Douk A5 Budget Stereo Amplifier Review

[Roland68]

If I compare TI PFFB circuit for TPA3255 and image then components in red circle fits directly as feedback resistors, values are directly from TI suggestion .

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Rest resistors are Douk specific, strangely number and values of them are not symmetric.

TI suggestion:
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But just because PFFB is additionally equipped, the remaining required components do not become obsolete.

 

[Kaameelis]

Good point, this helps to identify 4 more resistors, R48, R27, R53 and R57, same values as on TI suggested schematic, but 5 are still unidentified..

 

[Kaameelis]

As TPA3255 is balanced amplifier in BTL mode, it must cancel even harmonics when + and - channels have same gain (are exactly balanced). Imbalance can be anywhere from point where balanced signal start to the output. So it is very hard to find the real cause of high 2nd harmonic. But basically balance can be turned anywhere after balanced signal start to the output. So I have idea to make 2 opamp channels (one on left, one on right) gain adjustable what give possibility to balance amplifier based on 2nd harmonic minimum. This can be done with replacing one of 10k resistor with multiturn trimmer potentiometer.

 

[Kaameelis]

I measured opamps outputs and they are well in balance, same values with precision of 0,01 dB. THD in this points at 5W output is 10 times less than on amp output.

 

[DavidR]

I ran distortion RTA again for one Douk A5 amp, one and both channels driven. HD2 was most impacted with both driven. I also had to increase the Topping D50 level to achieve the 5W output from the Douk. No doubt it's due to lowering the input impedance as seen by the Douk. I encountered the same issue with the Scarlett 2i2 in other testing.

 

[DavidR]

This is a linearity measurement of one amp, 5W 4ohms both channels driven.

 

[Kaameelis]

On my A5 opamp output voltage measurement to get 5W on amp output, is below, volume knob was at maximum.
Voltage was about 0.58V, what mean TPA3255 gain in amplifier is 10.
Higher than 20kHz noises are from my PC.

 

[Richard G]

I was looking for multi-way desktop amplification. They needed to be small. No true multi-channel amps were available and/or satisfactory, so I decided that multiple 2-channel amps were the only considerations. Amir's review of the A5 prompted me to buy. Got lucky with Amazon sale price. I bought three and made distortion measurements with REW, Topping D50 III and E1DA ADCiso. I measured both channels of all three. Unit-to-unit consistency was excellent. There was only one slight outlier for noise floor.

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Thank you for sharing. With your setup is it possible to measure the distortion which correlates more strongly with audibility - IMD(?)

 

[DavidR]

I did run an IMD test a couple of times. @5W it was very poor, lower power was much better, but I attribute that to my test rig. I had similar issues when testing the V3 Mono, especially when using only the Scarlett 2i2 alone (it is too limited for this). The problem with my rig is (may be) that it cannot use a single clock that allows REW to use the rectangular window. The clock/window combination is extremely important for some tests in my experience so far. I don't trust my IMD test results, especially since the RTA and Stepped Sine tests closely match those from Amir.

Or course it is possible that the A5 IMD response isn't as good as the other tests, but I have no way to say one way or the other. I'm not as concerned because I plan to use these in a multi-way desktop DSP system that will use the A5 well below the 5W level of the tests.

I may do more testing later, but I'm occupied with other things for now.

 

[truth_soldier]

I bought my DSP for $ 30 dollars (Brazil)

The Wondom BRU5 is a very low cost TPA3255 amp with built in DSP for about US$35
This is a low cost option if you want to drive a pair of subwoofers.
I will try this; I'm currently looking for a pair of bass 18" drivers, made/sold in Brazil, for use in open baffle.

 

[truth_soldier]

Is known, what is different in V1, V2 and V3, besides heat sink, HPF and HPF scale markings?
I am sure there must be deference in amplifier components or schematic, other way is not possible to get so different THD figures and THD harmonic composition.

yeah, you can see The Douk A5 V3 circuit has many changes.
The below image of V2 (left side) and V3 (right side) has just the changes I saw at first glance, maybe there are more.
Note the size difference of the HPF dial pot and changes in the components near it.
The coupling caps after the opamps were changed as well as caps between the TPA3255 and the output filter coils.

The question is, is how does V3 sound now compared to V2?
It's kinda annoying to me because it invalidates subjective reviews that people did in the past of V1 and V2 if (big if) V3 sounds different.

 

[truth_soldier]

yeah, you can see The Douk A5 V3 circuit has many changes.
The below image of V2 (left side) and V3 (right side) has just the changes I saw at first glance, maybe there are more.
Note the size difference of the HPF dial pot and changes in the components near it.
The coupling caps after the opamps were changed as well as caps between the TPA3255 and the output filter coils.

The question is, is how does V3 sound now compared to V2?
It's kinda annoying to me because it invalidates subjective reviews that people did in the past of V1 and V2 if (big if) V3 sounds different.

these two chips in the V2 are gone in V3. These look like a 5532 opamp and maybe a 3.3V regulator.
What purpose did they have in V2 and why are they no longer needed in V3?

 

[DavidR]

yeah, you can see The Douk A5 V3 circuit has many changes.
The below image of V2 (left side) and V3 (right side) has just the changes I saw at first glance, maybe there are more.
Note the size difference of the HPF dial pot and changes in the components near it.
The coupling caps after the opamps were changed as well as caps between the TPA3255 and the output filter coils.

There is also an additional surface mount capacitor to the right of C89 at the right. No way to know if any others had value changes. I would expect combined these may improve the overall performance. Even the power output binding post quality appears improved (threads) if it's not just the picture making it appear that way as well as the soldering. I'm surprised at how much has changed. These ought to have some designator indicating version if significant changes are made.

 

[truth_soldier]

Amazon.de is selling old V2 version and V3 seems better.

seems better in what ways exactly?

 

[truth_soldier]

There is also an additional surface mount capacitor to the right of C89 at the right. No way to know if any others had value changes. I would expect combined these may improve the overall performance. Even the power output binding post quality appears improved (threads) if it's not just the picture making it appear that way as well as the soldering. I'm surprised at how much has changed. These ought to have some designator indicating version if significant changes are made.

actually the change is just R35 was added to the left column and C112 was moved from the left column below C110 to the right of C110 so the right column was shifted a bit but everything else looks the same.
@Kaameelis do you have any understanding of this circuit and change?

 

[Kaameelis]

Not in details. Upper 4 and lower 4 bigger SMD resistors and capacitors on last image are PFFB, same values as TI suggestion for TPA3255. Upper for right channel, lower for left.
Rest are other needed component, not directly sound passing.

 

[Kaameelis]

seems better in what ways exactly?

THD in measurements.

 

[truth_soldier]

THD in measurements.

measurements are what you can see. I"m more interested in what differences can be heard if any.
Or should we just assume the changes made, make it sound better than V2?
Why do you think the HPF knob/pot device is nearly twice the size of the one in V2?

 

[MCH]

actually the change is just R35 was added to the left column and C112 was moved from the left column below C110 to the right of C110 so the right column was shifted a bit but everything else looks the same.
@Kaameelis do you have any understanding of this circuit and change?

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they didnt add R35, they just moved it from its original position close to the opamp U12 (picture below) all the way up there. See how the other parts around U12 are still there (C124, C120, C8 etc) but R35 is not there anymore. The value is different though.

Maybe @Fosi Audio could clarify what is different between V2 and V3?

 

[Kaameelis]

measurements are what you can see. I"m more interested in what differences can be heard if any.
Or should we just assume the changes made, make it sound better than V2?
Why do you think the HPF knob/pot device is nearly twice the size of the one in V2?

HPF is remade in big scale. Probably filter involves more variable resistors than in V2.