I think I would be inclined to defer to Jocko's views. However I can suggest a couple of thoughts. The TDA1541 can source a maximum of 4.6mA. So you shouldn't really run into any trouble here. Some DAC chips can source or sink more than this, and would need a higher bias. Probably no harm it playing with bias, but it doesn't sound like the culprit to me.
One thing that has been in the back of my mind. You only have the I/V stage here. So far no reconstruction filter. We can buy into all sorts of arguments about these, but in general, if any downstream stage fed by this DAC gets into slew rate limiting due to the quite substantial amount of high frequency energy in the output, you will get serious distortion. So, you may well want to look toward implementing an approriate filter. The precise nature of the filter will depend upon exactly how you are connecting it, but in general need not be complex.
You haven't said how you are handling the output.
One thing that has been in the back of my mind. You only have the I/V stage here. So far no reconstruction filter. We can buy into all sorts of arguments about these, but in general, if any downstream stage fed by this DAC gets into slew rate limiting due to the quite substantial amount of high frequency energy in the output, you will get serious distortion. So, you may well want to look toward implementing an approriate filter. The precise nature of the filter will depend upon exactly how you are connecting it, but in general need not be complex.
You haven't said how you are handling the output.
Please see the schematic Francis.
I have soldered an IC base where the opamps are, IC 6306 and 6307 in the attached schematic. I broke an IC base and soldered the I/V stage input to a pin. I then can install teh opamp and listen to the player, and when Im ready to test the I/V, I just pop the opamp and stick the I/V input pin to IC base pin 2 on IC 6307.
Perhaps where Im going wrong is that the weird feedback circuit that connects to this pin is in circuit, perhaps this is the problem? Maybe I should cut the track and test it, that way its straight from TDA1541 pin 25 to I/V input.
The output is using aligator clips from I/V output and ground to my preamp via rca cable.
I appreciate your help with this.
regards Arthur
I have soldered an IC base where the opamps are, IC 6306 and 6307 in the attached schematic. I broke an IC base and soldered the I/V stage input to a pin. I then can install teh opamp and listen to the player, and when Im ready to test the I/V, I just pop the opamp and stick the I/V input pin to IC base pin 2 on IC 6307.
Perhaps where Im going wrong is that the weird feedback circuit that connects to this pin is in circuit, perhaps this is the problem? Maybe I should cut the track and test it, that way its straight from TDA1541 pin 25 to I/V input.
The output is using aligator clips from I/V output and ground to my preamp via rca cable.
I appreciate your help with this.
regards Arthur
No GIFs should really never compress. But that is another story.
OK, lets have a look at what you have. The output of the DAC goes stright to the -ve input of one half of the 5532, which is set up as a pretty conventional transimpedance amplifier. The 1k8 resistor and 2nF cap are pretty much what one would expect. The output voltage you expect is simply the current from the DAC as if it was placed across the resistor. The mess also across the op-amp - the FET and the other network is almost certainly the de-emphasis network. With luck you can ignore it. CDs are never encoded with emphasis, and so the FET should never be tunred on, and the whole horrid thing never actually in circuit. After that the transimpedance amplifier feeds the reconstruction filter. This is made up of the 2k4 resitors, 470uH inductor 2n2 and 1nF cap along with the second half of the op-amp. After that you simply have some muting effected by the two transitors and out.
However, you do have a heap of junk hanging off the input to you I/V, and with the op-amp missing, and therefore the virtual earth gone, one can only begin to guess at what the entire train of junk is doing. I am especially worried by that link that goes off to the "filter circuit". I can't begin to imagine what that might be, or what it is doing connected where it is. This would rather suggest you need to do something about breaking the connection from all this mess to your I/V. This gets messier. If you did this there remains the possibility of using the reconstruction filter and buffer you have now. But first things first.
The other question is how you are powering the circuit? In particular, the relationship of the 0v of the I/V's supply and the CD player's ground.
OK, lets have a look at what you have. The output of the DAC goes stright to the -ve input of one half of the 5532, which is set up as a pretty conventional transimpedance amplifier. The 1k8 resistor and 2nF cap are pretty much what one would expect. The output voltage you expect is simply the current from the DAC as if it was placed across the resistor. The mess also across the op-amp - the FET and the other network is almost certainly the de-emphasis network. With luck you can ignore it. CDs are never encoded with emphasis, and so the FET should never be tunred on, and the whole horrid thing never actually in circuit. After that the transimpedance amplifier feeds the reconstruction filter. This is made up of the 2k4 resitors, 470uH inductor 2n2 and 1nF cap along with the second half of the op-amp. After that you simply have some muting effected by the two transitors and out.
However, you do have a heap of junk hanging off the input to you I/V, and with the op-amp missing, and therefore the virtual earth gone, one can only begin to guess at what the entire train of junk is doing. I am especially worried by that link that goes off to the "filter circuit". I can't begin to imagine what that might be, or what it is doing connected where it is. This would rather suggest you need to do something about breaking the connection from all this mess to your I/V. This gets messier. If you did this there remains the possibility of using the reconstruction filter and buffer you have now. But first things first.
The other question is how you are powering the circuit? In particular, the relationship of the 0v of the I/V's supply and the CD player's ground.
Hi Francis,
I have stuff to do tonight so I wont get a chance to do anything on the cd player.
I will do as you suggested and go straight from tda1541 to I/V with all else out of circuit, ie cut tracks. Then straight to RCA out.
The PSU is + - 24V 560uF caps 7818 and 7918 with a 6.2V zener at the common(Sorry bout cheap regs Jocko). 47uF decoupling and 0.1uF polyester at the I/V stage. The 0V point is not earthed and I use this ground to connect to the RCA shield. Im not sure about this but can experiment if it hums. The Output of the I/V obviously connects to the RCA.
So far Im blown away with the opamp, TDA1541 upgrade to A variant and Tent XO clock. I prefer it to my stock Marantz cd67IISE OSE.
cheers ab
I have stuff to do tonight so I wont get a chance to do anything on the cd player.
I will do as you suggested and go straight from tda1541 to I/V with all else out of circuit, ie cut tracks. Then straight to RCA out.
The PSU is + - 24V 560uF caps 7818 and 7918 with a 6.2V zener at the common(Sorry bout cheap regs Jocko). 47uF decoupling and 0.1uF polyester at the I/V stage. The 0V point is not earthed and I use this ground to connect to the RCA shield. Im not sure about this but can experiment if it hums. The Output of the I/V obviously connects to the RCA.
So far Im blown away with the opamp, TDA1541 upgrade to A variant and Tent XO clock. I prefer it to my stock Marantz cd67IISE OSE.
cheers ab
Luke said:
The question is how the ground of the I/V PS meets the ground for the DAC. Or to really ask the right question: what is the return path for the DAC output current? This is something that needs to be got right, and not, as sometimes happens, a path that wends its way all over the place, possibly only circulating by the most baroque of routes. I would be looking to tie the 0V of the PS right to the ground of the DAC. And keeping the paths as short as is reasonable. There is still much RF energy in that return current, you want to treat it well.
No, just that you did not go through this (?):
http://www.diyaudio.com/forums/showthread.php?postid=378072#post378072
http://www.diyaudio.com/forums/showthread.php?postid=378072#post378072
dc servo ?
Hi,
Has anyone implemented a dc servo into Jocko's design ?
Rbroer implemented a dc servo for IV http://www.diyaudio.com/forums/showthread.php?postid=262355#post262355
Kind regards,
Ashley.
Hi,
Has anyone implemented a dc servo into Jocko's design ?
Rbroer implemented a dc servo for IV http://www.diyaudio.com/forums/showthread.php?postid=262355#post262355
Kind regards,
Ashley.
What transistors to use ?
Hi,
I've just modelled Jocko's IV in a LTSpice to compare op amp IV.
1. Standard opamp IV distortion on input rises rapidly from about 10 Khz (LT1364)
2.Current feedback op amps show a bit better on the input compared to voltage feedback. (LT1395)
3.My model of Jocko's IV appears flat upto 2Mhz on input.
I need to order some transistors so I can build the IV stage. Then when it's built I can add a filter.
Any recommendations for transistors? (I can't get those in Jocko's schematic)
Hi,
I've just modelled Jocko's IV in a LTSpice to compare op amp IV.
1. Standard opamp IV distortion on input rises rapidly from about 10 Khz (LT1364)
2.Current feedback op amps show a bit better on the input compared to voltage feedback. (LT1395)
3.My model of Jocko's IV appears flat upto 2Mhz on input.
I need to order some transistors so I can build the IV stage. Then when it's built I can add a filter.
Any recommendations for transistors? (I can't get those in Jocko's schematic)
Ashley
You can get those transistors from Cricklewood Electronics for pence I need to get some too.
http://www.cricklewoodelectronics.com/Cricklewood/customer/home.php
Pete
You can get those transistors from Cricklewood Electronics for pence I need to get some too.
http://www.cricklewoodelectronics.com/Cricklewood/customer/home.php
Pete
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