IV-converter
This whole thread seems to explore and reverse-engineer and possibly improve the circuit by Jocko.
I strongly disagree with you Thorsten that a discreet circuit would be worse than integrated. Why on earth would Jocko use four dozens of transistors on his IV-board if he was satisfied with a simple AD844?. BTW that is real current feedback amplifier.
I constructed as a first attempt to a discrete design a circuit as in the OPA627 datasheet and it sounded a lot better than the original.
The whole idea of the IV-converter seems to circle around the input impedance of the IV-converter as the output of the DAC chip has limited compliance. What does that mean?
Well to qoute from Horowitz:
"<B>Ideal current-to-voltage converter</B>
Remember that the humble resitor is the simplest I-to-V converter. However it has the disadvantage of presenting a nonzero impedance to the source of input current; this can be fatal if the device providing the input current has very little compliance or does not produce a constant current as the output changes" [due the current]
The transresistance configuration can be used that is a opamp with non-inverting input grounded and the current going to the inverting input. A feedback resistor is connected between the output and the inverting input. The opamp does not need to be IC-type. It works very well with a discrete opamp. This is the configuration I am using with cascoded JFET's.
Now for Jocko and many others feedback seems to be a ugly F-word so I asume Jocko is using a current feedback configuration without the feedback resistor just as you do Thorsten.
As for the offset there are ways around Jocko wrote. I see some IC's on Jocko's board and suspect these are servoamps for offsetcountermeasure. A full complementary circuit would also be a asset.
In the datasheet of the AD844 it is explained that the current feedback amplifier has some advantages over a normal voltage feedback amplifier as a IV-converter. This is too technical to repeat or explain here but has to do with phase margin and stability. Also in fig. 28 of the datasheet is a simplified schematic in which the first part is almost excactly the scheme by Thijs. (the other part is the double diamond outputbuffer, LH002 style) I have been using the AD811 and OPA603 current feedback amplifier during some years in my DAC. After reading the Pooge Chronicles by Walter Jung and Hampton Childress. Also tried the AD844. Thereafter I switched to my discrete circuit as it sounded superior to the IC-currentfeedback amps. See Sonny Andersons thread.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=1794&highlight=sonnya+AND+amplifier
(SCM.zip)
His circuit is similar to mines.
Of the three transistor configurations possible the common base amplifier (input goes to the emittor) has the lowest inputimpedance. This happens to be the inverting input of the current feedback amplifier.
With the transresistance configuration a virtual ground at the inverting input is created.
Hi Thorsten, congratulations, you just constructed a current feedback amplifier but without the usual feedback resistor.Kuei Yang Wang said:
This whole thread seems to explore and reverse-engineer and possibly improve the circuit by Jocko.
I strongly disagree with you Thorsten that a discreet circuit would be worse than integrated. Why on earth would Jocko use four dozens of transistors on his IV-board if he was satisfied with a simple AD844?. BTW that is real current feedback amplifier.
I constructed as a first attempt to a discrete design a circuit as in the OPA627 datasheet and it sounded a lot better than the original.
The whole idea of the IV-converter seems to circle around the input impedance of the IV-converter as the output of the DAC chip has limited compliance. What does that mean?
Well to qoute from Horowitz:
"<B>Ideal current-to-voltage converter</B>
Remember that the humble resitor is the simplest I-to-V converter. However it has the disadvantage of presenting a nonzero impedance to the source of input current; this can be fatal if the device providing the input current has very little compliance or does not produce a constant current as the output changes" [due the current]
The transresistance configuration can be used that is a opamp with non-inverting input grounded and the current going to the inverting input. A feedback resistor is connected between the output and the inverting input. The opamp does not need to be IC-type. It works very well with a discrete opamp. This is the configuration I am using with cascoded JFET's.
Now for Jocko and many others feedback seems to be a ugly F-word so I asume Jocko is using a current feedback configuration without the feedback resistor just as you do Thorsten.
As for the offset there are ways around Jocko wrote. I see some IC's on Jocko's board and suspect these are servoamps for offsetcountermeasure. A full complementary circuit would also be a asset.
In the datasheet of the AD844 it is explained that the current feedback amplifier has some advantages over a normal voltage feedback amplifier as a IV-converter. This is too technical to repeat or explain here but has to do with phase margin and stability. Also in fig. 28 of the datasheet is a simplified schematic in which the first part is almost excactly the scheme by Thijs. (the other part is the double diamond outputbuffer, LH002 style) I have been using the AD811 and OPA603 current feedback amplifier during some years in my DAC. After reading the Pooge Chronicles by Walter Jung and Hampton Childress. Also tried the AD844. Thereafter I switched to my discrete circuit as it sounded superior to the IC-currentfeedback amps. See Sonny Andersons thread.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=1794&highlight=sonnya+AND+amplifier
(SCM.zip)
His circuit is similar to mines.
Of the three transistor configurations possible the common base amplifier (input goes to the emittor) has the lowest inputimpedance. This happens to be the inverting input of the current feedback amplifier.
With the transresistance configuration a virtual ground at the inverting input is created.
Hi Elso,
Sorry, but I want to ask you, what is the (current) feedback without the feedback? No feedback, no current feedback. Simply. But signal goes to emitter. That is not feedback at all. That is common base. The result is buffered DAC’s current output, and its voltage compliance is not critical afterward. That is all. And not every current feedback monolithic opamp can serve for this purpose. But OPA660 and AD844 can.
However, it is little dubious if cfb opamp like AD844 is really useful this way as it’s inverting input impedance is 50 Ohms (non-feedbacked, typicaly, according to datasheet, SPICE model actually shows 65 Ohms), generally low, but higher than we would like. Btw, OTA’s emitter of OPA660 has some 7 Ohms input impedance.
Pedja
Sorry, but I want to ask you, what is the (current) feedback without the feedback? No feedback, no current feedback. Simply. But signal goes to emitter. That is not feedback at all. That is common base. The result is buffered DAC’s current output, and its voltage compliance is not critical afterward. That is all. And not every current feedback monolithic opamp can serve for this purpose. But OPA660 and AD844 can.
However, it is little dubious if cfb opamp like AD844 is really useful this way as it’s inverting input impedance is 50 Ohms (non-feedbacked, typicaly, according to datasheet, SPICE model actually shows 65 Ohms), generally low, but higher than we would like. Btw, OTA’s emitter of OPA660 has some 7 Ohms input impedance.
Pedja
Re: IV-converter
Hi,
Actually, Burr Brown constructed (and manufacturerd) the device, so congratulation for that to them.
And congratulations to you for my newest favourite oxymoron, namely:
"a current feedback amplifier but without the usual feedback resistor"
BTW, I did not construct ANYTHING LIKE what you say, but it sounds good. Congratulations.
I don't know about "the circuit by Jocko", but I do have in my little black book copies from Audio in the earlier part of the 80's of the last century which show the same principle as discussed here and if I may be so bold, much better implemented than anything I have seen in this thread.
I know you do.
I also wish you a good time selecting dynamically matched complementary transistors (and matched at least a few MHz so this whole shebang really works) and to minimise parasitics in the layout. I for one have long given up discrete solid state circuits, in the basic area where I need them to work I find the right IC's applied competently outperfom OBJECTIVELY AND SUBJECTIVELY ANY discrete circuit presented to me.
But that's just me and I have only been playing this game for two decades, so WTFDIK.
Beats me. But IIRC he prefers oversampling too. Maybe an innate prefernce for the complex over the simple.
It takes all sorts.
Please qualify your remarks by saying:
"To me, in my system and based on my personal preferences and prejudices it sounded better."
Or if that is not correct state the "rahmenbedingungen" (cornerstones may be the best contextual translation of rahmenbedingungen). unqualified remarks assert usally superiorty that does not exist even in subjective terms, never mind the type of subjectivism usally falsely peddled to us as objectivism (see Hume).
Feedback is feedback.
What I propose does not include ANY form of feedback, not even local degeneration. Not knowing "Jocko's Circuit" I shall not speculate about what it is and what it not is. I do know what the circuit I propose is however and it does not include feedback in the I/V conversion..
Well, I prefer to do without servos. I have in purely subjective sonic terms (though my sample size in these investigations considerably exceeds the number one it is not large enough to allow statistical significance at a .05 level) always found servos on the liabilities side of the balance sheet.
I am proposing to use one. And indeed one that due to the special process used is truely complementary, unlike ANY discrete circuit based on so called "complementary" types, selected for equal Ft and Beta (or transconductance, treshold voltage and Ft in FET's) or not. So what is your problem. Apart from the fact that the suggestion came from me, that is?
Again, you fail to sufficiently qualify your remarks. I personally prefer to use a combination of transformers and valves for I/V conversion (incorrigable reactionary is me). But what holds true for me is not the universal truth.
Sayonara
Hi,
Actually, Burr Brown constructed (and manufacturerd) the device, so congratulation for that to them.
And congratulations to you for my newest favourite oxymoron, namely:
"a current feedback amplifier but without the usual feedback resistor"
BTW, I did not construct ANYTHING LIKE what you say, but it sounds good. Congratulations.
I don't know about "the circuit by Jocko", but I do have in my little black book copies from Audio in the earlier part of the 80's of the last century which show the same principle as discussed here and if I may be so bold, much better implemented than anything I have seen in this thread.
I know you do.
I also wish you a good time selecting dynamically matched complementary transistors (and matched at least a few MHz so this whole shebang really works) and to minimise parasitics in the layout. I for one have long given up discrete solid state circuits, in the basic area where I need them to work I find the right IC's applied competently outperfom OBJECTIVELY AND SUBJECTIVELY ANY discrete circuit presented to me.
But that's just me and I have only been playing this game for two decades, so WTFDIK.
Beats me. But IIRC he prefers oversampling too. Maybe an innate prefernce for the complex over the simple.
It takes all sorts.
Please qualify your remarks by saying:
"To me, in my system and based on my personal preferences and prejudices it sounded better."
Or if that is not correct state the "rahmenbedingungen" (cornerstones may be the best contextual translation of rahmenbedingungen). unqualified remarks assert usally superiorty that does not exist even in subjective terms, never mind the type of subjectivism usally falsely peddled to us as objectivism (see Hume).
Feedback is feedback.
What I propose does not include ANY form of feedback, not even local degeneration. Not knowing "Jocko's Circuit" I shall not speculate about what it is and what it not is. I do know what the circuit I propose is however and it does not include feedback in the I/V conversion..
Well, I prefer to do without servos. I have in purely subjective sonic terms (though my sample size in these investigations considerably exceeds the number one it is not large enough to allow statistical significance at a .05 level) always found servos on the liabilities side of the balance sheet.
I am proposing to use one. And indeed one that due to the special process used is truely complementary, unlike ANY discrete circuit based on so called "complementary" types, selected for equal Ft and Beta (or transconductance, treshold voltage and Ft in FET's) or not. So what is your problem. Apart from the fact that the suggestion came from me, that is?
Again, you fail to sufficiently qualify your remarks. I personally prefer to use a combination of transformers and valves for I/V conversion (incorrigable reactionary is me). But what holds true for me is not the universal truth.
Sayonara
Oxymoron?
Hi Pedja and Thorsten,
I meant the <B>topology</B> of the current feedback amplifier of the course. The AD811 has 14 Ohm inputresistance on the inverting input and the OPA603 30 Ohm according to the datasheets.
I tried to make clear the similarity between Jocko's circuit, Thijs's & Rudolf's schematic, OPA660 and common current feedback topology. Apparently it is not appreciated by all
Thorsten I am surprised by your reaction. Just before you left the AudioAsylum you wrote that "most" solid state was fatally flawed.
http://db.audioasylum.com/cgi/m.pl?forum=tweaks&n=50504&highlight=thorsten+fatally+flawed&session=
Now the right IC's are OK?
Hi Pedja and Thorsten,
I meant the <B>topology</B> of the current feedback amplifier of the course. The AD811 has 14 Ohm inputresistance on the inverting input and the OPA603 30 Ohm according to the datasheets.
I tried to make clear the similarity between Jocko's circuit, Thijs's & Rudolf's schematic, OPA660 and common current feedback topology. Apparently it is not appreciated by all
Thorsten I am surprised by your reaction. Just before you left the AudioAsylum you wrote that "most" solid state was fatally flawed.
http://db.audioasylum.com/cgi/m.pl?forum=tweaks&n=50504&highlight=thorsten+fatally+flawed&session=
Now the right IC's are OK?
Hello Elso,
Opamp’s open-loop input impedance is important if you could use it in open loop. AD8009 also has low inverting input impedance (8 Ohms, available only in SO package though), but the question is how to use it (and AD811, AD8001, OPA603) without the feedback. You only have their final (voltage) output available for connection. The only way to use them as I/V is to use them like classic opamp. Or I am wrong?
Again, that no_feedback approach can be meaningless if feedback doesn’t shorten bandwidth, and cfb doesn’t (… and if bandwidth is what we need here ).
Pedja
Opamp’s open-loop input impedance is important if you could use it in open loop. AD8009 also has low inverting input impedance (8 Ohms, available only in SO package though), but the question is how to use it (and AD811, AD8001, OPA603) without the feedback. You only have their final (voltage) output available for connection. The only way to use them as I/V is to use them like classic opamp. Or I am wrong?
Again, that no_feedback approach can be meaningless if feedback doesn’t shorten bandwidth, and cfb doesn’t (… and if bandwidth is what we need here
).Pedja
IV-Converter
Hello Pedja,
I only used the AD811, AD844, OPA603 and my own discrete opamp in the transresistance configuration. I.e with feedback applied. {Non-inverting input grounded and the current going to the inverting input. A feedback resistor is connected between the output and the inverting input. }
With the TDA1543 you can connect Vref to the non-inverting input of the opamp just as in the datasheet. You can even null the offset at the output by adjusting the resistor from Vref to ground but I found the sound better without the resistor.
I am not sure it will work without feedback. Maybe it works with the OPA660. Kuei Yang Wang, is your schematic a <B><I>working</B></I> circuit?
As said earlier I am not against feedback.
Hello Pedja,
I only used the AD811, AD844, OPA603 and my own discrete opamp in the transresistance configuration. I.e with feedback applied. {Non-inverting input grounded and the current going to the inverting input. A feedback resistor is connected between the output and the inverting input. }
With the TDA1543 you can connect Vref to the non-inverting input of the opamp just as in the datasheet. You can even null the offset at the output by adjusting the resistor from Vref to ground but I found the sound better without the resistor.
I am not sure it will work without feedback. Maybe it works with the OPA660. Kuei Yang Wang, is your schematic a <B><I>working</B></I> circuit?
As said earlier I am not against feedback.
Hi Elso,
I had been using opamps for I/V like most others, and much preferred the LM6172 as least evil above others like OPA2604, AD811, LM6181, AD797, OPA627, AD828, AD826, AD825, etc. etc.
About 1.5 years ago I made a real big step forwards using circuit shown; applied it to TDA1541 and TDA1543 cd players.
Didn't use opamps as I/V anymore since then.
The common base circuit is another step forwards and to quote Droopy "I'm happy".
If the Thorsten proposed circuit SOUNDS better, I'll use it, it is minimalistic enough for my DIY activities.
So, if anybody could send me a few OPA660's, I'll try it, compare it to my current common base circuit and post the results.
In the mean time, I started this thread to let other DIY know they can get good sound from a very simple discrete circuit, using generic parts and sounding so much better than the opamp I/V circuit used in many cd players and dacs.
As always, I'll try to pick up as much hints and suggestions as possibe, to complete my quest to audio nirvana...
I had been using opamps for I/V like most others, and much preferred the LM6172 as least evil above others like OPA2604, AD811, LM6181, AD797, OPA627, AD828, AD826, AD825, etc. etc.
About 1.5 years ago I made a real big step forwards using circuit shown; applied it to TDA1541 and TDA1543 cd players.
Didn't use opamps as I/V anymore since then.
The common base circuit is another step forwards and to quote Droopy "I'm happy".
If the Thorsten proposed circuit SOUNDS better, I'll use it, it is minimalistic enough for my DIY activities.
So, if anybody could send me a few OPA660's, I'll try it, compare it to my current common base circuit and post the results.
In the mean time, I started this thread to let other DIY know they can get good sound from a very simple discrete circuit, using generic parts and sounding so much better than the opamp I/V circuit used in many cd players and dacs.
As always, I'll try to pick up as much hints and suggestions as possibe, to complete my quest to audio nirvana...
Attachments
Re: IV-Converter
Hi,
If Burr Brown writes accurate application notes - yes. You can simply check their app note for the common base circuit, it's pretty much electronics 101.
As said, I would not use this type of circuit myself for any number of reasons, however if anyone wants a zero feedback, fully complementary I/V converter using the OPA660 is the best bet, plus you get your own buffer on chip too, so it is a single chip soplution for a feedback free I/V conversion.
Yet I too have not much against feedback,if applied apropriatly and in apropriate circuits, again, it is not WHAT you do that matters, it is HOW you do that matters.
Sayonara
Hi,
If Burr Brown writes accurate application notes - yes. You can simply check their app note for the common base circuit, it's pretty much electronics 101.
As said, I would not use this type of circuit myself for any number of reasons, however if anyone wants a zero feedback, fully complementary I/V converter using the OPA660 is the best bet, plus you get your own buffer on chip too, so it is a single chip soplution for a feedback free I/V conversion.
Yet I too have not much against feedback,if applied apropriatly and in apropriate circuits, again, it is not WHAT you do that matters, it is HOW you do that matters.
Sayonara
Re: simple i/v
Hi,
This will not be so straightforward. First of all, the common mode Voltage of the TDA1543 will cause offset and the voltage compliance of the OPA660 ios too limited. It would require an unusual arrangement of Powersupplies to start with, namely +8.3V/-1.7V, then the base (pin 3) needs to be connected to Vref. This will ten give 3.3V DC offset on the output, requiring a coupling capacitor or some other form of DC nulling. Sounds like a lot of hassle..
Sayonara
Hi,
This will not be so straightforward. First of all, the common mode Voltage of the TDA1543 will cause offset and the voltage compliance of the OPA660 ios too limited. It would require an unusual arrangement of Powersupplies to start with, namely +8.3V/-1.7V, then the base (pin 3) needs to be connected to Vref. This will ten give 3.3V DC offset on the output, requiring a coupling capacitor or some other form of DC nulling. Sounds like a lot of hassle..
Sayonara
Fully agree. In fact there is no life without feedback. This holds for electronics as well. The debate about feedback in electronics is as old as there are electronic amplifying devices around. The large internal feedback of a triode was the reason for developing tetrodes and later on penthodes.
Concerning the IV stage with an op-amp: What counts is the slewing capability of the input stage without going into saturation and the open-loop corner frequency to handle the fast transients coming out of the DAC. Some BB app notes put a large cap directly at the output of the DAC to tame this. Unfortunately this impairs the noise gain of the amplifier.
Maybe a pair of J-Fets or even an ECC88 as the input stage of a discrete op-amp is better option for the input stage in an IV converter with “feedback”
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