“Model 7” Amplifier

– You can buy this amp for $NZ1250.00 or talk to us about changes or enhancements, to either features or aesthetics –

The idea of the Model 7 was conceived during a visit to English Acoustics. There they had a single EL95 PP stage set up being fed through an unholy mess of wires from a preamp across the room. It was playing into a single Klipsch speaker, and I was impressed with the dynamics from such a low powered amplifier.

A deal was struck which saw me walking away from that visit with a quad of EL95s and a pair of output transformers of their design.

Over the following few months the design of the Model 7 came to fruition. The idea was to create a small form factor amp that would fit into the same size case as the previous 3 channel preamp. This would necessitate the whole amp – including power supply – to be made onto a single PCB. I discounted point-to-point construction out of preference. Plus with a PCB it makes it easy to make another unit if the need arises.

The Model 7 amplifier

About the name
The name reflects several attributes of this design:
– The amplifier power (7 watts/ch)
– The tube count
– It’s the 7th amp designed by Atrad Audio
– It weighs (almost) 7kg (Well, 5.7 to be exact, but there’s a 7 in there at least!)

Design and Construction
The circuit design is simple. Working backwards form the output, the design of the pushpull stage was dictated by the transformers: 9.1K Ra-a with 20% UL taps. So the EL95s would be used in UL mode at the maximum specified screen voltage in that mode, dictating a B+ voltage of 300V. Cathode bias was chosen for simplicity.

The preamp stage was designed using the tubes to hand. I have a large stock of Russian NOS 6N1, 6N2 and 6N6 types. An initial gain stage followed by a LTP was the topology selected, and the 6N1 for both stages gave enough open-loop gain to allow around 14dB of negative feedback.

So the circuit was designed by drawing the loadlines and choosing the correct operating points.

From there, a test rig was constructed to verify the design and thereafter the PCB was designed. I use DesignSpark PCB from RS for my PCB designs.

After the board was designed I sent it out for manufacturing. I wanted a professionally made dual-sided, silkscreened board. I am happy with the result.

Testing
Performance testing on the amp was undertaken using Room EQ Wizard (REW) and a Creative Soundblaster Pro. The pictures are worth a thousand words

These graphs tell us
– the noise floor is around -75dB at 1W, or -83dB at full power
– the Freq response is flat across the audio spectrum
– the distortion is at 0.2% at 1W and 5% at full power

Analysis and Listening Tests
The amp is a delight to listen to. Into efficient speakers, the sound belies the small power rating. With no tone controls you are at the mercy of the input signal, and being line-level those signals are highly likely to be from a digital source, so unlikely to need tone correction.

Listening to a variety of genres the sound is clear and neutral and full of detail. A great example of the laid-back yet rich and detailed presentation that tubes are renowned for. Listen all day with no fatigue.

The only drawback from initial testing – as expected, putting so many tubes into a small chassis, this thing radiates rather strongly in the infra-red. Translation: it gets quite warm.


Specifications

Topology Line-level amplifier, grounded cathode gain stage, AC-coupled
longtail pair phase inverter, push-pull EL95 output in class AB using cathode-bias ultralinear topology, global negative feedback. 300V Plate Voltage.
Valve complement Gain stage: 1 X 6N1
Phase Inverter: 2 X 6N1
Output: 4 X EL95
Power Output (measured) 7W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured) 0.2% THD at 1W, 5% THD at 7W, 20Hz – 20KHzboth channels driven, resistive load
Output Impedance 8Ohm only
Input Impedance 100 KOhms
Input Sensitivity 1.0v p-p for rated power (0.35v RMS)
NFB 14dB
Frequency Response 6Hz – 35kHz ±1dB @ 1W
15Hz – 35KHz ±1dB @ 7W
Signal:NoiseNoise Floor: -75dB @ 1W, -83dB @ 7W
Power consumption 230v 50Hz 90w nominal

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