On Tue, 13 May 2008 08:22:52 -0500, John Byrns <byrnsj@[EMAIL PROTECTED]
>
wrote:
>In article <kk4i24t13io43j65n6jgmbjnpbr7juna56@[EMAIL PROTECTED]
>,
> flipper <flipper@[EMAIL PROTECTED]
> wrote:
>
>>
>> Btw, the OTL doesn't 'require' the 120 ohm resistor and it works just
>> fine as a 'low impedance' output.
>>
>> You could just remove the 120 ohm, and then adjust the FB resistor for
>> the changed Vo, but there's also a nice trick can be played. Instead
>> of the FB R and blocking cap. as shown, put a 12ohm under the existing
>> Rk on V2A and take the headphone return to that junction. That
>> eliminates the DC blocking cap.
>
>If I am following your description of this mod correctly, doesn't that
>convert the negative feedback from voltage feedback to current feedback,
Yep
>with the expected effect on source impedance?
Yes, but then a moving coil device is actually current driven so I'm
not sure what the 'net effect' would be.
I liked the idea of eliminating one of the high voltage blocking caps
and there's another version with CF, put the 120 ohm in parallel with
the headphones. You get the same 'semi constant' Po vs headphone
impedance but it increases the uF needed for the output coupling cap
because the minimum load is then 120 || 32 instead of 152.
>
>> With low impedance headphones there's a teensy improvement in
>> distortion because, without the 120 ohm, output swing is less but
>> it's negligible with 300 ohm headphones.
>>
>> Like I said, plenty of room for tweaking ;) But the basic topology
>> looks good and 6CY7s are rather cheap.
>>
>> You felt the 6AW8s looked 'gloomy'. Well, lemme tell ya, the 6CY7s are
>> anything but and those suckers glow like Christmas tree lights.
>>
>> Oh, something I noticed after posting everything. When I compromised
>> with 200V on the phase splitter I forgot to readjust the R3 current
>> soak resistor. It's there just to suck up the idle current V1A puts
>> onto the output rail, and at 200V it's less than at 300V, so R3 should
>> be more like 250k. Doesn't really matter all that much, though.
>
>I was wondering about that, I had assumed that the 120k value shown on
>the schematic was for the 200 volt supply feeding the phase inverter on
>the "breadboard", and wondered why you hadn't also shown the correct
>value to use with the intended 300 volt supply, now I see that I had it
>sort of backwards.
Yeah, I screwed up. I drew it up as an exercise then, later, suddenly
got a bur under my saddle to build the thing and was 90% done when I
got to the 'connect to 300V' part and, well crap, I had forgotten the
300V was in there. LOL
But at that point, hey, what the hell? Might as well try it at 200V.
The 'saving grace', so to speak, is it doesn't have to produce a huge
voltage swing so it still works at the reduced voltage.
>Regards,
>
>John Byrns
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