Modding Taobao digital amps

Modding STA-based digital amps

My recent blog posts on digital amps from Taobao, specifically one based on the D2-41051 (Intersil) and the other based on STA333BW (STM) have generated a fair amount of interest so I figured it might be time to start a thread rather than simply entertain comments on the blog.

The SQ of both of these has impressed me, but not in stock form. I've focussed so far on mods to both the power supply and the output stage as this is the low-hanging fruit. Other mods are not being excluded simply because of my initial focus on these areas, there may be digital/clocking mods which turn out to be worthwhile too.

Come to think of it, I think this thread might be more appropriate to classD as its about what's in effect a power DAC.

The STA333BW board has a maximum power supply voltage of 21V (from the datasheet of the chip) so adapting it to work with step down transformers doesn't give much latitude before the available power output becomes impracticably low for speakers (though still oodles for headphones). I have two 2:1 step downs re-wound on EI 15VA cores which give this amp a very relaxing 'analog' or 'valve' sound when the power supply is beefed up - shown in the snap below. However the available output power then is under 10W per channel.

The 'hexacap' under the board is an array of 65 680uF/35V caps and there are short 1mm dia solid wires linking the cap bank to the underside of the STA333BW chip. The 'valve' sound arises in part due to HF roll-off caused by the output transformers having rather too high leakage inductance. I've measured this and its fairly substantial - something like a first-order pole around 4kHz.

<update - changed thread title to reflect only STA-based amps, my mods to the Intersil-based amp I'll open another thread for>
 

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If the D2-41051 chip accepts I2S it is likely that the USB input will be outputting I2S so maybe it just takes some investigation of the datasheet and PCB to find some locations to tap in. I may still buy one of these things regardless as, to be honest, although I am thinking of a spare room system my interest is now piqued as to how, with some fettling, it would work with my 15ohm Lowther horns (maybe I'll get teased about that too:D). An I2S input would allow me to hook into my preferred network audio approach and would open the door to reclocking the incoming data.

Lowther horns gets respect from me, not teasing :D 15ohms might work fine without an output trafo but potentially modify the output filter to suit the higher impedance. The rule of thumb is increase the inductance and decrease the capacitance by the ratio of the new impedance to the old. So you'll want something like 22uH and 220nF for the new filter caps. Of course it'll still work with the old values you just get a peak in the ultrasonic region due to the lack of damping of the filter - most likely totally inaudible.

Anyway, the cost of the unit you linked to on UK Ebay is £99, though shipping is free (not sure of Taobao shipping cost to the UK?). I will contact the fellow DIY Audio member as you suggest to see if I can get some hints.

I had a quick look on US-eBay and couldn't find it, how did you go about searching for this? I tried 'digital amplifier' and 'D2-41051' neither of those terms turned it up. Taobao shipping to UK depends on what you ask the agent to do, suspect that DHL will be in the region of UKP30 but don't quote me!
 
I had a quick look on US-eBay and couldn't find it, how did you go about searching for this?

I don't remember exactly what search I used, I think I just cut and pasted something from the Taobao translation. Anyway, here's a link to the UK ebay listing that should help;

POPU D2P DSP Digital Power Amplifier DAC HiFi Decode USB/COAXILAL/RCA 24Bit/192K | eBay

The price has reduced overnight and if you add in shipping at the rate you suggest it's still a bit high but maybe not quite as bad as my first impression.

I couldn't find an ebay listing for the STM board though. Update: Just seen Jean-Paul's post with a link though.

More investigation required.

Ray
 
Thanks, the blurb there does indeed look pretty much word for word from the Taobao Chinese version, including the pictures. Though there's a remote shown there and none of mine have come with remote. Interesting to get the background on D2 Audio which went right over my head when it was in Chinese... :p
 
I'm going to buy a couple of the STA boards - what the heck!

I'm pretty sure I can see the I2S connections to the STA chip running across the bottom of the board. A quick look at the datasheet shows it'll accept upto 192KHz 24bit input, though it appears to process at 96KHz internally? Shouldn't be difficult to bypass the USB input and replace with a decent reclocked input.

Ray
 
I bought one. But then one wasn't enough, so I now have 6 :D

@Ray - yes my reading of the DS is that its doing all its internal processing at 96kHz (or maybe 88k2 for 44k1). And I figure the upsampling filter at the front end isn't very good. But now JP has mentioned the CM108 only does 44k1/48k I'm confused how I got it to sound better with upsampling to 96k....
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You will regret when you move houses again :)

Designing a small PCB with a 24 bit 96/192 kHz USB chip with SPDIF input should not be too hard. I just tried to check if C-Media produces a pin compatible high res version of CM108 but apparently they don't.
 
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I bought one. But then one wasn't enough, so I now have 6 :D

@Ray - yes my reading of the DS is that its doing all its internal processing at 96kHz (or maybe 88k2 for 44k1). And I figure the upsampling filter at the front end isn't very good. But now JP has mentioned the CM108 only does 44k1/48k I'm confused how I got it to sound better with upsampling to 96k....
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I will ignore/disable the USB stage and feed 96/192KHz I2S straight into the STA chip (I will upsample in my player software, I use HQPlayer but others will do it too). In my experience I2S input will give you the best results. I would also use a reclocker to get the incoming I2S into good low-jitter shape too, maybe use a RPi and one of these?

https://volumio.org/product/allo-kali-i2s-reclocker/

There's a whole thread about it here;

http://www.diyaudio.com/forums/digital-line-level/294940-new-fifo-buffer-rpi-sbcs.html

I have a spare Acko SO3 reclocker available that I'll experiment with.

Without reading all the information, I suspect the onboard volume control solution might lose resolution so I will probably use the lossless volume control in my player software.

Anyway, I've ordered two of the STA boards from ebay.

Ray
 
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Lowther horns gets respect from me, not teasing :D 15ohms might work fine without an output trafo but potentially modify the output filter to suit the higher impedance. The rule of thumb is increase the inductance and decrease the capacitance by the ratio of the new impedance to the old. So you'll want something like 22uH and 220nF for the new filter caps. Of course it'll still work with the old values you just get a peak in the ultrasonic region due to the lack of damping of the filter - most likely totally inaudible.

abraxalito, is this filter suggestion for the STA or Intersil amp - not sure if the context of the different amp boards will change the requirements for the filter values?

Ray
 
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Maybe discussing two different amplifiers in one thread is a bit confusing ? I suggest two new threads with correct thread titles "modding a cheap STA333BW Taobao board", "modding the POPU D2P Digital Power Amplifier" or the like. In my limited experience this makes it easier for all certainly when one wants to find back information after a while...

Both threads should be in the Class D section for now. I kindly asked the mods if a new category can be added:"DDX/FDA amplification" as this will be the dominant technology. IMHO this is not the same as Class D with its analog inputs and DDX/FDA very likely being the dominant technology in consumer electronics from now it deserves its own category. If you read this and think the same please endorse the idea by asking the mods or, maybe better, react in this thread. I suppose the mods will react here as I reported my own post and send an email with the same questions.
 
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Who except Harman Group used this "dominant" technology from 1999? Is there growth in number of products they use this "dominant" technology, or is it on the decline ?

Harman bought B&O automotive and B&O Play recently, Icepower is used there exclusively offcourse. So Harman has started to sell Icepower. I haven't seen contracts, conditions. But in general all Harman classD I heard are inferior to Icepower products I heard.
 
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Check manufacturers of DDX/FDA chips and what they have in their growing product line please. In low cost devices DDX/FDA will be dominant, I never said it already is dominant and what does 1999 say ? Tubes from the fourties are still used today ! Not dominant though :D

Full Digital Amplification surely is rising as it is cheaper and you know what technology "wins". I trust you can Google yourself what products come to the market. Many new devices are FDA, not only cheap chinese devices but also A brands FWIW. Sometimes the products are not labelled FDA so the industry has a task in branding the technology somewhat more universal. DDX, FDA, PowerDAC, Sound Terminal etc. are confusing for end users.

With slight logic thinking: why would one (either manufacturer or end user) produce/use analog inputs, expensive switches, relays and potentiometers when the world is digital and most new devices are digital ?

Thinking FDA is declining is phase of denial ;)
 
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Possibly but not necessarily. STA333BW has less powerful outputs and it seems an older design if the date on the datasheet says something. This cheap board it is also limited to 16 bit 44.1/48 kHz operation contrary to other FDA amps I have so comparing is not fair.

D802 is a good example how FDA amps can perform. No denial possible that it is a very good sounding amplifier. It even sounds better by just looking at the price tag ;) If I had to choose between the various TPA3116/3118 amps I have and the D802 I would not hesitate for a second.

Too bad yours broke down. With the right tools replacing the STA326 shouldn't be a problem.
 
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Newer DDX designs cant really compete qualitywise, (even with older DDX), also because to simplify/lower cost the design deteriorated rather than improved over last ~15 years. DDX=STM. High quality STM/DDX doesn't exist. So starting DDX section anno 2016 is odd imo.

TI has some superior closed loop digital input these days, not DDX, but those are inferior to their analog input models too, probably because feedback is still harder digitally.
 
Possibly but not necessarily. STA333BW has less powerful outputs and it seems an older design if the date on the datasheet says something. This cheap board it is also limited to 16 bit 44.1/48 kHz operation contrary to other FDA amps I have so comparing is not fair.

D802 is a good example how FDA amps can perform. No denial possible that it is a very good sounding amplifier. It even sounds better by just looking at the price tag ;) If I had to choose between the various TPA3116/3118 amps I have and the D802 I would not hesitate for a second.

Too bad yours broke down. With the right tools replacing the STA326 shouldn't be a problem.

I, however, prefer the 3116 (still waiting for Weiner board from Gmarsh, hoping it would another step up from the Chinese variety) with a transformer input bypassing the coupling caps and a cheap Chinese tube preamp.