Phonoclone 3

[rjm]

Hello everyone. I guess it's to cold weather that does it, but at long last I've spared a little time to review the Phonoclone design.

I've always felt that the weakest link was the voltage regulators, and this is where most of the new stuff is. The phono circuit is unchanged.

Regarding the voltage regulation, to make any improvement one has to first identify what's wrong with the old version. With the standard 3-terminal fixed and adjustable types, I can spot two main areas right from the data sheets, confirmed by measurements:

1. They are noisy. Output noise is measured in mV p-p.
2. They are slow, the bandwidth under which they do their thing is about 2 kHz. Above that they stop being regulators, and let the noise through, too.

As far as their primary functions of removing ripple voltage and keeping the output voltage constant, however, the work just fine. Furthermore, the output capacitor nicely covers for the regulator at higher frequency. However, we have to start from somewhere, so we'll accept that a better regulator will have wider bandwidth and lower noise.

It's been rattling around my head for a while now, and actually I'm pretty sure I floated the idea here some time ago, but I'm actually going to go ahead and try it out this time: namely, try using the same OP27s used in the phonoclone circuit to also provide a low impedance, low noise power source for that circuit.

Since playing around in Eagle is easier than actually soldering stuff on the bench, I've gone ahead and designed the new board. Full size 16x10 cm, and double sided with a full ground plane, just like the old BE boards. The next step is to prototype the regulation unit and measure it to make sure is does actually work as advertised. After that I'll finalize the board and order it from Olimex.

If anyone wants to join me at the guinea pig stage, you are welcome to drop me an email. I'll order extra boards and send them out to you, at cost.

Anyhow, without further ado, here's what the new beasties look like:

both layers shown


bottom layer (signal trace)


Eagle brd and sch files attached.

regards,

/rjm

[rjm]

The output voltage of the regulator section, Vout, is determined by the resistors R9-R16 and the input voltage Vin.

Vout=Vin * R9/(R9+R11) * R15/R13

Assuming R9 = R13 and R9<<R11

Vout=Vin R15/R11

To adjust the ratio, change R11.

To preempt and answer the inevitable in advance: Yes, you can use the Jung Superreg instead, or any number of other "high end" voltage regulator circuits. I'm not ... because... [Singing] I (wanna say I) did it .... MY WAY.

[rjm]

Just a quick update: the regulation circuit is not working out as well as I thought it would. Going to have to rethink this...

[Werner]

What's the problem? Regulator doesn't start? Excessive noise?
Oscillation? Instable output voltage?

[rjm]

That would be "C" : excessive noise.

If you have a look at the circuit attached, which is pretty much as I bench-tested it, you'll see the op-amp is run from a single-sided supply. The negative rail is connected to the circuit common... the source of my woes, as it turned out.

All the ripple on V_in feeds into the opamp positive power pin (7). I originally estimated the PSRR figure of ~85 dB from the datasheet PSRR vs frequency plot. This is even better than the attenuation of the RC filter of the voltage divider network R3/R4 so the contribution at the output would, I reasoned, be inconsequential.

No. Connected to a single supply, the PSRR would seem to be less than 20 dB, certainly not anything like 85 dB. For 100 mV ripple on V_in about 10 mV will appear at V_out. Ouch.


Opamps make good voltage amplifiers but poor regulators. When used in a traditional series regulator with reference and pass transistor, its not stated explicitly but the voltage amplifier itself is powered by a separate, heavily filtered or regulated line. Or there's that trick Walt did and have it powered by the output voltage of the regulator. Any which way, it has to be a clean voltage.

So, what options? One is to backtrack and connect the opamp to a split supply, by bringing the V- rail up to the opamp pin (4). This should bring the configuraiton back to teh default datasheet PSRR.

Fine as long as the total voltage doesn't exceed 44V.

Another possibility is to switch tactics and go into a simple shunt regulator mode, either a zener, zener + pass transistor, or something like the TL431.

Finally, we could pre-regulate the input voltage. Either a LM317, or a zener. I don't much care for this, since if you need regulation for the opamp input, you might was well get rid of the opamp entirely and feed into the phonoclone circuit directly, as it hardly seems to be doing anything worthwhile now.

So, split supply connection for the OP27s, or move to zener-style shunt configuration? What should it be?

[Werner]

Originally posted by rjm

No. Connected to a single supply, the PSRR would seem to be less than 20 dB, certainly not anything like 85 dB.

Nope.

The opamp doesn't know its lower supply is at ground level. Opamps are ignorant of ground.

Check your reference voltage generation (or rather the lack
thereof). That's how the ripple gets into the output.

You may want to replace R4 (in the latest schematic) with a
zener or better still LM329, and then adapt R3 for proper biasing. Even so a zener or LM329 has insufficient dynamic impedance to give good filtering with a shunt cap, so try to squeeze a resistor between the reference node and the opamp+/filter cap node.

That's how it is done ;-)

Even so I would expect the OP27 to be unhappy during power-on and during operation as its naked output sees the cap load and has to deliver serious current. Opamps don't like that.

[rjm]

Originally posted by Werner

The opamp doesn't know its lower supply is at ground level. Opamps are ignorant of ground.

Check your reference voltage generation (or rather the lack
thereof). That's how the ripple gets into the output.

Thanks. I was just coming back to the forum to post that after doing some research on the subject this afternoon. Stupid mistake on my part, shortcutting the full calculation the R3/R4/C2 divider. In a nutshell: it's not nearly as good as filtering the 120 Hz ripple as I thought it was.

Back in a bit with the next iteration...

[rjm]

so we could do the traditional approach, reference of some sort, bypassed for lower noise, opamp voltage gain, pass transistor... opamp running from the regulated output voltage...

[Werner]

I'm afraid that one is not even going to work: the opamp's output can't get above the reg's output, but yet it has to reach the input voltage minus Vbe.

Just feed the opamp from the raw input, possibly with a bit of filtering.

And there's still not a lot of noise filter on that reference diode!

This is something I made for a phonostage:



More complex than what you want, but then it performs a little trick in that the output impedance is resistive to beyond 20kHz.

Mind, this was NOT made for the lowest ripple rejection: it's dangling off a pair of emitter followers.

[rjm]

I'm not going to pretend I follow all of that, but I do see what you are doing with the filtering of the reference voltage.

One thing I think I could do, going back to my original concept, is just add more filtering to the voltage divider. The circuit below should manage about -90 dB or better (really this time I calculated it properly!) at 120 Hz., from both the reference voltage and opamp.

The time constant of the divider should limit the inrush currents through the op amps.

I've got it ready on the bench for testing later today...