Audio T Verdict
The QUAD QII-Forty proves that choosing a valve amplifier doesn't have to mean settling for single-figure power outputs or limiting yourself to extremely sensitive loudspeakers. Each monoblock delivers 40W RMS from a pair of KT88 output valves, giving one dedicated amplifier to each channel in a conventional stereo system.
There is genuine QUAD heritage here, but this isn't simply an old circuit reproduced for nostalgia. The QII-Forty was developed as a modern evolution of Peter Walker's original QUAD II philosophy, using a revised circuit, modern components and newly engineered transformers to provide substantially greater power and compatibility with contemporary loudspeakers.
Audio T Manchester has used the QII-Forty with Dynaudio Contour 30 and Contour 60 loudspeakers and was particularly struck by the sense of drive, bandwidth and bass extension. If you've dismissed valve amplification as inevitably soft, warm or underpowered, these are worth hearing before making up your mind.
QUAD QII-Forty Mono Valve Power Amplifiers
The QUAD QII-Forty is a dedicated valve monoblock power amplifier inspired by one of the most significant products in QUAD's history, but engineered to meet the demands of considerably more modern loudspeakers.
Audio T supplies the QII-Forty here as a stereo pair. One monoblock drives the left loudspeaker and the second drives the right, giving each stereo channel its own amplifier chassis, power supply, valve complement and output transformer.
- Supplied as a stereo pair of monoblock power amplifiers
- One dedicated amplifier per loudspeaker channel
- 40W RMS output from each monoblock
- Tim de Paravicini QII Series circuit design
- Inspired by Peter Walker's original QUAD II philosophy
- Two KT88 output valves per monoblock
- Push-pull KT88 output configuration
- Two 6SH7 input valves per monoblock
- One 5U4G rectifier valve per monoblock
- New-generation output transformer design
- 4-ohm loudspeaker tap
- 8-ohm loudspeaker tap
- 1V RMS input sensitivity
- 10Hz to 30kHz frequency response
- 0.03% THD at 1W / 700Hz
- 0.3% THD at 40W / 700Hz
- Damping factor of 20 at 700Hz
- Residual hum and noise better than -80dB
- 180VA maximum power consumption per monoblock
Important: These Are Monoblock Power Amplifiers
Each QII-Forty contains one amplifier channel. A conventional stereo system therefore requires two QII-Forty amplifiers, and this Audio T product is supplied as that stereo pair.
The amplifiers do not contain volume controls or source selection, so they require a suitable preamplifier or another source component providing a properly implemented variable analogue output.
The QUAD II Heritage
The original QUAD II emerged in the 1950s and became one of the defining amplifiers of Peter Walker's career.
Its significance was not simply that it used valves. The design reflected Walker's broader philosophy that an amplifier should interfere as little as possible with the signal it was asked to reproduce.
The QII-Forty belongs to the later QII family developed to preserve that philosophy while increasing the power and practical loudspeaker compatibility available from the basic concept.
Designed by Tim de Paravicini
The modern QUAD II Series circuit was developed by renowned valve-amplifier designer Tim de Paravicini.
His work retained the fundamental simplicity and cathode-loading philosophy associated with Peter Walker's original output stage while developing the circuit, components and transformer design for a new generation.
The result is not intended to be a component-for-component replica of the original QUAD II. That role belongs more closely to the QII-Classic.
A Modern Development, Not Simply a Reissue
QUAD is explicit that the QII-Forty circuit topology differs somewhat from the original.
The amplifier uses significantly newer components and a newly engineered transformer design, allowing the power output to increase to 40W while retaining the fundamental valve-amplifier principles of the QII family.
40W RMS From Each Monoblock
Each QII-Forty delivers 40W RMS.
Because this Audio T product comprises two monoblocks, each loudspeaker receives its own dedicated 40W amplifier rather than two channels sharing a single stereo amplifier enclosure.
What Does 40W Mean With a Valve Amplifier?
As with any amplifier, wattage should not be considered in isolation.
The suitability of the QII-Forty depends on factors including:
- Loudspeaker sensitivity
- Nominal impedance
- Impedance behaviour across frequency
- Listening distance
- Room size
- Desired playback level
A reasonably sensitive loudspeaker in a normal domestic room can require surprisingly little continuous power, while less efficient speakers or very large rooms may demand substantially more.
Don't Choose a Valve Amplifier From Wattage Alone
A 40W valve amplifier and a 40W transistor amplifier may have very different output architectures and interfaces with a loudspeaker. What matters is whether the complete amplifier and speaker combination works properly together. Audio T can demonstrate the QII-Forty with your intended loudspeaker type rather than relying on a single number.
KT88 Output Valves
Each QII-Forty uses two KT88 output valves.
The KT88 is a high-power audio output valve and allows the QII-Forty to develop considerably greater power than the KT66-based QII-Classic while retaining the traditional valve output-transformer architecture.
Across a stereo pair there are therefore four KT88 output valves in total.
Push-Pull Output Configuration
The two KT88 valves in each amplifier operate in a push-pull configuration.
In a push-pull output stage, the output valves work together across opposing portions of the signal waveform before their contribution is combined by the output transformer.
This topology allows substantially greater output than would normally be available from a single-ended arrangement using one comparable output valve.
The Output Transformer Matters
The output transformer is one of the most important parts of any traditional valve power amplifier.
The high-voltage, relatively high-impedance operating conditions of the valves are very different from the low-impedance load presented by a loudspeaker.
The transformer provides the electrical interface between the output valves and the speaker, making its design central to frequency response, distortion, damping and power transfer.
A New Transformer Design
QUAD developed a new transformer design for the QII-Forty rather than simply reusing the transformer specification from the original lower-powered QUAD II.
This is an important distinction because the QII-Forty was intended to offer a genuine increase in usable power and compatibility with modern loudspeakers rather than merely reproduce the original amplifier's historical specification.
Cathode-Loaded QUAD Philosophy
The wider QII Series continues the QUAD principle of cathode loading within the output-transformer arrangement.
This concept forms an important part of the design lineage linking Tim de Paravicini's later QII work with Peter Walker's original approach.
Rather than reducing the QII-Forty to the generic description 'a KT88 valve amp', it is this relationship between the valves, output stage and transformer that gives the amplifier its particular engineering identity.
Two 6SH7 Input Valves
Each monoblock uses two 6SH7 valves in its input circuitry.
These handle the low-level incoming signal before the KT88 power-output section takes responsibility for delivering sufficient energy to the loudspeaker.
5U4G Valve Rectification
Power-supply rectification is handled by a 5U4G rectifier valve in each monoblock.
Unlike a semiconductor rectifier hidden inside the chassis, the QII-Forty retains valve rectification as part of the amplifier's power-supply architecture.
Valve Complement Per Monoblock
Each individual QII-Forty uses:
- 2 x 6SH7 input valves
- 2 x KT88 output valves
- 1 x 5U4G rectifier valve
A complete stereo pair therefore uses ten valves across the two amplifier chassis.
4-Ohm and 8-Ohm Loudspeaker Taps
Each amplifier provides separate 4-ohm and 8-ohm output-transformer taps.
This allows the output transformer to be better matched to different loudspeaker loads.
The appropriate connection should be selected according to the loudspeaker and system rather than assuming that one tap is universally superior.
Which Speaker Tap Should I Use?
An 8-ohm loudspeaker will commonly begin on the 8-ohm tap and a 4-ohm design on the 4-ohm tap, but nominal impedance does not tell the whole story. Real loudspeaker impedance varies with frequency, so where there is any doubt we recommend comparing the manufacturer's speaker specification and, where useful, listening to the appropriate configurations in-store.
Damping Factor of 20
QUAD specifies a damping factor of 20 at 700Hz, referred to the 8-ohm output.
Damping factor describes the relationship between the amplifier's effective output impedance and the loudspeaker load.
It should not be used as a stand-alone sound-quality score, but it is a useful technical characteristic when considering how an amplifier interfaces electrically with a loudspeaker.
Wide 10Hz to 30kHz Frequency Response
The QII-Forty is specified from 10Hz to 30kHz at +0dB/-0.5dB.
That is a particularly wide and tightly controlled response for an output-transformer-coupled valve amplifier.
The specification also illustrates why the design of the output transformers is so important: the transformer must maintain its behaviour across frequencies extending well beyond the conventional audible band.
0.03% THD at 1 Watt
At 700Hz and an output of 1W, QUAD specifies total harmonic distortion at 0.03%.
This is a useful measurement because much domestic listening takes place at average power levels far below an amplifier's maximum rating, even when short musical peaks demand considerably more.
0.3% THD at Full 40W Output
At the full rated output of 40W and the same 700Hz test frequency, QUAD specifies 0.3% THD.
Publishing both the low-power and full-output figures gives a much clearer picture than quoting a single distortion number without its operating level.
Residual Hum and Noise Better Than -80dB
Residual hum and noise are specified as better than -80dB.
Keeping hum controlled is particularly important in a valve amplifier because its power supply, heater circuits, transformers and high-gain input stages all coexist within the same design.
1V RMS Input Sensitivity
Full output requires an input signal of approximately 1V RMS.
This makes the input sensitivity an important specification when choosing the preamplifier.
A suitable preamplifier must be capable of providing adequate output voltage while still giving a useful and controllable volume range.
A Separate Preamplifier Is Required
The QII-Forty is purely a power amplifier.
It does not contain:
- Volume control
- Source selection
- Phono stage
- DAC
- Network streaming
- Bluetooth
A suitable variable-output preamplifier is therefore required ahead of the pair.
Do Not Connect an Uncontrolled Fixed-Level Source Directly
A conventional CD player, DAC or streamer with only a fixed full-level analogue output should not be connected directly to the QII-Forty pair unless another component provides safe listening-volume control.
Natural QUAD Preamplifier Partners
The current QUAD QII family includes dedicated valve preamplifier options, while a modern analogue preamplifier from elsewhere in the QUAD range or another manufacturer can also be used when electrically suitable.
The key points are adequate output voltage, sensible gain structure and the quality of the complete source/preamplifier signal path.
True Monoblock Channel Separation
Using two individual amplifier chassis creates complete physical separation between the left and right power-amplifier channels.
Each channel has its own:
- Valve complement
- Power-supply circuitry
- Output transformer
- Loudspeaker output
- Mains connection
This is fundamentally different from a conventional stereo power amplifier containing both channels in one enclosure.
Why Use Monoblocks?
Monoblocks make it possible to isolate the two amplifier channels physically and electrically.
They can also be positioned closer to their respective loudspeakers where the installation allows, potentially shortening speaker-cable runs.
The trade-off is straightforward: two amplifier chassis require more space, two mains connections and more careful installation than a single stereo power amplifier.
Suitable for Modern Loudspeakers
The QII-Forty was specifically developed to deliver more usable power than the original QII concept and QUAD describes it as suitable for modern loudspeakers, including its own electrostatic designs.
That does not mean every loudspeaker is automatically an ideal match.
A 40W amplifier should still be assessed against speaker sensitivity, impedance and required listening level.
QUAD Electrostatic Loudspeakers
The current ESL X generation provides an especially interesting system-matching opportunity because QUAD specifies the ESL 2812X and 2912X around stable amplifiers in an approximately 40W to 100W/8-ohm operating range.
The QII-Forty's 40W rating therefore sits at the lower boundary of that recommended power range.
Room size, listening distance and required level still matter, so we recommend demonstrating the intended combination rather than choosing it purely from the figures.
Valve Power Versus Transistor Power
The QII-Forty should not be approached as though the only decision is whether 40W is 'better' or 'worse' than a transistor amplifier with a larger number printed on its specification sheet.
The output-transformer interface, loudspeaker impedance, amplifier clipping behaviour and complete system gain structure all play a part.
The sensible comparison is between complete amplifier/speaker combinations at the listening level you actually use.
Audio T Listening Experience
Audio T Manchester spent several weeks using a pair of QII-Fortys with Dynaudio Contour 30 and Contour 60 loudspeakers.
The store's listening notes specifically highlighted the amplifiers' bandwidth, bass extension and sense of power and drive, challenging the common assumption that all valve amplifiers must sound soft or lack authority.
You can read the full store experience in our Audio T Manchester QII-Forty blog.
Valve Amplifiers Need Space
The QII-Forty uses exposed high-temperature valve technology and should be installed with sensible free space around each monoblock rather than tightly enclosed between other components.
A stereo pair also needs enough supporting space for two separate power amplifiers and access to two mains connections.
Audio T can help plan the installation as part of a complete system demonstration.
Valve Ownership
Valves are operating components and will eventually require replacement as part of long-term ownership.
When replacement or servicing becomes necessary, use appropriate valve types and have any work beyond straightforward user maintenance carried out through a suitable specialist service route.
The QII-Forty should be treated as high-voltage audio equipment rather than as a product for owner modification.
QII-Forty or QII-Classic?
QII-Classic
- Faithful recreation of original QUAD II philosophy
- 15W RMS per monoblock
- Two KT66 output valves
- 8-ohm output
- Designed around the classic lower-power concept
QII-Forty
- Modern development of QII philosophy
- 40W RMS per monoblock
- Two KT88 output valves
- 4-ohm and 8-ohm taps
- Greater compatibility with modern loudspeakers
QII-Forty or QII-Eighty?
QII-Forty
- 40W RMS monoblock
- 2 x KT88 output valves
- Push-pull output stage
- 4/8-ohm speaker taps
- 180VA maximum consumption
QII-Eighty
- 80W RMS monoblock
- 4 x KT88 output valves
- Two KT88 pairs in push-pull
- 4/8-ohm speaker outputs
- Designed for substantially greater output
The Forty provides the middle ground between the historically faithful lower-powered Classic and the much more powerful Eighty.
Who Is the QII-Forty For?
The QII-Forty makes particular sense for listeners who want:
- A genuine valve power-amplifier system
- More power than traditional low-output valve designs
- Separate monoblock amplification for each channel
- KT88 push-pull output stages
- 4-ohm and 8-ohm speaker matching
- A wide-bandwidth transformer-coupled amplifier
- A modern interpretation of classic QUAD engineering
- A system built around a dedicated preamplifier
- The option to partner suitable contemporary loudspeakers
- The visual and engineering appeal of traditional valve amplification
Key Features
- QUAD QII-Forty
- Supplied as stereo pair
- Dedicated mono power amplifiers
- 40W RMS per monoblock
- Tim de Paravicini circuit design
- Peter Walker QII design philosophy
- Push-pull output architecture
- 2 x KT88 output valves per monoblock
- 2 x 6SH7 input valves per monoblock
- 1 x 5U4G rectifier per monoblock
- New-generation output transformer design
- 4-ohm loudspeaker tap
- 8-ohm loudspeaker tap
- 1V RMS input sensitivity
- 10Hz - 30kHz frequency response
- +0dB / -0.5dB response tolerance
- 0.03% THD at 1W / 700Hz
- 0.3% THD at 40W / 700Hz
- Damping factor 20 at 700Hz
- Residual hum and noise better than -80dB
- 180VA maximum consumption per amplifier
- One amplifier per stereo channel
- Separate power supply per channel
- Separate output transformer per channel
Frequently Asked Questions
Is the QUAD QII-Forty still current?
Yes. QUAD continues to list the QII-Forty in its current QII and power-amplifier ranges.
Is the Audio T price for one amplifier or a pair?
This Audio T product is supplied as a stereo pair of QII-Forty monoblocks.
How many QII-Fortys do I need for stereo?
Two. Each QII-Forty contains one amplifier channel.
How powerful is each amplifier?
40W RMS.
Does that mean a pair is an 80W-per-channel amplifier?
No. Each loudspeaker channel receives 40W from its own dedicated monoblock. The power figures of separate left and right channels are not added together to create a per-channel rating.
Who designed the QII-Forty?
The modern QII Series circuit was designed by Tim de Paravicini, building on the engineering philosophy of Peter Walker's original QUAD II.
Is it identical to the original QUAD II?
No. QUAD states that the circuit topology changed somewhat and that newer components and a new transformer design were developed for the QII-Forty.
Which output valves does it use?
Two KT88 valves per monoblock.
How are the KT88 valves configured?
They operate in push-pull.
Which input valves are used?
Two 6SH7 valves per monoblock.
Does it use valve rectification?
Yes. Each amplifier uses one 5U4G rectifier valve.
How many valves are there in a stereo pair?
Ten in total: four 6SH7 input valves, four KT88 output valves and two 5U4G rectifiers.
Does it have 4-ohm speaker outputs?
Yes. A 4-ohm transformer tap is provided.
Does it have an 8-ohm speaker output?
Yes.
Which speaker tap should I use?
Start with the tap corresponding most closely to the loudspeaker manufacturer's nominal impedance, then consider the actual speaker impedance behaviour and system requirements where appropriate.
What is the input sensitivity?
1V RMS for the manufacturer's rated operating condition.
Does it have a volume control?
No. The QII-Forty is a dedicated power amplifier.
Do I need a preamplifier?
Yes, or another suitable component providing a properly implemented variable analogue output.
Does it have a DAC?
No.
Does it have a phono stage?
No. Turntable functionality belongs in the preamplifier or an external phono stage.
Does it have balanced XLR inputs?
QUAD's current QII-Forty specification does not publish balanced XLR connectivity, so Audio T does not advertise it.
What is the frequency response?
10Hz to 30kHz at +0dB/-0.5dB.
What is the THD at 1W?
0.03% at 700Hz.
What is the THD at full 40W output?
0.3% at 700Hz.
What is the damping factor?
20 at 700Hz, referred to 8 ohms.
What is the residual hum and noise specification?
Better than -80dB.
What is the maximum power consumption?
QUAD specifies 180VA maximum for each QII-Forty monoblock.
Can the QII-Forty drive modern speakers?
Yes, where speaker sensitivity, impedance and required playback level are appropriate. QUAD specifically developed the Forty to provide greater power for contemporary loudspeakers.
Can it be used with QUAD electrostatic speakers?
QUAD specifically identifies its ESL loudspeakers as suitable applications for the QII-Forty. The exact ESL model, room and required playback level should still be considered when matching the system.
What is the difference between QII-Forty and QII-Classic?
The QII-Classic is a lower-powered 15W design based closely on the original QUAD II and uses KT66 output valves. The QII-Forty is a later 40W development using KT88 output valves and selectable 4-ohm or 8-ohm speaker taps.
What is the difference between QII-Forty and QII-Eighty?
The QII-Eighty doubles rated output to 80W and uses four KT88 output valves per monoblock, while the Forty uses two KT88s and delivers 40W.
Does QUAD currently publish dimensions and weight for the QII-Forty?
These figures are not included in QUAD's current dedicated QII-Forty technical specification, so Audio T does not quote unverified dimensions or weight here.
Related QUAD Products and Audio T Guides
Why Buy the QUAD QII-Forty From Audio T?
Valve monoblocks benefit enormously from being demonstrated as part of the complete system rather than chosen from specifications alone. Loudspeaker sensitivity and impedance, the choice of 4-ohm or 8-ohm output tap, preamplifier output level, room size and required listening volume all affect whether the QII-Forty is the right amplifier for you.
Audio T has first-hand experience of the QII-Forty with modern loudspeakers and can help you build a system that makes sensible use of its 40W output rather than simply assuming that more or less power is automatically better.
- In-store demonstrations
- Valve-amplifier demonstrations
- Loudspeaker sensitivity checks
- Loudspeaker impedance matching
- 4-ohm versus 8-ohm tap guidance
- Preamplifier matching
- Input-sensitivity matching
- QUAD system demonstrations
- Electrostatic loudspeaker matching advice
- Modern dynamic-loudspeaker matching
- Speaker-cable and termination advice
- Monoblock placement guidance
- Valve ownership and servicing advice
- Long-term system upgrade guidance
- Ongoing after-sales support
Contact your local Audio T store to arrange a demonstration or discuss whether the QUAD QII-Forty pair is the right match for your preamplifier and loudspeakers.