Ultimate preamp IC ?

[youyoung21147]

Now I'm convinced by power chip amps, I'm willing to add a buffer to my bridged LM4780 amp.

The bridged mode has a quite low input impedance, <1kohm on one of my amps, and I feel like my soundcard doesn't give its best through such a load.(it is a **** laptop soundcard, but I will replace it with a TransitUSB)

Anyway, I was wondering : what would be the best ICs (semiconductors, not valves) to drive my LM4780 ?

I have found the Analog Devices AD797 to be an excellent choice :
- 0,9nV/Hz input noise
- 25µV offset, allowing a output-capacitor-less design
- -120dB distorsion through 1kohm@1kHz

But I didn't see any well documented DIY preamp using it, so I'm wondering it there was anything better to drive chip amps.

Is there anything better than it ? I know carlosfm likes the AD815, and a french shop sells a preamp based on the AD825.
There are also opamps from Burr-Brown like the OPA2604, which seems to be liked very much also.


Any comment ?

[owen]

Try the LF353 in a unity gain buffer. The specs are nearly identical to the LM3875 chip (except power dissipation).

I like then as they sound very 'natural, detailed but un-hifi ... and cost next to nothing!

Use one chip perchannel - just dont connect the remaining opamp to the circuit (the LF353 is a dual opamp) - or at a push use a single chip for both channels, but imaging suffers a little..

Owen

[youyoung21147]

Do you think this cheap thing can be better than costly High-end opamps ? (Burr brown, AD...)

[jackinnj]

I guess I am in a little different camp -- I have used Analog Devices and Linear Tech -- the AudioXpress mod of the Adcom GFP565 used ADI devices and sounds wonderful. I have used BB(TI), but just have more stuff from the former 2 vendors.

It may be deemed "heretical" but I think that in the realm of high perf audio opamps the world flattens out and 1) the quality of the supply rails and 2) good layout dominate performance.

the AD797 is a great chip for low noise applications, but it can be a bit tricky to work with -- and it is quite expensive. You can get really great performance from a less expensive chip if you use 2sk170/2sj74 as a pre-pre.

[youyoung21147]

You mean I could achieve greater performance with a discrete design ?

Nuuk seems to have a big offset voltage which I'm quite afraid of. I'd like a capacitor-less signal path if possible.

Have to study PSU bypassing. I will surely build a double regulated PSU.

Got some NE5534 arround to play with, but no regs and no support to hold the opamp. They are said to be unstable at unity gain.

[pinkmouse]

I like AD811s. Current feedback, so you have to design around them, but they are fast, quiet, and can drive a lot of current if you need to, so are ideal for driving long interconnects or difficult loads.

[mlloyd1]

do you need gain or simply need impedance conversion?

dual jfet buffers (i.e. discrete) work quite well and can be made with excellent offset performance fairly easily.

mlloyd1

Originally posted by youyoung21147
You mean I could achieve greater performance with a discrete design ?

Nuuk seems to have a big offset voltage which I'm quite afraid of. I'd like a capacitor-less signal path if possible.

Have to study PSU bypassing. I will surely build a double regulated PSU.

Got some NE5534 arround to play with, but no regs and no support to hold the opamp. They are said to be unstable at unity gain.

[youyoung21147]

I basically need impedance conversion, but gain could be welcome with some sources, such as walkmans...

To take advantage of the preamp, I assume gain will help covering the IC's noise while keeping the amplifier's gain low and avoid DC offset issues.

But maybe adding a resistor to ground will reduce the sound quality ?

[peranders]

Originally posted by youyoung21147
I have found the Analog Devices AD797 to be an excellent choice :
- 0,9nV/Hz input noise
- 25µV offset, allowing a output-capacitor-less design
- -120dB distorsion through 1kohm@1kHz

I must remind that you can't achieve so good performance as some parameters suggest. You must know how your signal source is in order to choose the right opamp.

Noise is in most cases uninteresinting. Most modern opamps won't add any noise if we talk line level signals.

My Gainclones use AD8620 or OPA2134 but there are many many more to choose from. I recommend an opamp with JFET inputs.

NE5534, much hifi for the price
OPA134/2134
OPA627 (a bit expensive)
AD8610/8260 (only SMD)
AD825 (only SMD)
AD745 (only SMD)
many many more.

[youyoung21147]

Noise is in most cases uninteresinting. Most modern opamps won't add any noise if we talk line level signals.

What noise treshold can be considered "correct" and not causing "hiss" on the speakers ?

AD745 (only SMD)

I thought of this one too, it is more adapted to 1->10k input impedance sources, allowing a pot before the opamp.

What where your criterias for choosing your buffer opamps ?