2: Known issues with the design, it's origins and how to fix them
Let's just get right to it and here are the schematics for the ES-1/2 amps, as compiled from my amp and a number of others. There is some variation in parts used (and their values) but the basic circuit stays the same. First off is the amp section and then the PSU:
If you've spent any time looking at the schematics of electrostatic amps then the circuit will seem very familiar and that's because it is.
This is a very familiar site to many of us and pops up in most searches on the subject. Anyone with even no understanding of electronics can see that the ES-1 schematic is very similar, the same in most regards. This amp circuit, often called the Stax SRX, was given away for free by Stax in 1968 in the Japanese instructions manual for the then brand new SR-3 earspeakers. I happen to have that instructions manual and here is a shot of the schematic there:
It is a sound circuit but Mikhail did a poor job upgrading it. A shockingly poor job in fact. The biggest issue is the single filament supply which is grounded on top of that. Look at the Stax schematic and it's clear that the first two tube stages should share a grounded filament stage (AC or DC doesn't matter) but the last stage has to be left floating on its own. That's because the high cathode voltages will run most tubes out of spec. All tubes have a rated voltage gap which shouldn't be crossed at any time and that is the filament-cathode limit. Run the tubes past it and the tube will start to misbehave though it is hard to predict what will actually happen, since it varies from tube to tube. To make this a non issue then the filament is left floating. Luckily on most of the amps there are two filament windings which can supply enough current to drive the tubes so this is an easy fix. The next one isn't but that will have to wait...
2.1: How to fix the amp Added Dec 2009
It's been a while but I finally had some time to work on the ES-1 over the holidays. What I had to do was to completely strip down the amp section, have the top parts of both chassis repainted (simply too much work to gut the PSU as well so the bottom parts will stay black) and then rebuild it properly. Just about nothing from the original amp will be reused, the HE90 socket, attenuators, XLR inputs are just about the only things not scrapped. It may seem drastic but there are very few parts in an ES-1 so why not go crazy and get the really good stuff? First off, here is the schematic of the fixed amp courtesy of Dr. Gilmore.
As you can see it is pretty much the same but there are some crucial changes. The extra 5K pot on the input stage is there to decrease distortion and the two 50k pots on the output stage do work now and can limit the DC offset down to zero. For this you do need a new PSU but we'll get to that later. It is possible to simply modify a point-to-point wired amp according to the schematic above and the same should be true about the PCB versions. It will involve cutting and rerouting wires/traces but there is really no need for a complete rebuild.
As for the main PSU I chose to use the Blue Hawaii unit designed by Dr. Gilmore. This is a PSU design I've used for years and I had both PCB's and ample supply of parts for it. It's also easy to work with and it takes a lot to blow it up, always a good quality to have in a HT PSU. The design can be easily found over on Headwize but I'll include it here:
I built it just like that except the capacitors I used are 500uf/500V JJ units. Since the transformers are rated at 340VAC don't even think about using anything less then 500V caps, at least in the filter posistion (340V*1.41=~480VDC).
In the schematic above there was now a need for a new C- PSU giving use roughly -450V (-440V was enough for me but if you want to use 3d21a's then you need the extra voltage). The simplest solution here is to simply take a 50V supply and have it floating on top of the -400V supply. Lucky for use the Plitron transformers used by Mikhail have a 40V/100ma bias winding sitting unused which is perfect for this. You could simply wire up a bridge and some capacitors (hell I tested it with 3*9V batteries connected in series but this is not recommended as the batteries are now floating at 400V) but I really prefer a regulated PSU. The ideal regulator for this job is the TL783 and the PSU is just lifted directly from the data sheet.
As you can see there is no ground here, you take a wire from the -400VDC supply where the earth would normally go and that's it. There is also no need to use 500V rated parts as the components only care about the delta voltage and not voltage to ground. Here everything is lifted to -400VDC so 63V caps would be fine. I used just what components I had in the parts box so the values reflect that but also what the data sheet recommended. The formula to set the output voltage is in the data sheet so no need to repeat it here.
Since I had one extra 40V/100mA winding left unused in the amp I thought I'd use it to power the two indicator led's on the front panels. Mikhail powered them off the DC filament supply but I'd rather not do it that way so I built a simple PSU with a bridge, small filter cap and a voltage divider to give me roughly 14V. Again I just used parts I had at hand and there are many ways to do this. You could even put in a regulated PSU here too...
