Bridge Recitifer voltage drop question

[imJonWain]

    I was taking some voltage measurements inside my Laney AOR today and came up a little puzzled by the numbers in one part.  There is a seperate CT winding of the transformer for powering the SS portion of the amp with +/-12 Volts (see link to schematic).  Here are the measurements I got...

AC Voltage from transformer: 23.65V across or 11.81V to CT   < So it's a 24v CT winding

Rectified After diodes: +/-13.42 V    < 11.81 being the RMS value, shouldn't this be higher?     11.81 * sqrt(2) = 16.7 - (2*.7) = 15.2V ?

  Is this due to measuring rippled DC and the filtering being after the resistor? 

  The link below lists Vdc (without loses) = .647Vmas = .9Vrms
  That plus the voltage drops makes the numbers quite close but am I taking this out of context?


82ohm Resistor Drop: .988 V  <  This implies the circuit is drawing ~12mA, correct?

for +/-12.3V



Schematic:
http://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-AOR50-Schematic.pdf

What I am reading.
https://www.electronics-tutorials.ws/diode/diode_6.html

[PRR]

It works out right.

The 82r resistor voltage is very narrow spikes. Your meter may not read tight. Also it makes the effective full-cycle resistance much higher than 82r, more like 300r.

The load is three dual-opamps, say 9mA at nominal 30V. Which is about 3K equivalent. 300r series makes a non-negligible droop into 3K load.

Just like that, I get 14.8V DC.

But then there are the "15V" Zeners. And they are rated up to 100mA! They may be drawing a few mA even at less than 15V. If I assume they suck 6mA or 7mA (7% of rated current) at 13.4V, it comes about right.

Simulator does not have the +/- supply topology so I simmed half of it, since the other half works the same.

The Zeners will really suck-down the ripple tops and suck less on ripple dips. The actual waveform will be different.

It isn't very critical. You probably want over +/-9V for decent headroom. You must-not-ever go over +/-18V or chip life will be short. If the wall voltage is high and these rails try to go over 15V, the Zeners will clamp authoritatively, so all is safe; as they brownout sag below say 12V the Zeners will quit sucking and opamp rails won't drop far; some un-regulated stage will sound strained before the reverb does. Ugly waves will not bother the high-PSRR chips much.

[imJonWain]

Awesome, thanks for such a thorough answer!  I didn't even think about the the voltage drop due to the load's resistance/impedance, derp, makes sense to me.  That's interesting to know about zeners too.

So this question came up in relation to the PCB I am working on to replace the Effects Loop/Reverb PCB in the same amp as the above schematic.
http://www.diystompboxes.com/smfforum/index.php?topic=119509.0

I was planning on playing with the PS component values (smaller R, schottky diodes) here to try and get a bit closer +/-15V but do you think it's fine as is and not worth it?  I've been running the recovery stage on +/-12 for almost a year now and haven't had any issues.

[PRR]

> try and get a bit closer +/-15V

Why? It's only a reverb path. It may not ever need headroom. The change from 13.5 to 15 is not enough to hear. Leave it be.