Glasshouse 300BSE Amplifier Description

Introduction

To launch the range of Glasshouse valve amplifiers and pre-amplifiers, the first product had to be something special, it had to carry a strong signature look. As kit builders become increasingly aesthetic conscious, the Glasshouse range aims to reflect this in simple and elegant product design. This is complimented by the simplicity of its circuit design resulting in a jaw dropping sound. We decided to feature a classic valve,the 300B, well loved, regularly used in the industry and in manufacture today. An extremely linear directly heated valve, and in the right circuit a real stunner. The best way to show off the true character of a 300B is in single ended configuration. Our 300B single-ended amplifier is stereo and is available in two versions, the Glasshouse 300BSEi, the "i" integrated version that can receive 6 line-level sources. The power amplifier version is the Glasshouse 300BSEp playing from a pair of inputs from a pre-amplifier, be it passive or active. We have already begun work on the Glasshouse pre-amplifier, a one box item that will house a pre-amp section, a tone control and phono stage each with its own power supply unit. All modules will be available separately, this gives you the option to buy the front panel and knobs to fit the modules you will be using. The output power is 11 watts into a 6ohm load, so speakers of 89dB and above will be required.

The Design

Our designer, Andy Grove, of Audio Note, designed the circuit. Andy was challenged with creating a design whereby the input sensitivity had to be enough to allow the amplifier to run directly off a line level source.The output noise/hum needed to be low for the growing army of high sensitivity loudspeaker users. His circuit can be viewed from the Pdf file at the bottom of the page. Andy goes into detail of how the circuit works on the circuit description page. (left hand column links)

The valve heaters within the Glasshouse 300BSE series circuit power supply, apart from the rectifier valve, are supplied with a regulated DC voltage to keep hum to a minimum, this is achieved by using the LM323 voltage regulator chip. A large device on a T03 package that looks somewhat like a power transistor. The main HT rail is valve rectified using the 5U4 valve, smoothed by a 10H choke and a combination of 4 reservoir capacitors in a Pi network. This powers the 300B and the EF86 valves. The driver valve, the 5687 has its own dual supply +195Vdc and -195Vdc, formed by a solid state supply.

As mentioned, the input valve is the famous EF86 pentode valve, an unusual choice you may think, in that it is not a triode, but Andy raves about valve type mixed circuits working superbly together and we are in total agreement. No doubt in the future we will create another version of the circuit to incorporate a double triode to work with all the major parts. The driver valve is the double triode 5687, a rugged though beautiful sounding 9 pin valve. And last but by no means least, you have the 300B valve sitting happily on the output stage, feedbackless of course. Please note that the output transformer fitted at the rear are huge, built on a 120 lam, they have a 2.5 inch stack. This guarantees a good bandwidth, measured at +/- 1dB, 16Hz - 20Khz, with superb control and depth at the low frequencies.

The Chassis

The chassis measures 440mm(width) x 430mm(deep) x 230mm(height), and is made up of 5 pieces (excluding knobs). An important feature when it comes to designing enclosures for kits, is accessibility, you don`t want to be struggling to fit any of the parts inside. Spare space is essential, people invariably upgrade with better components after the initial build, these parts tend to be bigger, some a whole lot bigger, so your chassis needs to be able to accommodate these. Layout is critical, especially in a valve amplifier. A mains transformer sat next to an input valve or a mains wire loom will increase hum noticeably. A mains transformer sat too close to an output transformer will result in electric and magnetic field interaction. This is why the output transformers sit with a slight turn on them to minimise hum that such interaction causes. Additional hum prevention is provided by a 1.5mm thick copper faraday shield around the core of the mains transformer. (Please note the pictures illustrate a mirror finish copper plate, for the finished product the plate will be grained copper which will be more durable and aesthetically appealing).

The main chassis is made of aluminium, 2.5mm thick and with a durable eggshell, black powder coat finish. All front projection corners have a 10mm radius, as I for one, get tired of the sharp edges of everyday hi-fi separates. Aluminium is non-ferrous (non-magnetic) so it does not interact with the magnetic field of the transformers, unlike a mild steel chassis. As a result you get a far better musicality with aluminium and even better with copper. This is the reasoning behind the copper top plate, measuring 1.2mm thick. It also looks beautiful. The finished product will have a subtle grain from side to side and coated with baked on clear lacquer to prevent any oxidisation of the copper surface. The top plate attaches to the main chassis with the help of all the nuts and bolts that hold the hardware in place. 

The transformer cover is made of 1.6mm mild steel, in a black powder coat finish, this acts as a part magnetic shield for the transformers. This is side vented to allow ventilation, ample as the transformers run reasonably cool for valve amplifier standards. The transformer cover is fixed from within the chassis by 8 off M3 screws. The top front corners have the 10mm radius.

The valve cover is also made of 1.6mm mild steel, with an eggshell, black powder coat finish, the fitting of this is optional. The cover is highly vented on the front, sides and top, about 65% coverage, so the valves are visible when the cover is fixed. Fitting is in a similar way to the transformer cover. The top front corners have the same radius. This is an optional extra.

The chassis base is made of 2.5mm aluminium, with an eggshell, black powder coat finish. This fits snugly in the recess of the underside of the main chassis.

The front plate is a beautifully finished grey/silver anodised 6mm thick aluminium alloy. The knobs are made of the same material and share the same finish to ensure a exact match with the front plate. The knobs fit in a 2mm circular rebate on the front plate. Again all four corners have a radius of 10mm.

(please note all the pictures are of the prototype chassis, the finished product will be printed with the Glasshouse brand name - photos to follow)

The Kit

The passive components supplied are of good quality, but will not break the bank, for a full inventory have a look at the parts list. Money well spent is on the chassis and the transformers that are of high quality and in our opinion do not need improving on.

The Glasshouse 300BSE series amplifiers are hardwired with exception of the selector switch in the 300BSEi. The mains and two output transformers are supplied on open frames and actually fit on top of the main chassis, allowing more space inside the amplifier than the normal drop through variety would allow. This provides better electrical and magnetic separation from the passive components too. The flying leads pass through the chassis for internal wiring. The 10H choke is positioned internally, in the centre of the amplifier. 

The passive components are fitted on six tag boards, four for the power supply unit (PSU) and one signal tag board for each channel. The PSU capacitors, consisting of 4 off 100uF 450V and 6 off 100uF 250V, are axial types and fit on the 4 tag boards. C5, the larger 82uF 500V capacitor fits on a capacitor clip. Please note there is plenty of room for mounting numerous capacitor clips to hold large caps to replace the axials at a later date if you so desire. The signal tag boards are in close proximity to their related valve bases for short hook up wire lengths. The signal section has a low component count so you will have plenty of room to play with.

You will see from the internal picture that you have a bank of four LM323 5V regulator chips, these fit directly onto the chassis, using the isolation kits to avoid electrical contact with the chassis as the body of the device acts as their common. Being fitted to the chassis assists in heat transfer from the device as a result they run lukewarm and well within spec.

The selector switch sits on a printed circuit board at the rear of the amp, close to the phonos. This makes excellent sonic and build sense, no multi runs of screened wire connected to a fiddly selector switch at the front panel. The silver plated contact selector switch is physically connected to the front panel by an extension rod.

The connector hardware is fitted on the rear panel, 6 signal input phono pairs and a "tape-out" pair. Here we use insulated silver plated phono sockets far better sounding than the gold type. Also we have the Audio Note heavy duty silver plated speaker posts, the earth post and the fused IEC mains input. This is wired to a rocker mains switch that is positioned 10cm on the left side. Humbucker potentiometers for the 300B heaters are not required as we have a 5Vdc regulated supply so there is no AC waves to buck.

Power Output; 11 watts into a 6ohm load

Input Sensitivity; 370mV

Hum/Noise; 1.2mV

Bandwidth +/-1dB; 16Hz - 20kHz

Build Requirements

To build the 300BSE Series Amplifier you will need to:

• know how to solder a good join
• be aware of safety practices when it comes to electronics
• have an understanding of valve electronics
• be able to read a circuit diagram
• own a multimeter and a dummy load (old pair of speakers, or high power rating 6-10R resistors. Additionally,a oscilloscope and variac are useful but not essential.
• have patience, rushing creates mistakes

To help you build the kit you will be given an instruction book. This will soon be available as a pdf file on the site. A detailed run through the build process step by step, each step will be represented by a clear photograph for reference along with text. Numerous diagrams will also be provided, along with the start up procedure and a fault finding section. If you do have problems, we can help via the telephone or e-mail or you can return her to base for repair. 

At the end of the experience you will have a kit that not only looks amazing but it will sound awesome for many years to come. 

We regret to say that this product is no longer available