On Fri, 09 May 2008 00:43:25 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>On Thu, 08 May 2008 20:27:22 -0500, John Byrns <byrnsj@[EMAIL PROTECTED]
>
>wrote:
>
>>In article <KKLUj.38101$AL4.35754@[EMAIL PROTECTED]
>,
>> "Ian Iveson" <IanIveson.home@[EMAIL PROTECTED]
> wrote:
>>
>>> Thanks. One or two details might be worth discussing; I'm
>>> always interested to know how ppl make decisions.
>>
>>Hi Ian, I hope you don't mind my jumping into your discussion.
>>
>>> Why did you decide not to direct-couple the voltage amp to
>>> the concertina, and to share the valve halves by channel
>>> rather than by function?
>>
>>Conversely why would one want to direct couple voltage amp and the
>>concertina stage? By avoiding direct coupling the design of each stage
>>can be optimized. The only down side is that somewhat greater care must
>>be taken in the choice of the three low frequency time constants.
>>
>>> What's going on with the connection of the concertina's
>>> cathode load to the cathode of the voltage amp, rather than
>>> to ground? How well is the splitter balanced, given the
>>> different total loads?
>>
>>Flipper explained that it is positive feedback used to increase the open
>>loop gain of the amplifier. This is common in phono preamp stages, and
>>not unheard of in power amps. As far as precise balance of the
>>concertina loads goes, aren't you being a bit anal, they are close
>>enough.
>>
>>> I can't see how the output valves' bias is adjusted.
>>
>>I assumed it isn't adjusted, why should an adjustment be provided?
>>
>>> Why do Zobels go to anode supply, rather than to screens? I
>>> have read quite a lot of waffle around this subject but
>>> nothing very convincing. What, exactly, are those Zobels
>>> for?
>>
>>When I use Zobels I connect them to the anode supply as flipper did, why
>>would you want to connect them to the screens?
>
>My first thought was obviously the same, since I drew it that way, but
>after seeing his post and thinking about it I'm wondering if it's
>simply for the tie point convenience.
>
>
>>> Using a resistor in series with headphones isn't ideal, to
>>> my mind. My opinion is that a head amp should be have an
>>> output impedance of no more than 5 ohms, or one tenth of the
>>> 'phones' impedance.
>>
>>I'm not into headphones and hence don't understand how to properly
>>interface them. That said I was surprised that flipper didn't find some
>>attenuation necessary beyond that provided by a series 51 Ohm resistor?
>
>I think in practice I ended up using 56 ohms. That was also just a
>'starter' value and I don't use headphones much at all so I haven't
>gotten around to 'fine tuning' it but they do work. I was going to pop
>them back in and see how close the volume match is but seems I
>inadvertently left them at my brother in law's last time I was over
>there. So that took care of that, for the moment.
>
>The thing to keep in mind is amp power goes almost exclusively to the
>dummy loads with the headphones getting essentially a 'voltage' from
>it and at 3 watts Po 32 ohm headphones (mine) will get about 100mW
>(after the 56 ohm drop). Now, that's peak and average program
>material, with 6dB peaking, will be closer to 3/4 watt, which would
>put 25mw into the headphones.
>
>Problem is, SPL in both the headphones and speakers vary so widely
>there's no real way to know without listening to the ones in use.
>
>>I would think that a different circuit could be found that would meet
>>your 5-Ohm impedance criteria, perhaps by simply tapping the headphone
>>output doen on the 8 Ohm load resistors and eliminating the 51 Ohm
>>series resistor. Not understanding headphone circuits I would be
>>interested in learning what flipper's criteria were for the design of
>>this part of his amplifier?
>
>As I mentioned in the other post, 'copy cat' ;)
>
>And the series resistor will 'boost' the voltage up, to some degree,
>with higher impedance headphones so power doesn't drop off so much as
>it would if fed the same voltage level (like off a tapped 8 ohm load).
>
>Btw, I think 100mW 'max', at the rated amp power, into 32 ohm
>headphones is what I used to pick the starter values with the 51 ohms
>being back when Po was running a tad low and 56 ohms after some amp
>tuning got me to the target 3 watts. In fact, it peaks over 3 watts so
>I might increase that series R some more, if I ever find the blooming
>headphones.
>
>
>>> Nice pictures. Speakers are beautifully done. Amp looks
>>> functional. Valves look a bit gloomy. Transformers look
>>> cheerful but very small. How's your bass?
>>
>>The transformers do look "cheerful", but I don't understand why you say
>>the "Valves look a bit gloomy", they look quite bright to me?
>>
>>
>>Regards,
>>
>>John Byrns
I found the headphones. Turned out they had simply slid out of sight
in the car.
So I tried them again and I'd say they're too loud with the 56 ohms in
series. Works but it would be nicer to have a little more range on the
low end so I'll end up increasing that value.
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