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Modify Your "Phlanger"

for Lower Noise

Article from Polyphony, April/May 1978

I have become rather enamoured of the flanging effect, and use the PAIA Phlanger not just for flanging but for chorusing, pseudo-doubling, and vibrato. However, one aspect of the Phlanger where there is room for improvement is the noise level. Bucket brigade devices like the SAD 1024 are inherently noisy, so there's not much you can do about that part of the circuit; but there is also some noise contributed by the input stage, IC1c, and the output mixer, IC1d. By replacing these stages with special, low noise op amps, it's possible to obtain a dramatic decrease in the Phlanger's noise level.

This is not a particularly messy modification, but you will have to be patient and careful... remember that the goal of this exercise is to improve the Phlanger, not destroy it! So, follow the instructions carefully as we go along.

Here are the items required for the modification:

1) 4739 dual low noise op amp. Don't confuse this with the regular 739, which requires external compensation and is unsuitable for our application. The 4739 is manufactured by Raytheon (second sourced by Exar) and may be purchased from local stocking distributors, or is available by mail from Bill Godbout Electronics, (Contact Details) for $1.65 (orders under $10 add 50£ handling). See figure 1 for a pinout of the 4739.

Figure 1: 4739 pinout

2) A 14 pin IC socket for the 4739. It's best to use a mechanically strong circuit for this application... low profile types are not recommended; use 2 level wire wrap types or some other sturdy socket that can hold up to having its leads soldered to and generally messed with.

3) A .1 to .22 uF ceramic or mylar capacitor; any working voltage over 10V is fine.

We might as well start the modification by doing the roughest part first. We need to disable and remove pins 8 through 13 of IC1 on the Phlanger board from the component side. IC1 is either a 3900 ot a 3401, and is located towards the right hand side of the Phlanger. I luckily had the forethought to mount this IC in a socket when I built the unit, so disabling the pins simply involved unplugging the IC, cutting off pins 8-13, and re-inserting the IC back into its socket. If your IC1 is soldered to the board, you can either use a very fine point diagonal cutters to clip off leads 8-13 very close to the board, then bend the remaining part of the pin up horizontally (see figure 2); or, if you have the patience, unsolder the IC (or at least the side with pins 8-13) from the board, cut off the appropriate pins, and reinsert into the board. Be careful when unsoldering the IC that you don't overheat it (which can cause failures) or overheat the board (which can cause the traces to lift up). Also, make sure during this operation that when it's all over pin 14 is still connected to the board, since that's the pin that carries power to IC1.

OK, now we have pins 8-13 disconnected from the circuit. Next step is to remove R16, the 470K resistor, and replace it with a .1 uF capacitor. This modification adds extra decoupling on the bias line, and keeps noise carried on the power supply lines out of the input stage after the 4739 is wired in.

Next step is to take your 14 pin socket and solder eight 1-1/4" — 1-1/2" lengths of flexible, thin, stranded wire to pins 1, 5, 6, 7, 8, 9, 13, and 14, as shown in figure 3.

Now, from the foil side of the board, hook up these wires to the pads formerly associated with pins 8-13 of IC1, in accordance with table 1. Then hook up the power and ground lines, pins 14 and 7. The wires coming from pins 5 and 9 of the socket should have their ends twisted together; put these combined leads inbetween pads 12 and 13 of IC1, and let solder flow over both pads and both pieces of wire (see figure 4) so that pin 12 and pin 13 are connected to each other as well as the two wires. After checking that all the wires from the 4739 connect to the right pads associated with IC1, plug the 4739 into its socket.

You are going to have to re-calibrate the Phlanger to complete the modification. I recommend using a sine wave generator or PAIA VCO for calibration. Plug in the Phlanger, listen to the output on an amp, and feed the generator into the input. Obviously, the case has to be open to do this, so make sure you don't come into contact with any "live" parts of the circuit.

Set the controls so that speed, accent, center, and span are fully counterclockwise, with the mix control set full clockwise. Turn up the sine wave generator to a low level; don't be surprised if you don't hear any sound yet, as the bias trimpot, R5 must be properly set for everything to work. Adjust R5 until you obtain a clean, undistorted sound. You will note that turning the trimpot to the left or right of this position will give distortion, and eventually the sound will poop out altogether at either extreme. Now, turn up the signal generator a bit more, and readjust the trimpot for minimum distortion. Eventually, you will reach an input level where there is no un-distorted setting of the trimpot. Leave the trimpot where it is — you now have minimum distortion consistent with maximum level. If you don't have a signal generator or VCO (although the odds are admittedly remote that you don't if you're reading this!), plug an instrument like electric guitar into the Phlanger's input and simply adjust R5 for minimum distortion consistent with maximum level. While you're at it, you may want to readjust R6, the balance control, since R5 and R6 are somewhat interactive. Referring to page 10 of the Phlanger instructions, adjust trimpot R27 and turn up the span control (with the speed control full clockwise) until you obtain a "wheep-wheep-wheep-wheep" sound; then adjust R6 for a minimal amount of "wheep" level.

Well, your Phlanger is now modified for low noise operation. Before putting the case back together, sandwich a piece of foam around the 4739 and its associated socket so that it's held firmly in place between the foil side of the circuit board and the case bottom (see figure 5).

The difference in noise level is most dramatic, and shows up the most under studio conditions where every decibel counts. Now instead of having one of the neatest flangers on the block, you can have one of the quietest ones, too.

P. S.: You may have noticed that grounding the cancel control line gives a bit of a thump at the output. You can remedy this by locating the end of R10 that connects to the R11-C5-R12 junction, and inserting a .22 uF capacitor in between this end of R10 and the junction (see figure 6). Also add a IM resistor from point C to ground. Now your cancel line is as quiet as your front end and output mixer.

(Click image for higher resolution version)

Table 1: Wiring the 4739

Connect the wire from pin 1 of the 4739 to pad 9 of IC1 on the foil side of the board.
Connect... pin 5 of the 4739 to pad 12 & 13 (see figure 4) of IC1
Connect... pin 6 of the 4739 to pad 8 of IC1
Connect... pin 7 of the 4739 to pad 7 of IC1
Connect... pin 8 of the 4739 to pad 11 of IC1
Connect... pin 9 of the 4739 to pad 12 & 13 (see figure 4) of IC1
Connect... pin 13 of the 4739 to pad 10 of IC1
Connect... pin 14 of the 4739 to pad 14 of IC1

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Publisher: Polyphony - Polyphony Publishing Company

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Polyphony - Apr/May 1978

Donated & scanned by: Mike Gorman

Feature by Craig Anderton

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