Dynaco SCA-80, design discussion - Page 1
Forum section: electronics / general

author: GeoffreyF
author_rank: Regular Contributor
timestamp: May 09, 2019, 02:04:27 pm
content: https://phonobar.se/fileadmin/_processed_/7/d/csm_Dynaco_SCA_80_Q_open__3__1781efbae5.jpgLook at the linked to image or some other for the SCA-80. You will notice two fairly massive capacitors which are part of the final output of the amplifier. You will also notice that the capacitors also have a wire around them to form an inductor, also part of the finals. Back in the day, I built a couple of these for friends.Here is a schematic.https://www.updatemydynaco.com/pictures/stereo_120_later_schematic.pngC7 is the capacitor in question and L1 is the inductor. I am not looking for a discussion of why the schematic is done this way, as it appears. However, I have always wondered if there is some electronic value in having an inductor and capacitor mechanically integrated. My suspicion has always been that this was simply a mechanical convenience, a handy way and place to locate/form the inductor. I wonder if there is some point to having an electrostatic field within a magnetic one.Please confine responses to exactly the last two sentences.

author: cvanc
author_rank: Frequent Contributor
timestamp: May 09, 2019, 02:45:19 pm
content: Quote from: GeoffreyF on May 09, 2019, 02:04:27 pmThis. It was just easy to make this way, especially as it was sold more often as a kit than factory assembled.

author: GeoffreyF
author_rank: Regular Contributor
timestamp: May 09, 2019, 04:47:47 pm
content: I agree but it would be interesting if someone can offer thinking on the other side of the coin.

author: floobydust
author_rank: Super Contributor
timestamp: May 09, 2019, 07:26:44 pm
content: The output inductor's B-field can induce voltages into nearby parts and PCB traces, wiring etc.Modern power amplifiers with low PPM distortion goals, designers found the inductor's radiated B-field was getting into parts of the circuit and causing distortion.Dr. Edward Cherry did a1981 AES paper on mutual inductance distortiona long time ago, covering the PCB traces and wiring. It woke up some designers to a side effect few consider.The fix is to rotate the inductor perpendicular to traces, wires and leads and keep it away from sensitive parts.That is all hard to do, so Dynaco just moved it away from everything.SCA-80 is same as the ST-120, 10 turns on the output capacitor. I expect ~1uH but the (old) capacitors may have some steel in them.

author: cvanc
author_rank: Frequent Contributor
timestamp: May 09, 2019, 11:11:08 pm
content: Quote from: floobydust on May 09, 2019, 07:26:44 pmWell, yeah, sure, but I don't think they were thinking about that when they designed this in the late nineteen sixties

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2019, 04:04:51 am
content: The conductive shell of the aluminum electrolytic capacitor acts as a shorted turn but I do not know if the effect is significant. The big capacitor makes for a convenient coil winding form.

author: floobydust
author_rank: Super Contributor
timestamp: May 10, 2019, 04:50:23 am
content: Dynaco manual "... The output capacitors also serve as convenient forms for small value air-core chokes in the output which, in conjunction with an R-C circuit, roll off the response inthe HF region (above 500,000Hz), reducing interference and affording absolute stability under all circuit conditions."Dynaco is particular about winding them clockwise so I gave them benefit of the doubt on that being non-cosmetic. I can't see a shorted-turn, and it's supposed to be 10-turns/2" dia. cap but people just used all the kit wire they had or something. Some pics are like 20 turns.Silicon-chip magazine has moved the output inductor around and away from the input diff amp. Ultra LD amplifier, one rev. was flat on the PCB and another perpendicular, so it does affect distortion at higher output currents through mutual induction.

author: GregDunn
author_rank: Frequent Contributor
timestamp: May 10, 2019, 04:53:38 am
content: I actually had the opportunity to meet and talk with Ed Laurent, the man who designed the SCA-80, ST-80 and ST-120 (as well as several other Dyna components).The main focus on their topology was to simplify the design and make sure it was as reliable as possible. They spent a lot of time having people build the kits, provide feedback, and incorporating that into the production units; additionally, listening tests were a vital part of the approval process. I didn't specifically discuss the Zobel (R-L output) network design with him, but he said that this particular series of amp was designed with performance and simplicity in mind; I believe the use of the cap for a coil form was simply a way to reduce parts count and save space without affecting performance. I'm comfortable with saying that they didn't specifically use the capacitor shell to determine or calculate the inductor's characteristics, especially since they were typically basic Sprague aluminum cans. The prototypes probably were wired, tested in circuit, measured and then reproduced in the production line.The cap was there in order that they could use a single-ended PSU; and in case of an output failing, it wouldn't throw DC on the speaker. The cap was put inside the feedback loop to minimize the effect on output impedance. Remember, in the early 60s when this was designed, power transistors were not as reliable as they were just a few years later, and adding a reliable protection circuit would have added significantly to the amp's cost. It was basic.The -80 and -120 did undergo engineering revs to prevent HF oscillations in the driver and output stages, but these were unrelated to the capacitive-coupled outputs or the Zobel network. I got that info from Ed and their service manager Jim Elliott.

author: GeoffreyF
author_rank: Regular Contributor
timestamp: May 10, 2019, 02:20:35 pm
content: This is why it's still fun to think about it. For another day, to try some experiments and see if there is an effect, even one unintended by the Dynaco designers. I will also say that the manual is just giving the builder a reason. It's not necessarily electrical theory. At Audio frequencies, probably does not matter but ...https://en.wikipedia.org/wiki/Displacement_current

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2019, 05:59:47 pm
content: Quote from: floobydust on May 10, 2019, 04:50:23 amThe shorted turn is the conductive aluminum can of the capacitor itself; the effect is to lower the Q of the inductor which is likely desirable anyway. Usually a low value resistor, say 2 ohms, would be in parallel with the inductor to lower its Q but it is possible that the shorted turn from the capacitor can makes this unnecessary.

author: floobydust
author_rank: Super Contributor
timestamp: May 10, 2019, 06:52:43 pm
content: Hmmm, I can't see a shorted turn in a solenoid. The capacitor is like an aluminium pop can inside a solenoid coil but with no metal top cover. Flux is perpendicular to the cup. I don't know what I'm missing. Pic is of eddy currents.I blew up a ST-120 while the amplifier power was off.Connecting speaker leads, I think the output coupling capacitor discharged back into the output stage, thump! and turning on power, the channel was dead. I think it open-circuited an output transistor. It's a quirk working with these that C7 can be charged to 35VDC and backfeed into the circuit for some time (R9, R10) with power off, without two protection diodes.

author: duak
author_rank: Super Contributor
timestamp: May 10, 2019, 08:25:47 pm
content: Floobydust, the shorted turn would be the can of the capacitor directly underneath the coil. I don't think the bottom of the can makes much of a difference - it's the cylinder formed by the sides that forms a one turn secondary of a transformer.I grew up with power amps like this. I remember seeing woofers pump when power was applied - the output cap had to be charged to half Vcc.

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2019, 12:49:02 am
content: Quote from: duak on May 10, 2019, 08:25:47 pmExactly, the outside of the can is a wide strip of metal parallel to the windings or a shorted turn. It opposes any change in magnetic field strength. Some transformers include similar structures to control the magnetic flux.I suspect the benefit of using the capacitor for the coil form was accidental but the principle was well understood when these amplifiers were designed.

author: floobydust
author_rank: Super Contributor
timestamp: May 11, 2019, 06:05:26 am
content: Yes, I get the capacitor's bottom and the chassis steel bottom (probably the top too) would see some flux.But I figured it's over 2" away and not important. I'd have to do the math and see what 3A RMS does thorough air that far away.On the other question, I can't see displacement current and electromagnetic waves being important at these low (audio) frequencies.It might bring up some audiophoolery though.I do find an output inductor important in a new way. Loudspeaker wiring and crossover-network inductors are a terrific antenna.An output inductor attenuates RF from getting into the amplifier and rectified into beeps and boops. AM radio, cell-phone and 900MHz is a real bear to get rid of.

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2019, 01:13:48 pm
content: The inductor has nothing to do with audio frequencies. It prevents reactive loading of the output stage transistors from causing local parasitic oscillation at 100s of kHz. Without it, there is often a length of speaker cable (transmission line) which will cause the output transistors to break into oscillation and destroy themselves.

author: floobydust
author_rank: Super Contributor
timestamp: May 11, 2019, 10:35:04 pm
content: The output inductor does play a role at audio frequencies - for amplifier stability driving capacitive loads.Douglas Self says the Zobel network (Boucherot cell) is for stability, to prevent VHF oscillations with inductive loads.Dynaco's feedback network is a little hairy, by including the output capacitor - which is surely trouble with ESR and DA, and LF rolloff.

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2019, 04:38:33 am
content: Quote from: floobydust on May 11, 2019, 10:35:04 pmC8 and R18 are the Zobel network which compensates for the expected (inductive) load characteristics. A regulator's output capacitor serves the same purpose.L1 prevents VHF oscillations due to capacitve loads which includes the transmission line connection between the amplifier and speaker. Usually L1 would include a damping resistor in parallel but like I said, it is possible that the shorted turn produced by winding the output inductor around the output capacitor serves the same purpose; it will certainly lower the Q of the inductor. L1s effects are significantly above the audio band.Essentially this gets back to the emitter follower negative resistance oscillator. The low impedance reflected back through the transistor creates a negative resistance so if the loss in the parasitic elements is lower, it will oscillate. The same methods of preventing oscillation there work here.Power MOSFETs are especially susceptible to this problem because of their higher capacitance and higher frequency response.QuoteThe output capacitor has very little effect because feedback is taken after that point.