Silencing ALS-1306 Fans

This document describes how to apply NG7M's information on controlling fan speed in the ALS-1300 amplifier to the ALS-1306. Before reading this, you should read Max's original work here: https://groups.yahoo.com/neo/groups/ALS1300/conversations/messages/262

While the ALS-1306 cooling fans are not loud compared to the typical tube amp, I always thought it odd that the four fans in the amplifier cabinet ran at full speed all the time, whether the amp was cold orwarm. It turns out that the amp design includes thermistor control of the fan speed, but, even when cold, the circuit applies around 9.5 VDC to the fans causing them to run near full speed. NG7M described how to add a thermistor controlled fan speed controller to the ALS-1300 amplifier, which allows the fans to stop when the heat sinks are cool, but rise to maximum speed as the heat sinks heat up. In this document, I'll describe how to adapt that change to the ALS-1306, along with another change to the power supply fan.

The Amplifier Fans

The amplifier has two separate but identical PA units, each with two 80mm fans. At least one 1306 owner has reported that his fans do change speed as the amp temperature rises, but mine come on at what sounds like full-speed and there is no discernable speed change as things heat up. The amplifier control board includes circuitry to control the fan speed with temperature, but in at least some instances, that circuit is not working properly. While my fans start reliably at 3.2 VDC and are relatively quiet even up to 6V, the existing circuit applies 9.5V to the fans when cold and that's well within the voltage range where the fans sound like they're running at full speed.

Pending a permanent solution from Ameritron, here are a couple of options that solve the fan noise problem. Note that when the heat sinks are warm, the fans must run near full speed for adequate cooling so don't expect silent operation when the amp is being used heavily. In my case, with casual DXing on SSB or CW, the fans rarely reach a speed where the noise is excessive. On the other hand, during RTTY operation or in CW contests, the fans will run at full speed.

NG7M's document describes adding a fan speed controller which uses a thermistor to sense the temperature of one of the heat sinks. Unfortunately, the PA heat sinks in the 1306 are inverted and reversed compared to those in the 1300, and so the little window in the panel that allowed Max to attach the thermistor to the forward heat sink is not accessible in the 1306.

By removing the bottom panel of the amplifier, you gain access to the screws that fasten a chassis panelto the heat sinks and you can mount the thermistor to one of those screws (see Illustration 1). Note thatwhen you remove the bottom panel with the cabinet removed, the front panel is not supported by anything, so be careful with that.

Illustration 1: Thermistor mount to one of the heat sinks in the 1306.

While I was waiting for the control module obtained from eBay to arrive from China, I looked around for a fan control circuit I could experiment with, and ran across the circuit shown in Illustration 2. I obtained the parts from Jameco and built the circuit on a small piece of Velleman circuit board, using the same connections to the amplifier described by NG7M. Note that installation of this circuit or the Chinese controller can be done with NO soldering or modification of the existing amplifier circuit

board. You just insert the controller between the fan lead plug and the socket on the control board, and mount the thermistor to the heat sink.

Illustration 2: Thermistor controlled fan speed control circuit. Q1 is a IRFZ34N MOSFET (Jameco #669935), Thermistor is Jameco #207037. Use a 10 turn mini-pot for R1 to set fan turn-on point.

The thermistor used was a small unit resembling a disc capacitor and was mounted by expoxying it intoa wire lug for mounting to the heat sink. The 12 volt supply used was the center pin of the connector the fans are currently plugged into on the control board. The circuit ground goes to the chassis of the 1306.

I adjusted the pot, R1, so that the fans are just at the point of beginning rotation when the heat sink is atroom temperature. I used a 10 turn 5K Bourns pot for R1 to make the adjustment easier. With a small heat sink mounted on the MOSFET, it runs very warm with the less than 1 amp current draw of the fansat full speed.

With the amplifier fans no longer running at power-on, I discovered that the power supply fan runs all the time and the 3,000 rpm fan used generates a whine just loud enough to be irritating. So, I decided

to see what could be done to tame that noise.

The Power Supply Fan

The power supply has an 80mm muffin fan, secured inside the rear of the cabinet with four screws. The fan is a NMB model 3110KL-04W-B40, which is capable of 35 CFM at full speed while spinning at 3,000 rpm. This fan comes on when the amplifier is turned on and remains on until the amplifier is turned off. It emits an annoying whining sound, not at all unusual for a high volume 80mm fan using ball bearings. One solution to the problem would be to use a larger diameter fan that runs at a lower rpm. The problem with replacing the fan in the power supply with a larger one is that the rear panel space is completely used up and the line fuse holders and power cable passthrus are directly below the muffin fan opening. I first tried just replacing the metal screws holding the fan to the panel with rubberfasteners designed to minimize the transfer of fan noise to the panel, and while that reduced the fan whine a little, it was still far too loud.

I happened to have an unused 120mm silent-running muffin fan in my parts bin and set out to see how Icould adapt that to the power supply. As luck would have it, you can buy a converter or adapter which allows you to mount a 120mm fan where an 80mm fan is currently used. The adapter won't fit the power supply, however, due to the proximity of one of the line fuse holders to the fan opening, and the slight overhang of the cabinet top. Those problems were solved by removing the fan mechanism from an old 80mm muffin fan and using just the fan body as a spacer between the panel and the fan size adapter. (see Illustration 3 for details). I had some of the rubber mounts used in place of screws for fanmounting and I used those along with a rubber gasket to mount the spacer to the power supply panel. With the silent-running fan I used, that was probably overkill as there is very little vibration from the fan.

For this fan modification, there's no need to drill any holes or make any circuit modifications. You willof course have to unsolder the existing fan leads and solder the new leads to the same points. Make sure you have the 120mm fan mounted to blow into the cabinet and that you observe the polarity of the new fan leads. The fan won't run at all if you have the leads reversed. Since my 120mm fan had a connector on it, I used a mating connector to make the connection to the power supply board to make future fan changes easier.

With a silent running 120mm fan, all you'll hear when you turn the amplifier on is a very low level sound of air rushing through the power supply. There is NO whine from the fan. The fan I used is a Nanoxia DF1202512SELN, which spins at 1300 rpm and is capable of 60.1 CFM. The static pressure

of this fan is 1.64 mm H2O compared with 3.27 mm H2O for the stock fan. At this point, I don't know

if that static pressure is adequate to produce enough air flow, but during the recent CW SS the supply ran cool. You can find silent 120mm fans at any of the websites dedicated to silent-running PCs, such as www.quietpc.com, www.quietpcusa.com, or www.frozencpu.com. Noctua makes a 120mm fan that runs at 2,000 rpm with a static pressure higher than the original 80mm fan (NF-F12 industrialPPC-2000) and that one may be worth trying. The 120mm to 80mm fan adapter I used is from Nexus (https://nexustek.us/accessories/cfa-300) and similar adapters are available elsewhere.

Illustration 3: This spacer is an old 80mm fan with the motor and supports removed. It's used to maintain adequate clearance for the cabinet top and for removal of the upper line fuse.

Illustration 4: This photo shows the fan size adapter and 120mm fan mounted on the 80mm spacer. The new fan assembly extends 3” from the cabinet.

Note that there are no permanent modifications made to the power supply circuit or cabinet and the original fan can be reinstalled if necessary.

Floyd Sense – K8AC

November 14, 2016