Originally Posted by reddog
You never fail to amaze me with your knowledge of audio/ tube amps. I never thought about modifying the lyr. I think it would be neat to get a used lyr and modify it. Thanks again for sharing your knowledge sir.
I'm hoping it'll be a nice step-up. The other mods like changing out the Bridge Recs to Hexfreds would be my next 'stage'. Then some of the more difficult stuff like:
Cut the two long traces from the center contact of the input RCA connectors to the volume pot. Use X-acto and cut out a short piece of trace right at the RCAs and then right at the Pot. 4 cuts total. Then on the bottom, wire these to the same places using cotton-insulated pure silver wire from VHAudio. Twist two wires, one signal and one ground from connector to pot. One pair for left and one pair for right. 4 total wires.
The Lyr uses a unique circuit design - in lieu of the typical coupling caps for the tube stage they use a DC servo. With a typical coupling cap arrangement (like on my Modded Lite DAC60, and Bada Integrateds) the caps have to be big, on the DAC60 they're 2.7uf and the Bada 4.7uf! These on the Lyr are 1/0th the size, so I assume they work within the DC servo circuit. I' hoping the greatly improved film caps (Mundorf) have the same effect on the SQ.
Another very important part of the Lyr design is the 'Adaptive Class A Output Stage'. As their website explains:
It’s a current-sensing adaptive output topology which allows the amplifier to dynamically adjust to the headphone load. The primary benefits are essentially single-ended Class-A operation for high-impedance headphones, moving seamlessly to push-pull Class A and finally into Class AB as current needs increase. This provides much higher overall efficiency.
What this means is that the Mosfet outputs change their biasing characteristics as the load increases - from single ended Class A to push-pull Class A to Class A/B.
Single ended Class 'A' will always give the purest, smoothest, cleanest sound. Followed by push-pull Class A. Lastly A/B is the least pure - suffering from what is known as switching distortion.
So on high impedance HPs like the HD800s (300 ohms) the amp will operate in the first two Class A modes almost all the time - unless driven very hard - say during transient peaks. On a very low impedance HP like the AKG 702s (62ohms) the Lyr will almost always be in the 2nd and 3rd operating modes - giving a less natural and pure sound.
This white paper explains it pretty well
Simplicity is not the only reason for the use of the single-ended topology. The characteristic of a single-ended gain stage is the most musically natural. Its asymmetry is similar to the compression / rarefaction characteristic of air, where for a given displacement slightly higher pressure is observed on a positive (compression) than on a negative (rarefaction). Air itself is observed to be a single-ended medium, where the pressure can become very high, but never go below 0. The harmonic distortion of such a medium is second harmonic, the least offensive variety. It is occasionally misunderstood that single-ended amplifiers intentionally distort the signal with second harmonic in order to achieve a falsely euphonious character. This is not true. Low distortion is still an important goal, and it is my observation that deliberate injection of second harmonic into a musical signal does not improve the quality of sound.
Single-ended amplification is distinct from push-pull designs in that there is only one gain device for each gain stage, and it carries the full signal alone. Linear single ended designs operate only in Class A. In contrast, push-pull designs share the signal between two opposing devices, one concentrating on the positive half, the other the negative half. This positive/negative half of an audio signal is an artifice imposed by the desire to efficiently handle an AC only signal, with no DC component. Most Push-pull Class A designs offer energy efficiency of twice that of most single-ended designs, and they also offer a measure of distortion cancellation. A well matched push-pull pair of gain devices will have lower measured distortion due to cancellation, and will concentrate the harmonic content into third harmonic and other "odd" harmonics, reflecting the symmetry between the plus and minus halves of the waveform. Operation is possible in Class A, Class AB, and Class B modes. The most linear of these is Class A, in which the circuit will dissipate at idle more than twice its rated output.
Edited by rb2013 - 4/2/15 at 8:30pm
The Mosfet designs on the market are also Class AB designs. The transfer curve of Mosfets reveals serious nonlinearities at low bias currents, resulting in crossover nonlinearity in push-pull designs. This design flaw makes for a sonic signature that many have referred to as "Mosfet mist", where a loss of detail is apparent. To fully realize the benefits of Mosfet transistors, they must be operated in Class A.