ST70 or ST120 kit for driving Proac 2.5 clone speakers

Hi

I am inclined to build an ST70,but I am a little bit concerned about the output required to properly drive a pair of Proac 2.5 clones.
These are kind of low efficiency ,probably lower than 86db, but relativelly very easy load (8ohms). Would it be enough a St70 in a medium size room and non concert hall levels? or should I look more into KT88 version, ST120?

Also, if anybody can answer me about the topology of the VTA board for St70. I understand that the first stage is not directly coupled to the LTP like some other designs like the KT 88 Bill (http://www.triodedick.com) and it doesn't use a CCS source for the LTP like DIYTube design for ST70 (www.diytube.com).
Is there any advantages in case of the VTA board compared with the other two similar designs? (Bill is more similar with St120).


Regards
Dragos

Hi Drago,
the power needed to drive speakers properly is very subjective, especially when using tube amps.
I have had power amplifiers of all power sizes, from 2 watts to 400 watts output, connected mostly to either normally efficient (~89-91 db) to low efficiency (~85-88db), and have never really had a problem, it depends on the type of music, how loud you like to listen, etc.
My former neighbor had ProAc 2.5s, and he first ran them with Audio Research 150 wpc tube amps. I loaned him my VTA70 for a week and he was wowed that my $500 amp sounded as good as his $3000 amp. Later I loaned him my DIY ST35 and he loved it so much he wanted to buy it from me.
He and I both listen mostly to jazz, at a medium volume.
The ST35 isn't quite enough for higher volumes. The VTA70 was fine except at ear splitting volumes.
I've also run 2w tube amps with jazz at normal volumes, but my VTA70 does make a significant difference in sound quality and power reserves. Back in the early 80s I ran 200wpc SS into JBL studio monitors, which was OK, but I can tell you 100w of tube sounded better. 350wpc into JBL monitors was astonishing, even more so because I couldn't blow the JBLs up, even though I could hear the drivers exceeding their cone travel limits in each direction (can you imagine the Doubie Brothers at FULL volume with the VU meters peaking at +3db above 350 watts!!
At any rate, I consider my 40wpc VTA70 adequate for MOST listeners, but again, if you like ROCK or symphony music EXTREMELY loud into ProAcs, then go for the VTA120 amp from Bob.
Now, the VTA circuit was adapted from Vacuum Tube Logic circuits back in the mid-80s specifically to fit the Dyna ST70. I've had 20 years to play around with the topology, including trying CCS on the bottom of the LTP, with NO improvement. Other circuits can design for no coupling caps, but at the expense of comprimises in operating points of the circuit. The better solution is just to use a great design with good components, that all work together with a synergy that makes great music come thru. Premium caps can make a difference, as can premium tubes.
Lastly, excuse any spelling errors above, I'm pretty tired today!!
Roy www.tubes4hifi.com

Thanks Bob and Roy,

A lot of good info in your replies.
I have one 100W SS amplifier driving my Proacs and I am pretty satisfied with its power reserve for the type of music I have in my collection (jazz, classic but some rock too).
Is just that the low level music is not as good as I remember from my last tube amplifier.

I will probably go for a VTA120 since it provides a little more headroom. By design, the 2.5 has a raised treble and bass response, so I was thinking initially that EL34 might match them better than KT88
But on the other hand I like the deep bass the ProAcs have, so KT88 might be a better solution at the end, with more grip.

Have you ever a SN7 for the first stage and ECC99 in the LTP stage? I was intrigued by some of its reviews, one of the link I was mentioning in my first posting, Triode Dick, is using such combination for a very similar KT88 amplifier.
Also he is using quite some big capacitors, 500uF for each channel, just for the final stage.
But probably is a little more than necessary.


Regards
Dragos

Hi

I just finished building the ST120 kit and definetivelly it has enough guts to drive my Proacs insanelly loud.
I am thinking now that even the St70 would have been enough, at least for me
Bass is there, for sure, highs are there, clear up to the upper levels. I am still trying to find the best position for the speakers in my room, so I cannot comment to much about midrange

Good sound, even though my SS amp is not that far behind, at least to my ears.
I will try to get some hours on the ST120 though, my SS amp took some time to get better sound.

Thanks Bob and Roy

Regards
Dragos