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

[Spirantho]

Hi people,

While trying my new ZX81 composite output back porch thingy, I tried saving a test program... and lo, there was no output.
Tried various things, still nothing.

OK, thinks I, let's get a working one to compare.

Same thing.

Grab another ZX81.

Same thing.

That's issue 1 and issue 3, 3 machines, all do the same.
This is connected to a tape recorder, or to the PC... same applies.

So what's going on here? The signal's there, I can measure it on the PC but it's tiiiiiny.

Has anyone found a solution.. or even the cause? What happens if we lower the resistance between the ULA and MIC port? It's a 1MOhm resistor, pretty big.
What about if we put an amplifier transistor on the MIC port? Is the input too low?

I guess most people don't actually save anything on ZX81s any more, so maybe not many people notice.... but I do.

Any ideas?

[zx80nut]

Hi.
The original "mic" sockets on cassette recorders are designed to have a dynamic (moving coil) microphone plugged in.
The output of these microphones was absolutely tiny, so the mic socket has a huge volume gain.
As a result, to make the ZX computers (and others) compatible, the mic output is heavilly attenuated. As you point out, this is via a 1M resistor, then into a 1K internal load, therefore reducing by a factor of 1000. This doesn't take into account the very small capacitor, and also the load that you then connect the computer to.

The main reduction is the 1M resistor, so reducing this, or taking the output before the resistor would give a higher signal.

Regards.

Grant

[Paul]

The signal is supposed to be very low.
Are you using an old-fashioned mono tape recorder? They are the only ones that work reliable with zeddys.
For Line-in you would need a preamplifier, just as you would need on a Soundcard of a PC.
There were already some suggestions for this, but (on the last post I remember) the schematics of a working preamplifier is still missing...
I would be careful about changing connections at the ULA, it's so damageable.

[Spirantho]

Thanks for the replies!

I'm actually connecting it to a Mattel Aquarius tape recorder... these tape decks are very good and reliable. It's connected with a standard ZX81/Spectrum MIC/EAR lead.

I compared the output from the ZX81 to the output from a Spectrum and it's much softer. The Tape recorder records exactly the same as the PC does (when the PC is set to full gain, and using the microphone port), i.e. lots of hiss and a vague bit of audio.

I was thinking perhaps of modifying it to the circuit used by the ZX96... but I'm scared of increasing the load on the already fragile ULA!

[Spirantho]

Just to reply to my own post for a moment, with my findings ...

I want to report that indeed this is normal for a ZX81, as I've replicated it at home, with a normal cassette recorder.
It seems that the Aquarius cassette deck doesn't have the same impedance or something as a normal mono deck, and it's this impedance that's making the ZX81's output loud enough for the tape recorder.

Incidentally, I tried it with my ZX80 too, same thing happened.. although my ZX80 wouldn't load back from either deck (but that may just be the ZX80).

[Paul]

the resistors in the zx80 are different. you need a higher volume to reload on the zx80. there are just a few models that proved to work.
maybe zxpand is an alternative for zx80 once the rom has been adapted to work with it.

[Andy Rea]

All but 1 of my 4 zx81's don't have R27 (1k) fitted... the 1 that did have it fitted was the one i built from a kit, once i found it didn't work well with the tape recorder i have used sucessfully with the other 3 i wondered what the difference could be, R27 soon got snipped out...

Andy.

P.s. it looks like R27 were never fitted on the other boards.

[yerzmyey]

Aaaah, at last somebody was able to answer my question from some 2 years back.

YES - the MIC problem IS annoying. Also - there are programs that playing music with the MIC (like on Spectrum) and I actually can't hear almost _anything_ from the port.
So it IS typical after all. A pity.

I thought the signal might be boosted by the same methode that was used in ZX Spectrum...?
If we had a 'working' MIC, then we would have normal sound output with possible of making some music/sound fx on it.

[siggi]

Some 20 years ago there has been published a small modification to the ZX81 to give a louder MIC output signal. The modification was to solder a new R/C circuit in parallel to the original one (R29/C12) to give a higher output signal, while maintaining the filter caracteristics (the values for the new RC were 100 kOhm and 470 pF).
It was quite easy to do such a modification by soldering the new R and C to the soldering side of the motherboard. I did this modification and then could SAVE programs more reliable (*). And using some active speakers (with internal audio amplifiers), I can play the sounds, made by bit-banger programs using the MIC-port, like those stored on my server at

http://zx81-siggi.endoftheinternet.org/d:musik/mi/

Siggi

(*) nowadays I prefer MMC cards and USB sticks to save/load programs

Addendum: Scan of the orignal article from 1983:

Save.jpg (67.8 KiB) Viewed 10785 times

[kokkiklhs]

Some 20 years ago there has been published a small modification to the ZX81 to give a louder MIC output signal. The modification was to solder a new R/C circuit in parallel to the original one (R29/C12) to give a higher output signal, while maintaining the filter caracteristics (the values for the new RC were 100 kOhm and 470 pF).
It was quite easy to do such a modification by soldering the new R and C to the soldering side of the motherboard. I did this modification and then could SAVE programs more reliable (*). And using some active speakers (with internal audio amplifiers), I can play the sounds, made by bit-banger programs using the MIC-port, like those stored on my server at

http://zx81-siggi.endoftheinternet.org/d:musik/mi/

Siggi

(*) nowadays I prefer MMC cards and USB sticks to save/load programs

Addendum: Scan of the orignal article from 1983:

Save.jpg

Hi'y'all, ZX fans and happy new year!!
Freshly registered in the forum, but very old user of all the Sinclair range machines (except of the ZX80, no spare money for such an expense)
I'm digging this thread out after a looong time, kindly asking Siggi to give some more details of this modification, and if possible, some photo(s) of the modified PCB with the soldered resistor and condenser! The schematic on the magazine's page is quite simple and self-explanatory, but a good actual picture would be very helpful for anyone who wishes to do this for real!
Talked with Yerzmyey recently and he told me that the lack of music production on the Zeddy is mainly due to this low-voltage MIC problem, which prevents musicians and coders from trying to make some more music demos, and even try multichannel (which I believe COULD be done)!
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! The Z80 chip itself is powerful enough to give some nice performance even on the Zeddy (since we have 9 channels on a plain Speccy), but some serious sound output must be achieved first...
Siggi, we will be waiting to read your advice/experience, and perhaps some more detailed help on the project!