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Direct Reactance Drive (DRD)
SET Monoblocks
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Read the Enjoy The Music.com Review Here |
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The DRD Design
The circuit topology of the DRD amp is very unique as far as single-ended amplifiers
go. It consists of an Ultrapath output stage whereby the return path of the output transformer connects to the output tube's cathode instead of
the power supply. This topology results in greater detail and it is believed this is a result of better coupling between all parts of the circuit
and less interaction with the power supply components.
The choke-loaded driver stage allows better gain and a larger voltage swing than a
resistively coupled driver, and the choke also provides a lower DCR path to ground for the output tube's grid. Because the circuitry is so interactively
coupled, all of the components ride together with external voltage fluctuations thus minimizing shifts in bias and thus resulting in improved stability.
The circuit is just plain simple too, which means there are very few components in the signal path.
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As with all Welborne Labs kits, you receive a well made nice looking product. The DRD amps are comprised of solid Alder hardwood chassis with
0.09" thick anodized aluminum top plates and 0.06" thick powder-coated aluminum bottom plates. Another aluminum plate on the rear of each
amplifier provides access for all of the external connections including: input, output, on-off switch plus an IEC connector with detachable power cord.
The input and output connections are via high quality Cardas Audio connectors. All internal wiring is solid core silver plated and Teflon coated
OFC hookup wire. Rugged high quality turret boards are setup inside the chassis to facilitate easy point-to-point wiring.
The Direct Reactance Drive Amplifier - low volt version
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6N1P driver tube shown with TJ meshplate 300B |
All versions of the DRD amplifiers are stocked with the renowned Electra-Print output transformers. Our power transformers are custom made to
Welborne Labs' exact specifications and specifically designed to operate quiet and cool for extended periods of time. These transformers aren't
the wimpy things you find on other kits that buzz and run hot to the touch....these power trannys are massive weighing in at over 12 lbs. each with
additional laminations incorporated to operate vibration free even at 50Hz. These power transformers are designed to operate at all voltages: 100,
110, 115, 230 and 240V at both 50 and 60 cycles.Various option packages are outlined on the following pages
including the choice of solid-state versus tube rectification 45 or 300B...take your pick.
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All kits include good quality components like JJ-Elna-Xicon capacitors, ASC Oil Ultrapath capacitors, Cardas binding posts and rca jacks, solid
core Teflon coated hookup wire, Dale metal film resistors, Hammond PS chokes and ElectraPrint plate chokes, IEC power connector with detachable power
cord, tube sockets, ElectraPrint output transformers, Sovtek 6N1P driver tubes and Welborne Labs solder.
Available parts upgrades include Caddock foil resistors, silver wire and silver ElectraPrint output transformers.
I have spent many hours prototyping, listening and voicing both the 300B and 45 versions. The
component "types" and "values" have been selected and optimized for the best sound, maximum output and greatest bandwidth from each tube.
Power output of the 300B DRD is approximately 7 watts and the 45 DRD is 1.8 watts. 7 watts will
drive many speakers in the 90dB sensitivity range. If you opt for the 45 DRD then get yourself a set of high efficiency horns.
The 300B DRD design utilizes ac filaments for the 6N1P and dc filaments for the 300B. The dc
filaments on the 300B provides a super low level of hum (which is important) and this amplifier typically measures well below 0.5 millivolt of hum and
noise. The 45 DRD design utilizes all ac filaments and is extremely quiet too typically measuring below 0.5 millivolts too!
Input sensitivity for both models is approximately 1.5 volts so you can easily drive these amplifiers
with either an active or passive linestage.
The -3dB frequency response of both the 300B DRD and 45 DRD is approximately 10Hz to 48KHz.
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The SoundOK, let's start with the million dollar question: tube versus solid-state
rectification. The theory....tube rectification is quieter than SS rectification because the tube rectifiers do not generate switching noise.
The switching noise generated by SS rectifiers (at least in SET amplifiers) seems to add a slight characteristic hardness, or brittleness, to the sound.
The reality: with the use of ultrafast diodes or schottky diodes much of this switching noise is
reduced. Include a well designed snubber network across the diodes and provide adequate filtering in the power supply and the majority of this
switching noise is eliminated.
The benefit of SS rectification? Benefit#1: It's simpler and less expensive to implement.
Benefit#2: SS diodes don't limit the dynamic current like tube rectifiers do, so with SS rectification the low frequency response of the amplifier will
be more dynamic. Benefit #3: without the tube rectifier you have less current being pulled through the power transformer so it will run
cooler and generate less EMF. Benefit #4: the audio signal of the output stage does not pass through the power supply (remember? DRD design) so it is
less likely to be affected by power supply quality....thus the reason I've left the Black Gate power supply caps out of the Ultimate version.
"Whoa!!! hold on there Ron, sounds like you prefer the SS rectification".....YEP, in the DRD amp I do!
45 versus 300B. This debate is pretty much a badly beaten horse in my opinion. Everyone has their preference and opinions as to which
tube sounds better. Suffice it to say; the DRD designs will not get in the way of your love affair with your favorite tube. The 45 DRD
sounds like a really good 45 amplifier should and the 300B DRD sounds like a really good 300B amplifier should. Just remember to select your
amplifier based upon the speakers you'll be using and don't come crying to me when your DRD 45 amp just doesn't sound right on your Proacs, Quads,
Wilson Watt Puppies, etc.
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The DRD 300B gets reviewed in
Hi-Fi News magazine.
Click on the picture for the review |
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Take a look at the inside layout of the DRD amp.
Is this the cleanest layout you've ever seen? Harvey would be proud of me.
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** The Direct Reactance Drive amp design was first published in Vacuum Tube Valley #14 |
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| OK, with the details out of the way.....pick you flavor
below; 300B? or the ultimate flea power 45.
If the TERRAPLANE or 45 STAR CHIEF versions are a little out of your price range, don't get hung up
over this, go with the DRD300B or DRD45! With the Direct Reactance Drive design and the component quality of our products, you will be
hard-pressed to find a better sounding SET amplifier today. |
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| DRD300B |
DRD45 |
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TERRAPLANE 300B |
45 STAR CHIEF |
