On Fri, 09 May 2008 08:50:59 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>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.
Did a little research and it turns out IEC has a 'spec', IEC 61938,
for headphone 'interfaces'. Source impedance of 120 ohms driven by
5Vrms max. And, if the load were 8 ohms, 5Vrms would be, drum roll
please, 3.125 Watts... exactly (close enough for government work) what
my amp is putting into the dummy load. So a 120 ohm in series will be
'according to spec'.
Can't get much more convenient than that.
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