At your risk and at your responsibility.
Με δικό σας ρίσκο και δικη σας ευθύνη.
Such circuits can create electromagnetic interference.
Τέτοια ηλεκτρονικά κυκλώματα μπορούν να προκαλέσουν ηλεκτρομαγνητικές παρεμβολές.
The Pleiades one electron tube EC92 FM transmiter sounds better but it is much more bulkier, heavier, power consuming. For this transmiter see nearby posts.
Later addition: if the varicap diode is biased to its linear part of capacitance reverse voltage characteristic better sound quality should be possible. See next day's posts on how varicap biasing is done on the schematic of the electron tube oscillator.
One day, walking by Dr. Smith's (vacuum devices course) laboratory at Warwick university's physics department...
The door of his lab was open as always.
So naturally curiosity led me in.
He had been building an amplifier which looked almost like a strange insect. All components soldered together without box, without circuit board. It was like a small paper after wrapping it in your fist before throwing it to the bin. A 3D amplifier.
When asked why, he casually replied that he wanted the highest possible bandwidth. So stray capacitances or inducstnces should be minimized by shortest possible leads, no box capacitance etc.
How high was the bandwidth?
It can not remembered but maybe the answer to the high side might had been on the GHz area.
But it is very clearly remembered when he was asked if he can measure how low the low cutoff frequency is, he replied...
You can't live long enough to take a measurment.
It took some time to understand the answer.
But then it was realized that if the low cutoff were for example 0.000001Hz that would mean the period (f=1/T) or time from crest to consecutive crest of a sinewave would be 1 million seconds.
If frequency is much lower, it can be years.
So we burst to laughs.
He needed very low frequency response so that he could measure response of materials at such low frequencies.
Many years later on,..., inspired from this way of constructing an amplifier, a small 2N2219 FM oscillator had been built by a soldering components in such as way.
It worked but it did not sound as good as expected and as always with such projects, frustration got in and the little bug was abandoned in a special FM drawer.
But today after so many more years in worked great and sounded as big or bigger than a commercial radio station. With nice treble detail too.
The only thing that needed to be added was a Pleiades high pass filter, an (R,C) filter, to emphasize treble or reduce bass or mid heaviness so that sound is correct on the de emphasized commercial FM radios.
It is a similar situation to RIAA treble boost on cuting of vinyl records for reducing HF noise.
This baby transmiter is very simple and operates with just a 9V battery.
No input transformer is needed as full FM modulation can be achieved by the line out of a CD Walkman for example.
Signal path, setup:
I Want to Know What Love is - Foreigner CD - Sony CD Walkman line out - Pleiades (R,C) filter - one 2N2219 transistor FM oscilator - no antenna - JVC Nivico vintage radio 9425W - Pleiades linoleum made Auratone 5C type monitoring speaker with Tesla 5in fullrange drive unit
The R,C treble boost or high pass filter is just a 160KΩ resistor in parallel with 390-470 pF.
The FM modulated oscilator consists of a 2N2219 transistor in grounded base configuration as follows:
A circuit similar to Jostkit HF65 but no input transistor is used. A variable capacitance diode or varicap is used instead.
There is a BNC female chassis connector so that the likes of Orban vintage analog optimod pressesors can be used but it may not be needed. The simplest the signal path the best the sound quality can be.
This BNC is the input of the FM exciter. A very small core coil or solenoid, around 25 turns is used to bloc RF in or out. After this choke a 10KΩ resistor is connected (in series). After this resistor s rectangular small varicap (dot mark at thus side) is connected. After this resistor a small 10μF 63V capacitor is also connected at its negative terminal. The other side of said electrolytic capacitor to transistor base.
Other side of varicap is connected to ground or metal part of BNC female socket. To this ground are also connected: Batterry minus, ie battery holder black lead. An 100Ω emitter resistor the other side of which is naturally connected to transistor emmiter. A 10KΩ whose other side is connected to base.
To base is also connected another 10KΩ whose other side goes to battery plus terminal. Use a fuse in series with any battery for fire hazard safety. Each of these 2 10KΩ base biasing resistors which form a potential divider have a 470pF ceramic capacitor in parallel. So base is effectively grounded base to ac signals. So oscillation can take place by positive feedback from collector to emitter by the use of a 3.9pF capacitor from collector to emmiter.
Frequency of oscillation is dictated by a tank resonant circuit connected to collector. It is a coil, diameter 1cm, of 4 close turns of tinned wire in parallel with a small variable capacitor of 20pF. The other side of this CL (equivalent of spring and mass) is connected to plus supply (remember battery fuse).
This nice little transmiter which sounds rather very nice was held on hand while writting this post.
No comments:
Post a Comment