FAQ: What output attenuator settings will give me the best possible sound with the DAC1?

The output attenuators of the DAC1 and the DAC1 USB optimize the signal to noise ratio (SNR) of the interface for maximum performance. They also determine the volume control position at normal listening levels. The output attenuators are factory preset to -20 dB. This setting is required for many active speakers and power amplifiers but may not be optimal for your specific equipment. Please note that the attenuators do not affect the levels at the RCA outputs, which are designed for consumer-level signals. This article explains how these attenuators should be programmed.

Amplifiers, preamplifiers, and active speakers are all manufactured with different input sensitivities. Many clip at levels that are substantially below that of typical “+4 dBu” studio levels. Line-level analog interfaces in most recording studios are designed to handle levels as high as +24 dBu to +29 dBu. Power amplifiers and active speakers often clip at levels that are 20 to 30 dB lower than these peak studio levels. This huge difference in input sensitivity can cause poor audio performance if components are not properly matched. The DAC1 and DAC1 USB are equipped with programmable output attenuators that allow them to drive a wide variety of devices without any loss in performance. Below is a step-by-step guide to optimize your DAC1 attenuator settings.

Optimize the Interface Level: Reduce the Gain of the Driven Device

A high signal level between audio components maximizes dynamic range by keeping audio levels well above the thermal noise of each audio stage; and well above the level of any power line interference. The following procedure will maximize the signal level between the DAC1 and the driven device.

When driving a power amplifier (or powered monitors), reduce the input gain of the amplifier, if possible (usually to the amplifier’s minimum setting).

Optimize Volume Control Position: Set the Attenuator Jumpers

Once the amplifier’s gain is reduced, adjust the DAC1 attenuator jumpers so that a comfortable listening level is achieved when the DAC1’s front-panel volume control is near mid-rotation (or slightly higher). If the volume is too loud at mid-rotation, increase the attenuator setting. If the volume is too soft at mid-rotation, decrease the attenuator setting. When the optimum setting is between attenuation values, select the higher attenuation value.

Once the attenuators are set, you should have plenty of volume control range to accommodate variations in program material and desired listening level .

Optimize the Sound: The Attenuator Settings Can Effect Sound Quality

The attenuators will not affect the tone or quality of the audio at the outputs of the DAC1. The attenuators preserve the full dynamic range of the DAC1, do not add distortion, and do not change the frequency response of the DAC1. Nevertheless, you may detect an improvement in the sound when the attenuators are properly set. These differences are due changes in the gain settings on the driven device. In other words, if attenuation is decreased inside the DAC1, less gain is required in the amplifier to achieve the same playback level, amplifier noise levels may be reduced, amplifier distortion may be reduced, and amplifier frequency response may improve. These sonic differences underscore the importance of the DAC1 attenuators.

Other Considerations: Capacitive Loads

The DAC1 attenuators are precision low-impedance passive attenuators and have no direct effect on the sound unless the load is highly capacitive.

High- capacitance loads, such as long cables and/or devices with high input-capacitance, may cause some high frequency roll-off due to the RC (resistor-capacitor) network between the source and the load. The amount of roll-off, and the frequency at which it is rolled-off, is determined by the amount of capacitance in the cables and device being driven, as well as the output impedance of the DAC1.

The output attenuator settings of the DAC1 have known impedances, and the following charts are provided in the DAC1 and DAC1 USB owner’s manuals. These charts show the output impedance for each attenuator setting and specify some very conservative limits on cable lengths. Please note that the attenuators on the DAC1 USB have a lower output impedance than the attenuators on the DAC1. This change give the DAC1 USB the ability to drive increased cable lengths. The first chart below applies to the DAC1 USB, the second chart applies to the DAC1.

Balanced Output Drive Capability:
Attenuator Setting (dB)	Output Impedance	Maximum Cable (ft)	Loss in dB at 20 kHz
0	60	680	0.1
10	425	96	0.1
20	135	302	0.1
30	43	949	0.1

Unbalanced Output Drive Capability:
	Output Impedance	Maximum Cable (ft)	Loss in dB at 20 kHz
	30	1360	0.1

Balanced Output Drive Capability:
Attenuator Setting (dB)	Output Impedance	Maximum Cable (ft)	Loss in dB at 20 kHz
0	60	680	0.1
10	1600	26	0.1
20	500	82	0.1
30	160	255	0.1

Unbalanced Output Drive Capability:
	Output Impedance	Maximum Cable (ft)	Loss in dB at 20 kHz
	30	1360	0.1

These tables can be found in the respective manuals for the DAC1 USB and DAC1:
DAC1 USB (table is on page 14):
http://www.benchmarkmedia.com/manuals/DAC1_USB_Manual.pdf

DAC1 (table is on page 8):
http://www.benchmarkmedia.com/dac1/DAC1-Manual.pdf

USER BEWARE!: Will ‘hot-plugging’ a microphone kill your pre-amp?

‘Hot-plugging’ a microphone – that is, plugging or unplugging a microphone while phantom power is on – will damage the performance of some preamplifiers, and may completely destroy them. The damage usually begins as a rise in preamplifier noise. Every ‘hot-plug’ event can add further damage to the noise performance of the preamplifier.

Defective microphone cables and microphones can create severe phantom-induced damage. If the mic cable shorts between one of the signal conductors and the reference conductor / shield (ground) while phantom power is on, it can be devastating to the life of the pre-amp. It is important to find out if your mic pre-amp is susceptible to this type of damage, as many are.

The reason phantom power can cause so much damage is because the + and – conductors (the XLR input pins) each have a voltage of +48 Vdc when phantom power is on. These conductors are often isolated from the pre-amp components (op-amps / transistors / tubes) by DC blocking capacitors. These DC-blocking capacitors are charged to 48 Vdc and store enough energy to damage the protection components and/or the sensitive components in the input stage of the preamplifier. When a microphone is ‘hot-plugged’, or when there is a short between the signal conductor and the shield (ground), the signal conductors are instantaneously dropped to 0 Vdc. This causes the capacitors to transfer the 48 Vdc potential to the pre-amp side. The pre-amp will be slammed with -48 Vdc. This can cause severe damage to many pre-amps.

The PRE420 (www.pre420.com) is one mic pre-amp that is built to withstand ‘hot-plugging’ and cable-shorting without any damage to the PRE420’s performance. When John Siau (Director of Engineering at Benchmark) was designing the PRE420, he decided to design a bullet-proof protection network that would make the PRE420 immune to ‘hot-plugging’. This network absorbs all of the stored energy and prevents damage to the sensitive input transistors whenever the 48 Vdc shorts to ground. The PRE420 can absorb an unlimited number of +48V signal short-circuit events without degrading the performance of the pre-amp.