Justblair's DIY Audio and Electronics Pages

Balance Zen Line Stage Pre-Amp

 

image

A pre-amplifier is used to control the volume and input selection to your power amplifiers, this can be something as simple as a switch and a stereo volume pot.  However there are advantages in using a powered pre-amp.

A line stage can be used to match impedances between a source and a power amplifier stage.

Brendon Lee opted for a design by Nelson Pass, a familiar name in the DIY audio community.  Nelson’s design is a flexible design that can be used to convert un-balanced signal lines to balanced.

This article has been re-published with kind permission of Brendon Lee, originally published on Brendon’s Geocities hosted site.  The article was republished to preserve it beyond the closure of Geocities

 

This is Nelson Pass's Balance Line Stage pre-amp. Some people call it the "Bride of Son of Zen" but I like to refer it to the latter.

Completion Date: November 20th, 2001

PCB

image Power Supply Board

image Main Board

Specifications:

4 CHANNEL! - 2 * RCA  &  2 * XLR
Transistor: IRF610
Power Supply Rails: +/- 80 VDC
Mosfet Drain Voltage Rails: +40 VDC
Electro switch switched attenuator
Weight: approx. 9 Kg - 20 lbs

image image image image image image

How Does It Sound?

Sound Off Update - January 10th, 2002

My friend was in the market for a proper pre-amp so he had borrowed one from the local Hi-Fi shop. This gave me the opportunity to test my pre-amp against consumer grade units.

Main Setup:
  • Naim CDX CD Player
  • 300B SET power amp
  • B&W 6P Speakers
Preamp #1:
  • Naim Model 72  with SuperCap External Power Supply (Retail Price $2,400 USD)
Preamp #2:
  • BLS 4 Channel pre-amp (2 channel configured for RCA singled ended mode).
  • Built-in power supply. Unit handicapped to run at 40VAC from a Variac - to eliminate the newly found ground hum noise - yes I had no time to trouble shoot the boards in singled ended.   (Total Construction Cost excluding Labour $700 USD)

Hands down the BLS sounded better than the Naim. How much better? Well let's just say that there was "no contest" between the 2 units. I could not believe how flat sounding the Naim pre-amp was and how much it lacked in low end bass. The BLS pre-amp had cleaner sound through-out the frequency band while the Naim was a bit muddy all around (probably a bad mix as the Naim may not like driving the 300B tube amp - impedance load miss-matching). The 1st thing we noticed was the mid-range vocals came out alive with lots of energy. Seeing how poor the Naim measured up, it makes me wonder how much better would my BLS perform if it used an external power supply? Nevertheless, my friend has plans to built the same pre-amp but this time with external power supply and only 2 channel RCA.
Word of wise, TRUST YOUR EARS !!!!

Engineer's Notes:

Only negative I find is I'm getting lots of gain in the pre-amp and running sources that are +4 dBU will yield LOUD volumes. I've played around with the value of R15 and for some reason it doesn't sound as good when I reduce the gain (300 ohms or higher). Normally as gain decrease, there should be less distortion as per the graphs shown in the plans. BUT my ears seem to prefer values of 200 ohms or less where gain and distortion will be higher. Also clipping was a problem when using R15 values over 400 ohms when increasing the volume.

To compensate for the loud volume level, my attenuator had to be wired with very low values starting at 1 ohm in 1st position (to keep within a 10k attenuator).

The excessive gain may be caused from running 40 VDC on the mosfet drain instead of the 30 VDC shown in the schematics. I thought running the boards at a higher voltage rails is a good thing (less distortion).

Special Thanks to Koyaan (Steve Eddy) for helping me on this project!