Saturday, March 14, 2009

Quad 606 Transformer Swap

François' (Besancon, France) Quad 606-1 started making a rumbling sound....

Some QUAD 606 MK1's start to make a rumbling sound after some years - from the amplifier itself, not through the speakers.

This means that the transformer is worn out and needs replacement (we know the same problem from the Quad 405 where we can replace the old transformer with a Toroïdal version).

The problem with the 606-1 is that a 400VA 2x 80V Toroïdal transformer won't fit as the diameter is much too large. Quad uses a Toroidal transformer in the 606 MK2, the 707 and the 909 but they developed a special PCB for it that wont fit the 606-1 chassis.

The only alternative seemed to order a "New Old Stock" "classic" replacement transformer for the 606-1 but this appeared to be very expensive and we don't like the quality of this transformer neither.

So we thought about using 2x 250VA 2x 40V Toroïdal transformers. This offers the advantages of extra power headroom (500 instead of 400VA), (even) better channel-separation as we use two different transformers and the possibility to use larger PSU capacitors.

We decided to use 2 Noratel 250VA 2x 40 Volt transformers. They are 50mm high and 120mm diameter.

But how to build them into the Chassis? Chris Devrieze has been puzzling with the question during several days and came up with a very good solution.
  • We remove a part of the old transformer mounting-frame on the bottom of the (upside-down) chassis so the new transformers will fit in between

  • As we can't mount the capacitors and the diode's on top of the transformers anymore - there is no space - we glue them with 2-component Epoxy on both sides of the transformers

  • Both 115V primary windings are soldered in series (for Europe) or in parallel (for the USA & Canada) and the secondary windings are soldered in series to obtain 80V

  • We drill a hole in the top cover-plate and we use a M6 x 120mm screw to fix both transformers to it after connecting all the wires. We put 4 rubber feet on the bottom to avoid all movement of the transformers once they are fixed

  • We mount the cover-plate with both transformers to the chassis with M5 x 40mm screws in the four corners

Of course we also did a full revision of the amplifier-boards to bring the 606 to a Quad 909 level and we replaced the connectors.

Click on the pictures to enlarge.

Dada Electronics will offer the 606-1 transformer swap as an option to the 606 revision service and will sell the Noratel 250VA transformers in the Ebay-shop.

Any questions?

Stefaan, Chris

Wednesday, March 04, 2009


Most of the hum problems are related to “earth” loops in external cabling systems.
Input "hum" can arise from small ground potential differences between signal inputs and outputs.
The grounds between amplifiers are established by the AC outlets, and through signal cables. Various factors can cause small ground potential differences between the sound input (power amp) and output (pre-amplifier).
A one-volt difference, while seemingly insignificant, can cause large loop currents to flow through the ground conductor of the signal cable. Depending on the circuit configuration of the audio hardware, the audio signal impedance and the impedance of the signal cable ground, the currents may cause induced voltages in the audio circuitry.

Source: Channel D

This is a source of the familiar 'hum' problem.
In the “old” days, like the Quad 33 and 303 combination, the 303 has no separated ground connection. Grounding was done trough the signal cable between the 33 and 303.
The advantage of such a “star” system: no hum loops. With modern safety regulations such a star system is highly illegal in most countries, and more important, not safe.
In the multi channel (Stereo) world the problems are even more pregnant. Also in most cases the aerial connection of the tuner is also connected to “ground”. In professional installations signal transport is isolated from ground connections (balanced connections); in normal audio systems engineers mix signal ground and chassis ground.

How to avoid hum loops?

Use only one wall socket for the mains connection, if possible. Use the preamp and the associated cabling in the “star” configuration. Separate the safety ground and the signal ground in the power amp, so the chassis is connected to the safety ground of the wall socket, but not connected to the signal ground of the preamp. The same recipe for other equipment connected to the preamp (not always possible!).
General advice; use cables as short as possible. Use shielded cables for the power cables. In some cases, a so called “isolator” is unavoidable; first test one in the aerial connection of the tuner. The tuner signal is FM modulated, so an isolator does less (no) harm to the signal quality. In RCA cables a small signal transformer play this role. Don’t spend a lot of money on special cables and end up with a cheap isolator in between.
In some cases increasing the resistance of the shielding of the signal cable has a positive effect, apply this only if the amplifiers have a proper installed safety earth connection!

How to detect and solve hum loops?

In general; a hum loop generates hum independent of the volume settings. If the hum is dependent on the volume setting; the hum is equipment borne in most cases. But the hum could also be a combination of both situations! The elimination of a hum loop is a painstaking process. Trial and error and applying logical thinking is the best solution.
Start with only the power amp and the loudspeakers, no signal cables connected. Then rebuild the configuration item for item. A cable is also an item! Once the source of the hum is known, a measure can be determined.


In a Quad 405 and 303 the chassis ground can be separated from the signal ground. In both amplifiers they are bonded together at the DIN input chassis part. In a 405 with factory fitted RCA inputs the two grounds are already separated. The same applies for a 606 and the higher numbers. In Quad tuners it is difficult to perform this trick, but in most cases the hum loop is caused by the aerial connection. A coaxial cable isolator will fix this problem. For testing and analysis it is acceptable to use a non earthed power extension cable for detecting the source of the hum, but always restore the safety earth connection to the chassis afterwards for your own safety! So you should *either* have 3-core mains leads from every piece of equipment *and* signal cables with the shielding disconnected at *one* end, if separation from the signal ground and chassis ground is not possible, or else a single mains connection and everything earthed via the signal leads, as in the case of a 33. Read also the comments before you change anything to your equipment!

A Jensen coax cable isolator

A simple DIY HF isolator!

Joost Plugge

Labels: , , , ,