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Tubed CCDA I/V Amp for TDA1541(A)
Tubed CCDA I/V Amp for TDA1541(A)
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Old 16th January 2010, 12:32 PM   #11
regal is offline regal  United States
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Join Date: Jan 2004
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First I am so happy to see someone working on a passive I/V +tube DAC without the stupid SRPP which has no place in a DAC for the reasons you mentioned. I still think your goal should be the lowest I/V resistor possible with the highest tube gain that is quiet. I have long been considering a PCM1704 with a 10 ohm I/V and two CDA stages (ala Broskies simple phono stage minus RIAA.)

Kudos for pushing the tube DAC envenlope. So for my best results have been a PCM63K with a 6n6p anode follower CCS loaded (similiar to what tent labs discovered.) It had about -80db 2nd harmonic, but the I/V resistor was 47 ohms which is I think too large.
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Old 16th January 2010, 03:03 PM   #12
Radioman62 is offline Radioman62  Sweden
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Default Amasing findings to get really low THD and only 2nd.

Hi regal
I am glad to see that there is at least some souls out there thinking this is interresting.

You have really good THD figures on your setup. 47 ohm is not to large though, so you can sleep safely . 56 ohm gives about 84mV. TDA1541A puts out around 1.5 -1.6mA. I have done exhaustive tests on this lately and they tells me you are safe with 80mV.

For my piggybacked setup (another TDA1541A on top of the other) I now use 27 ohm and then I have 82 - 84mV -0dB 1kHz. FFT shows around -100dB measured on the resistor. You never get better. The spec says -95dB for one TDA1541A.
That means you only need one CCDA stage.
I have done a few more tests lately. I have som excellent E88CC Siemens SQ tubes I wanted to use. They only produce slightly more then 1Volt RMS -0dB but I think that is enough, at least for me using a 4X control preamp after, where I can choose input sources.
This tube gives a super clean sound. I didn't thought it would be possible to hear difference beween -65dB and -87dB K2 measured at -0dB, but it is.

and my latest tests showed some very interresting things.
I got the hint from my simulator program (Multisim 10) and have now a practical knowledge of the phenomenon.
There is a sweet spot when the two tubes is in current balance. The first tubes cathode resistor (R2 on the schematic first post) should be a potentiometer so that you can adjust it if the tubes are not 100% matched.
There is another sweet spot which was really amasing and quite shocking discovery (I did'nt get really frighten though )

The E88CC setup of this CCDA amp really likes to be loaded a bit. My tests showed around 4.1k for the R6 resistor outside the output capacitor. It dropped the THD from around -77dB to -87 dB! And more, no other harmonic then the second order was visible! In my case, using a R2R ladder volumepot after which loads the source with 10k I have to choose right resistor there as well or maybe have a pot. I now have THD figures with this tubed I/V CCDA, that runs circles around OP amps. I didn't even in my fantasy expect to get that good measuring figures out of such a simple circuit, not even using global feedback.

The sound is also fantastic. If you have good CD recordings it is very clean. It's also easier to find which recordings are not that good.

I have to go back to the bench again. Documentation on this last findings will be updated later on on my homepage.
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Last edited by Radioman62; 16th January 2010 at 03:06 PM. Reason: typo
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Old 16th January 2010, 04:54 PM   #13
Radioman62 is offline Radioman62  Sweden
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Default FFT on calibrated E88CC CCDA circuit

This is a FFT from last night findings, described in the post above.

Click the image to open in full size.

Today I also did both channels calibrated with those E88CC tubes (6DJ8, 6922, or ECC88 is equivalents) and is right now listening at it .
And because it's saturday I also have a few beer as companion to the music. It sounds even better then (subjective I know )


Couldn't get quite that low third order THD today though.
On the other hand I don't have any LP filter at all, running NOS. I'm intending some -12dB at 44.1kHZ with the finished project.
I also don't have any main AC filtering at all on any of the tests done so far.
Maybe a 220V AC cleanup could bring a few dB, I don't know.

Have a nice weekend and please, go get it for yourself with the solder iron! I mean the Nirvana of satisfaction when you built something of your own that whips out most of the competitors

Over and out for some time.
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Last edited by Radioman62; 16th January 2010 at 04:58 PM. Reason: forgot the FFT graph....
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Old 31st January 2010, 05:16 AM   #14
habsrock93 is offline habsrock93  Canada
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Hey, I just stumbled onto this thread. Great work so far I have the same dac kit and am in the process of setting up my soundcard to do the same type of measurements. I had planned on building the output with an anode follower. Could be interesting comparing results!

Dave
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Old 31st January 2010, 01:58 PM   #15
Radioman62 is offline Radioman62  Sweden
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Default project is progressing

Hi habsrock93
This DAC is awsome.
I have since my last post done some Eagle CAD and etched a PCB which I piggybacked on top of the four OP IC-holders. On top of that card I have the I/V resistor and a LP filter for the tubecircuit. There is also two High quality relays so that I can switch between tubed output or SS with the OP's.

I have settled for the lower amplifying device E88CC. So now my DAC gives 1.1V RMS, both with the LME49710 and the tubes. I have also altered the LPF and feedback on the OP's to match the tube circuit. So when switching there should be very little difference in amplitude or frequencey response. Just comparing other signatures which is hard to measure.

I'm not finished but I have even lower THD figures know, with both the OP's and the tubes. Superclean sound and very high revealing of details. My latest LPF approach which gives -12dB att 44.1Khz, still maintaining -0.5dB at 20Khz did a bit to the upper frequencys for this NOS setup I use. I handpicked and mesured all of those components to get exat the same phase response on both channels.

More info and pictures later on. FTM I'm in the middle of doing a genealogy death register of people that died between 1901 and 1946 in the Parish I live. Talk about different hobbies ...
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Old 31st January 2010, 02:01 PM   #16
Radioman62 is offline Radioman62  Sweden
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Quote:
Originally Posted by habsrock93 View Post
I had planned on building the output with an anode follower. Could be interesting comparing results!
Yes it will. I think you will have problems with a to high output impedance. Put a cathode follower after and you end up like me .... with a CCDA amplifier. Nothing to be ashamed of
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Old 31st January 2010, 05:53 PM   #17
Radioman62 is offline Radioman62  Sweden
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Lightbulb Fast switching between OP amps or tubes.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.
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Old 1st February 2010, 03:20 PM   #18
Radioman62 is offline Radioman62  Sweden
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Thumbs up Back on track again

I'm sorry if I have bothered you with other stuff after my first post in this thread. It's so many things to test before you finalize the project .
So now I'm back on track again.

I here present the final (?) schematic on my CCDA tubestage I have done research on lately.
This is the one I'm listening to right now on the testbench.
The schematic is for the output of one TDA1541A. For the moment I use two piggybacked DAC's and therefore have 27 ohm as an I/V resistor.

I have my tubes including the first 470k gridbias resistor and the 100k gridstopper connected trough screened cables to the other parts with the CCS and LPF filter. (Those parts sits on my new, also piggybacked, toggling card. The one I mentioned above where I now can switch between tubes and OP's.)
Schematic is telling the rest. Please feel free to try. You won't be dissapointed.

Click the image to open in full size.

Click the image to open in full size.
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Old 2nd February 2010, 04:13 PM   #19
ide2003 is offline ide2003  Indonesia
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Hi Radioman,
for sure you have brought me on track . tried SRPP on TDA1541, and looking for something different and better like your CCDA for PCM56..I'm new to all this..so..don't mind if I just read & learn..thanks
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Old 3rd February 2010, 09:42 AM   #20
regal is offline regal  United States
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SRRPP, CDA, both sound not as good a a simple anode follower with an INXY CCS, you juts have to use a tube like a 6n6p that has a low enough Rp and have an amp or preamp with an input impedance of >=50k. Believe me Ive tried.
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