HI-FI NEWS   DRD300B Review

Take a look at DIY internet bulletin boards and you'll find that interest in valves is being maintained by a considerable hardcore of enthusiasts, but there is also new blood joining this group. A great way to get to know about valve technology is to build a valve amp.  You can learn something and save money!

As with any 3008 single-ended triode design, the Welborne Labs 300B DRD kit reviewed here needs to be used with suitable loudspeakers. there's more to this than hooking it up to a transducer with 90dB sensitivity or more: to match well a speaker also needs a relatively flat impedance curve and a non-complex crossover.

If your room is small, speakers with sensitivity ratings below 90dB can be acceptable, but SETs distort much less at low outputs than when driven towards their limit. With a 300B, you really want to be doing most of your listening with the first watt; the further up the power scale you go, the less SET magic you will experience.  For much of this review I used Adire Audio HE10.1 kit speakers, a reflex-loaded design using a 10in. paper cone bass/mid with a coaxially mounted compression tweeter.   Sensitivity is 95.5dB and impedance is nominally 8 ohms with a 6.5 ohm minimum;  these speakers are SET friendly.

Ron Welborne offers two output valve choices, so there are 300B and 45-based kits.  The 300B version here outputs 7 watts per channel with zero negative feedback and it's all Class A.  It has an input impedance of 100kohms and an input sensitivity of 1.5V.  The 45 amp is similar but with 1.8W output - so really efficient (sensitive) speakers would be required here.

WHAT IS A DRD?
These figures do little to tell you what the DRD is all about.  DRD stands for Direct Reactance Drive.  Jack Elliano of Electra-Print developed the DRD circuit a few years ago.  The original design pumped out 12W from a 300B valve, where 8W or 9W is more typical.  But Ron Welborne prefers the sound with the DRD circuit working a different operating point and delivering 7W.  The 300B then runs at a plate (anode) dissipation of just 20W, which is really quite gentle.  Several options are available.  The kits can be purchased without valves, and with valve or solid-state rectification, while other options include Teflon valve sockets, silver wire and Caddock resistors.  Welborne says he prefers the sound of the solid-state rectification, so I went this route and added the Caddocks and Teflon valve sockets.

Prices start from $1010 for a 'nude' kit, that is without chassis or valves.  The basic complete kit is $1540, with Alder wood chassis and TJ meshplate premium 300B valves.  To this you need to add $240 for shipping to the UK and then import duty/VAT.

If a lack of time or skills means that you need a fully assembled pair of amps, you can have this for an extra $350.  But I strongly urge you to have a go at DIY with the DRD.  If you can put up shelves, change a tap washer or assemble flat pack furniture then you can easily learn to solder with a low-cost kit from Maplin and be able to stitch together a DRD. 

There are two particularly interesting circuit features.  One is the 'Ultrapath' connection which takes the power supply to the 300B cathode via an ASC oil capacitor.  This is said to change the way the circuit 'sees' the power supply, reducing the latter's importance to sonic performance.  That's not to say the power supply is downgraded; it isn't.  It uses ultrafast recovery diodes for the HT (B+), each having a snubber network comprising a capacitor and resistor.  There follows a film capacitor, making this a traditional capacitor-choke-capacitor (CLC) power supply.

The DRD uses a single choke-loaded driver stage (this is a Sovtek 6N1P dual triode valve with the two halves running in parallel) but the second unusual aspect of this circuit is that it does away with the coupling capacitor normally found between driver and power valve.

There's also another chunk of iron in the choke for the 300B heater supply, which uses Schottky diodes.  Generally, the more iron the better so this looks good.  What's more, deleting the coupling capacitor takes away the component that causes most grief when trying to voice an amp.  Even the cathode bypass capacitor for the power valve is dispensed with.  In line with its heritage, the DRD uses ElectraPrint output transformers.

START BUILDING
Once you've unpacked all the parts it's time to peruse the instructions.  These are truly excellent, step-by-step descriptions, with clear colour diagrams for each stage of assembly.  One challenge is that the left and right monoblocks are mirror images of each other.  Most of the diagrams show only one channel, so you need to perform lateral transformations in your head for the opposite amp.  But this isn't as tricky as it sounds.
 

One of the first connection tasks is to fit the valve sockets and turret boards, which enable point-to-point wiring.  When you get to wiring up the Teflon driver socket, take care when attaching the AC heater wiring; the stranded wires are thick, and you must make sure no strands touch the adjacent socket pins.  There are quite a few wires to attach;  the hookup wire is solid-core OFC and Teflon covered.  Solder is provided.

When wiring the power and output transformers you'll need to select your mains voltage from 100V, 120V, 220V or 240V and the required speaker impedance setting from 2, 4, 8 or 16 ohms.  Unused transformer wires need to be cut short and insulated with either adhesive sticky PVC tape or some heatshrink tubing.

When you get to the resistors, capacitors and diodes, there is little unusual save for the size of the leadouts of the 300B heater schottky diodes.  These take a fair bit of heating, and here it pays to have a powerful 40W or 50W temperature-controlled soldering iron.  With only a 20W iron you will have to heat those joints for a long time.  When you come to the wiring under the two power supply electrolytics, remove the capacitors rather than risk melting components with the side of your soldering iron element.

THE MOMENT OF TRUTH
I use a dummy load for the first power-up, then some old speakers if the voltages checkout.  I run these about 20 minutes with music before connecting to my main speakers.

When the amps are switched on, there is a hum for a few seconds before everything settles down. The next thing to notice is that there is absolutely minimal hum, the solid-state rectification and ground scheme do their job very well indeed.  These are really quiet amps;  I'd have no qualms about hum when using efficient horn speakers.

An initial listen with just 20 minutes running revealed a harsh sound that I trusted would vanish.  It did.  The ironware and capacitors need around 10 hours to get close to their potential, 100 hours and you're just about there.  Most of the time I used the TJ meshplate 300Bs but I also used Western Electric, JJ, EH Gold Grid, Svetlana and Valve Art Tubes.  Not one valve sounded poor; indeed the DRD possesses a very good balance between being revealing and smooth.  I believe the lack of capacitors in the signal path, excepting arguably the Ultrapath capacitor, gives great detail resolution without the downside of any tiring artifacts.

COMPATIBILITY
To check compatibility I tried out my Mordaunt Short Performance 860 floorstanders, which have a sensitivity of 90dB with a 6 ohm nominal impedance (minimum 3.8 ohm).  The sound was loud enough but the synergy was far from perfect; the 95dB Adire was a far better match.  With the MS floorstanders, the system sounded more like a typical music system, and it had little of the SET magic; and with just 7W on tap it was all too easy to overdrive the amps.  My room is 18 x 15 ft, but with the Adire speakers, power was not an issue.

For a pre-amp I mainly uses an S&B-based transformer/volume control; these TX102 transformers simply let so much music through.  In my system there was just enough gain to allow me to use my passive transformer setup with CD and a typical phono stage.  But if you need to be sure you have enough gain for all circumstances you may want a pre-amp with a few dB of gain (I also use a Transcendent Sound Grounded Grid pre-amp with 12dB of gain).  This is a tough call and depends on source output level, room size and how loud you listen.  Welborne has a battery powered dual-mono Ultrapath pre-amp with 10dB gain, which ought to be ideal for the DRD.
 

My first priority for a SET amp is that it should deliver a magical soundstage, openness and sense of space.  There is no question that the Welborne Labs 300B DRD does this.  In the mid it's slightly darker and even more soulful sounding than some other 300B SETs that I'm familiar with.  I found the top end well extended with a good clarity.  No complaints about the bass either, as you'd hope with solid-state rectification.  If you live on a diet of bass-heavy music, then maybe a SET amp is not your best option anyway.  The DRD does do bass, New Order's 'Blue Monday' was rendered powerfully; sure 200W of solid-state power would produce a more challenging effect but the DRD did not disgrace itself.  DRD bass says more about the shape of the notes versus the absolute drive of the solid state.

Does the DRD prefer small-scale music?  Well, if it does, it could be more to do with small-scale music often being simply better recorded, less processed and therefore retaining more atmosphere.  But the DRD does not fall apart with more complex material.

The DRD is an excellent example of a well-developed SET design, sounding wonderfully smooth, atmospheric and detailed.  Ultimate output power is a little lower than is typical for a 300B, but anyone listening to a SET amp at near max power is using the wrong speakers anyway.  Partner with sensible speakers, and enjoy.

Clive Meakins  Hi-Fi News

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