Buzz's "Residual" Preamp Project

BuzzK

AK Member
Subscriber
I've collected a lot of stuff over the years, especially parts and pieces.

I think I've bought almost every Broskie preamp and line stage PCB (and some complete kits) that he made, as of a few years ago. I used his power supply and a pirated PCB of his "Aikido" line amp, and really liked them in my "Ultimate" preamp (once Dave G. help me to get rid of the bugs, including a "missing" trace on the pirate PCB. I'm sure he felt like an exterminator by the time he was through). :)

Among the "Broskie collection" is the Tetris All-in-One Phono Preamp, and an Aikido line stage. (I have two Aikidos -- one is an "all in one" that includes power supply regulation; the other is just the line stage).

I can't fit both "all-in-one" boards in a 17" rack-mount enclosure, so I'm going to use the All-in-One phono stage to provide filament and B+ rectification and filtering for both. Broskie never intended that configuration.

I also have a Mikkel Simonsen remote 50K motor-driven pot for the volume control. I've added a K&K phono step-up transformer & PCB, a new toroid power transformer.

I had an old distortion analyzer that was obsolete -- it couldn't measure below 0.1% THD, so I stripped it for the enclosure, which is extremely well designed. Decent enclosures for DIY preamps are pretty scarce. I've attached photos below. I promise that it ain't gonna look anything like that when I'm finished.

Stay tuned, if this sort of project interests you.

Buzz


Pre chassis front.JPG Pre chassis rear.JPG
 
The Aikido line stage I built uses 12au7's, could use 12bh7's I guess. It sounds very nice. Tons of gain though.
 
The next step is to try to optimize the internal and external layout.

The back panel was originally "concave," but I had to flip it around to make it "convex", so that there is enough room for the PCBs. This will require fabricating a new part; the PIM nuts and holes don't line up. (The RCA jacks mounted on the rear panel will be "concave", i.e. Neutrix connectors).

I'll also need a 1/8" aluminum substrate to anchor everything, including the toroid. The original chassis had a partial substrate; the one I have in mind covers the entire bottom.

To get the input and output jacks to fit, I'll need to replace one of the 150 MFD @ 450V caps on the Tetris board to a smaller profile (the one on the right is optimal).

Also shown is the IR receiver and motor driver for the volume pot; a small PCB with the Lundahl MC-MM step-up transformers (upper right), and an unpopulated input switching relay board.

I'll make drawings on lined graph paper for the sheet metal folks. I have yet to do the front panel. My fab shop contact is on vacation, so in the meantime, I'll need to finish the phono PCB and buy some other misc. parts.

PS: I just noticed that in my photos, the phono board needs to be flipped around so that the tubes are away from the toroid.



Layout1.JPG Layout2.JPG
 
With all of the random preamps I've built from plans, kits or from diyaudio threads, I am still drawn back to Broskie's designs. They sound great and the quality of the boards can't be beat
 
With all of the random preamps I've built from plans, kits or from diyaudio threads, I am still drawn back to Broskie's designs. They sound great and the quality of the boards can't be beat

They are certainly wonderful, unless you have to desolder parts from them. :)

For some reason, Broskie's documentation leaves a lot to be desired. This is surprising, given the amount of time and detail he puts into his TubeCAD series.
 
Is the front face plate just held on with four screws? If so it would be easy to have a new aluminum one cut on a sheer if you have a local metal shop.
 
The last two I had made came with engraved lettering. The sheilded sockets for the line stage is a nice touch.
 
Any news on this project Buzz?

I put it to aside (temporarily).

I was trying to iron out errata in Broskie's documentation. Apparently he cuts and pastes from one PCB rev to the next, and there are multiple revs of some PCBs. The docs I had didn't correspond.

He's nearly unresponsive . . . I did hear from him but he did not fully address the problem. I sent him a few more e-mails and then finally one in which I suggested he not sell his products if he can't provide accurate documentation and / or is unwilling to fix errors.

No reply. :)

So I'll eventually get back to it.

Jon
 
Let's jump right in shall we?
Lots of tubes there to light. 12bh7 for the pre, what about the Tetra?
You will want to set filament voltage at 12.6 and use the jumpers on the tetra to series connect any 6.3 volt filaments.
You may be close to the limits of the LD1085 filament regulator if you want that to power all the tube filaments used here so we need to know the rest of the tube complement you want to use
 
Let's jump right in shall we?
Lots of tubes there to light. 12bh7 for the pre, what about the Tetra?
You will want to set filament voltage at 12.6 and use the jumpers on the tetra to series connect any 6.3 volt filaments.
You may be close to the limits of the LD1085 filament regulator if you want that to power all the tube filaments used here so we need to know the rest of the tube complement you want to use

Good idea! I'll revisit it and get back to you. Thanks for your help & encouragement.
 
Back
Top Bottom