Devices sharing a power supply

[Albatross]

Hello all. I'm hoping for a bit of advice.

I have a small diy amp running off a 12v camping battery for busking. Its good.

I also have a small delay unit based around a PT2399 board which has until now been outside the main box and running off a 9v pp3.

I would like to build the delay into the amp casing a bit more permanently and run it off the same 12v camping battery.

Question is, can I do this and if I can how should I wire it? Series or parallel, does there need to be some kind of isolation between the two devices being powered?

Be very grateful for any advice. Many thanks in advance.

[James Perrett]

I think we need more information. Can you give us the datasheet for the PT2399 board that you are using and a schematic or datasheet for the DIY amp?

[sonics]

I'd knock up (or buy) a 9V supply that uses the 12V battery. Voltage regulator (or zener) and a few components. I made a few of them a few decades ago back when I wielded a mean soldering iron. Then you've got power for future 9V pedals.

[Albatross]

Thanks Sonics.

I think we need more information. Can you give us the datasheet for the PT2399 board that you are using and a schematic or datasheet for the DIY amp?

Hi James.

The PT2399 datasheet is here https://electricdruid.net/datasheets/PT2399.pdf but it shows a supply voltage of 6v I believe.

The board is one of these https://www.ebay.co.uk/itm/254999495831 and it has a voltage regulator on-board and so will run off 6 to 15v.

the amplifier is a slight variation on one of these https://www.ebay.co.uk/itm/126234129967 and is based around the TDA7297. Datasheet is here https://www.st.com/resource/en/datasheet/tda7297.pdf

Really simple little busking amp and I have two home made preamps drivng that TDA7297 board. One j201 for the guitar and 2n3904 for a microphone.

I was just going to take two more wires off the battery switch and send them to the delay board and then I thought hang on I'd better ask in case it blows up with me sitting on it or something.

Very grateful for you guys time.

[James Perrett]

With careful wiring you could probably use both running off the same 12V supply. It looks like the reverb unit has an on-board regulator so there's no need for another one. You need to pay attention to grounding to avoid noise. There will be substantial currents going through the ground wire to the amplifier so you need to wire it up so that these ground currents don't affect the audio. I would suggest wiring the main power cable to the amplifier and then taking a power cable from the amplifier to the reverb unit.

Also, don't expect too much bass from the power amp. The suggested circuit shows a fairly small input capacitor which will filter out quite a bit of bass. Normally a capacitor around ten times that size would be used.

[ef37a]

A 78 9V regulator is cheap as chips and so it might be as well to fit one? Yes, the power wires need to go back to the 12V supply terminals so that no PA current runs in a common earth wire. The device draws only tens of mA so the dissipation in the regulator will be naff all.

James, the input resistance of that amp is stated as 25k minimum so a 0.22uF cap will give a -3dB point at just under 30Hz. For guitar use it might even be useful to come up an octave and use 100nF?

Dave.

[Folderol]

... and that's without taking into account the source impedance, which will also tend to lower the cutoff.

[Philbo King]

It's not a great idea to run gear at a voltage higher than it was designed for; there's a fair chance you'll smoke it.

So you need some sort of voltage reducing component. This might be a resistor, a zener diode circuit, several 1V silicon diodes, or a voltage regulator IC circuit.

The thing to watch about reducing supply voltage is heat buildup in the voltage reducing components.

You need to measure the current (I in amps) that the 6v or 9v device uses, and multiply that by how much the voltage is reduced to get the power wasted (P, in watts) by the voltage reduction.

P = (Vsource - Vload) x I

where Vsource is 12V
and Vload is 6 or 9V

If that power exceeds what rating for the voltage dropping component(s), the parts will fry. One simple and fairly robust way to do it is to put several 1N4148 silicon diodes in series, which typically drop a bit less than one volt each (so 3 or 4 diodes for a 9 Volt load).

[James Perrett]

Everyone seems to be missing the point here - the PT2399 board already includes a voltage regulator so there's no need for another one.

[Hugh Robjohns]

The delay module has its own onboard regulator and will happily accept a 12V battery supply without modification.

Just follow James' advice regarding the power cabling and all will be good.

EDIT — Simultaneous post crossed with James!

[ef37a]

"The delay module has its own onboard regulator and will happily accept a 12V battery supply without modification."

True Hugh but an inline regulator will reduce the dissipation in the onboard device and, more importantly, reduce any possible supply crosstalk. Many times have I read of weird things happening when lots of things are hung on a pedal power supply.

Dave.

[Wonks]

It will run off a 12v battery and there's just the amp and the pedal connected.

[Hugh Robjohns]

True Hugh but an inline regulator will reduce the dissipation in the onboard device and, more importantly, reduce any possible supply crosstalk.

How about just trying it the easy way first?

No need to invent and solve imaginary problems.

[ef37a]

True Hugh but an inline regulator will reduce the dissipation in the onboard device and, more importantly, reduce any possible supply crosstalk.

How about just trying it the easy way first?

No need to invent and solve imaginary problems.

They are not imaginary, people experience them. But "taking it easy" is good idea.

Dave.

[Albatross]

Thanks very much everyone. I had the back off today but forgot to look at the input cap on the amplifier board, it sounds all right though. I've been using it for quite a while and I'm happy enough. It's good for what it is.

I'll have a go at connecting my little delay board power supply in series with the amp and see how it goes.

Thanks Dave for pointing out potential issues. I always have an ear cocked to your wisdom on these matters.

Thanks again all, on we go.

[Wonks]

In parallel, not series!

In series you could possibly do a lot of damage by trying to draw too much current (amp plus pedal requirements), and neither device will get the voltage it needs as you might be getting say 7 v across one and 5 v across the other.

Always parallel.

[ef37a]

Heed Wonks Albatross!! Parallel, and thanks, nice to know someone appreciates this old valve jockey!

Dave.

[Albatross]

Now I'm confused, I was taking James' advice and taking it to mean 'in series'.

...I would suggest wiring the main power cable to the amplifier and then taking a power cable from the amplifier to the reverb unit.

Parallel is easier for me as I just take two more wires from the power on/off switch.

Am I reading James' advice wrong or is it a matter of differing opinions?

[Wonks]

James means parallel. But there are two ways to do it. You can star out from the switch to the amp and to the pedal with two cables, or you can daisy-chain to the amp and then the pedal. But in both cases you wire in parallel.

if you wired in series you’d go from switch +ve to amp +ve and then from amp -ve to pedal +ve, and pedal -ve to switch -ve.

[Albatross]

Thanks very much for clarifying that, Wonks. I daftly thought that daisy chaining meant 'series'. I'm grateful and enlightened. Thanks again.

btw, those little pt2399 boards are quite good for what they are and cheap as chips. For anyone interested there's a good clear write-up of how to mdify them to get at extra parameters here https://www.anexium.com/instruments/PT2399-Mods.html

[Hugh Robjohns]

Thanks very much for clarifying that, Wonks. I daftly thought that daisy chaining meant 'series'.

Nothing daft about it. Context is critical in situations like and the terminology is ambiguous.

Daisy-chaining does normally imply connecting devices "in series". However, power supplies are normally wired in parallel.

In your case, the issue is where to derive power for the delay board. The obvious option would be to connect both the amp and the delay board directly to the battery — positives to positive, and negatives (ground) to negative.

However, you're working with unbalanced signals which use the negative terminal as the ground reference, and it's critical that the amp and delay board have the same reference. Unfortunately, the amp will be pulling big currents, and big currents through the power wire results in a voltage across that wire, so the negative terminal at the amp will be at a different voltage to the negative terminal at the battery. If the delay board is connected directly to the battery, its ground reference will be different to the amp's, and you have a ground-loop!

The solution suggested by James is to power the delay board from the amp's power terminals, so any voltage developed across the cable back to the battery applies equally to both amp and delay board. They therefore have the same ground reference and there's no ground loop.

I hope that adds to the clarity

[Albatross]

It does and I think I understand although I'm still a bit vague until I can think it through. Certainly brings a bit more sense to my confusion. Thank you.

[James Perrett]

The solution suggested by James is to power the delay board from the amp's power terminals, so any voltage developed across the cable back to the battery applies equally to both amp and delay board. They therefore have the same ground reference and there's no ground loop.

I've been struggling with the tacho on an MTR-90 capstan motor all today so I'm glad Hugh jumped in and confirmed my thoughts.

[Albatross]

Cheers James and thanks for taking the time. I hope your machine is ok.

[Albatross]

Thanks James and Hugh et al. I got that all rigged as you suggested and its bloody great. There's no noise or hum at all and eveything tickety-boo.

Thanks again.