Are you certain you're not using a current feedback amplifier? The feedback resistor sets the frequency response and must always be present and the correct value even in a simple voltage follower. I haven't used CFB amps for audio but they're excellent for high def video.
G²
I use the LME49713 for audio and it sounds excellent. As output OP in a linestage i like it better then conventioanal voltage feeback OPs. Yes it needs always a feedback resistor. a bonus is the high oupt current of 100mA so you can drive a low impedance headphone quite well.
You can find a lot of information about why a current feedback OP needs that resistor in the spec sheet and elsewhere on the Natioal site.
My sugestion to put in a resistor in a voltgae folower is to avoid common mode distortion. The OPA134 works without that resistor but produces high frequency distortion used that way. In Douglas Selfs book about Small Signal electronics you can find an excellent analysys of the situation. A voltage follower is actually the worst case for common mode distortion but the huge amount of feedback helps to a certain degree. Some Fet Opamps like the OPA627, OPA827 and ADA4627 do not have that problem so pronounced because the input Fets are cascoded thus the miller capaciatnce is not driven into unlinearity. The OPA1641 supresses common mode distortion in a diffent way. The input Fets just have a current mirror and are not cascoded but the transistors in the mirror are from a new SiGe process that makes them suffer lesss from miller effect.
You can find a lot of information about why a current feedback OP needs that resistor in the spec sheet and elsewhere on the Natioal site.
My sugestion to put in a resistor in a voltgae folower is to avoid common mode distortion. The OPA134 works without that resistor but produces high frequency distortion used that way. In Douglas Selfs book about Small Signal electronics you can find an excellent analysys of the situation. A voltage follower is actually the worst case for common mode distortion but the huge amount of feedback helps to a certain degree. Some Fet Opamps like the OPA627, OPA827 and ADA4627 do not have that problem so pronounced because the input Fets are cascoded thus the miller capaciatnce is not driven into unlinearity. The OPA1641 supresses common mode distortion in a diffent way. The input Fets just have a current mirror and are not cascoded but the transistors in the mirror are from a new SiGe process that makes them suffer lesss from miller effect.
I am certain (absolutely sure) that I am referring to VFB opamps.
I don't have and have never used nor experimented with any CFB opamps.
I just finished a headphone amp w/servo based on that CF op-amp. I will post a pic of it when I get home from work.
What is the best way BW limit this IC in a non-inverting configuration where the +input is directly coupled to a 10k pot?
What type of op-amp is best suited for servo duty (I used a THS4032; too fast of an op-amp maybe?)?
The rail bootstrap technique shown in the book is a very attractive alternative, in particular for variable source resistance. I will apply it in my pre-amp.
Normal choice would be a FET input device, the TL071 is fine. You can use the offset null to reduce offest down to 0.00 if needed. It's more important to get the "references", ground and so on correct that the integrator works too.
Not a great idea to have the +ve input directly connected to a pot, from the pov of RF pick up. Use an RC filter (-3dB @ 150-200kHz) but ensure the cap goes to a clean ground from the star point , not one on which the input cable ground is also local to.
One that's both low noise and happy working with high impedances - this usually means a JFET input stage. CMOS could work but they tend to be much noisier at lower frequencies. I think the TI OPA2141 is an excellent choice owing to very low 1/f noise as JFETs go (higher frequency noise is less of an issue as a servo is acting as an integrator) and reasonable price. OPA827 and OPA627 are way overkill
Well, all grounds share a common ground plane, so I can't separate them. I think I will swap the THS4132 out for either an OPA2132 or OPA2211 though.
For the RC filter, wouldn't varying the pot change the Fc?
I've attached pics of the board. It has ~80mV offset on one channel for some reason; the other channel is about ~2mV.
Attachments
Yet you're using the (expensive) metal can versions of the NS CFB opamps -that to me suggests you really want to get the best sound. Although I've not used these particular parts, my experience with various other amp chips is that attention to grounding is important for ultimate sound quality and that ground planes don't cut it, in general.
Yeah, good point - didn't think of that first time around.
Just have to estimate the C value given the minimum impedance of the pot (2k5 for 10k, assuming zero source impedance). That sets the lowest corner frequency of the filter, all other pot positions will give higher. May be wise to add some series resistance (perhaps 500R or so) so even at full volume you're still getting a bit of filtering.I'd be inclined to suspect low-level oscillation with that amount of offset.
Yeah, proper grounding eludes me so I just throw a ground plane at everything. I call it the "I have no clue what I'm doing" approach.
I'll try a RC filter off 500R/220pF and change the servo op-amp to OPA2211 and report back.
BTW, trying to stay a little on topic here, the LME49713 does sound excellent.
Success!
I decided to keep with the NI theme and put in a LME49860 for the servo and I installed the input RC filters.
Output offset is now ~9mV for both channels and the board sounds amazing. I can already tell I like it better than the LME49710/LME49600 amp I built.
And about the previous mention of my not-so-great grounding method; the board is as silent as silent gets. No hum, hiss, noise whatsoever.
Thanks for the help fellas.
Bias to class A?
hi Andrea, please explain how to bias to class A, i don't quite understand this - TIA!
hi Andrea, please explain how to bias to class A, i don't quite understand this - TIA!
Analog Device ADA4627BRZ
I've tried this relatively new opamp from Analog Device
I got the samples from them and I had to make tiny pcboard to make them fit in my DAC since the opamps are mono
my impression it is better than the plastic LM4562NA , more air, smooth and rich mid, wider sound and more open, I haven't compare it to the can LM4652HA, but right now I have this opamps in my MSB link III and LM4652HA in my highly modified Musical Fidelity A324
Anyone ever tried this opamps? I interest on your opinion
thanks
I've tried this relatively new opamp from Analog Device
I got the samples from them and I had to make tiny pcboard to make them fit in my DAC since the opamps are mono
my impression it is better than the plastic LM4562NA , more air, smooth and rich mid, wider sound and more open, I haven't compare it to the can LM4652HA, but right now I have this opamps in my MSB link III and LM4652HA in my highly modified Musical Fidelity A324
Anyone ever tried this opamps? I interest on your opinion
thanks
Buongiorno Luigi,
Generally it's not good design practice to have a DAC directly drive cables of any length, or another separate stage- it will perform poorly, even if it's a voltage type output. Either select a very good sounding op-amp as a driver/follower, that will have low noise, wide bandwidth, high slew rate, low output Z, and low input current, or build a discrete design with FETs or bipolars- more difficult. The BB family of op-amps like OP27 and similar are really nice sounding, quietand you can run the supplies up to (+/-)18 or 20 volts, which gives lots of headroom. What is the preamp and what is the first device the signal hits when arriving at the pre-amp input? Input impedance? Nominal line level expected is -10dBv or +4dBM? ( home & garden vs professional audio levels) Some of the big recording consoles use video op-amps as buss drivers because of these qualities plus the ability to drive low Z such as 50 Ohms. Please send the data sheet for the DAC- that would help for further advice.
Ciao,
Gregg
Generally it's not good design practice to have a DAC directly drive cables of any length, or another separate stage- it will perform poorly, even if it's a voltage type output. Either select a very good sounding op-amp as a driver/follower, that will have low noise, wide bandwidth, high slew rate, low output Z, and low input current, or build a discrete design with FETs or bipolars- more difficult. The BB family of op-amps like OP27 and similar are really nice sounding, quietand you can run the supplies up to (+/-)18 or 20 volts, which gives lots of headroom. What is the preamp and what is the first device the signal hits when arriving at the pre-amp input? Input impedance? Nominal line level expected is -10dBv or +4dBM? ( home & garden vs professional audio levels) Some of the big recording consoles use video op-amps as buss drivers because of these qualities plus the ability to drive low Z such as 50 Ohms. Please send the data sheet for the DAC- that would help for further advice.
Ciao,
Gregg
re-pcm56
thanks for your reply .
i give you more information :
i have yamaha cdx 10000 with pcm56p
but i would like to direcly the dac to input transformer of my preamplifier
this is data sheet of pcm56p
and my preamplifier is push pull transformer circuit
imput transformer is 30k-30K
i have connected pin 9 of dac to +plugs cable
and 2 pin to - plugs cable
the volume gain is good
but the sound goes up and down as if there was anyone that lowers and raises the volume
thanks for your reply .
i give you more information :
i have yamaha cdx 10000 with pcm56p
but i would like to direcly the dac to input transformer of my preamplifier
this is data sheet of pcm56p
and my preamplifier is push pull transformer circuit
imput transformer is 30k-30K
i have connected pin 9 of dac to +plugs cable
and 2 pin to - plugs cable
the volume gain is good
but the sound goes up and down as if there was anyone that lowers and raises the volume