Help on MIC-Output ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
Moggy
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Re: Help on MIC-Output ZX81

Post by Moggy »

1024MAK wrote: Sun May 30, 2021 3:17 pm In this application we mainly need voltage gain rather than power gain. But the amplifier must be a low noise type. The voltage gain has to be in the order of about 5600!

That kind of gain would get you to about 2.8V AC.
A 74LS14 input needs 2V DC to ensure it sees a logic 1 input. But it’s possible to bias the signal from the amplifier so that a 2.8V AC signal when positive goes above 2V DC, and when negative does not go below 0V.

I’m guessing that the idea is that there should be no modification to the ZX81, right?

A possible source of a suitable DC power supply would be to use a splitter cable, and use the +9V DC (nominal) supply from the Sinclair PSU. Use a 1N4001 diode (or similar) followed by a suitable electrolytic capacitor to further smooth the ripple out.

That should provide a suitable power rail for any audio amplifier chips.

Mark
In my experience designing and building amplifiers, if used in an audio capacity op-amp based pre-amps should use a fully regulated power supply rather than a simple diode fix as the "buzz" at the output will be considerable and has to be heard to be believed. A few commercial amps I have come across, mainly older designs, tried the Zener and resistor method in their pre-amp stages and it was always unsatisfactory albeit cheap.
All my stage and guitar amps have separate regulated supplies at the pre-amp stage and are very quiet as a result, so would have thought low noise a priority here that's why I suggested the the audio grade NE5534s in the first place.

Power op-amps however are more tolerant supply wise and use no regulation, hence my suggestion rather than a need for a gain in power but it should be noted that once a pre-amp has done its job it will need to be fed into a power amp type device to trigger the zeddies loading sequence if that is the route he is going down, in the manner of a cassette players earphone output. A pre-amp on its own will never feed the input of the 81 no matter what it's voltage gain without the speaker driving "thump" that ear sockets produce hence the devices made for loading the spectrum that surface from time to time featuring power amps not voltage ones.

The banked pre-amp stages I use kick out at at least 3 p-p but wouldn't even power the smallest of speakers as the current is small however when that signal goes through the power amp stage the current goes up the voltage down in relative terms and as the zeddy is usually fed from a speaker driving source then power must come somewhere into the equation rather than pure voltage.


If however he just wants to feed a chip of some sort with 5v rather than the zeddy directly itself then yes agreed no actual power gain as such needed only voltage.
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1024MAK
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Re: Help on MIC-Output ZX81

Post by 1024MAK »

All good advice Moggy :D

The question that was asked was about the ZX81 Mic output (for saving) rather than the ZX81 Ear input (for loading). Hence my bias to only talking about the Mic output and the requirements to get it to a suitable level for the input of a 74LS14 logic chip.

I agree if using sensitive preamps, then a proper voltage regulator should be used as a smoothed unregulated supply will still have some 100Hz ripple on it. These days there is no point in using a zener diode even with a current boosting transistor. Voltage regulators are often just as cheap as a zener diode and transistor. I was going to write more in my earlier post, but decided against it as this was going beyond the question that was asked...

But anyway since we are on the subject, I would do something like this:

Code: Select all

ZX81 PSU —— Y cable splitter —— 9V DC (nominal) supply to ZX81
                    |
               1N4001 diode
                    |
      Electrolytic smoothing capacitor 
                    |
                    +—— to power amp (if needed)
                    |
            Voltage regulator
                    |
   Regulated supply to preamps/op-amps
As the diode is just separating the DC supply, the output is full wave rectified DC. So the electrolytic smoothing capacitor does not have to be massive.

A 7805 or 78L05 voltage regulator could also be used to provide the +5V for the 74LS14.

Mark
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Moggy
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Re: Help on MIC-Output ZX81

Post by Moggy »

Apologies Mark I got my sockets mixed up. :lol:

Your diagram is the way to go from a power supply point of view and should be quite noiseless.

I am curious as to where he is deriving his 3.3-5v supply from given what the Sinclair power brick kicks out.

@Werner.

Could you supply a schematic or diagram of what it is you are intending to do ,the power supply you have etc, then we can work from that . :D
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zx81vga
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Re: Help on MIC-Output ZX81

Post by zx81vga »

@Moggy

here is a quick Schematic...

The LOAD"" Section works well... (EAR_OUT to ZX81)

I need the MIC_IN and have no Clue :(
zxvtape.png
Werner
Last edited by zx81vga on Wed Jun 02, 2021 7:47 pm, edited 2 times in total.
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mrtinb
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Re: Help on MIC-Output ZX81

Post by mrtinb »

Sorry I haven’t followed this thread. Big delivery at work.

I haven’t made any special hardware to load files. I simply have a home made TZXduino, which standard firmware outputs 0V or 5V on a pin. This is fed directly into the EAR of ZX81. If you measure this on playback you’ll get the average 2.5V. So the 0V/5V signal from the Arduino fits the ZX81 or Lambda 8300 very well.
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BarryN
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Re: Help on MIC-Output ZX81

Post by BarryN »

mrtinb wrote: Mon May 31, 2021 9:11 pm Sorry I haven’t followed this thread. Big delivery at work.

I haven’t made any special hardware to load files. I simply have a home made TZXduino, which standard firmware outputs 0V or 5V on a pin. This is fed directly into the EAR of ZX81. If you measure this on playback you’ll get the average 2.5V. So the 0V/5V signal from the Arduino fits the ZX81 or Lambda 8300 very well.
I cheat, I use my ZXPand. :)
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zx81vga
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Re: Help on MIC-Output ZX81

Post by zx81vga »

@mrtinb

as i write in my last Post the EAR_OUT Section works well. (LOAD"" on the ZX81)
I need the MIC_IN... (SAVE "XYZ" on the ZX81)

Werner
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1024MAK
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Re: Help on MIC-Output ZX81

Post by 1024MAK »

Although it’s about a device to make loading from tape easier, have a look at this (rather long) topic.

Especially have a look at the circuit schematic for it.

The operational amplifier chip used has four sections, of which three are used.

The first section is a used as an amplifier. The resistors and capacitors at the input act as a filter. Negative feedback is used to control and set the gain. The feedback circuit includes a capacitor which reduces the gain for unwanted high frequencies.

The second section further amplifies the signal, but also has some hysteresis to ‘clean up’ the signal and to reduce the effects of unwanted noise. As a result it also outputs more of a square wave compared to the input signal. Which is what is wanted.

Although the circuit you want needs to start by amplifying a very low level signal, after this, something similar to the circuit described above is needed before feeding to a 74LS14 input.

Mark
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zx81vga
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Re: Help on MIC-Output ZX81

Post by zx81vga »

@1024MAK,

i think here is a big misunderstanding.

As i wrote the EAR_OUT in my Schematic works well.
I select a File with the ZXVTAPE press LOAD"" and NEWLINE on the ZX81 and the File load flawlessly.

I need a Cirquit for the MIC_IN of the ZXVTAPE.
I want to save Programs from the ZX81 to the ZXVTAPE.

The Link you posted is very interesting but it is only for LOAD :x

Werner
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1024MAK
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Re: Help on MIC-Output ZX81

Post by 1024MAK »

zx81vga wrote: Thu Jun 03, 2021 5:28 am The Link you posted is very interesting but it is only for LOAD :x
The device in the link I posted IS for loading, but some of the principles used in the circuitry do apply to your application of amplifying the low level signal from the Mic output on a ZX81 (for saving) and filtering and shaping the signal so that it can drive a input on a 74LS14 (as mentioned by you earlier on in the topic). Obviously you can’t use the design as published, as it is unlikely to have an input that is sensitive enough.

Mark
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