This weekends projects

Discussions about Sinclair ZX80 and ZX81 Hardware
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XavSnap
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Re: This weekends projects

Post by XavSnap »

Moggy,

No, nothing to do with your inversion of resistance, I had never seen this "R" notation on the French diagrams!
:shock:

470R was not French ! :lol:

For your inversion, it is very quick to make a mistake because the rings have different colors between 4 and 5 lines ... And the colors can be mistaken during assembly.
But here, we are on the amplification stage, and erroneous values are not necessarily a problem.
On the Mageco card, the digital output is mounted directly on a speaker in the commercial version.

Glad I unwittingly helped you fix this card ! … many years after its building.
:oops:
Xavier
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XavSnap
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Re: This weekends projects

Post by XavSnap »

mcarlson_sb,

Be careful, the card is an Issue 2 !

The diagram may be the issue 1...

Component face:
pcb_co.jpg
Copper face:
pcb_cu.jpg
(photoshoped from computinghistory.org.uk)
[edit] Date : 1979 ??? Maplin Electronics issue 6 (may 1983)

O.JPG
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Xavier
mcarlson_sb
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Re: This weekends projects

Post by mcarlson_sb »

I keep trying to figure out why the pre-amp circuit is so complicated. And, so far I've only come up with "to make me learn about transistor audio pre-amp circuits" :D

What I can piece together about how it works so far:
R2 is a pull up resistor on the input signal coming out of the SP0256-AL2 making sure we have a reasonable signal to work with
C4 is a DC filter capacitor to cut out signal noise
(I'd guess from switching, power, other chips etc.)

C9 and C10 looks like they are there to smooth out power fluctuations. I guess that makes some sense for C10.
But C9 looks like it's going to "round out" the input signal rather than allowing it to have nice sharp square wave edges.

R3 + R4 form the base resistance which will determine Ib
(why two resistors - and why put C9 between them?)
R7 + R6 form the collector resistance which will determine Ic
(again why two resistors and why C10 between them?)
I'm pretty sure Ib, Ic, and the fact that Ie has no resistance will help determine Vce and therefore the amount of amplification - but I haven't worked out how yet.

C11 seems to be another DC filter to cut out any last signal noise before it goes to the speaker

That leaves R5 which is providing the feedback loop for the signal being amplified.
And C12 which would seem to be smoothing out the signal for some reason that isn't apparent to me.

Lots of signal smoothing seems to be going on here - does that cut down on the sharp crackling noises?

If anyone who actually knows anything about audio pre-amp would care to correct my wild guesses I'd be happy to learn.

Cheers,
Matthew
Moggy
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Re: This weekends projects

Post by Moggy »

XavSnap wrote: Fri Aug 14, 2020 11:57 pm Moggy,

No, nothing to do with your inversion of resistance, I had never seen this "R" notation on the French diagrams!
:shock:

470R was not French ! :lol:

For your inversion, it is very quick to make a mistake because the rings have different colors between 4 and 5 lines ... And the colors can be mistaken during assembly.
But here, we are on the amplification stage, and erroneous values are not necessarily a problem.
On the Mageco card, the digital output is mounted directly on a speaker in the commercial version.

Glad I unwittingly helped you fix this card ! … many years after its building.
:oops:
Yes thanks for the help with the card XavSnap. The trouble is I didn't build it I bought it like this and I had the same problem with an EPROM burner that was a self build kit. I bought it ready built and a simple mistake in one resistor value caused quite a few problems.
Moggy
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Re: This weekends projects

Post by Moggy »

I agree that this is the most unnecessarily complex pre-amp for such a simple circuit I have come across in over 45 years of building audio amplification.

The cheetah sweet talker uses a simple 386 op-amp (although needlessly ramped up to x200 amplification when its own onboard x20 biasing is more than adequate) and there is no unnecessary out put filtering except for a DC blocking electrolytic capacitor and no switching noise to be heard.

The Maplin board uses a simple single transistor pre-amp in class A mode and can only assume the extra "baggage" is down to either a designer with ideas of hi-fi grandeur or Maplins wanted to sell a ton of components. :lol:

A common 386 schematic I use for small mini-amps, which to be honest I don't even bother with the zobel network cap and resistor on the output.

It should be noted that a 386 chip is more suited to driving a small speaker than general pre-amp use and a simple 741 or ne5534 op-amp design is preferable for pre-amp output levels with the caveat that unlike a 386 they won't work at voltages as low as 5 volts.
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1024MAK
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Re: This weekends projects

Post by 1024MAK »

The idea of using a letter in component values is supposed to reduce possible errors with a decimal point being mis-printed (as in, not being clear, hence being missed).

R is used in place of Ω because before modern computing, Ω was not available on typewriters and computer technology of the day.

Hence 2R7 = 2.7Ω

4u7 = 4.7uF

Etc...

With regard to the Talkback amplifier circuit, this is a breakdown of the circuitry:

R2 is a bias resistor, designed to bias the voltage at the output of the speech chip at the correct level so that C4 can recharge after it has been discharged. Presumably the circuit designer came to the conclusion that the chip could not pull its output high.
C4 is a coupling capacitor, it lets through the (wanted) AC signal but blocks the DC voltage. If a DC voltage is allowed through, it will mess up the DC bias point of the base of transistor TR1.
R3 is part of a low pass filter, in conjunction with C9, it reduces the signal level of higher frequency signals.
C9 is part of the low pass filter, it ‘shorts’ higher frequencies to ground.
R4 both limits the current to the base of transistor TR1 and increases the input impedance of the amplifier so that the filter formed by R3 and C9 works correctly.
R7 and C10 filter the +5V supply to try to remove the digital switching noise that is present.
R6 is the collector load resistor for TR1, it pulls the collector high (and hence the output) when the transistor is not conducting very much current.
R5 forces TR1 into partial conduction, it puts the transistor into part of its linear amplification area of operation.
C12 is another filter, it affects the amplification of higher frequencies.
C11 is the AC coupling capacitor. It stops the DC voltage from upsetting any input stage on any equipment you connect to the output.

Why all the filtering? Well, it appears that the intention is to round off the ‘square’ wave of the output from the speech chip. A square wave is made up of an infinite number of higher frequency harmonic sine waves. If you feed a square wave through a low pass filter, the filter will reduce the amplitude of the higher frequency components of the signal, producing a signal that is less aggressive to audio amplifiers. Some audio power amplifier stages really do not like square wave signals (it can cause distortion and may lead to overheating if left with a continuous square wave input).

Whether it is needed here, and how effective it is, is a matter for debate, because it depends on the audio amplifier you feed the signal into.

Note that human speech only really needs up-to about 8kHz of audio bandwidth. And older humans can’t hear much above about 12kHz anyway (depending on age and how much hearing loss an individual has).

Interestingly enough, for the Oric Talkback (Maplin Magazine December 1983, issue 9, page 24 onwards), a different filter arrangement and slightly different class A single transistor amplifier is used.

The VIC20 Talkback (in the same issue as the ZX81 Talkback) uses an even more complex circuit...

Mark
mcarlson_sb
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Re: This weekends projects

Post by mcarlson_sb »

Mark,
Thank you for the great explanation.
I was pretty sure some of my guesses were off.
Great to learn how it's actually working.

Cheers,
Matthew
Moggy
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Re: This weekends projects

Post by Moggy »

Although the bias resistor R2 and R7 were swapped around on my board luckily no damage done, as for the filtering this is such a basic circuit I still think it somewhat over the top but hey opinions differ and it made Maplin some cash so who am I to argue.

Should you go onto a 386 design, square waves shouldn't be too much of a worry as, for example, AY boxes like the Zonx 81 feed the chip with continuos squares waves when in use and they seem to do ok.
mcarlson_sb
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Re: This weekends projects

Post by mcarlson_sb »

Huh.
It works really well with my displays internal speakers without the pre-amp.

https://youtu.be/d10XCm6UQ4Y

I'm going to build the pre-amp anyway. If nothing else just to hear the difference.
mcarlson_sb
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Re: This weekends projects

Post by mcarlson_sb »

mcarlson_sb wrote: Sun Aug 16, 2020 1:03 am I'm going to build the pre-amp anyway. If nothing else just to hear the difference.

I can confirm that the pre-amp works as written.
It (quite a bit) more than doubled the volume.

I didn't realize the little display's volume was turned all the way up when I did the test without. I figured that out very quickly this time.

:shock: :shock: :shock:
That was - loud.
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