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Hidden Treasure: Bringing Out The Very Best of Dynaco's SCA-35

My SCA35 is long gone.
If I started from scratch again ( that is, non/ semi non working version needing a little to a lot of tlc) Dave's continuing evolution of it and Jon's nice casework would be a nice option.
Thinking aloud, I think I'd like 4 ohm as well. If the power jack had the built in fuse it'd save some space. I haven't had any tape decks in 30 years as well. I wouldn't lose sleep over no tape monitor.
Clearly each user has his own wants and needs.
 
PAS is easy, it already has a DC heater circuit. AC in the SCA.

I have a reel to reel but I don't record on it. I use my two tape jacks for a rumble filter and another input respectively.
 
The buffer and the line amp stage I designed are both op-amp based, and both do in fact operate single rail style from the output tube cathode voltage. Because the power supply rejection ratio is so high for the op-amps, a separate regulator is not even needed. The changes in voltage that appear at the cathode (increasing with increasing power output with the original design, decreasing with increasing power output with EFB) have no effect on the gain of the designs -- it only affects the maximum output voltage, which for the buffer is always at least 3.0 vac rms -- plenty for the Tape Output jacks.

The question then is not whether a buffer should or could be used or not, but whether Tape Output jacks have gone the way of the Blacksmith and video store. Are they really necessary anymore? Maybe not really, but still a nice feature to have?

With the modifications, the Filter switch is recommended to be turned into a Tone Control Bypass switch. On the other hand, the Stereo/Mono switch could be used for a Tape Monitor input, which would then really make Tape Output jacks useful. With the elimination of the Phono High and Ceramic Phono inputs, the jacks are there to make it work. That switch is also in the perfect position physically (and wiring wise) for a Tape Monitor switch. I mean, how often do we really need a Stereo/Mono switch anymore anyway???

Dave
 
I was thinking decoupling more to avoid any potential LF feedback issues if it got used as part of a tape loop. Output power swings the supply voltage, if any of that got into the op amp and made a similar low frequency shift on it's output, would it start feeding back on itself? Maybe it won't, dunno, was just a thought.
 
The tests I did showed that the op-amps, and the stability of the amplifier were completely unaffected by the power coming from the cathode bias voltage. The cathode bypass cap certainly acts to limit any coupling, and the EFB regulator (if installed) acts to limit any coupling even further.

Dave
 
nj pheonix said:
My SCA35 is long gone.
If I started from scratch again ( that is, non/ semi non working version needing a little to a lot of tlc) Dave's continuing evolution of it and Jon's nice casework would be a nice option.
Thinking aloud, I think I'd like 4 ohm as well. If the power jack had the built in fuse it'd save some space. I haven't had any tape decks in 30 years as well. I wouldn't lose sleep over no tape monitor.
Clearly each user has his own wants and needs.
Click to expand...

Thanks for your input. The SCA35 only offers 8 & 16 ohm taps. I normally use combo IEC w/ built-in fuse, but the clearance on the rear is pretty limited, so I'd rather opt for something with less depth.
 
spantou01 said:
BuzzK,

Good thinking and great ideas. But I have suggestions on two points.

1. I believe a lot of people will want the option to use 4 Ohms speakers, so, “Ground, 4, and 8 Ohm” outputs per channel (unless this is just for your personal use).
2. Are you thinking of using a small IEC C7 receptacle for power entry? No room for a C13 that will fit all those standard and exotic power cables out there? If not I would opt for the attached power cable using a nylon cord grip like these. https://www.elecdirect.com/media/specsheets/NPT-Dome-Cap-Cable-Gland-Connectors.pdf
Click to expand...

Problem is, the SCA only offers 8 & 16 ohm taps. I think if I went through the trouble to design and prototype one for myself, I might as well make some for others. :-)

I like the idea of a detachable power cable, but YMMV.
 
peterh said:
Don't forget that "tape out" will frequently be used to record with a computer. What is needed is some kind
of buffering.
Click to expand...

Peter -- I kinda like the idea of a buffered preamp out. Question: Would it be worth swapping that for one of the Aux inputs, leaving only two?
 
nj pheonix said:
Somebody made a buffer output for a pas3 using existing voltages and an op amp a while back . I don't remember the implementation :idea:
Click to expand...
Yes, i bought one and disassembled it. ( ebay 3 years ago)

It is powered by the +-12Volt filament rails, thus not applicable in the SCA35.
 
BuzzK said:
Peter -- I kinda like the idea of a buffered preamp out. Question: Would it be worth swapping that for one of the Aux inputs, leaving only two?
Click to expand...
You don't need additional RCA connectors, just a buffer to the existing "tape out".
The buffer ( FET or IC ) could be powered from rectified and doubled filament or from the cathode voltage
with proper filtering ) Nothing fancy needed, could be built in a lab board and stuffed at the RCS connectors.
 
BuzzK said:
Problem is, the SCA only offers 8 & 16 ohm taps. I think if I went through the trouble to design and prototype one for myself, I might as well make some for others. :)

I like the idea of a detachable power cable, but YMMV.
Click to expand...

I forgot it didn't have 4 ohm taps.
I don't remember seeing a 16 ohm speaker in over 30 years so it wouldn't be necessary for me.

I also forgot no dc filament to work with. Like I said it's long gone . Sorry about the sidetrack.
 
Pio1980 said:
I think the main point of interest is how common and available these are, compared to the Stereo 35 amp.
Click to expand...
I sort of have a pipe dream to build a from scratch ( an ST35) with the audio regenesis efb boards like njcanuck did a while back. Two things holding me back is lack of time and lack of need.
Still I have a yearning . Maybe after I retire and move.
I still have a set of eico st 70 iron that I intended to build one of the hotrodded 6550 versions that Gordon built. I don't need that one either. I really have to figure out what I will actually do and cut bait with every else.
 
A true preamp output/power amp input feature would be very difficult to do with the SCA-35. That's because a preamp is expected to (typically) exhibit a line/tone stage gain of 14-20 db, and output about 2.0 vac rms. This provides a gain of about 5-10, and an output voltage that will drive virtually all commercial power amps. At the point representing the output of the preamp section in the SCA (also the same point as the power amplifier input), the sensitivity is such that only 0.16 vac is required at that point to drive the internal power amplifiers to full power output. If you bring this point out to traditional pre out/pwr amp in jacks and jump them together for normal operation, then all if good. On the other hand, if the jumper is removed for use with external components, there would not be enough output to drive a conventional stand alone power amp, and any conventional preamp would have way too much gain for use with the internal power amps. You could solve that by adding a gain stage to bring the output of the internal preamp/control section up to a reasonable level, but then then you'd also need to add an mirror image attenuator to the input of the power amp section, so that when the jumper is reinstalled to produce normal operation, the gain would be normal again. But it all just starts to get away from the original Dynaco philosophy of simplicity and what the SCA-35 is if you go down that road.

The SCA is what it is. My effort here with the Group 1 modifications is to make it be the best it can be within what it is. Group two goes outside that box, and does include a buffer for the Tape Output jacks. In today's world, that represents a tap if you will into the sources selected by the input selector switch. Without complementing Tape Monitor jacks, the Tape Output jacks are of limited use today, but still might be nice to have.

For Buzz's work, it might be nice to have options to configure the new rear panel RCA jacks to suit a number of different possibilities according to each one's needs -- no Tape Output, Tape Output and a new Tape Monitor input, or the original plan of just providing Tape Output jacks. But again, where do you stop? Making up the modifications presented from this project weren't easy -- I see no reason why Buzz's should be any easier! :)

Dave
 
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