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Douk VU3 Review (VU Meters)

amirm

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This is a review and detailed measurements of the Douk / Little Bear VU3 dual analog VU meter and 2-way amplifier/speaker switcher. It is kindly sent to me by testing by a special member. VU3 costs US $140 from Amazon including Prime shipping.

The VU meter are the "paper back" ones which I like from my audiophile youth era:

Douk VU2 Review VU Meter.jpg


When you light them up, they turn a nice shade of yellow. The sensitivity control for the meters is poorly done in that it does nothing for a bit and then has massive effect and then not much else. On the other hand, the AB momentary toggle switches have a nice feel followed by a reassuring relay click. A cheap looking remote is provided which I did not use but is appreciated.

Power is provided by a USB-C connection:

Douk VU2 Review VU speaker selector Meter.jpg


I used a stand-alone USB power supply for it. Wish there was some trigger option to turn the thing on and off when the amplifier is powered on.

The binding posts are better than I expected although as usual, they are too close together to turn if you are using bare wires as I was.

Douk VU3 Measurements
To detect if there is an impact on the amplifier performance, I decided to use the Purifi reference design amplifier with it. Here is that amp measured again for this test:

Purifi amplifier measurements.png


SINAD is the same as what I measured before but third harmonic is worse. Wonder if it is aging or my load box is. Anyway, that is not for this article. Let's now route the output from one channel through the VU3 and see what happens:

Douk VU2 Purifi amplifier measurements.png


We have a pretty sharp rise in second harmonic and a mains hum. I played around with grounding, rotating the AC adapter, etc, and nothing would change the nature of it. I can't figure out how we lost 0.2 watt. The box must have rather low impedance to impact the amp this way.

To see the effect across the full power range, I ran the 4 ohm sweep:

Douk VU2 Purifi amplifier measurements Power into 4 ohm.png


There is definitely an impact here. So not good although you could argue SINAD is still > 100 dB so practical impact is low. Then again you paid so much for this Purifi amp and you dragged it down by some 15 dB.

Conclusions
Both this meter and its sister VU2 version seem to put undue load on the source causing distortion. There needs to be a buffer stage with high impedance that doesn't do this. It is a shame as the rest of the functional with remote controlled AB/amp switching is very good.

I personally would not want to degrade the performance of my system by any amount to have some "bling." So I can't recommend the Douk VU3.

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As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 

nyxnyxnyx

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Such a shame it fell short in the technical aspect, because everything else looks quite neat especially for the price.
Thanks for the great review sir Amir, I hope one day you'll do some content about opamps in the grand scheme of audio devices.
 
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amirm

amirm

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@tomchr, any chance you would take a shot at designing a proper one of these and able able to sell it near these prices? I will go up to $199 if it works well.
 

pma

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We have a pretty sharp rise in second harmonic and a mains hum. I played around with grounding, rotating the AC adapter, etc, and nothing would change the nature of it. I can't figure out how we lost 0.2 watt. The box must have rather low impedance to impact the amp this way.

My guess - the relay contacts, if there are relays inside to switch the binding posts. I have measured distortion related to relay contact resistance nonlinearity many times. You may also look into Douglas Self book.
 

polmuaddib

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I apologize for offending anyone, but do we really need to waste Amir's time and ASR space with these silly products?
Need VU meters? Buy Accuphase or Mcintosh, whatever... Can't afford it? Then get some virtual VUs on your display or tablet and don't mess with extra cables or waste SINAD.
 

solderdude

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Looks like there is some leakage between the Douk's internal power supply's sensing circuit and the amp's output.
It does have to work with both bridged amps and normal amps.
Maybe a sensing circuit with small transformers separating the signal and sense circuit could have been done.
The increased 2nd harm. could indicate that the sensing circuit loads the speaker output asymmetrically (so no bridge rectifier but a single) above 4.5V or is it the mains hum (45uV at 4.5V input voltage) that is shown ?

Given the fact that there are no calibrated steps telling what voltage belongs to 0dbVU and we have no idea how fast the meter is and what it represents (peak or averaged) it is safe to assume the meter is merely decoration.

On the other hand... it could be useful when AB'ing 2 amps (remote control) and one can use the meter with a constant low frequency to ensure they are level matched.

When the power is off is there a default amp A to speaker A connection or is no sound coming through ?
In the latter case how does the circuit load the speaker outputs (test on speaker outs before the meter)

What does the distortion plot look like at say 1W (2V) out and perhaps at 50W ?
 

pma

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I vote for relay contact nonlinear resistance - both re power drop and distortion rise. It is logical.

Below an example from Douglas Self book
Self_relay.png

Yes it is higher however contact material and aging are important.

I measured, years ago, the distortion of relay contact voltage drop when driven from 1A constant current source. The distortion was time dependent (improving with longer test run). Low when re-calculated to overall output voltage, however adding something similar as shown by @amirm .

rele-mereni-Cordell-1Amp.PNG


rele-mereni-Cordell-1Amp-2.PNG
 
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amirm

amirm

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When the power is off is there a default amp A to speaker A connection or is no sound coming through ?
I didn't actually measure or play it that way but did use the multimeter to see connectivity and B set is active input to output when powered off.
 
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amirm

amirm

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I vote for relay contact nonlinear resistance - both re power drop and distortion rise. It is logical.
Ah, my mind hadn't gone there but yes, that could very well be the cause.
 
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solderdude

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The issue with using power relays instead of signal relays is that power relays kind of depend on some minute 'welding' of the contacts so require a very small amount of 'arcing' when activated while signal relays with different contact materials rely on pressure contact and also switch well when no signal is present.
Maybe cheap power relays are used.
Don't know if this also explains the hum, I suppose that is leakage.

With some amps using output relays that have become faulty a channel might 'crackle' until you shortly turned up the volume substantially and from there on works well until the next time it is switched on.
 

audio_tony

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The best place for this kind of device is between pre & power amps or in an EQ loop (where present on an integrated amp), with some simple method of calibration. It would also benefit from a decent PSU and not running it off cheap switch mode power supplies.

Putting any additional resistance (cheap relays*) in the speaker chain is never a good idea.

*those relays might be cheap iron framed relays which Self has demonstrated introduce a lot of distortion because the signal is being passed through the iron frame.

I currently switch between two pairs of speakers with a Beresford TC-7210 - this has a couple of decent quality rocker switches on the front panel.

It's actually intended to switch 2x amplifiers to one pair of speakers, but works just as well the "wrong" way round.

1632475425045.png
 
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Rick Sykora

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Thanks Amir!

I found this meter before Amir had tested the VU2. @AdamG247 was looking for some meters to use for monitoring power in his home theater and so Adam purchased. I have built some amp VU meters previously, but it was more expensive to do than the $140 cost of a VU3. I had done some measurements to check and found distortion that changed with load. I gave it a listen on one of my systems and it seemed to sound ok. After the VU2 tested poorly, Adam decided the VU3 should get Amir’s scrutiny. So, I sent it to Amir.

@pma’s suggestion of relay contact resistance makes sense as I had experienced this in an aging Proton amp. After hearing low level distortion, I had sent the amp for repair twice. Each time it came back with a no problem found. Afterwards, had done some internet searches and found the relay complaint. I bought a couple of relays and replacing them fixed the issue.

Anyway, the VU3 does use traditional relays and perhaps could be fixed with solid state ones. Here is a pic of the VU3 interface board…

E1293235-DEEA-4030-9B0A-82828BC2C6C6.jpeg


Adam bought it, so is his call on what happens next. Along with the VU2‘s issues, would like to hear what Douk Engineering‘s response might be. :confused:
 

nagster

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@tomchr, any chance you would take a shot at designing a proper one of these and able able to sell it near these prices? I will go up to $199 if it works well.
How about a buffer amplifier that has a through output and an isolated output (for meters)?
Each person can connect their favorite meter, with or without adverse effects.
 

abdo123

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I apologize for offending anyone, but do we really need to waste Amir's time and ASR space with these silly products?
Need VU meters? Buy Accuphase or Mcintosh, whatever... Can't afford it? Then get some virtual VUs on your display or tablet and don't mess with extra cables or waste SINAD.

There is nothing silly about these, and it's not your call to say what's silly and what's not when you're not doing the sending, testing or paying for the website's hosting costs.

These sort of replies help no one.
 

pma

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How about a buffer amplifier that has a through output and an isolated output (for meters)?
Each person can connect their favorite meter, with or without adverse effects.

However the issue here are almost certainly relays in the speaker signal path. Buffering meters would not help because relays would remain there. Better relays or SSR to be used. Then it would be more expensive and probably bigger. This is just another toy.
 
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