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Old 17th January 2009, 03:31 PM   #1
tomcuk is offline tomcuk  United Kingdom
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Default Magnetic tape head voltage output

I am currently working on a cassette tape deck project trying to build a unit entirely from scratch. I have the microprocessor controlled motor and control systems completed and have partially completed tone control and balanced output stages done and am now starting on the tape head amplifier and frequency compensation stage.

Unfortunately I am as of yet unable to get any accurate measurements from the tape heads I have been using (not sure if they are still in working condition) and am still trying to hunt down some good replacements.
In the meantime I am keen to keep working on the project so was wondering if someone could give me a rough mV (I am presuming that is the range I should be looking at) values for what I should be expecting from the tape heads so I can start the circuit design?

Many thanks in advance for any help, also any general advice on tape head equalisation would be appreciated
Tom
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Old 22nd January 2009, 07:14 AM   #2
dangus is offline dangus  Canada
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Millivolts sounds right. I'm pretty sure there were plans for a tape preamp in Wireless World back in the '70s, possibly by Linsley-Hood. A university library should have bound back-issues, or access to microfilm. I'm pretty sure National Semi. included a tape playback circuit with the app note for the LM381 or the LM387. IIRC, the normal curve is NAB, but I think that's only for regular ferric (low noise), not metal and chrome.

Making a cassette player seems a little quixotic, though, when there's stacks of decent cassette decks in thrift stores for 1/10 to 1/100 of their original selling price. $15 got me a Harman-Kardon that was so recent, h-k had a pdf of the manual on their site.
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Old 23rd January 2009, 01:27 PM   #3
tomcuk is offline tomcuk  United Kingdom
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What a fantastic response, thanks dangus Ill be sure to check out the sources you mentioned (and thanks for introducing me to a new word, quixotic would make a good name for the project :-) )
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Old 27th January 2009, 03:37 PM   #4
tomcuk is offline tomcuk  United Kingdom
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Thanks again for the response, I checked out the LM381/387 and as you said they have some great typical application examples. The only problem I have now is that as it is an old IC I cant get any, also the searches I have done to find an equivilent all bring up obsolete IC too.

Would the LM833 be a suitable replacement for it? Or maybe the LM4562?
I can’t work out if the LM381 can be substituted with a standard op-amp or if it has totally different characteristics
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Old 29th March 2009, 05:16 PM   #5
tomcuk is offline tomcuk  United Kingdom
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Hello again all,
I am nearing completion of my project and am basing the analogue tape compensation section on a circuit i have found (hopefully attached) from the NE5532 application notes. I was initially planning on using the design from the LM381 but it is a very hard to find IC and has some extra internal capacitors which I believe mean it cannot be directly replaced with a standard op amp.

My first question is : on the circuit attached there is a input cap labelled as being 0.22 which I presume is farads which seems a freakishly large capacitance to use for an input cap, could anyone recommend a value that could be used instead?

Secondly there is a input resistor RSL, does anyone know its function? I am trying to work out a suitable value (or if indeed it is necessary?)

Lastly the feedback capacitor is stated to be 0.003 which is 3mF which is again a very large capacitor, is it possible there might be a u or p missing from the end? I only ask as I have now ripped open a fair few tape decks and have never seen a capacitor this large outside the power supply, perhaps they are just using a better compensation circuit.

Thanks in advance for any advice
Tom
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Old 29th March 2009, 06:02 PM   #6
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All capacitor values are microfarad, 0.22uF input (=220nF), and 0.003uF feedback (=3nF).
RsL is the head loading resistance, and is tailored to the specific head you use. It's likely to be around 180k. With the wrong value, the high frequency response will be poor, or over accentuated.
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Old 29th March 2009, 06:27 PM   #7
tomcuk is offline tomcuk  United Kingdom
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Fantastic, simply fantastic
Thankyou very much
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Old 29th March 2009, 10:02 PM   #8
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By the way, what transport (tape drive mechanism) are you using? Or are you building THAT from scratch too?

A side note, the NE5532A variant is lower noise. Might be important if you want really quiet playback.
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Old 29th March 2009, 10:19 PM   #9
tomcuk is offline tomcuk  United Kingdom
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I was planning on using OPA2134s as I have a few spare from a previous project, do you think the NE5532A would be more suitable?

As for the transport I’m using a old 90s Sony deck with modified spools (not driving the capstan but the spool instead), I won’t go into the drive details as I’m sure my approach will upset some analogue purists, safe to say it’s not a normal implementation.
Thanks again for the help it really has been very useful
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Old 29th March 2009, 11:06 PM   #10
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Well those 2 ICs are different; each has its own plus points as far as figures go. I guess it all becomes a matter of personal preference.
The NE is quieter, but has poorer slew rate.

Are you saying you aren't using a capstan+pinch roller *at all*?
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