Technical Update
 

    
 Last
    Modified: July 30, 1999

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Re-voicing the 1998 NP220.

"Thank you, Thank you, Thank you for the info on how to upgrade the 1998 NP220's voice to 1999 standards. This amp sounds incredible, a giant leap upward in sound. Soundstaging is just great, and it sounds a lot more like the musicians are in the room with me. Money and time well spent." - Greg Hughs

With the help of a few good ears, including those of renowned composer Michael Ruszczynski, I have recently developed a new tuning procedure for the NP220. It greatly opens up the high-frequencies, without adding any edginess. A re-tuned amplifier delivers more sparkle and "shimmer" in the highs, and the sound is completely smooth. This results in faster transient response "snap," and a far greater sense of spatiality and "air."

The procedure is divided into two main sections, with an optional third section. Before beginning, I recommend that you check my website to be certain that you have the latest version. See www.altavistaaudio.com/np220voice.html and check the date in the upper right corner.

 

Disclaimer: Section 1 and 3 require soldering skills. I don't recommend that you do this work yourself unless you are very skilled at working on electronics. I strongly urge you to get the work done by someone who you can trust to do a good job. A television repairman, or the technician at at hi-fi shop will usually provide good workmanship and has the troubleshooting skills necessary if things go awry. I cannot offer much support if you run into trouble except to ask you to send the amp to me. If you damage the amp through a mishap, it will not be covered under warranty. If you'd prefer, I will be happy to install Upgrdes 1 and 2 in my shop for $100 plus shipping, but as you know, shipping these amps is expensive. That's why I'm providing these instructions, so you can investigate having the work done locally. It's not an ideal situation, but the alternative is to not tell you about this upgrade. Rather than that, I'd prefer that you knew about it because it really makes a big improvement. If you do decide to do the work yourself, I assume you know what you are dong and that you have the requisite tools, like a good-quality 30 to 40 watt temperature-controlled soldering iron. If you have worked on circuit boards before, then you know you have the proper tools.   The second part of the procedure, Section 2, is easier to do. Even if you can't do Section 1, you'll get some positive results by doing Section 2 alone. But the real magic happens when you combine Section 1 with Section 2.   The third, optional section at the end of this procedure, shows how to install better-sounding capacitors in the signal path. Soldering skill is required for that, too.

Section 1

Tune the Pre-Driver Stage.

This part of the procedure requires sixteen 210 ohm resistors. Ordinary 1% 1/2 or 1/4 watt metla-film resistors will do, but you should use better-sounding resistors, like Resista MK3's, or Holco H4's. I prefer the sound of the Resistas, and find the Holcos to be unreliable. I stock the MK3's and a set of 16 -- enough to do both channels -- can be obtained from me, for $20, which includes postage in the continental U.S.; or from Michael Percy Audio at +1 415 669 7181. Of course, you can use even better resistors, like the Vishay VSH Bulk Foil ones, which are available from Michael Percy for $3.75 each.

Start by disconnecting all interconnects and speaker cables from the NP220, and unplug it from the AC Mains. Take off the top cover.

Locate the two output boards. One is mounted to the right heat sink, the other to the left heat sink. You won't need to remove the boards from the heat sinks: all the work can be done with the boards attached. On each board there are eight 186-ohm resistors, R36, R10, R57, R59, R37, R14, R58, R60. Locate them.

Remove these resistors.

Here's the board with all eight resistors removed. This board has been worked on a few times! Yours should look cleaner. Note that under each resistor is a second, unused, set of holes under it. You can use these to solder the new resistors into.

Prepare all 16 new resistors like this. Bend the leads and cut them to length.

Solder the new resistors in place. You do not need to use the old holes. Instead, use the two unused holes located between each resistor's old holes. The photo shows one resistor in place. Seven more need to be installed, for a total of eight per board. Be careful that the leads of the resistor do not protrude far out the other side of the circuit board and short against the heatsink.

All eight resistors in place.

This completes Section 1 of the modification. Be certain to do this to both channels. If you did this section, you must proceed to Section 2 to make the final bias adjustment.

Section 2

Re-bias the Output Stage.

First, we need to measure the current as it is. Connect the amplifier to the AC Mains (don't turn it on yet). Do not connect your speakers or any input cables.

Connect a digital multimeter as shown, with the black lead to the right-hand side of R11, and the red lead to the left-hand side of R6. Set the meter to measure DC volts (200mV range if available).

Turn on the amplifier. You should get a voltage reading somewhere between 20mV (0.02 volts) and 40mV. As the amplifier warms up, the voltage will be changing, first going up then going back down. Let it go for about 5 minutes then start adjusting it.

Adjust the bias pot VR2 with a small screwdriver. You are targeting a voltmeter reading of around 28mV (0.028 volts). As the amplifier warms, the reading will continue changing, but it will begin to settle. A clockwise turn will increase the bias, a counterclockwise turn will decrease it.

As the amplifier warms, the reading will continue changing, but it will begin to settle. Just keep adjusting until you are close.

For your final adjustment, measure the voltage across all four resistors. Each resistor will have a somewhat different reading. Adjust the bias so that the average of all four readings is 28mV (0.028V). You can leave the black voltmeter lead connected as shown.

Section 3 (optional)

Install a Better-Sounding Capacitor.

The Infinicap capacitor that I provided with the 1998 NP220 rebuild is a very-good sounding capacitor, and it is cost-effective. However, it is not the last word in terms of fluid, liquid sound. It tends to be a bit dry-sounding and flat. You can make a bit upward step in sound quality by upgrading this part. I like the VTV Ultratone silver foil in mineral oil cap (www.vacuumtube.com). They are extraordinarily liquid and musical. I can provide a pair of the 0.1uF/650V Ultratone caps for $60, which includes postage in the continental U.S.

Here's the old capacitor in location C6. Remove it.

Solder the new cap as shown, with the red lead connected to the pad nearest the orange muting relay.