Please bear with me. I do not have a strong grasp of the complex interactions of my audio equipment. I have never designed or built a new circuit from scratch.
I have been considering adding a tube output to my Philips CD880 for about 10 years now. I have spent a many, many hours trying to learn the technical requirements of I/V conversion and how to build a working tube output. Today, I ran across Regal's thread from 2011 https://www.diyaudio.com/community/threads/new-look-at-tube-analog-for-tda1541.194134/ and a light went on.
Why has everyone focused on building line level outputs instead of MC phono level outputs? I have never seen any discussion of a MC cartridge level output. My Ortofon Cadenza Black cartridge only has an output of 0.3mv and my CAT preamp and ARC D70 MkII have no problem driving my Martin Logan Sequel speakers to good volumes.
It seems to me that there would be more options to build a low level I/V conversion output that outputs 0.3V than 2.0V. What are the technical barriers to converting the TDA1541A's 0.4ma PP to 0.3mv and outputting that to the phono in of my preamp? Is there an insurmountable impedance mismatch problem between the chip and a preamps phono in? Even if I cannot eliminate the tube output, wouldn't I have more options for which tubes I use and which amplification cuirt I use if I am trying to build a phono level output?
I hope someone who has the knowledge will help me with this.
I have been considering adding a tube output to my Philips CD880 for about 10 years now. I have spent a many, many hours trying to learn the technical requirements of I/V conversion and how to build a working tube output. Today, I ran across Regal's thread from 2011 https://www.diyaudio.com/community/threads/new-look-at-tube-analog-for-tda1541.194134/ and a light went on.
Why has everyone focused on building line level outputs instead of MC phono level outputs? I have never seen any discussion of a MC cartridge level output. My Ortofon Cadenza Black cartridge only has an output of 0.3mv and my CAT preamp and ARC D70 MkII have no problem driving my Martin Logan Sequel speakers to good volumes.
It seems to me that there would be more options to build a low level I/V conversion output that outputs 0.3V than 2.0V. What are the technical barriers to converting the TDA1541A's 0.4ma PP to 0.3mv and outputting that to the phono in of my preamp? Is there an insurmountable impedance mismatch problem between the chip and a preamps phono in? Even if I cannot eliminate the tube output, wouldn't I have more options for which tubes I use and which amplification cuirt I use if I am trying to build a phono level output?
I hope someone who has the knowledge will help me with this.
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Haha, the phono has equalization riaa , you cannot do this.
Use an opamp to convert current into voltage
Use an opamp to convert current into voltage
Thanks, good point. I hadn't thought of about the RIAA equalization but it is not insurmountable.
I was thinking of something like this. I think the very low impedance levels make it somewhat impractical; in particular, the resistances in the RIAA correction network get so small you are more or less forced to realize them as the wire resistances of the inductors, which are not very accurate. You could use higher impedances and a voltage divider, though. You will also have to watch out for hum pick-up in the inductors.
Thanks. I was thinking of using Vishay Z Foil resistors for I/V conversion. I suspect that I might have to amplify the resulting voltage to get it up to at least 0.3mv but don't know enough to calculate how much voltage I will have after the I/V conversion. I have two 10 ohm Z Foils (spendy little things) that I bought for one of Vlad1980's Lampizator PCBs. https://www.diyaudio.com/community/threads/project-dac-tda1541-with-tube-output.325664/
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Wait a minute. When I read the comments about RIAA I got it backward in my brain. The LPs have RIAA pre-emphasis equalization applied in the mastering process and my preamp has an inverse RIAA equalization filter in it. Sooooo... I would need to apply RIAA equalization to the output of the I/V converted signal from my DAC
The wheel has already been invented and runs just fine.
Why are you trying to reinvent this wheel ?
What is your end goal ?
Why are you trying to reinvent this wheel ?
What is your end goal ?
Careful with 10 ohms.
You might burn the TDA by drawing too much current.
Lukaz (Lampisator) has tried various values, and the best result (with lowest distortion) came from
91-100 ohm, directly mounted from I-out to ground.
Have done "Lampisation" on a Philips CD-160 (among others) with 91 ohms, directly feeding 6N2P-EV, and it works
like a fxxxxng charm. Sound is awesome.
Btw..... why not just use an active MC-headamp??/passive step-up?
This should give you your average 5-10mV signal to work with. Designng 40dB low noise amplification around a decent opamp
shouldn´t be that difficult, and you´ll avoid the hazzle dealing with riaa-EQ 😉
You would have to reverse RIAA Eq and then cut it 40 dB so that you could ruin the noise floor to 65db and add distortion in the process.
I am hoping to get the signal from the TDA1541A in my CD880 to my preamp with the least intervening components between them as possible. Will it sound better than the op amps in my CD player? I don't know but I want to find out if it will. As far as the wheel analogy goes, do you have an example of this type wheel being tried before? I can't find an example of anyone even attempting it.
I am still trying to find a passive RIAA pre-emphasis circuit to pass the signal trough so I can send it too my preamp's phono inputs which go into an inverse RIAA (deemphasis) circuit. I don't understand why I would have to change the dB of that signal. I just need a signal of at least 0.3mv and up to about 5.0mv with RIAA re-emphasis. My preamp will do the rest.
The TDA1541A is a current output device. It is happiest into 0 ohms.
Probably because those who could know enough to know there are better ways of doing things.
https://www.tubecad.com/2012/06/blog0233.htm
Broskie is your best friend, without knowing it...
I would build on that big time and try to improve from there with THD analyzer/scope
There is no other way.
Read the article 100 times and then build it, built it overkill.
The resistors need to be manually adjusted as well as the DC servo. If you are not too sure how to optimize it for TDA send Broskie and Email or just copy what is there, calculate your filters.
Broskie is your best friend, without knowing it...
I would build on that big time and try to improve from there with THD analyzer/scope
There is no other way.
Read the article 100 times and then build it, built it overkill.
The resistors need to be manually adjusted as well as the DC servo. If you are not too sure how to optimize it for TDA send Broskie and Email or just copy what is there, calculate your filters.
Thanks. This is a great article and if I give up on my current quest to see if it is possible to do a simple resistor I/V conversion and go into my preamp's high gain MC phono input with as few additional passive as possible , I will read it until I completely grok it.
You could be right but impossible is just a state of mind. New things never result from starting with the belief that something is impossible. 🙂