RIAA preamplifier headroom

[Studio263]

How much headroom should one allow for when designing an RIAA equalising preamplifier for use with a typical range of MM and MC cartridges?

It was once normal practice to run this part of an amplifier circuit from the highest supply voltage available inside the equipment, even though the signal from the cartridge is tiny. It was explained to me that this was because when the stylus hits a piece of dirt or some other imperfection in the groove that a signal much larger than the normal programme would be generated, and although the event itself was a short one if the feedback controlled stages of the amplifier were asked to process a signal so large that the feedback system ran out of room to control the circuit properly the recovery time would make the disturbance much more obvious.

The supply voltage chosen was normally in the 30 > 50V range, this being easily obtainable in transistor amplifiers with ratings between 40 and 75W. There are oddities of course, the Sony TA-5650 had a specially arranged 100V supply for its V-FET RIAA stage. However, one now sees many separate RIAA equalising preamplifiers which run from very low voltages - 9 or 12V, despite being expensive and having pretensions of quality. As a case in point, the new SME 'Synergy' turntable has a built-in Nagra designed RIAA equaliser which runs from a single 12V DC supply (and exhibits poor overload performance as a consequence according to Noel Keywood's measurements and review in Hi-Fi World magazine).

Has something changed? Are records now better pressed and more resistant to dirt that they used to be, or has the correct way of doing things been set aside or forgotten? Is there a standard figure that designers should aim to achieve?

[Nickthedentist]

Thanks for bringing this up, Tim.

I have in my "stores" a handful of RIAA pre-amps which I've salvaged from scrapped Japanese music centres, and although I'd carefully labelled the connections and supply voltage, it surprised me that their supplies were all so high, yet cheap Chinese standalone units sold nowadays by CPC (and until recently, Maplin) etc. often have an external plugtop power supply with an output of just 6-12V.

I look foward to more responses.

N.

[Radio Wrangler]

You don't need huge supply voltages for large transients due to muck etc because you don't want to reproduce the muck faithfully. Clipping would be OK. What you want to be sure of is that the circuit recovers cleanly and quickly from the transient. Some circuits fail very badly at this.

You don't so much need the high voltage to give headroom for the cartridge's output, rather for the RIAA circuit's output. It has to have rather high gain especially at the LF end. Cartridges (magnetic) are velocity sensitive, so give more voltage on HF components from clicks etc... but clicks can be clipped, if they are clipped cleanly.

You need to handle what can really be cut onto a record. You can clip anything else, but you must not spread it out and make it worse.

THere was a fashion amongst Pioneer etc to use very high rails, high voltage FETs etc. In the UK Cambridge had their active volume control and passive EQ. Neither approach caught on. Pioneer was into brute force, Cambridge was more elegant, but their gain variation and its location in the preamp chain did silly things to the tape recording outputs. This could have been sorted at cost of more complexity. Nobody did.

+/-15v and NE5534s ought to be fine unless you're competing at on-paper specmanship.

David

[Studio263]

Originally Posted by Radio Wrangler

+/-15v and NE5534s ought to be fine unless you're competing at on-paper specmanship.

So you are saying 30V between the rails then? This is still greatly more than many modern designs use. The Aurex SY-C15 pre-amplifier uses split 15V supplies for its (discrete transistor) phono stage and has always struck me as a good performer.

http://www.thevintageknob.org/toshiba-SY-C15.html

[PJL]

A transistor datasheet may specify a 'Storage Time'. This is the time it takes to recover from saturation. For a general purpose transistor, I doubt it would have much effect at audio frequencies. The higher supply voltages may help with linearity.

[Craig Sawyers]

Shure in the manual for the V15V show a graph that suggests that the peak velocity on a "hot" recording is 50cm/sec peak velocity at 1kHz, so ten times nominal of 5cm/sec rms.
If the moving coil (say) gives 300uV at 5cm/sec, it will give 3mV peak at 1kHz.

The gain of a moving coil stage is about 60dB, so 3mV peak translates to 3V peak output voltage. So for normal signals there is plenty of headroom with +/- 15V rails.

The peak velocity at 100Hz is about 10cm/second (from Shure), and the boost is about 14dB - which is again 3V peak at the output.

If we assume that three transients at different frequencies overlap (because we have to pick a figure) that will result in a musical transient of 9V. Which is a truly extreme circumstance with an extreme recording - and even that is OK with +/-15V rails.

We have to assume that there is sensible rumble filtering to take out not so much rumble at ripples in the vinyl and warps. Because if not that can give rise to clipping problems.

Craig

[Radio Wrangler]

Storage time in transistors is eclipsed by changes in stored charge in decoupling capacitors etc. Bootstrap capacitors are particularly evil when things get hammered into the rails. A spike transient of several microseconds can result in a recovery curve of orders of magnitude greater area.

Minority carrier lifetime and stored charge in the transistor junctions is usually not a problem.

Good circuit design makes the difference.

David

[Valvepower]

Hi,

I’ve had good results with 22V (+/-11V) with a few designs.

When technical bravado is the order of the day, I’ve gone up to 48V (+/-24V) though.

Terry

[Beobloke]

Originally Posted by Craig Sawyers

Shure in the manual for the V15V show a graph that suggests that the peak velocity on a "hot" recording is 50cm/sec peak velocity at 1kHz, so ten times nominal of 5cm/sec rms.
If the moving coil (say) gives 300uV at 5cm/sec, it will give 3mV peak at 1kHz.

The gain of a moving coil stage is about 60dB, so 3mV peak translates to 3V peak output voltage. So for normal signals there is plenty of headroom with +/- 15V rails.

Unfortunately, your calculations are a little bit awry.

The Shure is a moving magnet design, not a moving coil, and its output is 3.2mV. This equates to 32mV on peaks given your 'ten times' theory and the gain of most MM stages is around 45dB. This gives a peak output voltage of just under 6V.

Whilst this may seem OK, the Shure is actually considered to have a fairly low output for an MM and 5mV is considered the norm. Some models can even go as high as 7-8mV which any decent phono stage should be designed to cater for. Put 8mV into the above calculations and suddenly your maximum output is heading past 14V. As a result, I would say that +/-15V is an absolute minimum for a phono stage supply.

[G6Tanuki]

What about just sticking a pair of back-to-back diodes (suitably reverse-biased if necessary) across the input to 'clamp' any transients?

[Radio Wrangler]

You can play the game backwards.

If you have a higher output cartridge, or a lower one, then you need less or more gain in the RIAA stage else your volume control has to be in a less mainstream position.

So you could consider the level the rest of the preamp needs from the RIAA stage and then add a chosen amount of headroom. Pick the gain to get this from your chosen cartridge.

You don't have to engineer the top end of the dynamic range to accommodate the full variation of the music, your variations in chosen listening levels plus differences in cartridges.

Quad's use of a plug-in 'biscuit' to allow equalisation/gain to be customised to suit individual cartridges has advantages.

Cambridge and Pioneer may look better as figures on a spec sheet, but do you actually use the difference?

David