The Liquid Gold is finally coming alive and even though its production run is scheduled for late Spring, it's time to start telling you about its history and design philosophy. More to come soon.
The idea for a solid state amp, which we originally called Liquid Gold (LAu), has been around for quite a few years now. I have had many private requests for this amp, but with other things taking precedence in the CA lineup, LAu was pushed to the background. I did prototype a design about 2 years ago. It was a single ended amp and when I listened to it my response was ... well, it was just an ok amp. After scrutinizing the circuit design for a while I had one of those, "oh yeah I see why this not so good" moments. Then, with the very high demand for LFs I let it drop until some better thinking could be done.
Late summer 2012 dBel84 strongly suggested a fully balanced LAu and since things were quiet at the time, LAu was resurrected. This time with a whole new set of ideas on its inherent design.
There are at least two ways to do a balanced amplifier. The first way is a genuine fully differential circuit and the other is two monoblocks bridged together at their outputs, but driven out of phase. A fully differential amp was the only way to get the results I was hoping for and so that's what we have. Fully differentilal amps take advantage of genuine common mode rejection and the lower (even harmonic) distortion that can be obtained.
There are some challenges, however, associated with a fully differential amp, especially the feedback network and secondarily making sure that both sides of a single channel are reasonably matched all the way through to the output stages. Another challenge is gain. Fully balanced mode essentially doubles the gain. Some headphone amps already have too much gain, especially when dealing with the wide range of impedances and sensitivities of modern headphones. Adjusting gain also involves the global feedback loop. And then there is the Nelson Pass patent on what's called Super Symmetry that applies to differential amplifiers. Some thinking needed to be done.
(No) Shunt Feedback
Nelson Pass' Super Symmetry patent has a number of circuit features. The one that is relevant to balanced differential amplifiers is the use of what's called shunt feedback. Shunt feedback takes the output from the amp and feeds it back directly into the input grid/gate/base. Shunt feedback has been around since the 1930s. Fender used shunt feedback in many of its guitar amps. The Cavalli Bijou DIY amp uses shunt feedback. I recall a big discussion on the DIY Audio forum about the patent and shunt feedback.
There are a number of ways to introduce global NFB in a single ended amp, but it's a little harder in a fully balanced amp and shunt feedback is the obvious way to provide the global feedback loops.
As I understand the patent, it does not specifically cover the use of shunt feedback, but it does mix shunt feedback with the other features of Super Symmetry and so the issue is fuzzy. In order to stay out of this problem entirely, I decided to find a way to use global feedback without using shunt feedback. And I did.
The method of using NFB in the Liquid Gold allows for really simple and non-intrusive gain switching. Prototype testing so far indicates that the gain can be switched while the amp is on and operating. If this continues to hold after more testing, the production LAu will have a two position gain setting switchable using a simple toggle switch on the front panel. No opening the box to set jumpers. Nothing complicated.
Output DC Offset Detection
The Liquid Fire has a DC offset detection and protection circuit. The design is a CA design, although it has similarities to other detectors which have been designed and used in other equipment and DIY projects.
There is a signficant difference, however, between an offset detector for an SE amp and a Balanced amp.
An offset detector looks at the average (integrated) DC at both SE outputs. It has an internal threshold for the DC so that if the DC exceeds the threshold for a pre-determined period of the time the detector disconnects the amp fromt the load (headphones). The DC in a SE amp is measured with respect to ground.
One has to be careful when designing an offset detector so that it doesn't just sum the two SE signals because then there is the (rare) possibility that one DC offset will go positive and the other negative with just the right amounts to cancel each other out and, therefore, not trip the interrupt. The CA SE offset detector does not sum the signals, but looks at them independently, to prevent this admittedly rare possibility from occuring.
In a Balanced, amp however, things get really complicated for two reasons: 1) There are now four outputs (two per channel) which have to be monitored with respect to ground and 2) The difference between the positive and negative outputs on each channel also has to be monitored. So, when going from SE to Balanced offset detection we don't just go from two to four monitoring points, we go from two to six. Furthermore, two of these monitoring points are difference measurements with respect to pairs of outputs and not with respect to ground.
Complicated. But, the original CA SE detector has been augmented to handle all six monitoring situations so that the Liquid Gold, just like the LF and LG, will not damage headphones because of DC.
CA offset detectors also include a built-in delay of about 20 seconds after rails are applied to give the circuit time to settle down.
Here's the board layout for the SE offset detector:
And here is the LAu offset detector board layout:
Much more complicated, but worth the effort to ensure that today's very expensive headphones are not damaged by Cavalli amplifiers.
How Deep in Class A
The LAu will very likely have its output stage biased at 150mA. This is where the prototype is biased. Biasing at 150mA keeps the amp in class A mode for a long time. For 50 ohm headphones, the amp will deliver class A power up to a little more than 550mW continous sine wave power (approximately). Even with inefficient headphones, this is a fair amount of power. Along with this the amp will move gracefully into class AB to make its full 6W into 50R.
Inputs and Outputs
The LAu will have at least 3 inputs, two Balanced and one SE. The SE input will be converted internally to balanced using the TI DRV134 SE to Balanced chip. Using the TI chip prevents loss of gain typically associated with SE into a balanced amp. The possible downside is that it does put an extra chip into the signal path when in SE mode.
The LAu will have 4 outputs: 4 Pin XLR, two 3 Pin XLRs, two separate TRS jacks. This array of output jacks will make it possible to use just about any modern heaphone termination with this amp.
More to come...