Congrats on deciding to build your own mic pre-amp! This is something the will be wholly unique to you, something with our own sound signature that you can take pride in. The Silver kit comes with all of the basic parts to start your circuit. These are standardized parts like the output, phantom, and DI circuitry. The rest of the parts are dependent upon which op-amps and transformers you select. They are also dependent on how you want your build to sound. Let's review the components not included with the kit. Start by downloading the schematic.
In addition to the main op-amp, DI op-amp, and in/out transformers, you'll need to decide on these parts:
1. The four components in this section form the power op-amp rails filter/decoupling network. R100 and C104 sit on the +16VDC rail and R101 and C105 sit on the -16VDC rail. C106 and C107 also are part of this network, however, the standard 0.1uF value caps are included in the base kit and are built into some op-amps.
Suggested values
R100/101: 10Ω
C104/105: 100uF (Nichicon VP/NHG/HE)
2. This section contains the input transformer load resistor (R501) and Zobel network (R500 and C500). These values are dependent on the transformer used. Typically, the datasheet for your transformers will contain the suggested values. In some cases, you may not need all three components or even any at all. The load resistor sits in parallel with the secondary windings. If there is no load resistor needed, leave it empty. The Zobel network is a resistor and capacitor in series that sits in parallel with the secondary windings. If there is only a single capacitor or single resistor for the suggested Zobel, you'll need to short the remaining component that is not used to complete the parallel circuit. If neither is needed, leave them both empty. The goal of the Zobel is to provide proper damping. This can be checked with a square wave generator, oscilloscope, and R/C substitution boxes. The procedure can be found here:
http://groupdiy.com/index.php?topic=32441.msg393923#msg393923
Suggested values
See transformer datasheet. Hairball Audio recommends polystyrene and MICA capacitors for Zobel networks.
3. This section contains the output transformer load resistor (R607) and zobel network (R608 and C600). These values are dependent on the transformer used. Please refer to #2.
4. These components form the feedback/gain network. C505/506 are DC blockers. C505 prevents DC from entering the gain pot and creating noise (or damage). C506 prevents DC at the op-amp output from entering the output fader circuit. R506 provides HF load isolation for feedback loop stability and may not be necessary. R504 closes the op amp's feedback loop and determines minimum gain. C504 provides HF filtering and compensates for HF phase lag, the value is dependent on the op-amp used. R505 limits the max gain of the gain pot and is necessary for stability.
Suggested values
C504: See op-amp data sheet. Generally 33pF-220pF/50V (COC/NPO) or omitted.
C505: 470uF/25V (Nichicon VP). May also be a short if DC offset is negligible.
C506: 220uF/25V (Nichicon VP). May also be a short if DC offset is negligible.
R504: 20k
R506: 10Ω – 100Ω
R505: 20Ω – 50Ω
5. These components sit between the input transformer secondary (after the load and zobel) and the non-inverting input of the op-amp. C501 is a DC blocker. It blocks DC offset at the non-inverting input to avoid input transformer damage. R503 may not be necessary (if in doubt, short) but can help to protect an op-amp by limiting max input current, and may also be used to build out the source impedance, optimizing for a specific op amp's noise performance. If C501 is present, then R502 is very necessary, as it drains the DC blocking cap and provides a ground referenced input bias point for the op-amp.
C502 is a filter capacitor that is dependent on the op-amp used. Only a minority of specific op-amps will require this capacitor. Most will perform better without it.
Suggested values
C501: 0.47uF/50V polyester film. May also be a short if DC offset is negligible.
C502: Check op-amp datasheet or app note. Generally omitted.
R502: 200KΩ
R503: 1.2KΩ
6. C503 is only required if an IC is used instead of DOA, and only if the IC used is not unity gain stable and/or not internally compensated.
Suggested values
C503: Omit
7. C302 is added amplifier stabilization for the DI IC. C302 is only required if IC300 is not unity gain stable and/or not internally compensated.
Suggested values
C303: 33pF/50V (COC/NPO)
8. C601 is a DC blocker. R600 limits max attenuation of output pot.
Suggested values
C601: 220uF/25V (Nichicon VP). May be polarized depending on detectable DC bias voltage. May be replaced with short if DC offset is negligible.
R600: 100Ω (for -40 dB)
9. These three resistors form the PAD switch's u-pad attenuator.
Suggested values
20dB Pad
R404: 620Ω
R405: 620Ω
R406: 150Ω
15dB Pad
R404: 330Ω
R405: 330Ω
R406: 150Ω
10dB Pad
R404: 160Ω
R405: 160Ω
R406: 150Ω
10. R606 is used to limit output short circuit current, and to drive the output transformer less efficiently for audible effect.
Suggested values
R606: 10Ω – 150Ω
Assembly
For instructions on enclosure, front panel, and Grayhill assembly, please reference the main Elements build guide.