PSU amperage question...

[iomegaman]

So I'm rebuilding the psu for the Amerimex StudioMaster II console without a schematic...

https://groupdiy.com/index.php?topic=72810.0

The pcb itself is a narrow board with three voltage regulators (that at most can 1.5 amps each) where the AC transformer is offboard (which I did not have nor can I find) this is a rare board with no real history trail)...

Anyway it has 26 channels of TL071 (6 each) which require -/+ 15 plus another 8 chips for cue and headphone strips brings me to 164 TL071's...

My Soundcraft has 2 TL072 per channel at 32 channels...plus another 8 or so...so 72 X 2 for dual chip = 144

According to the data sheet these chips BOTH can use between 1.4 -2.5 mA's per amp (TL071 is single)...so doing my math:

The Amerimex would draw 410 mA's and the Soundcraft would draw 360 mAs but the difference in these power supply boards seems huge.

...and the Soundcraft using the same voltage requirements for the chip is a +/- 17 volt NOT +/- 15 as TI recommends for these chips...

What am I missing here? Even if the Amerimex uses a higher amp transformer the regulators (7815's/LM38OT15) can only output up to 1.5 amps of current and BOTH TL07XX chips draw the same current at peak...and the most either of these would draw is less than half that...

The Soundcraft psu I have is built like a tank, and Amerimex CONSOLE is built like a tank but it seems the onboard PSU (Which is a Channel on the board so its like 2" wide)is kinda puny...but looking at the chips specs its not like we even need a whole amp here...

What am I missing?

I know the Soundcraft also powers the 48 phantom (so does the narrow Amerimex strip) it also drives the Vu's (there are only 4) so it does +/- 17, 5v, 24v, 48v...but the Amerimex powers its HUGE 10 Vu's from a seperate power supply (which I have a medical grade unit that works fine)...

I get that the Soundcraft is using an OEM special multiple out transformer donut the size of a small car tire...but I did get this Amerimex to fire up a handful of channels using off the shelf 16v AC transformers at .625 amps but it is not nearly enough...

On another note both the TL071/TL072 can handle +/- 5-15 supply voltages...why is SOundcraft supplying +/-17 ?

I'm scratching my head here trying to get it around the spec sheet (ideal) and the reality on the ground plane...

 

[ruffrecords]

The current data you are using for the TL072 is the quiescent  or idle current with no signal present. Notice the data sheet says Vo = 0 and no load. Like most op amps, the TL072 has a class B output stage which means it draws a lot more current when there is signal present - how much depends on the signal level and the load. So it is not unreasonable to need to design a power supply to be able to handle several times the nominal quiescent current of the op amps.

Cheers

Ian

 

[iomegaman]

The current data you are using for the TL072 is the quiescent  or idle current with no signal present. Notice the data sheet says Vo = 0 and no load. Like most op amps, the TL072 has a class B output stage which means it draws a lot more current when there is signal present - how much depends on the signal level and the load. So it is not unreasonable to need to design a power supply to be able to handle several times the nominal quiescent current of the op amps.

Cheers

Ian

Thanks Ian, makes sense I guess they have no idea how you will use it so some spec about "0v" is appropriate, by the way your article on grounding was awesome, you should write more, or if you have send me links, I learned more reading your article than ten years of trying.

So in response where does one start in trying to determine what size transformer/psu Amp rating one needs?

The Soundcraft will suggest 60 dB of gain on the channel strip whereas the Amerimex only provides 40dB...I know you can "over-volt" a chip hence the design warnings...(I guess I should actually put a meter on the Soundcraft and see what the chips are drawing...)

 

[ruffrecords]

It would be instructive to measure the current on the Soundcraft. You could try it first with no signal which will give you an ide of the minimum. The run a few channels and wind up some of the outputs well over the nominal +4dBu and see how much the current rises. It would at least give you a feel for things.

Since both existing PSU design seem to use regulators designed for at most 1.5A then it would be safe to use that figure for the transformer design. You might end up with a transformer a little bigger than you need but it will at least not be the limiting component for performance.

So you need +-15V at 1.5A. Standard regulator chips typically need 3V across them in order to regulate properly so you need to be inputting a raw dc voltage of about 18V to each one. Many people use 15V secondary transformer because they are readily available. With a capacitor input filter, this will have a peak output voltage of 1.414 x 15 = 21V which gives 6V across the chip. This is plenty but it does mean the maximum dissipation in the chip is now 6V x 1.5A  = 9 watts so it could need a big heatsink.

So the maximum required input power is 21V x 1.5A = 31.5 watts so for two rails it is 62 watts so you could just use a 60VA rated transformer. However, rectifiers take short pulses of large currents which I have found can easily lead to transformers getting really hot. So I now habitually over rate transformers by about 60%. In this case I would use a 100VA rated transformer.

This may end up being total overkill which is why it is quite important to know the exact current demands the power supply is likely to face.

I am pleased you found grounding 101 useful. Right now I am writing one on audio transformer design.

Cheers

Ian

 

[iomegaman]

Thanks so much Ian, full disclosure I wanted to be a rock star but ended up raising my five kids as a single dad, so audio is my vanity hobby...40 years in construction I am a hack at this and I am not ashamed to say I am kludgy at times, but my kids are all engineers, attorneys computer scientist and social workers, so I won.

This is my fun...although I have learned to curse more lately...

You have been extremely helpful as are almost all here...by far my favorite internot watering hole.

 

[iomegaman]

Turns out the Soundcraft PSU fired this thing up like a champ...I checked the voltage and as it goes the PSU provides +/-17 volt rails but those get filtered down to about 15.45 volts on the actual chips...

When I got the Soundcraft (200B paid $50.00 for it a year ago!) there was no OEM cable and it used that weird and outdated Amphenol 10 pin big circular connector...no way in hades I was going to pay $175.00 for a cable so I ordered parts and made my own...popped a new hole into the back today and wired up a Switchcraft FM-7 pin (only need 5 but its what was built into the board, 2 were for power switch which I won't use)...so now my Soundcraft PSU is interchangeable between the consoles and I only have room for one at a time so...we're golden.

Something wonky with the cue channel and I have about three channels that I have disconnected because they keep peaking the gain LED with nothing in them, phantom works, the Monitor send channels are incredibly quiet...(4 L/R) the mains have some high frequency hiss that I think is in the cue channel, but it only has 4 TL071's so it won't be much to track it down...

My ears are ringing because of feedback into the Mackie HD824's (which do not come with a gain setting!Blech!)...so tomorrow I will either go old school with my JBL 15" floors or the KRK 6" so I can keep the feedback from piercing the air...

Without a manual its a bit of guesswork and punching big buttons to figure out the signal flow...

 

[ruffrecords]

That is very handy being able to work either mixer from the same supply. The only thing that puzzles me is your mention of a switchcraft FM-7 pin connector. I can't find this listed anywhere on the switchcraft site.

Cheers

ian

 

[iomegaman]

Got it local surplus electrical store...

They sold it as a "Switchcraft FM-7" maybe their own label system, a bin full of "em.


They may have labelled it wrong...

https://www.elliottelectronicsupply.com/switchcraft-7-pin-female-xlr-inline-connector.html







The other thing that was kinda odd about this console is it had a IEC plug port on the back, but it was male and one of the pins was a 1/4" plastic looking dowel so there was no way to plug in a regular cord to it...the other two terminals go to the switch on the board and I assumed it was the way you turned on whatever psu they provided and there must have been some female port on the psu specially designed for a special cord...

 

[ruffrecords]

Got it, it is a regular 7 pin XLR.

Cheers

Ian