Technics SA-800 Pt. 2 - Power Select Circuit Madness

[@user-mv5bu2kk8b]

Thank you for the many hours of brilliant tech videos wishing you and your family a wonderful Thanksgiving 😊

[@rolfts5762]

Thanks for interesting schematic-La'electrones video Tony, Happy T.G. over there :)

[@Achim188]

Well, the descibed switching is obvious very complicated. At least there is more effort going into the switching than the signal path. Nowadays you would do the detection of the load and the switching by a cheap mikrocontroller. Actually I thought of switching the supply voltage in a headphone amp. Here the load varies between 15 and 600 Ohm!

[@sguttag]

Well, that was interesting.

[@johnfitzpatrick2469]

G,day from Sydney Australia. Very detailed troubleshooting with schematics. Q1. Can you spray anything in the Hz tuning pots? 🌏🇦🇺

[@Sans_Solo_]

In the marketing brochure, on this receiver, found at hifiengine....it talks about a lower THD number at half power and a different/higher THD at full power. Maybe during the receiver wars they were experimenting here with ultra low distortion power.....But wouldn't that be true of any amplifier - a lower THD at lower outputs? Later on Technics had the "new Class-A" amplifier typology.

[@xraytonyb]

That's just what I was thinking. It's all about the numbers.

[@kenielsen]

I'm only halfway through the video but felt for a quick comment: For silly ±10VDC the SOA of this amplifier must be really poor... If I owned this amp this "voltage function" would scream for a bypass😂... Back to the video👌

[@poormanselectronicsbench2021]

A lot of amps with smaller PS transformers will, run out of steam when trying to drive 4 ohm impedance speakers at full power. Amps with stronger transformers will usually have at least a 20% or higher output into 4 ohms from rated 8 ohm power. An example of a amp with a marginal transformer is an old Dynaco ST120, which drops from 60 WPC @ 8 ohms to about 45 @ 4 ohms, partially due to the transformer, and partially due to a power supply with a limiting / folding design to prevent damage.

[@poormanselectronicsbench2021]

It just seems that they designed a uber-sensitive cascading feedback mess to try to solve a problem that isn't really a problem. I'd doubt if designers like Bob's Carver and Cordell or Nelson Pass, would ever say "And now, I need to figure out how to limit the power into an 6.5 ohm load".

[@denbo1064]

Tony, just curious - did you try hooking up a single ‘raw’ 8 ohm driver? As we know, just about all loudspeakers present a ‘variable’ impedance to the amp (sometimes even varying with frequency) due to the woofers, tweeters, midranges and crossover components, etc. That’s why the majority of loudspeaker companies rate their impedance as ‘nominal’.

[@mdelabo]

Hello Why it is impossebel to translate the video on youtube? Thanks M from Belgium

[@jdmccorful]

Interesting sleuthing. Enjoyed the learning curve! Thanks! Happy Thanksgiving!

[@jjsant3250]

I think your "8ohm" speakers may be 6ohm since they measure 4ohms DCR. I would use ~5ohm resistors for the calibration. It seems that the writer of the procedure forgot that 8ohm speakers have ~6ohms DCR.

[@richardk7256]

I don't know why it's acting the way it is, but remember that when you measure the speaker's DC resistance, you're probably measuring the input to internal crossover and not directly measuring the voice coil. Not sure how would affect a direct DC resistance measurement, but it's something to think about.

[@paulb4661]

DMM gives you the correct value of series inductor's Rdc and Voice coil Revc and unless someone's gone mad trying to inrease alignment Qt and went for a high Rdc coil, the ratio should not exceed 1:10 ie. I wouldn't expect series inductor's or (inductors') resistance to e more than 10% of that of the driver and closer to 1% for the more elaborate designs. Of course, speaker wire resistance will become significant in the latter case. Hope that makes sense.

[@ibrahimkocaalioglu]

I agree it is over complicated poor design. Placing temperature varying resistors maybe an easier solution.

[@mbwatson1000]

Why not defeat that comparator circuit by adjusting it to select higher voltage mode with 4 ohm speakers, and allow the protection circuit to do its job if the current is excessive? I mean, if 8 ohm speakers can have DC resistance of 4 ohms, then presumably 4 ohm speakers could have a DC resistance of 2 ohms. It seems to me the concept is brain damaged because you cannot rely on the DC resistance of speakers giving an indication of the speaker impedance.

[@NiHaoMike64]

3D print a cam to put on the volume control shaft, then have a microswitch that flips the relay to full power when the gain is high enough to actually make use of the higher voltage.

[@poormanselectronicsbench2021]

I would consider re-adjusting that setpoint to about 3.2 ohms, as that may limit current / power going into 4 ohm impedance speakers, which may be a weakness of the power supply transformers in that receiver. Limiting the power to the most common impedance speakers on the market ( 8 ohm) is pretty ridiculous.

[@jeffadams5510]

Since most speakers are 8 ohms, what they SHOULD have done is simply have that relay switch when speakers B are selected-problem solved-duh-smdh..........