Amplifier Types DC Offset & Distortion Protecting your Speakers

[@rogermccormick5248]

Hello Audiophiles! I am a vintage audio restorer. My Amp is a full restorated 1973Mac1900 hooked to many speakers thru an array of switch resisters but my prize is Rogers Ls3/5a. Truly an interesting experience.

[@m.9243]

Great explanation of circuit and concepts in the theory part. I wish you were one of my teachers (many years ago), when I was studying electronics. Thanks for the video!

[@assistantto007]

A fantastic lesson in how it works. Thank you very much.

[@rotemcohen6239]

Man you rock my day!! I was lucky to watch your video today! I had a kenwood kr-4070 that i triied to fix fee hours and couldn't find faulty transisotors ( in the transistor checker they found good) I notice the 2sc1400 in your list of bad transistors, and replaced it although it tested good. That fixed my receiver! Thanks 😊

[@dhpbear2]

26:18 - We used to quip back in the day "Put in a bigger one. Why blow a perfectly good fuse?" :)

[@poormanselectronicsbench2021]

Looooong ago, I did just that, as a Altec Lansing model 1569A - 80 Watt mono tube amp for our high school's auditorium, just came back from a shop "check up" and when we reinstalled it in the rack and powered it up, the fuse blew. We replaced the fuse with another proper rated one, that one smoked as well. Running out of properly rated fuses, we, well, I did it, put in what was a waay over rated fuse, powered it up, and the rectifier tube lit up all kinds of pretty colors. Needless to say, the shop that checked it out, probably just took the school's money, and did nothing, and the amp made a trip home with me to have a shorted filter cap replaced, taking an 8 mile, 2 bus each way trip to and from home to fix it. After it was actually "serviced" , it made it through the year, then, I graduated and it was someone else's issue.

[@robertcalkjr.8325]

Excellent lesson, Tony! And thanks for the transistor tips also! When I build kits I rarely use the caps that come with it. I normally just throw them away. Now that I have the equipment, I can make my own boards.

[@Johnathan_Waters]

Thanks for the video! Also your fur baby Bella is adorable!!

[@mohimadvani6425]

excellent video. very informative 👏👌👍

[@davidmonk7773]

What a fantastic and informative video 👏

[@matthewbrown5677]

best audio vid on U tube learnt so much thank you.

[@JelckedeBoer]

Great video, very helpful!

[@dhpbear2]

nd to add further confusion, the circuit is drawn upside-down! :)

[@markbuxton2368]

great stuff, hi bella!

[@bobdoritique7347]

Merci for this video. Very interesting!

[@alfredneumann4692]

I love Bella. And your videos. :-)

[@jimgross5]

Installed the speaker protect board you showed on this video on a Sansui R-50 that I purchased new in 1980. Used the 12Vdc regulated supply to power the board. Noticed that it seemed to take a wee bit to long to come out of protect mode. Connected an external power supply to the board with 12.5Vdc and found that the output of the 12V regulator was outputting only 10.9 volts. Re-watched your video and noticed a pop up that said 14Vdc to something when using DC. Used a DC boost converter with an input of 12.5Vdc and set to an output of 14.5Vdc and the oputput of the 12V regulator is at 12.2Vdc and the circuit comes out of protect quickly and when I use a AA battery on the input it goes into protect much faster. The protect circuit only draws 47ma of current and found that it is possible to boost the voltage to the protect board albeit only if the DC supply you are using from your amp is cabable of handling it.

[@xraytonyb]

Thank you for sharing this information!

[@johnnytoobad7785]

As someone who built a power amp..Soft start circuits and speaker protect circuits are essential. Unfortunatley the most elegant soft start and speaker protection circuits were *not* found online, but in a book.."The Audiophiles Project Source Book" (2001) by G. Randy Sloane (R.I.P.). But there may be some newer designs out there..

[@acidchristo]

Very helpful.🔥

[@abdelkaderelbachir9523]

To be honest I never experienced that situation but that was interesting to know about

[@sherjeelshoaib89]

Great work

[@stevent1074]

Great video! Thank you for making this

[@timp9848]

As a beginner I have learnt so much watching your videos and this video was another excellent tutorial. In regards to dc offset, I am assuming the output voltage needs to be stable as I have a Marantz PM520DC and at power on it is 20mv and overtime 30mins later it slowly fluctuates down to -7mv, is this normal?

[@Panslapper]

You dont want to go over 35mv on your speaker terminals so you are in good stead with your amp.

[@izzzzzz6]

Did you ever do the video on the snubborn network the anti oscillating stage? If so what is it called?

[@williamsquires3070]

Another trick (for class-A amps) for the really cheap: double the capacitance value of the DC blocking cap at the amplifier output, and round to the nearest common value. Get two capacitors of that value, and put them in series with the speaker (if the caps are electrolytic, wire the “+” terminals together), and now both caps will have to be leaky in order to have a problem with DC on the speaker terminals. BTW: the reason for doubling the capacitor values is that caps divide in series, just like resistors in parallel, so by doubling them, then putting them in series, you get roughly the same capacitance as originally called for. So now you’re not depending on only one cap to save your speakers! 😁

[@paulmoniz6048]

Not following your logic...if 1 of the 2 caps gets leaky (low resistance/short) you still are passing DC...it basically shorts out the 2nd cap in parallel.

[@JohnAudioTech]

I've heard of people having their speakers burst into flames when their amp's output went to a rail due to some failure. I've been lucky so far but I've bought some amp IC's that had a rail on the output right from the start (counterfeit junk).

[@pulpanochamaster]

Awesome video, noob question, isnt there a way or does exist a device one can put between the speaker and the receiver/preamp/amp connectors, instead of doing an internal job? (A device for noobs like me?) Cheers!

[@poormanselectronicsbench2021]

You could build that same kit that Tony did, but then install it into a project box, adding "From Amp" and "To Speaker" connections, but then, you also have to provide it with the power it needs for that circuit to operate. I'd imagine some can be found on an auction site, or other websites. Most would consider an internally installed option, as it's not another device to add wiring to, you install it and hopefully it does it's job, but I understand the unwillingness for some to do that.

[@rotemcohen6239]

Thanks for this great video. I wanted to ask why isn't always allowed to use a Dim Bulb Tester? What damage it can do? And in what cases? I always use it and didn't know that it's only in some models allowed. (I mostly using sansui, akai, luxman and marantz from the 72-79 years) Most of them has a speaker relay, or fuses to protoect the speakers

[@km6hvu622]

Great videos, thanks for taking the time to make them. Where is a good site to get service manuals?

[@willyhaug549]

Hi, I bought a Dartzeel NHB-108 clone from China and measured the offset on both outputs. The right output shows 0mV DC and the left shows 155mV DC. Should I be worried about the left output or is it so small that it won't affect the sound or anything? It seems that it is not possible to adjust this with a potentiometer

[@Zonfeair]

During the test with the dummy load and scope connected are you running the amp at full line current or still on the dim bulb limiter? Also are you inputting a signal with a generator or just an antenna?

[@timothyhillchammandan]

How can we measure the quality of a capacitor?

[@gillanland]

I didn't check DC offset when i got Pioneer VSX-452 receiver and it damaged my Optimus 77Pro speaker. The receiver has variable DC voltage(30-200mV) on main outputs. It has protection curcuit which didn't save it as Tony says and it probably has leaky transistors.

[@bmboldt]

I have been wondering what class of amp my SX-6000 was and came across this video. Mine is similar to this and now I know it is a single supply push pull. The single supply was throwing me off. I was thinking possibly class A since there was only one supply voltage but class A just seemed wrong.

[@johnnytoobad7785]

The only issue I have with these speaker protect "kits" is that the relays that they use are generally too small. Once you exceed 60 watt/ch you should use 10-12amp relays depending on how much power you are going to draw from each amp. channel. You should use DPDT relays and tie the output poles together for higher currents.

[@poormanselectronicsbench2021]

The relay max current issue is / was most likely a problem in the Harmon Kardon Citation Sixteen original configuration ( with Op Amp differential stage) It has a DC protect / delay start relay with contacts only rated at about 8 amps, but, it could output 8+ amps per contact into a 4 ohm load when driven to full output ( most likely in sound reinforcement applications) . There was a service bulletin that "advised" a service tech to remove a resistor that provided the DC offset signal to the darlington transistor that switched the coil on, due to the relays having issues, then, by the 16A model, they totally removed the DC offset function, only using the relay as a start delay, but IMO, its still a weak link in the amp, as it has to add resistance that could be reduced with a better relay / higher current contact. I have worked out a board with 30 Amp relay contacts ( TE Model T92S11D12-12), using, but slightly modifying the original HK design, hopefully I can share the info soon.

[@dhpbear2]

45:54 - Each resistor is rated at 100W. Wouldn't your addition of a heat-sink and a fan raise that rating?

[@Eric-uf8zx]

I believe the 100W rating is with a heatsink, I doubt they would last very long without a heatsink while dissipating 100W.

[@Slartibartfas042]

As with transistors these ratings are always with maximum cooling equipped. you may see for example with Vishay power resistors: www.vishay.com/docs/31883/ah.pdf. Here you can see a 100W power resistor would be rated at 50W only when being mounted without external cooling. A 300W power resistor would even derate to only 75 W of maximum power consumption when no cooling is mounted.

[@hosac99]

Great video thank you

[@MichaelBeeny]

Just a point regarding the speaker protect boards. Amplifiers with bridged outputs are not suitable for these units, as the grounds are connected together which will blow up the amplifier. They need a floating ground. The negative outputs are also linked together on the cheaper boards. This can sometimes be an issue.

[@larryshaver3568]

what if you also put a capacitor in the negative side of the speaker terminals? would that stop or minimize dc offset?

[@tomicajovevski]

Thanks,could you privide the links for the protection relay? I have the same issue with bub sound on pioneer sx 626.

[@Discretesignals]

I wonder if using a buck converter will be more ideal then a linear voltage regulator? They make the buck converter circuits really small.

[@paulmoniz6048]

potentially adding a source of noise.

[@jutukka]

Nice video. 👍

[@mmaranta785]

How long does it take ( in seconds) to blow up a speaker when you give it too much power?

[@DAVIDGREGORYKERR]

What about 2x BD132 power transistors driven via BC182L feedback is from the emitter of the top transistor to the emitter of the BC182L via a 470K resistor. you will have a 1K resistor of the emitter of the BC182L to ground, just a suggestion for a small headphone amp if you wonder where I found this google for 5W Class A power amp.

[@erguy6319]

Great video !!

[@williamsquires3070]

I don’t know why I never thought of this before, but... what if you hook up a 6V power supply off the 12 volts (Vcc) with a 78L06, and a filter cap, then hook the other end of the speaker to this? Then - at quiescent state - you have 6 volts on both sides of the speaker; you can then remove the DC blocking cap from the circuit, which introduces a frequency-dependent impedance (and phase shift) to the frequency response of the class-A amplifier described at the beginning of the video. You’d probably need to put in a “DC bias” adjustment pot so you could adjust the voltage at the collector of Q1 to be 6 volts out with 6 volts in. 😊

[@martinda7446]

Yep, that works...

[@Slartibartfas042]

Nope, it won't work at all as - even if you do adjust the DC bias perfectly, the moment the temperature of your transistor sets will change the actual output voltage of the circuit will vary a bit. And this would result in you having DC voltage across your speakers again. The working point of an amplifier, especially an class-A or A/B mode one will never stay absolutely stationary but be wandering around with ambient temperature, with actual power dissipation situation from audio signal and so on. Without a much more complicated regulation circuit you would never be able to keep it absolutely stable. So, do yourself a favor and *don't* do the trick with the "power supply".

[@martinda7446]

@@Slartibartfas042 I was going to add...but didn't bother, because it was nit picking, that as with any output stage that uses transistors and a split supply, you are gonna have to take care of thermals. No problem. It works fine..

[@VIPINSAINI_20]

This is the one I was talking about

[@jimmychua6529]

How to test the DC offset. No source of sound no DC offset but if there's a source of sound it has a DC out and I don't know what class amplifier is this brand Ace LX20 and Joson Saturn. But my Technics class A no DC offset even there a source of sound.

[@eggshellskullrule7971]

Your Bella looks sweet. Mine, also a King Charles Cavalier, named SweetieSweetie, went in 2016 at age 14. She is the one in my icon picture on youtube and google now.

[@larryshaver3568]

putting a small resistor ahead of the 1mfd capacitor would slow down the current

[@82abn34]

Yes! A transistor cheat sheet. Screenshoted.

[@mikeconnor3602]

Love this, I have learned a lot, thanks I have an Adcom amp that has a hum at the speakers. Could this be evidence of DC offset? If I measure across the speaker terminals at idle, I should get zero volts is that correct?

[@esci8978]

You should see as close to 0 mv as possible.. maximum voltage would be +/- 100mv.

[@mikeconnor3602]

@@esci8978 thank you, I am getting between 4 and 7 mv on the speaker terminals. The hum was a mistake I made with my preamp connection. Quite frankly the Adcom GFA-545II is outstanding, the imaging and bass is noticeably better than my Hafler 220 and my B&W DM2000s seem to come alive, but I plan to do some critical listening next weekend when the family is away. The bass seems to be tighter and more controlled with less cone excursion. Thx

[@esci8978]

@@mikeconnor3602 That’s Great Mike..! It’s very difficult to get better than what you have measured in DC offset… Good that you found the source of your hum as well… Cheers..!

[@1959Berre]

A couple of years ago my son blew up my wife's amp, a low-end Technics SU-22. The left channel of the STK output module fried itself and destroyed the voice coil of the woofer in the process. 39 VDC is just a little over the limit. It made one loud bang, followed by a deep silence. BTW, the fuse was blown along with the voice coil. Those STK modules are notoriously unreliable when low frequency 'music' at a higher volume level is played. When the output is driven into serious clipping, the speakers face someting which resembles a square wave, containing a DC-like component, both at the positive and the negative top of the wave. Something a voice coil does not like. Maybe I will replace that dead STK module with a selfmade output stage one day, since the original parts are unobtainable and I don't feel like ordening a chinese fake one.

[@matthewbrown5677]

is a AKAI AA5810 a class A

[@Homer19521]

👍👍👍

[@larryshaver3568]

field coil speakers had a transformer attached right to them

[@dhpbear2]

24:32 - Those are both emitter followers. they have no gain and don't invert.

[@martinda7446]

They have no voltage gain (or less than unity), but have power gain. Don't know any fuse with that property )) Edit oops I got confused with your fuse statement....Silly me.

[@Digital-Dan]

Talk about why it's OK to plug headphones into these high power outputs.

[@Slartibartfas042]

Even if they may be providing signal from the same output stage like the speaker terminals, usually phone outputs do have resistors in series that will be limiting the current through the voice coils of your headphone. Additionally most of the (better?) headphones are using higher ohm rated voice coils, naturally limiting the current and therefore the output power on their own. If you remember when Tony spoke about the output of the circuit he always did talk about voltages at the output point, right? This means using higher resistance voice coils (usually about 400..600 ohms) or using a series resistor would limit the current through the coils. Therefore you would not send the full 25W of audio power through your headphone high-ohmed voice coils but only a small fragment, not much more than maybe 1 percent or even less. So, connected high-ohm headphones to the relatively high-powered low-impedance speaker terminals may be OK, but be aware that common ground of the 3,5mm or 6,3mm stereo phone jacks might blow your amplifier too (remember what Tony said about isolated ground amps?) . Connecting some of that lowcost walkman earphones for example with their impedance of 4 ohms would be also strict no-go for speaker terminals as they then would get much to high powers they would not be able to handle at all!

[@tahwnikcufos]

1 hour? not on a weekday man... see ya Sunday, if it rains ;-)

[@larryshaver3568]

i had to buy another receiver my receiver was blowing speakers when the volume was only at a reasonable level

[@muhammadawaisarain4172]

My marantz pm 14 have problim dc voltage 2vdc spike in speakers.. What is problim any body help me

[@R_31_111]

Bahaya itu ke speker jika play lama

[@stupidboybg]

hard clipping burns tweeters not because of DC, but because of resulting high freq with really high amplitude. think again why tweeters but not lows or even mids, and why 'soft clipping' results are different. those relay protections usually are not intended to protect against clipping. as you demonstrated in video, they didn't trip even at serious hard clip, at which tweeter is already dead! clip detection/protection is completely different story. also, those boards usually are powered by AC directly from transformer because they have 'missing mains detector', disconnecting speakers when you switch off the amp much earlier before amp start to develop DC (most split supply amps will).

[@frankgeeraerts6243]

Quasy complementary push pull .................( capacitor coupled output ) 1) No DC offset... 2) The 2 output transistors are the same , just like tubes ............tonal structure doesn't change .... 3) The capacitor must be high quality....

[@swinde]

Why block thumbs up/down?

[@mcsniper77]

Hey Tony you don't need any excuses. If we wanted to watch scripted content we would watch TV.

[@chaber6419]

That solid aluminum blue Sanyo has got to go. For a different video but another member of the failure prone club.

[@blade3120]

THE BEST SOLUTION TO AVOID HIGH DC OFFSET TI IS EARTHING THE APLIFIER EMEDIAETLY! GREECE ATHENS!

[@1959Berre]

Chinese power protection? Hm, maybe something Donald Trump may like... The price will rise.

[@robertcalkjr.8325]

President Trump just wants them to stop stealing everything and start trading fair. The thieves and liars will go kicking and screaming...

[@RaulHernandez-lg5nw]

You don't know what you are talking about! If it was a Darlington configuration there would be no emitter resistor in the driver transistor! It's emitter an emitter follower drive to the power transistor!!!

[@garydirkse9900]

You are right, it is not a Darlington configuration but, be kind and gentle correcting people. Most of us make mistakes and oversights when we are hyper focused trying to explain things occasionally.