By studying a magnetic tape reel to reel electron tube recording amplifier schematic (example Ferrograph series 6) one observation is:
That the high plate resistance of the driving tube is directly connected to the recording head coil. So we have a high pass filter, rising in voltage at 6dB per increasing octave, defined by the output resistance of the tube stage at the inductance of the recording head.
This is exactly the same way the Pleiades microphone filter operates, creating a rising 6db responce defined by the microphone output impedance (usually 200Ω) and the Pleiades filter inductance. The R and L define the 3 dB cutoff point to compensate for the proximity bass increase effect.
Now back to magnetic tape recording.
We have a rising recording voltage characteristic, or constant current into the coil. The is the premphasis. Cutoff points are usually defined by the time constant in microseconds. For example the IEC standard for 7.5ips speed is 70microseconds. (Strictly speaking the curves defined by standards are playback characteristic curves, of course the same time constant must exist during recording).
It is very easy to calculate R or L from the time constant. We know from high school physics that Farad x Ohm is Second and Henry decided by Ohm is Second.
So inductance is time constant x resistance. L=TxR
So for a 200Ω microphone for T=70μS we need a Pleiades filter of
L=70μS x 200Ω = (70 / 1000000)S x 200Ω = 0.014 SxΩ = 0.014 H
So we need 14 milliHenries
An audio (input for example) transformer with 14mH primary inductance is very easy to make as it needs just a few turns for the primary.
So we have a characteristic similar to recording on tape. Then after the signal passes through some tube stages, (the preamp itself) we integrate or high cut to get back to flat frequency response.
As is done on tape playback amplifiers.
This process should do instantaneous peak limiting with rounding of the waveform from the high cut filter.
In fact a very similar process is used by Mike Oldfield to get his guitar sound as explained on an interview.
Since for the mic there is also the proximity effect to take care off we must stop integrating at the time constant of the proximity effect which depends on the distance for source. This may be done by adding a resistance in series with capacitor which is used for high cut at playback
Even for an omnidirectional mic this may about 300-380Hz. (For an MD21 a Pleiades filter of 82mH makes a very natural male singing sound).
The above example is for using the tube gentle curvature for peak limiting and increasing average level.
If we need to to do this magnetically as in tape recording we much arrange to saturate gently a transformer in the signal path. In constant current high pass way as explained above.
In the meantime how would a Pleiades V5 preamplifier sound connected directly to the high impedance output of a portable Sony CD player.
The time constant should be very small corresponding to what premphasis is used for speeds as high as 30ips and more.
A capacitor in series with a resistor must be connected across the anode resistance of the V5 output tube to get back to flat frequency responce with the added advantage of rounding any instantaneously peak limited waveform as on tape recording and playback.
References:
Sound Recording Practice - Borwick
www.261.gr
That the high plate resistance of the driving tube is directly connected to the recording head coil. So we have a high pass filter, rising in voltage at 6dB per increasing octave, defined by the output resistance of the tube stage at the inductance of the recording head.
This is exactly the same way the Pleiades microphone filter operates, creating a rising 6db responce defined by the microphone output impedance (usually 200Ω) and the Pleiades filter inductance. The R and L define the 3 dB cutoff point to compensate for the proximity bass increase effect.
Now back to magnetic tape recording.
We have a rising recording voltage characteristic, or constant current into the coil. The is the premphasis. Cutoff points are usually defined by the time constant in microseconds. For example the IEC standard for 7.5ips speed is 70microseconds. (Strictly speaking the curves defined by standards are playback characteristic curves, of course the same time constant must exist during recording).
It is very easy to calculate R or L from the time constant. We know from high school physics that Farad x Ohm is Second and Henry decided by Ohm is Second.
So inductance is time constant x resistance. L=TxR
So for a 200Ω microphone for T=70μS we need a Pleiades filter of
L=70μS x 200Ω = (70 / 1000000)S x 200Ω = 0.014 SxΩ = 0.014 H
So we need 14 milliHenries
An audio (input for example) transformer with 14mH primary inductance is very easy to make as it needs just a few turns for the primary.
So we have a characteristic similar to recording on tape. Then after the signal passes through some tube stages, (the preamp itself) we integrate or high cut to get back to flat frequency response.
As is done on tape playback amplifiers.
This process should do instantaneous peak limiting with rounding of the waveform from the high cut filter.
In fact a very similar process is used by Mike Oldfield to get his guitar sound as explained on an interview.
Since for the mic there is also the proximity effect to take care off we must stop integrating at the time constant of the proximity effect which depends on the distance for source. This may be done by adding a resistance in series with capacitor which is used for high cut at playback
Even for an omnidirectional mic this may about 300-380Hz. (For an MD21 a Pleiades filter of 82mH makes a very natural male singing sound).
The above example is for using the tube gentle curvature for peak limiting and increasing average level.
If we need to to do this magnetically as in tape recording we much arrange to saturate gently a transformer in the signal path. In constant current high pass way as explained above.
In the meantime how would a Pleiades V5 preamplifier sound connected directly to the high impedance output of a portable Sony CD player.
The time constant should be very small corresponding to what premphasis is used for speeds as high as 30ips and more.
A capacitor in series with a resistor must be connected across the anode resistance of the V5 output tube to get back to flat frequency responce with the added advantage of rounding any instantaneously peak limited waveform as on tape recording and playback.
References:
Sound Recording Practice - Borwick
www.261.gr
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