tape head amp issues

[AdamB]

Thanks,

On the DC offset - I can't seem to kill it with decoupling capacitors, have you any advice on that? I tried both poly film and electrolytic (I figured poly film would be best as it's an AC signal?) but yea no change.

Good plan on putting my transport aside for electronics development - I have a tape transport here that is just a tape head and motor for cassettes (I'm using it to build a cassette synth but can easily connect it to this circuitry for testing). I'll use that to just blindly record to the tape and then try to play it back and see what the fidelity is like. I'll keep developing my transport alongside but stick to the off-the-shelf transport for testing for the time being.

I don't have any gear that already has a tuneable inductor, so I've ordered a set of various sizes of inductors - I'll try and just match the closest I can. Still trying to figure my way through the maths for it!

Again, thanks for the help

[jatalahd]

Polyfilm caps are fine in this case and ceramic too, but if any cap does not work, then there might be something funny with the scope. I checked the specs on that scope, it has only 1Mohm probes, so it will show a bit lower reading on the amplitude. Also the max input is 50 Vpk, what ever that means, but that explains the faulty trace on the erase head case, it provides too high peak voltage to your scope.

If you use the pot in the oscillator to attenuate the bias with the erase head connected, do you still get the dc offset?. If not, then better to use the erase head as the inductor.

But if you have caps on both sides of the tape head, you can be quite sure that there is no dc offset there. Also note that on the oscillator side, some caps need high voltage rating due to the large voltage swing.

It is good if you can test the recording on normal cassette transport, it might work better already without any changes to the circuit. Report back how it turns out.

[AdamB]

OK yea I think the DC offset is just weirdness from the scope. If I attenuate down the bias signal then the DC offset disappears. I've put a couple of 100n film caps around the head anyway to be sure.

Parts for the bias trap turned up - that helped, however for some reason it only works if I add an emitter follower to the output of the recording amp (between the recording amp output and the bias trap). I went with 10mH inductor and 330pF cap for now as that seems to give the best result from the parts I have at hand. I'm now getting around 24V rms sine at the head with everything connected, at 64Khz.

So, I went ahead with that and recorded to another tape machine to see what would happen - first just connected the head to the record amp and played some guitar in. Then rewound the tape, connected the head to the playback amp and turned on the motor - it plays back quite nicely. It's very noisy and thin but it clearly wrote the sound to the tape which is a step in the right direction. Here's a video:

Trying the same thing with my transport I get nothing at all at the moment - just occasional clicks or grungy noise. So must be something about the tape alignment/pressure on the heads or something about the tape heads I'm using on that transport. I tried shimming the heads to raise/lower them relative to the tape but that doesn't seem to make much difference. Next thing I'm gonna try is redesign the transport top-left corner to fit in another tape guide to try and keep the tape straight and add in some posts to drop those little head pressure-pads that are built into cassettes into place to keep the tape against the head.

I'll report back once I get there!

Edit: On the emitter follower buffer - I did already have a Tl072 unity gain buffer in there but for some reason that caused the bias oscillator to get loaded even with the trap in. Not sure what's going on there but we can look into this stuff to try and get optimal quality once I get the transport to work now.

[jatalahd]

Ok the bias should be good now. Definitely we need to improve the quality when you are ready for it. For that I would need to see clear pictures of your circuit, mainly how the recording side is connected now to the tape head and bias oscillator junction. I could not make it out from the video.

I had plans to design a simpler recording amp, it would consist of two transistors and the latter would be the emitter follower driving the signal to the head. I could work on that in the mean time and see how that comes out. Definitely we want the quality to be so that a clean sound guitar sounds just as clean on the echoes.

[jatalahd]

Hi Adam,

Just tested a simpler recording amplifier on the breadboard and noticed some issues...

The noise you are getting is most likely coming from the recording part of the circuit (or how it reacts with the rest of the circuit). I tested a single transistor gain element with emitter follower buffer (with the bias oscillator and playback amplifier connected) and got the same noise issues you have showed on your videos. The noise covers mostly everything and all I got was one distorted repeat as echo. Just like you did. After facing the issues, I re-built the recording amp which is on my schematic to the breadboard, and the noise is cleared. The circuit sounds the same as the one I have soldered to veroboard and hidden inside the metal box.

So, unfortunately, currently it seems that the circuit must be built exactly as drawn in the schematic  . I did not try how it sounds with a PSU, just used a battery and that 1000 uF capacitor parallel to it. If I take the filtering capacitor out, the noise is just like a jet plane would rocket itself to the sky... the fine line between noise and silence ...

Oh and another thing, I tested placing an electrolytic cap after the recording amplifier and got 2 volts DC offset to the tape head. Then I replaced it with ceramic (or poly film) cap that I normally use (1 uF or 2.2 uF) and the DC offset was gone. So strange things going on with the caps as well. My advice would be to use non-polarized poly film or ceramic caps where ever possible.

I will send you a breadboard photograph to help build the recording amp from my schematic tomorrow evening (24 hours from now). Currently the board is too messy to show anything clearly and I need to rest a little ...

Sorry for the inconvenience, your transport might work just fine. The circuit noise issue needs to be solved first.

[AdamB]

OK great. I did get much better recordings with the cassette transport so I suspect maybe it's a combination of electronics noise and bad transport. This is the latest version of the transport (now with those little felt pads to press tape to the heads, tweaked roller and added tape guides)

The quality is real low. I'll try rebuilding the recording amp as you suggest and then revisit once the noise is sorted.

[jatalahd]

In the previous post I promised to send pictures of building the recording amp (the one from the schematic) to the breadboard. I took 2 pictures, one from the input side to the inductor and the other from the inductor onwards. I use two 820uH inductors in series. Here is the input buffer circuit, where the guitar is connected, and the recording amp high-impedance buffer using JFET:



The red and black wires on the left go to the input jack. The red and blue wires on the middle indicate the placement of the 10k pot. The 2nd transistor from the left is J112 JFET, so the pinout is according to that component (gate on the left). For BF245C, the pinout is reversed with respect to the gate pin.

Then the rest of the circuit:



There are a lot of components, all connections are not that clear, but hopefully that helps. Again, the red and blue wires are reservation for the 10k pot to adjust the recording attenuation. The rightmost components are the bias trap inductor and cap, connected directly to the tape head stud.

I tested also using a 9 volt DC power supply (takes mains voltage in, puts 9V DC out). So I needed to add some more filtering for the noise. It is very surprising that the recording amp seems to be the cuplrit for adding most of the noise to the circuit. By adding a 150 ohm resistor and 100uF cap to the collector of Q8, the circuit remains stable and relatively quiet when using a PSU. Also, there might be noise issues on the breadboard even when using a battery, so the same resistor + cap combo should fix that as well. The 150 ohm resistor is shown in the second picture, but the capacitor is not connected, because it would have blocked the visibility of other components.

For this reason I made yet another addition to the original schematic and here is the latest version with that resistor + cap combo added:



Now that I know most of the supply filtering needs to be targeted to the recording amp, I might still try to find a simpler circuit to replace the recording amplifier. But for the time being, that is the best I can come up with. The original recording amp is good because it has a feedback loop to stabilize it and lots of places to add filtering to the bias oscillator signal.

[jatalahd]

In addition to what I wrote above, you could test the isolated circuit noise so that the playback amp, recording amp and bias oscillator are connected together and all powered by the same power supply and tape heads connected to the circuit. Then just take the output to a guitar amp and listen to the noise. The noise should be in such a level that it can be compared somehow to other pedals, a little bit higher of course, but not much.

[AdamB]

OK, so. Latest update is that I got myself a Bush tape recorder - handily the head in it is connected via PCB header, which makes it super easy to switch out which head it's connected to. It also has an aux in - so it's super easy to connect a guitar up to and record to tape. But, you can also connect a guitar up to record and then plug a different head (on a different transport) in and use the recording amplifier/bias from the bush which we know works. I've also tested playing back using the bush transport but outputting it's head to the playback amp on the breadboard and again that works fine.

So I know this electronics setup is all good - but still I get terrible results with the 3d printed transport. There's two main problems;
-Very weak signal and lots of noise
-Extreme wow

I've spent some time trying to solve the wow problem but it's hard to tell whether things are better or worse when the signal is so low. So I'm concentrating on getting a better write/read level out.

To do that, I've redesigned the head mounting on the transport to allow for height and azimuth adjustment; basically the heads are now mounted on screws that pass through a couple of springs that allow us to adjust both read and write heads independently. Here's a picture:


Another great thing about this is that I left enough space between the main body of the transport and the heads so that you can also put in there head "adapters". So if you have heads of different size/shape/mounting positions you can just make a small adpater that connects the head to the transport, but still allows for azimuth/height adjustment. So if this next step doesn't work out my next step will be to replace the heads to be sure it's not just bad heads (being as I've never used them in anything but this machine who knows if they're any good!).

I'm still waiting on the last few parts to print, and then I'm going to fire it up and see whether we can tweak the head positions to give a better output.

[jatalahd]

use the recording amplifier/bias from the bush which we know works.

Well, this is good that you now have a reliable rec amp + bias. But then I assume that you could not get the rec + bias from my schematic working 

The azimuth/height adjustment is a must have feature, so good to have that there now. Different heads give different output levels, depending on their impedance and head gap structure. I have tested now 3 different head types and one of them gave clearly lower output, it seemed that they did not make proper contact with the tape. Therefore switching the heads would be good to try out.

[AdamB]

But then I assume that you could not get the rec + bias from my schematic working

I haven't tried your recording amp yet. I know your bias works, because I used it with my own simple TL072 4x gain buffer and that worked fine when using an existing tape transport to record to tape.

So right now my priority is figuring out why my transport doesn't work, coz it seems like that's the weakest link currently. So for now I'm using off the shelf electronics that I know work perfectly, so as to make it simpler to debug the transport.

Once I prove out that the transport can work, then it's back to trying out your recording amp on the breadboard!

I tried the new transport last night and if possible I've made it perform worse lol. I get no output at all with the new head adjustments in - not quite sure how that's happened. So I'm gonna just assume these heads are bad and I've made some adapters to put in heads I took out of tape machines that I know worked - gonna try that tonight. Currently looks a-like this:


Fingers crossed this time I get something decent out of it!

[AdamB]

Nope, big old fail.

Not sure where to take this transport design from here. Perhaps a redesign of the pinch roller to use a sprung mechanism that holds the wheel against the steel rod, I've seen that kind of thing in a few machines. Time to do some thinking.

[jatalahd]

 

It would be best to design the mechanism in this phase so that it would be easy to debug. I would return to supporting standard cassette format and also not worry about fitting in the erase head at this point. Just have the record head and play head connected or just one of them, test recording and playback individually on the transport, as to cross check --> Record to cassette on your transport, play back on normal cassette deck, record to cassette on normal cassette deck and play back on your transport. That way you could isolate the problem better. For example, if your recording made on the transport plays well on normal cassette deck, but not on the transport, then the problem is most likely in the play head alignment. I know that you already made this kind of cross check, but maybe it should be more systematic...

Also the thing you mention always about the noise. As I noticed later on in my breadboard, large noise problems in the circuit causes the echoes to sound very faint and distorted. So the noise issues need to be cleared as well.

[AdamB]

Right, getting there. This is with an off the shelf transport (I purposely chose a machine that's easily available off of Amazon for around 25 quid), 3D printed chassis and mounts for the extra heads. The little tape guides are stolen from the insides of a cassette.

So not the fully 3D printed masterpiece I want, but still easily recreated DIY.

Electronics are a TL072 recording amp (fixed gain of ~4x) with Jatalahd's bias oscillator + bias trap. The playback amp is from the original tape machine (the jacks and power on the original machine are very easy to desolder, so I figure might as well reuse what's there for now).

Motor is controlled via the Pi Pico with squishy keys for now, via a mosfet motor driver on breadboard.

So next steps; It's still got lots of flutter where the tape action isn't completely smooth, but it's at least starting to be useable in a musical way. I'm going to PCB-up the Pi pico motor control, the recording amp and oscillator, and add on an output mixer to mix dry and repeats (and allow for feedback for multiple repeats) and rebuild this with a more complete enclosure. That'll be version 1, and I'll run that on my vocal board for a while to see how it holds up. I'll post a new demo when I get that far, and perhaps release the V1 BOM/STLs/Gerbers.

[jatalahd]

Nice progress!

[AdamB]

Small update; now with output mixer and feedback that allows for multiple repeats.

There is an issue with the repeats beginning to sort of oscillate that I've yet to figure out - anyone have an idea what's happening here? Here's a video to show the issue in isolation:

As the repeats continue you hear it sort of focuses in on the high end and then start to oscillate as it dies away.

[jatalahd]

As the repeats continue you hear it sort of focuses in on the high end and then start to oscillate as it dies away.

Well, I would not say it oscillates, but just emphasizes the high end on every repeat. Does that happen if you slow down the tape speed? Because the faster the tape moves past the tape head, the high end becomes more and more prominent. If you need/want to keep the tape speed high, you could try to balance the frequency response by amplifying bass in recording and adding a simple low pass filter at the end of the playback signal chain. 

[AdamB]

OK so hit on a problem; I've made up the first pass PCB and started to test with that but having trouble with the bias oscillator.

I tested the oscillator first in isolation (mounted all its components first and I included test pads on the PCB to check it). When powered by a mains 9V supply this worked fine, exactly the same behaviour as on the breadboard.

However, now that I have everything else mounted - Arduino, motor controller, relay switching, recording amp, playback amp (from the tape machines PCB) and output mixer - the bias oscillator no longer works; it oscillates correctly for a few moments and stops, then works for a few moments and then stops again and so on.

What's most interesting though, is that if I switch from mains power to a 9V battery the oscillator works exactly as expected again - it just seems to hate the mains supply now. Obviously, I can't just use batteries because the arduino and the motor will drain that too fast to be practical, so I need to figure out what's happening here.

One thing I also notice is that if I power the circuit from the mains, but lower the voltage to around 5V then the oscillator stabilises, albeit with a PP voltage and frequency that is too low to be useable (and also the Arduino can no longer power up as it needs >7V to function).

First best guess is that the oscillator is very picky about the quality of the supply voltage? I notice Jatalahd that you have a massive 1000uF filtering cap on your power supply - is that for this same reason?

Edit: Running on battery I've managed to successfully test the whole delay and it "works" (it's a bit low-fi but I think that's mainly the transport which I need to work on more). I had to hack the power supply for the playback amp - I was using the 3v3 supply from the arduino but that didn't fly - I assume the arduino can't supply enough current for it. Powering that separately everything works OK. If I switch to mains power it "works" but the repeats are super distorted where the oscillator isn't functioning right as detailed above. So if I can sort the oscillator-on-mains-power problem I can move on to revision 2 of the PCBs.

Thanks!
-Adam

[jatalahd]

First best guess is that the oscillator is very picky about the quality of the supply voltage? I notice Jatalahd that you have a massive 1000uF filtering cap on your power supply - is that for this same reason?

Well, in my circuit the 1000uF cap was used for DC filtering and reservoir for the whole circuit, mainly to avoid some motorboating noise arising from somewhere...

Lately I have started to build a home-made cassette recorder/player. I found some proper erase heads from aliexpress and currently testing those for recording music on cassette. Later I finally found also the traditional tape transport mechanisms from aliexpress, but it seems again I need to wait for several months for them to arrive...

But anyway, related to that, I built the same oscillator using the erase head as the inductor and bounced to the same issue you are facing. The oscillator did not want to keep running. For me the simple solution was to place a 100 uF electrolytic across the power pins close to the oscillator. I  can't say for sure it will solve your problem. I built my own mains power supply giving 12 volts unregulated DC and the oscillator worked fine with that, there was some excessive low-frequency ripple, but it kept running smoothly. So it should not be picky about "quality", but it requires "quantity" (of current). Also noticed that 12 volts is the absolute maximum voltage to run the oscillator, the transistors started to overheat at that voltage.

Please note that the oscillator takes up to 0.1 - 0.2 amps of current. Also note that most likely the motor will take close to 0.1 amps as well. So your mains adapter needs to supply enough current for the circuits. And you might need to add filtering to the motor's DC line, as it produces quite nasty electromagnetic noise.

Hopefully you get it working.

[AdamB]

WIP enclosure design is coming along.



I need to redesign the transport mount to take account of holding the reels in place (I already designed little caps that go over the tape guides to hold them in place, they're not fitted in this picture). I imagine it'll take the form of some narrow arm that goes over the reel and fits into the center of the reel where it meets the spindle. Might ruin the look of the reels a little but I'm not sure how else to handle that problem other than glueing the reels in place - which means the machine can never be serviced. Another option might be to mount the glass on the inside of the hole in the top of the enclosure so that the bottom face of the glass can be mounted just a fraction above the reels, but the tolerances on that are probably a bit tight to make work with an FDM printer.

Should have the latest PCB built up in the next few days so I can report back on where I'm at with that.