Dyna SCA-35 Especificaciones

1A New Look At an Old FriendImproving the performance of Dynaco’s SCA-35 & ST-35 amplifiersBy David C. Gillespie Rev.2In 1965 I was 13 years ol

10New Output Stage Operating ConditionsThe development of the EFB circuit provides new artillery in the arsenal of weapons to achievereally great ampl

11conditions in both channels. We have seen what that compromise produced: under-biased tubesat quiescent conditions which shortened their life, and o

12body, and then soldered stranded, colored, #22gauge wire to each terminal. This allowed theterminals to then be easily extended and con-nected to th

13The second point is obscure, but relates to a feature that Hafler always championed. The DynaBiaset resistors in his amplifiers serve as a current m

144. The 100 mfd/25v cap on the output of the EFB cathode regulator circuit is actually the originalcathode bypass cap used in the Dynaco design. Its

15except at the very lowest frequencies, where compliance for THD was in line down to 25 Hz.Below about 22 Hz (for my particular transformers), core s

16increased, but flattened as well. In short, it has fully succeeded in achieving the original goal ofallowing both channels to perform at the same pe

17Since EFB provides not only fixed bias operation, but also the same distortion reduction abilitiesas does operation from regulated supplies, it is c

18to each power amplifier module as well toimplement this change. The original bias potwould then be used to bring the bias voltage inrange, and then

2In classic Dynaco fashion, the Z-565 transformers (at left) used in the SCA-35 are excellent performers. The best examples are capable of delivering

3As power output from the driven channel increases, it draws more current from the powersupply. This drops the available B+ somewhat – to both channel

4Earlier, it was shown that when a single channel was driven and measured, the idle channel actu-ally helped contribute to the excellent performance p

5stereo operation, current draw conditions in both channels would rarely be identical, so giventhat understanding, the common cathode connection would

6deviating from that approach really takes away from what the original designers intended it tobe: a clean looking, simple, but high performing integr

7So existing efforts to regulate cathode bias are problematic at best or destructive at worst,because the problems they solve are done so at the expen

8the SCA-35: With reduced B+, the EFB regulator reacts accordingly to maintain it’s set relation-ship, so the lowest distortion operating point is aut

9Figure-2. Complete schematic of the SCA-35 modified with EFB. Additions are shown in yellow. Cathode resistorto be removed is circled in red.