question on preout to power amp without source signal

[BlackHole]

Hello friends,
My question is if I don't have a source song playing, is there a preout signal still going from the Class A pre-amp to the Class A power amp?

Windows 10 PC --> DAC --> Pre-amp --> Power Amp

If no song is playing there is no signal from DAC to preamp, correct? In that case what does the Class A power amp do to speakers?
I am asking this to understand what is the difference between having source signal vs without source signal how preamp and power amp work. I forgot to turn off system last night, so curious.

 

[whoaru99]

If it's all true Class A operation about the only material difference is the speakers are making noise or not.

Whilst there would be no signal if there were no song playing, the nature of true Class A gear is that it essentially works just as hard at idle with no signal as it does at full output.

 

[BlackHole]

Thanks
I make sure that source is playing music and dac and preamp are turned on before switching on sumo ‘the nine’ class a power amp.

Sometimes I hear a thumping sound while power amp is switched on while some times i don’t hear the thumping sound. I can’t figure out what is causing the difference.

Also I am told that this power amp has sliding bias which I assume means that according to preout signal voltage amplitude, the amp outputs power so when volume is at zero on the preamp there should be no current going to speaker causing amp to heat up ??. I turn it on for 30 minutes or so before listening. Is my understanding correct?

 

[Binkman]

Sometimes I hear a thumping sound while power amp is switched on while some times i don’t hear the thumping sound. I can’t figure out what is causing the difference.

Welcome to A.K.
Usually class A.. the mains caps are still holding a charge when you don't hear the thump.
Warm up? 20 min.. I guess maybe more?
@whoaru99 answered your question about Class A power consumption

true Class A gear is that it essentially works just as hard at idle with no signal as it does at full output.

 

[BlackHole]

Thanks
That makes sense that sometimes the cap is still holding charge even after powered off for a day or two but sometimes cap has drained fully. So the thumping noise is dependent on that. Thanks for the details.

When in idle mode, a class A amplifier is supposed to dissipate all energy into heat. So if I leave the power amp on for couple of hours without any music playing the power amp should feel warm or hot to the touch right? But this one is always cold even after few hours of idle time. So I am not sure if this is normal or a deviation (efficient design)??

 

[bowtie427ss]

When in idle mode, a class A amplifier is supposed to dissipate all energy into heat. So if I leave the power amp on for couple of hours without any music playing the power amp should feel warm or hot to the touch right? But this one is always cold even after few hours of idle time. So I am not sure if this is normal or a deviation (efficient design)??

It will be argued(with merit) that most sliding bias schemes are not true class A.

 

[whoaru99]

When in idle mode, a class A amplifier is supposed to dissipate all energy into heat. So if I leave the power amp on for couple of hours without any music playing the power amp should feel warm or hot to the touch right? But this one is always cold even after few hours of idle time. So I am not sure if this is normal or a deviation (efficient design)??

I would normally expect an amp said to be Class A or even sliding Class A (or for that matter high bias AB) to be fairly warm, if not hot, after few hours idling time.

My Class A and high bias AB amps are quite warm, some may describe as hot, after a few hours even at idle.

That said, it seems Sumo The Nine is fan cooled (yes?) so that likely skews the feeling of temperature. Because of that I cannot say in this case if what you have is normal or not normal operation, as my Class A or high bias amps are not fan cooled.

 

[gadget73]

Is it a thump through the speakers, or is it a thump in the amp itself? Thump in the amp may just be the transformer charging up. Depending on exactly where on the 60hz sine wave you close the switch, there can be a lot of inrush current. My big Phase Linear sometimes sounds like it got whacked with a rubber mallet when I turn it on, other times nothing. Its just how it is.

 

[BlackHole]

I would normally expect an amp said to be Class A or even sliding Class A (or for that matter high bias AB) to be fairly warm, if not hot, after few hours idling time.

My Class A and high bias AB amps are quite warm, some may describe as hot, after a few hours even at idle.

That said, it seems Sumo The Nine is fan cooled (yes?) so that likely skews the feeling of temperature. Because of that I cannot say in this case if what you have is normal or not normal operation, as my Class A or high bias amps are not fan cooled.

Hi @whoaru99 thanks for the response. Yes, the NINE is fan cooled. I don't think there is a heat sink in the NINE. Funny my preamp feels more warm to the touch than the NINE. The NINE manual says it wont have thermal runaway at any situation, which means they are using some kind of stabilization technique to route the current to prevent the transistor from burning out in case the fan fails to run or over heating happens. Someone who has worked on the NINE should have better understanding than me. My knowledge on transistors is very limited. The fan is little bit louder when it runs, not quiet. May be heat sink would have been better option for quietness. Fan is located in the back side of the unit and I have sufficient room for ventilation. What amps do you own? Do the ones you have use heat sink? Thanks!

 

[BlackHole]

Is it a thump through the speakers, or is it a thump in the amp itself? Thump in the amp may just be the transformer charging up. Depending on exactly where on the 60hz sine wave you close the switch, there can be a lot of inrush current. My big Phase Linear sometimes sounds like it got whacked with a rubber mallet when I turn it on, other times nothing. Its just how it is.

Hi @gadget73 thump is on the amp itself. Most likely same situation as yours. Question begs, is the once daily turning on and thump caused due to inrush current deteriorating the amp over time? should i just leave the amp on all the time assuming the unit never over heats? Electricity cost would go up by little bit. But there is the risk of fire because it is vintage? I can try leaving it on for 3 or 4 days and keep check on the unit regularly for warmth. If it stays cool, then it might be safer to leave it on. But I hate wasting electricity because I am environmental conscious and don't want to be irresponsible. LOL...What amps do you own?

 

[gadget73]

Not really a problem just how they work. Big inrush makes noises. My PL has its original switch and has never blown up so it can't be that much of a concern. Was built in 1982.

I run tube gear mostly. Not really willing to waste the power or tubes by leaving them on all the time.

 

[BlackHole]

Not really a problem just how they work. Big inrush makes noises. My PL has its original switch and has never blown up so it can't be that much of a concern. Was built in 1982.

I run tube gear mostly. Not really willing to waste the power or tubes by leaving them on all the time.

wow that’s amazing. 40 years old with original parts that’s incredible! My amp also is of the same age but not sure who owned it prior and I don’t know which parts are stock or what mods have been made to it. But it has been serviced and cleaned and recapped few months ago. I will do the same as you do. I will turn it off at end of the day. That’s best!

 

[whoaru99]

... What amps do you own? Do the ones you have use heat sink? Thanks!

Oh, I have quite a few different amps. These are just some, but they're the ones most relevant to the gist of the topic.

The ones that run Class A are Pass/FirstWatt F6 clone and several Yamaha M-series amps that can be switched to run Class A at low power. A couple of the high bias AB amps are Audio Research 100.2 and Levinson No. 23.

They all have a fair bit of heat sink. The F6, 100.2, and No. 23 have external heat sinks, the Yamaha M-series have internal heat sinks.

 

[gadget73]

wow that’s amazing. 40 years old with original parts that’s incredible! My amp also is of the same age but not sure who owned it prior and I don’t know which parts are stock or what mods have been made to it. But it has been serviced and cleaned and recapped few months ago. I will do the same as you do. I will turn it off at end of the day. That’s best!

I did replace the caps, but they were not bad when I changed them. The rectifier got changed / upgraded at the same time to go with the bigger caps but it was also still fine, and I actually used it in something else.

 

[llwhtt]

If your amp was serviced recently was the bias set correctly? If it was biased correctly the heatsinks on each side should be hot after a 1/2 hour or so. Very few people know how to bias this amplifier. To adjust this amplifier correctly a Harmonic Distortion Analyzer with a balanced input is needed as the outputs of each channel are both hot. I have a diagram draw by James Bongiorno himself showing how to do the adjustments. Short of having the correct test equipment you can set the bias to 900ma across one of the fuse holders in each channel. There are two trim pots and they must be kept even. That's the bias adjustment, the other adjustment is the "peak current" adjustment, adjusted for 3A at either fuse, 1KHz @ 60W into 8 Ohms.

Craig

 

[BlackHole]

Thanks a lot Craig @llwhtt
My apologies I was not checking the four corners of the unit, they were super warm like you said. I was testing top and back sides of the unit which were not warm to touch. My bad!
I will check in with the local shop who worked on this unit on how biasing was done. They wrote notes on the repair work that was completed.
Thanks a lot for your feedback.