Hello DIY Audio Community,
I'm in the process of designing a stereo hybrid headphone amplifier that leverages the rich tonal qualities of a tube stage combined with the robust performance of solid-state MOSFETs. The goal is to achieve at least 1W per channel into 32Ω headphones and 300mW per channel into 300Ω headphones. I'm seeking your expert insights and suggestions to refine this design.
I'm using KiCad for schematic capture and PCB design, and plan to have the assembled prototype tested by a certified electrician for safety compliance.
---
### Project Overview
The amplifier is intended to deliver at least 1W per channel into 32Ω headphones and 300mW per channel into 300Ω headphones. The design emphasizes simplicity by minimizing components in the signal path to preserve the tube’s sonic character while ensuring reliable performance across various headphone impedances.
---
### Design Specifications
---
### Bill of Materials (BOM)
#### 1. Tube Stage Components
#### 2. MOSFET Follower Stage Components
#### 3. Power Supply Components
##### A. High-Voltage B+ Supply
##### B. MOSFET Supply (Voltage Doubler)
#### 4. Signal Path and Control Components
#### 5. Grounding and Protection Components
#### 6. Chassis and Mounting Components
#### 7. PCB and Miscellaneous Components
---
### Seeking Your Expertise
I would greatly appreciate feedback on the following aspects of the design:
1. Component Selection:
- Are the chosen components optimal for the intended performance and reliability?
- Suggestions for higher-quality alternatives, especially for critical components like capacitors and resistors?
2. Circuit Design:
- Any improvements or modifications to the tube stage or MOSFET buffer for better performance or stability?
- Recommendations for additional filtering or decoupling to enhance audio fidelity?
3. PCB Layout:
- Best practices for minimizing noise and interference in a hybrid tube-MOSFET design?
- Tips for effective thermal management on a 2-layer PCB?
4. Safety Considerations:
- Additional safety features or precautions that should be incorporated?
- Suggestions for ensuring reliable isolation between high-voltage and low-voltage sections?
5. Overall Design:
- Any potential oversights or areas that could benefit from further optimization?
- Advice on achieving the best balance between simplicity and performance?
---
Thank you in advance for your valuable insights and suggestions!
Looking forward to your feedback to refine and enhance this amplifier design.
I'm in the process of designing a stereo hybrid headphone amplifier that leverages the rich tonal qualities of a tube stage combined with the robust performance of solid-state MOSFETs. The goal is to achieve at least 1W per channel into 32Ω headphones and 300mW per channel into 300Ω headphones. I'm seeking your expert insights and suggestions to refine this design.
I'm using KiCad for schematic capture and PCB design, and plan to have the assembled prototype tested by a certified electrician for safety compliance.
---
### Project Overview
The amplifier is intended to deliver at least 1W per channel into 32Ω headphones and 300mW per channel into 300Ω headphones. The design emphasizes simplicity by minimizing components in the signal path to preserve the tube’s sonic character while ensuring reliable performance across various headphone impedances.
---
### Design Specifications
Code:
+----------------------------+----------------------------------------------+
| Specification | Details |
+----------------------------+----------------------------------------------+
| Amplifier Type | Hybrid (Tube + MOSFET) |
| Tube Used | Tung-Sol 12AU7 (Dual Triode) |
| MOSFETs Used | IRLZ44N N-Channel MOSFETs |
| Power Output | ≥1W/channel into 32Ω; ≥300mW/channel into 300Ω |
| Power Supply | AnTek AS-1T150 Transformer |
| PCB Design Software | KiCad |
| Chassis | WHAMMY Case (80mm H x 210mm W x 230mm D) |
| Volume Control | Single Blue Velvet 10kΩ Audio-Taper Potentiometer |
| Testing | To be conducted by a certified electrician |
+----------------------------+----------------------------------------------+---
### Bill of Materials (BOM)
#### 1. Tube Stage Components
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| 12AU7 Tube | Tung-Sol 12AU7 (Dual Triode) | 1 | High-quality audio tube |
| Tube Sockets | 5-Pin Ceramic Tube Sockets | 2 | One per triode channel |
| Cathode Resistor (Rk) | 100Ω, 1W, Metal Film | 1 | Sets baseline cathode bias |
| Cathode Bias Potentiometer | 1kΩ, 1W, Audio-Taper (Blue Velvet) | 1 | Single potentiometer for both channels |
| Decoupling Capacitors (Ck) | 0.1µF, Ceramic | 2 | One per channel; stabilizes bias voltage |
| Grid Resistors | 1MΩ, Metal Film | 2 | Connects RCA inputs to tube grids |
| Coupling Capacitors | 10µF, 400V, Film (e.g., Panasonic EEFUE0G101R)| 2 | Blocks DC; allows AC audio signal to pass to MOSFET gates|
| Stability Capacitors | 100pF, Ceramic | 2 | From MOSFET gate to source for high-frequency damping|
+------------------------------+---------------------------------------------+----------+--------------------------------------------+#### 2. MOSFET Follower Stage Components
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| IRLZ44N MOSFETs | N-Channel, Logic-Level | 2 | One per channel; consider heatsinks if necessary|
| Gate Resistors | 10kΩ, 1/4W, Metal Film | 2 | One per MOSFET gate for stability |
| Source Resistors (Rs) | 1.7Ω, 10W, Wirewound | 2 | Handles current buffering; ensures low impedance output|
| Output Capacitors | 100µF, 50V, Film (e.g., Nichicon UFW100C) | 2 | DC blocking at headphone outputs; non-polarized film capacitors|
| Heatsinks for MOSFETs | Suitable for IRLZ44N (e.g., Velleman AN20) | 2 | Ensure proper thermal management |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+#### 3. Power Supply Components
##### A. High-Voltage B+ Supply
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Transformer | AnTek AS-1T150 | 1 | Provides 150V AC @ 0.33A and 6.3V AC @ 3A |
| Rectifier Diodes | 1N4007 | 4 | Full-wave bridge rectifier for B+ supply |
| Filter Capacitors | 220µF, 250V, Electrolytic (e.g., Nichicon UG2J221MPD) | 2 | Smoothes B+ DC supply |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+##### B. MOSFET Supply (Voltage Doubler)
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Schottky Diodes | 1N5819 | 2 | Voltage doubler configuration; low forward voltage drop|
| Electrolytic Capacitors | 1000µF, 25V, Electrolytic (e.g., Panasonic EEU-FC1C100) | 2 | Filter capacitors for voltage doubler |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+#### 4. Signal Path and Control Components
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Volume Potentiometer | Blue Velvet, 10kΩ, Audio-Taper | 1 | Controls overall volume before tube stage |
| RCA Input Jacks | Shielded, Mono | 2 | One per channel; high-quality for audio fidelity|
| Headphone Output Jack | 3.5mm Stereo, Gold-Plated | 1 | High-quality, gold-plated recommended |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+#### 5. Grounding and Protection Components
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Fuses | 1A Slow-Blow, 250mA Slow-Blow, 500mA | 3 | Protects AC input, B+ supply, and low-voltage DC|
| Star Ground Bus | Bus bar or Heavy Gauge Wire | 1 | Central ground point to minimize ground loops|
| TVS Diodes | Suitable for AC Mains Protection | 2 | Protects against voltage spikes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+#### 6. Chassis and Mounting Components
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Chassis | WHAMMY Case, 80mm (H) x 210mm (W) x 230mm (D) | 1 | Accommodates all components; ensure proper ventilation|
| Mounting Hardware | Screws, Standoffs, Thermal Paste for Heatsinks | As Needed | Secure components within the chassis |
| On/Off Switch | SPST, Rated for AC Mains | 1 | Controls power to the amplifier |
| Fuse Holders | Inline Holders for Respective Fuses | 3 | Mounts for 1A, 250mA, and 500mA fuses |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+#### 7. PCB and Miscellaneous Components
Code:
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| Component | Specification | Quantity | Description/Notes |
+------------------------------+---------------------------------------------+----------+--------------------------------------------+
| PCB | 2-Layer, Custom-Designed | 1 | Designed in KiCad; ensure high-voltage isolation|
| Connectors | Screw Terminals for Transformer Connections | 1 Set | Facilitates power connections to PCB |
| Wires and Cables | Various Gauges, Shielded for Audio | As Needed | High-quality for signal paths; proper gauge for power|
| Thermal Vias | For Heatsink PCB Areas | As Needed | Enhances thermal management on PCB |
| Mounting Hardware | Screws, Standoffs, Spacers | As Needed | For PCB and component mounting within chassis|
+------------------------------+---------------------------------------------+----------+--------------------------------------------+---
### Seeking Your Expertise
I would greatly appreciate feedback on the following aspects of the design:
1. Component Selection:
- Are the chosen components optimal for the intended performance and reliability?
- Suggestions for higher-quality alternatives, especially for critical components like capacitors and resistors?
2. Circuit Design:
- Any improvements or modifications to the tube stage or MOSFET buffer for better performance or stability?
- Recommendations for additional filtering or decoupling to enhance audio fidelity?
3. PCB Layout:
- Best practices for minimizing noise and interference in a hybrid tube-MOSFET design?
- Tips for effective thermal management on a 2-layer PCB?
4. Safety Considerations:
- Additional safety features or precautions that should be incorporated?
- Suggestions for ensuring reliable isolation between high-voltage and low-voltage sections?
5. Overall Design:
- Any potential oversights or areas that could benefit from further optimization?
- Advice on achieving the best balance between simplicity and performance?
---
Thank you in advance for your valuable insights and suggestions!
Looking forward to your feedback to refine and enhance this amplifier design.
Last edited:
Correction: It will be just one pot inside the case for the daring user to adjust, not one for each channel.
Plate (P1) ---- B+
|
[ ]
[ 12AU7 Triode 1]
[ ]
|
(Cathode)
|
+---- Rk ----+
| |
| [Potentiometer]
| |
| |
Plate (P2) ----+ |
| |
[ ]
[ 12AU7 Triode 2]
[ ]
|
(Cathode)
|
|
GND
Plate (P1) ---- B+
|
[ ]
[ 12AU7 Triode 1]
[ ]
|
(Cathode)
|
+---- Rk ----+
| |
| [Potentiometer]
| |
| |
Plate (P2) ----+ |
| |
[ ]
[ 12AU7 Triode 2]
[ ]
|
(Cathode)
|
|
GND
Had to edit the main post for clarity. Please disregard my previous comment, as I corrected that in the main post already.
I have started the schematic in kicad. I can't seem to upload it. I'd like to show you what I've got down so far, so you can tell me where I have already gone wrong. It doesn't show up as available in the directory when I go to upload it. Anyone know why?
I can see some problems already. For example, it calls for 2 5 pin tube sockets. That's weird. Obviously, we only need a single 8 pin tube socket.
It's my very first attempt at using kicad, as well as my very first attempt at drawing a schematic. So, on the one hand, you're gonna laugh and tell me it's hopeless. On the other hand, I need to learn how to do this correctly. I assure you, I am going to build this amplifier if it takes me the next 3 years. I will learn anything I need to learn to make this happen. I want the learning and growing experience of it as much as I want the amp.
I realize it's very incomplete. Even most of what I do have on the schematic still needs a footprint. I'm going through the process of getting all the datasheets, symbols, and footprints for all the components.
It's a start. Look at some schematics here and see how they are composed.
Here's a random example. https://www.diyaudio.com/community/threads/will-this-schematic-work.416086/
Schematic and pcb design has a steep learning curve, so just keep going.
Inputs at left, outputs at right.
Positive supply at top, negative supply at bottom.
Keep the parts in each sub-circuit close together.
Space apart the various sub-circuits.
This makes it easier to understand how the circuit works.
Here's a random example. https://www.diyaudio.com/community/threads/will-this-schematic-work.416086/
Schematic and pcb design has a steep learning curve, so just keep going.
Inputs at left, outputs at right.
Positive supply at top, negative supply at bottom.
Keep the parts in each sub-circuit close together.
Space apart the various sub-circuits.
This makes it easier to understand how the circuit works.
Excellent. This is great guidance. Thanks! I will keep working on it. I hope to have just the schematic finished this week or early next. Then, I'll assign footprints and get started on the pcb.
Just guessing, would you suppose the Whammy chassis (80hx210wx230d) would be the right size for this amp?
It's best to make a proto board first and see how things look. Avoid cramming things into the box too tightly.
A good schematic can be surprisingly similar in appearance to the pcb parts layout.
This is because good trace routing and parts placement will tend to make them similar.
A good schematic can be surprisingly similar in appearance to the pcb parts layout.
This is because good trace routing and parts placement will tend to make them similar.
Tubes like the 12AX7 have a total of 9 pins, including the heater center tap connection.
Somewhere in the library there will be a 9 pin miniature dual triode part.
To minimise distortion, you need to have a plate load which is at least 2.5 times the plate impedance.
A 12AU7 has a plate impedance of about 6.5k, so that would be a plate load of 16k.
You will have a lot of gain, around 13 with no cathode bias, around 22.6dB. For a HPA, a gain of around 10dB would be better - just over 3.1.
You have a surplus section there. Why not use a SRPP topology? This will give you a very similar gain but a lowish output impedance. If you want very low Zout for low Z HP, you would use a single ended mosfet source follower, as you are intending. For lower impedance Zout using only tubes, you might have to change your tube to something with a very low impedance, maybe even a pentode like a good sounding 6V6. The Salas preamp, on this forum, is a very good design, very well tested and documented.
HD
A 12AU7 has a plate impedance of about 6.5k, so that would be a plate load of 16k.
You will have a lot of gain, around 13 with no cathode bias, around 22.6dB. For a HPA, a gain of around 10dB would be better - just over 3.1.
You have a surplus section there. Why not use a SRPP topology? This will give you a very similar gain but a lowish output impedance. If you want very low Zout for low Z HP, you would use a single ended mosfet source follower, as you are intending. For lower impedance Zout using only tubes, you might have to change your tube to something with a very low impedance, maybe even a pentode like a good sounding 6V6. The Salas preamp, on this forum, is a very good design, very well tested and documented.
HD
I have been searching for a way to do this. I need some way to do both high and low voltage. My town has one of the few remaining Radio Shacks in Texas, so I went in there, and that guy didn't know. Not that he was a dummy, it's just there aren't that many hobbyists messing around with high voltage these days. I will keep your advice in mind, and check back with it. Thanks!
Good catch! Thanks! Yeah, this is the danger of relying on AI, why I really need yall's help. I was pretty sure it had been saying 9 pin, but today it started insisting it was 8 pin. Once you catch it being wrong, it gets ornery.
No, I'm drawing it from scratch in kicad. AI is walking me through it. I uploaded it in a previous comment.
Thus sayeth the AI:
To address your observations and feedback:
### 1. Plate Load and Distortion Considerations:
- You are correct that for optimal performance and minimal distortion, the plate load should ideally be at least 2.5 times the plate impedance of the tube. Given that the 12AU7 has a plate impedance of around 6.5kΩ, a plate load of 16kΩ or more would be ideal. This ensures that the tube operates in a more linear region, reducing distortion.
- If you currently have a lower plate load resistor, you might want to adjust this to ensure the tube operates with sufficient load. You could also consider using a current source for an even more optimal and linear plate load, though that might increase complexity.
### 2. Gain Reduction:
- A gain of 22.6 dB (~13x voltage gain) is indeed quite high for a headphone amplifier. For typical headphones, a gain of around 10 dB (a gain factor of about 3.1x) would be more suitable. This will give you better control over the volume range and prevent excessive amplification.
- One way to reduce the gain would be to:
- Increase the cathode resistor (Rk), or
- Add a global feedback loop between the plate and grid, but that would alter the amplifier’s sonic character.
- Alternatively, you can adjust the gain in your MOSFET follower stage (or even in the feedback around it) to maintain overall gain control without changing the tube's characteristics.
### 3. SRPP Topology:
- The SRPP (Shunt Regulated Push-Pull) topology is an excellent suggestion. This would use the extra triode section of the 12AU7 to provide a self-regulated push-pull gain stage with a lower output impedance than a single triode stage.
- SRPP has advantages in linearity and current driving capability compared to a single-ended triode, making it a good fit for driving headphones, especially with medium-to-high impedance.
- Gain: SRPP circuits typically have gains closer to what you're looking for (~10-15 dB), which would be a much better match for headphone amplifiers.
- Output Impedance: While SRPP doesn’t give you ultra-low impedance, it lowers the output impedance compared to a simple common-cathode stage.
### 4. MOSFET Source Follower:
- If your goal is very low output impedance to drive low-impedance headphones, a single-ended MOSFET source follower is indeed an excellent choice. The MOSFET follower stage will provide current gain and maintain a low output impedance, ensuring that even low-impedance headphones can be driven effectively.
- This approach would give you a robust design that combines the tube’s voltage gain and the MOSFET’s current driving capabilities.
### 5. Using a Different Tube for Lower Zout:
- If you wanted to lower the output impedance using only tubes, you'd need a tube with a lower plate impedance. While the 12AU7 is versatile, its plate impedance is relatively high for this purpose. Switching to something like a 6V6 pentode in triode mode or another low-impedance triode like the 6SN7 could give you a lower output impedance.
- However, this approach may add complexity and require a larger power supply voltage or heater current. It’s often easier to stick with the 12AU7 (especially in SRPP) for its linearity and simplicity, and use the MOSFET follower for low-Z headphone driving.
### 6. Salas Preamp Design:
- The Salas preamp is indeed a great, well-tested design. It incorporates a high-quality tube stage and might serve as a useful reference for optimizing your own amplifier design. You could borrow ideas from the power supply or biasing techniques used in the Salas preamp to further refine your own circuit.
### Recommendations:
- Use the SRPP Topology: This will reduce gain, lower output impedance, and use the extra triode section in a balanced way. The SRPP will give you the benefits of reduced distortion and a more robust drive without adding MOSFETs for impedance matching.
- Keep the MOSFET Source Follower for Lower Impedance Loads: If you expect to drive low-impedance headphones (e.g., 32Ω or less), keep your MOSFET follower as planned. This will ensure that you have enough current capacity to drive those headphones effectively.
- Adjust the Plate Load: If you're sticking with the simple triode stage, ensure you use at least a 16kΩ plate load to minimize distortion.
Let me know if you'd like help implementing an SRPP stage or making these adjustments!
What do you think of that?
Low and high voltage are routinely routed on the same PCB.
There are creepage and clearance guidelines to follow. Some examples:
https://www.protoexpress.com/blog/importance-pcb-line-spacing-creepage-clearance/
There are creepage and clearance guidelines to follow. Some examples:
https://www.protoexpress.com/blog/importance-pcb-line-spacing-creepage-clearance/
I mean just on the prototyping. I haven't been able to find a breadboard that can safely handle 210V.
I am concerned about the recommendation to switch to push-pull operation, as I had read that single-ended preserves more of the tube character. Having an adjustable tube character is a core feature of this design.
I am concerned about the recommendation to switch to push-pull operation, as I had read that single-ended preserves more of the tube character. Having an adjustable tube character is a core feature of this design.
From the AI:
You’re absolutely right to be cautious about switching to push-pull operation if preserving the tube character is a core feature of your design. Let's break down the considerations regarding single-ended versus SRPP (Shunt Regulated Push-Pull) and how each impacts the "tube sound" you're aiming for.
### 1. Single-Ended Triode Operation:
- Tube Character: Single-ended triode (SET) amplifiers are widely loved for their harmonic richness, especially in the even-order harmonics (2nd harmonic), which contribute to the "warmth" and "musicality" that tube amplifiers are famous for.
- Simple Topology: The signal flows through one active device, in this case, the 12AU7 triode, giving a more direct and unfiltered tube sound. Any distortion characteristics of the tube are preserved, which can give the amplifier a pleasant, organic sound, especially at lower power levels.
- Distortion: While single-ended designs typically have more distortion than push-pull designs, this distortion is mostly of the even-order harmonic type, which is often perceived as pleasing to the ear. This distortion is part of what many audiophiles seek in tube amplifiers.
If you want to preserve and control the tube character, sticking with a single-ended design makes sense.
### 2. SRPP (Shunt Regulated Push-Pull):
- Balanced Harmonics: The SRPP topology tends to cancel out even-order harmonics, which gives you a cleaner and more linear sound. While this results in lower distortion overall, it also removes some of the harmonic richness that gives tubes their characteristic "sound."
- Higher Gain and Output: SRPP is better at delivering more power and handling lower-impedance loads because it’s a more efficient design. However, this comes at the expense of some of the tube warmth, as it’s a quasi-push-pull operation.
- Lower Output Impedance: SRPP gives a lower output impedance compared to single-ended designs, which is why it's often used for driving headphones or lower-impedance loads without additional buffering.
### 3. Single-Ended with MOSFET Buffer:
- Since you're concerned about maintaining the tube character while still achieving low output impedance, the combination of a single-ended triode stage for voltage amplification and a MOSFET source follower for current buffering is a great compromise.
- Preserves Tube Sound: The single-ended triode stage will give you the desired tube harmonics and warmth, while the MOSFET stage (which doesn’t contribute much to the sound character) will provide the necessary current to drive low-impedance headphones.
- Adjustable Tube Character: If you want to fine-tune the tube character, you can achieve this by:
- Adjusting the cathode bias to shift the operating point of the tube.
- Adding a cathode bypass capacitor (Ck) with switchable values or even removing it entirely to adjust local feedback and harmonic distortion.
### 4. Final Recommendation:
- Stick with Single-Ended Operation: To preserve the tube character you value, keep the single-ended triode stage (SET) for your voltage amplification.
- Use MOSFET Source Follower for Buffering: The MOSFET buffer will ensure that you get low output impedance and adequate current drive for low-impedance headphones, without interfering with the tube's harmonic signature.
- Adjustable Tube Character: Implement an adjustable cathode bias or use different bypass capacitor values to allow control over how much of the tube's character (i.e., harmonic content) is present in the sound.
### Steps to Enhance Tube Character Adjustability:
1. Cathode Bypass Capacitor (Ck):
- You can make the bypass capacitor switchable or variable. This would allow you to adjust the amount of local feedback in the tube stage, which directly affects harmonic distortion and gain.
- No capacitor: More local feedback, lower gain, lower distortion.
- Large capacitor (e.g., 470 µF): Less feedback, higher gain, more tube character and harmonics.
2. Variable Bias Control:
- Adding a bias adjustment potentiometer (as you already have) can let you move the tube’s operating point up or down, changing the harmonic profile and providing more control over the sound.
### Why This Approach Works:
- By maintaining the single-ended triode configuration and pairing it with a MOSFET follower for current buffering, you’re getting the best of both worlds:
- Tube harmonic richness from the single-ended stage.
- Low output impedance and the ability to drive a wide range of headphone impedances from the MOSFET stage.
You’re also retaining flexibility by making the tube character adjustable through bias and bypass capacitor options.
### Conclusion:
To meet your goal of adjustable tube character, it’s best to stick with single-ended operation and implement adjustable cathode bias or bypass capacitor switching to control the sound. The MOSFET source follower is an excellent choice for maintaining low output impedance without altering the tube’s sonic signature. Let me know if you'd like further guidance on the bias adjustment or bypass capacitor switching circuits!
You’re absolutely right to be cautious about switching to push-pull operation if preserving the tube character is a core feature of your design. Let's break down the considerations regarding single-ended versus SRPP (Shunt Regulated Push-Pull) and how each impacts the "tube sound" you're aiming for.
### 1. Single-Ended Triode Operation:
- Tube Character: Single-ended triode (SET) amplifiers are widely loved for their harmonic richness, especially in the even-order harmonics (2nd harmonic), which contribute to the "warmth" and "musicality" that tube amplifiers are famous for.
- Simple Topology: The signal flows through one active device, in this case, the 12AU7 triode, giving a more direct and unfiltered tube sound. Any distortion characteristics of the tube are preserved, which can give the amplifier a pleasant, organic sound, especially at lower power levels.
- Distortion: While single-ended designs typically have more distortion than push-pull designs, this distortion is mostly of the even-order harmonic type, which is often perceived as pleasing to the ear. This distortion is part of what many audiophiles seek in tube amplifiers.
If you want to preserve and control the tube character, sticking with a single-ended design makes sense.
### 2. SRPP (Shunt Regulated Push-Pull):
- Balanced Harmonics: The SRPP topology tends to cancel out even-order harmonics, which gives you a cleaner and more linear sound. While this results in lower distortion overall, it also removes some of the harmonic richness that gives tubes their characteristic "sound."
- Higher Gain and Output: SRPP is better at delivering more power and handling lower-impedance loads because it’s a more efficient design. However, this comes at the expense of some of the tube warmth, as it’s a quasi-push-pull operation.
- Lower Output Impedance: SRPP gives a lower output impedance compared to single-ended designs, which is why it's often used for driving headphones or lower-impedance loads without additional buffering.
### 3. Single-Ended with MOSFET Buffer:
- Since you're concerned about maintaining the tube character while still achieving low output impedance, the combination of a single-ended triode stage for voltage amplification and a MOSFET source follower for current buffering is a great compromise.
- Preserves Tube Sound: The single-ended triode stage will give you the desired tube harmonics and warmth, while the MOSFET stage (which doesn’t contribute much to the sound character) will provide the necessary current to drive low-impedance headphones.
- Adjustable Tube Character: If you want to fine-tune the tube character, you can achieve this by:
- Adjusting the cathode bias to shift the operating point of the tube.
- Adding a cathode bypass capacitor (Ck) with switchable values or even removing it entirely to adjust local feedback and harmonic distortion.
### 4. Final Recommendation:
- Stick with Single-Ended Operation: To preserve the tube character you value, keep the single-ended triode stage (SET) for your voltage amplification.
- Use MOSFET Source Follower for Buffering: The MOSFET buffer will ensure that you get low output impedance and adequate current drive for low-impedance headphones, without interfering with the tube's harmonic signature.
- Adjustable Tube Character: Implement an adjustable cathode bias or use different bypass capacitor values to allow control over how much of the tube's character (i.e., harmonic content) is present in the sound.
### Steps to Enhance Tube Character Adjustability:
1. Cathode Bypass Capacitor (Ck):
- You can make the bypass capacitor switchable or variable. This would allow you to adjust the amount of local feedback in the tube stage, which directly affects harmonic distortion and gain.
- No capacitor: More local feedback, lower gain, lower distortion.
- Large capacitor (e.g., 470 µF): Less feedback, higher gain, more tube character and harmonics.
2. Variable Bias Control:
- Adding a bias adjustment potentiometer (as you already have) can let you move the tube’s operating point up or down, changing the harmonic profile and providing more control over the sound.
### Why This Approach Works:
- By maintaining the single-ended triode configuration and pairing it with a MOSFET follower for current buffering, you’re getting the best of both worlds:
- Tube harmonic richness from the single-ended stage.
- Low output impedance and the ability to drive a wide range of headphone impedances from the MOSFET stage.
You’re also retaining flexibility by making the tube character adjustable through bias and bypass capacitor options.
### Conclusion:
To meet your goal of adjustable tube character, it’s best to stick with single-ended operation and implement adjustable cathode bias or bypass capacitor switching to control the sound. The MOSFET source follower is an excellent choice for maintaining low output impedance without altering the tube’s sonic signature. Let me know if you'd like further guidance on the bias adjustment or bypass capacitor switching circuits!