The EBU SQAM disc is a test disc, created to include clean recordings of lots of instruments. It's not a normal music CD, but it does manage to contain a variety of hard to encode content. Lots of easy stuff to though.
http://sound.media.mit.edu/mpeg4/audio/sqam/Apart from that, why not PM Guruboolez (a member here) to ask which solo harpsichord CD he would recommend for this purpose? You could look through the listening tests section to see what he has reported in the past.
Going back even further, the founder of this forum, Dibrom, worked hard to try to make mp3 encode his own choice of electronic music. He concluded that mp3 wasn't really up to the job, so some of his favourite albums might be good choices. You'll have to search back a long way in HA to find anything about this.
Myself, I don't know of any hard to encode
discs - for example, I have the original CD with fatboy.wav on, and that's just the intro to one track on the CD - the rest of the track is easier to encode, as is the rest of the disc.
Lame really has become very good; a lot of the samples that used to trip it up are less of a problem now. Some of the weak points in lame are not a problem for FhG, so you cannot say that mp3 itself is at fault (other than sfb21, harpsichord(!) and may be temporal smearing, which all require high bitrates to solve/improve).
Your English is perfectly understandable.
Cheers,
David.