cassette tape heads circuit

[Dimitree]

hi everyone
as an experiment, just for fun, I'm trying to build a cheap and simple tape echo.
Something ispired by this, for example:

As a starting point, I took out, from an old and cheap Aiwa hi-fi, the magnetic heads (erase head, record head and playback head) used on the cassette deck and its associated circuit.
I'm not going to use the cassette deck mechanism as it is, I'd like to build my own mechanism, more or less like on the previous example, where tape is not sealed on a cassette and attached to reels, but is a loop in the "open air".
For the moment my concern is not about the mechanical issues (like how to make a tape loop, how to move the loop, align heads, etc), so I'm assuming that these issues were already addressed.

What I'd like to focus on now is understanding how to simplify the electronic circuit used to write/read the magnetic heads.
Since the PCB that houses the original circuit is huge (since on the same board the power amp and the tuner are also present), I would like to re-build the needed section of the circuit, or alternatively build an equivalent circuit.

This is the circuit I'm talking about, with highlighted in yellow the tape section:
http://www.ed-sounds.com/prova1.png

The most obscure parts to me are that BD3881 IC and the fet switching.
I searched for a datasheet of that IC, I only found a really criptic one, and this is the link:
http://pdf1.alldatasheet.com/datasheet-pdf/view/87881/ROHM/BD3881FV.html

Is it something really special, or maybe I could build a simple op-amp equivalent circuit, and avoid the need of that IC?
Also, do I really need to use all those FETs, given the fact that I must continuously/simultaneously write and read?
what's for all that switching?

thank you for the support!

 

[JohnRoberts]

I had to deal with one 4 track cassette deck and it was enough of a PIA with dolby NR and being able to over dub punch-ins for one track while listening to the other three.

So yes your application is a little easier without switching but you need to still manage a bias oscillator for the record head, and perhaps bias traps to keep that out of the playback.

Do you plan to use NR?

I wouldn't try to make one on a bet... but it's your time and money.

Tape head preamps are low noise for lower impedance than phono carts but a decent modern opamp might work.

Look up NAB playback eq.

Have fun..

JR

 

[Dimitree]

thank you for the reply!

So yes your application is a little easier without switching but you need to still manage a bias oscillator for the record head, and perhaps bias traps to keep that out of the playback.

if I'm not wrong, bias is already generated in that circuit, so I guess bias traps too..
how switching is involved in that?

Do you plan to use NR?

do you mean noise reduction? if yes, then I think it's enough what the original circuit does..or not? I mean, it was intended to use as hifi player, so I think noise is not that bad.
Btw I will going to use a faster speed than the usual cassette standard, and I will put the 2 channels in series, that would improve S/N ratio if I understood well.

I wouldn't try to make one on a bet... but it's your time and money.

please explain  ;D

Tape head preamps are low noise for lower impedance than phono carts but a decent modern opamp might work.
Look up NAB playback eq.

what I still don't understand is: are the tape heads all equal? I mean, can I simply take a tape echo machine (that obviously doesn't use cassette heads) schematic and use my own heads with that schematic? if not, are all cassette heads equal? or maybe there are differences even between them?

 

[JohnRoberts]

thank you for the reply!

So yes your application is a little easier without switching but you need to still manage a bias oscillator for the record head, and perhaps bias traps to keep that out of the playback.

if I'm not wrong, bias is already generated in that circuit, so I guess bias traps too..
how switching is involved in that?

Bias is only used during record.

Do you plan to use NR?

do you mean noise reduction? if yes, then I think it's enough what the original circuit does..or not? I mean, it was intended to use as hifi player, so I think noise is not that bad.
Btw I will going to use a faster speed than the usual cassette standard, and I will put the 2 channels in series, that would improve S/N ratio if I understood well.

If you record and playback at the same time while using NR, you need to filter the bias frequency leakage out of the playback channel so it doesn't corrupt the playback NR tracking. 

I wouldn't try to make one on a bet... but it's your time and money.

please explain  ;D

I would not do that project on a bet...  I know enough about tape recorders to not design one for fun.

Tape head preamps are low noise for lower impedance than phono carts but a decent modern opamp might work.
Look up NAB playback eq.

what I still don't understand is: are the tape heads all equal? I mean, can I simply take a tape echo machine (that obviously doesn't use cassette heads) schematic and use my own heads with that schematic? if not, are all cassette heads equal? or maybe there are differences even between them?

No tape heads are not all the same.

JR

 

[abbey road d enfer]

hi everyone
as an experiment, just for fun, I'm trying to build a cheap and simple tape echo.

Not dissing you, but it's clear you don't know "tape recording 101".
The schemo you joined is not going to help you understand, because the functional blocks are all hidden in the square box.
I suggest you study some basic schemos, such as Echoplex, Echorec and Copicat.
Most tape recorder schemos are difficult to analyze because they are switching building blocks between record and playback (sometimes the power amplifier is rescheduled into a bias oscillator). 3-head types are somewhat easier, but really, dedicated echo units are very simple; there are 3 blocks: record amp, playback amp and oscillator, with very little interconnect.
As JR mentioned, not all heads are equal.
There are two distincive aspects:
First is technology, tube vs. solid-state. Typically, the former have a much larger impedance than the latter.
Second is function: optimally, playback heads have higher impedance and smaller gap than record heads. However, 2-head type tape recorders use a compromise. Erase heads are a different animal, with much larger gap that makes them unsuitable for dealing with audio signals, and very low inductance.
Before undertaking the task of making a tape echo unit, you must understand the principles. there is a lot of mechanical engineering going in tape machines. For example, the heads must be provided with some kind of height and azimuth adjustment; I suggest you identify that on an existing machine.
I would try to identify a cheap tape recorder and start with it as a mechanical donor; modding a reel-to-reel into a close-loop system is not an impossible task. You could lock it in record mode and use another identical machine as a donor for the playback electronics.

 

[Dimitree]

Not dissing you, but it's clear you don't know "tape recording 101".
The schemo you joined is not going to help you understand, because the functional blocks are all hidden in the square box.

that's why I came here 
I indeed would like to understand what's inside that box.. Datasheet doesn't help me, I hope you can.

As JR mentioned, not all heads are equal.
There are two distincive aspects:
First is technology, tube vs. solid-state. Typically, the former have a much larger impedance than the latter.
Second is function: optimally, playback heads have higher impedance and smaller gap than record heads. However, 2-head type tape recorders use a compromise. Erase heads are a different animal, with much larger gap that makes them unsuitable for dealing with audio signals, and very low inductance.

I understand that playback heads, erase heads and record heads are different between them.
What I'd like to understand, and I didn't manage to do it, is:
- 1/4" reel to reel (or similar) heads are obviously esthetically different from cassette heads. But what are other differences? impedance, bandwith, etc.. I mean, what are the measurable characteristics that distinguish 2 tape heads formats?
also, between the same family (I mean, between cassette heads) what could be the differences between different manufacturers? as an example, can I put a playback head from a sony deck insiede a philips deck without adapting the circuit?

Before undertaking the task of making a tape echo unit, you must understand the principles. there is a lot of mechanical engineering going in tape machines. For example, the heads must be provided with some kind of height and azimuth adjustment; I suggest you identify that on an existing machine.
I would try to identify a cheap tape recorder and start with it as a mechanical donor; modding a reel-to-reel into a close-loop system is not an impossible task. You could lock it in record mode and use another identical machine as a donor for the playback electronics.

as I said previously, mechanical issues are going to be addressed in a later stage.. I'm still trying the understand how it all works..not starting the project at all.. This is why I'm assuming that the mechanical part is already perfect so I can focus on the electronics for the moment.

 

[abbey road d enfer]

I understand that playback heads, erase heads and record heads are different between them.
What I'd like to understand, and I didn't manage to do it, is:
- 1/4" reel to reel (or similar) heads are obviously esthetically different from cassette heads. But what are other differences? impedance, bandwith, etc.. I mean, what are the measurable characteristics that distinguish 2 tape heads formats?

First, the tape width is different, 0.25" for reel-to-reel, 0.15 for cassette. After that most of the differences come from the fact that cassette decks have had a degree of industrialisation that reeel-to-reel never enjoyed. A typical R2R head would have a separate shield, a separate mount with 3-way adjustment, when cassette heads would have integrated shield and adjustment system.

also, between the same family (I mean, between cassette heads) what could be the differences between different manufacturers? as an example, can I put a playback head from a sony deck insiede a philips deck without adapting the circuit?

Some manufacturers have developped heads that helped them achieve specific performance; IIRC Akai had iron-glass heads, Pioneer developped heads that allowed them crossfield bias, but in many cases, you could use mixed and matched heads from different manufacturers. The main issue would be mechanical. But you must know the characteristics of the heads or at least deduce them from the analysis of the surrounding electronics.

 

[JohnRoberts]

Not dissing you, but it's clear you don't know "tape recording 101".
The schemo you joined is not going to help you understand, because the functional blocks are all hidden in the square box.

that's why I came here 
I indeed would like to understand what's inside that box.. Datasheet doesn't help me, I hope you can.

As JR mentioned, not all heads are equal.
There are two distincive aspects:
First is technology, tube vs. solid-state. Typically, the former have a much larger impedance than the latter.
Second is function: optimally, playback heads have higher impedance and smaller gap than record heads. However, 2-head type tape recorders use a compromise. Erase heads are a different animal, with much larger gap that makes them unsuitable for dealing with audio signals, and very low inductance.

I understand that playback heads, erase heads and record heads are different between them.
What I'd like to understand, and I didn't manage to do it, is:
- 1/4" reel to reel (or similar) heads are obviously esthetically different from cassette heads. But what are other differences? impedance, bandwith, etc.. I mean, what are the measurable characteristics that distinguish 2 tape heads formats?
also, between the same family (I mean, between cassette heads) what could be the differences between different manufacturers? as an example, can I put a playback head from a sony deck insiede a philips deck without adapting the circuit?

Maybe if you are very lucky.. The heads will be mechanically standardized wrt track width and location but not necessarily similar mounting attachments. Electrically they could use different bias oscillator frequency and exhibit subtly different frequency response .

Another wild card with cassette heads is 4T and even 8T cassette heads will be different than common 2T+2T consumer heads.  While the 4T and 2T+2T appear similar for track location, the 4T head has magnetic shielding between each track so you can record one of the 4 tracks at a time. The 2T+2T head only has magnetic shielding between the 2T pairs.

AFAIK the heads are (were?) only made by a small handful of companies, so it is possible to find similar heads inside different brands.

Before undertaking the task of making a tape echo unit, you must understand the principles. there is a lot of mechanical engineering going in tape machines. For example, the heads must be provided with some kind of height and azimuth adjustment; I suggest you identify that on an existing machine.
I would try to identify a cheap tape recorder and start with it as a mechanical donor; modding a reel-to-reel into a close-loop system is not an impossible task. You could lock it in record mode and use another identical machine as a donor for the playback electronics.

as I said previously, mechanical issues are going to be addressed in a later stage.. I'm still trying the understand how it all works..not starting the project at all.. This is why I'm assuming that the mechanical part is already perfect so I can focus on the electronics for the moment.

In 25 words or less... To record you mix in a HF bias tone (85-100kHz) to get the magnetic domains moving for improved linearity. For playback you just need to amplify and equalize the tiny signal coming from the head.

JR

 

[Rob Flinn]

Most of these tape delays have pretty primitive electronics.  The Watkins Copicat (valve version) & the early Binson had bar magnets for the erase heads.  They don't have great repro quality on playback, which is part of the charm because quite often it sounds better with less clarity on delayed content.  With that it mind it may be easier to just copy a circuit from a copycat.   

However, bearing in mind that you can pick up a copycat for a couple of hundred quid it hardly seems worth the effort recreating it.

 

[iomegaman]

I just picked up an old Fender Electronic Echo CHamber from a craigslist ad that I will rebuild for fun, schematic pretty simple everything seems to be intact except the cartridge, which has been modified to hold an 8-track tape...we shall see...I have located a cartridge in Italy but am a little skeptical at this point...

This schematic is close, not exact...