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Re: Dynaco SCA-35 phono input stages

192.169.41.42

Bob, this was some work I did on my 35 some couple years back.
Having lived with my SCA35 for some time now, I have always been less than satisfied with its phono section. The power amp part is fine, especially after suitable mods, which really makes this little gem shine. I finally got around to measure the freq response on its RIAA curve and found that there were some serious deviations :

freq gain (dB) RIAA standard deviation
------ -------------- ----------------------- ----------------

20 Hz 13.5 dB 19.3 dB 5.8 dB
50 Hz 15.2 dB 16.9 dB 1.7 dB
100 Hz 13.0 dB 13.1 dB 0.1 dB
200 Hz 9.5 dB 8.22 dB 1.28 dB
500 Hz 2.5 dB 2.65 dB 0.15 dB
1 KHz 0 dB ref 0 dB 0 dB
2 KHz -2.5 dB -2.59 dB 0.09 dB
5 KHz -8 dB -8.21 dB 0.21 dB
10 KHz -14.3 dB -13.7 dB 0.6 dB
20 KHz -23.1 dB -19.6 dB 3.5 dB

Also, the gain at 1KHz was about 52 dB, which I consider mighty high.
I hunted around the web looking for phono schematics, with the intention that with minimal effort, I should be able to improve the Dyna's phono. Well, after listening to a friend's Dyna PAS 3, and having studied its schematic, I came to the conclusion that here was something worth copying. Bothe models use the 12AX7 for the phono stages and both use 1 tube (2 triodes) for each channel. The major difference was the way feedback was implemented, and the fact that in the SCA35, the tubes also double as amplifiers for tape heads (which I think is now quite obsolete). I was also attracted to the idea of modifying the "tape head" part of the selector as a second phono input, thus allowing me to use 2 turntables! Well, if you are with me so far, on to the mods. BTW, several schematics that I got from the web for the PAS phono section showed a grave error in the feedback network. After consulting the original manual, I corrected the mod and everything was fine. It will be neccesary also to have a copy of the SCA 35 schematic, as I will be refering to part designations from there. Here goes :

Use 1/2 W resistors, and polypropylene caps if possible.
Only one channel (the right) is mentioned. The other channel's mod is identical.

(1) replace resistor R11 (47K) with a jumper wire
(2) remove cap C4 (330pF)
(3) replace cap C1 (0.1uF) with a 10 K-ohm resistor
(4) remove resistor R1 (1.2M)
(5) replace cap C2 (22nF) with a 0.05 uF (400V) cap (nearest currently available value would be 0.047 uF)
(6) replace resistor R3 (5.6K) with a 47 K-ohm resistor
(7) remove resistor R10 (100K)
(8) replace resistor R4 (120ohm) with a 1 K-ohm resistor
(9) replace resistor R5 (120ohm) with a jumper wire
(10) replace resistor R8 (390ohm) with a 1 K-ohm resistor
(11) now make up a feedback network to be connected between pin 8 of the 12AX7 tube and eyelet 14. This network consists of 2 resistors and 2 capacitors. The 2 resistors are connected to each other in series. Then ONE cap is paralleled with EACH resistor. The resistor values are 4.7 meg-ohm and 100 K-ohm, with the 4.7Mohm connected to pin 8 of the tube, and the 100 K-ohm connected to eyelet 14. A 2700 pF cap is paralleled with the 4.7 M-ohm and a 820 pF cap is paralled with the 100 K-ohm resistor. A copy of the PAS 3 schematic would make the explanation easier. Anyway, it should look like this :

--------------4.7 M-ohm-----------------100 K-ohm-----------------
| | |
|-------2700 pF-----------|---------------820 pF----|

The left end goes to the tube and the right end goes to the eyelet.

(12) Finally replace the wire that connects eyelet 14 with pin 2 of the selector switch, with a 0.1 uF cap.


There you have it. With the proper schematic, it can all be done in less than an hour. I measured the freq response of the new phono stage, and deviations were less than 1 dB throughout the 20 Hz to 20KHz range, with most of it within 0.5 dB of the RIAA curve. Plus, the gain at 1KHz is now down to a more reasonable 42 dB, with slightly less hum to boot.

And how does it sound? In a word, stellar. Great improvement over the original circuit. Bass line is firmer, thanks to a much tighter adherence of the RIAA curve. Try it. Aftre the mods are reversible if you don't like what you hear (unlikely!).

I also added one last mod to turn the "tape head" input into a "phono 2 " input to accomodate my 2 turntables. Simply remove resistor R9 (18 Kohm)! However this will only work if the above phono mods have already been done. Also, if you want a 47 K0hm input impedance for the second phono input, wire a 100 K-ohm resistor at the "tape head" input socket to ground. This will parallel up with the built-in 100 K-ohm to give you a 50 Kohm impedance. Good luck.

Adrian Chan
(Singapore)



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