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across the bench: an electronic menagerie of diagnostic oddities plus photos and design analysis

douk a5 pro - 1.jpg
digging into a recent acquisition, douk a5 pro (convertible edition or the sunroof is removable). scrap cooling fan not included, room temp chassis at any volume level is a nice thing.

douk a5 pro - 2.jpg
premium price tag (compared to a5 model) includes gan power brick which did not disappoint either when doing measurements

doukaudio.com

Douk Audio A5 PRO TPA3255 Bluetooth Amplifier HiFi Class D Stereo Audi

Features Upgraded Features for Versatile Use: The A5 PRO builds on the strengths of the A5, featuring upgraded Bluetooth 5.1 connectivity with a reset button, bass and treble knobs for precise tone adjustments, and a headphone output. It delivers clear, powerful audio, perfect for diverse uses...
doukaudio.com doukaudio.com
 
Screenshot 2025-08-11 at 17-42-57 Measuring Distortion on the Cheap – Neurochrome - YouTube.png
youtube.com/watch?v=0lc9ramAufY

useful comparison video in regards to measurement hardware shopping and software interface usability.

more than a few threads on ak talk about LM3886 amp projects.

neurochrome.com

DIY Audio Engineering Gear for Audiophiles | Neurochrome

Neurochrome is your go-to source for audio engineering gear and consulting. We make it easy for audiophiles to get the gear they need to get the sound they want.
neurochrome.com neurochrome.com
 
Screenshot 2025-08-11 at 18-40-38 The £25 000 Pre-Amp that went Wrong - Tom Evans Mastergroove...png
youtube.com/watch?v=-RJbpFSFziI

the beauty of using a thermal camera to see an issue quick and easy (tantalum cap, probably shorted).

bonus, chassis lid repair due to shipping damage.
 
douk a5 pro - 3.jpg
back to a5-pro testing. initially set signal generator to 1.5v rms but later adjusted to 1.8v rms so the amp could hit distortion near max volume.

douk a5 pro - 4.jpg
minus fan and its supply would be around 9w for the amp at idle
 
douk a5 pro - 5.jpg
sine wave starting to get fuzzy at the peaks (31.9v rms at 8r = 127w)

douk a5 pro - 6.jpg
testing one channel pulls around 150w from mains (127w+9w= 136w, /150w= 90.6% efficient single channel)

advertised 300w/ch will require some number crunching/conversion.

300w p-p = 150w peak = 106w rms and if we look at max output before the sine wave gets fuzzy it will be closer to 110w. so at 0.1% distortion the final output rating will most likely be above 106w. i dont have a distortion meter but an oscilloscope is good enough to see when the output wave changes shape.
 
douk a5 pro - 7.jpg
running both channels at max before flat top distortion occurs (fuzzy wave as shown earlier)

douk a5 pro - 8.jpg
more than noticeable clipping at 35v rms (153w rms per channel) which makes things interesting for the power brick. this was not a quick "one-shot" capture on the scope, the measurement lasted a few seconds or more.

douk a5 pro - 9.jpg
at the clipping output (sustained) the 48v 5a power mains connected power brick was giving more than 100%. interesting to note the power factor of 1.0 (100% efficiency of power consumed) and 3.0a which seems like a nice clean round number. the 117v mains is from the variac starting to sag at high current since its rated for 5a which i occasionally exceed, small diameter fuse turns into tube getter.

so the amp efficiency at clipping would be 153*2= 306, +9= 315, /353= 89% which seems a bit low but reasonably acceptable.

newer low voltage power supplies are achieving 95-98% efficiency even though amplifier designs are uniquely different and more complex circuit. should be interesting to see what innovation replaces gallium nitride (GaN) tech to reach 101% efficient and beyond. efficiency to the point of over-unity is a wonder to behold.
 
douk a5 pro - 10.jpg
test loads, pair of 8r 100w non-inductive resistors, are almost ready to boil water. a bit late in the day to put the kettle on, especially when its 95f in the shade outside.

douk a5 pro - 11.jpg
cooling fan makes a big difference when stress testing the amp. no idea how many cfm air flow is achieved when feeding a scrap 24v fan 9v but it is whisper quiet and highly effective, worth every penny to extend power cap lifespan (the large cylindrical thing next to the hot spot inductors).

definitely need a cap mode to enhance performance and longevity even further. the power brick aint gonna be happy about that.
 
douk a5 pro - 12.jpg
specs of the scrap cooling fan for the douk a5 pro


douk a5 pro - 13.jpg
modified variable dc supply feeding the active cooling apparatus, it has a different output tail vs stock
 
douk a5 pro - 14.jpg
its nice that the amp chassis design included a removable plastic window and there are vent holes in the sides and bottom. a cheaper knockoff of this amp omitted vent holes.

douk a5 pro - 15.jpg
63v 3300uf rubycon storage cap parked way too close to the major hot spot of the system but that can be modified. adding another cap in parallel to increase energy reserve would be easy. although testing output improvement, if any, would be a good idea. temporary test jumper connections would be best before any soldering/desoldering occurs.
 
douk a5 pro - 16.jpg
front end op-amps can be changed if a person is feeling adventurous in regards to sonic flavor

NE5532 data sheet, product information and support | TI.com

TI’s NE5532 is a Dual, 30-V, 10-MHz, low-noise operational amplifier for audio applications. Find parameters, ordering and quality information
www.ti.com

most likely hear a difference between chips when using headphones due to the uniquely different loading and chaotic impedance curves (16r/32r/64r/300r/600r/123456r...)

www.audiosciencereview.com

Objective Guide to Op-Amp Rolling - Part 1

As per moderator, here is full article from the site: Objective Guide to Op-Amp Rolling - Part 1 July 30, 2021 Op-Amp Test Board This blog article/post is a deep dive into currently available 8-pin DIP dual op-amps, for those that like to swap op-amps in their designs and/or DIY projects. The...
www.audiosciencereview.com www.audiosciencereview.com
*general q&a about revising testing method and loads, original poster gets schooled in a few areas

*large pdf with distortion info

www.audiosciencereview.com

Does Op-amp Rolling Work?

When I post my last video on review of Douk A5 amplifier, I mentioned that rolling (changing) op-amp ICs in there is fruitless. I got have a dozen comments under that video in youtube asking why so I thought I experiment again with the A5. Note that I have done the same testing with both DACs...
www.audiosciencereview.com www.audiosciencereview.com
*amir and some measurements

*potential upgrade although some adapters will be needed plus soldering (spec comparison shopping). extremely expensive chip i.e. gonna need a fourth mortgage and a co-signer (or not).

not certain there will be enough space between the pcb sockets for a double wide adapter. need to do some measuring.
 
douk a5 pro - 17.jpg
moved original rubber feet from amp to the scrap cooling fan. gave the amp a lift kit using something that was floating around in the junk collection (unused, full health adhesive). should be able to do some off-roading and the occasional fire trail.
 
6c33cb - 1.jpg
looking through the warehouse bottle collection, new/used/nos, and found this unusual pair in bubble wrap

6c33cb - 2.jpg
6c33c-b with ulyanov logo, most likely zero hours on this glassware. probably make a nice pair of mono block amps from these or something.


How to get the best out of the 6C33C-B?

The 6C33C-B is an attractive tube; 15-20W output, relatively inexpensive at $16 each in Russia and, at 200V, relatively low voltage. The downside? The 6C33C-B requires a relatively high current, typically 220mA at 200V. And it has stinkin' hot heaters. Many of the published 6C33C-B circuits use...
www.diyaudio.com www.diyaudio.com

6C33C-B-ULYANOV

This is the super military contract 6C33C-B that is made by the Ulyanov factory and is the best sounding 6C33C-B ever made and is simply the best for all audio applications. The Ulyanov factory specialized in making valves for use in military aircraft electronics. So these valves are of...
www.watfordvalves.com
 
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