Class A biasing of opamps

[DrewP]

I'm planning to use the NE5534 (or replacement) in an MC phono stage hence the desire for low noise.

Eventually, I was planning (funds premitting) that it'll be upgraded to an AD797 or similar with mods where applicable to the circuitry,

As was said previously "pin compatible (sort of)"

At this point it's more in the prototype "will it work" kind of stage and hence I'm not planning of filling it with ultimate level chips and caps just yet.

The very first cheap metalfilm resistors and MKT polyester caps version actually sounds quite well, I was just wondering if the Class-A mod might also improve things.

It's a work in progress

drew

 

[cocolino]

read over the datasheets, but I couldn't find the difference in the two (627 and 637). Anyone know?

Hello Brian:
difference between 627 and 637:
627: unity gain stable
637: stable in gain =>5
Hope this helps

 

[mlloyd1]

Brian:
OPA627 has an internal compensation capacitor for unity gain stability.
Given 20 each? You have NICE friends These aren't cheap, thanks to the DI mfrg process.
Michael

I have a bunch of opa627 and opa637 that a friend gave me (20 of each). ..... I read over the datasheets, but I couldn't find the difference in the two (627 and 637). Anyone know?
Thanks,
Brian

 

[peranders]

I'm planning to use the NE5534 (or replacement) in an MC phono stage hence the desire for low noise.

If you plan to connect a "normal" MC pickup the NE5534 is way too noisy (OK a little bit too noisy). You should look for noise below 1 nV/sqr Hz. You can also use a preamp with a 2SK170 JFET. Check the Pearl phono amp.

 

[jean-paul]

Brian,

OPA627 and OPA637 are probably the best you can get at the moment. 2134 you can't compare to these fantastic opamps.


OPA637 is faster and has a higher slewrate but is, as mentioned before, not unity gain stable.

 

[PhopsonNY]

Try the ad 8610 of ad8620

I am sure not all will agree, but I have had excellent success with the new AD opamps as replacements in my SCD player

I/V stage;
Replaced opa2604 with 2*ad8610s with broundog adaptor
(gave smoother mids, less grainy highs and deeper bass)

Mixamp stage (converting balanced signal to unbalanced)
Replaced opa2123 with ad8620
(got rid of upper bass hump and a more linear response)

Buffer/Servo stage (one half of dual opamp did both)
Replaced ad712 with ad8620
(gave a more dynamic response and greater transparency)


I then applied current loading (Biasing into class A) in succession with;
1. 1kOhm r - greater bass articulation
2. 1kohm r with CRD (1mA) - as above, but with smoother mids
3. 100ohm r with CRD/FET (1mA) - as #2, but slightly better
4. 1kohmr with cascoded JFET (1mA then 2mA) - much better than any other option - took a while to break in, but was very linear and articulate with very clean highs... any residual graininess totally eliminated.

I have not done a head to head with the opa627, but others at Head-fi and Headwize have carried out extensive comparisons and the majority consider the 8610/8620 to be much better.
Check out ppl's posts

One thing that comes out of their comparisons is that this is only worth it if the rest of your system is of high enough resolution to distinguish the difference.

Good luck with whatever you choose.