(for me) Strange design in power-amp

Thread Starter

Joppnl

Joined Dec 29, 2023
9
Hello,

I own some rather old loudspeakers, the Infinity Overture 2.

These speakers have a powered sub so only the lower part of the audiospectrum is being amplified to some speakers in the cabinet.

One of the amps broke down and as I was looking into the schematic I can't say I understand that much of it, escpecially this circuit (see attachment#1).

What is the intention of this schematic, how does it work?

The input is an audio-signal being amplified by some filter-opamps.

For me it's rather strange that this signal is being fed into the emittor of Q6/Q7.
Sure this will make Q6/7 active but what kind of signal will be found on the base of Q6/7 as this signal is going into U2A.

My thinking would be that the current into the emittor-collector of Q6/7 divided bij Hfe is available at the base and so at the (common)R53.


So, anyone who has a clue how this works, what's the function of U2A and what kind of dignal comes out of that (Vb).

As you can see in the second attached picture this Vb is being used to do something with the previous opamp-circuits (I think the first one is a low pass filter), (the input signal enters the scematic at the left into that transformer)


For those who like to see the total schematic, service manual can be found here: https://elektrotanya.com/infinity_overture_2.pdf/download.html (page 33-35)


Thanks!
 

Attachments

AnalogKid

Joined Aug 1, 2013
11,041
Without a more complete schematic, it is hard to say.

The left schematic has a corner frequency of less than 0.16 Hz, plus an integrator driving the output. It could be part of a DC servo used to keep the power amp output centered about GND without coupling capacitors in the signal path.

ak
 

Thread Starter

Joppnl

Joined Dec 29, 2023
9
Thank you both for your help.

I read the common base article, didn't know what it was how it worked but now I've seen it, good background info but I do not think it is applicable to the circuit I posted.

However, with the remarks from AK I came up with a 2nd thought, may-be a solution:

So, my initial thought was that the ACsignal should find it's way up and through U2A but now I think that isn't the purpose of this circuit.

I now think it's purpose is 2 functions:
first a delay anti-startplopper, the loud noise you can get when powering on an amp and second it will provide (delayed) DCvoltage to some opamps and circuits to first shutdown (delay) and then operate with that DC voltage (Vb as in the schematic).

My thougts (not sure they are correct)

the 2 diodes in the 1st diagram, D4 and D6, when Q6 and Q7 are active: a positive half of the AC signal will be grounded via D4/Q6 and a negative ACsignal will be "filled up" by the +24V/Q7/D5, resulting in a 0V (may-be a little more, may-be 2* 0,7V diode + some E/C voltage) but the result is just a DV voltage say around 2V being blocked by a capacitor in the inputstage of the power section.

I think that there is a timing circuit because of R49 and C24, when C24 is full Q6 will shut down, not sure how this works for Q7 but they should work parallel so also shut down. When the timing is done the diodes will no longer cancel the incoming AC signal.

Then there is some timing in R53/C25 and may-be also C27/R54 at the output of the opamp.

So, not sure how everything works exactly but a new thought which makes more sense then my first one, thanks for your help,
 

Ian0

Joined Aug 7, 2020
9,814
That's quite a scary circuit. The whole thing runs on live mains, and the audio input is coupled in with a transformer.
sch1 is the bias generator and a clamp for the input signal (not a common base amplifier), and it's not a servo for the output, because the output IS capacitively coupled to the speakers.
sch2 is the audio transformer input, and presumably, some compensation for the transformer frequency response.
If you are working on this circuit, use an isolating transformer (especially if it is the 230V version).
 

MisterBill2

Joined Jan 23, 2018
18,491
Without a more complete schematic, it is hard to say.

The left schematic has a corner frequency of less than 0.16 Hz, plus an integrator driving the output. It could be part of a DC servo used to keep the power amp output centered about GND without coupling capacitors in the signal path.

ak
The circuit you show us IS NOT the AUDIO circuit. It is some small portion of a bias control scheme, which, given that it is decided that we do not need to see any more, I can provide no more explanation except that "It does not look like an audio amplifier circuit because IT IS NOT an audio amplifier circuit. "
For any additional explanation we will need to see the rest of the circuit.
 

Thread Starter

Joppnl

Joined Dec 29, 2023
9
First thanks for your reply and offer to help me understand it.

I attached the 3 schematics which makes this total amplifier and I gave the Vb where it's found a red dot making it easier to localize.

The part of the schematic I posted before is found in the amplifier schematic, left upper corner.

It does 2 things: it generates Vb and it does something with the audio going into the power amp section.

The Vb is also used in the filter section: it is attached to the positive side of the opamp but also in the loop of the filter of the first opamp.

So my questions are: what's the purpose of Vb, how is it generated and how would this signal look like.

And...what's the purpose of connecting Vb into the middle of the loop_filter in U1A.

I tried to model this total section in Tina-TI but get very strange results I do not trust...
Thanks!
 

Attachments

Ian0

Joined Aug 7, 2020
9,814
So my questions are: what's the purpose of Vb, how is it generated and how would this signal look like.
Vb is the ground for the power amplifier. It looks like a fixed voltage that is between the 320V and 0V rails. It has to be sufficiently above 0V to accommodate the biggest negative-going peaks of the input signal
 

Thread Starter

Joppnl

Joined Dec 29, 2023
9
Well...not so sure about that...

The ground in all of these schematics is represented by a triangle facing down, the Vb in the powerschematic is used to switch Q8 and in the other schematics also to switch something with the opamps.

My guess would be that Vb is something between 0 and 24V as it comes from U2A which is being powerd by 24V-GRND

The input of U2A is R53 and if I am right: U532 = ((320V/ (300K+3k01+3K01+9k09)) * (3K01+9K09) = 4,23V.

So that should be the input of U2A, output depends of what U2A is doing with it...

What I think for normal operation Vb should be 0V: in the power section that will make Q8 operate but a positive (my calc: 4,23V) on the positive input of an opamp does not make a zero volt on it's output so...well, more questions comping up the more I try to understand :)
 

MisterBill2

Joined Jan 23, 2018
18,491
Based on that earlier circuit schematic that shows the Vb source, it appears that Vb is a center reference voltage for audio signals. It also appears that the circuit was originally for an electric organ. Vastly more complex that even a top end music amplifier. With so much more circuitry it is no wonder that something failed. The active adjustment scheme may be to assure that some direct coupled portion avoids clipping and avoids putting DC into the speakers.

I do not like direct coupled audio amplifiers because a failed small front end transistor will usually destroy a very expensive output transistor, or both of them, or a non-replaceable output module.
 

Thread Starter

Joppnl

Joined Dec 29, 2023
9
Yep...

when an amp like this dies it's (at least) always the 2 mosfets because when 1 goes, with this high voltage, the other one goes as well. Good news is that a replacement part isn't that expensive (about euro 3,50 each) but the most difficult thing is to know what replacement mosfet will do the job.

Also (because they are rather old) when a capacitor starts leaking because of the high voltage the voltage may take down other components as well. Also the print has a lot of glue on the components which might do strange things when getting old.

Most safe way is to measure the components off-line so without connection to the mains.

On the other hand...they are old but as long as you use them they are pretty much ok...just when you have them stored away and try to use them again they start to fail.

But for me the exact -how it works- still remains a mystery....
 
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