Just picked up an older Denon DCD-2560 cd player , which uses 4(2 per ch) current output AD1862 dac's. Since there is not very much info and/or experience on this forum with passive I/V components(resistor values and such) with regard to these dacs, I would like to start with the op-amps. I have had extremely good experience with the dual OPA2107 as a buffer , but I'm unsure if it is up to the challange for I/V , as it has a relatively low slew rate, if that is any indication of usefullness here. Any suggestions are , as always , much appreciated.
Hi,
I'm using OPA2107 as an IV converter for my TDA1541A NOS DAC, and I think it sounds great (although I admit I haven't tried much else).
Why is slew rate more important for IV conversion than for buffering? I would have thought the criteria would be similar for both situations.
Have you heard the difference?
Cheers,
Phil
I'm using OPA2107 as an IV converter for my TDA1541A NOS DAC, and I think it sounds great (although I admit I haven't tried much else).
Why is slew rate more important for IV conversion than for buffering? I would have thought the criteria would be similar for both situations.
Have you heard the difference?
Cheers,
Phil
I-V conversion would require a high slew rate for accuracy - the DAC puts out step current change for each new word it receives; a low slew op amp would take some time to convert this, causing HF innaccuracies. Aside from slew rate, one should (I think) also be looking at settling time, noise, and current drive capability of the op-amp, given that the FB resistor used to set the I to V relationship tends to be pretty low value.
After LPF, the signals are into the audio band, thus a buffer need not be exceptionally fast. Low distortion is needed here though...
Cheers
After LPF, the signals are into the audio band, thus a buffer need not be exceptionally fast. Low distortion is needed here though...
Cheers
Hi georgehifi,
I'm not looking at the parameters of the AD825 at the moment, but I recall it was a pretty fast and good chip - well respected by many in the audio community. I have a very good reason not to bother looking at almost any AD product - it's not available here at decent prices... :-(
One thing I have tried is the THS4032 (texas instruments) - it's actually designed for video frequency I-V. We put this into a Pioneer CD/SACD/DVD-A player and the sound change simply blew our minds...
Cheers!
I'm not looking at the parameters of the AD825 at the moment, but I recall it was a pretty fast and good chip - well respected by many in the audio community. I have a very good reason not to bother looking at almost any AD product - it's not available here at decent prices... :-(
One thing I have tried is the THS4032 (texas instruments) - it's actually designed for video frequency I-V. We put this into a Pioneer CD/SACD/DVD-A player and the sound change simply blew our minds...
Cheers!
Interesting.
I will have to investigate this.
I was of the assumption that 4.5MHz GBP would suffice for IV. The OPA2107 has a slew rate of 18V/uS. The samle period (for NOS) is 1/44100 = 22uS. So initially it would seem it could easily handle a pk-pk voltage change within one sample period for NOS at least (unless, which is very likely, I have the wrong end of the stick).
I guess for an oversampling setup, requirements could be different, and generally, I can see how a faster settling time could well be of benefit.
I will have to investigate this.
I was of the assumption that 4.5MHz GBP would suffice for IV. The OPA2107 has a slew rate of 18V/uS. The samle period (for NOS) is 1/44100 = 22uS. So initially it would seem it could easily handle a pk-pk voltage change within one sample period for NOS at least (unless, which is very likely, I have the wrong end of the stick).
I guess for an oversampling setup, requirements could be different, and generally, I can see how a faster settling time could well be of benefit.
I think the parameter Walter Jung referred to was Vth given
by the Slew Rate divided by the Gain Bandwidth
philpoole if this use of a TDA 1541 is using a inverting opamp
followed by a non inverting ie- same device if a dual , check your
schematic and apply a 22k resistor globally around both ie:
from inverting input (also the Analog Dac output pin ), then of opamp 1
to output of non inverting opamp2. Note feedback arising from
output of opamp 2 then applies to correct two devices and not
just one device. Simply amazing when applied to a Marantz
CD50 using 2x opa627 per channel , close to if not the best
sound I have heard. Simple and effective
by the Slew Rate divided by the Gain Bandwidth
philpoole if this use of a TDA 1541 is using a inverting opamp
followed by a non inverting ie- same device if a dual , check your
schematic and apply a 22k resistor globally around both ie:
from inverting input (also the Analog Dac output pin ), then of opamp 1
to output of non inverting opamp2. Note feedback arising from
output of opamp 2 then applies to correct two devices and not
just one device. Simply amazing when applied to a Marantz
CD50 using 2x opa627 per channel , close to if not the best
sound I have heard. Simple and effective
Hi Chris,
Currently (if you forgive the pun), its only a single opamp per channel (its my first revision of this DAC, I'm still in the early stages of playing), but I can see how the global feedback for two opamps can be beneficial.
It will definitely go on my todo list (there's lots to do, and its great fun).
At the moment, I'm busy making a balanced I2S driver receiver, so I can connect the DAC to my CDP more sensibly.
Thanks for the tip!
Cheers,
Phil
Currently (if you forgive the pun), its only a single opamp per channel (its my first revision of this DAC, I'm still in the early stages of playing), but I can see how the global feedback for two opamps can be beneficial.
It will definitely go on my todo list (there's lots to do, and its great fun).
At the moment, I'm busy making a balanced I2S driver receiver, so I can connect the DAC to my CDP more sensibly.
Thanks for the tip!
Cheers,
Phil
hi
an other vote for you
opa2604 is one of the best for I/V and LPF after DAC.
thanks . afshin
Hi,
the AD1862 has a settling time of 350ns for +-1mA output current.
To accurately follow the possible steep pulses it required an extremly fast OPAmp.
Without hitting the brake pedal -using a cap to slow down the signal- the typical 'Audio'-OPAmps would be too slow.
The inputs could be overdriven and the OPAmp could run into openloop condition, with the associated increase in THD.
It's on the other hand easy to build discrete 'current buffers' that provide for the low input impedance the DAC requires and that feed into a I/V resistor.
These very simple grounded base or grounded gate circuits achieve very low THD figures without relying on global feedback and as such they are extremely fast.
Besides, music sounds more like music with these and not as HiFi as with OPAmps.
Look for example for EUVL's Zen IV and Sen IV.
It makes use of JFETs (very simple design) but the same basic circuitry can also be designed with bipolars.
jauu
Calvin
the AD1862 has a settling time of 350ns for +-1mA output current.
To accurately follow the possible steep pulses it required an extremly fast OPAmp.
Without hitting the brake pedal -using a cap to slow down the signal- the typical 'Audio'-OPAmps would be too slow.
The inputs could be overdriven and the OPAmp could run into openloop condition, with the associated increase in THD.
It's on the other hand easy to build discrete 'current buffers' that provide for the low input impedance the DAC requires and that feed into a I/V resistor.
These very simple grounded base or grounded gate circuits achieve very low THD figures without relying on global feedback and as such they are extremely fast.
Besides, music sounds more like music with these and not as HiFi as with OPAmps.
Look for example for EUVL's Zen IV and Sen IV.
It makes use of JFETs (very simple design) but the same basic circuitry can also be designed with bipolars.
jauu
Calvin
Hi,
the AD1862 has a settling time of 350ns for +-1mA output current.
To accurately follow the possible steep pulses it required an extremly fast OPAmp.
Without hitting the brake pedal -using a cap to slow down the signal- the typical 'Audio'-OPAmps would be too slow.
The inputs could be overdriven and the OPAmp could run into openloop condition, with the associated increase in THD.
It's on the other hand easy to build discrete 'current buffers' that provide for the low input impedance the DAC requires and that feed into a I/V resistor.
These very simple grounded base or grounded gate circuits achieve very low THD figures without relying on global feedback and as such they are extremely fast.
Besides, music sounds more like music with these and not as HiFi as with OPAmps.
Look for example for EUVL's Zen IV and Sen IV.
It makes use of JFETs (very simple design) but the same basic circuitry can also be designed with bipolars.
jauu
Calvin
Calvin,
What about the THS4032? It's fast an low noise.
I wonder why it's not more popular. People that tried it report notable improvements in audio quality all over.
Wrong implementation, perhaps? It does need good bypassing, particularly if using DIP adapters, and parts around it should be close to the pins.
1uF COG cap bypass right on the back of the DIP adapter might stabilize it, as well as more COG bypass to ground on the PCB, right by the pins.
The latter could improve things on most ICs, and should be an easy upgrade on existing boards. Those caps are not too small, and should be easy to manipulate.
Using single to dual, SOIC to DIP adapters might allow using single versions, like THS4031 or LME49990.
Carlos
I have played around with i/v opamps in my Adcom GDA-600 which uses PCM63 dac chips.
my favourites are
OPA1642 dual fet
LM6172 dual bipolar
LM6171 as I/V single bipolar followed by OPA1641 single fet for filter/buffer stage on a brown dog adaptor
They are relatively cheap and in this application stable.
Remember to check the new op amps to see if they are oscillating see Mooly's excellent thread
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
my favourites are
OPA1642 dual fet
LM6172 dual bipolar
LM6171 as I/V single bipolar followed by OPA1641 single fet for filter/buffer stage on a brown dog adaptor
They are relatively cheap and in this application stable.
Remember to check the new op amps to see if they are oscillating see Mooly's excellent thread
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
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