OPEN SOURCE DIY Mic Project - ORS 87 - Stripped Down u87

[micolas]

Pink one from a gen 2 C1.

It looks very similar to the capsule in my SP B1 microphones. I modded them, they were too bright, and I threw them in a box for several years.
Maybe I should try this capsule in a u87 circuit. Thanks!

 

[homero.leal]

There's a couple of photos of the Homero board build in the thread, one here:
https://groupdiy.com/threads/open-source-diy-mic-project-ors-87-stripped-down-u87.86814/post-1147018

Just an update @Wordsushi, V1.1 is already updated on PCBWay Shared Projects site, same link as before. Changes on this version are:
- Component names were updated to match original @OneRoomStudio schematic naming
- Added additional caps to allow playing with EQ tunning (220pf, 33n)
- Additional small cosmetic fixes

V1.1 PCB and Schematic:


Cheers!

HL

 

[Wordsushi]

Just an update @Wordsushi, V1.1 is already updated on PCBWay Shared Projects site, same link as before. Changes on this version are:
- Component names were updated to match original @OneRoomStudio schematic naming
- Added additional caps to allow playing with EQ tunning (220pf, 33n)
- Additional small cosmetic fixes

V1.1 PCB and Schematic:
View attachment 128811View attachment 128812

Cheers!

HL

Homero! This is fantastic! Thank you for continuing to evolve this board and this project. I love the new additions and I really do like the flexibility to experiment wildly with C7. Bravo for fitting this all on here so nicely.

 

[Papa Tango Charly]

Thank you for keeping the ORR87 Alive
I have order few PCB with an another french Groupdiy member. V1 version
I didn't really understand the changes for
C7 1uF (can tweak to taste. smaller = less bass)
4x1uf seems a lot of bass

C5: 33n (can go as big as 330n if you want more low-end)
Could tell us the 4 different existing value for C5 that would be worth trying


C6: 220p (treble cap - smaller = more treble, bigger = darker) could you tell us 4 different existing value for C6 that would be worth trying

I will you give my feedback as a singer

Thank you
Best regards peace out

 

[Khron]

I didn't really understand the changes for
C7 1uF (can tweak to taste. smaller = less bass)
4x1uf seems a lot of bass

You can't physically fit all four on the board at the same time anyway. They're just there for alternate footprint options.

 

[Papa Tango Charly]

Thank you again for all this input
I got it

I have tried electrolytic on a T84 I prefer good old polypropylene axial or square. Vishay MKP. I have the impression that Pio need more time to burn…

Do you have 4 standard alternative for C5 and C6?

Is R* an alternative for the trimmer?

 

[elskardio]

Do you have 4 standard alternative for C5 and C6?

https://groupdiy.com/threads/u87-ai-circuit-how-low-high-can-you-go-de-emphasis-capacitors.85730/

 

[homero.leal]

Thank you for keeping the ORR87 Alive
I have order few PCB with an another french Groupdiy member. V1 version
I didn't really understand the changes for
C7 1uF (can tweak to taste. smaller = less bass)
4x1uf seems a lot of bass

As previously mentioned, different C7 footprint options are only to allow the placement of several kind of capacitors, but you should use only one of these. Smaller footprint is for tiny electrolytic or tantalum caps. Larger footprints are for bigger film capacitor options. So, use only one of these.

C5: 33n (can go as big as 330n if you want more low-end)
Could tell us the 4 different existing value for C5 that would be worth trying

In my case, I have always used 33n, but I haven't had any problems. Lows are just fine with 33n, but it may depend on the capsule you use.

C6: 220p (treble cap - smaller = more treble, bigger = darker) could you tell us 4 different existing value for C6 that would be worth trying

I will you give my feedback as a singer

Thank you
Best regards peace out

I have only used 220p and 160p, and for me, 220p is the prefered option, but again, it may depend on the capsule you are using.
I would try 100p, 160p, 220p, and 330p for C6.

Regards!

 

[Papa Tango Charly]

Great fantastichen!!!! Guys i'm so grateful! so many things to learn!
Best
Peace out!

 

[Khron]

I would try 100p, 160p, 220p, and 330p for C6.

150p might be a more standard (E12 series) value? Just in case it's easier and/or cheaper to source than 160p...

 

[DaveLTX]

I see the fake TLM103 being mentioned here, I had a empty TLM103 and got the "TLM103" circuit by accident but whatever capsule i'm trying (I've got 4 and they sound all sound bad!) I just sound exceptionally nasally


My best guess is that it is just poor. I ordered the transformer TLM103 board and got this... The seller just sent out the right board but I don't have much faith in that either

 

[ubxf]

I see the fake TLM103 being mentioned here, I had a empty TLM103 and got the "TLM103" circuit by accident but whatever capsule i'm trying (I've got 4 and they sound all sound bad!) I just sound exceptionally nasally
View attachment 128907
View attachment 128908
My best guess is that it is just poor. I ordered the transformer TLM103 board and got this... The seller just sent out the right board but I don't have much faith in that either

Isn't TLM supposed to be TransformerLessMicrophone ?

 

[Barry Hufker]

Yes...

 

[DaveLTX]

Isn't TLM supposed to be TransformerLessMicrophone ?

Of course. But apparently TransformerLessMicrophone does not translate necessarily particularly well into mandarin

 

[micolas]

Isn't TLM supposed to be TransformerLessMicrophone ?

In the world of fakes, everything is possible.
If they put a circuit without a transformer in some fake U87s, who stops them from putting a circuit with a transformer in the TLM body?

 

[DaveLTX]

In the world of fakes, everything is possible.
If they put a circuit without a transformer in some fake U87s, who stops them from putting a circuit with a transformer in the TLM body?

i've seen a fake 87 that they claim is a tube mic but its actually a tube diode and doesn't do anything for obvious reasons

Yeah... As if that tube diode's gonna do anything

 

[micolas]

i've seen a fake 87 that they claim is a tube mic but its actually a tube diode and doesn't do anything for obvious reasons
View attachment 128913
Yeah... As if that tube diode's gonna do anything

Excellent!
DIY at the highest level!

 

[Wordsushi]

So I uploaded the original ORS87 schematic to ChatGPT to analyze.



This is what it told me, along with answers to the questions of what changing the values of C5, C6 and C7, along with C12, C13 would do.

How did it do? Below are its responses.

****************************

This is a schematic for a condenser microphone circuit, likely intended for a professional audio application given the use of an XLR output connector (J1). Here’s a detailed breakdown of the components and their functions in the circuit:

1. Microphone Capsule (MK1):
   • The microphone capsule is represented by MK1. It is a condenser type that requires a polarization voltage to operate.
2. Bias Resistors (R8, R9):
   • R8 (1MΩ) and R9 (7.5kΩ) provide the necessary biasing for the microphone capsule. The large value of R8 ensures a high impedance necessary for the condenser microphone.
3. Coupling Capacitors (C5, C6):
   • C5 (33nF) blocks DC voltage while allowing audio signals to pass.
   • C6 (120pF) is likely for high-frequency filtering, providing stability and preventing oscillations.
4. Field-Effect Transistor (Q1, 2N3819):
   • Q1 is a JFET (2N3819) that acts as an impedance converter, converting the high impedance of the condenser capsule to a lower impedance more suitable for further amplification.
5. Bias Network for Q1 (R7, R10, C8):
   • R7 (1GΩ) sets the gate bias for the JFET.
   • R10 (10kΩ) and C8 (20µF) are part of the source resistor and bypass capacitor network, stabilizing the operating point of the JFET.
6. Transformer (T1):
   • T1 is a step-down transformer with a ratio of 9.5:1. This matches the high output impedance of the microphone amplifier to the lower input impedance of professional audio equipment.
   • The primary side of the transformer is connected to the drain of Q1, and the secondary side provides a balanced output.
7. Output Coupling and Filtering (C7, C10, C12, C13):
   • C7 (1µF) and C10 (47µF) block any DC component from the output signal.
   • C12 (10µF) and C13 (10nF) further filter the signal to remove noise and unwanted frequencies.
8. Feedback Network (R13, R15, C9, C11):
   • R13 (330kΩ), R15 (300kΩ), C9 (470nF), and C11 (4.7µF) form a feedback network that stabilizes the gain and frequency response of the circuit.
9. Load Resistors (R12, R14, R16, R18, R19):
   • R12 (47kΩ) sets the load for the JFET.
   • R14 (47kΩ), R16 (56kΩ), and R18/R19 (2.2kΩ each) set the correct loading and impedance matching for the transformer and output stage.
10. Zener Diode (GR1, 1N4744A):
    • GR1 (1N4744A) is a Zener diode used for voltage regulation, ensuring a stable operation voltage for the circuit components.
11. XLR Output (J1):
    • The XLR connector provides a balanced output, which is standard for professional audio equipment, offering better noise immunity and signal integrity.

Circuit Operation Summary:​

• The microphone capsule picks up sound and converts it into an electrical signal, which is then impedance-matched and amplified by the JFET.
• The signal is further conditioned through coupling capacitors and a transformer to provide a balanced low-impedance output suitable for long cable runs to audio equipment.
• Feedback and filtering networks ensure the signal is clean and stable, maintaining audio fidelity.

This schematic provides a robust design for a condenser microphone, focusing on achieving high audio quality and stable operation through careful component selection and circuit design.


What is the effect of increasing the value of C5?

In the provided microphone schematic, C5 is a coupling capacitor that blocks DC voltage while allowing the AC audio signal to pass from the microphone capsule to the gate of the JFET (Q1). Its value can significantly affect the frequency response of the circuit.

Effects of Increasing the Value of C5​

1. Lower Cut-off Frequency:
   • The primary effect of increasing the value of C5 is to lower the cut-off frequency of the high-pass filter formed by C5 and the input impedance of the gate of Q1.
   • The cut-off frequency (𝑓𝑐fc) of a high-pass filter is given by:
   • 
   • where R is the input resistance at the gate of Q1, and C is the value of C5.
   • By increasing C, the cut-off frequency decreases, allowing more low-frequency signals to pass through.
2. Enhanced Low-Frequency Response:
   • With a larger value of C5, the circuit will have an improved response to low-frequency sounds, which can be beneficial for capturing bass frequencies in audio recordings.
   • This means that the microphone will be able to pick up deeper tones, making the audio output richer and more full-bodied.
3. Potential for Increased Noise:
   • A larger C5 could also lead to an increased susceptibility to low-frequency noise, such as rumble or handling noise, since more low-frequency content is allowed to pass through.
4. Impact on Transient Response:
   • Larger capacitors can affect the transient response of the circuit. While the effect might be minimal, it could potentially cause a slight smearing of transient sounds due to the slower charging and discharging times of larger capacitors.

Practical Considerations​

• When choosing the value for C5, it's important to strike a balance between capturing low-frequency audio and avoiding unwanted noise.
• The original value of 33nF suggests a design that balances adequate low-frequency response without picking up too much low-frequency noise.
• If the environment is prone to low-frequency noise (e.g., vibrations, handling noise), it might be preferable to keep C5 smaller to filter out such disturbances.

In summary, increasing the value of C5 will lower the cut-off frequency of the high-pass filter formed with the gate input impedance of the JFET, thereby enhancing the low-frequency response of the microphone. However, it may also increase the susceptibility to low-frequency noise. Careful consideration and testing are necessary to optimize the capacitor value for the desired performance in the intended application.


What is the effect of changing the value of c6?

In the provided microphone schematic, C6 is a small capacitor connected in parallel with the microphone capsule. This capacitor is typically used for high-frequency filtering and stability.

Effects of Changing the Value of C6​

1. High-Frequency Response:
   • C6 affects the high-frequency response of the microphone circuit. Increasing the value of C6 will lower the cut-off frequency of the high-frequency attenuation, thereby reducing the circuit's response to higher frequencies. Conversely, decreasing the value of C6 will allow more high-frequency signals to pass through.
2. Stability and Oscillation Prevention:
   • C6 helps in preventing high-frequency oscillations. Increasing the value of C6 can enhance the stability of the circuit by providing more effective filtering of high-frequency noise or potential oscillations. This is especially important in high-gain circuits where parasitic capacitances can lead to instability.
3. Signal Integrity:
   • An appropriate value of C6 ensures that the microphone has a smooth and stable frequency response. Too high a value might excessively attenuate high frequencies, making the audio sound dull or muffled. Too low a value might not provide enough filtering, potentially allowing high-frequency noise or oscillations to affect the audio signal.

Practical Considerations​

• Current Value (120pF): The current value of 120pF suggests a design choice that balances filtering out unwanted high-frequency noise while preserving the essential high-frequency components of the audio signal.
• Increasing C6: If you increase the value of C6, say to 220pF or 330pF, the circuit will have a stronger high-frequency roll-off. This can help in environments with a lot of high-frequency interference but may result in a loss of detail in the audio's high-frequency range.
• Decreasing C6: If you decrease the value of C6, say to 68pF or 47pF, the circuit will have a higher cut-off frequency for high-frequency attenuation, allowing more high-frequency components to pass through. This can make the audio sound crisper but might risk introducing high-frequency noise or oscillations.

Summary​

Changing the value of C6 will modify the high-frequency response and stability of the microphone circuit:

• Increasing C6: More high-frequency attenuation, better stability, less high-frequency noise, but potentially duller audio.
• Decreasing C6: Less high-frequency attenuation, crisper audio, but increased risk of high-frequency noise and oscillations.

Choosing the optimal value for C6 involves balancing the need for high-frequency clarity with the requirement for circuit stability and noise reduction. It may require some experimentation and consideration of the specific use-case environment to find the best value.


What is the effect of changing the value of C7?

In the provided microphone schematic, C7 is a coupling capacitor that connects the drain of the JFET (Q1) to the primary winding of the transformer (T1). This capacitor is responsible for blocking any DC component from the drain of the JFET while allowing the AC audio signal to pass through to the transformer.

Effects of Changing the Value of C7​

1. Low-Frequency Response:
   • C7, in conjunction with the primary impedance of the transformer and any parallel resistances, forms a high-pass filter. The value of C7 determines the low-frequency cut-off point of this filter.
   • The cut-off frequency (𝑓𝑐fc) of the high-pass filter is given by:
     
   • where R is the combined impedance seen by the capacitor, and C is the value of C7.
   • Increasing the value of C7 will lower the cut-off frequency, allowing more low-frequency signals to pass through. Conversely, decreasing the value of C7 will raise the cut-off frequency, attenuating more low-frequency signals.
2. Bass Response:
   • A larger value for C7 improves the bass response of the microphone by allowing lower frequency audio signals to pass through without significant attenuation. This can make the microphone sound fuller and more natural, especially for sources with substantial low-frequency content.
   • A smaller value for C7 will reduce the bass response, potentially making the audio sound thinner and less full.
3. Signal Integrity:
   • Proper selection of C7 is important to ensure that the microphone can accurately reproduce the intended frequency range. If C7 is too small, it may cut off important low-frequency components, leading to a loss of warmth and depth in the audio signal.
   • If C7 is too large, it could pass unnecessary low-frequency noise or rumble, which might not be desirable in all recording situations.

Practical Considerations​

• Current Value (1µF): The current value of 1µF is chosen to balance low-frequency response with practical component size and cost. This value ensures that the microphone can capture a full range of audio frequencies down to a reasonable low-frequency limit.
• Increasing C7: If you increase C7 to, say, 2.2µF or 4.7µF, the cut-off frequency will decrease, allowing even lower frequencies to pass through. This can enhance the bass response but may also introduce more low-frequency noise.
• Decreasing C7: If you decrease C7 to, say, 0.47µF or 0.22µF, the cut-off frequency will increase, attenuating lower frequencies. This can help reduce low-frequency noise and rumble but may also make the audio sound thinner.

Summary​

Changing the value of C7 will impact the low-frequency response and overall tonal balance of the microphone circuit:

• Increasing C7: Enhances bass response by lowering the cut-off frequency, allowing more low-frequency signals to pass. Potentially introduces more low-frequency noise.
• Decreasing C7: Reduces bass response by raising the cut-off frequency, attenuating more low-frequency signals. Potentially reduces low-frequency noise and rumble.

Choosing the optimal value for C7 involves balancing the desired low-frequency performance with the need to minimize unwanted low-frequency noise. This choice will depend on the specific application and the type of audio being recorded.

What are the effects of increasing the value of C12 and C13?

In the provided microphone schematic, C12 and C13 are capacitors that play roles in signal coupling and filtering. Let’s examine each one and the effects of increasing their values:

C12 (10µF)​

C12 is a coupling capacitor connected to the secondary winding of the transformer and the output circuit. Its primary function is to block any DC component while allowing AC audio signals to pass through.
Effects of Increasing the Value of C12:

1. Low-Frequency Response:
   • Increasing the value of C12 will lower the cut-off frequency of the high-pass filter formed with the output impedance, improving the microphone's ability to pass lower frequency signals.
   • This can result in a better bass response, making the audio output sound fuller and richer.
2. Bass Response:
   • A higher value capacitor will allow more low-frequency audio signals to pass through, enhancing the microphone’s bass response.
   • This can be particularly beneficial for capturing instruments or voices with significant low-frequency content.
3. Signal Integrity:
   • Proper selection of C12 ensures that the microphone accurately reproduces the intended frequency range. An increased value helps in maintaining the low-frequency integrity of the audio signal.

Practical Considerations for C12:​

• Current Value (10µF): This value is chosen to balance the low-frequency response with the physical size and cost of the capacitor.
• Increasing C12: If you increase C12 to, say, 22µF or 47µF, the cut-off frequency will decrease, improving low-frequency performance but potentially introducing more low-frequency noise.
• Decreasing C12: If you decrease C12 to 4.7µF or 2.2µF, the cut-off frequency will increase, attenuating lower frequencies, which might result in a thinner sound.

C13 (10nF)​

C13 is a capacitor connected to the ground and typically used for high-frequency filtering.
Effects of Increasing the Value of C13:

1. High-Frequency Filtering:
   • Increasing the value of C13 will lower the cut-off frequency of the low-pass filter, attenuating more high-frequency signals.
   • This can help in reducing high-frequency noise or interference, leading to a cleaner audio signal.
2. Signal Clarity:
   • While increasing C13 can help reduce unwanted high-frequency noise, it can also potentially attenuate desirable high-frequency components of the audio signal, leading to a duller sound.

Practical Considerations for C13:​

• Current Value (10nF): This value is chosen to balance high-frequency noise reduction without significantly impacting the desired audio signal.
• Increasing C13: If you increase C13 to, say, 22nF or 47nF, the cut-off frequency will decrease, providing more high-frequency attenuation. This can help reduce hiss and other high-frequency noise but may also reduce the brightness and detail of the audio.
• Decreasing C13: If you decrease C13 to 4.7nF or 2.2nF, the cut-off frequency will increase, allowing more high-frequency signals to pass through. This can make the audio sound crisper but might introduce more high-frequency noise.

Summary​

Increasing C12:

• Enhances bass response by allowing more low-frequency signals to pass.
• Improves low-frequency signal integrity but might introduce more low-frequency noise.

Increasing C13:

• Enhances high-frequency attenuation, reducing high-frequency noise.
• Can lead to a duller sound if it attenuates too much of the desired high-frequency content.

Choosing the optimal values for C12 and C13 involves balancing the desired frequency response characteristics with noise reduction and signal integrity. The specific values will depend on the application and the desired tonal qualities of the microphone.

 

[micolas]

In the world of fakes, everything is possible.
If they put a circuit without a transformer in some fake U87s, who stops them from putting a circuit with a transformer in the TLM body?

And yet....
If you want you can!


https://a.aliexpress.com/_EuaCfPV

 

[JeffKall]

So I uploaded the original ORS87 schematic to ChatGPT to analyze.

View attachment 128965

This is what it told me, along with answers to the questions of what changing the values of C5, C6 and C7, along with C12, C13 would do.

How did it do? Below are its responses.

****************************

This is a schematic for a condenser microphone circuit, likely intended for a professional audio application given the use of an XLR output connector (J1). Here’s a detailed breakdown of the components and their functions in the circuit:

1. Microphone Capsule (MK1):
   • The microphone capsule is represented by MK1. It is a condenser type that requires a polarization voltage to operate.
2. Bias Resistors (R8, R9):
   • R8 (1MΩ) and R9 (7.5kΩ) provide the necessary biasing for the microphone capsule. The large value of R8 ensures a high impedance necessary for the condenser microphone.
3. Coupling Capacitors (C5, C6):
   • C5 (33nF) blocks DC voltage while allowing audio signals to pass.
   • C6 (120pF) is likely for high-frequency filtering, providing stability and preventing oscillations.
4. Field-Effect Transistor (Q1, 2N3819):
   • Q1 is a JFET (2N3819) that acts as an impedance converter, converting the high impedance of the condenser capsule to a lower impedance more suitable for further amplification.
5. Bias Network for Q1 (R7, R10, C8):
   • R7 (1GΩ) sets the gate bias for the JFET.
   • R10 (10kΩ) and C8 (20µF) are part of the source resistor and bypass capacitor network, stabilizing the operating point of the JFET.
6. Transformer (T1):
   • T1 is a step-down transformer with a ratio of 9.5:1. This matches the high output impedance of the microphone amplifier to the lower input impedance of professional audio equipment.
   • The primary side of the transformer is connected to the drain of Q1, and the secondary side provides a balanced output.
7. Output Coupling and Filtering (C7, C10, C12, C13):
   • C7 (1µF) and C10 (47µF) block any DC component from the output signal.
   • C12 (10µF) and C13 (10nF) further filter the signal to remove noise and unwanted frequencies.
8. Feedback Network (R13, R15, C9, C11):
   • R13 (330kΩ), R15 (300kΩ), C9 (470nF), and C11 (4.7µF) form a feedback network that stabilizes the gain and frequency response of the circuit.
9. Load Resistors (R12, R14, R16, R18, R19):
   • R12 (47kΩ) sets the load for the JFET.
   • R14 (47kΩ), R16 (56kΩ), and R18/R19 (2.2kΩ each) set the correct loading and impedance matching for the transformer and output stage.
10. Zener Diode (GR1, 1N4744A):
    • GR1 (1N4744A) is a Zener diode used for voltage regulation, ensuring a stable operation voltage for the circuit components.
11. XLR Output (J1):
    • The XLR connector provides a balanced output, which is standard for professional audio equipment, offering better noise immunity and signal integrity.

Circuit Operation Summary:​

• The microphone capsule picks up sound and converts it into an electrical signal, which is then impedance-matched and amplified by the JFET.
• The signal is further conditioned through coupling capacitors and a transformer to provide a balanced low-impedance output suitable for long cable runs to audio equipment.
• Feedback and filtering networks ensure the signal is clean and stable, maintaining audio fidelity.

This schematic provides a robust design for a condenser microphone, focusing on achieving high audio quality and stable operation through careful component selection and circuit design.


What is the effect of increasing the value of C5?

In the provided microphone schematic, C5 is a coupling capacitor that blocks DC voltage while allowing the AC audio signal to pass from the microphone capsule to the gate of the JFET (Q1). Its value can significantly affect the frequency response of the circuit.

Effects of Increasing the Value of C5​

1. Lower Cut-off Frequency:
   • The primary effect of increasing the value of C5 is to lower the cut-off frequency of the high-pass filter formed by C5 and the input impedance of the gate of Q1.
   • The cut-off frequency (𝑓𝑐fc) of a high-pass filter is given by:
   • View attachment 128968
   • where R is the input resistance at the gate of Q1, and C is the value of C5.
   • By increasing C, the cut-off frequency decreases, allowing more low-frequency signals to pass through.
2. Enhanced Low-Frequency Response:
   • With a larger value of C5, the circuit will have an improved response to low-frequency sounds, which can be beneficial for capturing bass frequencies in audio recordings.
   • This means that the microphone will be able to pick up deeper tones, making the audio output richer and more full-bodied.
3. Potential for Increased Noise:
   • A larger C5 could also lead to an increased susceptibility to low-frequency noise, such as rumble or handling noise, since more low-frequency content is allowed to pass through.
4. Impact on Transient Response:
   • Larger capacitors can affect the transient response of the circuit. While the effect might be minimal, it could potentially cause a slight smearing of transient sounds due to the slower charging and discharging times of larger capacitors.

Practical Considerations​

• When choosing the value for C5, it's important to strike a balance between capturing low-frequency audio and avoiding unwanted noise.
• The original value of 33nF suggests a design that balances adequate low-frequency response without picking up too much low-frequency noise.
• If the environment is prone to low-frequency noise (e.g., vibrations, handling noise), it might be preferable to keep C5 smaller to filter out such disturbances.

In summary, increasing the value of C5 will lower the cut-off frequency of the high-pass filter formed with the gate input impedance of the JFET, thereby enhancing the low-frequency response of the microphone. However, it may also increase the susceptibility to low-frequency noise. Careful consideration and testing are necessary to optimize the capacitor value for the desired performance in the intended application.


What is the effect of changing the value of c6?

In the provided microphone schematic, C6 is a small capacitor connected in parallel with the microphone capsule. This capacitor is typically used for high-frequency filtering and stability.

Effects of Changing the Value of C6​

1. High-Frequency Response:
   • C6 affects the high-frequency response of the microphone circuit. Increasing the value of C6 will lower the cut-off frequency of the high-frequency attenuation, thereby reducing the circuit's response to higher frequencies. Conversely, decreasing the value of C6 will allow more high-frequency signals to pass through.
2. Stability and Oscillation Prevention:
   • C6 helps in preventing high-frequency oscillations. Increasing the value of C6 can enhance the stability of the circuit by providing more effective filtering of high-frequency noise or potential oscillations. This is especially important in high-gain circuits where parasitic capacitances can lead to instability.
3. Signal Integrity:
   • An appropriate value of C6 ensures that the microphone has a smooth and stable frequency response. Too high a value might excessively attenuate high frequencies, making the audio sound dull or muffled. Too low a value might not provide enough filtering, potentially allowing high-frequency noise or oscillations to affect the audio signal.

Practical Considerations​

• Current Value (120pF): The current value of 120pF suggests a design choice that balances filtering out unwanted high-frequency noise while preserving the essential high-frequency components of the audio signal.
• Increasing C6: If you increase the value of C6, say to 220pF or 330pF, the circuit will have a stronger high-frequency roll-off. This can help in environments with a lot of high-frequency interference but may result in a loss of detail in the audio's high-frequency range.
• Decreasing C6: If you decrease the value of C6, say to 68pF or 47pF, the circuit will have a higher cut-off frequency for high-frequency attenuation, allowing more high-frequency components to pass through. This can make the audio sound crisper but might risk introducing high-frequency noise or oscillations.

Summary​

Changing the value of C6 will modify the high-frequency response and stability of the microphone circuit:

• Increasing C6: More high-frequency attenuation, better stability, less high-frequency noise, but potentially duller audio.
• Decreasing C6: Less high-frequency attenuation, crisper audio, but increased risk of high-frequency noise and oscillations.

Choosing the optimal value for C6 involves balancing the need for high-frequency clarity with the requirement for circuit stability and noise reduction. It may require some experimentation and consideration of the specific use-case environment to find the best value.


What is the effect of changing the value of C7?

In the provided microphone schematic, C7 is a coupling capacitor that connects the drain of the JFET (Q1) to the primary winding of the transformer (T1). This capacitor is responsible for blocking any DC component from the drain of the JFET while allowing the AC audio signal to pass through to the transformer.

Effects of Changing the Value of C7​

1. Low-Frequency Response:
   • C7, in conjunction with the primary impedance of the transformer and any parallel resistances, forms a high-pass filter. The value of C7 determines the low-frequency cut-off point of this filter.
   • The cut-off frequency (𝑓𝑐fc) of the high-pass filter is given by:
     View attachment 128969
   • where R is the combined impedance seen by the capacitor, and C is the value of C7.
   • Increasing the value of C7 will lower the cut-off frequency, allowing more low-frequency signals to pass through. Conversely, decreasing the value of C7 will raise the cut-off frequency, attenuating more low-frequency signals.
2. Bass Response:
   • A larger value for C7 improves the bass response of the microphone by allowing lower frequency audio signals to pass through without significant attenuation. This can make the microphone sound fuller and more natural, especially for sources with substantial low-frequency content.
   • A smaller value for C7 will reduce the bass response, potentially making the audio sound thinner and less full.
3. Signal Integrity:
   • Proper selection of C7 is important to ensure that the microphone can accurately reproduce the intended frequency range. If C7 is too small, it may cut off important low-frequency components, leading to a loss of warmth and depth in the audio signal.
   • If C7 is too large, it could pass unnecessary low-frequency noise or rumble, which might not be desirable in all recording situations.

Practical Considerations​

• Current Value (1µF): The current value of 1µF is chosen to balance low-frequency response with practical component size and cost. This value ensures that the microphone can capture a full range of audio frequencies down to a reasonable low-frequency limit.
• Increasing C7: If you increase C7 to, say, 2.2µF or 4.7µF, the cut-off frequency will decrease, allowing even lower frequencies to pass through. This can enhance the bass response but may also introduce more low-frequency noise.
• Decreasing C7: If you decrease C7 to, say, 0.47µF or 0.22µF, the cut-off frequency will increase, attenuating lower frequencies. This can help reduce low-frequency noise and rumble but may also make the audio sound thinner.

Summary​

Changing the value of C7 will impact the low-frequency response and overall tonal balance of the microphone circuit:

• Increasing C7: Enhances bass response by lowering the cut-off frequency, allowing more low-frequency signals to pass. Potentially introduces more low-frequency noise.
• Decreasing C7: Reduces bass response by raising the cut-off frequency, attenuating more low-frequency signals. Potentially reduces low-frequency noise and rumble.

Choosing the optimal value for C7 involves balancing the desired low-frequency performance with the need to minimize unwanted low-frequency noise. This choice will depend on the specific application and the type of audio being recorded.

What are the effects of increasing the value of C12 and C13?

In the provided microphone schematic, C12 and C13 are capacitors that play roles in signal coupling and filtering. Let’s examine each one and the effects of increasing their values:

C12 (10µF)​

C12 is a coupling capacitor connected to the secondary winding of the transformer and the output circuit. Its primary function is to block any DC component while allowing AC audio signals to pass through.
Effects of Increasing the Value of C12:

1. Low-Frequency Response:
   • Increasing the value of C12 will lower the cut-off frequency of the high-pass filter formed with the output impedance, improving the microphone's ability to pass lower frequency signals.
   • This can result in a better bass response, making the audio output sound fuller and richer.
2. Bass Response:
   • A higher value capacitor will allow more low-frequency audio signals to pass through, enhancing the microphone’s bass response.
   • This can be particularly beneficial for capturing instruments or voices with significant low-frequency content.
3. Signal Integrity:
   • Proper selection of C12 ensures that the microphone accurately reproduces the intended frequency range. An increased value helps in maintaining the low-frequency integrity of the audio signal.

Practical Considerations for C12:​

• Current Value (10µF): This value is chosen to balance the low-frequency response with the physical size and cost of the capacitor.
• Increasing C12: If you increase C12 to, say, 22µF or 47µF, the cut-off frequency will decrease, improving low-frequency performance but potentially introducing more low-frequency noise.
• Decreasing C12: If you decrease C12 to 4.7µF or 2.2µF, the cut-off frequency will increase, attenuating lower frequencies, which might result in a thinner sound.

C13 (10nF)​

C13 is a capacitor connected to the ground and typically used for high-frequency filtering.
Effects of Increasing the Value of C13:

1. High-Frequency Filtering:
   • Increasing the value of C13 will lower the cut-off frequency of the low-pass filter, attenuating more high-frequency signals.
   • This can help in reducing high-frequency noise or interference, leading to a cleaner audio signal.
2. Signal Clarity:
   • While increasing C13 can help reduce unwanted high-frequency noise, it can also potentially attenuate desirable high-frequency components of the audio signal, leading to a duller sound.

Practical Considerations for C13:​

• Current Value (10nF): This value is chosen to balance high-frequency noise reduction without significantly impacting the desired audio signal.
• Increasing C13: If you increase C13 to, say, 22nF or 47nF, the cut-off frequency will decrease, providing more high-frequency attenuation. This can help reduce hiss and other high-frequency noise but may also reduce the brightness and detail of the audio.
• Decreasing C13: If you decrease C13 to 4.7nF or 2.2nF, the cut-off frequency will increase, allowing more high-frequency signals to pass through. This can make the audio sound crisper but might introduce more high-frequency noise.

Summary​

Increasing C12:

• Enhances bass response by allowing more low-frequency signals to pass.
• Improves low-frequency signal integrity but might introduce more low-frequency noise.

Increasing C13:

• Enhances high-frequency attenuation, reducing high-frequency noise.
• Can lead to a duller sound if it attenuates too much of the desired high-frequency content.

Choosing the optimal values for C12 and C13 involves balancing the desired frequency response characteristics with noise reduction and signal integrity. The specific values will depend on the application and the desired tonal qualities of the microphone.

Thank you for your concept and taking the time to share the results, sir !

This is an extremely intriguing and enlightening analytical scenario for ChatGPT !

I personally found the ChatGPT analysis to be extremely well constructed and educational (it helps to ask well worded questions … kudos to you again) !

Conceptually … I could certainly envision using AI (there may be a better choice than ChatGPT … but it was still impressive) as a very fundamental educational tool to help DIY Newbs like me learn the specifics of circuit design !

Thank you for birthing the concept, @Wordsushi !!!

Well done and greatly appreciated !