A Naim pre-amp PSU using DC-DC converters
best sounds I've ever heard from the Naim pre-amps was when
powered by six autonomous switch mode slabs. These are 125W DC to
DC converters made by Coutant, but they do cost over 200 UKP each.
This combination makes the Nac 52 plus Supercap sound
The quote above was the inspiration for this project; that and another long running project to build a Nelson Pass A75 power amplifier http://www.passlabs.com/a75prt1.htm, which had got hung up at the building of the power supply. The use of DC-DC converters suggested an off the shelf approach that could be quicker build and still sound good. I decided to build a prototype as a proof of concept to power a Naim pre-amp. The existing HiCap psu would be used to provide the unregulated DC with DC-DC converters replacing the linear regulators used in the HiCap.
Theory of operation
DC-DC converters take DC at one voltage and convert it to DC at another voltage (or a number of voltages, depending on the converter used). The technology is similar to that used in switched mode power supplies which convert AC to DC and are found increasingly in a wide variety of electrical equipment e.g. the power supplies of personal computers. These devices work by generating a high frequency AC (hundreds of kHz and above) which is then transformed, rectified and smoothed much as a conventional supply does. The use of high frequency allows the transformers, inductors and capacitors used to be much smaller in size and value than those required at the mains frequencies of 50 or 60 Hz yet achieve the same effect. The worry for Audiophiles is the use of such high frequencies, their entry into the audio system and their consequent deleterious effects.
The Naim HiCap is a psu using linear regulation to generate 24V. A cable connects the HiCap to a Naim pre-amp. An existing Naim HiCap unit was used as a source which was rewired to make the unregulated DC available on one of the unused DIN sockets. The DC-DC converter unit was housed externally and plugged into this socket with the 24V it generated fed back to the same socket. Further rewiring in the HiCap made the 24V available on a second socket using the same pin layout as the existing linearly regulated 24V. The feed to the Naim pre-amp could be changed from linear regulation to DC-DC converter regulation by unplugging from one DIN socket and reconnecting to the adjacent socket.
The DC-DC Converters
The DC-DC converters used were obtained as sale units from Amplicon http://www.amplicon.co.uk/power.html . A unit with +- 12V supplies was chosen to give a 24V 4A supply with an input voltage range of 18-36V. Two units were used, one for each stereo channel. As recommended in the application notes available at the Amplicon site additional output filtering was used comprising a CMC in a CLC arrangement. The chokes were 60uH, 3.3A, 0.05ohms from Farnell. The capacitors used were 10uF tantalum capacitors which had been removed from a Naim 42.5 amp in a previous updating project. The figure below shows the units used and their pinout.
The additional parts required are:
The circuit diagram of the output filter is:
and the layout of the HiCap sockets is:
The completed unit is shown below. As it's a prototype, it is held together with cable ties with milk bottle tops used as spacers between the two converters.
I have not put a scope on the PSU but I can say that the difference in output voltage between left and right channels is 0.1V in contrast with the linear PSU where the channels are 0.5V apart.
Let me first say that listening has been limited, only a few sessions at the time of writing, so only about an hour or two of listening. However I can say that the two units (the original linear regulation and the DC-DC regulation) do sound different and that the converter regulation does not appear to have the nasties that had been feared.
To put my opinions on the sound differences into perspective first let me describe the system. A Linn LP12 turntable with Avondale PSU, subchassis and bearing. A Wilson Benesch ACT-1 arm and Lyra Clavis DC cartridge. The pre-amp is a tweaked Naim 42.5 and listening was via Audia Technica ATH-7 headphones. For many years my power amp has been a Quantum 204 transistor amplifier of 1980 vintage but for the last few months it has been a recently built Electronic Tonalities Paraglow valve amplifier http://www.bottlehead.com.
I'd describe the difference between the linear PSU and the converter PSU as similar to the difference between the valve amplifier and the transistor amp. I have described elsewhere the change in sound moving from the Quantum to the Paraglow amplifier, a loss of air, a movement of the image, backwards, away from the listener and an integration of the individual sounds. I remember comparing the Quantum with a friends Naim power amplifier (it was not a 250 model). We both agreed that the sound from the Naim was more integrated while with the Quantum the instruments were more individual. My friend who is rather more forward than I advocated the integrated sound-stage to be better while both I and my wife when conferring later agreed that we liked being able to pick out and concentrate on individual instruments.
When compared with the linear PSU the DC-DC converter PSU moves away from the sound introduced by the Paraglow amp and back towards the transistor sound, the sound moves forward and is less integrated, i.e you are more able to concentrate on individual instruments. Further opinions will have to wait till I manage to get someone else to do a comparison or I do further tests, perhaps with CD and loudspeakers. However in summary, I have to say I'm quite happy with the DC-DC converter PSU; it moves my system towards the sound I'm looking for, so for me it works well though it may not suit every system.
I'm not yet prepared to say, hand on heart, that the DC-DC converter PSU is better that the linear PSU but I am prepared to believe it is, and will be looking further proof. I think that as a proof of concept the project has been a success and I will be building a PSU for the Pass A75 amplifier using DC-DC converters.
The cost of the project is dictated by the cost of the converters, which at sale price was £32 + VAT each, however list price is 3x higher.
A - amps, units of current
© Neil (J Mackie) November 2000.