Thursday, September 25, 2008

405 in the air

Justin Wood (UK) used the Dada 44 and 405 kits to upgrade his Quads. He wrote:

Thought you might like the photo below...

Perhaps the first time a Dada Electronics Quad 405-2 was given its own upgrade to 'Club Class'!?

As you know I did the upgrade work while flying S61 helicopters up in the Shetland Islands. At the end of the contract I flew the helicopter with the 405-2, back to our home base in Penzance, Cornwall, UK.

The 405 loved the trip!


Today Justin added:

Just back to Cornwall from Stornoway in west Scotland.

Plugged in the 44 ....and I'm very very pleased to say it works like a dream; perfect in every way. There is certainly a most worthwhile improvement in the sound -- much greater detail, transparency and channel separation. I am looking forward to plenty of great music as the winter nights draw in.

I am absolutely delighted with the 'Dada improved' 44/405-II combination as it sounds fantastic. I hope lots of other Quad 44 owners will take the trouble to do the Dada up-grade and re-discover how good this pre-amp can sound. We all know how superb your 405 mods sound!

The photo shows the freshly upgraded 44 ready to ride home to Cornwall on the instrument dash-board! ...actually it had to go under the seat for safety....

How about a challenge for the weirdest places to find a Quad?

Justin Wood []

Trying out the 1-day Quad revision...

The eternal problem with a Quad-revision is the transport. A 405 has a weight of around 10 kg, a 606 even more... So it is expensive to send it by mail. By the way, I had bad experiences sending amplifiers by mail ;-(

Pierre (from Calais, France - 250 kilometers from Antwerp) wanted to have his 405 upgraded and Burr Brown chips installed but he did not feel like driving 4 x 250 km.

So we invented the "Visit Antwerp & Upgrade your Quad" roundtrip.

If the Quad is delivered before 11 am it can be picked up around 6 pm IF all eventual problems are described via e-mail and a rendez-vous is made in advance (we have to make sure we have all the components ready).

Antwerp is an excellent city-trip - see


Wednesday, September 24, 2008

OPA134 pin 8 in Quad revisions / upgrades

When using a Burr Brown OPA134 opamp for revising / upgrading Quad amplifiers like the 34 and the 306 make sure Pin 8 is connected to nothing.

Pin 8 is the offset-input. If it is connected to pin 6 (like in some versions of the 306) this wil cause problems with the DC-feedback and result in a DC Voltage on the output.

You have to cut off pin 8 or cut the track on the PCB-board.

In general, when using the OPA134, OPA604 or OPA627 in a Quad revision project pins 1, 5 and 8 are not used and should be disconnected.


Saturday, September 20, 2008

Marcos Di Paolo uses a 303 with Dada's High-end boards

Marcos is one more professional musician who has chosen Dada's High-end boards in his studio for their excellent transparency and natural sound.

Marcos was born in Argentina and lives in Rotterdam today. He studies the guitar for more than 25 years and is now working as a full-time professional musician in several projects. See his website

He writes about his new High-end boards:

"And the most important,'s really a pleasure listening music with this amp!!, I've never heard a high freq range so clear like I'm listening at this very moment. I'm really happy with the sound of my new Quad, thanks a lot!!!."


Friday, September 19, 2008

Current Limiter in the Quad 405 and other limiters.

We decided to implement the 405-II current-limiters on request only. We have built the limiters with normal components. If you insist on having the limiters on your boards, ask for a quotation.

Why don’t we implement the limiter in our High-end boards?

It affects the sound quality, not only because of the topology of the limiter itself but also the way Quad implement it. In a 405, but still in the 909, the current trough the sensing resistor modulates the voltage of the positive power rail of the class A stage and the Opamp.

The Opamp is the least of our worries, because it has a high power supply ripple rejection ratio.

The modulation is done by a dirty half wave class C signal from the upper dumper transistor. In the original 1975 days this was not such a problem, noise, crosstalk and other distortions masked this problem. But in search for the highest quality, the limiter has to go. Also the current sensing resistors can now be short circuited.

The right way to implement a limiter with this topology: (This is a 405-2 example, the 405-1 is less complex)
The limiter in the negative part of the output circuit can stay as it is, it is part of the dumper section, the class A stage will compensate for errors via the current dumping principle!

Input limit is the role of the Opamp.

In the first production version of the 405 the power supply voltage of the Opamp was plus and minus 12 V. Also due to the DC stabilisation there was a DC offset at the output of the Opamp of 1.7 V. This means the theoretical input voltage of the Opamp stage is 486mV max, conclusion; with 500mV input no 100W in 8 ohm undistorted output power was possible!

The PSU voltage went up to 15V and the DC offset was lowered to 1.2V, still the headroom is only 2.3dB for undistorted output. So a popular en reasonable update/mod is raising the PSU voltage and selecting the proper Opamp. An OPA604 running on 24V gives a healthy 6dB plus headroom in the OpAmp stage.

The rest? Calculation shows that the maximum theoretical input voltage needed to let the dumper stage clip is only 1,9 dB above the nominal input, so the figure of 2,3dB implemented by Quad looks sufficient. So be careful with the input level, there is only a small overload margin.

Output limit.

The output can be limited to 20V by inserting a link in the OpAmp circuit; this was done to protect the ESL57 from overloading. We do not advice this option, this is not a soft clipping device, and feeding 20V square waves in to a fragile ESL57 is not a good thing to do. Buy a refurbished 303 or Quad II to drive the ESL57. Clipping of a 303 and an II is still possible, but only with high input levels.

DC detection and protection.

A low pass filter is placed in parallel with the output, when the DC voltage is about 6 volt, the Diac opens and the Triac will short circuit the output. Due to the characteristic of the Diac the output stays in short circuit mode, hence the name ”Clamp circuit”. It was the intention that the fuses would be blown, in practice also the output transistors will be thermically killed! But there was a problem anyway with the circuit. Low voltage Diac’s are hard to get in these days.
Dada Electronics' own DC protection circuit is relay based and has a switch on delay function built in to it. See Ebay-object 200243187533.


Wednesday, September 17, 2008

Dada's 405 High-end boards

We had our new High-end boards photographed by

Click on the pictures to enlarge.