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Grounded Grid Preamp Project And Possible Group PCB Buy

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Since the book "Audio Reality" is now out of print its high time to start a thread and pcb for building the GG. This of course with Bruce Rosenbilts blessing and a donation from any group pcb board sales going directly to Bruce.

Suggestions, answers, hints, comments, tips.... Let the thread begin.....

Mark
 
Hi Mark,

I have a GG pre-amp and will be using it on the front end of my KSA 50 clone when built. The amp is very quick and analytical, but still retains the smoothness that valves are known for.

It should be noted that complete pre-amps and kits are still available from Bruce through his website even though AR is now out of print so I don't know if this would be an issue.

In general, Bruce is against tweaking his products as he feels the stock item is basically as good as it gets. One option may be for us to develop the circuit a little and base a group buy on this 'super GG'.

One minor probelm I (and a number of other customers) encountered was that the unit was VERY microphonic, although this was cured by replacing the tubes that were supplied with the kit. I have a copy of AR so am happy to supply info or pictures of the GG if requested.

Cheers,

valvemaniac.
 
so, since the book has now dissolved into the ether, can anybody share info on what makes "grounded grid" special?

and please, i'm NOT asking for anyone to post any schematic info that will get anyone in trouble!

and yes, i did take a peek at the transcendent sound site.

thanks,
michael
 
Never mind; i found this link later:
http://www.tubecad.com/november99/page17.html


mlloyd1 said:
so, since the book has now dissolved into the ether, can anybody share info on what makes "grounded grid" special?

and please, i'm NOT asking for anyone to post any schematic info that will get anyone in trouble!

and yes, i did take a peek at the transcendent sound site.

thanks,
michael
 
Michael,

The GG has what I like to call a suave presentation, without introducing artifacts that would make it sound too much like tube gear. It is extremely quiet and impressively dynamic. For the past six years my reference pre-amp has been the Audible Illusions Modulus 3A, which is a well regarded design in the high-end category. I am glad to report that the GG compares very favorably to the M3A, with the benefit of being quieter and much easier on the tubes. The M3A has a well deserved reputation of being a tube eater. Even after re-tubing it with hand picked, low noise 6922s, it only takes four to five months with an average usage of 4 hours a day for it to show the hiss again. On the other hand I have left my scratch built GG on for over a month now and it still has a very low level of hiss.
Another benefit is the low cost of building one. Using all new parts, I built mine on a standard Hammond aluminum chassis for under $120.
 
Hi Moggly,

I'm currently using 12AU7's from Chelmer Valve Co in the UK. They do a range of 'Audio' tubes that I find to be very good.

www.chelmervalve.co.uk

The pre-amp was built in the UK from a kit before I moved out here to NZ. They do supply worldwide. I can't remember what tubes were supplied with the kit, they may have been Philips units. Tapping on the case produced some nasty noises from the speakers. I know that the standard answer to this is 'don't tap on the case!!!', but unless you can put the amp in a soundproof bunker, there are going to be problems:D

Check out the discussion forum on the Transcendent website. You should be able to pick up plenty of tips on what people are doing with their amps.

Cheers,

valvemaniac.
 
I use NOS Tung-Sols in V-1 and 2 and NOS Telefunken in V-3. That has been the best sounding combination with my KSA-50 Clone driving Dynaudios. This is the best line stage I've ever used period. Cost for me to build was about 85.00 including the NOS tubez. One can keep the transfromer costs down to about 6.00 each by using cheap isolation type trannys for the HV rails. The one below now sits in a nice quarter sawn oak base.....

Mark
 

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Just for sake of clarification....
The GG stage that is refered here ..... is this cathode follower driven????
Such as a diff input pair but with the first valve bypassed at the plate for de-coupling....
If so, I am not a real fan of the Cathode Follower in Hi-End audio applications.... anyone else feel the same...
The GG has nice slew/bandwidth...avoiding miller effect...

Chris
 
So, to build a GG from scratch, you have to get your hands on a 2nd hand copy of the book, is that right?

Well since the book is out of print you might be able to bribe someone with a copy to send you the schematic. The book is worthwhile and quite amusing to read through.

-------------------------------------------

The GG stage that is refered here ..... is this cathode follower driven????

Yep!

Such as a diff input pair but with the first valve bypassed at the plate for de-coupling....

And pretty much yep again but with some other major differences and another cathode follower type stage at the output..... so I guess you're doomed. It also has a somewhat different power supply from what you may think it has.

Some will say that no line stage is the best line stage and we can all say that we don't like something, however don't knock this one till you spend the 85 bucks to build it and can allow your comments to stand on their feet.... Danger, they won't if its constructed properly. I've owned so many types of commercial and DIY high end line stages over the last 20 years that I've lost count and the GG beats em all by a mile. Especially in terms of speed, and dynamics and complete lack of any grain. I also like the fact that it doesn't sound remotely similar to anything else out there and that alone makes it quite unique. Definately the biggest bargain in line stages there is.

Mark
 
Hi Barretter,

Yes the first stage is cathode coupled to the grounded grid, which is effectively the lower triode of the output pair. As I am sure you are aware, in most valve circuits the cathode is grounded and the grid modulated. In this circuit, it is the other way round. The big problem with this is the low input impedance of this configuration, hence the cathode follower at the front end to act as a high impedance input, low impedance output buffer stage.

The grounded grid was originally used in radio frequency applications, with very low noise and high linearity. In this particular circuit the frequency response is flat up to 300kHz. Its output impedance is around 300Ohms and with an ability to output over 20Volts, it should drive just about any power amp you hook it up to.

Hope this helps.

Cheers,

valvemaniac.
 
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