Help needed with Linn LK2 schematic

[lce]

As an owner of a Linn Lk2, I try to understand how it works.
Thanks to this very informative thread :
http://www.diyaudio.com/forums/showthread.php?s=&threadid=106226

I got the schematic, put it under LTSpice and now try to understand it ....
As it is not really simple and as I'm not really an expert, I need some help to understand the design choices ...

1) The input stage is a classical differential pair , with constant current source and current mirror.
I just wonder why they use 2 transistors (Q5,Q6) linked by R3 for CCS instead a more classic one transistor one..

2) VAS stage is another differential pair, cascoded input, CCS and current mirror.
Don't really understand the bias scheme (zener D1//(D2,D3,D4)) and why they don't link Q2 collector output
to Q8 base input in a more symmetrical scheme...

3) I must say, that I just don't understand at all the third stage (Q15-Q17). Seems to be a complex Vbe multiplier ,
but why so complex and why it is linked to amp output ?

4) Output stage seems to be a CFP with 2 current mirrors, but why the 2 output diodes D12,D13 ?

All this (26 transistors !) feed by a fully regulated power supply ! (when I say not really simple ...)

So lots of questions and some more :

I plan to recap it because of its venerable age.
Any idea about which cap for each function ?
Without changing all the topology, is there some gain to change some components (better output transistor,etc ... ?)

 

[lce]

Really nobody to discuss about this very "interesting" schematic. ?

 

[lumanauw]

Do you have the sim result of open loop gain curve for this amp?

 

[ticktock]

Some time ago - I'm interested though.

Did you read the service manual including "reliability mods" and compare
the LK2 schematic to the one of the LK280?

The front end seems silimar and I guess these mods also apply to the
older LK2 amps:

The unlikely star-candidate - Linn LK-100 - pink fish media

Another worthwhile read about tweaking an old LK2:

Linn LK2 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

Old vs New linn sound

http://www.diyaudio.com/forums/solid-state/106226-tweaking-classic-linn-lk1-lk2-lk280-6.html

 

[ticktock]

Any comments highly apprecciated.

 

[sbrads]

Off the top of my head with no simulations done........

Q15-Q17 purpose I think is to be precise with o/p stage bias so no adjustment pot is needed. Notice the same transistor is used, ZTX653 for Vbe matching plus a small voltage from R20/R23 to give the required voltage across the o/p stage emitter resistors, which is also why it's linked to the o/p.

The VAS stage Q2/Q8 issue I guess is that Linn wanted a particular distortion relationship between harmonics. e.g. less cancellation of 2nd harmonics. 2nd harmonics sound nice and all that.

D8/D9 might compensate for drop off of Hfe at high currents perhaps?

 

[ticktock]

What do you think about decreasing value of C1 and increasing C2/22?

 

[sbrads]

Quick rough calculation shows the bass -3dB is 8Hz which I would leave alone, but the top end roll-off is a worry. Fed from a low say <200R impedance you'll get something like 35kHz -3dB but this falls alarmingly if fed from a few kohm, so 2n2 might be worth trying.

 

[ticktock]

THX!
Guess I will go for C1 2.2 nF mica and C2/20 220 µF Silmic II parallel with 47 nF KP
as I have some 220 µF Silmics in the parts box, no less.

 

[ticktock]

Sadly there are very few doing mods to scottish amps. Linniacs fear modding,
many DIYer don't like Linn gear. Several sites covering Naim mods show the
way though.

This has beeen one of the starting points to think about decreasing value of
C1 and increase C2/C20 - measurements of the der LK260 on the Vintage-
Radio-Forum:

Left Channel:

Frequency response, -3 dB points: < 10 Hz and 37 kHz
Frequency response, -1 dB points: 18 Hz and 17.8 kHz
Frequency response, -0.5 dB points: 25.0 Hz and 12.0 kHz
Distortion: 0.012%
100 Hz spikes visible on output with no signal

Right Channel:

Frequency response, -3 dB points: < 10 Hz and 38 kHz
Frequency response, -1 dB points: 17.5 Hz and 19.0 kHz
Frequency response, -0.5 dB points: 24.0 Hz and 13.0 kHz
Distortion: 0.04%
100 Hz spikes visible on output with no signal

... and on Stereophile of the LK280, slightly better:

At a 1V output level, the '280 was ostensibly flat throughout most of the audio band, though
there was a slight droop at the edges, the response being 0.75dB down at 21Hz and 20kHz.

Measurements will probably better after mod.

Linn LK2 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

LK260:

Linn LK2 power amplifier - UK Vintage Radio Repair and Restoration Discussion Forum

LK100:

The unlikely star-candidate - Linn LK-100 - pink fish media

The later LK100 is a cheaper and simpler variant of the same of the almost
same circuit.

http://www.diyaudio.com/forums/solid-state/106226-tweaking-classic-linn-lk1-lk2-lk280.html



I'd strongly recommend to do these mods to any owner of a LK2/60:

- do the Reliability Mods (R33 und R49 > 2 W, C6 & C19, see below)
- heatsinks for Q10, Q11 und Q14 (the still get hot)
- evtl. heatsinks for LM317/337
- add the two 100 µF, the LK2/75 comes with
- renew several lytics, e.g. Pana FC, Silmic or Nichicon Muse
- consider to renew the big lytics, go for 10.000 µF
- if you remove the transformers and put them in an external case, you'd gain spce for additional capacitance
- swap C5, C6 & C19 for Micas
- decrease C1 to 2.2 nF, C2/20 > 330 µF + MKP (e.g. 150 nF)

Hope I didn't miss anything.



Does it sound better?

It does.

The modded LK260 play more authoritively, quicker and more dynamicly. The
former "british Sounding" has gone. If you like the old british day, don't do it.
If you like music and realism - do it !!
Probably there are more modifcations you could do, but there's already more
air and a deeper basement. I can listen to Marcus Millers Bass lines better.

I have to say I already have put the transformers into an external case
(similar to "Spark").
Hard to understand why Linn chose C1 so big and C2/20 that small. I cannot
understand als why there are no heatsinks on Q10/11/14. Even with some
Copper HS they still are 55° - 60° C hot. No wonder the melt the styrenes.
R49 also still gets hot. This one is prone to burn as it's just two 1/4 W types
from factory.


ps:
The later Klout and LK100 etc. have similar circuits. The Klout is a LK280 in a
more beautiful chassis, the LK100 has a simple PSU and no dual mono design.
Likely LK240 monos are similar. The later LK85/140 are not though.

 

[tiefbassuebertr]

Questions to different variations of Zobel Network/Boucherot Cell

Any comments highly apprecciated.

There are more (at least three) versions of PCAS003 board than described in the service manual on page 2 - go to attachement and under
http://www.acousticpsychos.com/Files/Linn-LK2-pwr-sm.pdf

1) the here not showed version use a 10nF cer. cap in series to a 10Ω resistor (C9+R29). Additional there two RC networks in use (27Ω 1nF) between base and collector of each output power devices (TIP35C/36C , sometimes BD249C/250C) - go to first schematic of previous post or the attached pdf LK2-60 schematic and the attached image (R101/102, C101/102)
2) also not showed is a version with only one capacitor C9 (100nF polyester) between output and GND without serial resistor R29 and without the RC networks (R101/102, C101/102, each 27Ω 1nF) between base and collector of each output power devices.
3) showed version: like previous version, but with additional RC Network 330nF/0,47Ω at the output (the value for R29 in the second schematic is 1000Ω instead 0,47Ω - a missprint so I think)

The most interesting question for me is to know the reason, why only a 100nF capacitor is in use for C in the second version (i. e. without additional resistor in serial mode). In such cases there is a shorting present at the output above a certainly frequency value (maybe to protect the tweeter in case of occur from unwanted oscillation ?) and why the resistor value for the additional RC-network in the third version is only 0,47Ω instead usually used values of 4R7-10R.

Thank you for advices.

And an additional question: which of the mentioned versions are the oldest ?
this URL don't provide the wanted information:
Linn LK2 power amplifier - UK Vintage Radio Repair and Restoration Discussion Forum