The amplifier "TRSAT-20" is result of thirty years of experience of author in construction of tube equipment. During the
period high goals were aimed for final result. The aim was to construct an amplifier with maximum bandwidth, less sensitive to
loudspeaker impedance changes, more linear...In short words, natural in audio reproduction.
The very first version was 300B + an output transformer. It sounded good, but...After many efforts with different transfor-
mers, included version with LF and HF transformers connected in series results were not satisfactory. All that was sounded nice
and pleased for ears, but sound was still "coloured". Sound was primarily depended of used output transformer. Unfortunately,
with all constructor's efforts the transformer is still the weakest component in this construction. Because of transformer's limita-
tion in frequency response and non-linearity it is impossible to obtain to make this amplifier to sound naturally. After all, the con-
clusion is only one. The amplifier mustn't have an output transformer.
However, it wasn't enough, at least the loudspeaker is not resistor. Its impedance is very dependable of frequency. Measure-
ments show that typical loudspeaker nominally 8 ohms has variable impedance between 3 ohms and 30 ohms.
If the amplifier has voltage negative feedback, it can be considered that constant voltage generator drives the loudspeaker.
In reality it means 10dB power difference. To eliminate this variation of power, it was necessary to eliminate this negative feed-
back. Yes, but, again but...New problems were raised as increased distortion and increased internal resistance of the amplifier.
Fortunately, these problems can be solved on another way. In theory, the simplest solution is "push-pull" connection, or in ver-
sion without an output transformer some version of the Cyclotron design. All these designs are generic low distortion. In these
designs even harmonics are attenuated, odd are doubled. The biggest component is distortion in second harmonic and this sys-
tem has less T.H.D. (Total Harmonic Distortion), but increased irritating distortion (odd harmonics). Furthermore, this circuit
has increased internal resistance because both tubes are connected in series with load (loudspeaker). From these, the question
was: "What's left?"
Distortion can be very effectively decreased (less than 1%) using non-linear diode attenuator. It's only way according the
author experience! To reduce internal resistance, it is necessary to use triodes with high transcondutance and low gain,
connected as cathode follower. Next request was high cathode current (2,5A @ 20W @ 8ohms). At market, this type of triode
doesn't exist, so it is necessary to connect few of them in parallel. It can be chosen between few types: 6080, 6333, EL519,
6C33. First two types have too low conductance, second two are pentodes. If used as triodes, trnsconduntance became too
low. The only choice is Sovtec's 6C33.
After all there is only one conception:
- Triode Single Ended
-No Output Transformer
-No Feedback
The amplifier is conceptually mono block. In each channel are 12 tubes (5X6C33 and 7XECC88). First tube is voltage amp-
lifier in SRPP circuit. This circuit was chosen because of an excellent linearity and low output impedance. Second tube, ECC88,
is connected as duo-diode and it is working as non-linear attenuator. Other five tubes 6C33 are working as drivers for voltage
follower. These are also connected in SRPP circuit. Output tubes 6C33 are working as voltage followers. Theirs cathodes are
connected in parallel for audio signal. Each 6C33 has separate bias adjustment. This is necessary because in first 50-100 hours
cathode current is very variable, even with paired tubes. After 100 hours of working, tubes becomes very stable and final adjust-
ment of bias can be taken. After this no further adjustments are necessary.
Output tubes are fed with DC via voltage very massive coil. It is an air coil, with maximum linearity. Total load of the output
stage is network consisting of coil, cathode resistors, coupling capacitors and loudspeakers. This network is keeping mostly res-
istive characteristic in very wide frequency bandwidth. Reactive component is very low so phase distortion can be neglected.
Because of this conception extreme wide bandwidth is obtained, over 2MHz at full power and almost 6MHz at 1W, tested
with colour TV signal amplified by this amplifier.
Very wide bandwidth, neglected phase shift and very low output dependence of loudspeaker's impedance is guaranty that
the amplifier will drive any kind of loudspeaker. This is very important for electrostatic loudspeakers. For these results primarily
are important an excellent Sovtec's tubes. Very impressive is 6C33. In technical data Sovtec is giving data for the worst sample.
In practice, these tubes are far better than given data, especially in internal capacities and maximum cathode current.
TECHNICAL DATA:
- Output Power @ 8Ohms 20W
- Bandwidth @ Full Power 10Hz - 2MHz
- T.H.D. <1%
- Input Sensivity 0,5V
- Damping Factor 2
-Slew Rate 400V/us
-Weight 25kg per channel
-Power Consumption 600W per channel
MUSICAL REFERENCE:
This amplifier is using in "MUSICAL ACADEMY INO MIRKOVIC", holder of license from Moscow conservatory,
situated in LOVRAN, CROATIA.
This amplifier will be made only on an individual request and agreement with author!
AUTHOR:
SLOBODAN SUPER
VIDIKOVAC 9
51000 RIJEKA
CROATIA
Tel: +385 (51) 400 971
Fax: +385 (51) 400 972
Mob: +385 (91) 540 3806
E-mail: slobodan.super@ri.t-com.hr