These are some very interesting parts!
In a sentence, Barrie Gilbert stated that his AFA (active feedback amp) "unlike the OPA (vfb opamp), has high open-loop linearity and excellent closed-loop linearity due to its distortion-canceling topology, and a very high degree of versatility arising from its dual fully-differential inputs, has the potential to eclipse the OPA in all applications involving the manipulation of purely voltage-mode signals."
The purpose of this thread is to get people to try out this topology.
Actual, working designs.
These chips can be used in line-level applications (AD830, LT6552) and head amp/ power amp (TDA856X). abraxalito even said there's one power amp which has a line driver mode selectable by a pin! So I didn't want to put the thread in amp or line level ... 'parts' seemed a good compromise but I hope it gets enough attention ...
Here are some of the chips: AD830, LT6552, TDA856X
(all these are still available)
Let's start with the AD830
AD830 High Speed, Video Difference Amplifier
View attachment AD830.pdf
from the spec sheet:
GENERAL DESCRIPTION
The AD830 is a wideband, differencing amplifier designed for use at video frequencies but also useful in many other applications. It accurately amplifies a fully differential signal at the input and produces an output voltage referred to a user-chosen level. The undesired common-mode signal is rejected, even at high frequencies. High impedance inputs ease interfacing to finite source impedances and, thus, preserve the excellent common-mode rejection. In many respects, it offers significant improvements over discrete difference amplifier approaches, in particular in high frequency common-mode rejection.
ADVANTAGEOUS PROPERTIES OF THE AD830
High common-mode rejection ratio (CMRR)
High impedance inputs
Symmetrical dynamic response for +1 and −1 Gain
Low sensitivity to the value of source R
Equal input impedance for the + and − input
Excellent high frequency CMRR
No halving of the bandwidth
Constant power distortion versus common-mode voltage
Highly matched resistors not needed
UNDERSTANDING THE AD830 TOPOLOGY
The AD830 represents Analog Devices first amplifier product to embody a powerful alternative amplifier topology. Referred to as active feedback, the topology used in the AD830 provides inherent advantages in the handling of differential signals, differing system commons, level shifting, and low distortion, high frequency amplification. In addition, it makes possible the implementation of many functions not realizable with single op amp circuits or superior to op amp based equivalent circuits. With this in mind, it is important to understand the internal structure of the AD830.
The topology, reduced to its elemental form, is shown in Figure 26.
Nonideal effects, such as nonlinearity, bias currents, and limited full scale, are omitted from this model for simplicity but are discussed later. The key feature of this topology is the use of two, identical voltage-to-current converters, GM, that make up input and feedback signal interfaces. They are labeled with inputs VX and VY, respectively. These voltage-to-current converters possess fully differential inputs, high linearity, high input impedance, and wide voltage range operation. This enables the part to handle large amplitude differential signals; it also provides high common-mode rejection, low distortion, and negligible loading on the source.
The label, GM, is meant to convey that the transconductance is a large signal quantity, unlike in the front end of most op amps. The two GM stage current outputs, IX and IY, sum together at a high impedance node, which is characterized by an equivalent resistance and capacitance connected to an ac common. A unity voltage gain stage follows the high impedance node to provide buffering from loads. Relative to either input, the open-loop gain, AOL, is set by the transconductance, GM, working into the resistance, RP; AOL = GM × RP. The unity gain frequency, ω0 dB, for the open-loop gain is established by the transconductance, GM, working into the capacitance, CC; ω0 dB = GM/CC. The open-loop description of the AD830 is shown below for completeness.
Precise amplification is accomplished through closed-loop operation of this topology. Voltage feedback is implemented via the Y GM stage where the output is connected to the −Y input for negative feedback, as shown in Figure 27.
An input signal is applied across the X GM stage, either fully differential or single-ended referred to common. It produces a current signal that is summed at the high impedance node with the output current from the Y GM stage. Negative feedback nulls this sum to a small error current necessary to develop the output voltage at the high impedance node. The error current is usually negligible, so the null condition essentially forces the Y GM output stage current to equal the exact X GM output current. Because the two transconductances are identical, the differential voltage across the Y inputs equals the negative of the differential voltage across the X input; VY = −VX or, more precisely, VY2 − VY1 = VX1 − VX2. This simple relation provides the basis to easily analyze any function possible to synthesize with the AD830, including any feedback situation.
The bandwidth of the circuit is defined by the GM and the capacitor, CC. The highly linear GM stages give the amplifier a single-pole response, excluding the output amplifier and loading effects. It is important to note that the bandwidth and general dynamic behavior is symmetrical (identical) for the noninverting and the inverting connections of the AD830. In addition, the input impedance and CMRR are the same for either connection. This is very advantageous and unlike in a voltage or current feedback amplifier where there is a distinct difference in performance between the inverting and noninverting gain. The practical importance of this cannot be overemphasized and is a key feature offered by the AD830 amplifier topology.
😎
And here are some of the circuits from the spec sheet:
Connection diagram
Gain of 2
AC coupled line receiver
Diff amp level shift
Much of this technical discussion is a bit beyond me so I'm hoping to attract some heavy-hitters to help summarize, explain, apply, and design some actual circuits. Now, if you want someone to poorly implement the design and then wax on about how good it sounds ... well I'm your man! 😉
Anyone have any ideas? Ways to apply these circuits? New circuits that use this topology/chip?
Here's abraxalito with an application:
Opamps tend not to like the high levels of RF coming out of DACs, I recommend passive (LC) filtering before the opamp - OPA627 is one of the better ones at dealing with RF. Better still though an AFA like AD830, this one is highly RF resistant and sounded great when fed with TDA1545.
http://www.diyaudio.com/forums/digit...hematic-3.html
Cheers,
Jeff
P.S. And here are Gilbert's two articles:
Are Opamps Really Linear?
View attachment Are Op Amps Really Linear.pdf
Nonlinear Effects of Radio-Frequency Interference in Opamps
View attachment Nonlinear Effects of Radio-Frequency Interference in Opamps opampsusc_01.pdf
Next up, LT6552 and TDA856X.
P.S.S. Here are some threads referencing the AFA that I'll draw from (if you know of any more, please add them):
http://www.diyaudio.com/forums/solid-state/19690-active-feedback-amplifier.html#post3392351
http://www.diyaudio.com/forums/digital-line-level/225960-ad1865-schematic-3.html
http://www.diyaudio.com/forums/power-supplies/229345-powering-high-end-usb-dac-2.html
http://www.diyaudio.com/forums/blogs/abraxalito/960-high-end-chipamp-build-project.html
Possibly the most frugal high-end sounding amp?
http://www.marchandelec.com/ftp/revpm48.pdf
In a sentence, Barrie Gilbert stated that his AFA (active feedback amp) "unlike the OPA (vfb opamp), has high open-loop linearity and excellent closed-loop linearity due to its distortion-canceling topology, and a very high degree of versatility arising from its dual fully-differential inputs, has the potential to eclipse the OPA in all applications involving the manipulation of purely voltage-mode signals."
The purpose of this thread is to get people to try out this topology.
Actual, working designs.
These chips can be used in line-level applications (AD830, LT6552) and head amp/ power amp (TDA856X). abraxalito even said there's one power amp which has a line driver mode selectable by a pin! So I didn't want to put the thread in amp or line level ... 'parts' seemed a good compromise but I hope it gets enough attention ...
Here are some of the chips: AD830, LT6552, TDA856X
(all these are still available)
Let's start with the AD830
AD830 High Speed, Video Difference Amplifier
View attachment AD830.pdf
from the spec sheet:
GENERAL DESCRIPTION
The AD830 is a wideband, differencing amplifier designed for use at video frequencies but also useful in many other applications. It accurately amplifies a fully differential signal at the input and produces an output voltage referred to a user-chosen level. The undesired common-mode signal is rejected, even at high frequencies. High impedance inputs ease interfacing to finite source impedances and, thus, preserve the excellent common-mode rejection. In many respects, it offers significant improvements over discrete difference amplifier approaches, in particular in high frequency common-mode rejection.
ADVANTAGEOUS PROPERTIES OF THE AD830
High common-mode rejection ratio (CMRR)
High impedance inputs
Symmetrical dynamic response for +1 and −1 Gain
Low sensitivity to the value of source R
Equal input impedance for the + and − input
Excellent high frequency CMRR
No halving of the bandwidth
Constant power distortion versus common-mode voltage
Highly matched resistors not needed
UNDERSTANDING THE AD830 TOPOLOGY
The AD830 represents Analog Devices first amplifier product to embody a powerful alternative amplifier topology. Referred to as active feedback, the topology used in the AD830 provides inherent advantages in the handling of differential signals, differing system commons, level shifting, and low distortion, high frequency amplification. In addition, it makes possible the implementation of many functions not realizable with single op amp circuits or superior to op amp based equivalent circuits. With this in mind, it is important to understand the internal structure of the AD830.
The topology, reduced to its elemental form, is shown in Figure 26.
Nonideal effects, such as nonlinearity, bias currents, and limited full scale, are omitted from this model for simplicity but are discussed later. The key feature of this topology is the use of two, identical voltage-to-current converters, GM, that make up input and feedback signal interfaces. They are labeled with inputs VX and VY, respectively. These voltage-to-current converters possess fully differential inputs, high linearity, high input impedance, and wide voltage range operation. This enables the part to handle large amplitude differential signals; it also provides high common-mode rejection, low distortion, and negligible loading on the source.
The label, GM, is meant to convey that the transconductance is a large signal quantity, unlike in the front end of most op amps. The two GM stage current outputs, IX and IY, sum together at a high impedance node, which is characterized by an equivalent resistance and capacitance connected to an ac common. A unity voltage gain stage follows the high impedance node to provide buffering from loads. Relative to either input, the open-loop gain, AOL, is set by the transconductance, GM, working into the resistance, RP; AOL = GM × RP. The unity gain frequency, ω0 dB, for the open-loop gain is established by the transconductance, GM, working into the capacitance, CC; ω0 dB = GM/CC. The open-loop description of the AD830 is shown below for completeness.
Precise amplification is accomplished through closed-loop operation of this topology. Voltage feedback is implemented via the Y GM stage where the output is connected to the −Y input for negative feedback, as shown in Figure 27.
An input signal is applied across the X GM stage, either fully differential or single-ended referred to common. It produces a current signal that is summed at the high impedance node with the output current from the Y GM stage. Negative feedback nulls this sum to a small error current necessary to develop the output voltage at the high impedance node. The error current is usually negligible, so the null condition essentially forces the Y GM output stage current to equal the exact X GM output current. Because the two transconductances are identical, the differential voltage across the Y inputs equals the negative of the differential voltage across the X input; VY = −VX or, more precisely, VY2 − VY1 = VX1 − VX2. This simple relation provides the basis to easily analyze any function possible to synthesize with the AD830, including any feedback situation.
The bandwidth of the circuit is defined by the GM and the capacitor, CC. The highly linear GM stages give the amplifier a single-pole response, excluding the output amplifier and loading effects. It is important to note that the bandwidth and general dynamic behavior is symmetrical (identical) for the noninverting and the inverting connections of the AD830. In addition, the input impedance and CMRR are the same for either connection. This is very advantageous and unlike in a voltage or current feedback amplifier where there is a distinct difference in performance between the inverting and noninverting gain. The practical importance of this cannot be overemphasized and is a key feature offered by the AD830 amplifier topology.
😎
And here are some of the circuits from the spec sheet:
Connection diagram
Gain of 2
AC coupled line receiver
Diff amp level shift
Much of this technical discussion is a bit beyond me so I'm hoping to attract some heavy-hitters to help summarize, explain, apply, and design some actual circuits. Now, if you want someone to poorly implement the design and then wax on about how good it sounds ... well I'm your man! 😉
Anyone have any ideas? Ways to apply these circuits? New circuits that use this topology/chip?
Here's abraxalito with an application:
Opamps tend not to like the high levels of RF coming out of DACs, I recommend passive (LC) filtering before the opamp - OPA627 is one of the better ones at dealing with RF. Better still though an AFA like AD830, this one is highly RF resistant and sounded great when fed with TDA1545.
http://www.diyaudio.com/forums/digit...hematic-3.html
Cheers,
Jeff
P.S. And here are Gilbert's two articles:
Are Opamps Really Linear?
View attachment Are Op Amps Really Linear.pdf
Nonlinear Effects of Radio-Frequency Interference in Opamps
View attachment Nonlinear Effects of Radio-Frequency Interference in Opamps opampsusc_01.pdf
Next up, LT6552 and TDA856X.
P.S.S. Here are some threads referencing the AFA that I'll draw from (if you know of any more, please add them):
http://www.diyaudio.com/forums/solid-state/19690-active-feedback-amplifier.html#post3392351
http://www.diyaudio.com/forums/digital-line-level/225960-ad1865-schematic-3.html
http://www.diyaudio.com/forums/power-supplies/229345-powering-high-end-usb-dac-2.html
http://www.diyaudio.com/forums/blogs/abraxalito/960-high-end-chipamp-build-project.html
Possibly the most frugal high-end sounding amp?
http://www.marchandelec.com/ftp/revpm48.pdf
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