DIY phono stages and some questions.

[fap]

G'day all, one of my main DIY interest areas in audio is phono preamps and I'm a big fan of the Elliott Sound Products P06 of which I've built quite a few for different applications over the years.

I realize that there are other excellent DIY phono stage designs as well but my favourite is the ESP P06. I never tire of it! A question on supply rail voltage and signal headroom/output level please.

These days I use either 15 or 18 volt split rail regulated power supplies and I've always believed that the higher the voltage (to the limit permitted) is always better. Yet I know of one very popular DIY phono stage that many are happliy using, powered by two 9 volt batteries.

To my way of thinking a phono preamp cannot develop enough signal headroom with such a small supply rail voltage. Any thoughts on this? Regards, Felix.

[TerryO]

Hi Felix,

Yet I know of one very popular DIY phono stage that many are happliy using, powered by two 9 volt batteries.

I'd like some feedback on this as well, the Hagtech "Bugle" does have an optional 15 volt power supply available and if any have gone from using the 9 volt batteries to the 15 volt PS, any opinions on perceived differences would be welcome. I haven't tried the PS myself, so I have nothing to add.

Best Regards,
TerryO

[fap]

G'day mate. Thanks for your comment. Yes purely from a 'gut feeling' viewpoint I simply cannot believe that any op amp can produce anything like optimum results in respect of things like distortion, output swing etc at 9 volts either side of the zero point on the supply rail.

In fact some years ago I recall reading in a local Australian electronics magazine about aspects of phono preamp design that it was stated, (at least for op amp based designs) that a 15 volt split rail power supply was essentially the minimum acceptable for reasons to do with signal 'headroom'. Even a 12 volt split rail supply was stated to be inadequate!

That makes sense to me. So how can 9 volts a side be ok? It obviously works, but surely a higher split rail supply voltage must be better. Certainly that is my personal experience with my DIY gear. Regards, Felix.

[Artie]

Take what I say with a grain-'o-salt, since I'm no expert, but wouldn't this depend on the opamp used? A phono cartridge only puts out around 5mV and the phone amp around 150mV's, so clearly, the application doesn't require that much voltage. So that just leaves the device. I'm currently prototyping a phone buffer that uses the Burr-Brown OPA333, which is designed to work with voltages as low as +/- 0.9 volts. (Yes, thats point 9.) This makes it ideal to operate off of a couple of hearing-aid size 1.5V batteries.

I'll let you know how the project goes.

Edit: Duh! I misspoke about my own project! Its not an RIAA preamp, but a cartridge buffer. I only need about 6db of gain, so my output should be around the 10mv range. Not quite the same concept.

[jcx]

Rod' spilt eq burns dynamic range - the single feedback RIAA eq circuits give better headroom from any given supply V and gain choice

I also consider it bad practice to have unbuffered output RC determine critical EQ time constant

you can read Walt Jung's Op Amp Applications book's chapter on RIAA preamps for free, Self's Small Signal Audio book has some phono preamp material too

another consideration for mm designs is op amp input nonlinear impedance at frequencies where the mm coil inductance & input loading C resonance gives very large source impedance
Fixing Phono Preamp Distortion due to Impedance Nonlinearity
the simplest solution would be to use the OPA637 - see http://www.fortunecity.com/rivendell...o/aaphono.html
(the output back-to-back electros are outdated - use bipolar electrolytics per Bateman's Capacitor Sound articles)

if you want to use 2 op amps, a CFA buffer in the loop lets you drive much lower Z feedback network without loading the input op amp - its also possible for the extra op amp to add loop gain at audio frequencies

[fap]

G'day mate. Thanks for your very interesting analysis. You raise some good points. Yes, re the dynamic trange issues with split EQ, I have often thought along similar lines myself and wondered, yet in actual 'listening' I have never perceived any limitations in that respect. Interestingly enough I've often thought that full feedback EQ phono stages I built were hampered by a lack of dynamics! Much to think about.

The output RC network issue is indeed interesting, and yes it's true and even Rod Elliott freely admits to this that some error to the RIAA EQ will occur if the following stage impedance is much below about 22 k. Indeed this was the case with an earlier NAD amplifier I had, its 'line level' inputs were only 10 k!

The potential MM cartridge non linearities are also interesting. Indeed at one point I did build two versions of a input 'buffer' stage, one designed around an op amp voltage follower and the other a FET source follower loaded with another FET as a CCS. The results were somewhat inconclusive. Rod Elliott tells me that 'input buffering' is built into the standard P06 circuit in any case. All very interesting, as regardless the P06 is a fine sounding phono stage to my ears. Regards, Felix.

[Mooly]

To go back to the original question... vinyl has no absolute limits on signal amplitudes in the same way that a digital system does... and so overload can be a very real issue. It's not so much average music levels (although some decent pressings can be very dynamic) but imperfections and scratches etc that can produce very high levels, which if they cause overload due to insufficient headroom can sound particularly bad during the time this occurs as the preamp is effectively blocked by this overload and takes time to recover as the overload dissapears.

I used to have a Pioneer A80 amp many years ago and that used a discrete front end on -/+ 45 volt rails.

I would recommend as high a supply as possible withinn the limits of the active devices.

[Calvin]

Hi,

vinyl has no absolute limits on signal amplitudes in the same way that a digital system does

This sentence is very clever formulated.
It says, that analog systems behave differently to digital systems under overload conditions, which is true. But it also implies that analog systems have no absolute limits on signal amplitudes, which is untrue.
Vinyl is restricted to less than 67dB of maximum dynamic range and the bandwidth is restricted by either the cutterheads limits, the Vynil´s or the pickup´s.
The analysis should be rather simple. Look which values of voltage with which possible risetimes might occur at which point of the circuit. Critical should be those stages before the 2120Hz break point. Behind the 2120Hz filter matters are rather non-critical. I´d opt for a 3 stage system with a linear amplifier which can freely be configured to accommodate mm as well as mc pickups and that supplies for most of the required gain. Via a DC-servo or a coupling cap a low cutoff of 20Hz (RIAA-IEC) or even lower can be incorporated here. This stage is then followed by a passive 2120Hz filter as 2nd stage and a third active medium-gain stage incorporating the 50Hz/500Hz filters. By splitting gain and filtering into several steps each stage can be optimized with regard to the pickup´s demands, noise, dynamics, linearity/precision and headroom. With such stages I never had any overload issues at all with supply voltages as low as 8V (the OPs might allow for ~12Vpp of output voltage then).
Since the total gain is split up the first linear gain stage doesn´t need to amplify as much as a single gain stage, which improves bandwidth, noise, et al. The output voltage of this stage would be far below the limits of the OP-amp´s output voltage limits of say +-10V (with suppplies of +-12V to +-15V). The passive 2120Hz filter stage is only restricted by the voltage limits of the devices, typically much higher than the power supplies voltages or signal voltages. The third, active stage amplifies just a couple of dBs and linearly above 2120Hz. Any critical signal (which would be high in level as well as in frequency) reaches this stage already lowpass filtered. So this stage is less sensitive to overload too.
Of course requires such a phono stage more effort than a simple one-gainstage amplifier, but to my taste the advantages in flexibility and reproduction fidelity are worth it. Since OP-amps are rather cheap nowadays, the advantages come with only a very small cost penalty.

jauu
Calvin

[Artie]

I think Mooly was focusing more on this statement:

Originally Posted by Mooly

It's not so much average music levels . . . but imperfections and scratches etc that can produce very high levels . . .

A scratch could easily be 1000x anything the cutter was capable of. Its in that vein that "analogue has no theoretical limit."

[Mooly]

Originally Posted by Artie

I think Mooly was focusing more on this statement:

A scratch could easily be 1000x anything the cutter was capable of. Its in that vein that "analogue has no theoretical limit."

I was