ForT

Class T alternative for GainClones...

Page 2: Construction

Last modified: 19-Nov-2006

Specs

The specification of the AMP5 is as follows (copy of 41Hz):

Some technical details

• High quality components are used throughout the kit. The PCB is double sided, with double weight, 2 Oz / 70 um, copper, as with all our boards. Component prints are on both sides.
• Resistors are metal film type for low noise.
• Alternate value resistors are supplied for setting input sensitivity.
• Capacitors are throughout of good quality without being "exotic". For example, ceramic decoupling capacitors are of X7R type and NPO for smaller values caps. The "near chip" bulk capaciotors are oversized 470 uF very low ESR/high ripple rating Panasonic FM capacitors.
• The supplied toroids are oversized. Copper wire is included, you have to wind them.
• The board has a power supply built in. You add a transformer, power switch and mains fuse.
• On the board is a +5V stabilized supply for the Tripath chip logics and low level signals. It uses a LM317 regulator and a bulk 1000 uF Panasonic FM capacitor plus a number of smaller decoupling capacitors for quiet operation. No external +5V supply s required.
• The board has a relay on the speaker outputs. This shuts off the outputs in case any error is detected and prevents speakers if the amp outputs are damaged /shorted. The relay is controlled via the Tripath chip hmute output.
• The standard version of AMP5 includes two high quality 10.000 uF, 50V Panasonic TS-HA bulk capacitors and a 8A rectifier. The bulk capacitors where chosen due to the high ripple rating, 7A at 85C, 4A at 105C, long life and low ESR.
• With the supplied components, rail voltages between +/-22V and +/-35V can be used. Under voltage threshold is nominally +/-20V and over-voltage threshold nominally +/-36V. Transformers of +/-18VAC to +/-24VAC are suitable. VA rating depends on speaker impedance and supply voltage. With a +/-24VAC transformer and 4 ohm speakers, a 300VA transformer is recommended. Lower voltage and higher speaker impedance reduces the required power/VA rating.
• The board has space for a pushbutton controlled volume chip (NOT included, and not available too until now in 2011).

Soldering the PCB

For this kit, no design work was necessary from my side. So the "only" thing I needed to do was follow the instructions and solder all the components. Hmm, since I like making my own designs making the amp had no priority and as a result it took more than 6 months to complete the board.

The gain for the amp is sort of selectable by chosing different resistor values in the input..

On the picture you can see the two heatsinks I mounted to the Tripath chip and the voltage regulator. These heatsinks were taken from an old PC power supply. Also shown on the left side of the board: I used terminals for connecting the power and speaker cables instead of soldering them directly to the board.

On the left is a 225 VA Amplimo transformer, the input RCA's are connected using twisted pair (UTP) cable just like the stand-by switch on the front panel.

Other components

After fnishing the PCB, I found the remaining components. The transformer is from Amplimo (2x22Vac, 5.11 A).

For cooling of the Tripath chip I used an aluminum heatsink which I took out of a broken computer power supply. I used a smaller version heatsink for the 7805 voltage regulator. The rectifier bridge doesn't get warm at all.

Adding the "high power" option

About twee weeks after finishing the amp I decided to buy the add-on to the kit: a second rectifier bridge and two large capacitors. The add-on components will provide the amp with more power during heavy use which is recommended especially when used in bridged mode and when "difficult" speakers are connected.

Not that I planned to use the ForT disco work, but for 20 euro's including shipment the package was delivered to my door, and it was one of those things that may not be available anymore when you change your mind just a little bit too late.

The aditional components are placed without problems on the open spaces on th PCB. However, one of the capacitors is soldered close to the 5V voltage regulator and I had to twist the regulator and its (small) heatsink other wise there was not enough room for he capacitor.

© Maarten_@_Platenspeler.com, 2006-2007