Some five years ago, The Audio Guild set out on a mission to see if we could not only capture the magical experience of the classic single-ended triode (SET) vacuum tube amplifier, but to actually improve upon it. And more to the challenge, to do it without the use of a single vacuum tube. This quest led to some very out of the box thinking and resulted in a radically different approach to single-ended amplification; a passive/active hybrid amplifier circuit that retains both the extreme simplicity and single-ended nature of the classic SET amp, but without the inherent limitations that prevent the classic SET amp from advancing the single-ended experience.
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THE 535SE STORY
AT A GLANCE
- Exclusive passive/active hybrid amplifier circuit
- Studio-grade input transformers provide ultra quiet, low-distortion signal amplification, ground isolation and high common mode rejection (CMR)
- Single-ended JFET/bipolar output stage provides low impedance, high current output
- Zero global feedback
- Gain: 14dB
- Maximum output: 14 volts peak/250mA
- Output Impedance: 1 Ohm
- THD+N: 0.05%
- Three inputs, preamplifier output
- Telecom-grade headphone jack with integral switching of preamplifier output
- Solid wood case
- Engraved Traffolyte front panel
- Genuine DAKA-WARE bakelite knobs
- 14" W x 7" H x 7.75" D
- Made in the USA
BETTER THAN SET?
The development of the 535SE was inspired by the classic single-ended triode (SET) vacuum tube amplifier, beloved not only for their wonderful sound, but also for their extreme simplicity and that they are able to produce such a wonderful sound in spite of measuring rather poorly compared to other amplifier topologies.
I have always loved the sweet sound of a good SET amplifier, but after a rather nasty "shocking" experience as a teenager I have since been quite the coward when it comes to high voltages and as such, have never designed with tubes. It was this which ultimately led me to explore the possibility of capturing that wonderful SET experience (and perhaps even improve on it) without using vacuum tubes.
There have been attempts at this, most notably Nelson Pass' Zen amplifier. The original Zen utilized a single power MOSFET device instead of a vacuum tube. And while the Zen has a certain charm of its own, it had always struck me as being a little too simple. Instead of the singular, "one hand clapping" of the Zen approach, I have always been more attracted to the dualistic Taoist approach. Yin and Yang. And so with Nelson having already walked down the Zen path, I chose the Tao.
The journey for me began by contemplating the classic SET tube amp. Here you have the active device (vacuum tube) at the input which then drives the output transformer. This being the case, the active device is charged with providing not only the voltage gain of the amp, but the current gain as well. In other words, the active device is responsible for all of the circuit's power.
Vacuum tubes are typically high voltage, low current devices and in a common cathode configuration have an output impedance that is much too high to drive a low impedance load like a loudspeaker or headphone. This is where the output transformer comes into play.
A transformer transforms impedances as the square of its turns ratio. This makes it possible for a transformer to take the high output impedance of the vacuum tube, and transform it to a much lower impedance suitable for driving a loudspeaker. However being a passive device, a transformer cannot do this without paying a price.
In this case, the price to be paid is voltage gain; in the transformation from a high impedance to a low impedance, voltage gain and maximum signal swing are likewise reduced. Fortunately it transforms voltage by its turns ratio instead of the square of its turns ratio so you're not giving up all of the voltage gain in order to achieve a lower output impedance.
Further, in the classic SET amp, all of the vacuum tube's DC idle current flows through the output transformer's primary, and if there's anything a transformer does not like, it is DC current flowing through its primary or secondary. To help ameliorate this, SET output transformers use steel laminations and are given an intentional air gap. This is done to help prevent the DC current from saturating the transformer's core. However the price paid for this is dramatically degraded performance compared to a transformer that is not air-gapped and uses superior lamination materials like nickel.
The Zen amplifier gets around the output transformer issue by virtue of the low voltage, high current MOSFET's output impedance being low enough to drive a loudspeaker directly. But it still suffers from employing a single device to deliver all of the amplifier's power, i.e. both voltage and current gain.
I had been contemplating these issues for some time before I finally had the revelation that would ultimately lead to the 535's design
Why not take the classic SET amplifier and turn it inside out?
This first involved moving the transformer from the output to the input, and instead of it being a step down transformer (in terms of voltage), make it a step up transformer.
Step up transformers have been commonplace in the recording studio for many decades, typically as microphone input and output transformers. When using nickel laminations and no intentional air gaps, a well designed microphone transformer can provide some of the quietest (no power supply required), most transparent voltage gain on the planet. Indeed, this is why many audiophiles prefer transformers for stepping up the outputs of low output moving coil phono cartridges.
The 535 uses a pair of the best 1:5 step up transformers available. These transformers are responsible for all of the amplifier's voltage gain. This is the Yin of the 535. Now we need to complete the other half of the equation with some Yang.
Voltage is only half of the power equation. The other half requires current. For those of you familiar with Ohm's Law, power is the product of voltage and current. As stated previously, vacuum tubes are typically high voltage, low current devices whereas transistors are typically low voltage, high current devices, so transistors were the natural choice for the 535SE's output stage.
But which? Bipolar? JFET? MOSFET?
As was stated previously, a transformer transforms impedances by the square of its turns ratio and as a step down, output transformers transform the vacuum tube's high output impedance to a lower impedance. But the reverse is true when you turn it the other way around. A step up transformer will transform the source's output to a higher impedance. Using a 100 ohm source impedance and a 1:5 step up transformer as an example, five squared is 25. The transformer will thus increase the source's 100 ohm output impedance by a factor of 25, or 2,500 ohms.
So the output device must have a high enough input impedance that the transformer's output will not be loaded down.
Power MOSFETs have an exceedingly high input impedance, but they also tend to have high input capacitances which can be problematic at the highest audio frequencies.
Bipolar devices can have acceptably high input impedances, but they also have current flowing through their bases which would cause DC current to flow through the transformer's secondary which as stated previously is bad Ju-Ju. Technically you could couple the transformer to the output device with a capacitor which would prevent the bipolar's base current from flowing through the transformer's secondary, but this would be a rather inelegant solution.
JFETs have a high input impedance and low input capacitance, but being small signal devices can't handle a lot of current.
What to do?
BEST OF BOTH WORLDS
The solution ended up being the 535's unique JFET/bipolar output stage. The 535's output stage combines a small signal JFET and a power bipolar device in what would otherwise be called a Darlington pair.
The JFET provides a very high input impedance with low capacitance (an ideal load for the transformer's output) and only has to handle a very small amount of current. It also isolates the transformer's secondary from the bipolar's base current. The bipolar's base current instead flows through the JFET device. And last but not least, this JFET/bipolar pairing improves the overall performance of the output stage just like a traditional bipolar Darlington pair.
The output stage is configured as an emitter follower. It offers a low impedance, high current output and its voltage gain is 1, or unity meaning we are not having to sacrifice any of the transformer's voltage gain.
"THD (Total Harmonic Distortion) is a largely meaningless spec."
This phrase has been a mantra within the audiophile community for many years. It was popularized, not in so few words, by the late J. Gordon Holt, founder of Stereophile magazine. And while many meaningless slogans can be found being bandied about in the audiophile community, this is one that has some truth to it.
Harmonic distortion refers to what comes out of a piece of audio equipment versus what was put into it. For example if you input a 1kHz tone, if the device is distortion-free, all you will see at its output is that 1kHz tone. However no audio device is truly distortion-free, so what you will see at its output will be other frequencies that are at harmonic intervals of that 1kHz tone, i.e. 2kHz, 3kHz, 4kHz, etc., hence the term harmonic distortion. THD is the singular figure you get if you add up the amplitudes of all of the harmonics and express them as a percentage of the original 1kHz tone.
And while this can give you a nice convenient figure to include on a spec sheet, it does not tell the whole story. What matters most is the nature of the distortion and that cannot be expressed as a simple singular number. And for that, we need to look at the distortion's spectra.
There is a psychoacoustic phenomenon called masking. Basically, if you introduce a second tone to a first tone, and the second tone's frequency is close enough to that of the first tone but at a lower level, the first tone will mask the second tone such that we only perceive the first tone. However as you move the second tone's frequency further and further from the first tone, the more sensitive we become to being able to perceive two frequencies.
And while some people will argue that even order distortion (second, fourth, sixth harmonics, etc.) is more pleasing than than odd order distortion (third, fifth, seventh harmonics, etc.), this isn't quite correct. If odd ordered harmonics were so objectionable, we simply wouldn't be able to bear listening to most musical instruments.
It is not an issue of even order versus odd order, but rather high order versus low order (seventh, eighth and ninth harmonics, etc. versus second, third and fourth harmonics, etc.). This is because as the harmonics get further and further away from the fundamental, the more sensitive we are to perceiving them as second tones, or in the case of a piece of audio equipment, distortion. And this is what led Norman Crowhurst and others in the 1950s to suggest weighting distortion products depending on how far they are from the fundamental.
So the Holy Grail for distortion is twofold. First, they must be low order (second, third, fourth) and second their levels should reduce monotonically from the fundamental, i.e. third lower than second, fourth lower than third, etc. SET amps can achieve this in good part due to their single-ended nature and helps explain why they can sound so good even if their THD figure might suggest otherwise. But the less than optimal air-gapped output transformer takes its toll and produces significant amounts of high order distortion products.
The 535 however admirably achieves this goal. Figure 2 shows the 535's distortion spectra measured while driving a 32 ohm load (an actual headphone in this case and not just a resistor) at nearly full output swing (the headphones were incredibly loud, and that was with them sitting on the test bench).
As you can see, the 535 only exhibits second, third and forth order distortion products and they drop rapidly and monotonically. And as a side note, the distortion shown in Figure 2 adds up to about 0.05% THD. This is pretty good performance even for a non-SET amplifier.
Note: The vertical scale in Figure 2 is logarithmic to better illustrate the harmonic components. If plotted on a linear scale, the harmonic peaks would be barely discernable blips. The largest distortion peak is several thousand times lower than the fundamental.
BETTER THAN SET? (REPRISE)
Have we achieved our hope of improving on the SET experience? We think we have, but of course tastes are personal and subjective so you will have to decide for yourself. What we can say with some confidence is that if you enjoy the SET experience, we don't think you will be disappointed in the 535's performance.
NOT FOR EVERYONE
The 535 gives much, but it does ask just a little in return and as such will not be compatible with all sources. Because the inputs go directly to the transformer instead of an active buffer, the source's output must adhere to the following:
- Maximum output should be no higher than 2 volts RMS
- Maximum output impedance should be no higher than 300 ohms
- The outputs must not have any DC offset voltage
Note: Exceeding these limits will not cause physical damage to the 535, but they will degrade audio performance.
DIRTY LITTLE SECRET
While the 535's inputs are only fitted with RCAs, the fact is that the 535's inputs are wholly balanced and feed directly into the input transformer's balanced primary.
We chose not to fit the 535 with XLRs for two reasons. One, typical balanced outputs on source gear output a level twice as high as their unbalanced outputs. The Redbook spec for unbalanced is 2 volts RMS and for balanced, 4 volts RMS which would exceed the input transformer's design envelope.
Two, there is really no need to feed the 535 from a balanced source. The whole reason for being of balanced interfaces is the ability to reject common mode noise due to interference picked up by the cable. While electronically balanced inputs can have excellent common mode rejection (CMR), their performance degrades rapidly if there is any imbalance in the source impedance. Indeed, it is not uncommon to find adjustments on the outputs of professional gear to precisely balance output impedances in order to achieve maximum CMR.
Transformers on the other hand do not have such issues. Because of their exceedingly high common mode input impedance, a transformer can maintain very high levels of CMR even when fed from a wholly unbalanced source.
However if you do have a source with balanced outputs whose maximum output level does not exceed 2 volts RMS, you may connect to the 535's inputs using a simple XLR to RCA adapter cable. However unlike typical XLR to RCA adapter cables which connect the XLR's pin 2 (positive) and pin 1 (shield) to the RCA, you would need to connect pin 2 (positive) and pin 3 (negative) to the RCA.