Cavalli Audio: Embedded Hybrid Power amplifier (EHPA)
Jun 10, 2009 at 9:49 PM Thread Starter Post #1 of 16

sachu

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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:

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:

EHPA One Channel 2.02 No Parts.gif


Notice that the B+ for the tubes is now 100V. Many of you will like this.
smily_headphones1.gif
And notice how the bottom triode replaces a bjt for the vas section.

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.
beerchug.gif


Roll call:

1> dBel84
2> sachu
3> digger945
4>
 
Jun 10, 2009 at 11:13 PM Post #3 of 16

bmwpowere36m3

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Quote:

Originally Posted by nux /img/forum/go_quote.gif
I would be really interested in making this, can I put my hand up? How much power output would it have?


Same here, what's its power capability, and output "tolerance" (4, 8 ohm loads). Any specific tubes in mind?
 
Jun 10, 2009 at 11:21 PM Post #4 of 16

digger945

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You got interest here bro. Wallet is open. Just read Alex's post's over on the EHHA thread. He seems to be pretty confident in the design. I don't really care what kinda tubes or outputs as it can no doubt be built to your liking, if not have some degree of flexibility as the EHHA does now.

I am now excited, smitten, and preparing to go broke in the next short while.
 
Jun 10, 2009 at 11:28 PM Post #5 of 16

sachu

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From what I can recall this can easily do 100W into 4 ohms..depends on your LV rails of course. AM planning on about 35V rails for the LV. With 40volts on your LV rails this can do 100Watts into 4 ohms.
 
Jun 11, 2009 at 12:44 AM Post #6 of 16

digger945

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Sigma22 is readily configurable for 36V, and I would bet Ti can tell you how to make it do 40 no problem. It would probably do 40 now with a 40V transformer and a change of one or two resistors.
 
Jun 11, 2009 at 1:38 AM Post #7 of 16

dBel84

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Quote:

Originally Posted by sachu /img/forum/go_quote.gif
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 to bump this into prototype phase.
beerchug.gif


Roll call:

1> dBel94
2> sachu
3> digger945
4>



phew, for a moment I thought I was on that list
wink_face.gif
I am out of the amp building loop for a while
redface.gif
..dB


I WILL build it ........ someday



EDIT

I should just add here that this has been around for a while but Alex has been resistant to release it due to his limited time availability and commitment to other projects at the moment. There is much grumbling behind the scenes and I think that discussion around this subject would be of great interest in general but we should all bear in mind that EHHA is only just gaining momentum and this power amp may take a little longer to stretch its wings
 
Jun 11, 2009 at 2:02 AM Post #8 of 16

sachu

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Quote:

Originally Posted by dBel84 /img/forum/go_quote.gif
phew, for a moment I thought I was on that list
wink_face.gif
I am out of the amp building loop for a while
redface.gif
..dB


I WILL build it ........ someday



umm..you are on that list cause I know you said you wanted a set of PCBs.
 
Jun 11, 2009 at 4:50 PM Post #10 of 16

TimJo

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I'm in, if it gets to the prototype phase, and it can drive my extra set of speakers...

In the meantime, I'm certainly interested in following along with the discussion and evolution of the design...
 
Jun 11, 2009 at 4:53 PM Post #11 of 16

TimJo

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Alex (or any others following along) - I do have a question about the new EHPA schematic.

I'm not seeing an obvious connection between the first half of the differential amp triode (the input signal) and the upper half of the newly configured VAS stage, other than through B+.

What am I missing here? (Remember I'm still a newbie. )
 
Jun 11, 2009 at 6:18 PM Post #12 of 16

runeight

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TimJo, the diff amp at the front is no longer a true diff amp because we are not using the first triode for gain.

Instead the first triode is acting purely as a cathode follower to drive the second triode as a grounded grid amplifier. So there is no connection between the first triode and the second current mirror.

So what is the second mirror for?? Well, look at the bottom tube. If you think about it the BJT/FET output stage is just a plate load for the triode. But this plate load needs to terminate in the V+ so that the O/P stage can swing the rails. Thus we need a load on top of the Vbe mulitplier and this load must have high dynamic impedance so that the tube is not heavily loaded (so we get bandwidth).

So...... the second mirror is merely a CCS except that it is formed by a tube CCS slaved to the opamp which sets the current in the mirror. Coincidentally, this acts like a servo to set the output DC to zero.

Does this make sense?
 
Jun 12, 2009 at 3:50 PM Post #13 of 16

TimJo

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Quote:

Originally Posted by runeight /img/forum/go_quote.gif
Does this make sense?


Yes, now I get it - well at least I can visualize the signal path.
redface.gif


I will study the schematic some more over the weekend and read more about this part of the design:
Thus we need a load on top of the Vbe mulitplier and this load must have high dynamic impedance so that the tube is not heavily loaded (so we get bandwidth).
Thanks for walking me through the logic Alex.
smile.gif
 

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