Hi everybody
(mistake in the title, DAC current...)
Just finish to build my DAC pcm1796 and now I want to decide about the output stage.
This DAC is Current Output
I have the jc-2 preamp and I wonder how to connect this pre to the DAC?
now I this only to take the output from the PCM (- R) and Through capacitor(its contain DC) to the jc2 preamp.
Any other suggestions?
thare is problem with the 1M input resistor?
Not need to modify the preamp? Can I avoid the cap?
thanks
(mistake in the title, DAC current...)
Just finish to build my DAC pcm1796 and now I want to decide about the output stage.
This DAC is Current Output
I have the jc-2 preamp and I wonder how to connect this pre to the DAC?
now I this only to take the output from the PCM (- R) and Through capacitor(its contain DC) to the jc2 preamp.
Any other suggestions?
thare is problem with the 1M input resistor?
Not need to modify the preamp? Can I avoid the cap?
thanks
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Last edited:
Hi,
me thinks the title is right on target, but not Your thinking. 😉
You think in voltage terms, but You need to think in current terms.
That means, You can't use a cap to DC-block the PCMs centre current.
The PCMs source a 'idle current' of iirc -3.5mA, which You need to take care of in the design of the iv-converter.
In general two methods apply:
- the use of a dedicated current sink in addition to the iv-stage, so that only the AC-signal current enters the iv-stage. This is the case here, where the iv shall be a complementary design.
or
- use the DACs idle current as biasing current for the iv at the same. This is easily accomplished with singleended designs like the Jocko and even simpler with OPCs one-MOSFET-only iv.
With the JC2 the DACs current output needed to be connected to a current sink (3.5mA) and the -In input, while the +In input is to be shorted to signal gnd.
Also the 4.7M feedback resistor needed to be reduced to a much lower value.
As the PCM swings 2mApp every 1kOhm generates 1V per 1mA.
Beeing peak values the rms value of the voltage is lower by 1/sqr2, hence 0.707Vrms/1mAp/1kOhm
1.41kOhm would then generate 1Vrms, 2.82kOhm would generate 2Vrms, and so on.
The JC2 is connected like a operational amplifier and as such does altering of the feedback resistor mean that You have to watch if it remains stable.
The small feedback cap will very probabely require trimming also.
As DC current sink a single degenerated N-channel JFET connected between DAC output and the negative supply line will suffice.
Since there is no dc- blocking cap at the output of the JC2 You get a output offset voltage if the current sink's current does not precisely match (or nulls-out) the DACs centre current.
To be honest, even though I like the JC2s simple design, I'd personally prefer to use a singleended stage here, that elegantly uses the DACs centre current to bias up itself.
jauu
Calvin
me thinks the title is right on target, but not Your thinking. 😉
You think in voltage terms, but You need to think in current terms.
That means, You can't use a cap to DC-block the PCMs centre current.
The PCMs source a 'idle current' of iirc -3.5mA, which You need to take care of in the design of the iv-converter.
In general two methods apply:
- the use of a dedicated current sink in addition to the iv-stage, so that only the AC-signal current enters the iv-stage. This is the case here, where the iv shall be a complementary design.
or
- use the DACs idle current as biasing current for the iv at the same. This is easily accomplished with singleended designs like the Jocko and even simpler with OPCs one-MOSFET-only iv.
With the JC2 the DACs current output needed to be connected to a current sink (3.5mA) and the -In input, while the +In input is to be shorted to signal gnd.
Also the 4.7M feedback resistor needed to be reduced to a much lower value.
As the PCM swings 2mApp every 1kOhm generates 1V per 1mA.
Beeing peak values the rms value of the voltage is lower by 1/sqr2, hence 0.707Vrms/1mAp/1kOhm
1.41kOhm would then generate 1Vrms, 2.82kOhm would generate 2Vrms, and so on.
The JC2 is connected like a operational amplifier and as such does altering of the feedback resistor mean that You have to watch if it remains stable.
The small feedback cap will very probabely require trimming also.
As DC current sink a single degenerated N-channel JFET connected between DAC output and the negative supply line will suffice.
Since there is no dc- blocking cap at the output of the JC2 You get a output offset voltage if the current sink's current does not precisely match (or nulls-out) the DACs centre current.
To be honest, even though I like the JC2s simple design, I'd personally prefer to use a singleended stage here, that elegantly uses the DACs centre current to bias up itself.
jauu
Calvin
Last edited:
Hi,
the schematics are small and of low quality.
I can hardly figure out what´s what. 🙄
Even so I ca say that "than plug the DAC to +in (before the 100R) and -IN" won´t work at all.
You still view the circuit in voltage terms not in current terms.
Think and analyse what happens to the currents.
How does the DAC current look like, where does it enter the circuit and where does it leave the circuit ... resp. search for the current loops.
jau
Calvin
ps. just spotted that the Pin of the 4k53, needs to be conneted directly to -In, instead of including the 2µF in the loop.
Connected as drawn in the sketch it´d be a verylow-NF amplifier with incredibly high gain .... a DC-Offset generator 😉
Compare to the connections in the phono section.
the schematics are small and of low quality.
I can hardly figure out what´s what. 🙄
Even so I ca say that "than plug the DAC to +in (before the 100R) and -IN" won´t work at all.
You still view the circuit in voltage terms not in current terms.
Think and analyse what happens to the currents.
How does the DAC current look like, where does it enter the circuit and where does it leave the circuit ... resp. search for the current loops.
jau
Calvin
ps. just spotted that the Pin of the 4k53, needs to be conneted directly to -In, instead of including the 2µF in the loop.
Connected as drawn in the sketch it´d be a verylow-NF amplifier with incredibly high gain .... a DC-Offset generator 😉
Compare to the connections in the phono section.
thanks calving
agree with you, i stack with the voltage terms like school 🙂
what about passive i/v? R to ground and cap > to the preamp?
second, if the JC-2 act like OP AMP, why not use it for i/v conversion like in the pic?
(forget about the diffrantial, i only use +iOUT leg)
about the Jocko i/v, look very nice, but i already buy and build the JC-2 and another TUBE pre amp 🙁, so i prefer to solve the problem with them, if its possible.
thanks 🙂
agree with you, i stack with the voltage terms like school 🙂
what about passive i/v? R to ground and cap > to the preamp?
second, if the JC-2 act like OP AMP, why not use it for i/v conversion like in the pic?
(forget about the diffrantial, i only use +iOUT leg)
about the Jocko i/v, look very nice, but i already buy and build the JC-2 and another TUBE pre amp 🙁, so i prefer to solve the problem with them, if its possible.
thanks 🙂
Attachments
Last edited:
Hi,
yes you can do that .... as described in#2.
Either connect the new feedback network in parallel to the 4M7/20pF, or replace that network alltogether.
Ground +In and add the required current sink to -In.
jauu
Calvin
yes you can do that .... as described in#2.
Either connect the new feedback network in parallel to the 4M7/20pF, or replace that network alltogether.
Ground +In and add the required current sink to -In.
jauu
Calvin
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