[regal]

Quote:

Originally Posted by cetoole /img/forum/go_quote.gif I would think I/V is actually performed by the two 300ohm resistors going to v- through that zener diode.

So its passive + a buffer with the DAC seeing a very high 300R load ?


Looking at the other designs posted here I am thinking only Twisted Pairs would handle the high DC offset of the new PCM179x chips. This is what I ran into with the Borbely I/V stage. Works great with my AD1862 but when I tried to work it with my EZDAC (PCM1794) it couldn't deal with the high DC offset (I remember volts not mV.) This is because the new chips are single rail supplied.

 

[cetoole]

Quote:

Originally Posted by regal /img/forum/go_quote.gif So its passive + a buffer with the DAC seeing a very high 300R load ? Looking at the other designs posted here I am thinking only Twisted Pairs would handle the high DC offset of the new PCM179x chips. This is what I ran into with the Borbely I/V stage. Works great with my AD1862 but when I tried to work it with my EZDAC (PCM1794) it couldn't deal with the high DC offset (I remember volts not mV.) This is because the new chips are single rail supplied.

No, not at all. Input is a common base amplifier, each input transistor is biased to ~2.7mA, making AC input impedance ~10ohm. They did a crappy job of biasing the bases of the input transistors, shoulda used at least current mirrors, so there is significant DC offset on the input. Input impedance starts rising around 200KHz, going extremely inductive. My simulation shows a peak at ~85MHz of ~150ohm, though this isnt precise at all, since I am using 2SC2240/2SA970 spcie models. I assume this rising impedance is due to the resistor biasing of the input transistor bases, just as a guess.

Forgot about the second part. Actually, all that is needed is to rebalance an existing design. High offset can be compensated for, you just cant slap a generic design meant for something else in and expect it to work.

 

[Cauhtemoc]

Quote:

Originally Posted by regal So its passive + a buffer with the DAC seeing a very high 300R load ? Looking at the other designs posted here I am thinking only Twisted Pairs would handle the high DC offset of the new PCM179x chips. This is what I ran into with the Borbely I/V stage. Works great with my AD1862 but when I tried to work it with my EZDAC (PCM1794) it couldn't deal with the high DC offset (I remember volts not mV.) This is because the new chips are single rail supplied.

The current conveyor I posted above can easily handle the -6.2 mA offset on the PCM1794. Just adjust R5 to supply 16.2 mA. That's it.

 

[Russ White]

Quote:

Originally Posted by Cauhtemoc /img/forum/go_quote.gif The current conveyor I posted above can easily handle the -6.2 mA offset on the PCM1794. Just adjust R5 to supply 16.2 mA. That's it.

That may be prone to drift, for a number of different reasons. Still it could be satisfactory.

Cheers!
Russ

 

[00940]

Quote:

Originally Posted by Cauhtemoc /img/forum/go_quote.gif The current conveyor I posted above can easily handle the -6.2 mA offset on the PCM1794. Just adjust R5 to supply 16.2 mA. That's it.

Thinking of it... shouldn't one adjust R5 to supply 10ma - 6.2ma and thus 3.8ma rather than 16.2 ?

 

[Cauhtemoc]

Quote:

Originally Posted by 00940 Thinking of it... shouldn't one adjust R5 to supply 10ma - 6.2ma and thus 3.8ma rather than 16.2 ?

From what I can tell from the datasheets the PCM1794 has a negative offset of 6.2 mA, meaning that it will suck up this much current at bipolar zero. So if you want 10 mA to go through the conveyor you need to increase the current source by 6.2 mA. I could be wrong of course, the offset could be positive, in which case you are correct.

So, is anyone going to build it?

 

[00940]

well, it's a bit complicated to me... The datasheet indeed speaks of a negative offset. On the other hand, still according to the datasheet, this offset translates after I/V conversion into a negative voltage. In ltspice, using an opamp for I/V, you need to source current in order to get that.

 

[Russ White]

I see someone else on another forum had the same concern about a voltage source in series with a resistor for a current source.

In reality most current sources are much more dynamic than that.

I am not sure there is an ideal way to simulate a DAC current output. At least none that i know of yet. The best way would be to have in depth knowledge of the inner working of the DAC itself.

Cheers!
Russ

 

[SnoopyRocks]

diyAudio Forums - Discrete Super Symmetric(I think) Opamp for I/V Etc...
Quote:

Originally Posted by Russ White /img/forum/go_quote.gif I see someone else on another forum had the same concern about a voltage source in series with a resistor for a current source. In reality most current sources are much more dynamic than that. I am not sure there is an ideal way to simulate a DAC current output. At least none that i know of yet. The best way would be to have in depth knowledge of the inner working of the DAC itself. Cheers! Russ

Russ, it's nice (refreshing really) to see a DIYer attempting to make a fully differential opamp with a propper common-mode feedback loop (CMFB). IMO your characterizations of it are overzelous to say the least. Nor is it new...I've seen all of this before. This does not take away from what you've got going here though. It is a very sophosticated design for an ametuer. If you want your design to be the best you can make it, the CMFB loop needs improvement. The primary problem is where (and how) you've chosen to feedback the CMFB error signal. Beyond that, proper compensation of the CMFB loop is just as important as the differential path, but quite a bit more difficult to accomplish. You won't find this knowledge reading this forum. If you would like some guidance, send me a pm. Nice work.

 

[00940]

Quote:

Originally Posted by Russ White /img/forum/go_quote.gif I see someone else on another forum had the same concern about a voltage source in series with a resistor for a current source. In reality most current sources are much more dynamic than that. I am not sure there is an ideal way to simulate a DAC current output. At least none that i know of yet. The best way would be to have in depth knowledge of the inner working of the DAC itself. Cheers! Russ

You mean this ? DIYHiFi.org • View topic - I/V and amount of current feedback

I think it's safe to quote diyhifi.org, we're not on diyaudio

 

[00940]

Quote:

Originally Posted by Cauhtemoc /img/forum/go_quote.gif So, is anyone going to build it?

I hope to. Got to have a working pcm1798 first

It would be easy to wire p2p. Here's a version with bc560c/bc550c and output buffer. It'd also be easy to add a servo to the output buffer btw. http://www.membres.lycos.fr/sst00940/schem%20cau.PNG

 

[Cauhtemoc]

You can save some parts by using the same bias for the output emitter follower as I used for the current source in the conveyor. In other words, connect the base of the output current source transistor to the same location as the base of Q12 and Q14 in my schematic. You could also cascode it in the same way I have done with Q11 and Q13.

 

[00940]

You're right

There's something to be said against copy-paste parts of schematics.

I updated the pics.