Technical & Editorial
- Written by Dr. David A. Rich
- Published on 29 January 2014
- AVR - Audio Video Receiver - Build Quality: Part II - Design of High-Performance AVRs and Pre/Pros
- Page 2: Electronic Volume Controls that Enhance Performance
- Page 3: Operational Amplifier Selection
- Page 4: Differential and Current Mode DACs
- Page 5: Multiple DACs Combined to Produce a Single Channel Output (Mono Mode)
- Page 6: Quasi Current-Mode Interface for ESS DACs with a Single Operational Amplifier
- All Pages
Electronic Volume Controls that Enhance Performance
While AVR and pre-pro marketing focuses on the highest-resolution component – usually the DAC chip - an audio signal chain is ultimately limited by its lowest-resolution circuitry. Often, that is the volume control. The difference between the THD of the best IC-based volume control ICs (Integrated Circuits) and typical AVR LSIs (Large Scale Integrated Circuits) is 2.5 bits using measurements sourced from data sheets with the control set to unity gain.
In a traditional volume control, a fixed gain stage [typically 20-26 dB of gain] follows a voltage attenuator (potentiometer). The fixed-gain stage introduces significant noise and distortion. For most of the range of the volume control, the attenuation exceeds the gain. Under these conditions, the gain following the volume control is unnecessary.
In some older SSI (Small Scale Intgration)-based multi-channel products and stereo preamps using an electronic volume control, an obsolete IC was used. This old IC contained only one resistor ladder and was a direct replacement for the potentiometer. This part has been discontinued, favoring a more modern design discussed below.
In a modern electronic volume control, two resistor ladders are integrated on the chip. One is placed in the traditional position before the amplifier. The other resistor string is used to adjust the gain of the amplifier. With this topology the amplifier is wired as a unity gain buffer (0 dB), and has much lower noise and distortion than an amplifier required to provide a gain of 20-26 dB).
Only when the attenuator before the volume control moves to the unity gain position does the second resistor string become active to increase the gain of the amplifier above unity. The added gain, with its associated noise and distortion, only occurs when it is needed. The complete circuit is called a "Gain Optimized Volume Control". Since this circuit requires two resistor strings, whose dual operation must be coordinated, it is not realizable using a passive volume control.
An SSI option for high-performance volume controls consisting only of resistor ladders is readily available now. The latest part introduction is from New Japan Radio. The part (the MUSES 72320) is found in the $1000 stereo Emotiva preamplifier reviewed here in Secrets.
The JRC MUSES 72320, or the similar, recently introduced, Micro Analog Systems MAS6116, has four resistor strings to allow for one chip to be used in a stereo system. These chips are not used in current Pre/Pros whose internal design I am familiar with. Instead, high-performance, dynamic range optimized, electronic volume controls in Pre/Pros with SSI designs are from Cirrus and TI. In addition to the resistor ladders, these parts include an operational amplifier (one per channel) internal to the chip. The chips are available with two to eight sections. These parts started to appear in the early part of the last decade.
The eight-section Cirrus CS3318 consumes the same amount of current as the LSI AVR chips, despite having fewer than half the op-amps. Since the Cirrus 3318 only functions as a volume control, each op-amp consumes more silicon area. The Cirrus operates at a higher power supply voltage of ± 9V. This all translates into better performance. Using typical numbers, at unity gain, the Cirrus data sheet reports distortion as 0.00025% THD+N, while the data sheets for the LSI AVR chip report 0.001% for the same output signal swing. This is a 12 dB improvement at 1 kHz. In bits equivalent, we shift from 16.5 bits to 18.5 bits. With eight channels of volume controls, the Cirrus is more accurately called a "Medium Scale Integrated" Circuit (MSI).
The TI PGA4311UA is a four-channel component that has lower distortion than the Cirrus between 600 mVRMS and 2 VRMS. Two of the TI chips provide the volume control function for the eight channels. Since twice the TI chips are required, the pair is more expensive than the eight-channel Cirrus part. Worst-case distortion of the TI part with an output voltage of 2 VRMS is 0.004%. A single chip AVR LSI has a specified worst-case distortion of 0.02% at 2 VRMS at its output. This is five times the distortion of the TI PGA4311UA.
Secrets' Audio Precision measurements of multi-channel products using two versions of the LSI AVR chip (Renesas and Rohm) and the Cirrus CS3318 confirms the Cirrus chip has lower distortion at 1 VRMS. The distortion vs. frequency measurement stays flat with the Cirrus chip, but the Renesas starts to rise about 2 kHz, moving to four times the mid-band value at 20 kHz. Complete THD+N vs. Frequency was not available for the Rohm chip. The SNR measurements of the AVR with the CS3318 show at least a 1/2 equivalent bit improvement in comparison to the Renesas and Rohm chips.
Electronic volume control performance can be improved with two volume controls sections in balanced mode, but does so at twice the cost.
Balanced mode minimizes distortion and expands dynamic range of the volume control. The op-amp at the bottom left inverts the incoming signal and the Single Ended to Balanced Converter restores the signal to unbalanced mode. The balanced-to-single-ended converter consists of resistors and one op-amp. The Emotiva XSP-1 stereo preamp, mentioned above, uses a balanced volume control topology.
Secrets' measurements of the XSP-1 preamp shows improved distortion performance over the single ended Cirrus CS3318. Distortion of the Emotiva at unity gain was -106 dB from 20 Hz to 20 kHz at 2 VRMS with almost not change at 5 VRMS.
In theory a Pre/Pro with balanced outputs could have a completely balanced signal chain from the DAC output through the analog volume controls to the balanced output connections at the back of the unit. However, practical issues, such as the balanced signals becoming mismatched as they travel through the signal path, present a roadblock. In addition, common mode signals that accumulate as the balanced signal moves through the unit are not removed.
The engineer has many choices for a fixed build of material when designing an electronic volume control. For example, the cost of using two sections of a Cirrus CS3318 in balanced mode might be similar to using the MUSES 72320 single ended with high quality op-amps.
None of my service manuals for multi-channel products pointed to the balanced technique. The technique was adopted in the B&K Reference 70 which, unfortunately, never made it to production. Designed by the extraordinarily talented Ed Mutka, the Reference 70 was truly innovative.
The most elegant configuration for an electronic volume control uses current-mode switching with bipolar devices. An example is the Texas Instruments DAC8812. The voltage at the current switch remains constant at ground potential, which introduces far less distortion than voltage-mode switching where a tap is selected on the resistor ladder.
The resistors on the Texas Instruments DAC8812 have an added layer of deposited thin film metal to reduce distortion, but this adds significantly to the price of the silicon. The resistors in the other volume controls are made from material already deposited to make an IC with MOSFET transistors. Typically, these are polysilicon wires that form the gate of the MOSFET. Distortion in polysilicon resistors occurs when the resistance changes slightly as the voltage across the resistor changes. This is called the voltage coefficient of the resistor, which is a specification sometimes found in data sheets for discrete resistors.
The current at the output of the selected switches is converted back to a voltage by an op-amp circuit functioning as a current-to-voltage converter. This method is used in high-end two-channel products by Accuphase and Simaudio Moon, although they may use other options than the TI chip mentioned above. It is unlikely an affordable Pre/Pro could include this chip. See the example in this link to the Accuphase C-2120 Stereo Control Center.