When using the ad844, are there any real benefits in adjusting the dc offset at the input of opamp? In case of the tda1541a, the dc offset seems to be created by the dac itself. Could trimming down to 0vdc affect the normal operation of the DAC. Pedja still used an output dc blocking cap after the ad844!
I am concerned about the dc adjustment circuit injecting noise directly into the signal path. Is the ccs with 2sk170 so good that it does not cause any additional noise?
I am concerned about the dc adjustment circuit injecting noise directly into the signal path. Is the ccs with 2sk170 so good that it does not cause any additional noise?
Best cap is no cap!!! If you can do things right have dc coupling from the dac chip output all the way your better of for it.
Especially when the rest of your system is dc coupled everywhere else, you can really tell what a cap sounds like if inserted anywhere into the signal path Any sort of cap, even those esoteric ones that cost an arm and a leg
Cheers George
Especially when the rest of your system is dc coupled everywhere else, you can really tell what a cap sounds like if inserted anywhere into the signal path Any sort of cap, even those esoteric ones that cost an arm and a leg
Cheers George
Hi guys,
I finally built Pedja's OPA861 IV+Buffer. You won't believe how much time it took me to build this simple circuit. (I spent my whole weekend on this.) The layout is important! In my first attempt, I got very noisy output (+-100mv noise) simply because the trace of IV stage is a bit longer. It took me many hours to figure that out.
Here is my impression on Pedja's circuit. It reveals much more details than previous setup (AD811 IV + ADA4627 + LME49990). Very nice. However, the sound is thinner and less in bass. The filter of IV resistor plays an important role. I found 1000pf is not enough. I replaced it with 2200pf silver mica, and it helps a lot. Finally, I ended up with 3000pf foil capacitor. The sound is better now, but HF still needs some kind of tweaking.
My supply is 15V OP based regulator. In order to supply +-5V, a pair of TPS7A4701/TPS7A3301 are used as secondary regulator. I suspect the output and bypass capacitors (both X7R ceramic) are part of the cause. I will replace them with electrolytic and film capacitors. Will report back.
I finally built Pedja's OPA861 IV+Buffer. You won't believe how much time it took me to build this simple circuit. (I spent my whole weekend on this.) The layout is important! In my first attempt, I got very noisy output (+-100mv noise) simply because the trace of IV stage is a bit longer. It took me many hours to figure that out.
Here is my impression on Pedja's circuit. It reveals much more details than previous setup (AD811 IV + ADA4627 + LME49990). Very nice. However, the sound is thinner and less in bass. The filter of IV resistor plays an important role. I found 1000pf is not enough. I replaced it with 2200pf silver mica, and it helps a lot. Finally, I ended up with 3000pf foil capacitor. The sound is better now, but HF still needs some kind of tweaking.
My supply is 15V OP based regulator. In order to supply +-5V, a pair of TPS7A4701/TPS7A3301 are used as secondary regulator. I suspect the output and bypass capacitors (both X7R ceramic) are part of the cause. I will replace them with electrolytic and film capacitors. Will report back.
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TDA1541A Offset
There is no noise with the 2SK170 current source.
Hi quantran, Yes. You want to counter the idle current on the DAC. Considering the low available current through the AD844's current mirrors it is good to get rid of this unwanted idle current. You can use the AD844's nulling circuit following the AD844 to null the output for direct coupling without a capacitor. It does not have enough range to replace the 2SK170 current source. I did this for a long time before moving on to the DDNF.
There is no noise with the 2SK170 current source.This is good point particularly if using breadboard. I think the I/V resistor and the parallel cap should be connected directly to ground pin of the opamp at the other end. This would minimize current loop.
How is the OPA861 compared to the AD844 (if you have built that one before)?
Distortion is a bit of a strong word for the sound of series caps in the signal path. I prefer to use the word colouration, even though it is a form of distortion.
A least you know when direct coupled what goes in comes out, and if you don't like it, look at problems elsewhere.
With a cap, they all sound different, and depending on the colouration "can mask" the above problems along with detail and transparency even the good expensive ones.
For bass, nothing beats dc coupling, all the way through if you can achieve it.
Cheers George
Agreed.... Direct coupling if you can do it is the way to go. I think it may be a challenge on the DDNF version for the PCM1704. I do plan to try.
I need to build a transistor tester of sorts and I need a better DMM. I am looking at Bench DMM's at this time. This will take me some time George. I will of course share my findings.Thanks, George and Torchwood421. I have connected the offset adjustment circuit and trimmed them down to about +/-2 mV on tda1541 output. Now I have about +/-8 mV at the output of the ad844. Is this good or can be better?.
The sound after adjusting seem tighter but I am not sure. Result is very good even through the capacitors. I still keep them for safety. I am afraid of temperature drift ...
The sound after adjusting seem tighter but I am not sure. Result is very good even through the capacitors. I still keep them for safety. I am afraid of temperature drift ...
BC557C and BC547C
Pedja I believe choose the C parts for the hfe of 500 to 900. That is current gain. The B parts are somewhat lower gain (400 or so). The purpose of the DDNF is to get a lot of current gain and then convert it to a voltage. I think the C grade transistors could be matched well just requires a lot of testing and grading.
AD844 offset....
I will be offline a few days as I will be having a medical procedure.
You should now add the null circuit in the datasheet for the AD844. I believe there is a typographical error there. The trimmer should be 20K Ohm as I recall. George can confirm that. On my TDA1541 I was able to get less then 1 mV at the DAC side and 0.1 mV's at the output of the AD844. You should allow the DAC to warm up and stabilize before trimming. It is slightly temperature sensitive as you noted. I would be happy with a couple of mV's on the output of the AD844.
I will be offline a few days as I will be having a medical procedure.Thanks Torchwood421. I have looked at the data sheet and remember that the nulling circuit has quite a few parts. I become lazy and I think I will just trim for zero V at ad844 output using the ccs in Pedja's design. it's not far from where I am now. I have both channels at 10mV when the output pin of tda1541a is at <1mV. I think if I trim for zero at the ad844 output I will have a very small dc at output pin of tda1541a.
this i/v stage is much easier to trim than the pass d1 which drifts a lot more and more difficult to get the D1 stage close to zero. It could be that irf610 is much hotter.
this i/v stage is much easier to trim than the pass d1 which drifts a lot more and more difficult to get the D1 stage close to zero. It could be that irf610 is much hotter.
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Hi Quantran,
Don't mean to hijack this thread. I tried the stacked AD844s without success. Last time, I was doing a simple mod on my DAC. I replaced the AD811 with AD844 and made the adjustment. The sound did come out, but it has too much bass. Perhaps the on-board LPF and buffer ruined everything. I should have built the circuit outside of my DAC like this time. I will try AD844 again. (I got plenty of AD844s)
I have limited experience on I/V stage, so I can only compare the OPA861 with my previous setup - AD811 I/V + ADA4627 LPF + LME49990 Buffer. Pedja's OPA861 provides much more details than before. Every instrument is very clear and well defined. On some favorite tracks, I was surprised to hear the 'correct' reproduction of the music. However, the bass is a bit weak than existing setup. (Could be the enhancement done by OP itself) Overall, I like Pedja's work.
I can't remember look back through these pages to make sure at my posts, I think it may have been 2k, it's there somewhere, and I don't have my unit here to confirm.
Cheers George
Torchwood's right, I found it was 20kohm not 20ohm as in the data sheet.
http://www.diyaudio.com/forums/digital-source/227677-using-ad844-i-v-60.html#post4417872
Cheers George
http://www.diyaudio.com/forums/digital-source/227677-using-ad844-i-v-60.html#post4417872
Cheers George
AD844 null...
The ideal situation is to zero the DAC output with the 2SK170 current source. Then null the AD844 with it's null circuit. In my builds the drift was very good. Not that sensitive. Maybe I was just lucky. The DDNF discrete circuit drifts a lot although I now have a few ideas. DDNF has it all, detail, bass slam and I like it quite a lot.
AD844 stack....
It works best to build the AD844 stage out board. I think you still had some of the original circuits filter intruding on the response. You likely made a bass booster by accident. AD844 works best in a no feedback situation. I did try the AD811. Never really liked it's sound. Dropping in a OPA627 in DIP was better for I/V. At least in my system.
the 8d844 is very stable now. once warmed up the tda1541 output pin always stay at <1mV. It is currently sharing the power (+-18v Kubota discrete regulator) with the DAC board. I will make a separate PS for it later but the sound is so good now. Just one observation that it has a typical AD tone in the lower midrange that is different from Burr Brown or Philips tone. It's not a bad thing but just different sound signature.
4 more AD844s are on their way to me for stacking up. Even with a single one now the sound is full on my system. I noted that the datasheet mentions the output stage can drive 50 ohm load with low distortion.
4 more AD844s are on their way to me for stacking up. Even with a single one now the sound is full on my system. I noted that the datasheet mentions the output stage can drive 50 ohm load with low distortion.
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