In exclusive partnership with top-tier modification house Parts ConneXion, Underwood HiFi is proud to offer the following carefully tested, factory-authorized performance enhancement packages for the Quad QC-24 and Quad 909.

By virtue of selling consumer-direct, we can offer these super-charged, brand-new models for a better price than the same modifications would cost retro-fitted - though we offer that, too.

The Quad 909 power amplifier is a wonderful 140wpc power amp (250 watts into 4 ohms), with a midrange unsurpassed in its price range which The Abso!ute Sound review compares to $4000 tube amps. The 909 uses a unique technology called current dumping which has a very high-quality but small class A amplifier driving a high-current efficient class B amplifier. The result is the sound of class A with the power of class B. Again the midrange performance is superb. Bass, treble and resolution are good for the price but not exceptional. In addition we have many complaints about the speaker binding posts. Our Level-2 mod turns the 909 into a superb amp with bass and treble on a par with its superb midrange. We also upgrade the binding posts to quality Cardas posts. Our mod was designed by Chris Johnson, founder and former president of Sonic Frontiers.
Sonic improvements with the Level-1 are:
• Bass extension and midbass slam are improved.
• Treble detail both micro and macro and dynamics are improved and now keep up with anything we have heard under $3K.
• Imaging is more focused while the entire soundstage is both deeper and wider.
• The upgraded binding posts now accept spades as well as bananas and bare wire.
• Transparency is improved , a fine layer of grunge removed. You get a cleaner look into the musical soundstage.
• The amp is quieter to resolve more low level detail and dimensionality.
The following details our Level-1 mod [$750 + freight retrofit; $1.890 + freight preinstalled]:
• 1 x dual configured SMT-style Burr-Brown OPA-627 op amp on a custom adapter module for the input/driver stage
• 2 x Multicap PPMFX metalized polypropylene input signal coupling capacitors
• 6 x Nichicon audio grade KZ (Muse) and FG (Fine Gold) electrolytic power supply capacitors
•4 x EAR small” compliant Sorbothane isolation feet h
• 1 x set (two dual) Cardas CCGR-Short Rhodium over Tellurium copper binding posts
• Ten feet of WBT solder
• 4.5 hours labor. The mod is quite labor intensive as the main PC board has to be totally removed to complete much of the above work.
Sonic improvements with the Level-2 are:
• Bass response and dynamics are dramatically improved.
• Treble detail both micro and macro is improved and now keeps up with amps selling for up to $5K.
• Imaging is more focused while the entire soundstage is both deeper and wider.
• The L-2 modified amp will now compare to $4.000 - $5.000 amps at a fraction of their cost. We kept the beautiful midrange and added the dynamics, bass and treble extension and transparency of much more expensive products.
The following details our Level-2 mod [$1.190 + freight retrofit, $2.190 + freight preinstalled]:
• 2 x dual configured SMT-style Burr-Brown OPA-627 op amp on a custom adapter module for input & driver stages
• 1 x pair of premium Vampire direct gold-plated over copper body” RCA input jacks
• 1 x set (two dual) Cardas CCGR short Rhodium over Tellurium copper binding posts
• 2 x Multicap PPMFX metalized polypropylene input signal” coupling capacitors
• 4 x Amtrans AMRG film & foil polypropylene feedback capacitors with gold-plated OFHC leads
• 12 x Nichicon audio-grade KZ (Muse) and FG (Fine Gold) electrolytic power supply capacitors
• 6 x Takman REX series carbon film signal series resistors
• 4 x EAR small compliant Sorbothane isolation feet
• 10 x feet WBT Solder used throughout
• 7 hours labor

These modified Quad 909 amps come with a 1-year parts and labor warranty.
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The Quad QC-24 is a wonderful $1.000 tube preamplifier from one of longest-lived, most famous names in High-End audio. It is a Golden Ear Award winner in The Abso!ute Sound. Our mod takes it to the next level and allows it to compete with anything under $2.500.
 
 
 
Sonic changes of our Level-1 mod
• Lower S/N ratio with blacker silences, better low-level detail and 3D ambiance retrieval
• Smoother yet more detailed presentation, better macro dynamics
• It seems to play louder, with improved microdynamic contrasts
• Tonal balance is more coherent from top to bottom
• In addition, the midrange is a little more liquid than the stock machine's
• Imaging is more defined, soundstage depth is enhanced
• Bass extension and impact are dramatically improved, weight and authority built out, sounding more like solid-state in this area
QC-24 Level-1 mod [$1,490 +freight, $600 retrofit]:
• 4 x small EAR brand Sorbothane isolation feet
• 2 x sheets of SoundCoat chassis damping material
• 3 x pairs of Vampire CM-1F/CB copper body, direct gold-plated RCA input jacks
• 4 x Auricap signal path, metalized polypropylene coupling capacitors
• 4 x MultiCap RTX Series polystyrene and tin-foil coupling bypass capacitors
• 4 x Film Cap power-supply bypass caps
•4 x Black Gate coupling bypass and power-supply capacitors
• 8 x Riken Ohm ½ and one watt signal path 1% carbon comp. resistors (gold-plated leads)
• DH Labs Revelation Series pure 99.99% silver Teflon-insulated input/output wire
• Fou4 x IR Hexfred ultra-fast soft recovery diodes
• TRT WonderSolder used throughout
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