Modding Taobao digital amps

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#81
 
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#84
abraxalito said:
You'll definitely want to do the tweak to the level shifting caps I detail on my blog. That's a design fault which could easily be responsible for the graininess on vocals you're hearing.
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Thanks. So can I just pull those 2.2nf caps out and replace with a 10nf cap. What do u think is the optimal value for that position?

Those 4 power caps will be replaced as well. Inductor will change to 22uH.
 
#86
jean-paul said:
Eh ?! The original ones are 2.2 nF and Abraxalito added 4.7 nF. Would 6.8 nF MKP not be a better choice ? Or just 4.7 nF as a starter ?

I mean a fourfold value could be just too much 😉

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For ref, the D2P schematic is here from a japanese site.

http://nice.kaze.com/popu_d2p_sch.pdf
POPU D2P
 
#87
ChuckT said:
Thanks. So can I just pull those 2.2nf caps out and replace with a 10nf cap. What do u think is the optimal value for that position?
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Hard to estimate an optimal value as FET threshold voltages are quite variable. I reckon the smallest that will fully enhance the FET (i.e. deliver at least 3.5V). 10nF I'd estimate to be a too large value but I'm not yet clear what the downside is going to be from going too large. 6n8 does the job admirably well so if you have a totally free choice I'd go for that.
 
#88
OK, finding schematics of a relatively new product is rare.

Some things that I see:

- Soldering in a low jitter 24.576MHz 3 x 2 mm XO with LC decoupling connected to 3.3V must be possible. Or a separate reg.

- I noticed SPDIF datas stream connected to the data stream of the USB module.

- That USB module seems to be chosen deliberately low in features, possibly to bring out a new version with high res via USB next year 😉

- USB module puts out SPDIF to a SPDIF input while the main chip also has I2S inputs ?!?!?

- Analog input is digitised by ADC (probably sounds like s**t). That ADC puts out I2S. Removing this ADC makes it possible to use WM8804/5 on a new PCB with more SPDIF coax/toslink inputs or just a very good high res USB chip with I2S outputs.

- Choice of regs is poor with AMS1117. There are pin compatible LDO regs with way less noise !

- Same with LM2596-5.0. Like in D802 this reg has hundreds of mV ripple. I solved this in D802 with a ready built module.
 
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#91
For the sta333 board, I change few caps and at the underside of board power trace near the IC I scrape away the 2 power trace to add 2 x 0805 10uF smd cap on each rail. I may add more to those 2 power rails, which is feeding the left and right power supply of ic.

Now I notice there are the VDD (pin15 3.3v internal reference) and VSS (pin4 internal ref) uses 100nf. I wonder if those can be improved by substantially increasing the cap value to 10uf or 100uf.

Next I will replace the 78M05 to the very low noise LT3088
 
#92
abraxalito said:
Where 'it' is the Popu amp? If you set that to 0 then you get nothing at all I believe. Transformers provide two functions - one being step down, the other is bal-SE conversion.
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The data sheet for the 333 has a pin for a set gain and 0 gain. I'd assume 0 gain means the input voltage would come out, not nothing. It's not a mute pin. Obviously it wouldn't exceed 3.3v from USB, at 0 gain if it means what it says.
 
#96
Yeah I noticed that - the typical input impedance of the ADC (due to some internal biassing resistors) is 15kohm, minimum 10k. So can't practically be very much higher without fitting a buffer first. I'd fit a discrete buffer myself, a few bipolar SOT-23s but it would probably need a local negative supply (ICL7660).
 
#97
AKM says it has internal HPF that also takes care of dc-offset, so dc blocking capacitor already? Just low voltage probably?

edit: it is written under analog input section, but they might mean the outputsignal ?
 
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#98
abraxalito said:
Indeed it does in stock form, no dynamics. But that can be fixed with an LC filter on the VA supply to the ADC. Then it sounds fairly decent to my ears.



Save​
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There is really no point of using the analogue input, its a full digital amp after all. My opinion is the tpa3116 with transformer input will be better for analogue input.

However, the A-D outputs I2S to the D2 ic, so I believe that can be utilize for external I2S input.
 
#100
jean-paul said:
+1

LJM CM6631A 24bit/192khz USB to Coaxial and Optical fiber SPDIF and I2S Board | eBay

How does the Popu name inputs ? By number ?
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I have already got it cover owning both an Amanero and a Xmos xu208 USB to i2S board.😀
I think the newer generation xu208 sounds a touch better than the Amanero but it could be due to implementation. And the Xu208 board also has an spdif out too.

I would recommended those over the older generation USB to I2S chips. They can be find easily in ebay/aliexpress/taobao.

The Popu input is selected by pushing the left knob: RCA, Spdif, USB
The left knob also use as bass control.
 
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