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Whitestone Audio P331 Tube Loading Amplifier

$5,999.00

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Description

Whitestone Audio P331 Tube Loading Amplifier

 

Whitestone Audio P331 Tube Loading Amplifier Demo

  Whitestone Audio P331 tube loading amplifier white

1940s 6SN7 Tubes and High-Voltage Linear Power Supply

The P331 employs 6SN7 octal (8-pin) double-triode vacuum tubes from the 1940s, chosen for their lower distortion capabilities that exceed the standard and ubiquitous 12AX7 variety. Introduced by RCA Radiotron in 1939, the 6SN7 is the true common ancestor of all modern double triodes used in high-fidelity amplifiers and remains in production after nearly 80 years for good reason.

The P331's linear power supply is the most critical component for audio integrity next to the actual gain structure and circuit topologies. High-voltage plate/anode tube power supplies start at around 320V DC and are filtered up to six times, smoothed with active regulators using only film capacitors and real inductors (chokes) to remove ripple and noise. This ensures the purest possible foundation for the P331's tube circuits.

 Whitestone Audio P331 Loading circuit Lift filter

Loading Circuit With Clean and Bloom Modes

The Loading circuit is part of the fully differential (balanced) Class A tube amplifier, designed to coerce an audio signal to hit the tube at different points on its response curve. As gain increases to the tube, the signal is padded down in equal measure, allowing precise and repeatable loads while ensuring minimal level gain or loss.

Clean (feedback) mode manipulates feedback to affect internal tube gain with subtle nonlinearities and even-order harmonics. Bloom (feedforward) mode uses plate loading manipulation for slightly-less-subtle nonlinearities. Auto-Padding (Pre, Post, Open) controls where padding occurs in the circuit or disables it entirely. The Lift circuit is a gentle parallel filter that enhances highs, lows, or both, passing audio unscathed through the balanced circuitry and blending it with harmonic content from the Lift and Loading circuits working in concert.

 Whitestone Audio P331 transformerless output amplifier

Digitally Controlled Analog With Hard-Wire Bypass

The P331 is fully digitally controlled analog — each parameter is fully repeatable and can be recalled via front panel rotary and toggle switches commanding over 80 sealed precision relays. No audio runs to the front panel, and only the finest components are used throughout the audio path, ensuring mastering-grade signal integrity.

The Xformer section allows switching in output transformers with two optional loading settings for additional subtle textures, though the P331 runs fully transformerless in its default state for the purest audio path. A low-distortion, fully balanced solid-state output amplifier provides 11 dB of boost or cut in 1 dB increments, with Resolution mode allowing .25 dB or .5 dB fine control. Each section includes hard-wire bypass, plus a global bypass to remove the entire unit from your signal chain — though you likely won't want to use it much.

Highlights

  • Mastering-grade tube loading amplifier for subtle harmonic enhancement
  • 1940s vintage 6SN7 octal double-triode tubes for lower distortion
  • High-voltage linear power supply with up to 6x filtering and active regulation
  • Film capacitors and real inductors (chokes) for clean power delivery
  • Fully balanced, Class A differential tube amplifier design
  • Digitally controlled analog with over 80 sealed precision relays
  • Loading circuit with Clean (feedback) and Bloom (feedforward) modes
  • Auto-Padding feature: Pre, Post, or Open modes
  • Lift circuit: gentle parallel filter for selective frequency enhancement
  • Xformer section with switchable output transformers or fully transformerless operation
  • Low-distortion, fully balanced solid-state output amplifier
  • Output gain: unity to +11 dB in 1 dB increments with Resolution mode (.25 dB/.5 dB fine control)
  • Individual hard-wire bypass per section plus global bypass
  • Frequency response: 10 Hz – 65 kHz (±1 dB)
  • Signal-to-noise ratio: 78 dB minimum
  • THD + Noise: <0.03% at unity setting
  • Two-channel stereo operation with balanced I/O

Whitestone Audio P331 Specifications

Specification Details
Processor Type Tube-loading amplifier
Number of Channels 2 (stereo, fully balanced)
Vacuum Tubes 6SN7 octal (8-pin) double-triode (1940s vintage)
Tube Amplifier Design Fully differential (balanced) Class A
Power Supply High-voltage linear (320V DC+), up to 6x filtering, active regulation, film capacitors, inductors (chokes)
Control Design Digitally controlled analog with over 80 sealed precision relays, full repeatability and recall
Loading Circuit Modes Clean (feedback), Bloom (feedforward plate-loaded)
Auto-Padding Modes Pre, Post, Open
Lift Circuit Gentle parallel filter (highs, lows, or both)
Xformer Section Switchable output transformers (2 loading options) or fully transformerless
Output Amplifier Low-distortion, fully balanced solid-state
Gain Range Unity to +11 dB in 1 dB increments with Resolution mode (.25 dB/.5 dB fine control)
Frequency Response 10 Hz – 65 kHz (±1 dB)
Noise Floor ≥-98 dBu (20 Hz–28 kHz unweighted, RMS) @ 0 dB unity gain; ≥-94 dBu with both channels output gain fully driven
Signal-to-Noise Ratio 78 dB minimum
THD + Noise <0.03% at unity setting
Bypass Individual hard-wire bypass per section (except Loading), global hard-wire bypass
Analog Inputs 2 x XLR (balanced)
Analog Outputs 2 x XLR (balanced)
Power Source Standard IEC AC cable
Model Variant Original P331 (white) - designed for subtle, unobtrusive mastering enhancement

What's in the Box

Included Items
Whitestone Audio P331 Tube Loading Amplifier
Power Cable
User Documentation
Shipping

Shipping & Returns

  • We deliver all US orders in 3–5 business days
  • Free cancellations
  • Returns accepted within 30 days (unused items)

Order Confirmation

Once your order is placed, you will receive an order confirmation email. This confirms that we have received your order and have pre-authorized your credit card for the purchase. After confirming availability with our suppliers, we will proceed accordingly:

  • If your item is available and ready to ship within 1–2 business days, we will process the charge and arrange for shipment.

  • If your item is on backorder or unavailable, the pre-authorization will be voided and you will be notified by email.

  • Please note: While most items ship out within 1–2 business days, total delivery time is typically 3–5 business days, depending on your location and carrier handling.

Order Shipment

If your item is in stock and your payment has been processed, your order will typically ship within 1–2 business days. Total delivery time is approximately 3–5 business days from the date of order. Once shipped, you will receive tracking information at the email address provided during checkout within 24 hours of dispatch.

If you have not received tracking information within 3–5 business days of placing your order, please contact us at support@proaudioreserve.com.

The Distributor-Direct Advantage

To ensure you receive the most recent hardware revisions and 100% factory-sealed units, Pro Audio Reserve utilizes a Distributor-Direct fulfillment model. Unlike big-box retailers that may ship "floor models" or gear that has been sitting in general-purpose warehouses, your equipment is pulled directly from the primary manufacturer hub, serialized, and dispatched. This process ensures the highest level of quality control for professional-grade recording equipment.

Shipping Policy

We currently offer free shipping within the United States only. If the checkout process on our website allows you to enter a shipping address and complete your order, your location is eligible to receive delivery.

Changing Shipping Address

If you need to change your shipping address after your order has been processed or shipped, we will do our best to accommodate the request. Please note that address changes may result in delivery delays. Any additional costs incurred due to address changes, including carrier fees or rerouting charges will be the responsibility of the customer. No exceptions.

Shipping Times

All shipping times provided are estimates and should not be considered guarantees. Most orders ship out within 1–2 business days from the time payment is processed. However, total delivery time is typically 3–5 business days, depending on your location, carrier performance, and supplier fulfillment timelines.

We work closely with our vendors to ensure timely deliveries, but please note that supply chain disruptions, high-demand periods, or weather events may occasionally cause delays beyond our control.

  • To avoid misunderstandings: The “shipping time” refers to the total time from order placement to final delivery, not just when the item leaves the warehouse.

  • Cancellations: If you choose to cancel your order because it has not arrived within the estimated delivery window, you may be responsible for applicable shipping charges, restocking fees, warehouse handling costs, and other related fees.

  • Chargebacks: Customers who initiate chargebacks due to shipping delays without contacting us first may be held legally responsible for losses. We strongly encourage open communication so we can resolve any delivery concerns directly and efficiently via our Atlanta-based support team.

Delivery Options

  • Threshold Shipping: Your item will be delivered to your front door, porch, garage, or a covered entryway.

 

Why Buy From Us

We get it. You've got a lot of choice when it comes to online retailers.
Here's why we think you should shop with us at ProAudioReserve.

  • Free Shipping USA wide. 
  • 30 Day Easy Returns. Not happy with your product, we'll take it back noquestions asked.
  • Great Service. We pride ourselves on the best service, anywhere.

Reviews

Customer Reviews

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Description

Whitestone Audio P331 Tube Loading Amplifier

 

Whitestone Audio P331 Tube Loading Amplifier Demo

  Whitestone Audio P331 tube loading amplifier white

1940s 6SN7 Tubes and High-Voltage Linear Power Supply

The P331 employs 6SN7 octal (8-pin) double-triode vacuum tubes from the 1940s, chosen for their lower distortion capabilities that exceed the standard and ubiquitous 12AX7 variety. Introduced by RCA Radiotron in 1939, the 6SN7 is the true common ancestor of all modern double triodes used in high-fidelity amplifiers and remains in production after nearly 80 years for good reason.

The P331's linear power supply is the most critical component for audio integrity next to the actual gain structure and circuit topologies. High-voltage plate/anode tube power supplies start at around 320V DC and are filtered up to six times, smoothed with active regulators using only film capacitors and real inductors (chokes) to remove ripple and noise. This ensures the purest possible foundation for the P331's tube circuits.

 Whitestone Audio P331 Loading circuit Lift filter

Loading Circuit With Clean and Bloom Modes

The Loading circuit is part of the fully differential (balanced) Class A tube amplifier, designed to coerce an audio signal to hit the tube at different points on its response curve. As gain increases to the tube, the signal is padded down in equal measure, allowing precise and repeatable loads while ensuring minimal level gain or loss.

Clean (feedback) mode manipulates feedback to affect internal tube gain with subtle nonlinearities and even-order harmonics. Bloom (feedforward) mode uses plate loading manipulation for slightly-less-subtle nonlinearities. Auto-Padding (Pre, Post, Open) controls where padding occurs in the circuit or disables it entirely. The Lift circuit is a gentle parallel filter that enhances highs, lows, or both, passing audio unscathed through the balanced circuitry and blending it with harmonic content from the Lift and Loading circuits working in concert.

 Whitestone Audio P331 transformerless output amplifier

Digitally Controlled Analog With Hard-Wire Bypass

The P331 is fully digitally controlled analog — each parameter is fully repeatable and can be recalled via front panel rotary and toggle switches commanding over 80 sealed precision relays. No audio runs to the front panel, and only the finest components are used throughout the audio path, ensuring mastering-grade signal integrity.

The Xformer section allows switching in output transformers with two optional loading settings for additional subtle textures, though the P331 runs fully transformerless in its default state for the purest audio path. A low-distortion, fully balanced solid-state output amplifier provides 11 dB of boost or cut in 1 dB increments, with Resolution mode allowing .25 dB or .5 dB fine control. Each section includes hard-wire bypass, plus a global bypass to remove the entire unit from your signal chain — though you likely won't want to use it much.

Highlights

  • Mastering-grade tube loading amplifier for subtle harmonic enhancement
  • 1940s vintage 6SN7 octal double-triode tubes for lower distortion
  • High-voltage linear power supply with up to 6x filtering and active regulation
  • Film capacitors and real inductors (chokes) for clean power delivery
  • Fully balanced, Class A differential tube amplifier design
  • Digitally controlled analog with over 80 sealed precision relays
  • Loading circuit with Clean (feedback) and Bloom (feedforward) modes
  • Auto-Padding feature: Pre, Post, or Open modes
  • Lift circuit: gentle parallel filter for selective frequency enhancement
  • Xformer section with switchable output transformers or fully transformerless operation
  • Low-distortion, fully balanced solid-state output amplifier
  • Output gain: unity to +11 dB in 1 dB increments with Resolution mode (.25 dB/.5 dB fine control)
  • Individual hard-wire bypass per section plus global bypass
  • Frequency response: 10 Hz – 65 kHz (±1 dB)
  • Signal-to-noise ratio: 78 dB minimum
  • THD + Noise: <0.03% at unity setting
  • Two-channel stereo operation with balanced I/O

Whitestone Audio P331 Specifications

Specification Details
Processor Type Tube-loading amplifier
Number of Channels 2 (stereo, fully balanced)
Vacuum Tubes 6SN7 octal (8-pin) double-triode (1940s vintage)
Tube Amplifier Design Fully differential (balanced) Class A
Power Supply High-voltage linear (320V DC+), up to 6x filtering, active regulation, film capacitors, inductors (chokes)
Control Design Digitally controlled analog with over 80 sealed precision relays, full repeatability and recall
Loading Circuit Modes Clean (feedback), Bloom (feedforward plate-loaded)
Auto-Padding Modes Pre, Post, Open
Lift Circuit Gentle parallel filter (highs, lows, or both)
Xformer Section Switchable output transformers (2 loading options) or fully transformerless
Output Amplifier Low-distortion, fully balanced solid-state
Gain Range Unity to +11 dB in 1 dB increments with Resolution mode (.25 dB/.5 dB fine control)
Frequency Response 10 Hz – 65 kHz (±1 dB)
Noise Floor ≥-98 dBu (20 Hz–28 kHz unweighted, RMS) @ 0 dB unity gain; ≥-94 dBu with both channels output gain fully driven
Signal-to-Noise Ratio 78 dB minimum
THD + Noise <0.03% at unity setting
Bypass Individual hard-wire bypass per section (except Loading), global hard-wire bypass
Analog Inputs 2 x XLR (balanced)
Analog Outputs 2 x XLR (balanced)
Power Source Standard IEC AC cable
Model Variant Original P331 (white) - designed for subtle, unobtrusive mastering enhancement

What's in the Box

Included Items
Whitestone Audio P331 Tube Loading Amplifier
Power Cable
User Documentation
Shipping

Shipping & Returns

  • We deliver all US orders in 3–5 business days
  • Free cancellations
  • Returns accepted within 30 days (unused items)

Order Confirmation

Once your order is placed, you will receive an order confirmation email. This confirms that we have received your order and have pre-authorized your credit card for the purchase. After confirming availability with our suppliers, we will proceed accordingly:

  • If your item is available and ready to ship within 1–2 business days, we will process the charge and arrange for shipment.

  • If your item is on backorder or unavailable, the pre-authorization will be voided and you will be notified by email.

  • Please note: While most items ship out within 1–2 business days, total delivery time is typically 3–5 business days, depending on your location and carrier handling.

Order Shipment

If your item is in stock and your payment has been processed, your order will typically ship within 1–2 business days. Total delivery time is approximately 3–5 business days from the date of order. Once shipped, you will receive tracking information at the email address provided during checkout within 24 hours of dispatch.

If you have not received tracking information within 3–5 business days of placing your order, please contact us at support@proaudioreserve.com.

The Distributor-Direct Advantage

To ensure you receive the most recent hardware revisions and 100% factory-sealed units, Pro Audio Reserve utilizes a Distributor-Direct fulfillment model. Unlike big-box retailers that may ship "floor models" or gear that has been sitting in general-purpose warehouses, your equipment is pulled directly from the primary manufacturer hub, serialized, and dispatched. This process ensures the highest level of quality control for professional-grade recording equipment.

Shipping Policy

We currently offer free shipping within the United States only. If the checkout process on our website allows you to enter a shipping address and complete your order, your location is eligible to receive delivery.

Changing Shipping Address

If you need to change your shipping address after your order has been processed or shipped, we will do our best to accommodate the request. Please note that address changes may result in delivery delays. Any additional costs incurred due to address changes, including carrier fees or rerouting charges will be the responsibility of the customer. No exceptions.

Shipping Times

All shipping times provided are estimates and should not be considered guarantees. Most orders ship out within 1–2 business days from the time payment is processed. However, total delivery time is typically 3–5 business days, depending on your location, carrier performance, and supplier fulfillment timelines.

We work closely with our vendors to ensure timely deliveries, but please note that supply chain disruptions, high-demand periods, or weather events may occasionally cause delays beyond our control.

  • To avoid misunderstandings: The “shipping time” refers to the total time from order placement to final delivery, not just when the item leaves the warehouse.

  • Cancellations: If you choose to cancel your order because it has not arrived within the estimated delivery window, you may be responsible for applicable shipping charges, restocking fees, warehouse handling costs, and other related fees.

  • Chargebacks: Customers who initiate chargebacks due to shipping delays without contacting us first may be held legally responsible for losses. We strongly encourage open communication so we can resolve any delivery concerns directly and efficiently via our Atlanta-based support team.

Delivery Options

  • Threshold Shipping: Your item will be delivered to your front door, porch, garage, or a covered entryway.

 

Why Buy From Us

We get it. You've got a lot of choice when it comes to online retailers.
Here's why we think you should shop with us at ProAudioReserve.

  • Free Shipping USA wide. 
  • 30 Day Easy Returns. Not happy with your product, we'll take it back noquestions asked.
  • Great Service. We pride ourselves on the best service, anywhere.

Reviews

Customer Reviews

Be the first to write a review
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