9/17/2015 cassette tape heads circuit

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DimitreeMemberPosts: 6cassette tape heads circuit« on: May 28, 2015, 05:51:30 PM »

hi everyone

9/17/2015 cassette tape heads circuit

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hi everyoneas 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 hi­fi, the magnetic heads(erase head, record head and playback head) used on the cassette deck and itsassociated circuit.I'm not going to use the cassette deck mechanism as it is, I'd like to build my ownmechanism, more or less like on the previous example, where tape is not sealed on acassette 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 atape loop, how to move the loop, align heads, etc), so I'm assuming that these issueswere already addressed.

What I'd like to focus on now is understanding how to simplify the electronic circuitused to write/read the magnetic heads.Since the PCB that houses the original circuit is huge (since on the same board thepower amp and the tuner are also present), I would like to re­build the needed sectionof 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.ed­sounds.com/prova1.png

The most obscure parts to me are that BD3881 IC and the fet switching.

9/17/2015 cassette tape heads circuit

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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 thelink:http://pdf1.alldatasheet.com/datasheet­pdf/view/87881/ROHM/BD3881FV.html

Is it something really special, or maybe I could build a simple op­amp equivalentcircuit, and avoid the need of that IC?Also, do I really need to use all those FETs, given the fact that I mustcontinuously/simultaneously write and read?what's for all that switching?

thank you for the support! Logged

JohnRobertsGlobal ModeratorMemberHickory, MS

Posts: 9581

Re: cassette tape heads circuit« Reply #1 on: May 28, 2015, 06:15:29 PM »

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

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

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 decentmodern opamp might work.

Look up NAB playback eq.

Have fun..

JR

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JR Logged

John Robertshttp://circularscience.comTune it, or don't play it...

DimitreeMemberPosts: 6Re: cassette tape heads circuit« Reply #2 on: May 28, 2015, 06:23:44 PM »

thank you for the reply!

Quote

So yes your application is a little easier without switching but you need tostill manage a bias oscillator for the record head, and perhaps bias traps tokeep 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?

Quote

Do you plan to use NR?

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

Quote

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

please explain

Quote

Tape head preamps are low noise for lower impedance than phono carts

but a decent modern opamp might work.Look up NAB playback eq.

9/17/2015 cassette tape heads circuit

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

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JohnRobertsGlobal ModeratorMemberHickory, MS

Posts: 9581

Re: cassette tape heads circuit« Reply #3 on: May 28, 2015, 09:12:13 PM »

Quote from: Dimitree on May 28, 2015, 06:23:44 PM

thank you for the reply!

Quote

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

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

Bias is only used during record. Quote

Quote

Do you plan to use NR?

do you mean noise reduction? if yes, then I think it's enough what theoriginal circuit does..or not? I mean, it was intended to use as hifi player, soI think noise is not that bad.

Btw I will going to use a faster speed than the usual cassette standard, and Iwill put the 2 channels in series, that would improve S/N ratio if I understoodwell.

If you record and playback at the same time while using NR, you need to filter the bias

9/17/2015 cassette tape heads circuit

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

Quote

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

please explain

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

Quote

Tape head preamps are low noise for lower impedance thanphono 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 Isimply take a tape echo machine (that obviously doesn't use cassetteheads) schematic and use my own heads with that schematic? if not, are allcassette heads equal? or maybe there are differences even between them?

No tape heads are not all the same.

JR Logged

John Robertshttp://circularscience.comTune it, or don't play it...

abbey road d enferGlobal ModeratorMemberMarcelland

Posts: 4804Re: cassette tape heads circuit« Reply #4 on: May 29, 2015, 02:48:27 AM »

Quote from: Dimitree on May 28, 2015, 05:51:30 PM

hi everyone

9/17/2015 cassette tape heads circuit

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hi everyoneas an experiment, just for fun, I'm trying to build a cheap and simple tapeecho.

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 functionalblocks 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 switchingbuilding blocks between record and playback (sometimes the power amplifier isrescheduled into a bias oscillator). 3­head types are somewhat easier, but really,dedicated echo units are very simple; there are 3 blocks: record amp, playback ampand oscillator, with very little interconnect.As JR mentioned, not all heads are equal.There are two distincive aspects: First is technology, tube vs. solid­state. Typically, the former have a much largerimpedance than the latter.Second is function: optimally, playback heads have higher impedance and smaller gapthan record heads. However, 2­head type tape recorders use a compromise. Eraseheads are a different animal, with much larger gap that makes them unsuitable fordealing with audio signals, and very low inductance.Before undertaking the task of making a tape echo unit, you must understand theprinciples. there is a lot of mechanical engineering going in tape machines. Forexample, the heads must be provided with some kind of height and azimuthadjustment; 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 reel­to­reel into a close­loop system is not an impossible task. You couldlock it in record mode and use another identical machine as a donor for the playbackelectronics.

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Who's right or wrong is irrelevant. What matters is what's right or wrong.Star ground is for electricians.

DimitreeMemberPosts: 6Re: cassette tape heads circuit« Reply #5 on: May 29, 2015, 07:24:35 AM »

Quote from: abbey road d enfer on May 29, 2015, 02:48:27 AM

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 thefunctional blocks are all hidden in the square box.

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

Quote from: abbey road d enfer on May 29, 2015, 02:48:27 AM

As JR mentioned, not all heads are equal.There are two distincive aspects: First is technology, tube vs. solid­state. Typically, the former have a muchlarger impedance than the latter.Second is function: optimally, playback heads have higher impedance andsmaller gap than record heads. However, 2­head type tape recorders use acompromise. Erase heads are a different animal, with much larger gap thatmakes them unsuitable for dealing with audio signals, and very lowinductance.

I understand that playback heads, erase heads and record heads are differentbetween 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 cassetteheads. But what are other differences? impedance, bandwith, etc.. I mean, what arethe measurable characteristics that distinguish 2 tape heads formats?also, between the same family (I mean, between cassette heads) what could be thedifferences between different manufacturers? as an example, can I put a playbackhead from a sony deck insiede a philips deck without adapting the circuit?

Quote from: abbey road d enfer on May 29, 2015, 02:48:27 AM

Before undertaking the task of making a tape echo unit, you mustunderstand the principles. there is a lot of mechanical engineering going intape machines. For example, the heads must be provided with some kind ofheight and azimuth adjustment; I suggest you identify that on an existingmachine.I would try to identify a cheap tape recorder and start with it as a mechanicaldonor; modding a reel­to­reel into a close­loop system is not an impossibletask. You could lock it in record mode and use another identical machine asa donor for the playback electronics.

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

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abbey road d enfer

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Global ModeratorMemberMarcelland

Posts: 4804Re: cassette tape heads circuit« Reply #6 on: May 29, 2015, 08:20:34 AM »

Quote from: Dimitree on May 29, 2015, 07:24:35 AM

I understand that playback heads, erase heads and record heads aredifferent 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 fromcassette heads. But what are other differences? impedance, bandwith, etc..I mean, what are the measurable characteristics that distinguish 2 tapeheads formats?

First, the tape width is different, 0.25" for reel­to­reel, 0.15 for cassette. After that mostof the differences come from the fact that cassette decks have had a degree ofindustrialisation that reeel­to­reel never enjoyed. A typical R2R head would have aseparate shield, a separate mount with 3­way adjustment, when cassette heads wouldhave integrated shield and adjustment system.Quote

also, between the same family (I mean, between cassette heads) whatcould be the differences between different manufacturers? as an example,can I put a playback head from a sony deck insiede a philips deck withoutadapting the circuit?

Some manufacturers have developped heads that helped them achieve specificperformance; IIRC Akai had iron­glass heads, Pioneer developped heads that allowedthem crossfield bias, but in many cases, you could use mixed and matched heads fromdifferent manufacturers. The main issue would be mechanical. But you must know thecharacteristics of the heads or at least deduce them from the analysis of thesurrounding electronics.

Logged

Who's right or wrong is irrelevant. What matters is what's right or wrong.Star ground is for electricians.

JohnRobertsGlobal ModeratorMemberHickory, MS

Posts: 9581

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Posts: 9581

Re: cassette tape heads circuit« Reply #7 on: May 29, 2015, 09:21:29 AM »

Quote from: Dimitree on May 29, 2015, 07:24:35 AM

Quote from: abbey road d enfer on May 29, 2015, 02:48:27 AM

Not dissing you, but it's clear you don't know "tape recording101".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'thelp me, I hope you can.

Quote from: abbey road d enfer on May 29, 2015, 02:48:27 AM

As JR mentioned, not all heads are equal.There are two distincive aspects: First is technology, tube vs. solid­state. Typically, the former havea much larger impedance than the latter.Second is function: optimally, playback heads have higherimpedance and smaller gap than record heads. However, 2­headtype tape recorders use a compromise. Erase heads are adifferent animal, with much larger gap that makes themunsuitable for dealing with audio signals, and very lowinductance.

I understand that playback heads, erase heads and record heads aredifferent 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 fromcassette heads. But what are other differences? impedance, bandwith, etc..I mean, what are the measurable characteristics that distinguish 2 tapeheads formats?also, between the same family (I mean, between cassette heads) whatcould be the differences between different manufacturers? as an example,can I put a playback head from a sony deck insiede a philips deck withoutadapting the circuit?

Maybe if you are very lucky.. The heads will be mechanically standardized wrt trackwidth and location but not necessarily similar mounting attachments. Electrically they

9/17/2015 cassette tape heads circuit

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width and location but not necessarily similar mounting attachments. Electrically theycould use different bias oscillator frequency and exhibit subtly different frequencyresponse .

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

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

Quote from: abbey road d enfer on May 29, 2015, 02:48:27 AM

Before undertaking the task of making a tape echo unit, you mustunderstand the principles. there is a lot of mechanicalengineering going in tape machines. For example, the headsmust be provided with some kind of height and azimuthadjustment; I suggest you identify that on an existing machine.I would try to identify a cheap tape recorder and start with it as amechanical donor; modding a reel­to­reel into a close­loopsystem is not an impossible task. You could lock it in recordmode and use another identical machine as a donor for theplayback electronics.

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

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

JR Logged

John Robertshttp://circularscience.comTune it, or don't play it...

Rob FlinnMemberSussex, UK

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Sussex, UKPosts: 3222

Re: cassette tape heads circuit« Reply #8 on: May 29, 2015, 01:34:55 PM »

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'thave great repro quality on playback, which is part of the charm because quite often itsounds better with less clarity on delayed content. With that it mind it may be easier tojust copy a circuit from a copycat.

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

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regards Rob

Capital letters. The difference between helping your uncle Jack off his horse, & helping your uncle jack offhis horse ........

iomegamanMemberChico Ca.Posts: 311

Re: cassette tape heads circuit« Reply #9 on: May 29, 2015, 05:50:23 PM »

I just picked up an old Fender Electronic Echo CHamber from a craigslist ad that I willrebuild for fun, schematic pretty simple everything seems to be intact except the

cartridge, which has been modified to hold an 8­track tape...we shall see...I havelocated a cartridge in Italy but am a little skeptical at this point...

This schematic is close, not exact... Logged

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