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DRAFT!! Page 1: january 11, 2010
In cooking, a syrup (from Arabic شراب sharab, beverage, via Latin siropus) is a thick, viscous liquid, containing a large amount of dissolved sugars, but showing little tendency to deposit crystals. The viscosity arises from the multiple hydrogen bonds between the dissolved sugar, which has many hydroxyl (OH) groups, and the water. Technically and scientifically, the term syrup is also employed to denote viscous, generally residual, liquids, containing substances other than sugars in solution. Artificial maple syrup is made with water and an extremely large amount of dissolved sugar. The solution is heated so more sugar can be put in than normally possible. The solution becomes super-saturated.
And of-course SRPP is an acronym for Shunt Regulated Push-Pull, but by adding some vowels I hope that my Syrupp will sound as sweet as Syrupp.
I´ve built several phono amps based on the SRPP design, but never came on the idea to use that for a headphone amplifier. Reading an article in Elektuur (special edition, dedicated it seems to amps of Menno van der Veen) and then continuing reading about his pre-amp that would match may tube amp UL40s2, I discovered that the headphone output of the pre-amp was based on an SRPP design.
Reading further on Tubecad about Broskie´s SRPP+ amp, he did somehing that I strugled with for a longer time. Taking the output from the anode of the lower tube or from the cathode of the upper tube sometimes made the difference and although I wondered why i did not come to the idea of using not one but two resistors (like Brodie did) between the cathode of the upper tube and the anode of the lower tube and connecting the output in the middle.
Since I still have the Headhunter amp on a piece of wood, it´s actually quite simple to use that Power supply for a litle experimental amp. Maybe in all its simpleness this amp would give the desired results.
What we have take care of though, is that the simple version will be suitable for one impedance headphone only. That´s because in the circuit described by Tubecad, the two resistors in the SRPP stage are calculated based on the impedance of the cans. And should be make a perfect amp that is capable of providing output to a number of different headphones or even a linestage, then we need a selector switch somewhere and adapt the values of the resistors accordingly.
So what are the best tubes (if tubes are doing good at all in an OTL design) to use in and OTL tube amp?
Compatible tubes with the board are of the 9A pin-out type. Suitable candidates for the 2nd power stage are tubes with above average power (current) output. A few well used types are:
| Tube | Va | Amplification | Current | Ri | Trans Conductance | Comments |
| 6N6P (6H6N) | 120 | 22 +- 4 | 28 | 1964 | 11.2 | looks like ECC99 |
| 6H30N/6N30P | 15 | 40 | 1974 | 18 | ||
| ECC88/6920 | 90 | 33 | 15 | 2640 | 12.5 | |
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ECC99 |
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| 5842 / 417A | 150 | 43 | 25 | 1700 | 25 | Single Triode |
| 6S3P |
Single Triode looks like 5842 |
Based on an Aikido design, there are not so many principle choices to make. The driver tube may be the most important. I´ve been looking at the octal tubes I have in stock as I like these tubes a lot. However, Apart from the 6H13C which has the same difficulties as the 6AS7 I do not have good candidates.
Broskie is convinced that this amp with a semi "SRPP" first stage and symmetrical loaded power stage has sufficient power supply rejection ratio so that the power supply can be quite simple. Regarding the heater supply I'm not quite sure. I'm convinded that DC gives best results and minimal humm.
Well, as you just read, the design of the amp itself is probably not going to be something quite special. But I need to do something special, and for me that means using a more exotic tube in the powersupply. I will use a voltage regulator tube, or more precisely two tubes, to regulate the power.
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