact is and would love to learn more.

If you only want a visual representation then you can decrease either C2 or C3 to nudge the level of the lows down.

I'll leave that voltage divider since I don't have another 4.5V source, and will experiment with C2 and C3.  Thanks for the tips!

[Rob Strand]

My googling and vocabulary is failing me here... what's a term or topic I can search for regarding this cap to the ground on the input, or the effect it's having.  I don't understand why this was here or what its impact is and would love to learn more.

This impedance of a cap decreases with frequency so it has an effect of increasing the loading at high frequencies which cases a darkening of the sound.

It's the same effect as having a long cable.   The cable capacitance loads the signal.    That's what PRR was referring to.

As to why the 1nF cap is in the MXR.    There's many reasons but giving a reason is speculation.

- The cap can prevent oscillations in high gain circuits.

- It stops RF getting into the front-end.

- If affects the tone.  So perhaps it could be deliberately added.

- For the first two cases the affecting the tone is an unwanted side-effect.
  To make matters works the original MXR wasn't true bypass to that cap stuffs up the
  signal even when the effect is bypassed!    That was the bad old days of tone-sucking pedals.

[mark2]

The loading is caused by the 1n.   You don't need that.

You could probably take out the 10k at the input as well.

I did some quick testing and the 10k makes a huge difference, and the 1n has a bit of an impact as well. I had to increase the Rv trim a lot, but was able to tune it in the 500k ballpark.  Removing both that resistor and cap makes it nearly transparent. I'll do more deliberate and careful testing later, but I strongly suspect it's in "good enough" territory.

Thanks for that, and for the explanations!

[mark2]

I have a remaining weird issue I'm curious if anyone has insight into: The meter reads with a continual DC offset. I tried a couple different meters and the voltage is slightly different, but still there (20-80 mV).

Assuming C4 wasn't doing its job, I removed it but it turns out even with no input signal when C4 is omitted, it still exhibits this behavior.

I didn't notice this behavior yesterday. I tried grounding the screws on the meters but that didn't do anything. Measuring between M+ and GND (rather than M-) actually shows a higher DC reading. I checked all the ground points on this part of the circuit and there's no voltage difference.

Any clues where this comes from or how/if the meter is introducing it?

[mark2]

Any clues where this comes from or how/if the meter is introducing it?

It goes away when I disconnect my DSO-138 scope. Hrmm.

[anotherjim]

Is your scope probe normal x1 or x10? You really need x10 for these jobs. x1 probe loads the circuit by 1Mohm but x10 is 10Mohm.