Audio Research LS8 mod. to MKII

[Osirison]

I'm having Audio Research LS8 tube preamp from 1998 and it is possible to mod it to MKII

Does anyone have a mod description of this modification?

I was not able to find one so far on Google.

[Osirison]

Since Audio Research did not respond on my fax requesting information about modifying a LS8 to MKII i am trying to find this out myself by the schematics of the LS8 and LS8 MKII and many other tube preamp designs.

LS8 Audio stage Left channel only:
src825.PNG

I don't understand why Q2 is in there? it seem they didn't know either so removed this in the MKII though.

What is with Q1? from my basic knowlege of tube and transistor amplifiers it should work fine? if the cathode of V1 is just connected to the ground with 300Ohm resistor.

Why Q3? looks to me it works like some sort of diode this way preventing some feedback?

I'm quite sure i don't get it at this point, so i hope someone out here can help me out.

Right now i'm already happy with this preamp i just wonder if it can get any better and i like to mod.

All ideas are welcome!

[flyingfader]

Hi,
I'm owner of a Ls8 too but unfortunately I can't help you for the MKII mod cause I was unable to get these informations too. But let me thank you for the audio part of the schematics you provided, I'm desperately looking for the Ls8 service manual for 2 years. I think this preamp has not been a hit in the ARC family. I'm keeping on digging the web to find any docs on the MKII mods, I'll keep you informed just in case...
BTW, regarding the schematic, to me Q1 is a CCS used to maintain a constant biasing in the first stage (as do Q3 in the last one) and Q2 seems to be a protection for V3 first triode during warm-up to maintain a safe V2/Vgk.
Regards
Fly

[hm4nine]

Though I cannot help with a new circuit diagram: Q2 is a Norton level shifter, working as a CCS to allow DC-coupling of V3 to V1 without attenuating the signal.

[kevinkr]

Originally Posted by hm4nine

Though I cannot help with a new circuit diagram: Q2 is a Norton level shifter, working as a CCS to allow DC-coupling of V3 to V1 without attenuating the signal.

Q3 is a CCS (constant current source) implemented as a cathode load on the output cathode follower. (Slightly better linearity and a predictable operating point for the CF)

If you are happy with it why not leave as is? I think of mods as something to address a perceived shortcoming.. Perhaps the output cap could be upgraded.. Or you could build something from scratch or from a kit which would be great learning experience. (Something like the Aikido line stage)

[Osirison]

Thank you for the replys, i'm glad i know now why these fets are in the circuits

I decided to keep it like it is indeed, like Kevin wrote

I have been thinking about replacing the output caps indeed, i'm not sure yet how since 2uF is hard to get (in a single cap) should i go with 2.2uF or 1uF

[flyingfader]

You can't go wrong with Auricap 2u2 EGA7956-45, they sound pretty fine.
Fly

[hm4nine]

This I found on AR´s website about the LS8 MkII:

......MKII component improvements include new regulator FET devices, higher-quality resistors, new Infinicap(r) coupling capacitors and our latest selected 6922 twin triode vacuum tubes. Mechanically, the MKII incorporates new chassis isolation feet, extensive internal chassis damping, and the proprietary clear tube damping rings also used in the LS16, LS25 and Reference 2 models. The cumulative sonic gains are exciting: less smearing with tighter focus of individual instruments, more lucid and natural-sounding upper frequencies, better sorting out of complex orchestral programs, improved dynamics and better bass control and impact. The overall sonic presentation is smoother, more extended in range and more convincingly authoritative.....

I think, there are useful hints how to upgrade the older modell.

[Osirison]

Originally Posted by flyingfader

You can't go wrong with Auricap 2u2 EGA7956-45, they sound pretty fine.
Fly

I'm gonna try to get these cap's.
Did you notice a difference when you changed them?

[flyingfader]

Originally Posted by Osirison

I'm gonna try to get these cap's.
Did you notice a difference when you changed them?

Hi Paul,
Again Thx a bunch for the S.M. of the Ls8's. To answer your question, yes... mainly because one InfiniCap had some DC leakage so a pretty loud bump occured every switch off ..... At this time it was a bit difficult to find some InfiniCap so I've decided to give a try to Auricap 2U2/400v, a way more easy to get. Sonicaly speaking ... well a bit more space and a tiny sweetness in highs maybe, not a huge gap! even after some months. Regarding the mods to MKII it's not as easy as it seems because the modification KIT is no longer available and some trans/fet references are mfg refs (color code)and originals refs earased on the component itself, the Resista MK2 and Mk3 resistors not so easy to find on small quantities and at last U9 seems to be customized too (not always the same prog revision) so ... stay tuned
regards
Fly