Replacement Toroidal Transformer

[RishiRishiRishi]

I still think a 15 volt transformer is to high.
30 Volt, bridge rectified will produce 30 x 1.414=42.42V minus 2 diode Volt drops gives about 41.02 Volts.
The regulation of the VTX-146-050-115 (if that is the one you ordered) is 14%, so maximum off load voltage could be 46.77 Volts.
This is more than the maximum rating of the two smoothing capacitors at 40 Volts!
Even if one allows for copper losses under load, I would think the final unregulated DC voltage will still be too high.

As is often the case, it would be prudent to replace the 40v Electrolytics here anyway, with caps rated 50v or 63v at 105oC.
And possibly the Zener diode.

So would the 15V transformer be ok, if I replace the caps?

What do you think of the proposed wiring?

 

[Walrus]

Even with uprated capacitors, there may then be the problem of excess dissipation in the regulator transistor as it only has a small heatsink.

Re the wiring, yes the vu meter connection looks right. But the Orange and Red wires need to go to the AC terminals of the bridge, not ones marked positive or negative!
A picture of the bridge rect would help.

 

[RishiRishiRishi]

Thanks! And apologies, they're not actually marked like that - one goes to ground which I was intending on connecting the orange wire to, and the other one, the red. Please find attached a picture of the bridge rectifier.

Also, why would the schematic specify 15VAC if this is too high?

 

[RishiRishiRishi]

Just an after thought, since this application is centre-tapped, would the V be 15, and not 30?

"For centre tapped - the first diagram above again shows how this can be done - you need to look at the bottom left. What you will get is a centre tapped wiring - the centre tap is where the word link is. In a 2x 12V transformer example, the configuration 12-0-12. The maximum current that can be pulled in this configuration is half that rated by the transformer (i.e., the rated current for each winding) as you are effectively getting two times the voltage." - Wiring Toroidal Mains Transformers

So here, with a 2 x 15V transformer that is centre tapped, it would essentially be 15-0-15. Some quick maths tells me 15 x 1.414 = 21.21 V

 

[MidnightArrakis]

I've purchased the toroidal transformer - I can fit in a 2x15V, 50VAC.

[50VAC] -- Showing "50VA" is all that you need to show when relating to the "VA" rating of transformers. "50VAC" indicates an AC-voltage rating.

Personally.....I'm not that crazy about having the mains "Earth GROUND" being connected to the signal electrical GROUND. I find it OK to have the mains "Earth GROUND" connected to the metalwork of the chassis, but not to the electrical GROUND of the circuitry.

You may also need to reverse the wiring on the input XLR Pins 2 & 3 as well, since things are now "Pin-2 HOT":


And, finally.....the bottom sine-wave symbol shown on the bridge rectifier is incorrect. The bottom sine-wave symbol should be shown as being mirrored vertically in relation to the top sine-wave symbol. I know!!! I know!!! Nitpicking!!!.....



/

 

[Walrus]

Yes, the terminal that is under the - negative symbol should connect to the same track as the first capacitor negative is connected to. And like wise for the
+ terminal., to the same track as the first capacitors positive terminal
The Orange and Red wires need to go to the tracks that are under the bridge's AC pins.

"For centre tapped - the first diagram above again shows how this can be done - you need to look at the bottom left. What you will get is a centre tapped wiring - the centre tap is where the word link is. In a 2x 12V transformer example, the configuration 12-0-12. The maximum current that can be pulled in this configuration is half that rated by the transformer (i.e., the rated current for each winding) as you are effectively getting two times the voltage." - Wiring Toroidal Mains Transformers

So here, with a 2 x 15V transformer that is centre tapped, it would essentially be 15-0-15. Some quick maths tells me 15 x 1.414 = 21.21 V

Notice the "as you are effectively getting two times the voltage" i.e. 30 V
Your required supply voltage is 24, how will you get this if there is only 21.21 going into the regulator?

 

[RishiRishiRishi]

I'm tempted to just try the 15V transformer when received and see it it get's it going, and try and measure certain reference points if it doesn't explode as soon as it's connected. I got another response off Allen and Heath:

"It needs +24VDC for the transistors and op amps. You'd need to count them and add up the total current draw to arrive at a number. Plus the 15VDC supply to the meter lamps. Add up their wattage to arrive at a number. The combined number would be the minimum power rating for a replacement transformer."

 

[NoisyIndividual]

It may be worth noting that a typical 50VA toroidal running at 50VA will reach a temperature around 42oC above ambient.
The figures aren't given for a 80VA toroidal running at 50VA, but I suspect it will be considerably less than that, given that it gets to 50oC above ambient at 80VA.

I know which I'd rather have in my desk if it would fit.


https://www.farnell.com/datasheets/3964074.pdf

 

[RishiRishiRishi]

I assume that would be at maximum draw? After some more maths a 2 x 12VAC it seems more appropriate. Also, the 15VAC on the meters look inappropriate as per schematic. Should be a floating ground

 

[NoisyIndividual]

My point is, that for the same current draw/piece of equipment, a smaller transformer at near maximum rating will probably produce more heat than a larger one at 60% or so max.

You might wonder why Allen & Heath chose a 15v transformer and not a 12v one.
Was this some kind of blunder then?

 

[Walrus]

You are ASSuming that the "15 V AC to meterlamps" designates a 15-0-15 transformer.
It maybe a 12-0-12 (or custom) transformer, and the actual measurement, taking into account regulation, at the centre tap to 0 V is 15 V AC.
Cheap 12 V transfomer with 20% regulation gives 14.4 Volts.

Agree about the heat from loading at max though.

 

[NoisyIndividual]

You are ASSuming that the "15 V AC to meterlamps" designates a 15-0-15 transformer.
It maybe a 12-0-12 (or custom) transformer, and the actual measurement, taking into account regulation, at the centre tap to 0 V is 15 V AC.

Why would anyone write that on a schematic?

And there is no need for insults.

 

[Walrus]

Utmost apologies, that was not meant as an insult, just it never pays to assume something on a schematic is correct.

I'll bow out of this thread.

 

[MidnightArrakis]

I'll bow out of this thread.

[I'll bow out of this thread] -- BUT!!!.....WAIT!!! >> YOU!! << ARE "THE WALRUS"!!! ---- Goo goo g' joob.. Goo goo g' joob, G' goo goo g' joob, Goo goo g' joob, goo goo g' goo g' goo goo g' joob joob!!!

(In case you just might be wondering, the above "Goo Goo's" were taken directly from The Beatles' website)!!!

/

 

[MidnightArrakis]

Utmost apologies, that was not meant as an insult

[that was not meant as an insult] -- You will need to learn -- AND TOLERATE -- that many of the members here in this forum may be "younger-generation snowflakes" who will take offense for even the most inane comments!!!

Like yourself.....I have posted a variety of comments and replies in the past within many of these threads where what I had written had basically "no ill-will" intent whatsoever!!! However, someone along the way took offense to my comment and then reported me to one of the moderators on this forum. Bottomline??? ..... I have had at least three of the moderators on this forum threaten to -- BAN -- me from this forum due to my postings!!! It was all I could do to try and explain to each of the moderators what it was that I had written and what my intent and/or meaning was behind my comments!!! It finally got to a point where, despite as much as I enjoy the various threads in this forum, it wasn't worth it to me to have to "walk on eggshells" whenever I posted a comment. Even as I write this response to you, I wouldn't doubt that -- SOMEONE -- in this forum will take offense to my comment written here and report me to yet another moderator!!!

This is how today's "CANCEL-CULTURE" works!!! If someone disagrees with what you say.....report them, destroy them and ban them!!! Then, the only opinion that is left is theirs and the world becomes their world-view!!!

HANG IN THERE, BUDDY!!!

/

 

[NoisyIndividual]

Utmost apologies, that was not meant as an insult, just it never pays to assume something on a schematic is correct.

I'll bow out of this thread.

Well I sincerely apologise if I misinterpreted your post, but I didn't know how else to read it.

 

[RishiRishiRishi]

Can we please not digress - everyone here is interested in electronics and let's stick to that. Everyone's input has been invaluable and I don't see any need to describe certain generation as something or other or to find reason to discuss something that irks them outside of the electronic landscape!

Let's stick too the more interesting debate at hand around transformers!

 

[Mr. Moscode]

The diagram says 15vac to meter lamps but that diagram has a mistake. You can't have the - side of the bridge and the center tap of the xfmer going to ground. It's a short through the bridge rect.

The center tap should go to ground and the 15vac comes off the blue winding. Then the lights get AC.

So that said, you need a 15 vac dual winding xfmer. I would think 50VA would be enough for this.

For the math:
30 X 1.41 = 42 volts unloaded at the ps input cap, which is underrated volts wise, should be 50 or 63V.

If you can cobble something in there you can get measurements for current. Either a xfmer or a power supply. Adjust for 40 volts at the first capacitor.

After you get it going the voltage drop across the 10 ohm resistor will tell you the current drawn by the circuit, V/10 = current, multiply that by 1.8 for peak current derating to allow for I2 R losses in the transformer for this kind of circuit. For ex: you measure 2 volts across the resistor, that gives you .2 amps, multiply by 1.8 = .36 amp rating for the circuit, add the rating of the bulbs, say .15 amps each X 2 = .3 amps. .36 + .30 = .66 amps at 30 vac = 20 VA fmer. You have to get your own measurements.

You should oversize the xfmer because the PS draws 1.8 times the current at the peak of the sine wave and that can cause the xfmer to radiate 60 cycle magnetic pulses. With a larger transformer you have more iron to contain the magnetic field so magnetic radiation will be less. Very important for a mic preamp with all that gain.

 

[NoisyIndividual]

It should further be noted that unless the fuse to the 24v 2SD regulator is blown, the mains transformer will always be fully loaded.

There is no on/off switch for the desk, just the mains on/off.

 

[13engrsapper]

https://www.sowter.co.uk/rectifier-transformer-calculation.php

Leave the meter lamp issue alone and get everything running first. I can hear my engineer biz partner saying "I don't give a F$%K if the lights work, I need sound."

I could be wrong, my ex-wives tells me I always am. But I will chime in on where I would start as I am stuck on my own project at the moment. With a dead supply I nomally would bring it up to voltage slowly on a desktop DC power supply to see what else is wrong with the whole unit while I fix the supply (bypassing the original supplies rectifier). Alternatively you could power it at the bridge rectifier with a Variac bringing up the voltage slowly until everything was functioing correctly at 24V then just measure the variacs voltage. You could also measure current draw to help better dial in which transformer to choose since it is powered.

On the transformer...
First you need to get to 24V plus regulation voltage minimum. Let's say 26V (smarter guys here can get you closer but this should get things rolling) - this puts you at an 18V transformer for a full bridge rectifier (see handy Sowter link above). I would find a dual 9 volt or center tapped 18V, and wire it as in the schematic.I would guess 20-30VA size range based on the fuse. Remember to aim higher on the amperage for a full bridge - refer to Sowter chart (add some for your meter lamps too).

Once the unit is running, setup lamps to run on the 9V and convert to LED as tungsten bulbs are getting harder to find (get tungsten colored ones or amber)

;-)