[Coco] WTB Orchestra 90 cartridge
Zippster
zippster278 at gmail.com
Wed Aug 3 08:39:10 EDT 2016
Yes, I’m reluctant to open the one Orch 90 cart I have here as well. :)
I’ve been wondering about the choice in value as well.
Using .1uF does seem like a really low value, until you take into consideration
that the output cap, in conjunction with the input circuit on the receiving end
form a high-pass filter. To see what the effects of the value chosen will be,
we need to know what that load impedance will be.
What I can gather on the internet indicates it is typically 10,000Ω or more,
at least for typical consumer gear, and walking around the house with a multimeter
here would seem to confirm that.
The lowest I could find were the small speakers shown in the CoCo2 test,
at 12,000Ω. One TV came in at 50K ohms, and everything else I could find
ran into many, many megohms.
So, in a worst case scenario (10K), you’re looking at a -3dB roll off of ~159Hz
using a .1uF output cap. At megohm input impedances, it’s negligible, certainly
beyond the range of human hearing (mine at least), and wouldn’t really matter.
If the schematics are correct at .1uF, I almost suspect Tandy might have intended
the high-pass formed from the connection to aid in filtering out low Hz noise,
but who knows. The other consideration I suppose would be just what frequencies
is this circuit even capable of producing.
I may bump the value up of the output caps to something like 10uF or more on
the final revision, not sure. BTW, I was able to find .1uF bi-polar electrolytic caps
at Digikey.
<http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=203424567&uq=636057666275979890 <http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=203424567&uq=636057666275979890>>
- Ed
> On Aug 3, 2016, at 4:51 AM, Jayeson Lee-Steere <cocolistemail at titaniumstudios.com> wrote:
>
> Very cool Ed.
>
> On capacitors, I think perhaps the schematic is in error with the 0.1uF
> non-polarized cap values. The smallest NP electrolytic I can find available
> anywhere is 1uF, and if you look at the pic of the Orch 90 board, what
> should be the 0.1uF NP caps are physically larger than what is supposed to
> be a 1uF NP cap. And then there is the question of, if 0.1uF is the
> intended value, why not use polyester caps as was done with the opamp input
> circuit?
>
> In fact, the caps look suspiciously like the polarized 10uF bypass cap on
> the 5V rail. This happens to be the same value as the output cap used for
> Tandy's Coco 2 and Coco 3 audio out. Not the best evidence since the output
> impedance of the Orch 90 DAC is indeed around 1/100th of the Coco 2/3
> circuits mentioned. Still, makes more sense to me than using a 0.1uF NP
> electrolytic where a 0.1uF polyester cap would do.
>
> Having the screw under the label sure is a curse, eh.
>
> Jayeson
>
> On Wed, Aug 3, 2016 at 6:47 AM, Zippster <zippster278 at gmail.com> wrote:
>
>> Thank you. :)
>>
>> After playing some tunes in your CoCo, you can just flip the switch and
>> plug it into a Dragon. Thanks to Pere Serrat’s adaptation of the Orch 90
>> ROM.
>>
>> <https://drive.google.com/open?id=0B1sgl615FlGnNzRhNC1xSjRBSFk <
>> https://drive.google.com/open?id=0B1sgl615FlGnNzRhNC1xSjRBSFk>>
>>
>> Just plugged into the TV speakers, but you can see it works…
>>
>> - Ed
>>
>>
>>> On Aug 2, 2016, at 5:38 PM, Bill Pierce via Coco <coco at maltedmedia.com>
>> wrote:
>>>
>>> <Applause> Good job!
>>>
>>>
>>>
>>>
>>>
>>>
>>> Bill Pierce
>>> "Charlie stole the handle, and the train it won't stop going, no way to
>> slow down!" - Ian Anderson - Jethro Tull
>>>
>>>
>>
>>
>> --
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>>
>
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