The Integrated 300B Amplifier - Part 1

Hi!

I repeatedly get asked about the possibility to offer an integrated amplifier and also headphone amplifier based on the 300B and preferably in a single chassis.

So I decided do develop one and see if there really is demand. I mocked up a test circuit using a new Lundahl output transformer with a new high bandwidth winding technique and which has optimum values to be used with Elrog ER300B.

Above the bread boarded test circuit, one channel completed. To fit all into a single chassis no interstage transformer and no transformer volume control. Volume control is done via a DACT stepped resistive attenuator (not seen in the pic above yet)

I selected the 6DN7 as driver tube. The two systems offer the right amplification factors and plate resistances for this job.

Initial measurements and listening tests with a bunch of lab supplies gave very good results. So time to bread board a power supply as well.

Some older metal plates got recycled for the test. The result is very pleasing. A headphone version of this will be offered as well with selectable impedances for 32/150/600 Ohm. Now everything will be fitted into a single box and the metal plates for that have to be designed.

Drop me a mail if you are interested to become one of the first owners of a 300B integrated amplifier or headphone amplifier. Other tubes will also be possible.

Best regards

Thomas

All DHT Headphone System

Hi!

I often get inquiries about headphone amplifiers. Although I do have a headphone amp in my portfolio with the E55L as output tubes. Many want a DHT.

The 10Y line preamp can perfectly drive high impedance headphones (300 Ohm or more) and is useable as a normal preamp at the same time. It works very well for example with the Sennheiser HD600 shown in the photo. This HP has an impedance of 300 Ohms. Existing 10Y line preamps can be used with headphones simply with a suitable adaptor. For new builds, headphone jacks can be added at the front and/or back of the unit.

For those who have lower impedance headphones, I am proposing to add a 'booster stage' which is then driven by the 10Y preamp. This booster stage can be built with any suitable DHT. For example 45, 46, 2A3 and of course 300B or even another 10Y or 801A. The booster stage would get it's own separate power supply.

Such a system could be used to drive any headphone on the market and even very high efficient speakers.

If you are interested in your own state of the art all DHT headphone system, drop me an email

Best regards

Thomas

The Headphone Amplifier 2

Hi!

Last month I showed the first headphone amp I ever built. That one was meant to be used with a preamplifier, hence it did not have input selector or volume control. This month I finished another version of that, this time with input selector switch and a volume attenuator at the input so sources can be connected directly to it.

It is based on the same circuit and component selection as the previous one, just with the selector added and a resistive ladder type volume control.

While the previous one had all the taps of the output transformers brought out to individual jacks, this only uses one of them to make room for the control switches.

Output tubes are triode wired E55L and the rectifier is the 6BY5.

Best regards

Thomas

The Headphone Amplifier

Hi!

In most cases my line preamplifiers can drive headphones without any problem. But certain headphones with 32 Ohms impedance are a too heavy load for a preamp. Typically they are too sensitive to be connected to a power amp, even if it is a flea power single ended one. So in such cases a dedicated headphone amp which acts as buffer after the preamp is the ticket.

This is how the E55L single ended headphone amp was born. It is meant to drive the Grado RS1 which is very sensitive (1mW for 96dB SPL) and has a 32 Ohms impedance.  The E55L triode connected delivers more than enough power for that and has just the right gain. So the amp can be kept rather simple with just a single E55L stage per channel.

The circuit follows my usual approach in power amps. Output transformers are Hashimoto. The E55L runs at a moderate 30mA. The power supply is tube rectified with a 6BY5 augmented with UF1007 diodes to create a full wave bridge. Several LC filter stages after rectification. This guarantees a hum free operation even with these sensitive headphones.

Top view:

There are 3 outputs connected to the 4, 8 and 16 Ohm taps of the output transformers. The headphones work on either of them. Selecting different output provides some additional gain control.

Just one tube per channel:

View from the back:

The E55Ls proved to be just right for this application.

Best regards

Thomas