ECC82 Pre-Amplifier

The amplifier described here is a little bit different than the usual you will find att the local radio shop. It is a bit nostalgia together with high-quality sound in a simple circuit. The heart of the amplifier is two vacuum-tubes - the dual-triode ECC82.
Why use tubes instead of transistors or Op-amps? First of all - they are proven to give a more gentle, warm sound compared to semiconductors. This is specially true to a power amplifier build with tubes. In a pre-amplifier, this is not as noticable due to the usage - the risk of distorsion in a pre-amp is less than in a power amp. Also, the pre-amp lacks the typical output transformer that almost all tube power amplifiers are equipped with. Still, the sound is different - to the better.

The design this amplifier is based on is called SRPP - Serial Regulated Pull-Push. This is a rather simple design that gives excellent perfomance. For more information about the SRPP design, you could read this article at the excellent tube web-magazine Tube CAD Journal: SRPP Decoded.

The rest of the amplifier is designed to minimize the impact of the surrounding at the signal:

* High-quality relays used in the input selector. The signal is never fed to the selector switch at the front.
* Alps audio potentiometer for volume control - also placed close to the inputs. A long brass shaft makes the volume operateable from the front.
* Gold-plated connectors where it is possible.
* High-quality components used in almost every part of the amp.
* Copper shielding between channels and to the parts where AC-voltage could be found.
* Extreme DC filters for the anode voltage - 2 x 470uF together with a 20H choke.
* Power transformers placed in a external box. This gave me an additional feature that allows it to power some future equipment that could be started at same time as the pre-amp.

Actually, there is one thing in the construction where look have been prioritized before function - the placement of the actual tubes (and the orientation of the amplifier board). A large piece of the amusement of a tube amp is actually seeing the tubes glowing. That is the reason why many tube amps (both pre- and power) are designed with all hardware on the top, fully visible. A bonus of this is that the cooling are not a problem (specially with power tubes). In my design, I wanted a amplifier that would fit in a regular cabinet, with the ability to put something else on top. So, the tubes-on-top design was not an option. Instead, I got an idea to make a window in the front panel and place the tubes in behind. As I remember, Luxman made an amp in that way. Therefore I had to design the amp so the tubes are placed close to the front panel. Due to the length of the box, the distance between the input selector and the actual amplifier circuit became rather long. A optimum placement would be with the tubes close to the back - but they wouldn't be visible from outside in that case. To compensate for the extra distance, I used high-quality screened audio cables between the amplifier section and the rear connectors and input selector. The plates of copper was also put in to screen off the amplifier components, both from external interference and also between the channels. A design bonus was the copper "mirrors" behind the tubes...

The Original Design
The original schematics for the amplifier stage. This design was originally published in Glass Audio 1991 by Bruce Rozenblitz.
The original drawing also showed a power supply and the input selector. These parts have been removed as I have made my own versions. Also, the design above have been slightly altered - the high impedance output have been taken away. I wasn't really in need of it. Then the anode voltage was increased - the amp is running at 350V instead.

Amplifier Pictures
The amplifier ready for testing. Still, there are some parts that still have to be added and the cabling aren't finished.
The part everyone want to see - the tubes. The two ECC82's from Ruby Tubes are shown here - one for each channel. The copper plates are acting as a shield but also as a part of the design. The surfaces behind the tubes have been polished to shine...
Rectifier with the large capacitors and the filter choke. First, it was planned for a tube rectifier using the EZ81 full-wave rectifier, but it was scrapped and replaced with a silicon diode bridge rectifier built with discrete diodes filtered off with high-voltage 100n capacitors. The avantage with silicone in this part is that the capactitors can be very big. Tube rectifiers have a limited capacity of filter capacitors (50uF for a single EZ81) - they can't handle the current rush when the capacitors are charged. Also, silicon diodes runs much colder than tubes, making the interior of the amplifier colder.
Top view of the amplifier board. The IC's visible at left, under the cables aren't for the sound. These are plain 555's controlling the delayed turn-on of the amplifier.
This is the regulator for the heaters and the logic. They both runs at same 12.6V. Using stabilized DC for the tube heaters really takes off the need of hum reduction or other strange solutions to reduce induced hum in the catodes from the heaters. The bonus is that the 12V relays and the timer circuits runs fine from it too - no additional supply was needed.
The final interior of the pre-amp. All cabling are done and a extra copper wall have been placed to screen off the parts containing AC voltage.
Another view of the final mounting - only the top cover is missing.
The final result - all boxed up and ready to be used.
The amplifier running - note the two glowing tubes in the window. It would be a mistake to hide these inside the box...

The red, fat cable shown behind the amp is that one that connects the transformer unit to the amplifier.
The external transformer unit. Not the best color of the box, but it was rather cheap and it will be placed out of sight anyway. Note the two AXL-connectors - the leftmost is for the pre-amp and holds also the remote start wires. The right connector contains same power connections as the left one but without the remote start. This can be used for some future tube equipment (like a RIAA-stage) that could be switched on at the same time as the amplifier.
Inside look of the transformer unit - it contains two toroidal transformes. The one closest to the viewer is the low voltage ring - it gives 2 x 15V for low-voltage circuits and (via regulator in amplifier) tube heaters. The other toroid is the high-voltage - a standard 2 x 115V transformer. The box also contains fuses for both primary and secondary sides. The circuit in the middle is the remote start.
This unit is connected to the amp via a 1m long lead containing screened cables - each voltage inside it's own screened cable. All three is hosted in a flexible tube and terminated in each end with angeled AXL-connectors.

Scope Pictures

These scope pictures was taken under same condition:

* Voltage per square: 5V
* Input signal level: 1Vpp
* Output volume at full (volume potentiometer at max).
* Only the time base have been adjusted to show the same amount of periods for the different frequencies.
1kHz square wave
10kHz square wave.
100kHz square wave.
20kHz square wave.
1kHz sine wave.
100kHz sine wave - can't hardly see any difference from the picture above.

Listening impressions

As with every piece of audio equipment, there is always interesting to give some opinion about the actual use of it. The practical usage of the amplifier is rather simple - volume control and input selector. This doesn't need much exercise to be able to use it. Therefore, the actual sound reproduction is more interesting - did it sound better?
The answer: Yes, it did.
Compared to my older design, this one have much better frequency response - this is specially noticeable at the high frequencies. A much clearer sound is enjoyed with this amplifier.
This amplifier have no problems driving any power amplifier - a input signal of 1Vpp gives 16Vpp out unloaded. That is a gain of 16 times. This makes this amplifier a perfect partner to my next project - the 10W SE Ultra Linear power amplifier based at the KT88 power pentode.

Schematics to download:
Amplifier Stage Show one channel, only the tube stage.
Power Supply Enough for both channels and all logic needed.