I recently built one and did not like the high gain and placed it for sale....but then after awhile I reduced the gain to 5x with a simple resistor modification.
Then a friend built one and used Sparkos SS3601 op amps and loved it...so I tried popping in some Burson Vivids and Classics and was astonished at the change in the amp and how well it works now for me.
Update I replaced the two gain resistors from 1K to 100 ohms and replaced the 552's with 551's and the volume level set has improved and its not at the first part of the mechanical travel on the pot...and the gain is right on for low and high impedance cans I have HD600 and Focal Clears...
So you're at 100R/100R now then for R9 and R10? (schematic) That's substantially lower than needed, really, the OPA551 has 14 nV/√(Hz) worth of input noise. You could easily put in 1k/1k and no-one would be the wiser, and the poor resistors wouldn't be stressed out as much. Even 10k/10k would only incur a 1.5 dB noise penalty, though the effect of impedance imbalance on distortion may be another matter.
BTW, I am not sure why the amp is lacking any and all input RF suppression. I would have thought at least something like 470R - 100...220 pF at the input and 220...470R in series should be standard... it's not like these components would be breaking the bank or adding inordinate amounts of noise.
BTW, with your current values simulation says that even the OPA552 should theoretically be happy (supply bypassing and layout permitting). For 1k + 1k the parallel C would have to be reduced to 47p (47-68p). I do believe that I have achieved a decent match of the opamp model with OPA552 datasheet performance.
I dont have any real measuring gear here and dont run simulations etc...but with the Butte
and having its original design gain of 11x the volume never got out of the really low end of its mechanical travel...so i first just paralled another 1K with R5, R9 and the gain was lowered to around 6x, this still didnt help with the volume pot travel. So I sent Pete a note and he repsonded:
Actually, both work.
Initially I used 121 ohms, but decided it was pulling more current from the output than was needed. So I changed to 1k. That still keeps the output stage in class A for most applications.
So yes both gain resistors are now 100 ohms and the gain is 2x. I had a couple of 1/2 watt 100's in my parts bin. The volume pot with high impedance cans HD600, T1 is now 10 oclock to 12...out of the low end etc..
I am not hearing any real difference in the actual output sound that I can detect with my ears. But Pete mentions drawing more current from the output..so this may impact this somewhat with high current demanding headphones?
I did read the datasheet from TI, and I noticed that the 552 can be used with lower gains than 5x, but there are other complications to consider if you do this, ie addition caps etc..