How to measure output voltage of a pedal?

Website · Post by **Lonkero** » 08 Nov 2011, 21:39

Been trying to measure output voltage of different OD pedals of mine cranked up by putting DMM to pos and neg poles of guitar jack, and using either AC or DC voltage measurement but I get nothing!!

Do you need some kind of special oscilloscope to measure that or is there something i'm doing wrong?

DWBH · Post by **DWBH** » 08 Nov 2011, 23:03

Lonkero wrote:Been trying to measure output voltage of different OD pedals of mine cranked up by putting DMM to pos and neg poles of guitar jack, and using either AC or DC voltage measurement but I get nothing!!

Do you need some kind of special oscilloscope to measure that or is there something i'm doing wrong?

It's kinda difficult just using a guitar and a DMM.
Ideally, the DC voltage on the output should be zero, or very very low/near 0v.
Measuring the voltage swing of a pedal (AC component) is hard on a DMM, unless you have a steady input, like a sine wave generator, or a function generator. And with a DMM, measuring the AC voltage means you're only measuring the effective component of it (Vrms). So, if you measure 7v on a DMM, on an oscilloscope the peak voltage would actually be 9.9v (7 * sqrt(2))

DrNomis · Post by **DrNomis** » 08 Nov 2011, 23:40

A Digital Multimeter has a limited response bandwith when it's switched to it's AC mode, furthermore it is actually measuring what's called the RMS (Root Mean Squared) value rather than the Peak-to-Peak value, to accurately measure the output voltage of your pedal, you need a Sine/Square Signal Generator and a standard Oscilloscope with a bandwith greater than DC to 20kHz, a 5Mhz Scope will do fine...

A good test signal to use is a 1kHz Sine wave, since 1kHz is right in the midrange frequencies and it's the midrange frequencies which give you a sense of loudness....

Website · Post by **Lonkero** » 09 Nov 2011, 02:09

Oh I see.

Well if someone here owns blackstar ht dual pedal, or actually any blackstar pedal plus oscilloscope please measure output voltage of it full cranked. It sounds so freaking powerful compared to my 9V stompboxes I wanna know if the reason is due the headroom like some have suggested.

diagrammatiks · Post by **diagrammatiks** » 09 Nov 2011, 04:39

you'd ideally need a vacuum tube voltmeter.

there shouldn't be any dc at all at the output.

i think they use half a triode as the last stage so that could account for the swing.

if they really are running 300v for the tubes then it'll definitely have a bigger ac swing potential then a 9v pedal.

DrNomis · Post by **DrNomis** » 09 Nov 2011, 06:34

Just out of curiosity, I measured the output of my Klon Centaur Overdriver clone stompbox, I was able to get it to produce a maximum undistorted output of 8 Volts Peak-to-Peak as measured on my Scope with a 1kHz Sinewave signal....

mictester · Post by **mictester** » 09 Nov 2011, 09:16

DrNomis wrote:Just out of curiosity, I measured the output of my Klon Centaur Overdriver clone stompbox, I was able to get it to produce a maximum undistorted output of 8 Volts Peak-to-Peak as measured on my Scope with a 1kHz Sinewave signal....

Considering that the input stage of most guitar amps is designed to take a guitar pickup signal of 10 - 200 mV p-p (from the weakest PAFs through to the massive outputs of Di Marzio humbuckers!) 8V p-p is silly!

My "hottest" output pedals give 4V p-p to drive cables, which is then passively attenuated to 100 - 400 mV at the input to the amp head. This configuration ensures that there's no noise picked up by long cables across a stage. The input to the backline amp is a sensible level so that the amplifier will handle the signal without distortion (though it can be turned up for that "amp sound"). Slamming the front end of a guitar amp with a big signal seldom sounds good - preamplifier distortion is NOT what you're looking for!

This is just an example of the badly thought-out "headroom feature" of the Klon.

DrNomis · Post by **DrNomis** » 09 Nov 2011, 10:01

The weird thing is that for some strange reason when I use my Klon clone to overdrive my 5 watt Class-A single-ended Legacy combo amp, it actually does sound good, especially when I max out the gain control on the amp Mictester, it just seems to work, I also tried adding a dynacomp stompbox between the guitar and the Klon in my signal chain, and that made it sound even better, I'll have to record a short clip and send it to you so you can hear what it sounds like, but I'll wait till I get my new Fender Standard Strat home first though....

Website · Post by **Lonkero** » 09 Nov 2011, 15:20

DrNomis wrote:The weird thing is that for some strange reason when I use my Klon clone to overdrive my 5 watt Class-A single-ended Legacy combo amp, it actually does sound good, especially when I max out the gain control on the amp Mictester, it just seems to work, I also tried adding a dynacomp stompbox between the guitar and the Klon in my signal chain, and that made it sound even better, I'll have to record a short clip and send it to you so you can hear what it sounds like, but I'll wait till I get my new Fender Standard Strat home first though....

Yeah it definetely sounds good! Many people on web have commented Blackstar tube pedals (they've confirmed they actually run at 300V measured)have a lot noise in signal especially when gain cranked up, mine as well, but with using noise gate you can live with it, and the extra headroom and powerful signal is definetely worth it

FiveseveN · Post by **FiveseveN** » 09 Nov 2011, 19:39

i think they use half a triode as the last stage so that could account for the swing.

if they really are running 300v for the tubes then it'll definitely have a bigger ac swing potential then a 9v pedal

After the HT tube stage the signal is attenuated 30:1 and goes through several opamp stages. However, the opamps themselves operate on a +/-15 V supply so there is a lot more headroom available compared to an ordinary 9 V pedal.
It is way overkill if you're using it in front of a guitar amp, as Mictester pointed out. But if it's used as a preamp feeding a poweramp (as it was probably intended) or mixer/recorder (balanced line-in) it probably makes sense.

Website · Post by **Lonkero** » 09 Nov 2011, 23:02

Has someone ever calculated or tried with oscilloscope how much those resistors in signal path after op-amp reduce output?

Take an example of MXR dist+, been played with it on bredboard on the fly and taking out 10k resistor after output cap makes it sound a bit better. Altough, the difference is really subtle

Makes me think, why in earth do almost every pedal maker put resistor to signal path AFTER amplifying the signal? Is it just because setting impedance or are there some other reasons as well?

mictester · Post by **mictester** » 09 Nov 2011, 23:43

Lonkero wrote:Has someone ever calculated or tried with oscilloscope how much those resistors in signal path after op-amp reduce output?

Take an example of MXR dist+, been played with it on bredboard on the fly and taking out 10k resistor after output cap makes it sound a bit better. Altough, the difference is really subtle

Makes me think, why in earth do almost every pedal maker put resistor to signal path AFTER amplifying the signal? Is it just because setting impedance or are there some other reasons as well?

The resistor at the output to ground is usually to discharge the output side of the output capacitor (to prevent pops and bangs when plugging the effect into something else). It can also be used as a pull-down resistor for a "Millennium Bypass" circuit. The resistor will be a much higher value than the output impedance of the effect, so it will have almost no effect on the output level.

Website · Post by **Lonkero** » 09 Nov 2011, 23:51

mictester wrote:
Lonkero wrote:Has someone ever calculated or tried with oscilloscope how much those resistors in signal path after op-amp reduce output?

Take an example of MXR dist+, been played with it on bredboard on the fly and taking out 10k resistor after output cap makes it sound a bit better. Altough, the difference is really subtle

Makes me think, why in earth do almost every pedal maker put resistor to signal path AFTER amplifying the signal? Is it just because setting impedance or are there some other reasons as well?
The resistor at the output to ground is usually to discharge the output side of the output capacitor (to prevent pops and bangs when plugging the effect into something else). It can also be used as a pull-down resistor for a "Millennium Bypass" circuit. The resistor will be a much higher value than the output impedance of the effect, so it will have almost no effect on the output level.

No, I don't mean the resistor to ground, but the one that signal just goes through.

eg. R8 in here http://www.joehartband.com/DOD250Schematic.JPG

What i dont understand why you wanna weaken the amplified signal by running it through 10k resistor

diagrammatiks · Post by **diagrammatiks** » 10 Nov 2011, 01:39

that isn't there for attenuation.

the 2 back to back diodes are gonna clip the signal more then the 10k resistor.

sending a signal through a single series resistor usually does not attenuate the entire signal very much at all until the values become very big.

Website · Post by **Lonkero** » 10 Nov 2011, 19:33

I'll see if I manage to calculate the effect on output with LTSpice or some other simulator without the resistor and with it.

Website · Post by **Lonkero** » 10 Nov 2011, 23:11

Intresting result. With taking out the R8 from the DOD 250 schematic I provided earlier, the output does indeed raise a bit

Red line is gain with 0.1 and Blue with gain full. Upper diagram is without the 10k resistor.

So if I've done this right, the difference which can also be heard can be seen also on numbers. Not much, but still!

Website · Post by **Lonkero** » 11 Nov 2011, 00:09

I used modded 250 circuit model, and noticed there's one extra 1n4148 diode for clipping section, that's why asymmetrical as seen on pic.

Taking the diodes out and raising the input voltage to higher you can get really high output voltages even from dod 250 i've noticed. But how much a normal transistor amp can take without frying it ?

mictester · Post by **mictester** » 11 Nov 2011, 00:31

Lonkero wrote:
No, I don't mean the resistor to ground, but the one that signal just goes through.

eg. R8 in here http://www.joehartband.com/DOD250Schematic.JPG

What i dont understand why you wanna weaken the amplified signal by running it through 10k resistor

You're not. The resistor serves a number of purposes - mostly it's protecting the output of the op-amp. When the diodes go into conduction, they can pass a lot of current (relatively) - they would look almost like a short when they are conducting. The 10k introduces some resistive loading so the output of the op-amp isn't directly shorted to ground when the diodes conduct. It also tends to "soften" the onset of clipping, reducing the extreme high frequency content and making it more "musical". Finally, the 10k also provides buffering between the output of the IC and the outside world when the volume pot is all the way up. It prevents possible damage if the output is shorted to ground.

You can try reducing the value of that 10k - you could go as low as 1k without detriment - but you won't find any output level increase because that maximum amplitude s determined by the forward voltage drop of the diodes (about 0.65V p-p at room temperature). You may find subtle changes in the tone that you might like!

mictester · Post by **mictester** » 11 Nov 2011, 00:33

Lonkero wrote:I used modded 250 circuit model, and noticed there's one extra 1n4148 diode for clipping section, that's why asymmetrical as seen on pic.

Taking the diodes out and raising the input voltage to higher you can get really high output voltages even from dod 250 i've noticed. But how much a normal transistor amp can take without frying it ?

The 0.65V p-p you're getting with the pot all the way up is more than enough for the input of any amp! If you really want to increase the level, replace the two diodes with back-to-back LEDs!

Website · Post by **Lonkero** » 11 Nov 2011, 00:48

mictester wrote:
Lonkero wrote:I used modded 250 circuit model, and noticed there's one extra 1n4148 diode for clipping section, that's why asymmetrical as seen on pic.

Taking the diodes out and raising the input voltage to higher you can get really high output voltages even from dod 250 i've noticed. But how much a normal transistor amp can take without frying it ?

The 0.65V p-p you're getting with the pot all the way up is more than enough for the input of any amp! If you really want to increase the level, replace the two diodes with back-to-back LEDs!

But how much would be the maxime?

When using leds are diodes, shouldn'vt I be able to leave the 10k resistor out, since it's doesnt sound like a short circuit when the leds start conducting since they lighten up? (vs. other diodes)

With leds the extra energy goes into light, and not shortcircuit therefore right?