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What we offer

We have developed a module that operates as a direct replacement for the amplifier modules used in the Quad 405 Power amp. These modules are made to the highest standards of design and manufacturing and take advantage of the latest developments from semiconductor manufacturers to provide the best performance available.

Details:

The module uses no less than six Motorola MJL3281A transistors in the output stage and is conservatively rated to provide more than adequate current drive into the lowest impedance speakers you are likely to come across. All parts have been carefully selected to give the lowest distortion and noise levels, including a Burr-Brown OPA134PA voltage amplifier stage, metal film and wire wound resistors, polystyrene, and non polarised capacitors in all signal processing stages, and high voltage capable devices everywhere. The board is designed to fit in the standard 405 space and includes a new heat sink bracket arranged to take the six output devices and drivers. The heat sink is fully insulated electrically and an extra lead is supplied if requested to provide the chassis ground connection.

Owners of Quad 405 and 405-2 amplifiers are able to use this as a drop in upgrade to their existing systems and enjoy both renewed reliability, superior performance, and greater satisfaction with their equipment in the knowledge that the amplifier has a new lease of life and more than matches the latest power amplifiers available today whilst retaining their investment in this classic piece of British engineering. The module could also be used to build a multi channel system for home theatre, and information or supply of suitable power supplies is available

For more information please send E-mail as above. To purchase please send an order to AudioLinear, 35 Monument Chase, Whitehill, Bordon, Hampshire, England, GU35 9QS. At present we must ask for cheque with order, but hope soon to be able to accept credit cards. We make to order, but hold stock of some bare PCB's and other long lead time parts. Assuming we have bare boards, our lead time from receipt of order is generally less than ten days.

Various angles of the 6100-1 amplifier modules showing connection points, mounting arrangement compatible with Quad 405 module, and 6 transistors for output stage.

6100-1 Amplifier Module £180.00 each

Last revised:

More information

The amplifier module is the same shape and size as the original 405/405-2 board, but mounts the output transistors and drivers on ‘L’ shaped heat sink brackets fitted to both sides of the board. This way we are able to fit 3xTO264 and 1xTO220 device side by side mounted to both sides of the PCB.  Each output transistor has its’ own 0.1 ohm wire wound emitter resistor to ensure current load sharing between the three devices. The drivers are Motorola MJE15033 devices selected for their high voltage capability, linearity, and gain, as well as having the right current, power, and package. The frequency response of these devices matches well with the Motorola MJL3281A output devices, and the rest of the circuit, although in this arrangement the lower driver is damped by a choke to ensure the overall response is matched.

The other modifications to the original are aimed at increasing the current limit and reducing noise and distortion – real or imagined. We have used 1% metal film resistors and wire wound to avoid question as to whether there is an audible contribution from carbon devices. We have also used the best available OP amp for this purpose, i.e. the Burr Brown OPA134PA, again to remove any blame for distortion from that area, but also to reduce the real measurable component contributed by the LM071 device of the original. The OP amp power supply is additionally regulated by transistors. In the area of capacitors, electrolytic types are used where the purpose is purely supply decoupling, combined with low value ceramics to improve RF rejection and prevent oscillation. In the signal path polystyrene and polypropylene devices are used where their attributes could make a difference.

To make the replacement job as simple as possible the board has been designed to fit within the 405 case, and the heat sink bracket is designed to mount using the same holes as the original. Connector tags are used to allow existing wiring to be used although the position of tags is slightly different (the tags are lined up on the board edge at the end opposite the heat sink). Because the heat sink is insulated electrically from the circuit an additional tag is used to provide chassis ground connection, and a suitable wire can be supplied complete with a screw tag that allows it to be connected either to the heat sink (spare hole for this) or to the power supply reservoir capacitor tags.

At present we have tested the modules extensively, mainly with real music input to real loudspeakers. Our favourite speakers for testing have been Celestion A3’s, which have a 200W rating and present a 4-ohm load. In A/B tests at normal listening levels most of our listeners have preferred the upgraded device. At high output the preference has been unanimous in favour of our upgraded board. Given the aim of renewing the life of the 405, and improving drive to modern low impedance speakers we are pleased with the results so far.

Installation

Before performing any work on the amplifier ensure that it is completely disconnected from the mains power source and has been left with the speakers connected for at least 10 minutes in order to discharge the power supply capacitors. Mains power can be lethal. Under no circumstances should the Amplifier be operated with the lid removed. The internal power supply contains high value capacitors that can hold charge for a significant time after power is removed from the Amplifier. To ensure the capacitors are discharged leave the speaker connected and the input signal feeding the Amplifier for a few minutes to discharge the capacitors after removing mains power.

Before you rip open your 405…

Please, first read all the way through in case we have written anything near the end that you later wish we had put at the beginning.

Opening the Quad 405

The Quad 405 has two screws holding the lid on. These are to be found on the rear underside of lid, and are plain cross head screws. After removing both screws the lid will slide back and then lift off.

Removing an Amplifier Module

Each module is held in place with four cross head M4 screws, together with shake-proof and flat washers. You will need a long bladed screwdriver for these, and they may be rather tight, so take care not to damage the heads! After removing these screws the modules will lift out, but may be a bit sticky due to the use of heat transfer compound.

Disconnecting the Amplifier Module

!    Before disconnecting the wires from the amplifier modules be sure to write down the colours of each wire and which terminal it was connected to.

You can disconnect the leads from the right hand module with the module still in place, but you will need to lift out the left hand module to get at the wiring and disconnect the leads. Left and right refer to the front view of the Amplifier, i.e. when looking at the heat sink fins.

Free up the cable loom wires

On the replacement modules the terminals are positioned at the end of the board. This differs from the original which has terminals at the side, and it is usually necessary to loosen the cord on the cable loom to allow the wires to reach the new position. The Quad loom is very neat and tidy, but there is no sonic benefit to tying together the wires.

Remove screws from module land on heat sink

Depending on model, after removing the old modules you will find two screws in the area under the heat sink bracket. You will need to remove these screws before fitting the replacement. If you have these screws then be careful to not move the Amplifier while the screws are removed as they hold the heat sink to the chassis, and with it the threaded slides into which the amplifier is screwed. If they are moved you can still get the screws in, but it is more fiddly.

Fitting the new Module

You will find it easier to install the new modules if you place the Amplifier heat-sink down on a stable flat surface. The anodised finish is durable and unlikely to be damaged but if you are concerned then place it on something soft.

Hold the new module vertically with the heat sink bracket down, and then place the screws and washers from the old module into the four small holes. Lower the board into the case and line up the screws with the holes.

Turn each screw a few turns to start the thread so that they don’t drop out and fall inside, then tighten each in turn.

Connecting the new Module

AudioLinear 6100 Modules have 6 x 2.8mm Faston terminals along the end edge of the board. With the modules fitted to the 405 chassis it should be easy to get at these and connect the leads. Each terminal is marked on the board with its’ function as follows:

+v        Positive DC supply line (nominal +50V)

In        Input

Gnd     Ground side of input lead, from braid or shield in coax lead

0v        Power supply zero volt line

Out     Speaker output line

-v         Negative DC supply line (nominal -50V)

Connecting the chassis ground lead

In the original 405 chassis ground to the module is via the heat sink bracket. AudioLinear Modules have a terminal for positive connection of chassis ground, and this may be connected via the supplied lead to the heat sink bracket using the spare hole occupied by a screw for this purpose, or may be soldered directly to the earth tag on the mains input connector.

Replace the lid

Although you may be tempted to try it out now please put the lid on first. Mains power is directly connected to the top of the transformer and easily touched with the lid off.

Powering up for the first time

Finishing off and optional items

Depending on the age of your 405, and whether or not the Power Supply capacitors have ever been replaced, you may benefit by replacing them. If you do this you can also upgrade them from the original 10,000uF items to 22,000uF parts. Thanks to advances in manufacturing these are now available in the same size cans and are a drop in upgrade.

If you replace the DIN input connector with RCA/Phono sockets it becomes a lot easier to mate up the 405 with commercial leads. Quad thoughtfully put two holes in the unit that can be used for this, but make sure you use the isolated type because otherwise you will create a short circuit between chassis ground and the input stage ground of the amplifier module. This will not create a safety earth problem but may cause earth loop problems.

If you want to go to town you could rewire the whole thing with higher grade wire internally. You are unlikely (in our view) to hear any difference, but you would feel good about having done it. About the only place that is likely to make any improvement is to use good thick low resistance wire from the power supply capacitors to the amplifier modules, but don’t spend too much on it…