tape head amp issues

[jatalahd]

Nice that you got it working! Meanwhile I integrated that playback amplifier with some minor component value changes to replace the old one in my tape echo prototype and made a v2.0 schematic:



I added also a buffer after the playback amp, because one needs to take care of attenuating the signal in the reverse direction. I also added more filtering to the playback amp and one more filter cap to the recording amp. The problem is that when the playback head and record head are near each other, the record bias signal is induced to the play head (also the wires must be separate!). This induced bias is then amplifier in the playback amplifier and in the worst case takes the output to the rails, muting the original signal from the tape.

I am interested to see your 3D printing results, so please keep us posted on your progress on the project. If you have some problems with the bias oscillator or the recording amplifier, I am happy to help. The recording amplifier can be made simpler, but since I wanted to add the high-freq boost, and the need to filter the bias frequency, and my limited skills, I cannot figure out a better one than the existing is.

[AdamB]

Thanks for the full schem. Will try my best to breadboard bits so that I understand what's going on, and then I'll PCB-it-up!

The buffer, is that the section around Q5?

This induced bias is then amplifier in the playback amplifier and in the worst case takes the output to the rails

So I guess this goes for the erase head, too? I guess that can go on the other side of the transport, far away from the other 2 heads. When I make up the PCB I'll be sure to route the connections to opposite sides of the boards to make it easy to keep the head wiring away from each other.

If I'm understanding the schematics right, the erase head would replace the inductor L2, right?

-Adam

[jatalahd]

The buffer, is that the section around Q5?

Yes, Q5 and R14 makes the buffer, then R17 + R18 is the feedback level control. Making R17 smaller, you will attenuate less on each echo repeat, making it too small will cause too much feedback and that makes the echo boost up into oscillation. If the feedback path is investigated from the guitar input buffer towards the playback amplifier output, you want there to be attenuation, so that the guitar signal does not interfere with the output. This is taken care of in the schematic, where the attenuation is in the ballpark of 1/100.

It might also work without the buffer, the output impedance of the playback amplifier without buffer is around 1k, which is not that bad, but I like to have it lower. With buffer it is less than 100 ohms. if you want to experiment leaving the buffer out, then just neglect Q5 and R14.

So I guess this goes for the erase head, too? I guess that can go on the other side of the transport, far away from the other 2 heads. When I make up the PCB I'll be sure to route the connections to opposite sides of the boards to make it easy to keep the head wiring away from each other.

Yes, erase head will radiate the bias frequency as well so keep it far away from play head. This is what I experienced when measuring the circuit with the oscilloscope and moving the heads around. This problem does not exist on a normal tape recorder, where the play and record are two separate functions and the bias signal will not stick to tape when recording.

If I'm understanding the schematics right, the erase head would replace the inductor L2, right?

Yes, correct. I have not managed to find any erase heads anywhere, so I have not had the change to measure how the signal behaves with that.  Actually, after I finished my build, I found this guy on youtube, who had used the same bias circuit with a real erase head. He got the bias frequency almost to 100kHz. He also incorporated the bias trap between the recording amp and bias oscillator, but I am not sure did he tune it correctly. In my schematic, the use of bias trap is avoided using the same reverse path attenuation method that I explained on the playback amplifier. This is beneficial in the sense that it can be built to work without tuning the bias trap with an oscilloscope, but somehow I feel that I don't get the same recording level (volume) as I would get when recording with a normal tape deck, without distorting the signal that is.

So here is the video of the guy making his tape recorder with GOOD bias

If you have more questions, just post them here and I (or someone else) will try to answer them

[jatalahd]

Oh and there was one mistake in my new schematic regarding connection of Q4 base junction. I edited the post #20 above and added a link to corrected image. Now it should be ok. If you saved the previous image, please delete that and take this new one from post #20.

[AdamB]

OK thanks for all the help, I'll be sure to update on here with progress - and there will definitely be questions. The recording/bias part seems a bit more involved so no doubt I'll have trouble with that, too!

I managed to get my first print done - it came out better than I expected it to. I printed the cliche benchy that everyone does. Really happy with the results for a first print, pic linked here:
https://www.dropbox.com/s/vx9b8lo7vupstfu/PXL_20210205_160736889.jpg?raw=1

There's the tinniest amount of stringing in there, but yea pretty happy with that. Next I'll try to print a simple mock up of a motor mount or a tape reel to see how accurately I can make the simpler parts and what degree of post processing might be needed. I'm not very experienced with CAD so I think there's gonna be a lot of printing small pieces until I get the dimensions etc. right.

[AdamB]

OK had a first crack at getting the bias/recording amp working and.... no luck. I hear the whirring of the motor and some general RF noise, and a very faint dry guitar signal - which in itself is interesting as the dry isn't hooked up so we should only be hearing the delayed signal.

We know that my playback breadboard is working fine (I left that as-is), so it's either something with my breadboard of the recording/bias or something about the recording head/transport.

Here's a picture of the setup:
https://www.dropbox.com/s/9hzak4wuz1q5bkt/fullsetup.jpg?raw=1

And here's a picture of the recording amp (with bias oscillator on a separate little breadboard above):
https://www.dropbox.com/s/c5va2d77qn7qsvj/recording_bias_breadboards.jpg?raw=1

I changed the input buffer to a TL072 unity gain setup because I didn't have enough transistors to go around, I shouldn't think that causes any issues though. I then fed that straight into the recording/bias circuit from your schematic, leaving off the connection between the output of the playback amp and the input of the recording amp (for zero feedback).

One obvious issue is that the recording head is loose - I'm playing some guitar one-handed while I hold the record head over the tape with my other hand - not ideal but the shims wouldn't hold it in place so I figure this would be fine for now just to show if the circuit is working. I'd expect it to not give me a fantastic signal but I'd expect to at least hear something. I also connected both L and R coils for the recording head together so that it writes to both channels. The playback head is just shimmed and hot-glued into the blank tape, it has a tape guide on the head and a soft pad to press the tape onto so I figure this is likely fine.

Anything else obvious I'm doing wrong here? Jatalahd, do you have any voltage readings I can check over on the recording amp?

[anotherjim]

Maybe a stupid question, but be sure the heads are the right way up. May be so long since this tech was common but cassette only uses half the width of the tape. It looks like you have swapped heads for recording/playback as separate tests.
Also, your breadboarding is much neater than mine. That could be where you're going wrong 

Being serious, can you pickup audio on the recording head (and if you have a scope can you also see the bias signal).

[AdamB]

Maybe a stupid question, but be sure the heads are the right way up.

Good question - I think so. One of the reasons I shimmed and glued in the playback head was so that I could turn it around. The recording head has this long PCB attached to it, so it would only go in the opposite way around to how the play head was mounted on the transport. So yea I just took the playhead out, turned it around and glued it in

I did try quickly turning the rec head around and trying to get it in to position and it didn't help (though it's hard to get it in position this way).

My breadboarding's only so neat because I'm copying the way Jatalahd did his breadboard

Being serious, can you pickup audio on the recording head (and if you have a scope can you also see the bias signal).

I can pickup audio on the recording head (if I stick the recording head wires into the playback amps input and then press the head up to a running cassette with some audio on).So hopefully that means the recording head is all good.

I do have a scope, but it's a cheap hobbyist one and it doesn't seem to go up high enough in frequency to pickup the 50Khz signal from the oscillator. It does show a sine signal at some settings but reads it's frequency out as around 50Hz (I guess it's aliasing?).

[anotherjim]

In theory, if you put the heads together face to face, you should hear the recording head signal back through the playback head - like a crude transformer coupling. I would be interested to know if that works.

You should be able to see which side of the tape the heads match. Since the deck never knows which side of the tape it's looking at, as long as the heads are all aligned with the same side of the tape, it should be good.

As to fixing the heads on the deck. You will normall see a fixed screw on one side of the head and a spring-loaded adjustment screw. The adjustment should allow you to tilt the head one way or the other. This makes a huge difference to sound quality. The heads usually have an elongated or slotted screw hole on one side for the adjustable one.
Look up "tape head azimuth setting". Some of the write-ups can get very technical and call for special equipment.

The easy way is to use a recorded tape you know sounds ok and do the adjustment while playing it. Then change to blank tape and record to it while listening to the playback. Adjust the recording head tilt to find the best playback sound.

[AdamB]

In theory, if you put the heads together face to face, you should hear the recording head signal back through the playback head - like a crude transformer coupling. I would be interested to know if that works.

That doesn't work, so suggests something bad in my recording amp right?

You should be able to see which side of the tape the heads match. Since the deck never knows which side of the tape it's looking at, as long as the heads are all aligned with the same side of the tape, it should be good.

On the one in the pics above I can't see the head because it's shimmed/glued into the tape lol. But when I did the pressing-heads-together test I used a different unit where I can see which way round the head is.

As to fixing the heads on the deck. You will normall see a fixed screw on one side of the head and a spring-loaded adjustment screw. The adjustment should allow you to tilt the head one way or the other. This makes a huge difference to sound quality. The heads usually have an elongated or slotted screw hole on one side for the adjustable one.
Look up "tape head azimuth setting". Some of the write-ups can get very technical and call for special equipment.

For now I'm not too worried about quality, as I'm going for a low-fi kinda vibe anyhow, but certainly once I've gotten the first unit working I could look at how to include adjustment for azimuth to optimise that.

While I work on getting this recording amp going, I've been messing with CAD and printing stuff. Here's a quick test print of a reel and spindle. If I work on individual parts and then figure out how to integrate it all and get the quality up I think that's a good strategy.

[jatalahd]

That is good progress that you have the whole setup laid out. It needs to be noted that each circuit block (playback, record and bias) can be tested as separate entities, before hooking them together.

Unfortunately it is getting close to my bed time, so I can only give quick instructions.

1. As said, the record amplifier can be tested separately. Just connect guitar to the input and output to guitar amp. If signal comes out amplified through the recording amp, then it is working sufficiently (at this point). You basically need only about 12 dB (x4) gain, maybe a simple unity gain buffer is also enough at this point.

2. You have wiring errors in your bias oscillator. That is why it won't work. I re-built a relatively clean breadboard layout for you, it is the one on the bottom part of the board in the image (rows A to E). The top part is the leftovers from my earlier proto:



The red one is the 1uF cap, the blue ones are the 100 nF caps, the gray one is clearly 10 nF, and the last green one is 100 pF cap. I built it with a 1 k resistor without the bias pot, for clarity. With this setup you get a very decent output of 41 Volts (rms):



You don't need a scope to measure the output voltage. A normal multimeter with AC measuring works fine. It directly shows the rms voltage, but because of limited probe impedance it will show only 25 to 30 volts rms for the output.

Once you get the bias oscillator running (and some signal out of the recording amp) let's hook them together. I have been still testing my own circuit and as I mentioned about the bias trap, I included that to my proto and it made things better. The recording amp seems to load the bias oscillator signal down, but even with an untuned bias trap, I mostly got rid of the loading.

But lets get back to this later. Time to sleep ...

[AdamB]

Thanks for all this Jatalahd, really appreciated.

So I tried isolating the record amp and going straight into the guitar amp with it, that didn't work. I know how to build a simple 4x gain opamp stage with that tl072 so I'll do that for now and revisit the record amp once the system is working.

So next thing to do for me is fix the oscillator - I'll copy your layout for that. I don't suppose you could you point out the error on my breadboard, I can't see it and it'd help me to learn if I can figure out where I'm going wrong with copying over the schematics.

Thanks,
-Adam

[AdamB]

Ah I think I see the problem with my oscillator breadboard - the missing connection between the inductor and the 1uF cap right? I'll rebuild it anyhow to be sure!

[jatalahd]

Yes, the inductor connection was missing and also I think you were missing the ground connection on the emitter of the npn transistor. It is easy to make mistakes like this, that is why I always triple-check my breadboard builds before testing them. And usually after that there still might be issues ... So just taking the time and being patient would be the way to go.

Op-amp gain stage is good for the recording amp, it has a low output impedance, so when connecting the output of the record amp with a 10k resistor to the junction of the record head and bias oscillator output, you will get a proper reverse signal path attenuation when looking from the bias oscillator towards the record amplifier output.

Hopefully you will get the circuit working soon.

I would still like to discuss about adding the bias trap, but that can be done later.

[AdamB]

I haven't had chance to work on the electronics more yet, however made some progress on the transport end of things. I've decided to try and make it work using standard cassettes (ones ideally converted into short tape loops). I figure this makes it easier to construct, though it means I'm stuck with the 3-in-a-line head positioning that will fit into the top of the cassette. But anyhoo...

BEHOLD! A 3D printed motor mount and flywheel assembly!

I notice that the rpm even without the gearing from an ordinary transport is actually not all that fast - it's probably about the top speed you'd want a tape delay running at. What this leads me to thinking is that as a first try I might just mount the spindle on the flywheel and be done with it - not sure what that'll do to the motors stability when running at lower speeds as I guess it'll have less torque but if I can keep the 3D printed parts simple it makes it easier for other people to put together.

Here it is with the spindle mounted on the other end of the flywheel shaft:
https://www.dropbox.com/s/39iwkg9y7d3f050/simple_assembly.jpg?raw=1

And boom, here's it driving a tape reel:

I've been doing some design work on how this might connect to a printable transport mount, it looks something like this currently:
https://www.dropbox.com/s/fnr3rxtzvz4h0ol/transport%20mount.png?raw=1

I'll try this and see how it lines up - though I think it might be a better idea to rework it so that it can be mounted inside a 1590BB enclosure, with a couple holes drilled to pass the spindles through the top of the enclosure, so that the tape is attached to the top outside of the enclosure. That'd look neat.

[anotherjim]

A frustrating thing about this project is that you can only click like once.
Keep going. This is popcorn munching stuff, and I don't like popcorn.

[jatalahd]

Looks very nice and professional. The higher the tape speed is, the better high frequency response will be (up to certain limit of course). But I am a bit afraid that turning the tape reel at the middle directly from the motor drive will cause pretty big wow and flutter problems, even if the motor would be using a speed stabilization circuit.

When the "traditional" pinch roller mechanism is used in tape transport, the very small variation in the motor speed is not so noticeable, since the radius of the capstan is so small and the motion is converted into linear pull of the tape due to the pinch roller with larger radius. In your current implementation, the large radius of the tape reel driven from the center will "multiply" the variations in the motor speed when considering variations in the angular velocity at the outer edge of the tape reel, which directly affects the linear motion of the tape itself.

Not sure if my writing above made any sense, but definitely looking forward to see the progress of this project.

[AdamB]

Keep going. This is popcorn munching stuff, and I don't like popcorn

I'll definitely give it a good try!

But I am a bit afraid that turning the tape reel at the middle directly from the motor drive will cause pretty big wow and flutter problems

Indeed - the original plan was to create a completely custom transport that could use a pinch roller setup and 3d printed reels, but I decided for now that using a standard(ish) cassette allows for interesting possibilities (such as different "prepared" cassettes with damaged tape and the like for making whacky sounds that can easily be swapped out) as well as making construction easier.

The problem with cassettes though is that if I want an erase head then there's nowhere to stick a pinch roller.

I'm a little worried that it won't have the torque to pull the tape through once all 3 heads are in place, but we'll see I suppose. If it has a lot of wow and flutter that's not necessarily a problem - I'm going for a lofi warts-and-all tape sound. One of the reasons I'm using a microcontroller to drive the motor via PWM is so that I can modulate the motor speed in interesting ways. I'll likely add a "broken" knob that controls how erratic the motor speed is.

[AdamB]

More more progress on the transport, here's the head engage/release mechanism.

It works using two 3d printed plungers (engage and release) that are mounted on metal springs and sandwiched between 3 layers of 3d printed plastic. The head just screws directly onto the engage plunger.

Here's a CAD picture showing a cutaway of how it works:
https://www.dropbox.com/s/v4stkpn9yt9svu5/tape%20head%20mechanism.png?raw=1

The head is a couple millimetres too far off the tape when engaged, so I need to shorten that plunger and re-print. It also jams sometimes, so I've ordered some PTFE lubricant to put on it to hopefully resolve that. I might also beef it up and make all the parts a bit thicker just to make it feel a bit more sturdy as the PLA I'm printing in is a bit too flexible - though I'll likely switch to printing in PETG once the design is finalised to add extra rigidity.

There's only one head (playback) mounted at the moment, but once I've had chance to test it working playing back a tape via the playback amp, I'll change the design to mount all 3 heads on that same plunger. It might need a couple extra "runners" left and right for the plunger to make it more stable when engaged but that shouldn't be too much of a problem.

[anotherjim]

Very clever stuff.

From the drawing, it looks like a 45deg angle between the movement faces. As only the head needs to push the release slider across and not the other way around, a shallower angle in favour of the head slide could be better?