Author Topic: tape head amp issues  (Read 1273 times)

AdamB

tape head amp issues
« on: January 18, 2021, 03:42:39 AM »
Hi,

So I'm messing with cassette tape, trying to get a playback amp working for an eventual tape echo. I saw there's a design on the forum already which I tried to recreate on the breadboard with no success - so, I thought I'd simplify a bit and stick to a simple op-amp design just to get me started making some noise.

I had a LM386 chip in the parts bin and hooked that up as in the following schematic:


Substituting in the tape head for the audio in jack.

All I get is noise, and occasionally a very weak signal from the head under the noise floor. If I increase the gain by adding a 10uF cap across the gain pins (1 and 8 ) I just get the same, but much louder. So I assume the amplifier is working as it should and something else is the problem. I also thought that schem was missing the virtual ground so I added a voltage divider on the input to see whether that made any change, but to no avail.

I realise this is not optimal for a tape head amp, as I need NAB equalisation and what not, but I'm confused as to why this doesn't work - should this work in theory, or am I being stupid somehow?

PS I've tried several transports and heads, and confirmed they work in their original circuits (I have a handful of portable tape players to tinker with).

Thanks,
-Adam

anotherjim

Re: tape head amp issues
« Reply #1 on: January 18, 2021, 04:44:55 AM »
It should be ok.
Are you using a battery for the 386 power and is the voltage good when connected?
The 386 isn't an opamp so don't worry about virtual ground stuff. It's designed to be that simple.
If you've become used to hooking up opamps like the 741, it's incredibly easy to apply +power to pin 7 by automatic pilot. Check again: +power on pin 6!


Croeso i Diystompboxes.

jatalahd

Re: tape head amp issues
« Reply #2 on: January 18, 2021, 05:32:28 AM »
If you get a weak signal with lot of noise and you can confirm that the contact with the tape head and cassette tape is "tight", meaning that there is proper tension in the tape transport to keep the tape firmly against the head without alignment issues, then I would suspect a grounding issue.

If there is a DC motor grounded to the transport mechanism and the tape head also grounded to the transport, the (most likely separate ground) of your (assumed breadboard) circuit might cause problems. In my build at first I got huge ground loop noise problems from using wall wart power supplies and in addition powering the guitar amp from mains.

To avoid noise from grounding problems, I found a solution to isolate the DC motor and the tape head grounding from the tape transport mechanism, so that tape head ground is the breadboard ground, and the DC motor is powered from separate battery with its own isolated ground point.

I would also like to know what design from this forum you could not get working? For tape head playback amplifier, (if you want) I could also share a reasonably simple discrete 3 transistor design, which will provide the required 50dB - 60dB (1000x) gain with relatively small background noise level.

Radical CJ

Re: tape head amp issues
« Reply #3 on: January 18, 2021, 05:39:17 AM »
Coincidence. I was just wondering whether I could make a small tape echo, and started pulling apart an old tape deck to have a look. Haven't progressed very far with the project though.

iainpunk

Re: tape head amp issues
« Reply #4 on: January 18, 2021, 07:02:45 AM »
the 368 needs a 100uF capacitor from ground to 9v, and possibly one extra 100u from pin 7 to ground

cheers, Iain
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AdamB

Re: tape head amp issues
« Reply #5 on: January 18, 2021, 09:11:15 AM »
Thanks for the replies;

Quote
The 386 isn't an opamp so don't worry about virtual ground stuff. It's designed to be that simple.
If you've become used to hooking up opamps like the 741, it's incredibly easy to apply +power to pin 7 by automatic pilot. Check again: +power on pin 6!

Ah OK, yea I don't have any experience with the 386 and assumed it was an opamp like the TL072. I'll leave out the virtual ground stuff then. I'll double check the power hook up - I think I copied the pin assignment in that schematic I posted but yea it's possible I've hooked that up wrong, so worth another check, thanks!

Quote
I would also like to know what design from this forum you could not get working? For tape head playback amplifier, (if you want) I could also share a reasonably simple discrete 3 transistor design, which will provide the required 50dB - 60dB (1000x) gain with relatively small background noise level.

Jatalahd, the one I was copying was yours! It's here:
https://www.diystompboxes.com/smfforum/index.php?topic=125858.msg1201030#msg1201030

Really inspiring build. What I want to do is repeat what you did, and then take it a little further by trying to 3D print a replacement transport. I got a 3D printer and now I need a difficult music-related project for it to keep me busy for the next year :P The eventual plan would be to open-source it all so people can order a PCB of the electronics and 3D print the transport parts to make an easily serviceable cassette tape delay without relying on hacking old existing machines (other than sourcing heads).

I actually started with your earlier forum post with the simplified schematics coz it was easy for me to see the individual components. I got the bias oscillator working - well I think it works, I have one of those hobbyist scopes and it shows a nice sine wave, but the P2P voltage seems quite low (less than 1V) and it reports a frequency of only around 50Hz - but that might just be coz it's a bad scope - I figured I'd assume it's fine for now until I get a playback and recording amp working.

I hadn't realised the gain required was as high as 1000x, the current setup only has around 20->200 gain depending on how I setup the LM384, so perhaps this is a bad route anyway? I do have a LA3161 I can try next, which if I can make work would be fine for now, but I want to avoid parts that are no longer manufactured so other people can copy it easily.

I tried building the playback amp from your build a couple times on breadboard, and once on PCB to be sure, but they all failed for some reason. It's almost certainly something I'm doing wrong; I've never had much luck with analogue circuits for some reason - anything digital I build works fine but for some reason my analogue circuits have a high failure rate. I'm a software dev by trade so I find the digital end of things much easier to grasp - this is definitely going outside my comfort zone! In the long term I'd like to make something that has low parts count and uses easy-to-source parts so that there's no worry about transistors being hard to find in the future or dealing with counterfeit parts or what not, but for now I'll take anything that I can get to work!

Quote
If you get a weak signal with lot of noise and you can confirm that the contact with the tape head and cassette tape is "tight", meaning that there is proper tension in the tape transport to keep the tape firmly against the head without alignment issues, then I would suspect a grounding issue.

So I think the transport/heads are all good in terms of them being in the right positions/tensions - the same setup works fine with the original electronics. I have two transports - one which is almost completely original (I just cut the wires to the head and soldered them to a breakout for connecting to the breadboard). The second setup is from another machine of the same make/model, but I dremel'd off all the case to fit a recording head and the motor is now connected to a STM32 dev board's PWM output via a mosfet driver.

On the grounds - I'll check that when I get home, I hadn't really paid attention to how the transport/motor/head are grounded to one-another, so that's one potential issue to sort! I'll also try running it on a 9V battery to see if that has any affect.

Powersupplies; of the two setups I have, they are like this:
-Setup 1; preamp on breadboard is powered by a 9V T-Rex pedalboard supply. Motor is powered by AA batteries (in original tape player casing).
-Setup 2: Same preamp on breadboard powered by the T-Rex 9V. Motor is powered by an STM32F dev board via mosfet driver - ultimately the power for this comes from the 5V USB from a PC. The motor is supplied at 3.3V - I did worry it would pull too much current for the USB but I tried it and it appears to work so I didn't worry about it  :icon_mrgreen:

Quote
the 368 needs a 100uF capacitor from ground to 9v, and possibly one extra 100u from pin 7 to ground
I hadn't really added any PS filtering and what not, so I'll make sure to add this tonight and see if that helps things, thanks!


jatalahd

Re: tape head amp issues
« Reply #6 on: January 18, 2021, 11:50:16 AM »
Quote
The eventual plan would be to open-source it all so people can order a PCB of the electronics and 3D print the transport parts to make an easily serviceable cassette tape delay without relying on hacking old existing machines (other than sourcing heads).

Sounds like a one year project  ;) ! For background knowledge I would recommend reading the book "Audio Electronics" by John Linsley Hood. The tape recording part of it (1st chapter) is almost fully readable from the Google Books preview:

https://books.google.fi/books?id=EOE1tx3CJ98C&printsec=frontcover

I purchased the full ebook, because that seems to be the latest and most compact introduction to cassette tape recording with full circuit examples available. But after chapter 1, there is no more information on that topic...

Based on the example on that book (Sony playback amplifier) I modified a 9 volt single supply version of a simple (my opinion!) tape head playback amplifier. The topology is the same as in my tape echo circuit schematic, but here is less parts:
 


The transistors are: Q1 and Q3 = BC559C ; Q2 BC549C

The frequency response with 4.7k resistor and 0.015uF capacitor included is:



For linear 60 dB gain, you can leave out the branch with 4.7k resistor and 0.015uF capacitor. This depicted here is more closer to actual cassette player playback amp response, because typically the high-frequencies are boosted when recording. I have narrowed down the bandwidth here using the 3300 pF cap in the input. This is because I want to effectively filter out the bias frequency at this point. Obviously, to drive a loudspeaker, you need a power amplifier after this, I just use my guitar amp (which is actually a bass amp) for that.

It is not necessary to reach the 60 dB, but when trying to match the signal level obtained from a guitar, then the gain should be at least 50 dB.

There is also a good example on using one op-amp as the playback amplifier in the audio electronics book (page 34), but again it uses bipolar supply. Hopefully these examples get you forward.

PRR

Re: tape head amp issues
« Reply #7 on: January 18, 2021, 02:23:44 PM »
...typically the high-frequencies are boosted when recording....

Actually mostly because the wound tape playback head works much better at high frequencies. We start with a complementary roll-off in the playback amp.

The LM389(?) app-sheet has a complete tape play amp plan. If you can't find the chip, it is LM386 plus three 2-penny NPN transistors.  https://rtellason.com/chipdata/lm389.pdf
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danfrank

Re: tape head amp issues
« Reply #8 on: January 18, 2021, 11:45:52 PM »
Also, take a look at the National Semiconductor data sheet for the LM387, a stereo preamp in an 8 pin IC. I've made the NAB circuit in that data sheet and it works well. I have no idea how hard LM387 ICs are to find now...

AdamB

Re: tape head amp issues
« Reply #9 on: January 19, 2021, 03:48:29 AM »
Quote
Based on the example on that book (Sony playback amplifier) I modified a 9 volt single supply version of a simple (my opinion!) tape head playback amplifier. The topology is the same as in my tape echo circuit schematic, but here is less parts:

I have those parts to hand so I'll get started on breadboarding that tonight and let you know whether I have more luck!

Quote
The LM389(?) app-sheet has a complete tape play amp plan
Quote
Also, take a look at the National Semiconductor data sheet for the LM387

I'll see if I can hunt these down - I originally chose the 386 coz it's still available from wholesalers (at least as an SMD part), but it might be a good idea to have more options to try.

AdamB

Re: tape head amp issues
« Reply #10 on: January 21, 2021, 03:39:36 PM »
Ok so...

Attempted the design you posted, jatalahd. I get the same result - lots of noise and no signal. Tried battery power and filtering in the 386 design - this does lower the noise but still get either very low signal or no signal at all, seemingly at random.

Since I know the transports and heads are fine (as I have several and they all work in their own circuits) I'm trying to figure out what else could be wrong.

Thoughts are that either
A) the circuits are wrong on the breadboard.
B) components I have are faulty.
C) there are errors in the design

A) I'm not sure about this - it seems unlikely i'd get it wrong so many times in a row but it's possible I'm making some simple mistake over and over. Not sure where to go here.
B) I have a few of each part, but they're all from the same supplier. I've come across problems using TH parts before and suspected they're fakes, how likely is this though in this case?
C) seems unlikely the 3 or 4 designs I've tried would all have errors in that prevent the circuit working at all.

Any other ideas?


anotherjim

Re: tape head amp issues
« Reply #11 on: January 21, 2021, 05:03:56 PM »
There have been fake LM386 chips around.
Some notes.
A healthy one will bias close to 1/2 supply voltage at its output pin 5.

The input is easy to work with. For simple experimenting, you may input to pin 2 or 3 (do you care if it's inverted or not?)
The unused input can be tied to 0v. Some say the inverting pin2 input is less noisy - but it can be an easier layout to use pin 2 anyway and ground pin3 next to pin4.

You don't have to have an input volume control. You can go straight in.

There can be a slight DC present on the inputs. It's probably not a good idea to get it on tape heads. Inputting via a capacitor makes things safer. The input impedance of the chip is 50k so any cap from 100nF up can be used.
You may well need the 10uF cap between pins 1 & 8 to boost gain, but if it gets too much, a 5k variable resistor in series with the 10uF can give a useful range of lower gains.

Pin7 allows an extra power filter cap to connect to 0v for the chips preamp section. It comes after an internal 15k resistor from the power input pin6 and there is little point having bigger than 10uF for this. Bigger caps give a slow startup when you apply power.

Pin 6 power should have at least 100uF capacitor to ground.

The low-value resistor and cap across the speaker output (a "Zobel" network) are to damp ringing caused by the speaker coil inductance. You can get away without it, but the ringing, which is usually supersonic, can have audible side effects so definitely worth fitting the network.

This simple amp chip can start to get complicated with all the bells & whistles added  ;D

Croeso i Diystompboxes.

jatalahd

Re: tape head amp issues
« Reply #12 on: January 22, 2021, 12:55:11 AM »
- lots of noise and no signal.

Could you please describe how does the noise sound like? Loud "SHHHHHHHH" (hiss)? Loud "RRRRRRRRR" (motorboating)? Loud "WHEEEEEEEE" (whistling)? Or something else?

I have had the motorboating problem a couple of times with these high-gain feedback circuits, it happens because the circuit picks up some interference signal (most likely 50/60 Hz hum) and starts to self-oscillate covering all other sounds underneath. Quite often adding a 100 ohm resistor in series with the battery and a 1000uF cap to ground after the 100 ohm resistor cures that problem (a low-pass filter that filters almost all frequencies from the power supply line). Also the whistling noise refers to circuit self-oscillation.

If it is only loud hiss and no signal, then I would suspect a wiring error at some point.

From the design that I presented, only I can say that I have had that circuit on my breadboard for a month now, I have tested it using signal sources such as an audio signal generator, a tape head and with reduced gain I have used it as a microphone pre-amplifier. I remember that when I had the bias oscillator circuit on the same breadboard, I got some interference from that, but isolating the circuit to its own breadboard has worked well for me. But I understand that saying "it works for me" does not help anything  :(

Maybe the way to go at this point would be to get that LM386 circuit working and identifying where the noise appears. After the noise problem is out of the way, you could try other circuits as well.

anotherjim

Re: tape head amp issues
« Reply #13 on: January 22, 2021, 05:17:38 AM »
Another tip.
If the tape head gives you separate terminals for both ends of its pickup coil, then a twisted pair of wires (or twin & screen cable) can directly connect across pins 2 & 3 of the 386 giving it a balanced drive. Any DC voltage on the pins will be very similar at both ends of the coil so won't do anything. This will also give it some freedom from common ground hum/noise.
Croeso i Diystompboxes.

AdamB

Re: tape head amp issues
« Reply #14 on: January 22, 2021, 10:38:04 AM »
Thanks for the notes anotherjim, I'll re-try the 386 design and check with your info as I go to see if that gets me closer! The only issue with the 386 is that it doesn't provide enough gain (it maxes out at 200). But, I could combine two of them (one set to gain 20, and the next one set to gain 50) and that would give me my 1000x gain. So if I can get one of them working without the cazy noise, then I could just bolt a second one on,

Quote
Could you please describe how does the noise sound like? Loud "SHHHHHHHH" (hiss)? Loud "RRRRRRRRR" (motorboating)? Loud "WHEEEEEEEE" (whistling)? Or something else?

With your design just get a quiet buzzing - this might just be background EM it's picking up (as there's no signal coming through it's hard for me to know how loud the noise is, as it's being amplified through my interface's instrument input so there's no frame of reference for me). But, here's a video I took after finishing your circuit on the breadboard;



It's the tape machine with the head connected to a breadboard of the preamp circuit you posted above^ Towards the end of the video I crank the gain on the interface so you can hear the noise clearly over the sound of the tape transport whirring away. It just sounds like typical background EM.

When used with the 386, instead it's a loud Shhhhhhh noise, occasionally I can hear the tape signal underneath quietly but it's not reliable.

Quote
From the design that I presented, only I can say that I have had that circuit on my breadboard for a month now, I have tested it using signal sources such as an audio signal generator, a tape head and with reduced gain I have used it as a microphone pre-amplifier. I remember that when I had the bias oscillator circuit on the same breadboard, I got some interference from that, but isolating the circuit to its own breadboard has worked well for me. But I understand that saying "it works for me" does not help anything  :(

Sure, it seems unlikely a problem with your schematics as I've tried 2 of your designs and 1 other using the 386 and all have failed in one way or another - most likely is that I'm making a mistake in breadboarding the designs and next likely is that the parts are in some way faulty.

I'll try both the 386 on one board and re-do your design on another side-by-side, so that way there's 2 chances I can get something working and figure out why I've been having problems.


AdamB

Re: tape head amp issues
« Reply #15 on: January 22, 2021, 12:05:30 PM »
Quote
Another tip.
If the tape head gives you separate terminals for both ends of its pickup coil, then a twisted pair of wires (or twin & screen cable) can directly connect across pins 2 & 3 of the 386 giving it a balanced drive. Any DC voltage on the pins will be very similar at both ends of the coil so won't do anything. This will also give it some freedom from common ground hum/noise.

You're saying put the "left channel" of the head into + and the "right channel" into - of the LM386? Twisting them together to ensure they pickup the same interference pattern? Or have I misunderstood?

jatalahd

Re: tape head amp issues
« Reply #16 on: January 22, 2021, 03:03:32 PM »
Thank you for the video. Regarding the design I shared, it sounds like an error in the connections. In the video I only hear normal background hum, which implies that the signal path is broken somewhere (or my ears are damaged).

I could not trace all your connections properly from the video, but please check the polarity of the second electrolytic cap. The minus side (-) should be towards the 100 ohm resistor. I could not see this properly from the video (my eyes are not that good either).

I also made another breadboard reference build to show you my layout. I can confirm that this build is working. The transistor order is the same as in the schematic from left to right. In the image below, the PNP transistors BC559C are facing south (downward) and the NPN transistor BC549C in the middle is facing north (upward). The image should have a decent resolution (if the service shows the original size) so you can zoom in to see the details better, also the polarity of the electrolytic caps.



The black/green/yellow wires disappearing to the top left are the tape head connection wires. The emitter of Q3 biases at 4.8 volts in this build, it can be measured for example from the left side of the output capacitor.

Hopefully this helps ...
« Last Edit: January 22, 2021, 03:09:53 PM by jatalahd »

anotherjim

Re: tape head amp issues
« Reply #17 on: January 22, 2021, 05:19:58 PM »
Quote
Another tip.
If the tape head gives you separate terminals for both ends of its pickup coil, then a twisted pair of wires (or twin & screen cable) can directly connect across pins 2 & 3 of the 386 giving it a balanced drive. Any DC voltage on the pins will be very similar at both ends of the coil so won't do anything. This will also give it some freedom from common ground hum/noise.

You're saying put the "left channel" of the head into + and the "right channel" into - of the LM386? Twisting them together to ensure they pickup the same interference pattern? Or have I misunderstood?
No, the idea needs each head channel to have 2 pins for both ends of each coil. So there are 4 pins total on the back of a stereo head. It would apply to either left or right channels, not both. You can't do it if there are only 3 pins for a common ground for both coils. Some heads have a little PCB over the pins that commons the grounds and I wouldn't recommend unsoldering the PCB from the head if yours has one.
What I'm suggesting is to treat it like a dynamic microphone with balanced XLR connection. That too feeds a +input and -input of a preamp and the grounded screen only provides screening. The twisted pair does do some noise reduction. The balanced connection also gives x2 extra amplitude before any amplification.

There are quite a few different tape preamp chips that have been sold on ready built PCB's. They seem to mostly be on auction sites now.


Croeso i Diystompboxes.

AdamB

Re: tape head amp issues
« Reply #18 on: January 23, 2021, 02:51:04 PM »
Quote
I also made another breadboard reference build to show you my layout. I can confirm that this build is working. The transistor order is the same as in the schematic from left to right. In the image below, the PNP transistors BC559C are facing south (downward) and the NPN transistor BC549C in the middle is facing north (upward). The image should have a decent resolution (if the service shows the original size) so you can zoom in to see the details better, also the polarity of the electrolytic caps.

Thanks for this, a massive help. I'm rebuilding my breadboard to be a replica of your setup, if I have trouble I'll post a picture and we can play spot the difference!

Quote
No, the idea needs each head channel to have 2 pins for both ends of each coil. So there are 4 pins total on the back of a stereo head. It would apply to either left or right channels, not both. You can't do it if there are only 3 pins for a common ground for both coils. Some heads have a little PCB over the pins that commons the grounds and I wouldn't recommend unsoldering the PCB from the head if yours has one.
What I'm suggesting is to treat it like a dynamic microphone with balanced XLR connection. That too feeds a +input and -input of a preamp and the grounded screen only provides screening. The twisted pair does do some noise reduction. The balanced connection also gives x2 extra amplitude before any amplification.

The heads in the machines I have do have 4 pins, but the grounds are bridged together. However, I have a bunch of other heads I grabbed from ebay and also from out of car cassette adapters - some of these have the 4 pins separated so should be able to give your idea a go with one of those.

AdamB

Re: tape head amp issues
« Reply #19 on: February 04, 2021, 03:42:03 PM »
OK I copied your breadboard layout exactly, jatalahd. And it works!

https://www.dropbox.com/s/15mhpqbpp8r0jkt/PXL_20210204_202057707.jpg?raw=1

I get a decent signal out of it, must of been me getting something wrong translating the schematic to breadboard. Next up I need to build the recording amp and check the bias oscillator I have on breadboard already works - once I have that going I'll try converting it all to a PCB layout and I'll post the gerbers and BOM for that here once verified. I figure even without the 3D printing stuff that might be useful for people to use as a drop-in replacement for making their own cassette delays.

In other news, my 3D printer is built and ready to start printing. I feel that end of things is going to be much harder - I'm gonna try printing my first calibration cube in the next couple of days, wish me luck!

I think for the first iteration I will print the enclosure, reels, motor mount and some transport parts and use pre-made gears (bitsbox has a bag of gears of various sizes I picked up to play around with). I think the gears are going to be the hardest part because the teeth are small and it probably needs printing in nylon, so I'll get on to that once I've got some printing experience and proven the rest of it can work.

Edit: changed image to link coz it was massive lol.
« Last Edit: February 04, 2021, 03:43:56 PM by AdamB »