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eXStata DIY Electrostatic Amp for Intermediate DIYers - Page 9  

post #121 of 2969
Quote:
Originally Posted by runeight View Post

So, to settle this concern, there will be. In fact, I've already created the web pages on Cavalli Audio website similar to the other designs there. They just haven't been turned on yet.
Will you be doing as much detail to the Exstata pages as for your other designs? Your page is very AMB like and a real boon to new builders.
post #122 of 2969
I was a bit confused by the earlier post. I thought the intention of the original amp thread was to make it diy. I must have missed some banter somewhere. Either way, glad to see this will be an available build at some point.
post #123 of 2969
Thanks for the impressions Larry. I just handed back my borrowed 007t today, so I'm very much looking forward to building one.
post #124 of 2969
Quote:
Originally Posted by ericj View Post
Ahh.

Sure is a lot more bling on the outside, though.
True but you can find those Woo cases cheap in China if you know where to look. Not that I'd use them for a SS amp but a Hifi-2000 Galaxy case would look great for a minimal outlay.

I'm all for a cheap and easy ESP amp but it would be wrong to think that the Exstata is the only alternative. With Justin now offering KGSS PCB's for sale (100$ for the set) you could build a KGSS for 600$ with some wise part choices. No 300$ 4-gang DACT's (30$ ebay stepper or a 40$ Alps RK27 balanced pot instead), matched singles instead of the rare 2sk389's, cheaper subs for the PSU sand as just some suggestions which will make the amp a bit cheaper to build but do little to sacrifice its quality.

Even cheaper would be to build an amp point-to-point such as the Stax SRX. Just take a metal plate and mount the necessary tube sockets plus some terminal strips and start wiring it up. There are some small tricks such as how to deal with the filament wiring and ground but these are minor obstacles.
post #125 of 2969
All these new more affordable stat amps have got me excited.
post #126 of 2969
Quote:
Originally Posted by HeadphoneAddict View Post
Let me put it this way, the improved soundstage, separation and power vs the GES is enough to make me want one even if the frequency response wasn't optimal in my rig. However, with the Marantz CD5001 > OPUS DAC > eXStatA I thought it sounded good with all the stats we tried. It sounds more solid state than tubey like my GES, but I still enjoyed it just as much. I liked the WES the most followed by the A-10 for their richer mids, but now it would be a choice between the WES and eXSstata if I was buying my first stat amp and didn't have one for my O2 Mk1. The KGBH SE mids were lacking a bit vs the others. The GES is a bit richer and refined sounding, so the eXSstatA wont be replacing my GES for my Jade, HE60, Smegma Pro, SR-404 LE, SR-Lambda, SR-5 gold edition and SR-003. On the other hand, the WES is good enough to replace my GES, but those cost 2x my maxed GES which has pro/normal bias and loop out.
Were the Exstatas at the RMAF of the SS or tube variety or both?
post #127 of 2969
SS version
post #128 of 2969
Thread Starter 

eXStata Amp Schematic

Time to start talking more about the amp now that the protos seem to be remaining stable.

Remember that the design goals were to create a reasonably priced, but good stat amp. Part of the original desire was to not require hard-to-get components and/or expensive iron. Well, it's hard to know when components will become hard to get, so you do the best you can with what there is currently around and what seems like it will be around for a while.

We also wanted easy-to-build and easy-to-bring-up for a moderately experience builder.

The goal became simple-but-good.

When this particular design concept came to thought I scoured the web for anything that I could find that would be similar or the same. I was not able to find anything using the keywords that made sense to me. But, the web is a big place and there are plenty of good designers in the world so I haven't really got a clue if there is an amp like this somewhere else. And, of course, I am not privvy to the commercial amps, so who knows. I'm sure that you all will not hesitate ....

Many of the stat amps that I have seen use three stages. Since stat amps are balanced they use a differential front end, an intermediate gain stage, and then an output stage. Sometimes the output stage is coupled to follower to reduce the Zo of the amp so that it can push more current easily. This also increases the speed in most cases.

This amp only uses two stages to simplify the topology. Andreas Rauenbuehler, however, has a nice two stage hybrid amp here. His transistor amp is a three stage amp as is the KGSS (although it could be argued that the diff amp and current tunnel are a single unit), for example.

All of these amps are good stat amps each with different qualities. And since I haven't heard any of them I can't say how they sound, but others can comment on this.

To make a two stage amp we only have two things to work with: the input stage and the output stage (followers are part of the output stage). So how do we do this?

Well, we know that the front end will be a diff amp and that the diff amp must translate the signal down to the bottom rail. The bottom rail because HV N-type devices are easier to find than P-types and so the output device will be at the bottom rail. Fortunately for us, we have p-type devices that can make the diff amp so that their collectors/drains point down and not up.

A classic way to handle high voltage with solid state devices is to use cascoding. Cascoding was first developed in tube topologies to generate high gain, but since tubes are usually HV devices they were not often used for HV protection. In sand circuits, however, cascodes are very handy for using an HV device to protect some LV devices that do the actual work.

Andreas uses a circuit with pfets and HV bjts. I did not know about this when I suggested the same idea in the previous thread, but really, there is hardly anything new in circuit design. Much of what we do today is assembling basic building blocks in ways that suit our purposes. Nonetheless, pfets with cascoded HV bjts is a no brainer for the front end of a simple-but-good stat amp.

Now that we have the signal at the bottom rail we have to reflect it back up to the top rail. One way is to provide a high impedance load for the diff amp and then to drive the grid/base of the output device. But this scheme has other drawbacks.

The most simple way that I can think of to reflect the diff amp off the bottom rail is to use a current mirror. And since we need a cascoding effect we can use a Wilson current mirror. Wilson mirrors have also been around for a long time, we're just coupling one to the front end.

And, that's about it for the amp topology. Schematic will be in the next post.

Current Sources

There was a great temptation to use current sources in this amp. The first, not-so-good design, used them. But after some thought, it became clear that for simple-but-good, we really don't need current sources.

In this amp, for example, the tail of the differential amplifer is connected to the positive rail which is sitting at 300V. If the front end runs at a total of 2mA the resistor at the tail can be 150k. And while this is not the 1G dynamic resistance of a current source, it is plenty good enough for the operation of this amplifier.

Same is true on the output followers. It is tempting to load them with CCSs, but it isn't really necessary to do that to get good performance.

Not having CCSs really simplifies the amp. Hence, this amp has no CCCs. In fact the amp has no caps either because it does not have huge open loop gain which, when tamed by NFB, leads to possible instability. It is stable without caps while still having a gain around 1000.

The tradeoff is that at the limits of its excursion it will evidence more distortion than an amp with high NFB and high OL gain. OTOH, high NFB has its own drawbacks to the sound of an amp so, for simple-but-good, this was an easy trade to make.

Next post, schematic.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
post #129 of 2969
Hi,
Any Glamor shots of the latest SS and Tube versions? Just to throw a little chum in the water!
post #130 of 2969
Damn, I need one of these in my life.

Not too sure whether I'd go with tube or SS though.
post #131 of 2969
Thread Starter 
Here's the schematic. Values are mostly left off for the moment because there are some tweaks happening. But, this is coming.

Please see below for further explanation.



You can see the simple diff amp/Wilson mirror combination.

The diff amp tail is loaded with resistors and trimpots for adjusting offset/bias.

The jfets are J271 because the are plentiful and easy to get right now and they have high enough transconductance for this amp.

The front end from the jfets down to the resistors below the KSA1156 is identical to Andreas front end except for choice of fets and some resistor values. The resistors on the collectors of the transistors serve two purposes: 1) to lower the voltage some on the KSA1156 and 2) more importantly to offer some protection to the source if the transistors should short out. This particular problem was pointed out by KG in the original thread.

Then, very simply, the HV BJTs feed the Wilson mirror. The primary mirror transistors are ordinary BC550 and you can see that they never see more than about two diode drops on them. The KSC5042 (Fairchild, 900V, 10W, 300mA) is the primary output devices. It does the rail to rail duty.

The KSC is loaded with a resistor and a follower. The follower is loaded with a resistor.

The mirror has a 3:1 current gain ration (680/220). This helps to relieve the dynamic current swings in the jfets and, hence, reduces the distortion.

Those of you who are paying attention will see that the gain of the amp is mostly dependent on the transconductance of the jfets. The balance is dependent on how well they are matched. Thus, it will be necessary to match the jfets to within about 10% for Idss. For the proto builders, with a batch of 20 jfets, it has been possible to get at least one and sometimes two quads of matched devices with Idss higher than 10mA. The higher the better, but nothing less than 10mA

There is just a tiny bit of NFB to bring the gain down to 1000, but not enough to introduce instability.

Now, because of the mirror, this amp is basically a current amp. In effect, the jfets are driving the load resistors at the top rail through a chain of devices that pass (and amplify) the current. With 1mA in each jfet there is, therefore, 3mA in the KSC devices. You can deduce many of the resistor values from there.

One downside to the location of the offset/bias is that they are adjusting the O/P DC through this long chain of devices, each of which will have thermal drift. This is one of the reasons for the great care in prototyping the design and not releasing immediately - to ensure that the bias stays stable over time after the amp burns in. So far, this seems to be true with the offset/balance staying less than 2V and usually less than 1V.

We could have used an auto adjust circuit, but then this just makes it more complex and more expensive with more potential problems.

As you can see, this is a very simple design. Transistors and resistors. And, so far, the listening tests have judged it to be very good. Theoretically, the amp is pretty fast. This would need to be confirmed on the bench. But the listenting tests indicate that it resolves very well and exposes sources in the same way.

And, it is simple enough for almost anyone to build if you take care. I am certain that this amp will come up properly if all of the right components are in the right places and none of them are defective.

The tube schematic will come after we're confident of more than one build. As well as other info about the next steps for a broader release.

I hope this helps everyone.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
post #132 of 2969
Subscribed

Looking forward to impressions of the tube version
post #133 of 2969
Quote:
Originally Posted by les_garten View Post
Hi,
Any Glamor shots of the latest SS and Tube versions? Just to throw a little chum in the water!
Chum in the water...

post #134 of 2969
I love how I'm taking a class that's essentially on transistor amplifiers and I can come online and start to understand everything.

The Wilson Mirror was a new one though.

Edit: I spy a BOM under there....
post #135 of 2969
Quote:
Originally Posted by pabbi1 View Post
Chum in the water...

Yum! Thanx!
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