“Model7” – amp-on-a-board

This is a project that was born from a visit to a fellow amp-building enthusiast. There was a small output transformer sitting in a pile of wires and upon closer inspection it was connected to a breadboarded pushpull stage, with the driver stage sitting across the room connected by a long cable.

The size of the output transformer and the diminutive size of the tubes struck me, particularly because they were at odds with the full, rich sound I was hearing through the Klipsch speaker.

The driver tubes were EL95 and the output transformers were custom-wound, 9.1K Ra-a with 20% ultralinear taps.

I quickly acquired a set of EL95s and a pair of the output transformers. Then I returned home and sat on them for a while before deciding on a circuit topology. I opted for a preamp stage based around the 6N1 tube since I had plenty in stock. So I designed a simple topology based around a gain stage followed by a long-tail pair phase splitter. I calculated this would give me around 15dB more open-loop gain than I needed to get full output at 1v P-P input. Thus allowing 15dB global NFB, which is a good amount for a push-pull design.

(click to enlarge)

The blue numbers in the schematic represent actual measured values in the built circuit.

The next step after designing the circuit was to bread-board the initial stages, and fine-tune the resistor values with experimentation. Note especially the Long-Tail Pair, the first half has 14K load resistor while the second half has 15K. This was calculated using Blencowe’s method to provide the best balance, which was validated by measurement. To make 14K I parallelled a 39K and a 22K resistor.

After breadboard validation, it was time to lay out the PCB. I had one very specific consideration for this amplifier: I wanted to fit it into a small form-factor size case, the same size as my previous 3-ch preamp project, in fact. This necessitated a limit on the PCB size, so I opted for 160 X 100mm. Therein lay the challenge: To fit the whole amp, power supply included, onto this size board.

In the end, I was successful, and after review I sent the board out to be manufactured. When it came back, I built it:

After building, it was time for some testing. This was accomplished using a 8R resistive dummyload and Room EQ Wizard, using a 100R : 20R voltage divider on the speaker output to allow accurate measurements at 1W and at full rated power (7W)

This shows freq response at 1W and full power, noise floor at 1W and full power, and THD again at 1W and Full Power. (Noise Floor levels as references to output level). (This was with the amp totally unshielded sitting on the bench with long flying wires everywhere, so I expect the noise floor to improve when it’s built into a chassis)

Now, it’s just a case of putting it into a chassis. The chassis design has been completed – the CNC pattern files in QCad and the PDF of the laser engraving in LibreOffice Draw. Once the chassis is back from the sheetmetal workshop, I’ll complete the assembly. Stay Tuned…

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