I just rolled a pair of 825's on a browndog in today. Initial impressions are very positive. This amp really responds to different op-amps. I have not listened to it enough to form a real opinion but im quite impressed thus far. I have not tried the 797's yet, I just got finished soldering them up though.
The OP275 was a popular op-amp in the 90's. But I think it's long been surpassed and there stands alot to be gained by swapping it out with something a little nicer.
Another nice benefit to these SOIC on browndog op-amps is they are very small and can easily fit under the lid without having to use washers to raise the top. They are actually smaller then a single dip-8 opamp.
edit: ok spent some more time listening to 825, still have not tried 797. This is easily the best op-amp i've tried in this amp yet. Clearly surpassing the PA2227, LME49860, OP275 and AD743JN. If you leave this op-amp stock your really missing out on what the EF5 can deliver. Im listening with the RCA Cleartop too btw. Different op-amps do synergize differently with various tubes so this comes into play as well.
Edit: been listening to the AD797's for awhile. They are very neutral sounding and clear, I think they are a step over the 825 as well. And that is saying alot, as the 825 was easily better then the above mentioned opamps. There's simply nothing to not like about the 797, it seems to do everything very well. When first putting it in, coming from the 825 it really shows how not-neutral the 825 is. That said, the 825 is still great, and very fun/musical sounding. It's a real toe tapper, if not the last word in neutrality. For those of you still using the stock op275 it's really time to roll that badboy, your missing out.
The 825's are available at Digikey for 4.23 each, the 797's are 9.45 a piece. You need 2 of these as they are single channel. You'll also need browndog 2xSOIC8 to DIP8 adapters, which are 2.39 each, with a 5 min order.
