Design Philosophies and Architecture Notes



The main factors differentiating microphone pre-amp systems are the differences in the gain stage designs, how the gain stage designs are implemented within the product, and how the product interacts with the entire recording system. The microphone-to-pre-amp impedance matching, resulting bandwidth effects, noise matching and mic loading consistency versus pre-amp settings are profoundly affected by the interface of the microphone to the pre-amp. Just as profound are the gain-stage effects on the transient response, distortion, and bandwidth as the gain stage amplifies the audio signal from microvolt levels to Volt levels. The microphone and pre-amp are especially critical since these two components have arguably the most potential to shape the sound of any component in the audio recording chain, excluding EQ and other deliberate effects. Together, the design techniques and components used for Ingram Engineering equipment realize tools for audio recording that maximize the potential of the recording situation.


We use innovative circuits and implementations to realize a combination of audio quality and features not found in the market, at any price point.  Circuit design, components or features that are critical to the audio or build quality are approached from the "cost is no object" model as much as is feasible, while overall product cost is kept as low as possible to provide the maximum value.


Our designs commonly use JFET topologies and transformer coupled input / output interfaces.  Several of the designs use the socketed gain block form factor popularized by the API2520 discrete op amp.  The pre-amps contain features that are not typically found in the majority of high-end audio gear, such as complete Direct Box capability, re-amping capability and the ability to control the impedance presented to microphones while maximizing dynamic range. 


While the equipment can be used in ultra-linear modes of operation and can provide transparent processing, strengths lie in the ability of the chosen topologies and components to add color to a recording situation if the situation demands it.


In total, the microphone pre-amp models provide highly flexible tools for use with any recording situation, providing superb sound using everything from the best boutique microphones and processing / recording gear to budget mics and equipment that require help in order to maximize the audio quality.  Additional technical and architecture details are given in the following sections.


Mic Impedances Loads


Audio system design techniques were originally influenced by the telephone distributions considerations, and power matching between the source and load used to be a requirement so that max power could be transferred to the load. Power matching dictated that the real part of the source impedance matched the real part of the load impedance. Under these conditions, optimum noise matching was also realized if there were no circuits that added electrical noise to the system. Systems used to be designed for 600 Ohms balanced pair distribution, specified at a measurement frequency of 800Hz to 1kHz. This impedance was determined by the physical size and arrangement of the wires and their shield and the need to match to telephone relay coils’ impedances. Power matching is not required today. Maximum voltage transfer to the pre-amp load is usually a goal, and mic to pre-amp interaction is optimized for voltage transfer, noise and impedance loading presented to the mic.  Detailed information regarding mic impedance loading is contained in this section. 


Not only does the load presented to a mic have an effect on voltage transfer and system noise, but the load affects the frequency response, dynamic response and distortion of circuits inside a mic, particularly in the case of some condenser and ribbon mic designs. 


Mic pre-amps in recording equipment commonly present a fixed configuration that is a good match for a specific mic type. However, because mic loading has such an effect on audio tones, changing the mic loading can therefore be used as an additional tool to control the audio tone.  The Ingram Engineering MPA681, MPA683 and MPA685 pre-amps are configured to present variable impedance to mics, therefore enabling tones to be optimized for the widest selection of mic types.  Impedance configurations can easily be changed via a front panel rotary switch.


The MPA681 is configured to provide very low to low-medium impedance to the mic and is tailored to passive mics and active mics that operate with low to low-medium load impedances. The MPA683 and MPA685 are configured to provide low, medium or high impedance to the mic and are particularly tailored to cover applications with condenser or ribbon mics that require a high impedance load for best operation. The MPA685 impedance configuration is the widest, as it can be configured via factory-installed options for any impedance configuration, from very low to high. Each transformer based Ingram Engineering design is configured to provide the transformer with constant and optimum load at the secondary for all pre-amp gain and attenuation settings, therefore realizing consistent, predictable mic load in all cases.


A summary of mic load impedance and ranges of optimal mic total noise vs. transformer tap position are shown below for several models.


Mic Impedance Selector Position Position 1 Position 2 Position 3
Load Presented to Mic, MPA681 80 250 700
Load Presented to Mic, MPA683 900 3.5k 14k
Load Presented to Mic, MPA685 (Option 1) 80 250 700
Load Presented to Mic, MPA685 (Option 2) 600 1.5k 2.5k

Table 1: Summary of Mic Load vs. Transformer Tap Position, MPA681, MPA683 and MPA685


The flexibility of system configuration provided by these pre-amps makes these pre-amp designs extremely useful tools for any recording situation.


Attenuation / Gain Configuration Advantages


The level setting features of the Ingram Engineering pre-amps allow the recording engineer to optimize dynamic range with minimum effort. The user may notice that the classic 20dB attenuator input pad is not used with the designs. Equipment that relies on a 20dB pad to protect the system from signal clipping can suffer a loss of dynamic range if the 20dB pad is activated to protect the system from infrequent audio peaks. For example, if infrequent peaks that are 3dB higher than max audio level occur, activating a 20dB fixed pad will result in a loss of as much as 17dB dynamic range. Only 3dB (or more, if margin is desired) was needed, but the pad value is fixed at 20dB, and the signal level is unnecessarily attenuated.


Perhaps more important is the fact that these Ingram Engineering pre-amps preserve the mic to pre-amp impedance interface for all settings, even when the attenuation mode is engaged. The majority of other pre-amp designs typically accomplish padding by switching in a resistive pad that completely changes the complex load presented to the mic and therefore changes the tones.


Instead, the Ingram Engineering pre-amps are designed with a single stepped switch that is used to set the input audio level using an intuitive, simple procedure. By adjusting the stepped switch so that the level is set as high as possible, but at least one step below the level at which the "Overload" LED illuminates, optimal signal to noise, dynamic range and headroom are realized. The stepped switch is configured to seamlessly switch in varying amounts of attenuation when required by high level audio program material, or remove the attenuators from the audio path and control gain, only, when required by low level audio program material. The transition between attenuation and gain modes is not ultimately important to the user and is not explicitly indicated. This technique, which mimics Automatic Gain Control (AGC) circuits, leaves no dynamic range or signal to noise dB "on the table". When the input level has been set, the output level control is used to set the mic pre-amp output level so that it is compatible with other processing or recording equipment in the audio signal chain.


Gain Stage Designs and the OA101 Discrete Op Amp


The microphone pre-amp gain stage has the job of amplifying microVolts of audio signal by 3000x or more and has the responsibility of determining the system noise performance while preserving the dynamics and frequency domain content of the audio or coloring the content in a somehow pleasing manner.


The heart of the MPA681, MPA683 and MAP685 is a discrete op amp design packaged into the modular API2520 form factor. The MPA201 uses a PCB mounted discrete op amp design.  The OA101 circuit is a minimal component discrete design, and consists of a low noise JFET differential input stage and Class A BJT output. The Ingram Engineering design is based on the ground-breaking work of Dr. Marshall Leach of Georgia Institute of Technology Dr. Leach pioneered work to understand transient intermodulation (TIM) distortion, then, design topologies that minimized their effect on audio signals. Most important, they sound great. The OA101 design has excellent transient response and low TIM, and has been judged to sound superior to a veritable who's-who of classic and modern audio designs in A-B listening tests.