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Building the ultimate NOS DAC using TDA1541A
Building the ultimate NOS DAC using TDA1541A
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Old 12th January 2019, 04:22 PM   #6871
Hanze Khronye is offline Hanze Khronye  Australia
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Quote:
Originally Posted by Zoran View Post
If You are changing the value of input pot it can lead to cutting out highs at lower pot levels? Because of conjunction with dynamic capacitance of the stage?
But also, if this amp have C at the output to speaker - maybe should check this value against the load and increase it for lower LF rolloff?
I dont know exactly the case only thinking loudly...
I agree with much of what you say. PP 2H cancellation, and also output impedance issues.

Further, to say you have a passive output to your DAC and just to add an additional gain stage, or any stage thereafter to make up the shortfall is like putting the cart in front of the horse.

There are only a few current model DACs using 1541a, and they all run single ended and with resistor IV. Zanden, AMR, Audial.. but what do they know, right?
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Old 12th January 2019, 06:20 PM   #6872
Zoran is offline Zoran  Serbia
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Quote:
Originally Posted by Hanze Khronye View Post
I agree with much of what you say. PP 2H cancellation, and also output impedance issues.

Further, to say you have a passive output to your DAC and just to add an additional gain stage, or any stage thereafter to make up the shortfall is like putting the cart in front of the horse.

There are only a few current model DACs using 1541a, and they all run single ended and with resistor IV. Zanden, AMR, Audial.. but what do they know, right?
Consider the most simple set up. just an Riv and coupling C to output. It is not good idea because for larger output V You forced to use larger Riv value. That will lead to much higher offset then stated in datas. (Some of folks even measure that, me too...). And You will stay with increased distortion? So no matter is it SE or BAL topology of TDA1541A, and passive Riv, I think that lower value of Riv should be used. That will cause low Voltage signal level that should be amplify to reach some standard value.
...
Other thing is dac chip have DA grounds. For my op. it should be a galvanic isolated to make "pure" analog ground. Generally, transformers have significantly lower self distortion with lower value of signal...
...
I think that Zanden have some active circuit to drive passive notch filter multiply network? Please take a look at the Zanden papers? Also I think that Audial have also an active circuit to drive isolation transformer? In some models it have a active IV conversion even? I dont have these schs to check now.
...
There are many solutions for TDA1541A end stages.
cheers
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Last edited by Zoran; 12th January 2019 at 06:23 PM.
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Old 13th January 2019, 07:47 AM   #6873
Hanze Khronye is offline Hanze Khronye  Australia
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Quote:
Originally Posted by Zoran View Post
Consider the most simple set up. just an Riv and coupling C to output. It is not good idea because for larger output V You forced to use larger Riv value. That will lead to much higher offset then stated in datas. (Some of folks even measure that, me too...). And You will stay with increased distortion? So no matter is it SE or BAL topology of TDA1541A, and passive Riv, I think that lower value of Riv should be used. That will cause low Voltage signal level that should be amplify to reach some standard value.
I don't think anyone is suggesting trying to achieve a suitable output voltage with a large value resistor and capacitor output. FFT analysis clearly shows 68 ohms as a sensible upper limit, and this has been covered many times now. I prefer 33 to 47 ohms.

Seems to me that overall THD level is perhaps not as critical as even to odd harmonic spectra, especially higher level harmonics, and this speaks also to my comment WRT 2H cancellation within balanced stages. This seems to be often overlooked, or misunderstood.

Quote:
Other thing is dac chip have DA grounds. For my op. it should be a galvanic isolated to make "pure" analog ground. Generally, transformers have significantly lower self distortion with lower value of signal...
The digital and analogue grounds are shared at some point. I understand your comment about galvanic isolation, and this can be done also at the output of the analogue stage. Very generally WRT the transformer comment, certainly not compared with a resistor, and then I think overall context becomes important. Consider E280F or D3a as triodes, suitably loaded noise and distortion will be beyond concern, load it with a step down transformer and you have a gain stage, buffer, LP filter and galvanic isolation with the use of two resistors, one tube and one transformer.

Quote:
I think that Zanden have some active circuit to drive passive notch filter multiply network? Please take a look at the Zanden papers? Also I think that Audial have also an active circuit to drive isolation transformer? In some models it have a active IV conversion even? I dont have these schs to check now.
Zanden 5000 uses a complicated filter, apparently to good effect, and I believe that with Fs 176kHz this becomes unnecessary anyway. Still the use of a resistor IV, you can use the output current of the 1541a, the mu of the tube, and the output voltage to determine what value of resistor this is.

WRT Audial, the use of OPA861 to rx the 1541a output current which is modulated by the Iq of approx 7mA then mirrored to the collector where a 1k5 resistor performs the IV conversion (4mA x 1k5 = 6vpp, or 2.1vRMS). Load to the 1541a at 7mA is approx 10 ohms. This resistor is then buffered by another opamp of the same type. The circuit uses current injection to null the offset to zero volts, in reality it is subject to minor thermal instability as expected. When nulled for zero at the output of the buffer, it can drive a 0mA type transformer, yet in my experience it does not respond well to direct inductive loading, I have lost a few to no apparent reason, and was surprised that things actually sounded better without the buffer and a series capacitor output in lieu of the transformer.

Quote:
There are many solutions for TDA1541A end stages.
cheers
Indeed, and I have built variations of all mentioned within this post, with the exception of the zanden filter, and I know which I prefer - and as before, a lot will be subjective, but I think some understanding as to why, might assist the builder with a direction, or at least a starting point.

Kind regards HK

Last edited by Hanze Khronye; 13th January 2019 at 08:17 AM.
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Old 13th January 2019, 10:19 AM   #6874
batteryman is offline batteryman  United Kingdom
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Building the ultimate NOS DAC using TDA1541A
Quote:
Originally Posted by Zoran View Post
I think that for the passive Riv You have to tap one end to ground - to have any conversion
In the direct Transformer load, You have I/V conversion at the highly reactive element. I listened more implementation like this and personally I was not satisfied. Most of them was from prominent brands. Modifying in this fashion i explained was significantly better. But I must say that was SE typologies? Cant remember now for sure...
.
For the passive IV conversion It is from significance to have "pure" resistive load. Like EC explained earlier on the topic. And give the option for the hand made Isotan wire resistor. I make it may self and it sound very good?
...
How was offset without current inj. and what value of Riv was in Your example please?
Cheers
.
I use one 100R 1% metal film on each of the 4 dac outputs. (to ground)

There is no need to cancel the offset because the transformer is connected across the + & - Dac I/v resistors (which are both at -200mv no signal, so no DC current flows through the transformer)

I have tried connecting the centre tap to ground but as I said, I could not hear any difference, so its left unconnected and I also don't want another path to ground which is frequency dependent.

I will experiment with non inductive wire wound resistors.
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Old 13th January 2019, 10:35 AM   #6875
batteryman is offline batteryman  United Kingdom
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Building the ultimate NOS DAC using TDA1541A
Quote:
Originally Posted by Hanze Khronye View Post

Zanden 5000 uses a complicated filter, apparently to good effect, and I believe that with Fs 176kHz this becomes unnecessary anyway. Still the use of a resistor IV, you can use the output current of the 1541a, the mu of the tube, and the output voltage to determine what value of resistor this is.

WRT Audial, the use of OPA861 to rx the 1541a output current which is modulated by the Iq of approx 7mA then mirrored to the collector where a 1k5 resistor performs the IV conversion (4mA x 1k5 = 6vpp, or 2.1vRMS). Load to the 1541a at 7mA is approx 10 ohms. This resistor is then buffered by another opamp of the same type. The circuit uses current injection to null the offset to zero volts, in reality it is subject to minor thermal instability as expected. When nulled for zero at the output of the buffer, it can drive a 0mA type transformer, yet in my experience it does not respond well to direct inductive loading, I have lost a few to no apparent reason, and was surprised that things actually sounded better without the buffer and a series capacitor output in lieu of the transformer.

Indeed, and I have built variations of all mentioned within this post, with the exception of the zanden filter, and I know which I prefer - and as before, a lot will be subjective, but I think some understanding as to why, might assist the builder with a direction, or at least a starting point.

Kind regards HK
There are certainly almost infinite ways to recover the audio from the TDA1541 and everyone has there own preferred method. However just because xyz commerical manufacturer uses a particular method does not imply it must be superior.

Do these 'high end' manufacturers use simultaneous mode and stop the bit clock after latching the data? Or have they just focused on the reconstruction filter?

I don't have the means, money & time to explore all the various I/V stages so I have tried to eliminate all the variables surrounding current injection, opamps, tubes, interstage transformers etc by sticking to the simplest - passive I/V and a transformer designed for the job.
I am very pleased with the result and impressed with what is possible from a couple of TDA1541s.

The only commercial dacs I have owned over the years were a Pink Triangle Ordinal (TDA1547 DAC7 bitstrem dac), a Audio Note Dac 1.1 (AD1865 NOS + tube output) and a Musical Fidelity XRAY - (Burr Brown PCM dac).

I do remember removing the tube output of the DAC1.1 and fitting my transformers to the benefit of the SQ.
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Old 13th January 2019, 11:38 AM   #6876
Hanze Khronye is offline Hanze Khronye  Australia
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Quote:
Originally Posted by batteryman View Post
I don't have the means, money & time to explore all the various I/V stages so I have tried to eliminate all the variables surrounding current injection, opamps, tubes, interstage transformers etc by sticking to the simplest - passive I/V and a transformer designed for the job.
If they were designed for the job they would be gapped for 2mADC.

What is the output voltage at 0dB FS, and the output impedance at the output terminals?.

HK

Last edited by Hanze Khronye; 13th January 2019 at 11:44 AM.
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Old 13th January 2019, 02:12 PM   #6877
koldby is offline koldby  Denmark
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Quote:
Originally Posted by batteryman View Post
I use one 100R 1% metal film on each of the 4 dac outputs. (to ground)

There is no need to cancel the offset because the transformer is connected across the + & - Dac I/v resistors (which are both at -200mv no signal, so no DC current flows through the transformer)

I have tried connecting the centre tap to ground but as I said, I could not hear any difference, so its left unconnected and I also don't want another path to ground which is frequency dependent.

I will experiment with non inductive wire wound resistors.
Now I have modified my DAC/Preamp so it now consists of 2 TDA1541A in balanced simultaneous mode.
First picture is of the DAC/Preamp open, where you can see the dual TDA1541a board and the small Sowter transformers. The Preamp/vol consists of bruno putzeys balanced preamp with Maya controller. Inside is also a second DAC that eventually shall feed a subwoofersystem.
From the next picture, you can see the front with the leftside display displaying the sampling frequency.

I was a little disappointed by the sound at first especially when the digital signal was high.
The main difference between how the DAC was before the modification, besides the fact that it is now balanced, is that before I had to bias the output from the TDA1541A to avoid DC flowing through the transformers. This is not necessary when two outputs are feeding the transformer.
BUT! When the TDA 1541A is loaded by 100 Ohm and have no bias, the output is ca. -200mV DC and if you to that add the AC when outputting 0dB digital , the output peaks at around -400 mV and this is too much for the TDA1541A. It will start to increase distortion. I have attached two pics from a (pretty noisy) PC spectrum analyzer, The first is with no bias and the second is with 2 mA bias.
What I did was to remove the four 100 Ohm resistors and the transformers from the outputs, and connect a trimpot from +5V to one output and trim it to 0 V DC and the measure the value, just to be sure. It turned out to be near 2K4 witch I had in my collection. I then reinstalled the 100 ohm and connected each of those to a 2K4 going to +5V and then reconnected the transformers .
It is quite obvious that bias is indeed needed when operating the TDA1541A with a 100 Ohm I/V resistor. If you lower this value considerably, you can do without the bias.
Attached Images
File Type: jpg IMG_20190113_111406.jpg (141.7 KB, 140 views)
File Type: jpg Front.jpg (48.7 KB, 140 views)
File Type: jpg 100Ohm without bias.jpg (107.1 KB, 139 views)
File Type: jpg 100Ohm with bias.jpg (97.8 KB, 136 views)
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Old 13th January 2019, 02:15 PM   #6878
lcsaszar is offline lcsaszar  Hungary
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Quote:
Originally Posted by batteryman View Post
You do still have I/V resistors?

Or no resistors and you have grounded the centre tap, in which case DC is flowing through the transformer? (not allowed for the Sowters)

I have tried my balanced setup with the centre tap grounded and not grounded and cannot hear a difference. (I have 100r I/V resistors)
The I/V resistor is the cathode impedance of the tubes, reflected to the primary. The center tap of the primary is grounded, but the -2 mA bias current cancel each other in the two halves of the primary, because the DAC chips work in push-pull.
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Old 13th January 2019, 02:59 PM   #6879
batteryman is offline batteryman  United Kingdom
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Building the ultimate NOS DAC using TDA1541A
Quote:
Originally Posted by koldby View Post
I was a little disappointed by the sound at first especially when the digital signal was high.
The main difference between how the DAC was before the modification, besides the fact that it is now balanced, is that before I had to bias the output from the TDA1541A to avoid DC flowing through the transformers. This is not necessary when two outputs are feeding the transformer.
BUT! When the TDA 1541A is loaded by 100 Ohm and have no bias, the output is ca. -200mV DC and if you to that add the AC when outputting 0dB digital , the output peaks at around -400 mV and this is too much for the TDA1541A. It will start to increase distortion. I have attached two pics from a (pretty noisy) PC spectrum analyzer, The first is with no bias and the second is with 2 mA bias.
What I did was to remove the four 100 Ohm resistors and the transformers from the outputs, and connect a trimpot from +5V to one output and trim it to 0 V DC and the measure the value, just to be sure. It turned out to be near 2K4 witch I had in my collection. I then reinstalled the 100 ohm and connected each of those to a 2K4 going to +5V and then reconnected the transformers .
It is quite obvious that bias is indeed needed when operating the TDA1541A with a 100 Ohm I/V resistor. If you lower this value considerably, you can do without the bias.
Interesting. I will try your suggestion although I can't say I have heard any obvious distortion. (probably I wouldn't given its -54db)

I assumed that a load resistor on the secondary would reduce the primary impedance (to 65R claim Sowter) which is in parallel with the I/V resistor, so although the output is at -200mv with no signal, at frequencies above about 10Hz, the 1541 sees a much lower resistance.
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Old 13th January 2019, 03:57 PM   #6880
Hanze Khronye is offline Hanze Khronye  Australia
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I note the prominent (third) odd order distortion in the FFT.
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