Signal path, setup:
Alan Parson Project CD - Sony portable CD player with line out - Pleiades Electra III v2 - Sennheiser HD580
The Pleiades Electra III is (per channel):
1 EF183 triode connected.
1 1μF coupling capacitor from CD player to a 47KΩ volume control logarithmic potentiometer.
1 1.6MΩ biasing resistor from anode to control grid to free electrons.
1 12V lead acid battery for series heaters and anode circuits.
The cathode is connected to battery minus, ground.
On the left channel the 300Ω headphone was connected directly as the load itself. Ie the battery plus was connected to the headphone common ground and the other side of the headphone coil directly to
the anode.
On the right channel a Sowter 8856a, (100H to 6H taps) output transformer was used in the anode circuit and the headphone connected to the 6H winding.
The music material was from the Alan Parsons Project definitive collection CD, Turn of a Friendly Card for example.
The sound quality was hard to believe. (When the other channel was connected to an output transformer the result was good but not as good as expected, see part 2 of the post).
The right channel with the output transformer sounded immediately about 2x as loud indicating an
increase in sensitivity, increase in power sensitivity by quite more than 2x from S.S. Stevens law.
The amazing smoothness of sound and high frequency detail was preserved.
The bass was better as it seems now that the reasonance of the headphones from driving them with relative high impedance is gone.
The inductance ratio of 100:6, approximately 16.6 means that the output impedance of the electron tube is reduced by 16.6.
Assuming that the impedance of the electron tube at the given conditions is 5KΩ, at the secondary of the output transformer it is reduced to (5,000/16.6)Ω or 301Ω. If this is the case, it is perfect for the input impedance of the Sennheiser HD580 headphones which is close to 300Ω.
And this gives maximum power transfer from the maximum power transfer theorem.
The output impedance has to be measured.
Next possible step is including a Sowter transformer to the left channel.
And also possibly winding 2 Pleiades transformers with large Magnetec Nanoperm Nanocrystalinne tape wound ring cores.
And also of course experimenting with cathode temperature for the best possible sound quality.
Part 2, next day.
The other channel was connected to the same type of Sowter transformer.
Sound quality and measurements:
A VTVM was connected across one channel of the headphones. After connecting the transformer the output increase is 3 dB ie there is a doubling in power sensitivity.
The frequency response goes -8dBm down at 20Hz and +1/2dBm at 20KHz.
This is possibly by the transformers saturating with DC current as they are not designed for that purpose and do not have an air gap. Using full instead of half of the primary winding the responce at 20Hz when up to -5dB but sound quality was more distorted. This indicates even more core saturation from the 2 mA DC current.
By direct stereo comparisons of directly connecting the headphones to anodes and through transformers the direct to anode connection was preferred. It sounded more open on HF and better on bass even having the disadvantage of +5dB broadly and centered at 70Hz from the headphones reasonance.
For these experiments air gaped cores have to be tried. Is there a way to have airgaps at tape wound ring cores?
The output transformers gave the opportunity to measure the output impedance of the triode connected EF183 at these conditions.
Using the 16.6:1 ratio stated above, when connecting the headphones showed a signal output voltage drop at 1 KHz of only 4dB. If 6dB was the drop (ie half the voltage) that would mean that the impedance of the amplifier is 300Ω. Only 4 dB means that the impedance is lower than 300Ω. And therefore the EF183 dynamic output impedance or ra is lower than 5KΩ.
(To repeat the operating conditions). They are. EF183 triode connected. Cathode connected to ground. 1.6MΩ between anode and control grid to free the electrons. 12/2 volts on heaters and 12 volts on anodes.
So the static impedance 12V/2mA ie 6KΩ gives a good indication of the dynamic output impedance which is lower than 5KΩ.
The transformerless version sounds fantastic. Possibly the most open and sweet treble ever heared.
It is extremely simple and inexpensive to make.
So at least for the time being the amplifier will stay as is.
The fact that an output impedance of less than 5KΩ is loaded by 300Ω does not seem to matter much save for the headphones reasonance.
On the Bang and Olufsen Beocenter 2600 power amplifier a series resistor of 220 ohms is used to drive the headphones from the loudspeaker output.
No comments:
Post a Comment