Need Help with this Circuit Please

[rievax_60]

Re: Need Help with this Circuit Please

That's right. I would set it very low on that amplifier. Try to set it to 2mv across the total emitter resistance. The main reason is to check that it isn't too high. Just need to be very careful with the meter probe to not slip and short out something.

[budm]

Re: Need Help with this Circuit Please

Typical idling current on the output transistor is about 15~20mA. What is the Emitter resistor value?

[keeney123]

Re: Need Help with this Circuit Please

Typical idling current on the output transistor is about 15~20mA. What is the Emitter resistor value?

Bud M is that set at a current just below turn on of the transistor or just after turn on?

[budm]

Re: Need Help with this Circuit Please

http://sound.westhost.com/articles/amp-classes.htm
15~20mA are what I see in most service manuals for setting up the idling current to reduce crossover distortion. You can see it on the scope.
http://www.eetimes.com/document.asp?doc_id=1274880

[rievax_60]

Re: Need Help with this Circuit Please

This amplifier is unusual. The drive transistor collectors are fed by resistors from the main rails. The manual explains how this provides short circuit protection at low output levels.
It's odd that the manual doesn't seem to mention setting the idle current.
The idle current should be set lower than usual on this amplifier. Under 10ma.

I worked on an old Pioneer model that had no idle current adjustment. The idle current was set to zero but with the BE voltage set to almost turning on. It sounded fine.

[keeney123]

Re: Need Help with this Circuit Please

http://sound.westhost.com/articles/amp-classes.htm
15~20mA are what I see in most service manuals for setting up the idling current to reduce crossover distortion. You can see it on the scope.
http://www.eetimes.com/document.asp?doc_id=1274880

Is this with a AB amplifier? I know from memory in an AB amp one wants to keep the amp on the verge of turning on. As I remember vaguely to turn off a transistor before one turns on in their crossover frequencies and yet have them on the verge of turning on so there is no delay in the turn on time which would eliminate dead frequencies response at the crossover point. I actually studied this online a couple years ago on Analog Devices Courses. They actually use to test you and you would get feedback as to how you did and if you passed and were OK to go on to the next level, but like with all things I study like this I forget almost as fast as I learned it. I guess I will assume that the 15 to 20 mA are on bipolar transistors in a AB push pull, is the E current, E= B +C current or virtually C current, on the verge of turning on the transistor out of is Knee region.

[rievax_60]

Re: Need Help with this Circuit Please

What ever the idle current should be, it is more important that it should be checked. Not a good idea to put an amplifier into service in an unknown state.
That is an informative link, http://sound.westhost.com/articles/amp-classes.htm

I met a very unusual design of output stage lately, Yamaha's floating balanced amplifier,

[tfs2015]

Re: Need Help with this Circuit Please

Everything is working good except for a popping sound is heard when the unit is powered down and I cannot figure out why any help would be greatly appreciated

[keeney123]

Re: Need Help with this Circuit Please

I have heard of this before can't remember the cause. You might take a DC voltage reading from the Speaker L to speaker ground same for the right speaker. There should be near O volts DC. The other thing might be the bounce of voltage when you turn off the switch. You might be able to check this by just unplugging the power cable when the amp is on and see if the amp pops. Just remember to turn the switch off before you plug it back in. It should not really matter just good practice, I think.

[budm]

Re: Need Help with this Circuit Please

Everything is working good except for a popping sound is heard when the unit is powered down and I cannot figure out why any help would be greatly appreciated

It sounds like the preamp section shuts down before the power amp section.

[keeney123]

Re: Need Help with this Circuit Please

It sounds like the preamp section shuts down before the power amp section.

Question, are you also saying that the power amp is shutting down to fast? I'm I wrong to think that there is some DC charge that is collapsing through the speaker?

[budm]

Re: Need Help with this Circuit Please

When you have preamp and power amp, the proper way to shut down the system is to turn off the power amp first then the preamp, on boot up mode the preamp should be on first and then the power amp.
That is one of the reason you can hear a thump, but you can easily verify that by grounding the input of the power amp and see if the thump is still present, if it is still there then he will need to look in the power amp section, if it is not there then he will have to look at the preamp section. He should also look with the scope to see how big the amplitude is going into the speaker.

[keeney123]

Re: Need Help with this Circuit Please

So there you go follow BudM's instructions. Let us know your results.

[tfs2015]

Re: Need Help with this Circuit Please

Ok when I turn the power switch on both the mixer and the power amp receive power at the same time, so all I can do is ground the input of power amp to see if it makes that same noise. If I remember correctly if I unplug the reverb board power that sound that is heard when turning the power switch to the unit off is not heard anymore but I need to confirm that and I will let you know.

[budm]

Re: Need Help with this Circuit Please

http://www.prosoundweb.com/article/t...er_sequencing/

http://www.behindthemixer.com/pow-po...e-right-order/

http://www.sweetwater.com/insync/pow...-off-sequence/

http://www.longbeachindependent.com/...-sound-system/
You should have power sequencer.

[budm]

Re: Need Help with this Circuit Please

Page 8 of the user manual.

POWERING UP
Turn the Power Switch to the ON position. The Power
LED will illuminate green and the system will turn on. If
other powered equipment is to be attached to the system,
it is always advisable to turn on your Passport last. In this
way any transient spikes and thumps caused by other
equipment will not be amplified and sent to your system
speakers. For the same reasons it is advisable to turn off
your Passport system first before turning off the attached
equipment.

[keeney123]

Re: Need Help with this Circuit Please

Seems a pretty convincing argument. I just wonder why they would not make it so there is no chance of this happening, no matter what was power up or down at any sequence. You know that some guitar players are not even going to read the manuals of the amp they are just going to use them. This would really be the case if they have a few drinks in them. Then they will wonder what went wrong with there amp and figured it is just cheap equipment and go down the street and pick up another amp. Seems the manufacturer would make it so this does not happen so there names would not be cursed at.

[tfs2015]

Re: Need Help with this Circuit Please

If I just turn on the Passport with no input going to it and then turn it back off, it is still making that sound through the speakers. If I unplug the Reverb board power and then shut down I don't hear that sound through the speakers. I notice that with my car amps when you shut them off, they turn off nice and quiet with no noise from the speakers.

[SteveNielsen]

Re: Need Help with this Circuit Please

Perhaps some leaky (electrically leaky, not physically) DC blocking caps north of or in the reverb unit.

[budm]

Re: Need Help with this Circuit Please

is the reverb unit a stand alone with its own AC power? is the output of the reverb unit feeding the the power amp inputs.
Lots of good pro audio processing equipment will have Muting transistor or relay on the audio line out so when power is shutdown the output will be disengaged, power amp will also have speaker protection relay that will disconnect the speaker when power is turned off.