Quad 306 - 606 Monoblock & Virtual Earth

Corected version 20-02-2010



A Quad 303 or 405 power-amplifier can be easily converted to a parallel monoblock that will deliver more power to speakers with a lower impedance, will be more stable at higher volumes and can handle difficult loads like ESL speakers in a better way.

All you need are a trimmer potentiometer to divide the signal between both channels and (for the 405) some 0,3R 5W resistors to connect the outputs together. The Quad monoblock instructions are available in the Download-section.

In the 606, 707, 909 it is more complicated.

These amplifiers have a non-conventional power supply with a “virtual mass”. The advantage is that no DC power can go to the output so the clamp-circuit is no longer needed and that you will only need one fuse to protect the power-supply.

Both channels in are completely separated, the power-supply as well as the earth. Normally this technique is only used in professional audio. In the 606 the masses of both speaker-outputs (the black binding-posts) are connected… via the preamplifier, we may consider this a serious design-error!

Fortunately the virtual mass circuits in the separated power-supplies in both channels will keep the power-supply voltage at the correct level.

But when converting to monoblock we don’t want separate voltage regulation in both channels.

Another issue is that both safety-circuits (R32 and C12) are now in parallel between the signal-earth and the chassis, resulting in an impedance that is too low. When converting a 606 to monoblock those components will have to be removed in one of the channels. In the 306 they are only mounted once so they don't have to be removed.

Because of the separate power supplies both channels in the monoblock will not always react the same way. We will have to remove one of the regulation circuits and connect both power-supplies together. Also we have to remove one safety coupling. Look at the specific diagram's.

The 306 has a slightly differant layout, the mass of the two channels is allready conected together and there is only one safety coupling to mass.
To modify the amplifier to a monoblock , this is what we have to do:

Connect C10 (positive tag) in both channels in parallel by soldering an isolated copper wire between them on the copper-side of the Printed Circuit-board.
Do the same with C11 (negative tag).
In one channel remove the regulator circuit by removing R28, R29, R30, R31, T11 and T12.

Now the amplifier has one power-supply (with twice the power), and one regulator-circuit. We can now replace the RCA or Din input(s) by one new input, add a trimmer to divide the signal between both channels like in the 303 or the 405 monoblocks (tip: Glue the multiturn trimmer to the inside of the back-panel beside the RCA input) and the resistors between the outputs of both channels and the red binding-post. If you want to retain the input connector (Din or RCA) which is directly connected to the PCB, than connect the output wires of the pot to R1 in both channels and solder the input of the pot to the connector.

Stefaan & Joost