Cassette tape head amplifier?

Started by WholeLottaDimitri, May 05, 2015, 09:01:44 PM

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WholeLottaDimitri

Just like the tittle says, I need a simple cassette tape head amplifier. It doesn't need to be hi-fi, I just need to boost the signal to run into a guitar amp.
What circuits are good for this?
The reason I ask is cause I'm making a cassette tape delay.
I have one recording head with all the circuitry, I found an old cassette player, and I have another playback head by I need to amplify it so it can run into a guitar amp.
So does anybody have a circuit that'll work well for this?

R.G.

The National Semiconductor Audio Applications book from about 1975 has a couple. I think they also had a single IC to do the cassette tape function. Probably not available any more, though.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

PBE6

For straight amplification you could use a simple opamp non-inverting gain stage. If you want to mimic a guitar, you could try the AMZ simulator.

Digital Larry

My first job out of university was at Ampex.  I did actually spend about a year testing heads and tapes for Type C video tape machines (1 inch wide tape) before venturing off into microprocessors.

That was back in the days of tape tape tape boy they made a lot of money on magnetic tape.  My brain's getting a bit cobwebby but I seem to recall that a regular magnetic head's response rises at 6 dB/octave until the wavelength gets close to the gap width.  So you might wish to roll it off using a low pass filter somewhere along the way.
Digital Larry
Want to quickly design your own effects patches for the Spin FV-1 DSP chip?
https://github.com/HolyCityAudio/SpinCAD-Designer

anotherjim

Most of circuits you'll find involve low power audio amp chips that are mostly now extinct. The important thing though, is the tape eq curve, and it shouldn't be too hard to fiddle some of those circuits to use your go-to op-amp. Another thing is power supply - what supply voltages do you need to work with?

This...

You don't need the tone controls, so can put a volume control pot after the 1uF leaving the amp.
For a single power supply and using an op-amp, you need to make the usual half supply volts reference (Vref) with a voltage divider.
The 100k from the amp (-) input connects to the Vref now instead of ground. The amp (+) input needs connecting to Vref via a resistor between 470k and 1M. Don't worry too much about getting the exact values shown in the eq network. The .003 (uF) can be 3.3n and so on.


PRR

> recall that a regular magnetic head's response rises at 6 dB/octave until the wavelength gets close to the gap width.

Yes, but "close" is a fine point which needs to be defined in context of the HF losses and the traditions of the tape-machine market. In audio, differences between a simple/standard playback EQ and reality are compensated on the record side. (In cassettes, with a massive record boost in the top-octave.)

The classic low-speed tape playback EQ is flat below 50Hz, falls 6dB/8ve to upper midrange, then flat to the intended upper limit.

The schem that Jim posted is the right topology, but the inflection points do not look right? (Also that chip is obsolete and its DC bias scheme is unique.)

The upper-mid inflection for cassette is---

120uS - 1,300Hz - Type I (rust)
70uS - 2,275Hz - Type II/IV (chrome)

This is the very basic tape preamp:
http://www.hqew.net/files/Images/Article/Circuit_Diagram/135-9630.png

"5.3K" should be 7K for Chrome, 12K for Type 1. For your purpose, I'd make it 4.7K fixed plus 10K trimmer, diddle for happy highs.
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jim68000

Is there anything special about the HA5101 in that schematic, or will any op amp do?

PRR

> anything special about the HA5101 in that

Nothing, except fairly low hiss.

You want low voltage hiss. TL072 or '4558 may not be ideal for a cassette head. '5532 or LM833 is excellent.
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anotherjim

I found the same scheme that Paul links, but for some reason linked the LM381.
Dimitri hasn't come back to say what power supply is available in his "found" deck.

The LM381 looks like some kind of ancestor of the LM386, without speaker drive capability. Input impedance is low (100/200k) -and not  the same for both inputs! Note, I'm not saying that the 386 is a substitute for 381 and I can't find any 386 tape amp circuits.


PRR

> LM381 looks like some kind of ancestor of the LM386

They come from the same rush of designs that National was pushing out. And a similar point in Silicon process history.

I recall them coming out the same year. Of course one must have come before the other, but I suspect more sibling than ancestor.

Yeah, I suppose the guts are quite similar, except of course the '381 output stage is far lamer than the '386's lame output. Also the '386 had Darlington PNP inputs so they can be held to ground, while the '381 is NPN(?) and want to sit +1.2V. And they cared about the hiss-level of '381, not the '386.

> not  the same for both inputs!

Designed-in function. One input goes to a tape head. Other input goes to a NFB network, which is typically hi-gain, so low-Z at the tap point.

> I can't find any 386 tape amp circuits.

The plain '386 input hiss is rather high, and its output grunt is far more than is warranted for the volume control pot you surely want between preamp and power amp. And '386 is by-design set up for specific gains, getting an EQ curve would be more work than we want.

Find the LM389 datasheet. This is/was a '386 with three NPNs thrown in. Fig 2 shows a tape recorder. Throw the switches the other way (Play), that is an adequate tape preamp. The supply voltage can be +9V or +12V with no change. Note the datasheet has not been updated in *20* years, the '389 never sold well enough to justify itself, and warehouse crums may be scarce. If you just need the preamp, use two common  jellybean Si NPNs; one of R.G.'s Rules applies. (Note also that this is pretty-much a FuzzFace, except gain is controlled by NFB instead of output cut-down with a tap on the load resistor.)

-- edit -- Ha! Apparently BG Micro got the leftover LM389s. Buck a pop.
http://www.bgmicro.com/lm389audioamp.aspx

A guy built a $5 LM389 guitar-amp:
http://workisforsuckers.org/prj/fet_amp
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R.G.

And now for something modestly different.

A cassette tape head makes a GREAT tool for debugging shorts on a PCB. It's designed to read magnetic fields that are really, really close to a tiny gap on the end of the head. If you feed a current signal through a wire or PCB trace, you can follow the path of the current with a cassette head that's properly post-amplified.

R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

jim68000


PRR

> Page 10 of the LM833

Yes; except this (and many others) are scaled for Open-Reel (fast) tape systems.

You can't shove heavy highs onto slow (cassette) tape, so cassette needs more high-lift (a lower corner frequency).

Or it does for "hi fi". Going through a guitar system, it may be moot, or you may fake-it with g-amp tone control.
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sokeman

BUMP!!!

i know its been a long time just wondering if anything was made. i want to do the exact same thing build a tape head amplifier. does anyone know the output of a cheap cassette tape head and also the gain i would need to get it to 1v pp?


thanks tim