Noob question about PS rail

Thread Starter

kiltro

Joined Oct 24, 2011
50
I'm looking into this (attached) schematic for a bass amp.

I'm not sure, why the low voltage supply (+-16V) has his own transformer windings, regulators etc. but then it's connected to the hugh voltage rail? (I've circled in red)

Thanks
 

Attachments

BobTPH

Joined Jun 5, 2013
9,138
The 16V+ rail is driven by a 15V regulator with a diode in the ground to make it about 15.7V. Then the high side is run through a diode to make it about 15.

Don’t have any idea why. Maybe someone else does.
 

WBahn

Joined Mar 31, 2012
30,242
It looks like there are two power sources for the circuit. My guess is that it is designed to work when either one is connected and if they are both connected the intent is that the 15 V source can't back feed the 16 V source (but it looks like the 16 V source could back feed the 15 V source). I suspect that figuring out the answer would require knowing more about the larger system of which this is just one part.
 

Thread Starter

kiltro

Joined Oct 24, 2011
50
It looks like there are two power sources for the circuit. My guess is that it is designed to work when either one is connected and if they are both connected the intent is that the 15 V source can't back feed the 16 V source (but it looks like the 16 V source could back feed the 15 V source). I suspect that figuring out the answer would require knowing more about the larger system of which this is just one part.
I can post the entire schematic maybe?
 

Attachments

dl324

Joined Mar 30, 2015
16,996
I'm not sure, why the low voltage supply (+-16V) has his own transformer windings, regulators etc. but then it's connected to the hugh voltage rail? (I've circled in red)
The rails you circled are coming from the +/- 15V regulators (that are increased to +/- 15.7V due to the diodes in the ground connections).

They're inconsistent in the use of those connection arrows:
1671724693968.png 1671724743006.png
1671724825738.png

It's also a bit strange that they put the polarity sign after 'V' instead of before the number.
 

Thread Starter

kiltro

Joined Oct 24, 2011
50
I still don't understand.
In the first file I've posted (Combo 115_2.jpg) there is a +-16V powersupply in the lower part of the page.

Senza nome.png

Then at the top of the same page (so I suppose coming frome the previous supply rail) I see +16V rail that through a led becomes (16-0.7) 15.3V? But then why it goes to the high voltage rail?

Senza nome 2.png
 

dl324

Joined Mar 30, 2015
16,996
But then why it goes to the high voltage rail?
The high voltage rail is generating 15V via the 1N4744 zener diode.

Don't know why they thought they needed to connect two different 15V sources together.
1671729886751.png
Also don't know why they're cluttering the schematic with terminal numbers on the diodes, resistors, and caps...
 

Lo_volt

Joined Apr 3, 2014
321
It looks like there are two power sources for the circuit.
It looks to me like the +/- 16V rails are low power for the pre-amp circuit. 7815 and 7915 typically can't supply more than 1 amp with a good heatsink. That's not enough to drive a power amp. The rails coming in from the "POWER XFMR HIGH VOLTAGE SECONDARY" are for the output section at higher voltage and power as well. Note that those (HV) rails feed the output transistor arrays. They are connected via a 2.7k resistor. That's not going to allow much current to flow, milliamps at most depending on what voltage the HV rails are at. I doubt that's enough to supply the pre-amp circuit.
 

MisterBill2

Joined Jan 23, 2018
18,970
In addition to the explanation in post #9, consider that the input stages are handling much lower power levels and are much more sensitive to small disturbances on their power rails. So a good design always tends to provide adequate isolation between power for the low level stages. The connection that I see does not make much sense to me, either, so it might be included for some initial testing of the amplifier that we do not see, or it might even be for an alternate configuration, not shown.
Also, there is a fair amount of unknown circuits tied to those multi-pin connectors, which we do not see.
 

PaulEE

Joined Dec 23, 2011
474
Original 30-second assessment:
Maybe the diodes allow the power from the 7815/7915 to help the output stage when the HV Zener-supplied +/-15V starts to be dragged down, while also allowing the HV Zener rails to absorb voltage spikes coming off the +/-16V. Just my 30 second thoughts...

Second assessment because I didn't like the first one and wanted to make sure I wasn't missing something (spoiler alert, I was):
If you look at the high-voltage transformer rectifier diodes, bleeder resistors, and capacitors, note the capacitors are rated 63V. This is definitely not just a +/-15V rail here of course. It is also well known that Zener regulators are not as stable as current fluctuates, which will definitely happen on a power amplifier output stage.

My second thought, therefore, is that the op-amps, Q102/Q103 and Q100/Q104 buffer the audio to prepare the output transistors (off schematic) with high voltage and moderate current. The feedback is 47K and 1.8K, which is a fairly stout ratio to get what is probably +/-45-50V down to just a few volts for the op-amp feedback. Those Q100/Q104 transistors, by the way, are rated 120V @ 3A each! (SJE5331/SJE5332)...PNP drawn incorrectly...and this bass amp is apparently rated 300W. V^2/R, where the speaker is say 4 ohms, gives you 35V to get that kind of wattage, and just shy of 49V in the case of 8 ohms. I bet those rails are around 50V before the "Zener'ing".

The diode-connected 7815/7915's are probably there to keep those lower-voltage rails "honest" in the event of large rail fluctuations to preserve biasing in the discrete output stages, but only kick in (demanding power dissipation in the linear regulators) when needed, helping to keep things a little cooler. They also probably prevent the 2.7K resistors from becoming smoke generators.

That's my assessment! And definitely not a "NOOB" question, as it turns out...thoughts anyone on this?

Paul
KI5VNH
 
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