Need Help Line Out To Tape Head Level - Page 1
Forum section: electronics / beginners

author: dellsam34
author_rank: Regular Contributor
timestamp: March 03, 2019, 04:41:09 am
content: Hello everyone first time here but I'm a subscriber to EEVBlog youtube channel.I have a weird request:According to the diagram attached bellow, what needs to be installed in terms of an electronic circuit that goes from a line level to a head preamp level in an effort to replace the old fashion magnetic head pulse reader with a signal generator?I will not remove the old pickup system, I will just use a selector switch in case I need to go back to the old system (embedded in the 3.5mm jack, Shown separate for simplicity).

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 03, 2019, 05:57:12 am
content: You likely need nothing more than a "pad" which is two resistors arranged to reduce the signal voltage. Perhaps best would be to use a potentiometer or variable resistor or "volume control".That way you could adjust the level of the signal coming out of your source so that it is equivalent to what comes out of the tape play head.

author: dellsam34
author_rank: Regular Contributor
timestamp: March 03, 2019, 07:00:59 pm
content: I was thinking about inputting the audio signal directly to the tape-head preamp but wasn't sure since the magnetic head is a passive component and the line out is an active component, Wasn't sure how an active component (line out) reacts to another active component (tape-head preamp), In other words, The tape-head preamp puts out a voltage and expects the magnetic head to modulate that voltage based on what's recorded on the tape, Can an external voltage (audio tone) modulates the tape-head preamp voltage? That's what I meant to say.If there is no harm with two voltages getting together I probably don't need a variable resistor because I can control the volume from the app from zero to the max.

author: Benta
author_rank: Super Contributor
timestamp: March 03, 2019, 07:37:35 pm
content: Are you writing or reading the tape?If you're reading, your question should be "how do I go from magnetic head level to line level".Please clarify.

author: dellsam34
author_rank: Regular Contributor
timestamp: March 03, 2019, 07:49:01 pm
content: Quote from: Benta on March 03, 2019, 07:37:35 pmI'm neither reading nor writing, I want to get rid of the playback magnetic head, I'm not making a magnetic head preamp, there are tons of them on ebay if I wanted one. The head preamp is already there it's part of the main board. I've described what I wanted to do in details in my first post.Here is a simplified diagram of what I want to do:

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 03, 2019, 08:48:27 pm
content: We still don't know whether you are trying to write or record ONTO the tape? Or whether you are trying to read or playback FROM the tape? Perhaps if you revealed the larger scenario here and described what you are trying to do. Otherwise, I'm not even sure if you are asking the right questions.

author: dellsam34
author_rank: Regular Contributor
timestamp: March 03, 2019, 09:17:10 pm
content: I already did explain, I want to replace the old magnetic pulse reader with an electronic pulse generator, But since I can't get rid of the magnetic head preamp which is part of a big circuit board, I will have to connect my tone generator to the head wires after getting rid of the magnetic head and tape mechanism all together. The last diagram shows exactly that.Think of it as a cassette adapter for a mp3 player hooked up to an old school cassette player, Instead of using a head to head contact, get rid of both heads and use something modern like an isolating transformer or something, I don't know. That's why I'm asking.

author: ebastler
author_rank: Super Contributor
timestamp: March 03, 2019, 09:17:25 pm
content: Seems pretty clear that the tone generator is meant to simulate playback from the tape, rather than recording.

author: Gyro
author_rank: Super Contributor
timestamp: March 03, 2019, 09:22:34 pm
content: If it's just pulses then it sounds as if you do just need a resistive attenuator. If it was audio then you would need frequency equalization.

author: dellsam34
author_rank: Regular Contributor
timestamp: March 03, 2019, 09:23:07 pm
content: Quote from: ebastler on March 03, 2019, 09:17:25 pmExactly, that's what I've been trying to explain.

author: Audioguru
author_rank: Super Contributor
timestamp: March 04, 2019, 01:46:57 am
content: In addition to the line level to tape head level attenuation, you would need to do the opposite of the tape player equalization so that the signal resembles the tape pulses output.

author: dellsam34
author_rank: Regular Contributor
timestamp: March 04, 2019, 03:32:13 am
content: Quote from: Audioguru on March 04, 2019, 01:46:57 amHow do I achieve that?

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 04, 2019, 05:12:52 am
content: Quote from: Audioguru on March 04, 2019, 01:46:57 amChances are pretty good that the waveform from the signal generator will be cleaner and wider bandwidth (i.e. more "square") vs. the signal from the playback tape head (more "rounded" corners of the pulse waveform). IOW, the signal generator signal could have nice sharp corners of the pulses because it hasn't been recorded to and replayed from tape.So, depending on what is the point of the experiment, you could simulate the roll-off of the record->tape->playback path with a simple high-frequency roll-off. The higher the frequency or the wider the head gap, or the lower the speed of the tape, the more high frequency roll-off you could expect.OTOH you could just leave it "clean" if it is not important to the overall exercise.If you are working with a regular waveform (like a sync clock), then a simple waveform (perhaps a square wave with the appropriate duty-cycle) out of a common signal generator would be good. But if it is a more complex waveform (like SMPTE timecode, etc.) then many modern signal generators (even low-budget models) will support generating arbitrary waveforms as appropriate for your experiment.

author: dellsam34
author_rank: Regular Contributor
timestamp: March 04, 2019, 10:27:43 pm
content: I haven't measured it, But it is for a consumer video cassette player, I believe it is either 59.94 Hz or 29.97 Hz square wave for the tape control track, Not sure about the duty cycle, I will have to look at the service manual, This is an effort to recover some video tapes where the control track area is completely damaged and the frame is rolling every few seconds.