Need help designing an audio circuit - Tape delay unit

DHughes said:

Thanks for all the advice so far everyone, this has been extremely helpful and enlightening.


While I'd definitely follow this path if I was short on time or ambition, at the moment I really just want to make this work. It's already been such an education so far and it's only been a few days.

While looking through some more schematics, I found one that seemed like it would be a lot easier to produce and understand. It does, however, use tubes rather than solid state components. Is there any way I could change the circuit to work with transistors? Here is the schematic;
http://www.vintagehofner.co.uk/britamps/watkins/schematics/copicat.jpeg

Also, I'd probably want to lose one of the inputs, and 2 of the 3 play heads. Can this be easily done?

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Absolutely. And certainly the easiest of the bunch.

Since 5 of the 6 tubes in the circuit are triodes (and the 7th, a tetrode as amplifier - easily replaced with a triode) could all have transistor replacements. The "Bias Oscillator" circuit could probably be replaced with a simple 555 timer circuit (simpler and without the LC tank).

You would, of course, have to alter the PS voltage(s) to appropriate transistor levels.

And relatively careful transistor selection, especially for the "Record Amplifier" would be necessary but not at all difficult.

I like the idea...

DHughes said:

Ahh, this is excellent. I think I understand this one. I have a couple of questions. What voltage is the rectifier outputting? In other words, if were to use some kind of DC power supply, what would I use? Also, how would I go about getting/making the bias transformer? Thanks btw, this is so helpful.

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As there's a 680 ohm feeding an 18V zener I presume the DC from the rectifier is about 24V or so?.

As for the bias transformer, I'd suggest using the one from the cassette deck you used to get the mechanics (and using that bias circuit as well).

A transistor circuit is completely different to the vacuum tubes circuit.
Cassette tape runs much slower and is much narrower than the old huge reels of tape 60 years ago, therefore the levels and equalization are also completely different.
You also need to match the equalization to the tiny tape heads you have which are also completely different to the huge tape heads in an old reel-to-reel vacuum tubes tape deck.

cowboybob said:

Absolutely. And certainly the easiest of the bunch.

Since 5 of the 6 tubes in the circuit are triodes (and the 7th, a tetrode as amplifier - easily replaced with a triode) could all have transistor replacements. The "Bias Oscillator" circuit could probably be replaced with a simple 555 timer circuit (simpler and without the LC tank).

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Sorry, but presumably you're not familiar with tape recorders? - the reason for the transformer is 'mostly' to give a high voltage erase/bias signal, and it MUST, MUST be sine wave, a square wave from a 555 would be useless.

You would, of course, have to alter the PS voltage(s) to appropriate transistor levels.

And relatively careful transistor selection, especially for the "Record Amplifier" would be necessary but not at all difficult.

I like the idea...

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Pretty silly idea when there's multiple transistor versions on the same site

You can't just replace valves with transistors, it needs completely redesigning (as the WEM engineers did).

Incidentally, I've repaired various Copicats over the years - but none recently.

Many years ago I built a tape echo machine. I would not do a mechanical machine now. But, as the OP seems to be determined to do that, may I suggest he/she starts of with a 3-head tape machine and mount the play-back head on a pivot (roller-guides on both sides) to allow for the delay adjustment and some effects (delay time, flanging, vibrato and tremolo). E

Nigel Goodwin said:

Sorry, but presumably you're not familiar with tape recorders?

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I am familiar with recorders.

Nigel Goodwin said:

Since T1 is a custom unit, you're going to have to come up with a PS. Pretty silly idea when there's multiple transistor versions on the same site

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Mentioned the need for a different PS.

Nigel Goodwin said:

You can't just replace valves with transistors, it needs completely redesigning (as the WEM engineers did).

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Of all the circuits thus far shown (the VintageHofner site), the tube type was by far the simplest. And, of course, I did not imply that a simple swap of transistor for tube was appropriate. But the basic design would easily lend itself to a solid state remake.

I might suggest to the OP that he get himself a SIM program (take your pick - I like TINA, but no one else on the board seems to) and, using the circuit under discussion as a templet, re-create it in the SIM using transistors. Might be a fun and enlightening project.

cowboybob said:

I am familiar with recorders.

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And yet you suggest using a 555, and no transformer, for bias and erase?.

Sorry, but that doesn't sound like you're at all familiar with them?.

Nigel, you are right, but do you have to be so abrupt about it? E

Nigel Goodwin said:

And yet you suggest using a 555, and no transformer, for bias and erase?.

Sorry, but that doesn't sound like you're at all familiar with them?.

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Analog audio tape recorders (and by extension, echo producers) are, in essence, very simple devices. Please don't impugn my familiarity based on what little I have offered to the OP or by what I have discussed.

Perhaps I need to explain that my proposals were in a general context. I was not precisely describing a circuit, rather options that the OP might consider (i.e., a 555 as the bias/erase oscillator circuit, in lieu of an LC tank) in his quest for ideas for constructing his own system. And, of course, most people would understand that the 555 could not possibly provide the current needed to drive the, in particular, erase head. I just liked the tube type schematic and despite what some members may think, I had no problems seeing an adequate conversion option to solid state as a possibility.

I've always assumed a certain level of understanding by the members of this forum for things "left unsaid" so as to not offend.

If I may be so bold as a mere member, I was responding to the OP's fairly clear request for help with his stated qualifications:

DHughes said:

...a medium level hobbyist with plenty of free time?...

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I did not go for his throat with my vast knowledge. I was trying to get a feel for his desire to engage in some electronics fun.

Nothing more, nothing less. I hope the OP is still with us...

Don't worry haha, I'm still here. These are all interesting ideas, and I appreciate your time.

I have one problem though, to do with the bias transformer and circuit. Basically, out of the 3 tape units I've taken apart, I can't find a single bias transformer. Are they much smaller than I might expect them to be? Or have all the tape decks I've dismantled, ranging from quite high quality to dirt cheap, used a DC bias circuit?

I can't find a single bias transformer. Are they much smaller than I might expect them to be?

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Here's one, with dimensions:
https://www.echotapper.nl/wp/?p=799

DHughes said:

I have one problem though, to do with the bias transformer and circuit. Basically, out of the 3 tape units I've taken apart, I can't find a single bias transformer. Are they much smaller than I might expect them to be? Or have all the tape decks I've dismantled, ranging from quite high quality to dirt cheap, used a DC bias circuit

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Entirely possible.

A DC erase bias aligns the magnetic particles all in a row. Upon re-recording there is a loss of audio quality as a result. Many people don't notice the difference.

AC biasing (for both erase and record) enhances the audio reproduction (especially the trebles).

Your call as to using either method.

Not knowing the recorders you've looked at, it's certainly possible that to achieve the higher currents needed for biasing that some manner of FET (or other) amplification/mixing circuit was used rather than a transformer.

And, strictly speaking, for re-recording it's not absolutely necessary to use an erase function at all, but the recording quality really does suffer. For your purposes you may not care.

Lotta ways to skin cats...

cowboybob said:

Analog audio tape recorders (and by extension, echo producers) are, in essence, very simple devices. Please don't impugn my familiarity based on what little I have offered to the OP or by what I have discussed.

Perhaps I need to explain that my proposals were in a general context. I was not precisely describing a circuit, rather options that the OP might consider (i.e., a 555 as the bias/erase oscillator circuit, in lieu of an LC tank) in his quest for ideas for constructing his own system.

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You're still ignoring the main problem (which is why I keep suggesting you know little about tape recorders), it's ESSENTIAL to have sine wave bias/erase. Current concerns from a 555 aren't the problem, it could probably provide enough current, feeding a transformer to give the higher voltage - but isn't a square wave (and as you still need the transformer, why use a 555?).

And, of course, most people would understand that the 555 could not possibly provide the current needed to drive the, in particular, erase head. I just liked the tube type schematic and despite what some members may think, I had no problems seeing an adequate conversion option to solid state as a possibility.

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But why? - when there's already a perfectly good and simple design from the original manufacturer? - I can assure you, it's not full of parts that aren't needed .

DHughes said:

Don't worry haha, I'm still here. These are all interesting ideas, and I appreciate your time.

I have one problem though, to do with the bias transformer and circuit. Basically, out of the 3 tape units I've taken apart, I can't find a single bias transformer. Are they much smaller than I might expect them to be? Or have all the tape decks I've dismantled, ranging from quite high quality to dirt cheap, used a DC bias circuit?

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They range upwards from about the size of a miniature IF transformer - and often look like one.

Nigel Goodwin said:

You're still ignoring the main problem (which is why I keep suggesting you know little about tape recorders), it's ESSENTIAL

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My underlining emphasis.
Nigel:
Why do you insist on slamming a member? Isn't it a better idea to just state your position and leave it at that? That sort of comment, under ANY circumstance, is enormously unnecessary.

That aside, a bias is not "essential". It is strictly used to enhance the re-recording process to achieve superior recording results. Without biasing you can still record, just with reduced fidelity.

I refer you to this, part 2.5, for an edifying explanation of the biasing concept.

cowboybob said:

My underlining emphasis.
Nigel:
Why do you insist on slamming a member? Isn't it a better idea to just state your position and leave it at that? That sort of comment, under ANY circumstance, is enormously unnecessary.

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My reasoning is that you're (either deliberately, or through ignorance?) giving completely wrong and misleading information - why do you keep advising using square wave bias?, even repeating so when your error has been explained?.

Incidentally, DC bias is 'possible' for low quality applications, but not in this case where quality is particularly important as you continually read and re-record the same information - and distorting it every time round would be be really bad.

DHughes said:

Don't worry haha, I'm still here. These are all interesting ideas, and I appreciate your time.

I have one problem though, to do with the bias transformer and circuit. Basically, out of the 3 tape units I've taken apart, I can't find a single bias transformer. Are they much smaller than I might expect them to be? Or have all the tape decks I've dismantled, ranging from quite high quality to dirt cheap, used a DC bias circuit?

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Just had a thought - all VCR's have bias oscillator transformers in them (for the linear audio track, as well as full erase), it might be easier to find one of those?.

Nigel Goodwin said:

My reasoning is that you're (either deliberately, or through ignorance?) giving completely wrong and misleading information - why do you keep advising using square wave bias?, even repeating so when your error has been explained?.

Incidentally, DC bias is 'possible' for low quality applications, but not in this case where quality is particularly important as you continually read and re-record the same information - and distorting it every time round would be be really bad.

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Can't recall (nor can I find any evidence of) my mentioning square waves. So I'm at a loss as to why you continually (and wrongly) persist in accusing me of "wrong and misleading information". All I mentioned was an oscillator. I'm sure most members know how to generate a sine wave from the output of a 555. Somewhat surprised that you don't seem to know that. I'd be glad to provide a schematic (in LTSpice) for you if you would like.

And your are right about the lesser quality of a DC biasing system which is what I previously noted, so on that we agree.

Anyway, I stand by my advice. I'm sure the OP can decide for himself what advice to pay attention to and what advice to ignore.


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A pretty good sine-wave can be made from the square-wave of a 555 (with extra parts to make it a very square 50-50) by using a multi-pole switched capacitor lowpass filter IC.

darn, i used to have a parts drawer full of bias oscillator boards, about cubic inch, with transistors and transformer all in a metal can. they had two power pins, and 3 output pins, one of which was ground , and a tap for the record head bias, and a pin that was the whole secondary for the erase head.... the oscillators were made by Toko (i think). they put out 90Vrms@50khz. i used one in a prototype miniature oscillator/voltage multiplier that put out 250V@0.5mA