Cassette Tape Record circuit - Bias OSC - Page 1
Forum section: electronics / projects

author: SamScwanch
author_rank: Newbie
timestamp: September 20, 2017, 03:02:23 pm
content: Yeah I know, ancient technology, but I couldn't think of a better forum to ask.I am designing a cassette tape delay project and I am determined to build it from the ground up for a few reasons, the most important being form factor, custom pcb shape ect...At this point I am figuring out the record circuit.I think I may go with the AN7105 IC which was designed by Panasonic.Attached picture was taken from:http://pdf1.alldatasheet.com/datasheet-pdf/view/157265/PANASONIC/AN7105.htmlHere is the question, as far as the BIAS OSC, I know I will need to generate a signal anywhere from 40 to 150 kHz and basically mix it with the L and R signal, can I use something as simple as a 555 timer? I will create somewhat of a sine wave coming out, with a variable frequency and amplitude to fine tune it for specific record heads.Also will I only need to passively mix the bias osc and audio signal together? Or will a proper summing circuit be required?Thanks guys.

author: Paul Rose
author_rank: Regular Contributor
timestamp: September 20, 2017, 07:31:47 pm
content: Seehttp://www.gammaelectronics.xyz/sound-engineering_28c.htmlin the neighborhood of section 4.2

author: Benta
author_rank: Super Contributor
timestamp: September 20, 2017, 08:57:08 pm
content: @ Paul Rose: great link, very interesting.Thanks.

author: SamScwanch
author_rank: Newbie
timestamp: September 20, 2017, 11:07:39 pm
content: Quote from: Paul Rose on September 20, 2017, 07:31:47 pmAh yes, thank you. I've been through this site and somehow missed this portion, this kind of explains how to modulate the bias and audio signal together properly. Thank you!

author: MattHollands
author_rank: Frequent Contributor
timestamp: September 20, 2017, 11:15:17 pm
content: I built one of these tape delay pedals a while back. What I found was that AC biasing the signal wasn't always the way to go. I took the tape head out of a tape player that used DC biasing (about 50mV) and I found that therefore DC biasing produced the best sound. When I tried AC biasing the signal (with 40kHz if I remember correctly), I got a weird buzzing sound, check out the link.To be fair, I don't think I did extensive research into the biasing techniques so I could well be wrong, but this is just what I found.My brief article: projects.matthollands.com/2015/03/12/project-analogue-tape-delay-writing-to-tape/Sound:https://soundcloud.com/user-530698836/ac-biasing

author: SamScwanch
author_rank: Newbie
timestamp: September 20, 2017, 11:18:56 pm
content: Quote from: MattHollands on September 20, 2017, 11:15:17 pmI think it depends on the head, I think*** that typical high end tape heads are designed for ACI think if you would have been able to dial in the frequency and amplitude you could have possibly made it sound much better. That is why I like the 555 timer idea, i'd literally tweak the frequency and amplitude, with a scope on the input and output delay to compare the two until its just right, or as good as it could get.

author: MattHollands
author_rank: Frequent Contributor
timestamp: September 20, 2017, 11:20:58 pm
content: Quote from: SamScwanch on September 20, 2017, 11:18:56 pmYeah I think you're right. I've been wanting to have another shot at this project for a while now.

author: SamScwanch
author_rank: Newbie
timestamp: September 20, 2017, 11:25:06 pm
content: Quote from: MattHollands on September 20, 2017, 11:20:58 pmWell Ill keep it updated here, Id be happy to share what I've come up with. Ive got everything mostly figured out, play head circuit, low pass filters, high pass filters (for delay), motor controller, proper active summing mixer. Dual play heads, adjustable distance, adjustable speed (which I now think I may need to tie to the bias circuit for proper biasing at different speeds.

author: MattHollands
author_rank: Frequent Contributor
timestamp: September 20, 2017, 11:26:24 pm
content: Great! Thanks. Btw where did you get your tape heads from?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: September 21, 2017, 12:11:14 am
content: http://www.hparchive.com/appnotes.htmhttp://www.hparchive.com/Application_Notes/HP-AN-89.pdf

author: Cerebus
author_rank: Super Contributor
timestamp: September 21, 2017, 01:44:38 am
content: Quote from: SamScwanch on September 20, 2017, 11:18:56 pmIt's not the head so much as it's the tape thatneedsAC bias. To put it in highly, highly, simplistic terms, the magnetic domains on the tape get 'shaken up' by the bias signal making it easier for the (lower frequency) signal to get them to 'settle' into the new magnetic field you're trying to impose on them.It ispossibleto use DC bias, but that only allows you to use less than half of the (magnetic) dynamic range of the tape. AC bias allows higher dynamic range and less distortion. DC bias in practical recorders is a historical curiosity that probably disappeared while magnetic recorders were still unheard of outside Germany. Basically: no bias = terrible distortion, DC bias = poor distortion and poor dynamic range, AC bias = best distortion and best dynamic range.To explain it properly you need to talk about coercivity, remanence, BH curves and a whole lot more that I'm not even going to try to get into, if only for the reason that this is ancient history for me and I'd have to pull out the textbooks and revise before evenattemptinga proper explanation.The only takeaway point that I'm going to offer is that proper bias strength and frequency is essential to linearising the magnetisation of the tape, and thus minimising the distortion inherent in the magnetic recording process. The head characteristics will largely set the operating frequency you can use (and physical considerations of tape and track width tend to determine the head characteristics), the tape chosen sets the bias level. Once upon a time I could do a 'line up' - getting the bias level and equalization set for a particular brand of tape - on a multitrack reel to reel recorder in my sleep* but it's so long ago now that I'd have to pull the books out just to work out which knob did what.And forget 555 timers, this is definitely sine wave territory, in fact low distortion sine wave territory. It's time to learn the joys of trying to get an apparently simple sine wave oscillator circuit to 1) oscillate, 2) do so stably, 3) not generate umpteen unwanted harmonics.Just had a, very quick, look at that HP app note. Those BH curves are on page 2-3.This is one of those areas where you can learn a lot from 'the masters' by studying old circuit diagrams. I know for a fact that all the old Revox and Studer service manuals are available online, complete with schematics. By reading the circuit diagrams from several machines you'll soon get a feel for the basic 'how to' and after a while hopefully begin to spot some of the subtleties that make the difference between a design that simply works and a great design that works well.*And usually did. Studio maintenance is an 'off hours' activity and I'm definitely an owl, not a lark.