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My second project was breadboarding a Grounded Grid preamp. This was the design found in Bruce Rozenblitz's book Audio Reality, and the amp itself can be purchased in kit form or assembled from Transendence Sound.

I tried a different approach on contruction this time around. The individual sections of the amp were point-to-point soldered on its own board. The boards are then connected via terminal strips. Breaking up the amp in this manner made the build a lot easier and I didn't have to worry about how many solder terminals I need or their locations. Also, each section can be moved around independently.

Parts are mid/hi grade. Nothing outlandish. Two Auricap coupling caps, Holco resistors, JAN Philips tubes. The only "mod" I made to the design was the use of poly bypass caps in the power supply. Other than that, the design came straight out of the book.

One of the things I like about the design is the powersupply. Bi-polar +/- 200V from voltage doubler rectification. Even though two transformers were required, it still reduced cost and size by quite a bit, compared to the use of one large 300+V transformer. The PS uses two 120V 6VA transformers which are about $12 each. As you can see from the photos, they're also very compact.

Other reasons why I choose this design is that it's simple and has very low gain (3X?). I find most preamps do not allow you to turn the volume pass 9 o'clock. This one, I'm listening at 12.

The tubes are horizontal mount and its my intent to build this as an enclosed stackable unit, under two and half inch in height. Seeing tubes peak out of a top panel in a "traditional" tube chassis is cool, but you can only have so many of those in your system before you run out of space. The parts run cool enough so I don't foresee a heat issue.

So, how does it sound with the Gainclone? Not much to report here. It sounds pretty much the same as when I connect my source directly into the Gainclone with only a pot in between. EXCEPT ... the sound is a lot more dynamic. With the "passive preamp" (ie. pot only) everything comes through. The only problem is that the music sounds "soft". Regardless of how loud the setting is, you're still not getting any "bite". The Grounded Grid improves on that by delivering the dynamics that's missing, while taking nothing away (at least nothing I can hear). Compared to my integrated, the Grounded Grid Gainclone combo is letting me "hear things that I've never heard before". Never thought I'd use that phrase but in this case, it's apt. I am getting a slight hum from the preamp, but I'm not too worried about it. It's only on an exposed breadboard afterall, with no shielding for the signal path.

I highly recommend this project for someone looking for a preamp. Simple enough for someone with a couple of projects under their belt, but sophisticated enough to satisfy those who are serious about audio.

The next step is to find an appropriate chassis, which I have not come across yet. Considering building my own from scratch.

Gar.

• Grounded Grid Breadboard

• Gar's K-502 amp photos

  
  
  

• Gar's grounded grid preamp photos

Regards, Russellc

Is there somewhere I can go to see an exposition of this circuit? I would not have thought that the basic characteristics of a grounded-grid circuit (high voltage gain, low Zin, high Zout) would be optimum for a line amp, but I've heard a good deal about people building this particular line amp. The original design is Rosenblit, no?

Poinz

It's different from most preamp designs out there. I've heard and/or built preamps in the common cathod, mu follower and SRPP topographies and was interested in hearing how this one would be different.

It's simple with relatively few part counts. Three tubes in total and six resistors per channel in the amp stage. I think I've also mentioned the PS which uses low cost parts such as 120V transformers and low value filter caps.

It's been reviewed very well. Bruce has a very loyal following on his own message board and most everyone who's build or heard it have very positive things to say about it.

But the most important reason why I selected this preamp was its low gain of only 12dB (4x). Prior to building this amp, I was toying with the idea of building an unity gain buffer (12SN7 cathode follower)as a preamp. With most power amps having an input sensitivity of less than 500mV, it doesn't make sense to put a high gain stage between source and amp. The gain provided by this preamp lets me listen pass the 12 o'clock position, and even into the 3 or 4 o'clock with low sensitivity speakers.

BTW, I'm still toying with the idea of the unity gain buffer.

Gar.

Sorry, the schematic is not public property, so I can't share it with you. You are correct that a GG circuit has low Zin. In Bruce Rozenblit's design, he uses a cathode follower buffer in front of the grounded grid gain stage to reduce Zin. Not sure if most grounded-grid circuits have high Zout, but this design has a Zout of only 300ohm (have not verify, but going by the book's description). As for gain, this design has a natural gain of 19dB, but total gain is knocked down to 12dB due to the use 7dB of feedback.

Gar.

Not many home audio components could drive a grounded grid circuit directly - typical input impedance of 600R - or less.