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Review by LEO SIMPSON ALTITUDE 3500-SS Stereo Valve Amplifier Amplifier Some readers may be annoyed by the sight of a review of a valve amplifier in SILICON CHIP – but we often get requests to publish a modern design. So we thought it would be useful to put this model through our usual performance testing. L et’s get one thing straight: we are biased! Most readers will already know that as we have always stated our preference for solid-state amplifier designs, even though quite a few people are interested in valve amplifiers. Having noted our bias, let’s try to set it aside while we check out the impressive Altitude 3500-SS amplifier. This very well presented amplifier is siliconchip.com.au made by Fountek Electronics Co Ltd, in China but has been carefully tweaked for our market by the noted Australian designer, Russell Storey, on behalf of WES Australia. It is fairly conventional in layout, with the valves on top of and toward the front of the chassis while the very large power and output transformers are at the back. These transformers have cylindrical housings but they are unlikely to be based on toroids – they are too tall for that. We think they probably have conventional E-I laminated cores. The chassis itself is especially impressive, being made from interlocking aluminium extrusions 7mm thick while the top and bottom plates are 3mm thick aluminium secured with multiple stainless steel countersunk screws. Everything has been precision June 2008  73 Underneath, the Altitude 3500-SS is very neat, just like the top side. The only real giveaway that it is a valve amplifier are the sockets on the PC board (and possibly the absence of ICs and transistors!). This shot also gives a good idea of the rear panel input/output layout with its gold-plated sockets. machined for close fit of all sections. The valve sockets are recessed into the top of the chassis and they sit on a large PC board underneath. The front panel knobs also fit into machined recesses on the panel. The valve line-up is two 12AX7 and two 12AT7, both twin triodes, and four EL34 pentodes, used as matched pairs in each channel. The valves are branded ElectroHarmonix, a US-based company but they are labelled “Made in Russia”, as are most valves these days. By the way, that is not a back-handed comment on quality – Russian-made valves are equal to the best that were made in the past. 74  Silicon Chip As far as we can tell, each channel uses the 12AX7 as a direct-coupled cascode preamplifier stage and the two triodes in the 12AT7 are used for the phase-splitter driver to the push-pull pentode output stages. The pentodes appear to be connected in simple push-pull and not ultra-linear, which implies transformer primary connections to the pentode screens. The output transformers have secondary taps for 4W and 8W connections so there are three gold-plated binding post terminals for each channel on the back panel. Also on the back panel are four pairs of gold-plated RCA sockets which are intended for line level signals and are labeled CD, Tuner, Aux1 & Aux2. The mains power connection is made via a 3-pin IEC socket, the same as used on computers and a lot of consumer electronics equipment these days. The front panel is very clean and simple, with two large knobs, Selector and Volume, flanking the centrally mounted push-on push-off power switch and the four LEDs which indicate the selected source. An interesting feature on top of the chassis is the inclusion of two stainless steel vertical plates which deflect heat from the valves away from the transformers. This is one of the modifications for the Australian market. siliconchip.com.au We asked Russell Storey for some details about the modifications for the local market. Apparently the stainless steel plates have two advantages. First, they greatly reduce the heat rise of the power transformer (“by at least 22°C”) and second, they reduce the induction of 50Hz hum into the output pentodes which has resulted in less output hum and intermodulation. Another significant difference in the Australian version (referred to as Altitude 3500-SS) includes a change to the power transformer primary voltage from 230V to 240V which has the effect of reducing filament voltages to the correct range for longer valve life and also reduces overall power consumption by about 9%. Other changes to the circuit include improved amplifier slew rate, matched EL34 valves and factory selection of the 12AX7 and 12AT7s for low noise and optimum gain. By the way, for those not familiar with valve amplifiers, they do get stinking hot. This will be quite obvious for old-timers familiar with valves but newcomers will be astonished. The output pentodes get hot enough to give you a serious burn if you are not careful. As the owner’s manual states: “Keep out of reach of children. Valves get hot!” Under the chassis Most of the circuitry under the chassis is mounted on a large doublesided PC board, as mentioned above. Audio Precision The only components mounted on the topside of the PC board are the ceramic valve sockets and the board is mounted so that the tops of the pentode valve sockets just sit flush with the top surface of the chassis. The smaller ceramic triode sockets sit lower so that the valves end up being slightly recessed into the chassis. There is a small vertical PC board associated with the RCA input sockets and this carries four double-pole relays and the associated transistors which are switched by the front panel rotary switch. All the power supply components are on the mains PC board, apart from the iron-cored choke which is mounted underneath the chassis. There are also small PC boards associated with the rotary selector switch, the dual ganged volume control and the four front panel LEDs which indicate which source is selected (1-4). Good quality components are used throughout and the overall standard of workmanship is excellent. The Altitude 3500-SS sits on three large feet, two at the back and one in the front, more or less corresponding to the heavyweight transformers. As an aside, we found that the feet are too hard and as a result, the amplifier will slide quite easily on hard surfaces. In fact, at one stage we almost had the amplifier slide off a small coffee table. We hate to think of the consequences. . . Apart from that little drawback, the Altitude 3500-SS ticks the boxes for Frequency Response 8 Ohm (1W) 04/18/08 10:38:25 The frequency response is very flat, less than –1dB down at 50kHz and ruler flat at the low end to below 10Hz. Those output transformers must be exceptionally good. siliconchip.com.au Audio Precision all the right features; it really is most impressive in its presentation. Specifications The Altitude 3500-SS is rated at 32 watts per channel into 8 or 4-ohm loads, for a rated total harmonic distortion of 0.63% at 1kHz. Its frequency response is 15Hz to 50kHz ±0.5dB (1W), while its power bandwidth is 10Hz to 90kHz at the -3dB points. Its signal-to-noise ratio is -76.2dB unweighted, with respect to full power (32W). Input sensitivity is quoted as 375mV RMS for full power. These are very respectable specifications for a valve amplifier, particularly with regard to signal-to-noise ratio. This is one of the quietest valve amplifiers we have ever come across and there is certainly no hum evident from the speakers – the noise is just a faint hiss. Significantly, there does not appear to be any sign of microphony either. To explain, microphony is a problem in high gain audio valve circuits and is an audible ringing heard through the loudspeakers, when any of the valves, particularly those in the small signal stages (ie, 12AX& etc) are tapped. Performance The six graphs included in this article give a pretty good picture of the amplifier’s performance. In general, we would have to state that it has come very close to meeting all of its specifications. We have also included a number of Cross Talk vs Freq 8 Ohm (1W) 04/18/08 10:35:40 The separation between channels. This demonstrates commendably low crosstalk in the small signal stages and very little crosstalk via the power supply rails. June 2008  75 The amplifier operating in the mid-power range with a 1kHz sinewave: the yellow and green traces show both outputs while the purple trace is the residual harmonic distortion from one channel, at around 0.5% THD. digital scope waveform grabs to illustrate the amplifier’s typical distortion performance. For example, the scope grab immediately above shows the amplifier driven with a 1kHz sinewave at 25W into an 8-ohm load. The top two traces (yellow and green) show the outputs from both channels while the blue trace shows the harmonic distortion waveform which consists of mainly lower order harmonics (about 0.6% THD). Other scope waveforms are included to show the performance with square waves at 1kHz and 10kHz (at 5W into 8-ohm loads), with and without a 2mF capacitor shunting the load. The 2mF capacitor is a severe test of amplifier stability and while it causes noticeable Audio Precision The square wave response of the amplifier at 1kHz. The yellow trace is the input signal while the green trace is the amplifier output. As you can see, the wave shape is pretty good with only modest overshoot and minimal ringing. ringing on the waveform the amplifier’s performance is commendably stable. No problems there. We should also note our measurement for signal-to-noise ratio. This was measured with respect to 32W into 8-ohm loads and gave a result of -76dB unweighted, with a measurement bandwidth of 22Hz to 22kHz. That’s a very good figure for a valve power amplifier. As always when we test an amplifier, we check its damping factor, particularly at low frequencies. Damping factor is the ratio of the nominal loudspeaker impedance to the output impedance of the amplifier and is an indication of how well the amplifier “controls” the loudspeaker’s cone THD+N vs FREQ 4 Ohm (25W) 04/18/08 10:42:43 Audio Precision excursions, particularly within the “piston” range below about 500Hz. Solid-state amplifiers generally come up with very high figures for damping factor and this is partly due to the fact they employ high orders of negative feedback and partly due to the fact that they are inherently low impedance sources which can provide lots of drive current. By contrast, valve amplifiers employ output transformers and these limit the amount of negative feedback which can be applied before instability becomes a problem. The Altitude 3500-SS evidently uses very high quality output transformers but they still have the same limitation. THD+N vs FREQ 8 Ohm (25W) 04/18/08 10:43:30 These two graphs show the distortion versus frequency at a power level of 25W into 4 and 8-ohm loads. Interestingly, one channel (red trace) was significantly better than the other, although both were within specifications. 76  Silicon Chip siliconchip.com.au This scope grab shows the square wave response of the amplifier at 10kHz. The upper trace (yellow) is the input signal while the lower trace is the amplifier output. This is very good for 10kHz and any ringing is very well damped. We measured damping factor at a power level of 1W into an 8-ohm load and the result is 1.68 at 1kHz and 1.64 at 100Hz. These figures are equivalent to an output impedance of about 4.7W (using the 8W connection). If the 4W connection is used, this output impedance should be halved. Power consumption We measured the power consumption of the amplifier at around 135W with no signal present, rising to around 230W with both channels driven to full power (32W; ie, a total of 64W). This level of power consumption is about what you would expect from an amplifier of this power rating. As an aside, we were pleased to Audio Precision Finally, this shows the amplifier driven with a 10kHz square wave with the output shunted by a 2mF capacitor. This is a stiff test for any amplifier and demonstrates a high level of stability with any ringing still well damped. note that the owner’s manual recommends against leaving the amplifier permanently switched on. This would not only waste electricity but greatly reduce the operating life of the valves. Also noted in the manual is the fact that the valves will need replacing after several years, depending on how much use they get – this applies particularly to the output pentodes which should all be replaced at the same time, as matched pairs. Sound quality We spent quite a lot of time listening to this amplifier with a range of CDs and feeding a pair of very good loudspeakers. And while this was never going to change our overall opinions about THD+N vs Power 4 Ohm 04/18/08 10:46:28 Audio Precision valve versus solid-state amplifiers, we have to state that this particular valve amplifier performs very well. As already noted, it is particularly quiet and it gives a good account of itself on a wide range of musical programs. If you are attracted to the sound of valve amplifiers, you will really like this one. Recommended retail price of the Altitude 3500-SS is $1899.00 including GST. It is available direct from Wagner Electronic Services Australasia, 138 Liverpool Road, Ashfield, NSW 2131. Phone (02) 9798 9233. www.wagner. net.au The amplifier can also be purchased from Stones Sound Studios www. stonessoundstudio.com.au SC THD+N vs Power 8 Ohm 04/18/08 10:48:44 Here is the distortion versus power for both 4 and 8-ohm loads, at a frequency of 1kHz from 50mW up to the level of clipping (where the THD values rise rapidly). Again, one channel is significantly better than the other siliconchip.com.au June 2008  77