sachu
Headphoneus Supremus
- Joined
- Oct 29, 2006
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Alright,
For those folks who have been following the EHHA thread, you know that there has been a design for a power amplifier based on the EHHA design.
Here's hoping there is enough interest in the power amplifier version of the design to start a prototype phase.
I will let Alex chime in on the details of the design, PCB availability, the power supply requirements, et al.
Here is a quote form the EHHA thread from Alex about the EHPA and a preliminary schematic.
Quote:
I am in for a set of boards and already have secured the case, transformers, tubes and bits and pieces for the amplifier.
Step up and let it be known that you want to get in on this awesome power amplifier build. Here's hoping there is enough interest to bump this into prototype phase.
Roll call:
1> dBel84
2> sachu
3> digger945
4>
For those folks who have been following the EHHA thread, you know that there has been a design for a power amplifier based on the EHHA design.
Here's hoping there is enough interest in the power amplifier version of the design to start a prototype phase.
I will let Alex chime in on the details of the design, PCB availability, the power supply requirements, et al.
Here is a quote form the EHHA thread from Alex about the EHPA and a preliminary schematic.
Quote:
Originally Posted by runeight /img/forum/go_quote.gif Gents, while I am away for a few weeks with little free time on my hands, perhaps I can contribute something to get your thoughts going. One thing I've always wanted to do with the EHHA is to make all of the gain stages tube. In the current version, of course, the VAS stage is symmetrical BJTs. The first problem right off the bat is that we don't have p-channel tubes. This means that the symmetry goes away. But we can utilize the p-channel VAS portion of the amp to create a different kind of offset servo. With only an n-channel device (a triode) we can only replace the bottom VAS device with a tube. But we still need a connection from the top of the Vbe to the V+ and this connection must be a high dynamic resistance. Usually in this type of amp topology this load is provided by a CCS, either a sand one or a bootstrapped one using capacitors and resistors. However, since I'm figuring we'll use dual triodes for all tubes, we can use a tube and a current mirror for the current source. Here's the schematic: Notice that the B+ for the tubes is now 100V. Many of you will like this. Because of the high voltages the simple current mirrors must be replaced by Wilson current mirrors. The third transistor is the HV device. It must be at least 250V because it will see the rail-to-rail B+/B-. The other half of the VAS triode is used as a current source for the other current mirror loading the Vbe. This triode is slaved to the opamp which is detecting the output offset and zeroing the DC. The VAS triode pair use cathode LEDs to eliminate bypass caps. The mosfets are fronted by BJT emitter followers to increase the load seen by the VAS triode. Otherwise the gate capacitance of the mosfets is too much for the tube to drive. But with the followers the amp is still pretty fast, although not quite as fast as the EHHA I. This amp will need an additional B+/B- supply which makes the PS a bit more complex than the EHHA I. OTOH, the amp itself is a bit simpler. |
I am in for a set of boards and already have secured the case, transformers, tubes and bits and pieces for the amplifier.
Step up and let it be known that you want to get in on this awesome power amplifier build. Here's hoping there is enough interest to bump this into prototype phase.
Roll call:
1> dBel84
2> sachu
3> digger945
4>