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The 405 amplifier is a classic power amp of enviable reputation across the globe. Although a very likeable amplifier, it isn't the most detailed but does produce an enjoyable listen without the fatigue of some designs. Although good, it is not perfect and so can be improved, firstly with some very worthwhile modifications which will add more listening enjoyment to your 405 or 405-2.

The NET Audio 405 Upgrade are slightly different for the MK1 and MK2 boards. Your 405 can also be upgraded to MK3 standard - please see this page  NET Audio 405 MK3

Firstly the input opamps are replaced with higher quality Burr Brown devices, the opamp power supply is improved and the gain reduced to improve the stability and noise and bass response. The other capacitors are replaced with high quality audio grade devices from Elna. The Mk1 circuit is altered slightly to be closer to the 405 MK2 including changes to the current limiting circuit, hence the slight extra cost for the MK1. Other circuit changes are also made to improve performance on both MK1 and MK2.

The Quad Current Dumping circuit relies on a bridge for the feedback and for correct functioning, the accuracy of this bridge determining the accuracy of the amplifier to a large extent. Part of the modification process is to use hand selected parts to ensure the bridge is as balanced as possible which lowers distortion and improves clarity. A new high power 9 amp output indictor is  fitted to prevent saturation of this component which would unbalance the bridge during music playback and create audible distortion. No matter how low in distortion the class A amplifier is which drives the output and the dumpers, if the output bridge is not balanced then gross distortion will occur directly proportional to the bridge imbalance so these components used in the NET Audio Quad 405 upgrade are carefully matched to ensure peak performance.

This upgrade can be fitted to all marks or 405. The sonic improvements are substantial, improved bass, greater insight and midrange detail, smoother more detailed finer etched treble. An overall much improved listening experience and great sonic value for money.

Once an order is placed using the PayPal links below, the postage address will be returned to you by email. You amplifier boards will normally be returned within a week fully modified and tested.


Quad 405 Fitting Charges

For customers who would like to have their 405 upgraded by NET Audio all that is required is for the two internal amplifier boards (easy to remove) to be posted to the address on the home page and they will be returned within a few days fully upgraded. Before posting, please clean the amplifier heatsink bracket to remove the old heatsink compound. When replacing the amplifier modules, smear a film of heatsink compound (a tube of which will be supplied with returned boards) onto the heatsink bracket to ensure a good thermal connection.

On the 405 MK1 there is a speaker protection board mounted on the rear of the speaker terminals. This should be removed - it can be quite difficult - so that it can be upgraded to match the upgraded 405 MK1 boards otherwise it may produce false triggering and damage the output stage.

The full 405 can be sent to NET Audio if you prefer, please see below.

Quad 405 MK1 Fitting Charges includes:-

Total Price excluding return P+P = £75 including the protection board if the boards are sent separately.

Quad 405 MK2 Fitting Charges includes:-

Total Price excluding return P+P = £75 if the boards are sent separately.

Optional opamp upgrade with OPA627AP devices - probably the best there is.

Opamp upgrade consists of

  • Two OPA627AP Opamps instead of OPA604s
Total Price only when purchased with the 405 Full Upgrade = £32 a pair

405 Full amplifier prices.

If you would prefer NET Audio to do all the work for you, then you may post your complete 405 amplifier. It will be returned to you by insured courier delivery.

Total Price excluding return P+P = £85 for full 405 MK1 Amplifier.


Total Price excluding return P+P = £85 for full 405 MK2 Amplifier.


Additionally the 4 output and 4 driver transistors can be upgraded for further sonic improvements.

Output stage upgrade includes:-

  • 4 Motorola (On Semiconductor) MJ15003 20A 250W Power Transistors
  • Four Motorola MJE15031 Driver Transistors
  • Labour charge for removal and fitting
Total Price when purchased with the 405 Full Upgrade = £35.