Decoupling in Phono stages.

[Jeb-D.]

I read somewhere earlier today that in multi stage Phono designs, each stage should have it's own RC decoupling network. What conclusions have you experienced phono guys reached regarding this matter? Is it a must or is it overkill?

[SY]

It depends on the stage topology- for some, it's a benefit, for others, a negative. Whatever you use, keep in mind power supply rejection (more is better).

[kevinkr]

These days I tend to use a single regulated supply feeding all stages with either resistive loading or most recently with active current source type loads in my phono pre designs. I don't use any additional decoupling.

In the old days I used RC filtering from a common supply and sometimes had problems with subsonic instability in multiple stage designs with both daisy chained rc decoupled supplies and sometimes even ones fed from a single supply mecca point if that point was not fed by a regulated supply with low source impedance below the audio band.

[Jeb-D.]

Awsome guys, just the type of input I was looking for!

Since were on the topic of phono power supplies. Have either of you had any improvements to the noise by adding a snubber cap to the transformer secondaries? It's supposed to reduce transformer ringing. Right now i'm using Hexfred rectifiers but plan on going tube rectified on the next version.

Just looking for small tweaks of improvment, or potential pitfalls that those who are more experienced have faced for the next version. It seems that to design and build a stable, trouble free, good performing phono pre, is more difficult than an amplifier.

[kevinkr]

Hi Jeb,
I generally use tube based series pass regulators with the supply on a separate chassis so noise has not really been a major concern. Most of my designs also use tube rectifiers, but not all - I sometimes use small caps where RF might be a concern, particularly with ss rectifiers, but nothing that could be considered a snubber in the sense you are talking about.

Low ESR caps sometimes need some additional series resistance to limit fast rise time transient currents like distant lightening strikes and load disconnects.

I haven't used the snubber technique on a transformer but have used it on chokes in choke input supplies.

[Shoog]

I got into the habit of snubbering my secondaries, but its mainly habit. I think my early experiments prodcued a softening of the sound - but its so long ago that I really can't remember. I would point to the trouble that TV /smps designers go to to snub their supplies.
It's so cheap and it can't do any harm, so I would go with it. I generally use those big red caps from TV's or computer monitors, as this is one of their main design applications,

Shoog

[Gordy]

Whether it be a stage in a phono amp, a line stage or a power amp output, the stage will have some degree of inherent Power Supply Ripple Rejection (PSRR). It is highly unlikely that the inherent PSRR will be sufficient in itself to combat any supply ripple. Also the current drawn from the supply of any stage can back-modulate the supply and hence affect other stages.

Also let us bear in mind what an amplifier stage does...

... an amplifier stage is presented with a wave of electrons at it's input and creates at it's output a facsimile of the input by modulating it's power supply according to it's transfer characteristic.

All of this suggests that the power supply is King and needs great attention. It makes me laugh out loud when I see a design with a highly detailed amp, and over the page a shallowly considered power supply. It's as if the designer believes that the amp is important and the power supply is just a hindrance. In reality the two are one.

Whilst I know little about valves (tubes) it seems as if you can get away with greater absolute ripple than with solid state as the supply voltage is (generally) higher, and hence the ripple proportionally lower. However good general design practice suggests that you consider separating the stages and reducing the ripple level, and reduce the impedance of the supply as low as possible.

If you can afford the cost of the parts you could try a regulator for each stage. However keep in mind the current loop, because what flows out must at some point flow back, and watch out for layout induced problems, especially with your 'grounds'.

[Jeb-D.]

It's so cheap and it can't do any harm, so I would go with it

True, it can alway be undone. I've seen some wright up from a guy that added them to a K-502 kit amp and it seemed to make a decent difference by looking at the waveforms

All of this suggests that the power supply is King and needs great attention. It makes me laugh out loud when I see a design with a highly detailed amp, and over the page a shallowly considered power supply. It's as if the designer believes that the amp is important and the power supply is just a hindrance. In reality the two are one.

That's quite true.

I was more-so inquiring about separating stages to prevent instability issues. Since ripple could easily be just as low with stages sharing a supply. Instabilities from stage sharing a supply would seem most likely in a phono stage. Since the attenuation is least (passive RIAA) in the low frequencies and that's also where the decoupling/reservoir caps become least effective.

As Kevin pointed out from his past projects, instability seems to only be a problem from shared stages if the power supply has high impedance.

That is where my error has been. Building a power supply with the only motive being to eliminate ripple as much as possible. Not paying much attention to impedance. That would explain the rise in frequency response below 10Hz, even though I put a rumble filter in. Also, there was a very small amount of sub sonic action going on (aside from the massive amount caused by my soldering irons heat cycle that I posted about before)

However good general design practice suggests that you consider separating the stages and reducing the ripple level, and reduce the impedance of the supply as low as possible.

So, I was forgetting a fundamental.

Thanks for your input guys!

It's so cheap and it can't do any harm, so I would go with it