Topic: Sansui Amplifiers

[rascal101]

hehe ... I'm from Bulacan ...

Sir TonyT, alam ko naman marami na kayong experience sa audio kaya kayo ang  "resource person". Ako naman, sumasagot lang ako base sa experience ko. Kung ano mali ko, sabihin niyo lang pero sana samahan ng nauukol na paliwanag. Gusto ko rin naman matuto kung bakit. Huwag naman yun walang lugar SMPS sa analysis. Lahat naman tayo natututo sa isa't isa.

 

[ATJr.]

Linear of SMPS, you still need a PSU in your amp and same principles apply.

agreed, but you are still out of line.

as i recall the discussion was about transformers for japanese amps, 100volts ac vis a vis 110volts remember?

now how does the smps psu come into the picture?

[ATJr.]

Isa pa nga,

For example on your amp,

Input power = 100W, Output power = 50W (standby)

Assuming input line voltage = 220Vac, input line current = 454.4mA, power factor = 1.

The equations are:

1. Efficiency = Pout / (Pin x power factor)
2. Power = voltage x current

Note: Pout = output power, Pin = input power

When input line voltage = 180Vac, what will be input line current to maintain output power of 50W?

hey rascal101 for Pete's sake just cut it out, please! :'( :'( :'(

[rascal101]

You didn't read the part where an undervoltage condition was being discussed.

And for Petes' sake, please read.  :'( :'( :'(

[ATJr.]

oh didn't i?    by simple ohm's law reckoning, current goes down in the transformer primary when voltage decreases. ie, when voltage goes from say 120 to 100 volts, power delivery by the amp decreases proportionately... this is as simple as it gets...

[rascal101]

Your AVR's undervoltage is below 10% of the expected values and AFAIK they can have detrimental effects on some appliances.  If you measure them at 90VAC without load, expect the voltage to further drop when loaded.

I think there's also such a thing as undervoltage shutdown or lock-out in power amps if the input voltage falls below a certain value.  This is entirely theory for me though as I have not experienced severe undervoltage conditions where I live.  I heard that feeding motors with lower than required voltage can cause them to run overly hot as increased current is forced to compensate for prolonged voltage sag. 

I would think that any undervoltage in the amp's voltage rails will have the same effect as reducing the speaker load impedance.  And that is to extract more current that could cause the protection circuit to engage when it could no longer produce the needed current to amplify the signal. 

But since you are using Automated Voltage Regulators, they are supposed to output the correct 100v or 110v, as the case may be.  The input household voltage sag must be so severe to cause more than 10% reduction in the AVR output .    See if you can up their output voltage to the proper level or borrow another that does,  and if your problem recurs despite this, then it's time to turn to a repair center. 

I think your amp can not easily be destroyed by under voltage - the only thing you can do to validate if your amp is still in good condition is to try to plug it in the correct voltage and operate it and observe.

This is where I took off ...

[JojoD818]

Pete's Law?

What is that?

  :'( :'( :'(

[ATJr.]

I think there's also such a thing as undervoltage shutdown or lock-out in power amps if the input voltage falls below a certain value.  This is entirely theory for me though as I have not experienced severe undervoltage conditions where I live.  I heard that feeding motors with lower than required voltage can cause them to run overly hot as increased current is forced to compensate for prolonged voltage sag.  

ok, now i see, guess i wasn't paying too much attetion on avphiles's post....

well, in all my years repairing amps and looking at schematics, i haven't seen any undervoltage protection in any amp that i know of, and the i understand the reason for that, there is no need for it whatsoever. i have explained in my previous post.

now motors are a different animal altogether, operating motors on undervoltage causes the rotor to move very slowly thus mimicking a short circuit. so here is where you get the idea! this have no correlation with amps....

[ATJr.]

Pete's Law?

What is that?

  :'( :'( :'(

LOL! :'( :'( :'(

[rascal101]

The undervoltage condition rarely does happen but it may happen. It is a good idea to have an undervoltage protection circuit both at the input and bulk (if budget permits and your market are 3rd world type).

Don't know if my Sansui amp has one.

[ATJr.]

it happens all the time, and the net effect is decreased power available to the speakers...

[JojoD818]

LOL! :'( :'( :'(

guys,

you have to excuse me... I probably woke up this morning on the flipside.  :'( :'( :'(



seriously, lowering the input voltage will also lower the input current. this is in accordance with Ohm's Law (again), if V=IR, and R is the primary resistance of the tranny which is relatively constant, then to satisfy Ohm's Law, I should automatically lower itself with a lower V. this should also apply to Joule's Law for it also use V and I parameters, thereby having a lower power.

I understand rascal's concerns for protection during low voltage situations, but such events rarely damages power amps with conventional power supplies.

[JojoD818]

btw, don't SMPS have this autovolt feature?

[ATJr.]

guys,

you have to excuse me... I probably woke up this morning on the flipside.  :'( :'( :'(



seriously, lowering the input voltage will also lower the input current. this is in accordance with Ohm's Law (again), if V=IR, and R is the primary resistance of the tranny which is relatively constant, then to satisfy Ohm's Law, I should automatically lower itself with a lower V. this should also apply to Joule's Law for it also use V and I parameters, thereby having a lower power.

I understand rascal's concerns for protection during low voltage situations, but such events rarely damages power amps with conventional power supplies.

amen....

[ATJr.]

btw, don't SMPS have this autovolt feature?

hey OT na tayo matagal na, but to answer you question yes, duty cycle is varied to cope with varying inout voltage..

[av_phile1]

well, in all my years repairing amps and looking at schematics, i haven't seen any undervoltage protection in any amp that i know of, and the i understand the reason for that, there is no need for it whatsoever. i have explained in my previous post.

Ofcourse,  I should have mentioned that undervoltage protection is fairly common in many Class D amps.  I read their nature of operation requires one.

now motors are a different animal altogether, operating motors on undervoltage causes the rotor to move very slowly thus mimicking a short circuit. so here is where you get the idea! this have no correlation with amps....

Another ofcourse.   But if you read my statements in the proper context of that post, it was more an example to support my premise that undervoltage "can have detrimental effects on some appliances."   And for the same reason, cited the existence of undervoltage protection circuits.   But for most lnear amps, I agree, undervoltage will just result in underpower.  But i think a severely voltage-starved amp, when you  increase the volume,   will just output current-starved square waves that are harmful to the speakers.  IOW, it will attempt to extract the current needed to amplify the signal as you go up in volume.  There being none, you get clipped signals.   Now if such adverse output condition will triger the protection circuit, it should, but I have no experience in that.

[karipas]

Thanks for the reply. You're lucky, konti lang meron nyan. That's an excellent amp unless you got a defective one. The other I mentioned, A-707, was the Saudi-type component amp once common here, yung mga binebenta sa international market nuon that accept speakers 8 to 16 ohms only. Ang alam ko marami dun nasunog na.

I was thinking the mains was your problem since undervoltage was common to your AVR and transformer. I'm using several AVRs and transformers now and measuring outputs on a monthly basis. Since I started buying Jap vintage hifi 5 years ago, I have limited my AVR and 100V transformer choices to two brands, Zebra and Panther, but there maybe better and cheaper brands now.

Sir Sandawa, thank you very much, I have seen many panther brands here in our place, maybe I will settle for that brand. Will a 500W will be ok for my amp?

You are right Sir, I have been using this amp for 3 years now and I really love this amp.

[JojoD818]

hey OT na tayo matagal na, but to answer you question yes, duty cycle is varied to cope with varying inout voltage..

oo nga OT na.

sorry po... pero para sa kapakanan naman daw ng Sansui Amplifiers yun under-voltage discussion...

palusot pa

[jhunB!]

Sir Rascall, Sir Tony... baka maulit ung S2N-HAnz showdown.... wag naman sana... anyway, both of you are correct with respect to your analogies in electronics, la me mai-share but lots of informations I gained from you both.... THank you very much!!... God Bless!!!..  (OT)

[aHobbit]

nawalan ako ng time sumilip dito ah!

recap ... I have yet to see amp destroyed by undervoltage ... speakers could be the one destroyed ... no idea with class D yet   

[rascal101]

Get a variac and for your 220Vac appliance set it to 180Vac and just leave it on.
Or, for your surplus amp set it to 70Vac. Same thing leave it on.

[av_phile1]

I think the guy wants to increase the AC voltage, not decrease it. 

[rascal101]

OT, AHobbit and TonyT said that they have yet to see an amp being destroyed by undervoltage. Best way to do it is have voltage at least 20% below low line eg
for 220Vac, low line = 198Vac for 100Vac, low line = 90Vac. This will show if there is undervoltage protection.

Going to the topic Sansui Amplifiers my AU-D907F gets very hot at 10 o clock volume position. Have checked biasing and measured around 25mV and 28mV at left and right channel, respectively. Is this normal?

[JojoD818]

OT, AHobbit and TonyT said that they have yet to see an amp being destroyed by undervoltage. Best way to do it is have voltage at least 20% below low line eg
for 220Vac, low line = 198Vac for 100Vac, low line = 90Vac. This will show if there is undervoltage protection.

Going to the topic Sansui Amplifiers my AU-D907F gets very hot at 10 o clock volume position. Have checked biasing and measured around 25mV and 28mV at left and right channel, respectively. Is this normal?

where did you measured the 25mV and 28mV?

[rascal101]

At the output jack terminal of the amp (going to the speakers).

[JojoD818]

At the output jack terminal of the amp (going to the speakers).

Ok, that is not the biasing. What you measured is your amp's DC Output Offset which by some standards can go as high as 100mV, but of course the lower the better.

Biasing is the amount of idle current flowing to the opt trannies, to get this you need to disconnect either the V+ or the V- and insert an ammeter or you can always measure the voltage drop accross any of the opt transistor's emitter resistor and use Ohm's Law to get the current.

[rascal101]

Ok. I used the wrong term.

What voltage bias do you apply to the base of the power transistors (sorry I typed gate earlier)? And, is the trim pot connected to it normally referenced to ground? Or is it a trimmer series type?

[ATJr.]

Going to the topic Sansui Amplifiers my AU-D907F gets very hot at 10 o clock volume position. Have checked biasing and measured around 25mV and 28mV at left and right channel, respectively. Is this normal?

i have answred this type of question several times in at least two threads here..

[ATJr.]

to understand clearly what jojo is saying we need to post the schematic of one of those sansui amps....any volunteer?
 
plus you need to understand how the circuit works....

it is not a simple matter of asking what voltage is inputted to the base or gate...

[JojoD818]

Ok. I used the wrong term.

What voltage bias do you apply to the base of the power transistors (sorry I typed gate earlier)? And, is the trim pot connected to it normally referenced to ground? Or is it a trimmer series type?

Oh yeah, these terminologies can be a bummer sometimes.

Back to topic, it is not really a question of what or how much base voltage is needed but how much current is flowing during idle. Common topologies of class AB ss amps puts the idle current in the range of 25-40mA per device (Bipolar). This of course must be confirmed by an oscilloscope to see if the crossover distortion has been removed/minimized for the whole freq spectrum and from 1 watt to max opt power.

Biasing your opts higher will guarantee a minimized crossover distortion levels but at the expense of heat dissipation.