Chinese DAC-AH new modifications

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Hi

I am starting this as a new thread because the already existing ones on the DAC-AH appear to be rather old.
I have recently aquired two of these units from Ken Chang / Hongkong. A first review of the circuit diagram invites to some other modifications than those already discussed here. Two have come to my mind, which I could read nowhere about so far.

1. The Lite DAC-AH has separate voltage regulators for each group of four TDA1543 chips. Thus it lends itself to a complete separation of silicon for left and right channels. Simply cut the AOL and AOR rails in half and increase the I/V resistors accordingly. This should reduce crosstalk and maybe improve reproduction of sound details.

2. Completely eliminate the Vref rail, actually leaving each Pin7 open. Rewire output op amps as inverting I to V converters like in TDA1543 data sheet. Delete R35 and R36 and connect AOR and AOL rails directly to op amp -inputs. Feed positive bias current via separate 1 K pots from +15 V to op amp -inputs. Bias op amp +inputs to approx. 2 V, fixed. Adjust bias pots for 0 V quiescent output on each channel. Use non polarized output caps or even delete output caps as output will be DC free, if the following equipment permits.
Expected result: improved DAC performance. DAC output sees fixed current sink point and therefore comes closer to what the designers of the DAC probably had in mind, see TDA1543 data sheet. Moving bias current source from within DAC to op amp summing junction eliminates possible cross talk between audio and Iref currents. Leaving each Vref pin open instead of tieing them all together eliminates random compensating Vref currents between the DACs, caused by generic Vref differences per chip and temperature drift.

Has anyone already tried these mods?
DACs run on 6 V, output op amps are AD847.

Thanks for your comments, Karsten
 
Has anyone already tried these mods?

I have done it my Philips CD614 extensively used to try new thing like this, quick end dirty. Opamp is LM833 at +/-10VDC supply rails, one TDA1543 at 5VDC. Opamp inputs are at 2VDC, but I have to use 10k multiturn pot from +10VDC to bias inverting input at 2VDC and trim output to within +/-15mV DC offset.

I can only confirm your finding sound vice, great mod for sure.:)
 
but I have to use 10k multiturn pot from +10VDC to bias inverting input at 2VDC and trim output to within +/-15mV DC offset.

I can only confirm your finding sound vice, great mod for sure.:)
Thanks for the reply. It is encouraging. I will start with the mods and report my results. Btw: in case of four parallel DACs a 1k trimpot should be required for the increased bias current.

Regards, Karsten
 
Have ordered parts for the planned mods. Will start work next week, probably.
In the meantime I have done two things:
1. Started to read (never ending?) thread on http://www.diyaudio.com/forums/digi...building-ultimate-nos-dac-using-tda1541a.html . Very interesting. Has convinced me that non OS best adheres to Shannon and has most appeal to DIYers because easily understandable and easy to build. Besides, as Shannon IS right, I expect it to give best results.

Learned from one post, that AOR/L ac voltage compliance is only within +- 25 mV as per TDA1543 data sheet. Had overread this. Therefore, original circuit 274 ohms and even my 150 ohms resistor I/V is far out of specs.

2. Started to poke around in original circuit with oscilloscope. Created 22050 Hz 0 dB (16 bit full scale) square on left channel (2 samples per sqare), digital silence on right channel. I/V resistors 150 ohms each. Right channel shows decaying oscillation: 50 mV pp max, total duration approx. 6 us, frequ. approx. 800 kHz. Amplitude varies with excitation. Vice versa when changing channels --> -34 dB (!!) crosstalk problem. Will keep an eye on this.
Also tried to identify any crosstalk from BCK/WS/DATA lines to AOR/L but could'nt find anything significant. Small decaying 75 MHz oscillations are probably caused by suboptimal scope probe coupling (German readers: wer misst, misst Mist).
Unfortunately, I have no means for digitally recording oscilloscope traces, can only photograph screen. As there is nothing exciting yet, I will save this for later.

BTW: your offset problem 500 mV. Simple R voltage divider should work best to create the 2 V op amp bias. Also, only 500 mV and if you still use output caps, is'nt that more of a cosmetic problem?
 
Hi,

I have created voltage divider via trimmer, so opamp +in sits at 2VDC, no problems there, but output offset is 500mV from cold start, when fully warm I can adjust it to +/-15mV, which is almost tolerable since there is no output cap anymore. Also, offset significantly change if I press forward or play for example, so I think offset problem is PSU related - voltage apparently drops when motor have to do something I guess, have to check this to be sure.
 
Current modifications

Following mods have been implemented:
  • Cut Vref rail and connect individual Ibias resistors to each Vref pin. Result: AOL(R) noise reduction.
  • Make existing op amp AD847 work as transimpedance amplifier followed by passive voltage divider giving approx. 1 Vss output at 0 dB fs digital input. Reason for passive divider: omitting it would require 100 Ohm feedback resistor which will degrade op amp performance (value too small). Result: DAC operates within specs: AOL(R) +- 25 mV AC compliance.
  • Make op amp +Vbias adjustable for 0 Vdc on output.
  • Connect 10 nF capacitor from AOL(R) to ground. Result: greatly reduced output noise. This size capacitor does not yet degrade output but only with this capacitor output remains inaudibly silent when other channel is excited with digital 0 dB fs.
  • Attach 47 nF ceramics across both DAC voltage regulator output elcos.
  • Change output decoupling elcos to 6.8 uF WIMA MKS2 type.
See circuit diagram for details.

Overall results in A/B comparison with second unmodified DAC: improved musicality, sounds silky, more "tube-like". Enhances piano and violin sounds, for example. Both outputs remain totally silent when CD player stops whereas unmodified DAC still emits faint but audible hiss.

Development work was carried out with LITE Audio PCM2704 based USB-SPDIF converter and Esser Audio TTG test tone generator here .
 

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I/V converter with LME 49713 cfb OP Amp and transformer output

After reading about current feedback op amps I tried an LME49713 as I/V converter. This amplifier has excellent reputation for its audio performance. It requires a 1.2 K feedback resistor, the total current from 8 parallel TDA1543 giving approx. 22 Vpp max output. To bring this high output voltage swing down to the common 2 Vpp range I use a 10:1 step down transformer and a servo amplifier to eliminate any DC component in the transformer primary. The servo circuit reduces the DC to less than 2 mV. This makes a coupling capacitor unnecessary.
When used in an inverting amplifier configuration, the LME49713 tends to ring. It can be easily "tamed" however with a 2.2 K resistor between current summing point and inverting input and a 470 pF shunt capacitor across the feedback resistor. With these measures impulse response becomes totally flat.
T1 is a cheap audio transformer made by German GT Elektronik. The Reichelt part number is NFU1-10, selling for 2.35 Euro. The 2.2 K resistor across the output suppresses any parasitic transformer oscillations caused by the typical high Z line load.
How does it sound: marvellous. Best non-oversampling DAC I ever heard. Also because English is not my native language I will not attempt to write any flowery or esoteric portrayals. Instead, go ahead and build this simple circuit and listen for yourself. Happy soldering! :)
 

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....

Learned from one post, that AOR/L ac voltage compliance is only within +- 25 mV as per TDA1543 data sheet. Had overread this. Therefore, original circuit 274 ohms and even my 150 ohms resistor I/V is far out of specs.

...

I confirmed this yesterday as I replaced R35 and R36 with 150 ohm resistors. The DAC sound became dead, like a good cassette player. No life. This is after trying the bias voltage at anywhere from 2.0 to 5.0 volts. Even the recommended 3.3 volts setting did not work. So back went in the 274 ohm stock resistors. Of course, the distortion is back too on very loud and complicated passages.

So, has anyone figured the correct ohm resistor? 230 ohm maybe?

I am not willing to go with the LM7808 regulators.

I did have a chance to tap in for the passive output using 2.2 uF caps.

Any suggestions for snap in Op Amps? I don't want to change the case types as some do.

Would not recommend this DAC to non-DIY people.
 
I will answer my own question :D since I found out by "resistor rolling" that 240 ohm resistors to replace the stock 274 ohm R35 and R36 provide the best balance on this DAC before you start to cut its legs off and also eliminate peak saturation and distortion on loud and complex recordings.

At least to my ears, the DAC now lives up to its reputation written about in all the threads. Up next is the passive mod and hear I am in for a treat.

So, to summarize 240 ohm resistors for R35 and R36 and about 3.0 - 3.3 V bias voltage.

The pictures show the now removed 150 ohm resistors. Also notice the CAT6 wire tap-ins for the passive mod that will be done next.
 

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use active I/V Converter

whaleman, you might want to try my circuit. I am using two such modified DACs for over a year now. They sound good.
If I ever do another one, I will use a 6:1 transformer from the same manufacturer, only for the lower winding capacitance. I doubt though, that there will be any audible difference.
See the updated schematic (attached) for reference.

-Karsten-
 

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whaleman, you might want to try my circuit. I am using two such modified DACs for over a year now. They sound good.
If I ever do another one, I will use a 6:1 transformer from the same manufacturer, only for the lower winding capacitance. I doubt though, that there will be any audible difference.
See the updated schematic (attached) for reference.

-Karsten-

Thanks Karstenpg, I may try this with a second DAC AH. For now I've added the passive mod using Jantzen 2.7 uF capacitors and an additional pair of RCA jacks.

The sound is very nice, the bass is very tight and the highs cleaned up nicely. This DAC has a lot of potential, so I will probably do another and consider your mod.

The caps can be inserted from the top and soldered from there. A little heat rash to nearby components is almost inevitable.
 

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I had a hum in the right channel at loud volume, so I removed the passive mod, I thought it was picking something up and I really was not using it. Hum still there. Replaced caps with good quality and higher capacitance. Hum still there.

Then looking at the DAC, I notice the transformer next to the right channel DAC and components, presto! I removed it and placed it as far away as I could from the components on the board. Now, hum 80% gone. The rest is impossible to remove unless the transformer is moved outside the box. I know some have found this out already, but it is cool when you figure it out for yourself. Notice the before and after photos.

As a side benefit, now there is some ventilation for the once sealed case.
 

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@ Cola: copper shielding will have close to no effect against magnetic stray fields.

@ whaleman: EACH DAC chip contributes equally to BOTH audio channels (see circuit diagram).
The stray field of this type of transformer is far too small to induce audible hum current through the I/V resistors. Therefore, it is unlikely that moving the transformer alone did really reduce the hum. On one channel only?? Is it possible that you changed the overall setup / wiring in your installation somehow when you reinstalled the DAC-AH after moving the transformer?
I would suspect some kind of ground loop as the real cause instead. Try using the optical SPDIF input (ignore any tales about the negative influence of optical SPDIF on audio fidelity. You won't hear the difference).
If you cannot use optical coupling, modify the coaxial SPDIF input, as this input is not isolated:
** Remove the RCA connector from the DAC pcb.
** Mount an isolated RCA jack in the now free hole on the rear panel instead.
** make a 1:1 pulse transformer by winding 2 x 6 turns of suitably thin enameled wire on an Amidon BN 73-2402 binocular core (available from Mouser) or salvage the pulse transformer from a scrapped Ethernet PC interface (the old kind with a BNC connector). I am using the homebrew transformer.
I have not tried the Ethernet transformer but I have read somewhere that it works as well.
** Glue the pulse transformer to the DAC pcb in place of the removed RCA connector. 2 component epoxy will make a secure joint. Solder the wires: primary start to RCA inner connector, primary end to RCA outer connctor, secondary start to former SPDIF input, secondary end to digital ground.

Also, make sure that the power cord protective ground is connected to the power transformer shield winding (shield between primary and secondaries) only and has no connection to the DAC PCB.

This will prevent any ground loop between DAC and connected equipment and should solve your problem.
 
Thanks Karstenpg for your detailed reply.

Moving the transformer is exactly what took away the hum since I did it in steps and that was the only thing that changed for that trial. The wiring followed the transformer along with the ground chassis wires. You can see the wiring in the before and after photos.

It has been my experience that with line level stuff like this, you can expect hum when getting a signal wire even less distant than the resistors were to the transformer, so I am not surprised if they were the "antenna" responsible for contributing the hum noise.

Currently, the only way you can hear faint hum is with the volume all the way up beyond 4 PM, where I never go past 11 AM on this amp, so for all practical purpose, this problem is solved.

This will be of benefit to future Lite DAC AH owners. Thanks for your tips, certainly of benefit.
 
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Adding USB to the Lite DAC AH

How can I add a USB connector (to accept cable pictured) into this DAC? Do I need a separate receiver? or will the CS8414 handle the new input?

This is so I can hook up directly to a laptop USB out direct digital (bypassing the laptop sound card).

Thanks.
 

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