Phono Preamp+PSU

[MTron]

Hey all, i'm nearing the end of another audio project. This time a couple of Phono Preamps, and their respective power supplies.

Here are some pictures...its all on breadboard, but instead of point to point with wire runs (like on my Tube Amp) i tried to do the layout so that i could make all the connections directly with component leads or with some solder bridges. I think my next project i might try etching and routing my first PCB




I do have some questions/issues

Each preamp-psu combo use exactly the same schematic (schematics courtesy of Elliot Sound Products - ESP) and while the layout is different in both of them, i am getting the same quirky issue with the PSU

On both the PSU's i get a solid -15.04 V on the negative rail, but on the positive rail i get roughly 14.85 volts....i'm not sure why i'm short 150mv when i'm using voltage regulators....7815 and 7915 regulators. I did check the input voltage to the regulator, and while i forgot what it was, it didn't raise any red flags, i think it was at least 18 or 19 v (well above the dropout voltage for the regulator to give 15v out) but ill double check tonight.

Schematic is here Its being fed by a 16V AC transformer...but runs a bit high, around 18-19v on idle, with a load still around 17vac

Any thoughts? or comments?

p.s One additional design question. The IEC power socket, is it worth it to use the one with the built in filter, as pictured, or just use a standard IEC socket to save space.

[DJPhil]

On both the PSU's i get a solid -15.04 V on the negative rail, but on the positive rail i get roughly 14.85 volts....i'm not sure why i'm short 150mv when i'm using voltage regulators....7815 and 7915 regulators. I did check the input voltage to the regulator, and while i forgot what it was, it didn't raise any red flags, i think it was at least 18 or 19 v (well above the dropout voltage for the regulator to give 15v out) but ill double check tonight.

You're not doing too bad at that voltage, most of the 7815s are specified to +/- 500mVDC. Also, the dropout for most plain 7815s is 17.5VDC, so if any part of your unregulated input (low end of ripple, for example) drops below, regulation suffers. National's datasheet assumes a nominal 23V input (14.4-15.6VDC out), for example, and derates output for 17.5V input (14.25-15.75VDC out). You'd have to check the datasheet for the specific manufacturer, as there's more than a few who make them. In the end I don't think it will matter, as I doubt you're counting on that extra 150mV of headroom in your amplifier.

Schematic is here Its being fed by a 16V AC transformer...but runs a bit high, around 18-19v on idle, with a load still around 17vac

It's my old friend, the full wave voltage doubler. Looks good to me. Most would say to remove R1 and . . R1 (?), however they can help with start up surge current. Just make sure they're rated to waste the heat they'll generate.

p.s One additional design question. The IEC power socket, is it worth it to use the one with the built in filter, as pictured, or just use a standard IEC socket to save space.

It won't hurt anything, but I don't think you'll notice the difference. Your transformer and bypass caps will do a decent job of isolating HF junk on the power rails both coming and going. I'd say absolutely definitely if you were using a switching supply. If you're worried about space, check the specs on one or two, they're quite small. Also consider the IEC modules with a built in fuse holder and/or switch.

Hope that helps.

[MTron]

That was a big help actually. Im not to worried about the 150mv, i just found it strange that 2 different Vregs would output the same voltage. i mean if one were 14.85 and the other 15.15 i wouldn't skip a beat. But both doing 14.85...i figured better ask then be sorry.

As for the IEC socket, i only have the 1 filtered ATM and it way overrated (8 amps) and was saving it for a power amp. If i can get away with a standard IEC socket then i will, i have tons lying around. As for a fuse, i suppose for safety i could include one.

[Zero999]

You shouldn't need a regulated power supply for an audio amplifier, providing it's been designed correctly.

What's the maximum power level you need?

What impedance are you driving?

With a 4 Ohm speaker, the maximum RMS power will typically be 9.68W, which is actually less than the RMS maximum output power with an 8 Ohm speaker which will be about 10.56W.

EDIT:
Ignore my previous questions. I missed the fact that it's a preamp but I've left them in for educational purposes.

Incidentally, if this were a power amplifier, do you know why the maximum power output would be lower with a 4 Ohm load than an 8 Ohm load?

Hopefully if you figure this out, you'll gain a greater understanding of why voltage regulators such as the LM78xx should be used carefully when powering audo amplifiers and should generally not be needed.

[MTron]

Incidentally, if this were a power amplifier, do you know why the maximum power output would be lower with a 4 Ohm load than an 8 Ohm load?

Hopefully if you figure this out, you'll gain a greater understanding of why voltage regulators such as the LM78xx should be used carefully when powering audo amplifiers and should generally not be needed.

A lower impedance load will increase the current, for IC amps the increase in current will likely trip the protection circuitry.

Normally i would never use a regulated power supply for an amp (i have built a few amps). The only exception to this is my other post about a regulated PSU for my LM3875 chipamp. I have a nice Toroidal transformer for it, but the rectified voltage would be too high to utilize the LM3875 with speakers less then 8ohms. So in this case, i plan to use a regulated supply to dial in the voltage according to the speakers being used at the time (i swap speakers often). But thats a whole different story all together.

This being a preamp though, and me using OpAmps within the circuit, i needed a stable well filtered and regulated PSU...which is what i built.

[NiHaoMike]

A lower impedance load will increase the current, for IC amps the increase in current will likely trip the protection circuitry.

Normally i would never use a regulated power supply for an amp (i have built a few amps). The only exception to this is my other post about a regulated PSU for my LM3875 chipamp. I have a nice Toroidal transformer for it, but the rectified voltage would be too high to utilize the LM3875 with speakers less then 8ohms. So in this case, i plan to use a regulated supply to dial in the voltage according to the speakers being used at the time (i swap speakers often). But thats a whole different story all together.

Using a variable power supply allows you to perform a trick known as "dynamic supply voltage". Basically, at a given volume control setting, the output signal will never exceed a certain value. By reducing the power supply voltage, switching losses and reactive pumping losses are reduced, along with an improvement in effective resolution as the Delta-Sigma modulators are working with greater changes in duty cycle. Where the envelope of the signal is known sufficiently ahead of time, it is even possible to vary the supply voltage in shape to the envelope for higher efficiency.

That trick is generally only useful for larger amplifiers. The small ones are very efficient as-is and are pretty limited in how much the supply voltage can change.