Quad 405 Mod to Quad 606 series big family

(This information ONLY for the people who know the power amplifier circuit.)

Quad's current dumping circuit amplifier is such a simple circuit, good sounding and powerful long lasting amplifier from the past to modern days.

In the Quad current dumping amplifier big family, only Quad 405 is using op-amp in the first input stage. When compare Quad 405 circuit to her big brother Quad 606, it's simple to modify Quad 405 to Quad 606 circuit by replacing the op-amp input stage with a single PNP transistor, change op-amp(use as DC servo control), resistor, capacitor, adding transistor and diodes on the same Quad 405 PCB.

The new component can be place on either side of the PCB according the component space needed and cut one PCB copper trace for the mod job.

I made the MOD and the result is good. Sounding is good and I like the all discrete current dumping circuit.

This MOD is not the last, we can have more MOD like adding one more pair output power transistors as the Quad 520 model which can handle better speaker loading.
 

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The schematic is from a Quad 520(for BBC) early model which is simple using zener diodes and resistors to form the current sources. Later models and Quad 606/909 using LM334 and J503 current diode for current sources. The rest amp circuit function is basic and same as other 606/909 models.

The original Quad circuit is very easy to oscillate, so that I changed some coupling capacitors to make the amp more stable in working, these coupling capacitors will not affect the audio.
 

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