Talk? GK-1 idea.

Malcolm Fear · « **on:** 26 Oct 2003, 12:51 pm »

Has anyone separated the solid state and valve section of a GK-1, and just used the valve section? I haven't done it yet. Just wondering.

andyr · « **Reply #1 on:** 26 Oct 2003, 09:12 pm »

Malcolm,

Could you possibly be a bit more informative with your Q ... for those of us who are about to build (but haven't yet) a GK-1.

What does the valve section do and why might you want to use it on its own?

Ditto for the solid state section?

Thanks,

Andy

Malcolm Fear · « **Reply #2 on:** 27 Oct 2003, 01:00 am »

The GK-1 has 3 distinct sections:
A solid state section
A volume control section
A valve stage section

A few have been fiddling with our volume sections, changing it into a shunt, by using a Riken Ohm resistor (works well).
A friend of mine (Bart), keeps asking me to put RCA sockets just before the volume section, and to bypass the solid state section. Bart doesn't like solid state.

I am happy with my system, and don't feel like muck around with it any more.

Just wondered if anyone had tried it.

Propstuff · « **Reply #3 on:** 27 Oct 2003, 10:04 am »

well yes, actually, I've done it...........
Apart from fiddling 'round with the valve "power supply compensation" section, (sorry Hugh-the Devil made me do it), and fiddling around with volume controls, I put in a switch to select whether the output passes through the valve or not.
I retained the S/S input section to maintain the buffering b/w the source and the pot.

The result is that I switch the valve in and out depending on what I'm listening to and what sort of mood I'm in.........-but we won't go there.

Currently its all in bits as it's going into a new enclosure, and I had to get the axe to the GK1 board to get it to fit.

Progress can be seen at http://www.objext.com.au/devotion.html

maybe by christmas???
cheers,

Malcolm Fear · « **Reply #4 on:** 27 Oct 2003, 10:52 am »

Ahh, that's the opposite of what I am wondering about. I am interested in the sound of switching the solid state section in and out.

Greg Erskine · « **Reply #5 on:** 27 Oct 2003, 11:53 am »

Great work propstuff. I like it. Sure beats my black/dark grey prisms.

I been trying to think how I could introduce stained glass into a suitable amp or preamp enclosure.

Propstuff · « **Reply #6 on:** 27 Oct 2003, 09:24 pm »

Malcolm,
I think you would find not much difference (?) That is; Hughs minimalist input stage is not "effecting" the sound. -transparent.
I seem to recall him saying that the input impedence of the valve stage is good for normal output impedences of sources, but you'd want to check that with him ratherr than take MY word for it :-)

What you will have to worry about is if the volume is in front of the valve, the subsequent impedence mis-match as you "twiddle the dial". (shunt volume??)

cheers

ginger · « **Reply #7 on:** 3 Nov 2003, 03:01 am »

When I wired my GK-1 I did the following:
1) The Passive Output from the Switch section which normally feeds the HT bypass I connected to the sub output instead. (the two pins labelled "From Wiper")
2) The "From Wiper" which normally feeds the Sub Output I wired to where the Passive switched output normally goes ie to the HT Bypass input
3) Main Out wired as per instructions.

This means my SUB Out is purely passive source switched output with no level control

My main Output now switches between Solid State Buffer ONLY and Solid State Buffer + Valve Driver via what was the HT bypass switch - BUT both are under control of the volume knob.

To accommodate this the 150R Mute resistors were wired between the volume pot and the mute switch at the front panel.

AS I was doing a bit of component optimisation this allowed me to hear which section to target for optimisations.

Also I didn't like the idea of volume going to flat chat if I accidentally knocked the switch.

Aside on the optimisations:
SS Buffer:
Parallel 4700pF Polypropylene cap across the 470nF input cap.
Replace the Electrolytic feeding the diff amp stage with Rubycon ZL Series
Valve Driver:
Parallel 100pF Polystyrene across 10nF input cap
Parallel 4700pF Polystyrene across the Output Auricap
Watch the Voltage rating on these - I used OLD caps pulled from a HP Pulse Generator I bought at auction for $10. It contained about $300 worth of valves and $100 worth of very high quality capacitors.

A non-starter mod for the Valve Driver:
Even thought the Valve Driver is basically a Cathode Follower circuit which means 100% LOCAL Feedback I had a purists Engineers reaction against using a Variable Mu Tube (The ECC189/6ES8). I therefore tried a pair of JJ Tesla ECC88 (6DJ8) which will substitue without the need for ANY component changes. (I very carefully checked that this was the case). The "engagement" noted below was lost. This is a mod I abandoned as a backward step.

Thats it - no other mods for my GK-1

By the Way wiring as above allowed me to assess the sound contribution from the Valve Driver since I could switch it in and out. The contribution is extremely subtle BUT definitely worthwhile. I think Hugh used the descriptor "engaging". I can't think of a better description so we'll stick with that.

Malcolm - When using my 845 SET Power Amp which has its own volume control I could use Passive Source Switch ONLY, SS Buffered with Volume Control OR The SS Buffer/Volume Control/Valve Driver Output. With the mods mentioned above I found the SS Buffer to be absolutely transparent - I don't believe you will get any real improvement using the Valve Driver Section by itself.

Cheers,
Ginger

EchiDna · « **Reply #8 on:** 3 Nov 2003, 04:06 am »

Quote from: Propstuff

Currently its all in bits as it's going into a new enclosure, and I had to get the axe to the GK1 board to get it to fit.

Progress can be seen at www.objext.com.au/devotion.html

maybe by christmas???
cheers,

OT, sorry! but....

that's a very nice job on the GK-1 Chassis Propstuff!

If only I had the tools available here to do that sort of thing *sigh*
did you do your own aluminium casting etc? most impressive!

Malcolm Fear · « **Reply #9 on:** 4 Nov 2003, 12:54 am »

Hi Propstuff.

From your last post, would it be a good idea to modify a TLP, by converting the volume into shunt mode?

AKSA · « **Reply #10 on:** 4 Nov 2003, 01:48 am »

Thanks Ginger,

Interesting post, truly a vindication of all the angles Darl and I tried back about 18 months ago. My sincere thanks.....

You mentioned the SS buffer was utterly transparent, and that the variable mu tube added 'engagement'. Absolutely my take as well; we worked damned hard to ensure it was exactly that.

A few points, confirming Ginger's post.

A preamp has three essential functions; source switching, a little gain, and a level control. To this engineering list I would add 'engagement', although it obviously can't be quantified, any more than taste, or pleasure, or relief. However, to us humans, these are valuable qualities, and worth striving for in the man-machine interface.

I found that a level control is extremely damaging to the music, particularly top end, sound stage, and 'engagement'. Some even muddy the music, introducing intermodulation. The audiophile community has found this too, and this is the reason for the recent plethora of DACTs, transformers controls, digital pots like the APOX from Dale and Craig, and ALPS Black Beauty etc etc.

The engineering behind the level control would point to a couple of ideal operating conditions, to minimise the 'damage'. The source should have very low impedance, insignificant compared to the pot, while the target, the circuit the pot works into via the wiper, should have infinitely high impedance. Even with our pot in shunt mode, abiding by these principles should give a better sound. This situation prevails in the GK-1. Zout of the solid state buffer is just 32 ohms, and Zin of the tube section is around 2 megohms, and it is the reason the level control is inside the preamp, rather than at the input, as in most preamps.

Clearly, then, removal of the SS buffer section will compromise this situation, particularly if the source impedance driving the buffer is much higher than 50 ohms. While I understand the revulsion many feel for SS circuitry, this one is good! ICs in the output stage of CD/DVD players have very low output impedance, but it is normally ameliorated with a short-protecting 100R resistor in series with the output. Furthermore, most ICs use push pull output stages operating in Class AB, and my own feeling is that at this delicate line level we should stick to single ended circuitry since the energy levels are very low and the inherent inefficiency of SE is insignificant.

Thanks, Ian. Nice post!

Cheers,

Hugh