High Quality Audio Transformers

At last they are here :

Our range of high quality audio transformers continuous to improve and expand. Over the past years we have spent an increasing amount of time fine tuning the materials and winding techniques employed in constructing our transformers. This in turn has lead to improvements in tone and musicality, even if measured response has changed little. With the introduction of Interstage Coupling Transformers four years ago we helped to open up an area of design and performance which is only now beginning to be recognised in the establish audio press.

We are now pleased to introduce chokes specially designed to be used as plate loads in active stages. These chokes are wound and manufactured in a different way from our PSU chokes to offer finer resolution and musicality. The use of a plate load choke in an LC coupled stage, or in the emerging "parafeed" configuration, offers excellent linearity and voltage swing, which results in high levels of inner detail, ease and expression from the music. Just take care to have a low enough Rp, ad a large enough choke to ensure that your low frequencies don't suffer in the process! as always, we will be pleased to assist with any designs you may be cooking up.

Plate Load Chokes

The latest "re-discovered" technology to hit the DIY valve circuit is the choke load, or LC coupled, audio stage. Here the anode (plate) resistor is replaced by a low dcr choke. This gives a very flat load line and thus low distortion and good voltage swing. They also form an essential part of the newly introduced "parallel feed" or "parafeed" circuits which have recently started appearing. The sound of a choke load stage is somewhere between RC coupled and transformer coupled. The main benefits the choke brings are greater midrange fluidity and ease, whilst retaining much of the strong dynamic bass that RC coupling is capable of.

As when using preamp output transformers, or interstage driver transformers, choke loads need to be very carefully sighted when used in small signal applications to avoid excessive hum field pick-up. For this reason we are looking to source mu-metal screening cans in the near future to offer as an enclosure for these devices.

Interstage Coupling Transformers

Our most exciting recent products are the expanding range of interstage coupling transformers. Previously only available as very expensive imports from the Far East, interstage transformers are believed to be the only true method of driving output valves by many fevered oriental audiophiles. In response to requests from our customers for quality transformers with which to drive output valves these carefully designed and built products have been assembled. They probably represent the greatest challenge to a transformer designer due to the dc current in the primary and the very high impedance grid circuit across the secondary.

The 1:1 ratio transformers have a very wide, flat, frequency response and excellent sonic characteristics. The 1:1.5 designs are more compromised, but still extend beyond 40KHz. The initial designs have been based around using the following valve types for drivers: 5687, E182CC, 417A, parallel 6SN7, parallel ECC82. Designed around a 15mA-20mA primary current, approx. 3K5 anode impedance, or below. Other drivers such as 3A/167M and ML4 have been used successfully. They can be used with a large range of output valves, including 211, 845, 300B, 6080, 2A3 etc.

The 1:1 transformers offer the best high frequency extension and drive capabilities. The frequency response of the it102, for example, is 10Hz-120KHz at -3dB, there are no out of band resonance peaks or dips either which we believe is unique. They are sonically far and away the best method of coupling your driver to output, no cap is even close.

The 1:1.5 are aimed primarily at high bias output valves to help reduce the voltage swing required from the driver valve e.g. 845, DA100, 6080 etc. They are capable of bandwidths over 40KHz.

The push-pull driver transformers are phase splitters running off a single driver valve. They offer an interesting alternative to all the valve based phase splitters, and can help produce a zero feedback triode push-pull amp. We now only supply a 1:1+1 unit as it offers better bandwidth than the 1.5 version previously available. It is good for around 20KHz before falling away.

Standard finish is black painted fully shrouded. Chassis layout needs to be thought about to keep the transformers away from mains transformer hum fields. Chrome plated shrouds are an extra cost item at £25 per transformer.

Line Pre-Amp Output Transformers

Since summer 1995 we have been developing some line stage output transformers. The idea of these was published in Sound Practices about 18 months ago and requests have been coming in for us to make ours available. We are now happy with the bandwidth and sound quality and are pleased to offer them.

We initially worked up a 5:1 step down ratio which was nominally designed for 15K:600R, however is sounds best into high value loads, like 100K from the power amp input section. A 20:1 step down version has recently become available. This is a nominal 15K:38R and offers a lower output impedance, lower overall gain, better bandwidth and greater immunity from hum than the 5:1 version. They are based around the same driver tubes as the interstage transformers and will give you a low gain and very very low output impedance line stage, without having to use feedback or capacitors.

Standard finish is black painted shrouds, as for the interstage transformers. Larger versions are also becoming available for other, higher current, valves.  Chrome plated shrouds are an extra cost item at £25 per transformer.

All models were measured using a 417A/5842 driver valve.

Output Transformers

Our recently expanded range of high quality output transformers is now available for your projects. Traditionally the most difficult component for the home constructor to source, and with the growing popularity of single ended designs the home constructor has found himself even more limited for choice. For several years the only available single ended devices have been imported in very small numbers and this has made their price prohibitive to most. We aim to offer affordable British products to, initially, cover the most likely requirements, with further designs becoming available as requests are made.

Designed with the benefit of almost 50 years of experience our transformers are hand wound to order and made from the finest materials. The designs are specified according to their reflected primary impedance, maximum recommended dc standing current and maximum power. Frequency response is typically within 1dB from 20Hz to 40KHz and 4 and 8 ohm secondary taps are provided. The power rating of the OPT should be twice the maximum power output of the stage. Open frame, fully shrouded or drop through can be supplied as required. Chrome plated caps/shrouds can be supplied for an additional œ12.50 per drop through, and œ25 per fully shrouded transformer. Unless otherwise stated transformers will be supplied with fully shrouded with black caps.

All our transformers are built from the highest quality grain oriented steel, and are all critically tested for measured and sonic performance before we offer them for sale.

Over the past two years we have been developing a number of more complex wound transformers to give better bandwidth and finer detail to the sound. These are all multi-section split bobbin designs built on slightly larger stack sizes. In all appropriate installations these will outperform even our standard range.

Standard Range

Single ended output transformers

Super Range

Single ended output transformers

All can have ultra linear taps at no extra cost, all can be supplied fully shrouded, open frame or drop through as required. Please enquire about any requirements not listed here.

Over the past couple of years we have wanted to produce a still higher quality range of transformers. We have been ultimately thwarted in are efforts due to availability of some of the core materials and wires we want to specify. The program of work did yield great results in terms of winding wires, insulation materials and construction techniques. Where possible the lessons learned have been applied to the Super Range above, making them an even better value product, and even better sounding.

Mains Transformers

Offering a comprehensive range of mains transformers is almost impossible. We can supply and quote for any mains transformers to meet your own requirements, and we wont charge the earth for design or because it's a "special". Our mains transformers are all hand wound from finest materials and are specified to a maximum 5% regulation. Open frame, fully shrouded or drop through types are available as requested.

Pricing for mains transformers is based upon having 3 secondary windings and the VA rating of all the secondaries added together.

For each additional secondary winding please add 2.5% extra above the VA rating.

Power Supply Inductors

Inductor smoothing is becoming popular again as more people realise it was abandoned for reasons of cost, not quality. Inductors afford higher smoothing factors, and better voltage regulation, giving quieter operation and better dynamics. Using chokes enables lower values of smoothing capacitance to be used, allowing higher quality types to be specified. We can supply any choke. Specifications should be for inductance, dc current draw, approximate DC resistance and application (if the choke is to be used as the input to a PSU i.e. choke input filter, then we will need to know as a different design is required.) A selection of the chokes we already have available are listed below. Chrome shrouds are available as an extra cost option, see Output Transformers for details. The ch109 has been specially developed for use in high current, high voltage, choke input filter power supplies. Its primary use is for choke input supplies for stereo 300B amps where you may be looking at 450Vdc rails at up to 200mA. It is available only as an open frame construction, but has been built to be quieter than normal chokes in a similar supply.

Transformer Sets

We have put together three transformer "sets" for some commonly available kit designs. These "sets" contain all the iron work required for you to put together your own version of the kit amplifier. These sets are put together at a special price for maximum value for money.

Set 1: suitable for push-pull 5881 kit: 2xOTX, 1xChoke, 1xMains cost £210

Set 2: suitable for push-pull 300B kit: 2xOTX, 2xChoke, 1xMains, 2xInterstage cost £420

Set 3: suitable for single ended 300B kit: 2xOTX, 1xChoke, 1xMains cost £245

Set 4: suitable for SJS SE108IT MK2 design: 2x mono blocks cost £800

Set 5: suitable for SJS SE108IT MK2 design: 1x stereo amp cost £600

Transformer Sizes

For the majority of the above iron-work we have quoted a lamination size and stack size. Using the enclosed Size Indicator the dimension of any of our transformers can be readily worked out. (Under Development)