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floobydust wrote:

The main component that limits low-frequency response in any SET is the OPT. For these initial designs, I used the smallest Hashimoto SE OPTs... their H-203S for the 2A3 and their H-507S for the 45 which are rated at 7-watts at 50Hz. Despite their small size they are excellent in quality, both mechanical construction, shielding and sound quality. Tailoring the low-frequency response for any amplifier should be a design factor so you don't over-run the OPT. Such is the case in my design for these. I purposely wanted to roll them off on the low end so you don't have issues with core saturation and/or field collapse within reasonable listening levels.

Yes, I know but I do not particularly care about it as the DSET concept absolutely eliminate all those frustrations with OPT.

floobydust wrote:

Some thoughts on direct-coupling.... for all of the pluses people rave about regarding sonics and such and the removal of the alleged "evil" signal coupling capacitor there are also some negative points which you have hinted at. First, DC coupling does present some additional challenges in the design and maintaining proper DC biasing of the cascaded stages. In many cases, additional circuitry and components are used to compensate for this, many with adjustable resistances to lock it into spec. So that raises one initial question... does the addition of several resistance components where they are of different construction (wire-wound pots, cermet-film pots, wirewound resistors, metal-film, etc.) and/or regulator circuits to maintain DC biasing compensate for the removal of one single coupling capacitor? Which is worse, the cause or the cure? Second, as you noted there is something different in the sound at times. One possible cause is the fact that ANY DC offset and/or low-frequency underlying signal (like record warp) can push the circuit out of it's linear operating region causing high distortions at these extremes and also push the OPT into core saturation and field collapse. This is yet another reason to tailor the low-frequency response of low output amplifiers so they are not pushed out of their linear operating region from signal anomalies.

Valid points, we just discussed with Dima exactly the same, he does not feel comfortable with the direct-coupling, thought I feel that higher frequency the channel operated in the more benefits the direct-coupling has.

floobydust wrote:

Multiple stages versus one. Well, again, a choice that has to be made somwhere in the signal chain from start to finish. Let's start at the output stage... using my little design above, and the 45 triode. In the operating point I use, grid bias is approximately -60volts. Simple math shows that you need to be able to drive the grid at +/- 60 volts or 120 volts peak-to-peak for maximum power without driving the tube into A2 on positive swings or cutoff on negative swings. Due to the multiplying effect of input capacitance, you also need a suitable driver for the output stage. If you go thru the required math and use 20KHz and 60 volts swing you can calculate the required slew rate and minimum driver current to ensure you drive the grid properly... there's no free lunch here. The signal level and drive current must be met or you won't have a clean output stage. In a single stage design, you are completely reliant on the driving source. How many preamps can provide 120 volts peak-to-peak under these conditions? In such a design, the overall sound quality now becomes completely reliant to the what's driving it. Granted, if you design the entire system and take all of these points into consideration for design and implementation than you can manage it. If you plan on using other components... you've lost control.

An interning point. “The signal level and driving current must be met”, I was under impression that we meet the criteria of driving voltage not driving current. I thought that the driving current is necessary juts to push the tube capacitance, which is extremely low in the tubes I use, and other then that the driving current is just the way to load the interconnect before the amp. You feel that driving current play more roles in my situation? Do not forget that I use the single stage amp with 4.5V on grid, so the operation sits deep in A1 with no greet contra-currents in presence (in theory)

floobydust wrote:

As for the input/driver stage I eventually settled on, it has some excellent operating attributes. First, the driving stage (one triode of the 5814A) has ~2.8ma of current and can easily drive 150 volts peak-to-peak cleanly into the grid, meaning it can drive the 45 or 2A3 fully and at wide bandwidth as the response is flat past 40KHz. Yes, you use more drive currrent, but it is somewhat akin to killing a fly... you can use a fly-swatter or a bazooka... end result is still the same... dead fly. You can also look at it's square-wave response and note that it's crisp and clean which supports my calculations on drive current versus required slew rate and voltage swing. The input stage is the other half of the 5814A. The output of this stage is DC coupled to the driver... but, the operating points of each stage are selected to ensure it does not push the driver out of it's linear region. The input stage is biased at ~1.2ma of current and can handle a input voltage of 8 volts peak-to-peak without being driven into A2 or cutoff. Of course, this would result in pushing the driver stage well beyond it's maximum output level, but the design goal is to ensure the input stage has no possibility of being driven out of it's linear region or placing an odd load on whatever drives it, as class A2 would result is increasing current demands from the input.

Hm, interesting, I never seen/heard seen/heard seen/heard this 5814A. BTW, if you have any curiosity then you might try my driver.

http://audiodiy.hangszoro.net/viewtopic.php?p=27004&sid=e009865a60357dfd6f073a61a8872467

Put is at 200V with plate resistor no less then 15K and drive though it is 15-30mA..

floobydust wrote:

Another part of the design is the input bypass capacitor being chosen for a specific roll-off point, again to help reduce large low-frequency input offsets like record warp or rumble. The driver stage bypass capacitor is actually calculated for 1Hz or less as I don't want it adding anything like phase shift but simply following it's input signal. And lastly, the coupling capacitor to the output stage (it's the only one in the design) is also chosen for a specific roll-off point. In all, there are 3 time constants and they are not stacked but each is calculated for a predictable response on low-frequency. The end result was as required, 1dB down at 25Hz, which is 1-octave below the spec'd power level of the OPT.

Good point. The concept is totally irrelevant in DSET application buy still valid generally. Still it is a big question what shell die first – the filter or the OPT from the bottom. I you looked my straggling I had with in past with implementation of MF coupling capacitor as a high filter then you would know what I mean. What whatever reason the inner stage coupling capacitor is not always work good as a bandwidth limiting devise.

floobydust wrote:

So, what am I driving with these little guys... it varies. I do have quite a few speakers at home. I have Quad ESL-63 US Monitors ,but they are far too low on sensitivty for any appreciable SPLs.... but they are gems in their own right with enough power. For single drivers I have a pair of Fostex F120A in 10-liter enclosures I built which are also on the low side, but a few dB more sensitive than the Quads. However, the Feastrex D5nf setup in solid red oak BVR enclosures does have sensitiviy around 95dB. The amplifiers are okay provided you don't push too loud. Granted, having 100dB+ speakers would be a huge plus, but I can monitor output levels with a Mytek DDD-603 and ensure the amps can never be driven into clipping (with digitial sources only). They get loud enough for some very critical listening but of course not loud enough for a symphonic orchestra level. I'm not entirely done on speaker choices yet, but have additional higher powered SETs on the design board. Overall I would certainly recommend the smaller 2-watt 45 amp for driving a high-sensitivity tweeter in a multi-amp system. It's overall linearity is excellent.

Ok, I got the message. Still, as I understand that you are not in over 100dB would? The over 100dB environment in my view set another set of rules. It is not necessary that it would reverse what you say but it kind of different. It would be very interning to hear your thought after you play with well over 100dB sensitivity and it has a different impact to the view what amp shell do. I think instead of having a higher powered SET on the design board you might fish something like JBL 2440 from the ebay pond and see what happen next…

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