Are passive volume controls totally transparent?

[Miguelón]

I’m interested on your opinions about volume analogue controls, specifically passive, partially because lots of comments at Thomann website about the change of tonality added by monitor controllers, but also some personal perceptions.

My experience (psychological or physical, but I lack of proper instruments to measure):

- I had at same time one Genelec G Three (B) and one Genelec 8030C active monitors and sounded really different till the point it was impossible to match them on a stereo pair. Genelec measurements and technician by chat confirmed that are acoustically the same speakers. None of the units were broken, I returned G Three and they confirmed was perfectly operational, I purchased another 8030C and matched perfectly my unit at home.
Both were connected by XLR balanced and same source, only different between those 2 speakers were a passive gain control in 8030C (reduced by 10 dB to match G Three B), none on the G Three side.

-I have an Ifi Zen DAC signature V2 (with an inbuilt passive volume control) which has a switch to bypass what Ifi calls the “preamp section” though is just an attenuator (reducing signal by around 6 dB at maximum position). Again shows tonality changes when using through the analogue path with respect to pure digital attenuation.

-A monitor control (Behringer) connected by one channel to one of my monitor and other channel directly from DAC output shows similar perception in tonality changes.

The general point of view, at Thomann (I try to exclude audiophile websites because a tone of beliefs, supposing professional stores customers more trustable) is that adding a monitor controller (majority are passive) change tonality.

What do you think or measured? Are volume controls introducing some harmonics that justify this tonality changes? Since I don’t hear audible noise I suppose harmonic distortion to be the cause, or just purely psychological effect…

 

[Keith_W]

All volume changes will alter tonality because of the Fletcher-Munson curve. So listening is a useless test to determine transparency.

 

[Blumlein 88]

How long is the cable between monitor controller and speaker? Passive volume control can roll off high frequencies when it is followed by a long cable. The higher output impedance and high parallel cable capacitance combine to form a gentle low pass filter.

 

[Miguelón]

How long is the cable between monitor controller and speaker? Passive volume control can roll off high frequencies when it is followed by a long cable. The higher output impedance and high parallel cable capacitance combine to form a gentle low pass filter.

About 2 meters

 

[Blumlein 88]

About 2 meters

Seems unlikely that would be enough to be noticeable.

 

[Miguelón]

All volume changes will alter tonality because of the Fletcher-Munson curve. So listening is a useless test to determine transparency.

We measured SPL with the mic.

Genelec 8030C is very sensitive, even reducing sensitivity to +4 dB position to give equivalence to G Three B (information given by the brand), we should reduce another extra dB to have same SPL at 1 m.

And even at lowest sensitivity (around 2 dB less than G Three), 8030C is more “audible” than G Three, that’s the reason I thought in harmonics: if added to fundamental, can give some impression of clearest mid- high range.

At least if what I remembered from my class of Acoustics, but that was when I was 15 years old: I’m 49 now!

My girlfriend insists that cannot be at all the same speakers, I trust more her ears than mines: she’s a cello player and can perceive minor tonality changes in an instrument in a second…

I explained her my theory of harmonics, but she’s not interested at all on technical aspects of music

 

[Miguelón]

Seems unlikely that would be enough to be noticeable.

I have an audio interface, feeding the input with output of DAC, recording and analyzing and then comparing with DAC through monitor passive controller should show if are a significant difference.

Am I correct? What software should I use to see the harmonics?

 

[Holmz]

I have an audio interface, feeding the input with output of DAC, recording and analyzing and then comparing with DAC through monitor passive controller should show if are a significant difference.

Am I correct?

No you are not likely correct if you are thinking it is harmonic distortion.

What software should I use to see the harmonics?

Just overlay two collections of a long pink noise section, and see if the frequency is altered as @Blumlein 88 pointed out…

How long is the cable between monitor controller and speaker? Passive volume control can roll off high frequencies when it is followed by a long cable. The higher output impedance and high parallel cable capacitance combine to form a gentle low pass filter.

If it is bent down with the passive then it indeed make a gentle low pass filter.

…

I explained her my theory of harmonics, but she’s not interested at all on technical aspects of music

Best you stop that…

 

[Blumlein 88]

I have an audio interface, feeding the input with output of DAC, recording and analyzing and then comparing with DAC through monitor passive controller should show if are a significant difference.

Am I correct? What software should I use to see the harmonics?

The pink noise suggestion would work. If anything is happening it is not distortion or harmonics. It will be a simple slight roll off in frequency response which would be visible in pink noise.

You could take the audio interface play thru the Monitor 1, and then record the output with the interface. If you use REW you could do a sweep with and without the Monitor 1 in the signal path and compare. REW would also show distortion, but unless something is badly wrong there won't be audible distortion.

 

[Miguelón]

The pink noise suggestion would work. If anything is happening it is not distortion or harmonics. It will be a simple slight roll off in frequency response which would be visible in pink noise.

You could take the audio interface play thru the Monitor 1, and then record the output with the interface. If you use REW you could do a sweep with and without the Monitor 1 in the signal path and compare. REW would also show distortion, but unless something is badly wrong there won't be audible distortion.

Thanks, was thinking about installing REW and a better mic, It was suggested that WiiM software is not so trustable to room correction and I’m using a very old berhinger mic that a friend let me.

Why don’t using direct analysis of the signal instead of trough the Monitor?

 

[Miguelón]

No you are not likely correct if you are thinking it is harmonic distortion.

Can you extend a little bit more the explanation please?

I only have a very small formation in electronics, resistors don’t have any frequency dependent at high frequencies through self induction currents?

 

[Blumlein 88]

Thanks, was thinking about installing REW and a better mic, It was suggested that WiiM software is not so trustable to room correction and I’m using a very old berhinger mic that a friend let me.

Why don’t using direct analysis of the signal instead of trough the Monitor?

What I am proposing is not using a microphone, though one could do that. I'd disconnect from the speaker and feed it back into the ADC on the interface. Once going thru the Monitor 1 and once direct. If you go the microphone route, the microphone doesn't need to be exceptional.

 

[solderdude]

I’m interested on your opinions about volume analogue controls, specifically passive, partially because lots of comments at Thomann website about the change of tonality added by monitor controllers, but also some personal perceptions.

My experience (psychological or physical, but I lack of proper instruments to measure):

- I had at same time one Genelec G Three (B) and one Genelec 8030C active monitors and sounded really different till the point it was impossible to match them on a stereo pair. Genelec measurements and technician by chat confirmed that are acoustically the same speakers. None of the units were broken, I returned G Three and they confirmed was perfectly operational, I purchased another 8030C and matched perfectly my unit at home.
Both were connected by XLR balanced and same source, only different between those 2 speakers were a passive gain control in 8030C (reduced by 10 dB to match G Three B), none on the G Three side.

-I have an Ifi Zen DAC signature V2 (with an inbuilt passive volume control) which has a switch to bypass what Ifi calls the “preamp section” though is just an attenuator (reducing signal by around 6 dB at maximum position). Again shows tonality changes when using through the analogue path with respect to pure digital attenuation.

-A monitor control (Behringer) connected by one channel to one of my monitor and other channel directly from DAC output shows similar perception in tonality changes.

The general point of view, at Thomann (I try to exclude audiophile websites because a tone of beliefs, supposing professional stores customers more trustable) is that adding a monitor controller (majority are passive) change tonality.

What do you think or measured? Are volume controls introducing some harmonics that justify this tonality changes? Since I don’t hear audible noise I suppose harmonic distortion to be the cause, or just purely psychological effect…

It will depend on the input resistance of the connected amp (or when an input transformer is used on the input impedance) combined with the resistance of used passive volume control (or potmeter). It is the combination of those that matters.
Also the output impedance of the source could matter and for long or exotic interlinks (with unusual high capacitance) could create volume control dependent roll-off (6dB/oct).

Not all passive volume controls are the same, not only in resistance value but also construction and thus input/output resistance value per attenuation. switched ladder types and log volpot types can differ substantially in that ratio.

The problem here is measuring this effect. The reason being when you change the volume more than just the volume changes, transducers will behave differently when less signal is going in. When measuring with a mic (like measuring length with a tape measure made of elastic band) one must also undo level changes which opens another can of worms for instance S/N ratio. Certainly when measuring distortion you will get results that are all over the place in your average listening room.

Soooo.... there are too many variables and using a mic is not recommended if you really want to see something when attenuation differs.
An experiment doomed to failure. Only electrical measurements at the input of the amp (with the actual gear connected) is possibly going to let you see real effects of the source-volume control-input combination you are testing.

 

[Miguelón]

What I am proposing is not using a microphone, though one could do that. I'd disconnect from the speaker and feed it back into the ADC on the interface. Once going thru the Monitor 1 and once direct. If you go the microphone route, the microphone doesn't need to be exceptional.

Yes, excuse me, I’m so stupid! I forgot that Monitor 1 makes reference to the interface and not to the physical monitors

 

[Miguelón]

An experiment doomed to failure. Only electrical measurements at the input of the amp (with the actual gear connected) is possibly going to let you see real effects of the source-volume control-input combination you are testing.

Thanks, it seems over my capacity in this moment!

I have the sensation of having returned the wrong speaker, if one have two idenitical amps and one has a -10 dB fixed attenuator seems to have less impact on the chain than a variable passive device as it exist in most studio monitors…

 

[solderdude]

if one have two idenitical amps and one has a -10 dB fixed attenuator seems to have less impact on the chain than a variable passive device as it exist in most studio monitors…

The word 'seems' is the most relevant part here.
Measurements comparing 2 different ways of attenuating the exact same amount should be done electrically or by instant switching between the different attenuators that are equal within 0.1dB.
When those conditions aren't met the word 'seems' is very applicable.

 

[Miguelón]

The word 'seems' is the most relevant part here.
Measurements comparing 2 different ways of attenuating the exact same amount should be done electrically or by instant switching between the different attenuators that are equal within 0.1dB.
When those conditions aren't met the word 'seems' is very applicable.

I’m not english native, I used the word “seems” in a wrong context.

I was intended to express if the more simplicity of the fixed attenuator changes less (if there’s a real change, I’m ancient enough on ASR to never neglect the possibility of a pure psychological bias) the signal than a variable one, that surely has more components.

 

[Wolf11Man]

If you use a well designed, active pre amp, impedance match should be no problem. With a passive one you may get a missmatch, depending on level. The sound will become more "flat" and somehow lifeless. That was my impression after many experiments, which made me use an active stage, even as in theory it should be worse than a passive one.

 

[ObiWanKenobi]

All volume changes will alter tonality because of the Fletcher-Munson curve. So listening is a useless test to determine transparency.

If an amplifier/preamplifier would be transparent we wouldn't hear or see it

 

[Holmz]

Can you extend a little bit more the explanation please?

I only have a very small formation in electronics, resistors don’t have any frequency dependent at high frequencies through self induction currents?

The output resistance (impedance) of the device is something like 100 ohms to 10k ohms.
The passive preamp makes that go higher and higher as the outpuit volume goes lower and lower.

(And) The cable has capacitance.

So you have this:
(That resistor would be the device’s output impedance, but it gets to be a bigger sized resistor with the passive preamp.