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SCELBI Tape Interface Mic Connection

K

kalinchuk

Experienced Member
Joined
Oct 25, 2024
Messages
61
Hello,

I've been looking at the SCELBI tape interface "write" schematic and I can see that S1-2 is used for the "write out" connection but I don't see a second connection (which I assume should be ground) going to the mic connection. I'm wondering if this is a bug and if the second connection should really be ground or not. Any ideas?

Thanks in advance

Screenshot 2025-09-27 at 1.05.24 PM.png
 
Also there is no value for the capacitor that sits between the mic output and ground.
Also a bit odd to use a resistor in series with the input impedance of the device to generate the correct voltage. It might kind of work with an input for a ceramic pickup too (that afaik usually tends to have a rather high impedance but also way higher voltage).

If you are considering building a replica you might want to place that unspecified capacitor at the cassette end of the wire, inside the connector.
 
I did some testing and I can hear a static tone on my tape recorder but if I plug into a "newer" laptop (2004) there is considerable static along with the tone. I'm wondering if maybe the laptop is doing something with the signal and if there is anything I can do to remove that static.
 
Static, hiss or a hum?

If a hum, you may find you have an earth loop.

If a hiss, do you have a tone control that you can adjust? Just be careful that you don't use the tone control to get rid of the actual signal.

Dave
 
It's more of a hum. What do you mean by an earth loop? The loop is basically what the SCELBI audio interface has (the 100K + 10K resistors). I did not include the capacitor.

Screenshot 2025-10-01 at 12.28.44 PM.png
 
Simplest thing to test is to unplug the charger from your laptop. Also unplug the earphone out from your laptop when using mic in.

But also, consider trying line in and if the level is too low decrease the value of the 100k resistor.

Mic/ear was a thing that computers had to do as (non-European) cheap tape recorders tended to lack line in and line out, but they always had mic in and ear/speaker out.
 
It is to do with the mains supply.

If you have two (2) things plugged into the mains, and there is a difference in the earth potential between the two, then a current can flow between the two earths via the 0V/GND signal between the two pieces of equipment.

In an audio system, this appears as a 50 or 60 Hz mains hum. In the audio world we have a 'ground lift' device to prevent the current flow whilst still being electrically safe.

As @MiaM states, try unplugging the device you are using to supply the audio signal to/from the computer (providing it runs on batteries of course).

We come across this thing quite frequently in the audio world when we bring a computer audio card near our mixer desk...

Dave
 
I don't have a battery for this laptop, unfortunately. I did try unplugging the SCELBI side from the wall but still got the hum.
 
Sure. I'm attaching a short recording of it here. This is with the SCELBI turned off and disconnected from the wall. With SCELBI running, I hear that hum plus the tone intermixed.
View attachment test.wav
 
That sounds like an earth loop - mains hum somewhere.

How many things have you got plugged into the mains that are interconnected together?

Dave
 
I just realized that I had a serial device connected to the TTY interface which, of course, has a shared ground. After disconnecting it from the TTY interface, the audio is now clear. I guess the question now is - how can I keep the serial device connected and not get that hum?
 
Ah,

The key thing is: are they all plugged into the same mains outlet, or different outlets?

Can you make a simple drawing of what you have and what is connected to what?

Dave
 
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