RIAA cartridge signals and phonostage headroom

[Co&]

Hello,

I've been searching the forum about this topic, but cannot find any good clues.

The questions is: what is the severest case for the input of a phonostage?

Usually cartridges are specified with their (peak?) voltage output at a certain frequency and cutting speed. For instance 5mV @ 1kHz @ 5 cm/sec. If I understand correctly this is also the specification of a 0dB RIAA signal at this frequency.
A +20dB signal would then be cut at a 10X higher speed (or amplitude) of 50 cm/sec, delevering 50mV into the phonostage.

How do we translate this specification into the voltages generated by a freqency of 20kHz? The RIAA curve 'amplifies' frequencies above 2 kHz, yeilding into a 10 times higher 0dB signal at 20KHz. I wonder if I'm correct in concluding that the cutterspeed for a 0dB RIAA signal @ 20KHz is 50 cm/sec and 50mV output....

If we want to design a phonostage with a headroom of 20dB for all frequencies the highest frequency leads to the most severe case:
An imaginary +20dB headroom on 20kHz yields to a cutterspeed of 500 cm/sec or a 0,5V output. It is beleived that on some recordings the RIAA freqency-'amplification' can extend to 50KHz, before the cutter is not able to handle the signals anymore (at least not at a +X dB level). The cutterspeeds are at +20dB even higher.

This leads to several conclusions:
- the physical headroom at a recorded frequency of 1kHz is far greather than at higher freqencies (500cm/sec @ 1KHz is 0,5V or +40dB)
- the maximum cutterspeed limits the maximum output signal (and the headroom of the cartridge of course)
- for a standad MM cartridge and +20dB of headroom the 20kHz case is the most severe and should be able to handle at least a (theorethical) 0,5V output of the cartridge.
- with a 100X first-stageamplification, the max-headroom outputsignal loaded by a passive RIAA is as high as 100X0,5 = 50 Volts!

There are also other reasons to have a large headroom on every stage.

Am I missing a or some points in my reasoning?

regards, Coen

[ThorstenL]

Konnichiwa,

Originally posted by Co&
I've been searching the forum about this topic, but cannot find any good clues.

Well, Douglas Adams had a good article on that. The bottomline is that the maximum level on LP occours between around 150Hz and 3KHz, with a more or less first order function rolloff above and below. This is without accounting for RIAA EQ, we are talking actually cm/s values. The maximum cuttable tends to be around 20db above 5cm/s @ 300Hz and trackable is often limited to 3 - 6db less so usual cutting levels will be +14 - 17db around 300Hz, less at lower and higher frequencies.

Sayonara

[Co&]

Hi Keui,

I am beginning to realise that my question could have been better!

If I may paraphrase your answer: the cutterspeed represents the product of frequency and amplitude, if the frequency decreases the amplitude rises. Probably the lower boudary (150 Hz) is limited by the maximum desirable record groovewidth and the higher (3KHz) by the inertia of the cutterhead. Theoretically the maximum possible recordable signal is at 300Hz (in the 150-3KHz region).

Rethinking about my question:
- How does this translate to the RIAA recording level? Is there a predetermined level for 0dB in RIAA on the record or is this up to the recording engineer?
- The possible headroom for the lower frequncies is with RIAA even higher than in my example above. It is not even possible to record a signal of 20kHz with 50 cm/sec.
- does the cartridge output differ at 300 Hz from 20kHz at 50cm/sec?

thanks, Coen

[ThorstenL]

Konnichiwa,

Originally posted by Co&
If I may paraphrase your answer: the cutterspeed represents the product of frequency and amplitude, if the frequency decreases the amplitude rises. Probably the lower boudary (150 Hz) is limited by the maximum desirable record groovewidth and the higher (3KHz) by the inertia of the cutterhead. Theoretically the maximum possible recordable signal is at 300Hz (in the 150-3KHz region).

Yup.

Originally posted by Co&
- How does this translate to the RIAA recording level?
Is there a predetermined level for 0dB in RIAA on the record or is this up to the recording engineer?

The official standard(s) where 0db = 5cm/s (3.45cm/S) @ 315Hz.

Now this translates into a given voltage from a cartridge, usually around 5mV @ 5cm/s for MM Pickups.

The Cutting engineer is in essence free to violote the stadard to his liking, you can cut a lot "hotter" and most cutting engieers did cut as hot as was possible.

The RIAA Pre-Emphasis is applied so that the output from an accurate cartridge will follow the specified curve. We don't really need to concern ourselves with the exact mechanics.

Sayonara

[Co&]

Kuei,

So 0dB cuttingspeed is 5cm/s at 315 Hz, maximum cutterspeed is +20dB or 50cm/s.

With a cartridge delevering 5mV at 5cm/sec @ 315 Hz, this cartridge will NEVER put out a recorded signal, regardless of RIAA, greater then 50mV (for 50cm/sec). So if my phono can handle the 50 mV at its input, all recorded signals, regardless of RIAA, should be able to pass through. Any higher acceptability is only good for plops, dust or other transients...

Am I on the right track if I state that if a record engineer decides to make a hot recording, RIAA will cause compression (and distortion) at higher frequencies?

regards, Coen

[AndrewT]

Hi,
some early low quality phono preamps had a headroom of about +20 to +24db. All were critisised for poor sound quality.
By the time you reach +30db headroom this criticism has started to pale below other accuracy problems.
Most good phono amps had +34 to +36db of headroom and imo this is what you should strive for.
A few phono preamps got close to +40db but these were exceptional.
Short term overload due to dust, scratches etc (as mentioned earlier) seem to be the problem and if your preamp can amplify them without latching and clipping then the final sound seems to be more acceptable. It does not seem to matter that the cartridge may distort badly at these extreme outputs its more that the electronics down stream stretch the bad sounding parts into an audible mush and all the intermodulation that might go with it instead of a very short lived peak that may be less audible.
Some designers have advocated high voltage mosfet stages and even valve preamps to overcome this very overload problem.
Aim high! I think you might regret doing otherwise.
Ben Duncan did a thorough series back in the 80's (W.W. then, new name now) and he looked in detail at the headroom vs signal at 3 frequencies at each stage of amplification all the way through to the final output. He updated a year or two later for newer opamps. These are now available much more cheaply.

[Co&]

Hi,

For the input stage a +40dB design headroom will assure that all signals from the cartridge will be handled. This means a maximum input signal of 500mV.

The second amplification stage will have a hard time with the lower frequencies if we continue the +40dB criterion. These are the least attenuated by the RIAA filter. At 315 Hz the EQ is about -15dB (or 5,6X). The firt stage is therefore not allowed to amlify to much, avoiding overloading of the secondary and following stages.

Conclusion: if my phono can handle +40dB at 315 Hz, there will be hardly a headroom problem.

Your comments are allways welcome!!

thanks,
Coen

[ThorstenL]

Konnichiwa,

Originally posted by Co&
For the input stage a +40dB design headroom will assure that all signals from the cartridge will be handled. This means a maximum input signal of 500mV.

In extreme cases you find cartridges with more output than 5mV, so aiming for 1V Input tolerance is desirable.

As other mentioned, clicks and pops can cause very steep slope high level signals at high frequencies, often further amplified by mechanical resonances in the cartridge/record system.

Originally posted by Co&
The second amplification stage will have a hard time with the lower frequencies if we continue the +40dB criterion.

But equally, they are not cut onto the LP at such levels.

For reference, most Op-Amp based single stage RIAA EQ Circuits have around 20 - 26db headroom on the output and are usually fine with that.

So, aiming overall for >>20db headroom on the input to output where 315Hz are at nominal 5mV and all other frequencies RIAA pre-emphasised (eg Boost the generator by 20db leaving the RIAA Pre-emphasis alone) will do okay, a little less headroom at high frequencies may be okay.

Sayonara

[scott wurcer]

As an aside, KYW (or anyone else) any good LP's to push the headroom question? I use a couple of RCA/JVC 45's in the RDC series, I picked them up for $1 used. One of them 'Summer Rain' has a level vs. time plot on the back that has a fairly extreme range. The other is Beethoven's 23rd piano sonata 'JVC-RDC4' which I find sonically great but mechanical in performance. I also have used the Sheffield Track Record, but again I don't really like the music so it is hard to use it in a listening test.

[ThorstenL]

Konnichiwa,

Originally posted by scott wurcer
As an aside, KYW (or anyone else) any good LP's to push the headroom question?

I find DJ 12" Singles (I used to DJ) are cut about as hot as it gets. They used every db possible and then some.

Sayonara