300B Dream

The DIY/Audiophile/Tube world seems to be filled with 300B designs. Many writers and builders place the 300B at, or near, the top of any 'most desireable' power output tube. Having never heard a 300B amp, good or bad, I have decided to follow the flock and build myself a 300B amp. There are a raft of designs around the Internet for the 300B, ranging from the original WE designs, RC designs (Angela, Audio Note Kit, Reinhardt, etc), exotic DC designs (Electra-Print), parafeed, transformer coupled, etc, etc. There are just so many!

Not being an audio designer, or electronics engineer, I have to rely largely on other people's designs, skill, knowledge and experience to put together something that will not; kill the builder, kill the end-user, burn or otherwise destroy property; and at the same time, sound decent and be reliable. It is therefore important that I not only use designs from people who really know what they are doing, but that I also understand the theory and design of the amp, so that I will not be simply soldering.

Not wanting to do the now conventional/standard RC approach, I decided that this amp should make use of technologies, designs, and topologies that are somewhat different. From reading the various information, it appears that the a 300B amp is best suited to its now 'classical' configuration, the Single-Ended (i.e., Class A) design. Further reading and research, suggests that the importance of the 'audio chain' begins backwards from the speaker. Taking this logical, the critical audio path, in regards to the power amp is as follows:

1. Output Transformer
2. Output Tube Stage
3. Power Supply Stage
4. Stage Coupling
5. Driver Tube Stage

To thus 'rough-out' the basic design requirements. The output transformer should be as high quality as possible. Some of the commonly touted high-end transformer manufacturers include (in no particular order); Plitron, Tango, Lundahl, Sowter, Electra-Print and Tamura and Audio Note UK. Some of these transformers are in the hundreds of dollars, and some special versions, even more. More information will be provided as I research more deeply into this area.

In regards to the output tube topology, there seems to be four designs; Direct coupling between output tube-transformer-power supply), Ultrapath, Parallel Feed and a combo of Ultrapath/Parallel Feed. My main reference is an excellent article in a back issue of Vacuum Tube Valley. This is an important area that I will explore later.

The coupling between the two stages can take place in a number of ways; direct coupling; RC coupling, transformer coupled, Loftin-White, using active (semiconductor) devices, and others. Given that capacitors are the components that colour the sound the most, it would seem logical to use or not use only with full and proper understanding of its implications. An area that deserves far more attention.

Finally, the power supply and driver stage will be looked into much more detail later.

After all of that, a preliminary list of the designs that are being seriously considered is as follows; (this may grow or shrink)

1. Electra-Print DRD 300B (A Loftin-White direct coupled 300B that also uses the Ultrapath technique for coupling the cathode to the output transformer).
2. Andrea Ciuffoli's 300B PSE/SE (Transformer interstage between the 5842 driver and 300B).
3. TubeLabSE (CSS 5842 Driver with a MOSFET stage between).

Headphone Amps

I originally started the DIY Audio scene with an interest in high quality headphones. My first project as Pete Millett's Low Voltage Headphone amp, followed by the Maple Tree Audio's EAR++ kit. Both these amps proved to be very nice to listen to through my Sennheiser HD-650's. And I continue to enjoy the intimacy of the listening experience that headphones offer. My upcoming trip has once again sparked an interest in headphones, but this time, in the mobile/portable hi-fi direction. So now, how can I mirror the quality/result of my current setup so that I can take it around with me? The search is now on for a small, low-power consumption (i.e., battery operated) and highest quality mobile/portable headphone and headphone amp combination. Some of the amp work has been extensively covered by sites such as Headwize and Tangentsoft to name a few of the popular sites. This would be a good starting place. The classic CMoy design is a very simple design, and having built a few of them, a great place to start. In regards to headphones, it appears that 'In Ear Headphones' or 'In Ear Monitors' are the best mobile solution. From reading various reviews, Etymotic ER-4S appears to be one of the best sets of in-ear headphones that are priced within reason (less than US$300). Other contenders include Shure and Ultimate Ears. The Headroom site offers a great selection of headphones, amps and information. Surprisingly, there are a number of Australian sites that stock various headphones and related equipment; Wicked Digital and Headphonic are some of note. More notes and progress to come!

MathCAD Spreadsheet for FE-127E

Have now included a PDF print of the MathCAD models designed and made by Martin King that has the parameters for the FE127E speaker. It has been rn in for over 50 hours now, and some of the 'shine' or 'tinny' sound has been reduced. Still amazed at the level of bass that this has, given such a small Sd. But let me say that the bass is not 'loud' by any stretch of the imagination. It is audible, not soft, not recessed, but clean and balanced. The imaging is very nice indeed. Just watching a DVD movie with only 2 speakers, in a Dolby Stereo, the was a wide sound stage, and though not fully surround, the sound did appear to come from the entire front wall, with some sounds appearing to come from outside of the speakers.

The next project is to build another pair of the same, but with a higher front baffle so that it can be floor standing, and have the driver at 1000mm above floor, which is at ear level.

PCB Power Supply Boards

In my ventures, I stumbled across these PCB from DIY Club, an online shop in Hong Kong that sells some rather interesting audio DIY related items. Two of which I found great interest in, are the TP4 regulated HV and LT PCB, and A19, a LT regulated PCB. TP4 is described as: "Gold Plated double side pcb, MosFet output, delay function on the high voltage output, with four heater voltage supply output adjustable from 6.3V to 15V". Whilst A19 is described as: "output voltage adjust from 3V to 20V input voltage 5V to 25V AC use LT1083 as volatge regulator". The prices were very reasonable, US$10.50 for the TP4 and US$4.00 for the A19. I bought a few, and here they are.

TP4 PCB

A19PCB

The boards seem nicely made, with a thick board and nice tidy traces. It was noted however, that two of the pads to the A19 that shouldn't be touching, are in fact touching. This seems to occur (see photo above) with the variable resistor. This is not of great concern to me, as I plan to use a fixed resistor. The TP4 boards will mate nicely with my Aikido PCB, whilst the A19 would work nicely in any future tube amp projects.

Irons arrived!

The Edcor output transformer and the Hammond choke has both arrived. See pics. The output transformer is definitely larger than I expected. The grey is not the standard finish, but I got it to match with the output transformer that is being 'custom' made by Edcor. The finish on the end bells is okay, not the best I've seen, but okay.

I have placed on order two custom power transformers from Edcor, with the following specs:

Primary:
230V 50Hz

Secondary:
400V-0-400V @ 150 mA
5V-0V @ 3.0A (5.0V @ 3.0A)
6.3V-0V @ 3.2A (6.3V @ 3.2A)
12.6V-0V @ 0.6A (12.6V @ 0.6A)

That should complete the last of the big stuff for this amp. The chassis will be an all timber affair, approximately 450x300 in size, about 112 deep, with the tubes and transformers on the top side, and choke on the inside. I am thinking of constructing it with 12mm MDF top and bottom, painted black, and the sides of 90x19mm KDHW (hardwood). The connectors, IEC plugs, fuses, switches and posts will be the next to-do.

The tubes have already been purchased. I went with JJ KT88 after some review of stuff on the net and from Vacuum Tube Valley mags. The driver tubes are NOS 12SN7 (hence the 12.6 heater) and NOS JAN 5U4G for the rectifiers.

Fostex FE127E ML-TQWT Speaker in Progress

After a few tweaks using Martin King's MathCAD models, I have configured a ML-TQWT using a Fostex FE-127E. The design response is approximately -3dB at 40Hz. The size of the enclosure is externally 900x300x225mm with the driver 200mm from the top, and the port 150mm from the bottom. It is stuffed from the top to the top of the port. The port is 50mm diameter and 100mm long. Approximate total stuffing weight is around 500g. See the response graph below.
The cabinet is made entirely of 16mm MDF, which is readily available in Australia. The size of the cabinet was designed to minimise cuts required. Two sheets of 900x600 and 1200x450 was required. See the pictures below of the speaker in progress. Stuffing used was a product called 'Polyloft' from the local rubber(!) shop.

The speaker is now complete, and being 'run-in'. They are being driven by the Chipamp completed earlier, and a PCB build of Peter Millet's Low Voltage Headphone amp. At the moment, I can say that the sound is really nice. The imaging is very precise, and the separation and articulation of instruments and notes is very clear. I have yet to install the ports in, but the bass is nice and balanced. Generally, the sound is very well balanced and should improve with time!

Chipamp Complete

After two years of completing the PCB and getting a working amp, I finally managed to put together a case for chipamp! This was the kit purchased from Chipamp.com and uses the LM3886TF. I have been running this in for the last few days and proved to be an excellent power amp. There is no hum and no noise. Quiet, and reproduces stereo very nicely. Even though it is not a 'mono-mono' PSU setup, the channel separation is very nice. The case is a fairly simple affair, with hardwood top and bootm panel, and the front and back made of MDF left over from renovations. The sides have been left open and allows the heatsinks to cool and air to enter into the case. So far, after running for 4 hours, the heatsinks are barely warm to touch, and the whole rig is very cool. The wiring may not be the best layout, primarily because the wiring was done without thought of chassis! However, as mentioned above, at normal listening levels, the floor noise is very low. As much advised by other people, the most difficult and expensive part of DIY power amp is building the chassis, and the transformers. It is very hard to get something that is suitable at a reasonable price.

Uniamp PCB almost complete!

This is a PCB build of Peter Millet's Uniamp, see his webpage for more info. I have decided to build this as my first true DIY/buy all the components and assemble yourself (except PCB) tube amp. These will be initially run with KT88. After a few months of delay, the PCB is finally assembled. All but 2 resistors have been soldered on and completed.

The parts used for this amp include:

• NOS Allen Bradley Resistors from eBay.
• Kiwame Power resistors (5W+) from DIY Hifi Supply.
• Solen SM Tin Foil Coupling Capacitors from Parts Connexion.
• Elna Cerafine cathode caps from Welborne Labs.
• Sowter phase splitter from Sowter.
• The power supply filter caps (Panasonic FC Snap-on) from Digikey.

Still working on an alternate design from Pete's webpage.