SAE Mark IIICM Amplifier - Part 2 - Making it Right

[@Bhamgurl1]

No way man I love that paper I love that noise it makes that's why I watch your channel honestly. Shame on anyone who goes against this. It's what schematics are for

[@hestheMaster]

Very impressive repair work along with troubleshooting on the fly. Tony, you know your stuff!😍

[@johnreel8944]

Tony, What a masterful group of supporters to remember and share ideas. I picked up some new insights so... I continue to learn and now have two books to enjoy.

[@heinrichlubbe488]

Excellent work as always, I love to watch your work Tony. Well done!

[@BobPegram]

Quite a job, but you were up to it Tony! Lessons learned: 1) BEWARE the guy.who was in there before you! 2) Don't believe everything the seller tells you - heck he may not know much about it! 3) Ultimately, a pox be on manufacturers who don't document their designs and especially those who don't tell you how to repair their products adequately!

[@BobPegram]

I will say this for SAE, that's a tough little / BIG amp to work at all, the way it had been butchered! Thank you again for this video series, I learned a bunch.

[@MarcelHuguenin]

I liked this series of videos on this amplifier a lot (including the short one on the cap discharge box). Interesting insights from start to finish. You really fixed it to better than factory condition!

[@PeterMilanovski]

There's two different types of technicians, the first time that I came across this channel, I kinda found it boring but I didn't understand at the time what was actually going on back then and transistors were an anomaly to me, I was taught that they are like switches and that's basically how I viewed them. Then I came across "Steel Wheels Down" channel and his description of transistors changed my life forever! Sadly he's no longer with us, cancer has taken him! But his channel lives on to teach people about Electronics, his last video is sad, he knows that he hasn't got much time left so it's kind like a good bye.. I highly recommend to anyone that hasn't seen his videos yet to check it out! Since then, everything on this channel made complete sense! This sort of work takes much longer than the other type of repair style, anyone who has seen the 12V Videos channel should have seen the difference, both channels have a type of customer that they service. Those that have something special and know it come to people like Tony, it costs more to repair but it's going to be done right and it's going to be as close to the new condition as possible, you will possibly only ever have to see Tony once for that one job. The next time it will be for another piece of equipment... 12V videos on the other hand, Pop's the case and with his many years of experience dives straight to where the failure is really fast! I remember screaming at the screen, wait wait wait, check the voltages! Power supply first! But he's servicing a different type of customer! His customers are not interested in making their devices last for ever, they just want it working for as cheap as possible because it's cheaper than buying a new one! In a way, he's keeping that piece of equipment out of the landfill for a while longer.. sure he's going to get it working, but something caused the failure of the dead component in the first place and even though it's now working, it's going to fail again... We get to see an hour of Tony working but the reality is that it probably took twice as long if not longer... I enjoyed every minute of every single video including all the noisy schematics, sure it could have been fixed faster, just replace what's broken and send it out, but as we saw clearly in this video, it would have came straight back with more destruction.. it takes time to get it right. Everything has to be double checked twice if necessary... I'm now used to the different ways that technicians work, and I understand that depending on how I feel about my equipment will dictate who I take it to, I have had bad experiences in the past with having equipment repaired, it would have been so much better if I was told in advance that I had a choice of how I wanted it to be repaired, or if they didn't do full restorations and that I would have to go somewhere else if I wanted that. They would just take the job, replace the obviously broken component as hand it back for a fee! 6 months later it's dead again! My electronics have had a lot of time put into them from the standpoint of research, I made the purchase to keep it forever, if something ever did go wrong with it, I want someone like Tony to work on it, but before KZbin was a thing, you only had word of mouth, oh he's really good and doesn't charge to much! Finding someone who works like this was impossible... At least where I am. And even today, I couldn't recommend anyone! I had a Denon cassette deck which had a failed 5v regulator, took it to the local hifi store (this was about 5 years ago) and they organised someone to fix it for me, $120 later it's back in my hands and it works! Great, but 6 months later it failed again and it was the exact same problem, I'm not going to call on him for any repairs ever again.. my Denon aint going to the landfill yet, so I just packed it away and put it aside and just got another working deck, later on, I learned how to fix it myself after watching a hell of a lot of videos, reading more books than I probably did whilst I was at school, I'm sure my teacher's would be proud of me if they can only see me now. In 5 years time, I have learned so much, I understand both solid state and Vacuum tubes... I can repair, design and modify.. KZbin might have started with rubbish videos but it's now become the best way to learn anything apart from self experiment.. lessons are hard earned in the later case that linger longer in the memory LoL... So thanks Tony once again, I appreciate your time and effort much more than even you realize...

[@nickkanderis630]

Hi Tony, I've been following this series with some interest,. Back in the mid-seventies I was lead bench tech at a high end audio sales and service center in Queens NY. We were an authorized warranty repair center for SAE, Phase Linear, Bose, B&O, Nakamichi etc. We had it comparatively easy, as we had the manuals and the parts directly from these companies, so it was a matter of simply identifying the bad components and reaching into the manufacturer supplied parts and replacing with the exact same part numbers. Something the previous repair tech on that SAE obviously did not have access to....lol Very nice work you did! Those high power amps could be VERY tempermental. Phase Linear 700 amps for example...350W RMS per chan--DIRECT COUPLED. Yep. put in DC, and you get DC out...lots of it...also tended to be a little unstable, like breaking in to oscillation at 100Khz at 350W....It wasn't pretty when it happened! The amps, and especially the speakers did not like that at all. But, I have to say, nothing sounded as nice as those big power amps, and good speakers. Anyway, just wanted to say STELLAR JOB!

[@1941lynnslb]

This video shows extreme professional talent in action. Installing the mods all made sense, for the proper affects. When someone has previously worked on an amp of this quality, and the seller may not have known what was done, that's a big red flag. Tony learned something about the darlington transistors being used in the output stage. The way that Tony was careful to know where the position of each output was supposed to be was excellent on his part. Enjoyed all three of the videos he made on the amp. We have personally repaired a McIntosh 2100 and that is a different output circuit, I believe a completely complimentary circuit. We checked the output transistors using the voltage drop voltage between the emitter and base to match the two drivers within a few thousands of a volt, then all the others as close as we could. We heard from the Dr. that owned the amp a couple weeks later and he said it sounded better than new. I was taught that the turn on point i.e. was how to match transistors.

[@mackfisher4487]

Tony great series, read the other comments I can understand the purest view of following the original engineers train of thought. But when you saw a potential problem explain it to the owner and received permission to make said modifications, that to me is a mark of a good technician who can send the unit back to the owner with a clear conscience. Good job.

[@nickpopa7260]

Another job well done. It is a real pleasure to follow you. I look forward to the next work. I hope it will be just as interesting. Congratulations!

[@kev-the-windsurfer.]

Excellent job!! I like the soft start circuit mod.....So often "the guy before us" has a lot to answer for, it seems I am forever putting things right and back to factory or better.......Interesting amp too....

[@petedazer3381]

Nice, I learned quite a bit! Thank you!

[@kylesmithiii6150]

The soft start is a fantastic idea! Please let me know if (and if not, why not): i) you installed a diode in parallel & reverse to the soft start relay's coil; ii) you installed an arc protection capacitor (e.g., 0.022uF/400V) in parallel to the soft start relay's contact, i.e., in parallel to the 15 Ohm/25W resistor in series with the trafo's primary wire. Thank you.

[@xraytonyb]

The purpose for a freewheel diode across the relay coil is to suppress the back-emf of the coil when the relay turns off. This can cause damage to semiconductors, such as transistors, as this voltage can exceed the max voltage of the transistor. As this relay is only connected across a capacitor, it won't make any difference. In addition, the dropping resistor will dissipate any back-emf from the relay coil. Thanks for the comment!

[@Vintage_USA_Tech]

Yeah, when I saw the clipping at 60 wattish I was like damn Bob was right. Damn good memory Bob wish you were my uncle. In my experience, all the schematics I ever looked at used a Darlington transistor symbol on them but bad schematics used to be the bain of technicians and we would have to make our own notes. But xrayTonyB is the best living example of what benchwork was like back in the 70's and 80's it was an awesome time to work in this field. Thanks, Tony.

[@kylesmithiii6150]

Thanks for the video, it is very good! I always learn from you. Thank you. I had a problem with a speakers relay too and overlooked that during my troubleshooting, only to find that out in the end. Lesson learned.

[@jcamp3606]

Fantastic clean work man!👌 Great tech right there

[@Chrissy4605]

Sadly this video would not start for me. Now after waiting almost an hour ZI have seen and heard Tony's excellent commentary!!!

[@sharepointpro479]

Awesome Tony, Thanks for everything you do!!!

[@johnnytoobad7785]

Great lecture on how to replace/upgrade an "unknown" relay.

[@14.1guy2]

Overkill on that. No need for the dropping resistor unless the driver circuit can't deliver an additional 10 ma of current - the relay will draw what it wants.

[@jeanpierre3193]

Another great job Tony,very informative video ,your repairs is awesome! Thanks to France!

[@jameswortley2515]

What a great series. And for the paper noise it never bothers me. Maybe try using a chalk board next time, that should get them going.😁

[@bob-05it91]

If I remember correctly the 3cm could do over 325 watts at 4 ohms. Around 1976, SAE came out with the 2400 and advertised is as the " $750 Alternative" which was $250 less than the 3cm , but I don’t think that the 2400 had the same current capability. I think that they took some stuff out of it to lower the cost, and they came up with a new look that most people preferred. My unmodified 3cm has been running for 45 years and was tortured for the first 2 years of its life before our band had the money to purchase a real PA system. It never blew up on the road and people readily did the "thumb" test on the inputs (we didn't know better back then). The protection circuit was disabled (we couldn't have momentary clipping take the vocals out for 5 seconds +) and one night the fan that we used to cool it got knocked over and thing got so hot that you could't touch it. It survived all of that and then blew up in 85 about 9 years later. I use it today for the front left and right of a home theater system. It still runs great with the original Bongiorno design. I talked to Jim around the year 2000 back when he was rebuilding SAE amps, but I decided to wait to see how long it will take for this thing to blow up. I am now retired so I have plenty of time to rebuild it , if it actually goes before I do. To me there is no need for soft start. This is part on the character of this amp. Part of showing it off in the 70's was showing people how it dimmed the lights. The current limiting is provided by the branch circuit wiring from you breaker panel.

[@bob-05it91]

@Phil Allison the first mk 2400 had the nice meters www.vintageaudioaddict.com/sae_2400/sae_2400.htm

[@johnnytoobad7785]

IMHO: Soft start circuits are "mandatory" for every amp I build (or will build) regardless of power output. I even built an external one to feed my three (soon to be four) external PSU's for my "add-on" DYI Audio projects. It really extends the life of equipment. I use 24VDC. You can build a "generic" soft-start (driven by the AC line) using a standard (400v, 4amp) bridge-rect, 1000uf 50 VDC cap, and two 1/2 watt 24 volt zeners, (makes the 24 DC power source) a 220 ohm (2 watt) current limit (from the AC hot) and a 1uf (250volt!) cap in parallel with a 1meg (2watt) resistor. The 220 ohm is connected between the AC and the 1Meg/1uf in par. The other end of that par. circuit connects to one AC input of the bridge. The other AC input of the bridge connects to AC neut. The 1000uf and the 2 zeners are in par. across the DC output of the bridge and drive the relay. You can tweak the time delay by adding series resistors to the relay power leads. For a 24 relay with 600ohm coils, I used 2 100 ohm (1/4 watt) res. One on each relay power connector. This gives about a 2 sec. (long) delay. The relays I used "trip" at about 18 volts. (purchased from JameCo) Other than the fact that the high voltage cap is a bit expensive..this circuit is easy to build, takes little space and is very reliable over time. You can stuff all the components in a small residential electrical junction box. The actual relay connected current-limiting resistors are 4*200ohm (5 watt ea) in par. I got this circuit from Randy Sloane's..Audiophile Project source book. It is *NOT* documented anywhere on the internet. The par. RC circuit (connected to AC-hot) takes advantage of the capacitive reactance of the RC network at 60hz. So the circuit itself takes very little power. I even "modeled" it in a circuit simulator program so I could examine voltage and current levels.

[@4arocker]

Excellent videos on the series with the SAE amplifier. Ironically, if SAE had built that protection in for the inputs at the factory, the cost would have been not too high and I'm sure many consumers wouldn't have had an issue with the minor cost increase.

[@markweingart7636]

Tony that was a masterpiece ... Thank You !

[@barrysheridan9186]

Very interesting Tony, thanks for posting.

[@BruceNitroxpro]

It is interesting that the FEELINGS about frequency response vis a vis what you can hear entered into the design criteria at the time of manufacture. I totally agree with you, and silly misapplication of FEELINGS about things that could ruin an amplifier can be dismissed.

[@dontcare563]

Very good video Tony! Great job.

[@ruipontes8071]

Great job. Fantastic

[@petergoose8164]

Very interesting and now better than new. Owner should be very appreciative.There must be very few people worldwide with the knowledge and determination to fix these old monsters properly. Skilled craftsmen in all disciplines are dying out fast. Your reference to grandson reminds us that it is mostly grey hairs (not only I'll admit) doing this kind of work. I remember seeing an Audio Research d350 which had been traded in and even the young me could not imagine using that power else lugging the thing around. My Marantz pm8005 which produces 70 a side into 90db speakers actually sounds very powerful so technology must have come some way since those days.

[@ctbcubed]

Back in the late 70s early 80s, high end speakers like the Ohm model F were gaining popularity. They were not very power efficient and had a recommended amplifier power rating of 100- 200 watts. By contrast, my Klipsch Horns (1940s design) are quite loud with only a few watts driving them.

[@NoiseStaticBlur]

Funnily enough, Audio Research did a lot of damage to repairmen by introducing "the potted circuit" That were small essential circuits that they enclosed in epoxy and completely erased markings off transistors and other parts replacing them with color codes. None of the necessary information for repair was handed out all in an effort to kill servicing outside of Audio research itself.

[@kevinbeckenham3872]

Sound is brilliant

[@H-77]

Cordell's book is excellent. I would probably add Douglass Self's power amp and small signal books to the list as well. All of this is, of course, in addition to Art of Electronics.

[@jked7463]

When i said a high speed amp can sound better, i meant the amps internals. Minimizing rf input would make the amp more stable for sure. That said the internals of the amp are still able to shuck and jive better than an amp with a lower high frequency limit. I understand that the low pass filter will limit the ultimate upper frequency limit but i was talking about transients caused by multiple lower frequencies canceling, not actual high frequencies. I can hear the groans that the high transient from the cancellation is still a high frequency. I can only say that the signal coming in is based off of lower frequencies.

[@THEtechknight]

This is why I dont like working on this stuff anymore. When you have to go behind someone else who has "tinkered" with it, its just super painful. I did car audio amplifier repair for a few short years, and I saw this all the time. So much that I ended up leaving that job. Its different when youre repairing a piece of equipment that another professional technician has serviced in the past. But its the shadetrees like the one who touched that SAE that I worry about, or worse... people using IRFZ44 in place of an IRF540 in an output stage. Some things I will just never understand. Sorry for the rant and if I sound harsh, but its just been my personal experience. :-)

[@MrElectrowhiz]

How will this amplifier react around some strong RF signals? Many amplifiers picked up strong signals along it's input, AC lines and even the speaker terminals which cause many headaches for Ham radio operators trying to get rid of the interference.

[@xraytonyb]

Radios are sensitive down to the microvolt range. The RF & IF sections have extremely high gain and can be affected by small signals. This audio amp is only sensitive into the millivolt range (with a gain of only 26x). RF would most likely not affect the input, but a ground loop could. Thanks for watching!

[@Digital-Dan]

Hey, you should sell some of those neat white XRayTonyB Cutting Mats (1:20 in this video). :-) Tell the people complaining about paper rattling to get old -- solves the problem completely.

[@Internutt2023]

@ 43:30 I don't know if Tony has ever bought a used car before 😁😁😁

[@simonparkinson1053]

Hi Tony, I'm enjoying this video series on the SAE. I think you overcomplicated the relay issue though. It simply connects to the +75V and is ground side switched by Q21 through R60 and D19. D18 being the back EMF diode across the coil. Nothing feeds back from the collector side of Q21 into the control circuit. Looks more like the original relay would be a nominal 60V coil rather than the more standard 48V, given the 75V supply, the coil resistance 820 Ohms and R60 being 200 Ohms. Would give very close to 60V across the coil. Maybe a 48V coil version like you used for the soft start would be the best choice along with the extra series resistor? Regarding the output stage, it's amazing how many mistakes were made but the amplifier was still able to work comparatively well!

[@xraytonyb]

My concern was more about the voltage drop and current through the relay coil. As the DC resistance is important, I felt it would more faithfully copy the characteristics of the original relay. I was also concerned about the pull-in and drop-out voltages for the relays. It may be overkill, but for the cost of a resistor, I think it was worth it. Thanks for the comment!

[@simonparkinson1053]

@@xraytonyb sometimes it's easier to see things from a distance and comment on something someone's already done. As I saw it, the transistor is acting only as a switch, the protection circuit characteristics being determined by the comparators. And with the 75V supply you would have way more than 24V across its coil, even with the extra series resistance you added. That was my main concern, overheating the coil of the new relay, and why I thought the 48V version you used in the soft start would have been a better match for the circuit. The comparatively low pull-in voltage of the original relay (assuming it was a 60V coil) is again made irrelevant because the circuit characteristics are determined by the comparators.

[@xraytonyb]

I'll have to take a closer look at the schematic. You make some interesting points. So far, the setup I have is working fine, but there's always a better way! Thanks!

[@bdg77]

Nice work, Tony!

[@merrittderr9708]

As long as we're poking sticks, it seems that when you are looking at a schematic and have the led light on it, the flicker in the light beats with the frame rate of the camera, giving us rolling bands of light and dark crossing the schematic. I first noticed this when you were talking about the input filter at around 42 or 43 minutes in the vidieo.

[@bobbybiggs4348]

I like the Christmas power supply.😀

[@josephcote6120]

Would it have been possible to replace the bias trimmers with a multi-turn trimmer with the screwhead facing up? (Facing towards the slots, I mean)

[@rjonzun5828]

Great series! 👏

[@marvinmartian7281]

Very clever tech indeed!👍

[@justincase3880]

Dave’s Platform .. “We’re Amateurs, we’re gonna make mistakes, Get Over It” ... (maybe) LOL ...

[@i-squared]

The new bias voltage setting should be 1.58 volts with the new 0.1 emitter resistors. The Shottkys you removed are intended to prevent the Darlingtons from dropping excessive voltage across the output finals as output current rises. Not a big deal, but it will run hotter without them. Lowering the emitter resistors also will require thev finals to drop more voltage at high output making things worse. Good work overall, very thoughtful. But you have reduced its ability to deliver rated output for extended periods. Would be glad to discuss more if you like off line. Be well.

[@bob-05it91]

Yes , unlike parallel output stages , you only have 1 set of emitter resistors in this series connected output stage. The original Shottkys were a little underrated in my opinion as this amplifier can deliver over 9amps into a 4 ohm load. Removal will also most likely affect the current limiting protect circuit as there would be more voltage drop across the current sense resistor when the Shottkys were in there. I don't think that any current or drive limiting testing was done on this amplifier. Or any testing at 4 or 2 ohm loads. Once you are above about 6 watts into 8 ohms or so, those diodes are bypassing the emitter resistors.

[@chefbink61]

Isn't it such a wonderful feeling listening to an amp that you brought back to life! Just a quick question,,, What specters are you using for testing? I don't have a lot of space so I use a pair of Overnight Sensations that I built years ago. There not the most efficient but they do sound good.

[@dhpbear2]

43:28 - I'm guessing the original owner never cranked it up and stayed below clipping?

[@radiotvrepair1059]

Thanks my friend Tony

[@jimc9823]

Great video, thanks!

[@thecarl168]

if the base metal is solid silver then you could polish it

[@Chuck-U-Farlie]

I find it can be very helpful to put a tough dog aside for a bit and move on to a relatively easy project for a short time. Sometimes you just need a win to remind yourself that you're not a complete idiot. I find it much easier to resolve myself to taking that whole damned beast apart again just to get to that one part i was so sure it couldn't be the day before. (or week before LOL)

[@glenjohnson3134]

Did you add a second LPF to the input. Looks to me like R2 and C2 already provide that, or am I misreading it? Instead of 68 ohm and 100pf, they used 1k ohm and 680pf.

[@andydelle4509]

In the Douglass Self book, a good audio amp book just like Bob Cordell's, he discusses speaker protection relay distortion. You may think, what distortion, it's a relay? Well I was surprised at what he measured. Of course it was minuscule but there was some measurable added distortion.

[@H-77]

The reason is that some relays pass the signal through the soft iron frame in the relay. Ferromagnetic material tends to be somewhat non-linear. With high currents it can certainly be measured. Self also notes that binding posts that steel binding posts are another red flag in that regard- brass is preferred. Again, Self was able to measure it.

[@jimomertz]

Did you ever verify the 3dB point of the low pass filter you added? Nice repair of a challenging project.

[@ctbcubed]

Calculates to 23 MHz

[@jdmccorful]

I think SAE owes you some engineering fees first, then the sound test was really good. All instrumentation sounded correct. The vibaphone really came across accturate. Piano solo in upper register was spot on for sound. Really,,,,, WOW! Great work. Thanks for the lesson.

[@rogervonschleusingen4603]

THANK-YOU, WHAT IS THE NAME OF THAT JAZZ PIECE AT THE END, I ALSO HAVE AN SAE 2500 AMP !!

[@bilmarutha]

Hi Tony very helpful video, thank you! . Tony have a question for you. Could you please willing to sell me a copy of your service manual? Recently I got a very damaged amplifier and wonder to restore it as better as possible. In advance, thank you so much for your support. BR. Bryan.

[@mitkothemacedonian]

Hi there Tony. The diodes you removed from the output stage were also there for signal clamping to the peak output voltage, that can sometimes protect the speaker, and also protecting the output transistors from back EMF (electromotive force voltage spikes induced by the speaker coil that goes back to the output stage and can be of much higher voltage than the rails). I think you should try jury rigging two schottky 100V 10A diodes (like B10100) per channel, from the output to the power rails in this way: Diode 1: Cathode to the output and anode to the negative supply rail Diode 2: Anode to the output and cathode to the positive supply rail It's not a big deal cost/time wise, it doesn't affect the sound, and can totally save the amplifier or the speakers. Like this: ibb.co/t8gZmmH

[@stojanbn]

How did You decide to use 15 Ohm 25W resistor on mains input to transformer in your soft start circuit? Ohm's law? 120V/10A (fuse is 10A) is 12 Ohms, that's close to 15 Ohm resistor You used... Thanks.

[@russellhltn1396]

Question about switching the meter leads - are you sure the left channel isn't on the left side as viewed from the speaker output terminals? Not everything for the left channel will be on the left side, otherwise the "left" output will be to the right of the "right" output. There has to be a crossover somewhere. The real test is if left channel input will deflect the left meter.

[@xraytonyb]

I looked behind the front panel and the wires were indeed swapped. I think it was done by someone in the past, as I could see where they were in there cleaning the switches at some point.

[@frankgeeraerts6243]

I don't wear a hat , but if I did , I'll take it off for you !

[@justincase3880]

R39 & R40 .. Could you have used fuses instead?

[@flashcorp76]

Real world amp debugging, I gotta make me another amp😁👍🏻 BTW, what’s the title you played at the end ? 😊 Thx.

[@williamchow1624]

How did the amp function with the wrong parts? Guess it never worked correctly. GREAT restoration Job, but what do you do that is not GREAT. Very musical sounding amp.

[@NoiseStaticBlur]

Previous owner was either oblivious or a lying bastard hoping to get it working just long enough to get rid of this ticking time bomb.

[@BC-fy1wn]

xraytomyb,you at all interested in working on my SAE FM tuner,needs a little biasing and most likely re capped?BCWhat state do you reside in?I am in KY

[@michaeleales7097]

would you be willing to work on my SAE A501 AMP and SAE P101 preamp?

[@someoneoutthere7512]

it seems that you could of just used some CL30 or 60 inrush limiters. Those on/off switches are very robust. I've seen them operate continuously for over 40 years.

[@xraytonyb]

The CL series current limiters will still have resistance when at operating temperature, which will still limit current to the amp when it is playing at high volumes. By having a relay, it will bypass all current limiting to the transformer, once the caps are charged. I was also concerned that the larger capacitors (24,000 uF instead of 10,000) will cause more stress on the switch contacts. In addition, the lights no longer dim on the bench when I turn the amp on! THanks for the comment!

[@chessapk5034]

@@xraytonyb IDK Tony, an Ametherm MS32 15012 which is 15 ohm, 12A, would be ideal here. Resistance @ 50% Max Current is 0.31 ohms, trivial. I would go this route every time but to each his own.

[@neverknowit11]

That’s why it was a first test!

[@gamerpaddy]

i see you got a owon digital powersupply aswell, do you have that weird constant current delay? mine takes a while to kick in. like when hooking up a led, limiting output to 20mA but setting CV to like 20V, when turning it on it wont go straight to 20mA it kicks out a lot of amps killing/damaging the led and then limits the current after a few miliseconds. ( and i dont mean hooking it to a allready running output so the output caps discharge ) i dont see this bevahiour on my other powersupplies. (DPS5020, DPS5015 and some old linear regulated one)

[@xraytonyb]

I've not noticed this issue on mine, but now I feel the urge to test that out. Thanks for the heads-up!

[@grahamtownsend3751]

I wonder how many relay or amp manufacturers have considered if there was a market for relays with solid silver or, heaven forbid, solid gold contacts, warranty periods and expediency versus cost as usual I expect. And they would be very expensive.

[@jdmccorful]

But probably the correct thing to do.

[@josephsirois2353]

Nice!

[@rogervonschleusingen4603]

HOW MUCH OF A COST FOR THIS REPAIR ??

[@larryshaver3568]

i would rather do something that may seem unnecessary than miss something

[@Washburn-rr5eh]

I thought you were joking when someone complained about moving the schematics around too much. Then I actually read the comment. What the hell is smoking!!!!!!!!

[@kgsalvage6306]

Why not use a thermistor instead of the relay to limit the inrush current?

[@andydelle4509]

IMO, that's OK for tube amps which are mostly limited to 60 watts per channel. Big solid state amps can pull much higher line current and even the hot resistance of a thermistor can be too much and act as a current limiter. This would tend to destroy the amps damping factor at higher power levels, just where you need it the most.

[@H-77]

@@andydelle4509 It probably wouldn't affect the damping factor much unless the filter capacitors were grossly undersized. There is a decent chance of the thermistor failing, however, and no matter what it will drop the line voltage more than is ideal.

[@andydelle4509]

@@H-77 Agreed. I never liked the idea of thermistors permanently in the AC line and don't use them myself. When necessary I always build a soft circuit with ample power resistors and a timed bypass relay. Not a fan of thyristor based designs either. I have repaired a few of those on stage amps. Peavey used them extensively.

[@t1d100]

Are we talking about negative temperature coefficient (NTC) thermistors, here? I am sure that we are, but just to be clear... I was thinking of them, myself.

[@kgsalvage6306]

I've used NTC thermistors in quite a few different projects. They make a wide variety of different current ratings and resistance. You just have to match them to your needs.

[@herbward5240]

Dude, you way overthought this. A ICL is all that is needed for soft start. Relays always fail in that use. Crestron’s big 16 channel amps use big ICL’s. Many of Bongiorno’s amp designs were prone to blow ups due to RF bandwidth. Anything above 50 khz should be filtered with a 6db / octave at the input. Also , a common addition to the LTP input pair can be open loop gain reduction above 50khz as well. Many of the later designs using fully complimentary LTP pairs used that design.

[@xraytonyb]

I originally thought about using them, but chose this route for two reasons. First, ICL's give off a lot of heat when they warm up and go to low resistance. I don't really want that much heat inside the amp. Some of the old McIntosh gear used them and they would get so warm that the wires around the ICL would get all crusty. Second, the ICL will go low resistance, but not usually any lower that 1Ω. This small amount of resistance still adds an element of current limit, even after the caps are charged, which could at least in theory affect the dynamic performance of the amp. Using a 15 ohm resistor, there is a fairly low current that will be switched by the contacts, so arcing isn't an issue. With all 3 contacts wired in parallel, the contact surface area is quite large, after the contacts close, so again, I doubt that they will wear out any time soon. The circuit consists of a simple relay, one 15Ω/25W resistor for the limiter and a 900Ω dropping resistor for the coil. All of this is pretty inexpensive and easy to implement and should last a very long time. Thanks for the comment!

[@H-77]

Countless amps have used the relay-based soft-start that Tony implemented here without issue. The MC2 MC series amps all used this type of soft start and it rarely if ever fails. ICLs are a nice and simple solution, but far from ideal, especially in amp that is pushed hard. Speaker protect relays, on the other hand, are a bit of a sore spot since few of them are actually rated to break the connection if an output transistor fails short. Few relays smaller than an industrial contactor can break 90V @ 25A DC with a reactive load.

[@gregnewberry4813]

What variack are you using?

[@tommybewick]

And the owner said this thing was in perfect condition!!? He's a complete liar,!!! You're very gracious Tony... I hope your friend didn't pay much for this?

[@johnsweda2999]

I'm not happy about you taking schottky diode out I think was the wrong approach! just to protect the transistors why should that matter, The designer put them there for the sound signature and by taking them out you change the sound signature now off the amp. The designer must had like the sound that's why he put them in, must make quite a bit of difference I think they're quite an expensive part back then would have been. Be nice if you done a comparison one side to the other with and without. How much is he asking for this amp what's the price they sell for.

[@johnsweda2999]

@RebelDeuce not really there's a Sonic benefit, he's change the sonic characteristics of the amp but to what degree that's the question?

[@xraytonyb]

The schottky diodes were not, in my opinion, a good choice in this circuit. Two of the four were shorted and the other two were reading in both directions (leaky). The purpose for the emitter resistors is to limit current and to compensate for differences in gain between the transistors. As the amplifier drives the speaker at higher amplitudes, the voltage drop across the resistor will increase and the current through the resistor will increase. The power imposed across the resistor will be dissipated as heat. The diodes have no influence until the voltage drop across the resistor exceeds the forward bias voltage of the schottky diode. At that point, the diode begins to conduct and the voltage drop remains at the forward drop of the diode from that point on. As the amplitude out to the speaker increases, the voltage remains the same across the diode, but the current increases. As the drop on the diode is less than the Vce drop on the transistor, the transistor will take the majority of the excess current and will have to dissipate it as heat. In addition, the difference in gain between the NPN/PNP transistors will also be dissipated by the transistors. I believe this is why so many viewers have reported that the output transistors often fail in these amps. By reducing the emitter resistor resistance and eliminating the diodes, the amp will still allow proper transfer of power to the speaker while still doing their job limiting the excess curren between the transistors. I highly doubt that the amp will sound different than it did, but I will hopefully be more stable. The schottkey diodes are also under a lot of stress in this configuration, so they will have a tendency to short, which will provide no protection at all for transistor imbalances, especially at idle. It is virtually impossible to find a set of PNP and NPN transistors that are perfectly matched with Hfe and Vbe, thus making the emitter resistors necessary.

[@johnsweda2999]

@@xraytonyb yes that's the thing aren't you limiting the current to the speakers by removing the diode and reducing the the slew rate as well, does that have any effect on slew rate?

[@ctbcubed]

@@xraytonyb You did a fantastic job restoring this basket case to better than new. What a packaging nightmare! Regarding the diode comment, your amp showed nice symmetrical clipping at full output, very low THD, flat frequency response and very good square wave response which is indicative of good transient response. It would be interesting to see the intermodulation distortion for this amp, but I think it would also be very good. The job of an amplifier used to reproduce a recorded performance is to do so without additional coloration. If anything, I would think those diodes would introduce distortion. Sound instrument amplifiers are another story because they become part of the instrument. For those amplifiers, coloration is usually desirable by the artist. A Fender tube based guitar amp will sound much different than a high quality solid state amp of equivalent power. Once the artist records a performance, their intent is for us to play it back without adding or subtracting anything from it. Otherwise it's kind of like making a Xerox of a Xerox.

[@daveturner5305]

Thanks for an informative video. One plea though - please stop picking up the schematics and shaking them around; it makes it quite difficult to watch. The video quality is good enough to read them when flat on the desk.

[@FarnhamJ07]

Gee Tony, you're such a softie; ya gotta learn to give your papers the proper discipline!