100khz-200khz Oscillator - any ideas ? - Page 1
Forum section: electronics / projects

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: March 17, 2022, 11:03:08 am
content: Hello, I am working on a project, a Reel to Reel tape deck of my own and right now I have hit the brick wall with a problem.Tape decks require high frequency bias. Its so that crossover distortion of the recorded waveform will be minimised, and also a lot of other factors that help THD and so on so fort during the recording. Furthermore it is required for erasure of the tape.The heads are essentially inductors with a given inductance. In my case, I will need two oscillators. One that will be powering the erase head to..obviously erase the tape, and the other one to power the record head. The erase current requirement is roughly arround 75mA RMS per system. There are two systems as its a stereo deck. So it has to be somewhat stable amplitude so when I switch off one system of the head it wont go haywire, and by switching off I mean literary disconnecting one side of the head.For the bias oscillator it would have to provide only a maximum of 1,5mA for each system. But this one has to be regulated absolutely perfectly so that when I switch one system off it wont raise the amplitude throwing off my bias calibration. Also the amplitude of the signal should be at leas 100V. I have a 24V power supply available in the design so far.I found one oscillator in a tape deck of mine. The design overall is working and rather nicely it tolerates changing the frequency, slight problem is that I have no idea how to change the amplitude of the output signal to better fit my needs. And stabillity is not a strong side of this contraption either (schematic attached)I wonder if anyone here has a better idea how to create these power supplies...I am open to winding custom inductors transformers and so on..Sooo 65V 100kHz for the erasing head (thats 2x75mA) and 100V 200kHz for the bias for the record head (2x 1.5mA), and if there is a possibillity for the amplitude to be adjustable by, for example a transistor switch, it would be amazing because I could then set bias levels for multiple types of tape and so on...Thank you a lot in advance.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2022, 11:55:35 am
content: The transistors T1 and T2 with the diode form a complentary emitter follower.T3 works as an amplifier in base configuration: so the emitter is the input and the collector the output. The resonantor is from C2 and the inductor in series resonance and this way can exceed the supply voltage for the voltage at the inductor.The impedance at the base is effected by the diodes D2/D3 and this way set the gain of the amplifier. More current through D2/D3 gives more gain for the amplifier. The diode D1 and C4 rectify the oscillation amplitude and do some feedback to D2/D3 to regulate the amplitude.

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2022, 03:05:43 pm
content: I have a vague recollection about an article in Wireless World sometime in the late 1960s about bias oscillators that emphasized the need for low second-harmonic distortion in the waveform. IIRC, the circuit involved push-pull BJTs in the oscillator. I'll try to locate the article.

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2022, 04:52:13 pm
content: It took me a while to find that article. It is by D Griffiths: "Low-distortion bias and erase oscillator",Wireless WorldJanuary, 1970, p 20 (24/162 of the pdf at the link below).https://worldradiohistory.com/UK/Wireless-World/70s/Wireless-World-1970-01-S-OCR.pdf