8038 LFO

marcusriley · Joined: Dec 21, 2019 Posts: 4 Location: seattle

Hi all -
I just came into a dozen 8038 ICs. I know they're a much-maligned chip, though there is a fair amount of literature defending its honor.

The Intersil Application note 13:
https://www.renesas.com/us/en/www/doc/application-note/an013.pdf
and the Thomas Henry circuit:
https://www.birthofasynth.com/Thomas_Henry/Pages/8038.html

Both make a claim of a 1:1000 frequency span.
So far, I'm not quite reaching those heights.

More importantly, I'd like to use this chip as an LFO. This is a territory that gets a little more muddy. I know the chip can produce low frequencies - I know that the waveform distortion increases at lower frequencies. I'm OK with that.. but without a good frequency ratio, I'm not sure I have much use for it.

At the moment, I'm experimenting with the circuit in the Intersil Application Notes design, switching out the capacitor for lower values. I haven't done an exact measurement, but it sounds a lot closer to a 1:100 ratio with the sweep.

Does anyone have any suggestions?

blue hell · Posted: Sat Dec 21, 2019 3:25 pm Post subject:

Hi and welcome,

just had a quick look at the first link you gave .. it has some suggestions for your question it seems, and assuming you've tried 'm, do they not work .. or .. maybe they do work but in the audio range only and not in the LFO range?

Just trying to get a hold on the nature of the failure.

marcusriley · Joined: Dec 21, 2019 Posts: 4 Location: seattle

Well, maybe I'm attaching the potentiometer incorrectly - but based on the actual schematic (figure 1 in the Intersil application notes), it's not quite working as expected at audio rate either.
It seems to completely shut off (no frequency) in the lower portion of the pot and it skips over most of the medium and lower frequencies in the span. I'm not able to get a stable frequency under about 1k.
Unfortunately, I'm also not using a log pot, but the results seem more severe than that.

PHOBoS · Posted: Sun Dec 22, 2019 5:09 am Post subject:

are they genuine 8038 chips ?
I know you can get them rather cheap from china these days but to my knowledge it is actually an obsolete chip.
The ones you can get do work but I wouldn't be surprised if the specs are off and a limited frequency range could
be one of the results of that. I know there is a similar problem with the XR2066.

I might get a couple of them myself. The range doesnt matter much for me and it seems like an easy way to make
a tunable tri-/sinewave generator.

blue hell · Posted: Sun Dec 22, 2019 5:49 am Post subject:

Maybe the C has to be higher valued and / or of better quality (no electrolytic maybe). Also the pot maybe "not so good" ?

The datasheet suggests the capacitor should be <as large as possible> as otherwise the charging and discharging happens with very low current (on the low frequency end), which with low quality caps (electrolytic) is like asking for trouble.

Seeing the TH uses some trickery to get things working nicer, mostly driving the thing between ground and -12V, but the comments are not extensive there. Otoh, this should not mak a big a difference, ecept when voltage controlled modulation is being used.