Very quiet ZX81 MIC.. What's... Uh the deal?

Discussions about Sinclair ZX80 and ZX81 Hardware
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yerzmyey
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by yerzmyey »

Kokkiklhs:

> The AY-option is always great, but I think that it would be even greater to have some more cool 1-bit music stuff on the Zeddy, like the ones we know already from the Spectrum!
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True. if only somebody could port any engine from Spectrum to ZX81! Ah well.

> The Z80 chip itself is powerful enough to give some nice performance even on the Zeddy
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Yes it surely is. I dunno about ULA 'though. This chip is different, as far as I know.
Anyway, Atari XL/XE people are able to port Spectrum48 BEEPER engines and they work 100% identically on Atari XL/XE. And there's only 1,7Mhz on the CPU (or something like that).
So You see. Must be possible on ZX81 too.

> (since we have 9 channels on a plain Speccy),
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It's 16 channels now. http://z80.i-demo.pl/Mister_Beep-Organists_Farewell.mp3

> but some serious sound output must be achieved first...
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Yes. That's another problem. Yes indeed.
IN NIHILUM REVERTERIS - a big text-adventure game for ZX81: http://tiny.pl/g2m6m
"MONOCHROME" issue 5 - (Spring 2014) free paper/PDF magazine about ZX81: http://tiny.pl/q2m44
ZX81 COMPETITIONS 2007/2009: http://zx81.republika.pl/
Moggy
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by Moggy »

With all due respect to Siggi who is a true zeddy guru but in his description he says that a louder output is achieved whilst retaining the same filter characteristics by using a passive resistor capacitor circuit.
In all my years of audio amp and tone circuit construction unless active elements are introduced a simple, (what looks at first glance like a high pass filter), R/C circuit like this will introduce signal losses or have I missed something?
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kokkiklhs
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by kokkiklhs »

Moggy wrote:With all due respect to Siggi who is a true zeddy guru but in his description he says that a louder output is achieved whilst retaining the same filter characteristics by using a passive resistor capacitor circuit.
In all my years of audio amp and tone circuit construction unless active elements are introduced a simple, (what looks at first glance like a high pass filter), R/C circuit like this will introduce signal losses or have I missed something?
Indeed this sounds strange, but since it's published on a magazine AND since Siggi says so, I believe neither of them has a reason to fool anyone! So we expect some more details from Siggi, hope he reads these lines and answers soon, and a little more light will shine over this obscure case!! :)
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1024MAK
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by 1024MAK »

The impedance has changed, yes. As far as anything connected to the socket is concerned, the circuit has a lower loss between the ULA and the external world (so higher output voltage). The filter cut-off frequency is mainly formed by the ratio between the resistor and the capacitor. The value of the resistor has been reduced by a factor of ten, but the value of the capacitor has been increased by a factor of ten...

Also the actual filter cut-off frequency is a soft curve, so the exact cut-off frequency is not that important.

So not worth worrying about :P

Mark
Moggy
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by Moggy »

1024MAK wrote:The impedance has changed, yes. As far as anything connected to the socket is concerned, the circuit has a lower loss between the ULA and the external world (so higher output voltage). The filter cut-off frequency is mainly formed by the ratio between the resistor and the capacitor. The value of the resistor has been reduced by a factor of ten, but the value of the capacitor has been increased by a factor of ten...

Also the actual filter cut-off frequency is a soft curve, so the exact cut-off frequency is not that important.

So not worth worrying about :P

Mark
Who's worrying? :lol:

Looks like I read it wrong, I thought Siggi was presenting it as a signal booster not an impedance changer and yes I get the filter bit having built and designed tone circuits for my own stage gear but nice explanation anyway. :D
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PokeMon
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by PokeMon »

Well the output voltage of ULA (5V) is reduced by factor 1000 to just 5mV at factory setting - which is suitable for old recording technologies but not for modern equipment. Choosing resistor and capacitor like shown from siggi will give an ouput of 50mV which can be easily recorded with smartphones or PC audio card for example and reduces noise as well. The resistor and the capacitor can be simply wired in parallel to the onboard components with no problem (at the solderside of the Zeddy as well). The changes provide the same filter characteristics.
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kokkiklhs
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by kokkiklhs »

Well, here's a photographic step-by-step tutorial for dummies, for the 50mV MIC modification as I successfully did it yesterday!
I hope it helps EVERYONE who owns a Zeddy to easily improve the quality of its MIC output, this is really useful!
Just follow the steps respectively to the 5 numbered pics that can be found in the attached .rar file!

1)This is my Timex Sinclair 1000 pcb upper side, it is EXACTLY the same board with a genuine Sinclair british ZX81 issue 3 (the most common pcb)... Original resistor R29 and capacitor C12 can be seen within the green circle.
2)This is a detail with R29 and C12 on the board. I noticed (from internet photos) that on issue 1 the same components are located at different places, but of course the same mod must be done, once someone finds their exact place on the pcb (indicated with white printed letters).
3)This is the bottom side of my TS1000 pcb, before the modification.
4)This is a detail of the bottom side, with the soldered edges of R29 (red ellipse) and C12 (purple ellipse), where the edges of the 2 new components can be soldered respectively (WITHOUT desoldering the old ones from the other side of the pcb, they should stay at their places).
5)And this is how it looks like after soldering the new 100kOhm resistor (over the solderings of the two R29 edges) and the new 470pF ceramic capacitor (over the solderings of the two C12 edges)...

It DOES improve the sound quality of the MIC output significantly and it's really worth doing!

You just need a soldering iron, one 100kOhm resistor and one 470pF ceramic capacitor to do the job! The latter 2 cost less than 1 euro/dollar together and can be found on any electronics shop.
Very easy, even for a totally inexperienced soldering noob! :mrgreen:
N.B.
a) I am not responsible if any damage occurs to your equipment during the operation, it worked fine for me!
b) Take care while removing and refitting the keyboard membrane edges from and to the pcb, they are very fragile and can be easily damaged!!!
c) Ceramic capacitors (unlike the electrolytic ones) do not have specific polarity, so you can solder them either way you like! The same with resistors.
d) I suppose that this should work on any other ZX81 issue pcb, you just have to locate the exact positions of R29 and C12 and do the same procedure. I haven't tried yet it on a different pcb, though...
e) Excuse me for the crappy quality of the photos, but my camera sucks! I hope they are rather helpful than confusing...
Attachments
ZX81 MIC output modification.rar
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yerzmyey
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by yerzmyey »

This is a really great news!
IN NIHILUM REVERTERIS - a big text-adventure game for ZX81: http://tiny.pl/g2m6m
"MONOCHROME" issue 5 - (Spring 2014) free paper/PDF magazine about ZX81: http://tiny.pl/q2m44
ZX81 COMPETITIONS 2007/2009: http://zx81.republika.pl/
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user@lab1
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by user@lab1 »

Simple and interesting modify. ;)
An old electronic magazine suggested to bring C12 from 47pF to 560pF and R29 from 1Mohm to 82Kohm.
Can anything be done to increase the sensitivity of the EAR jack? :?:

Thanks, Mic.
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kokkiklhs
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Re: Very quiet ZX81 MIC.. What's... Uh the deal?

Post by kokkiklhs »

user@lab1 wrote:An old electronic magazine suggested to bring C12 from 47pF to 560pF and R29 from 1Mohm to 82Kohm.
Can anything be done to increase the sensitivity of the EAR jack? :?:
A bigger capacitor and smaller resistor would simply increase the output voltage even more! Since a magazine suggests it, seems that someone has already tried it and it *should* be safe for the Zeddy! But as the 50mV seems to be enough for a good quality playback or recording of the sound produced, I'd say *not* to push this ancient machine's tolerances to their limits...
;)

As for higher EAR input sensitivity, may I ask "what for?"
You can already load anything flawlessly, I even use OTLA-converted turbo loading stuff, which does not allow input mistakes at all, and it works like a dream!!
8-)
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