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Old 16th Jan 2010, 11:23 pm   #1
Growlerman
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Default RIAA equalisation for Ceramic cartridges?

Hi Guys

Does RIAA equalisation apply to ceramic phono cartridges?

Does this account for the lack of bass and strident treble when playing "modern" records with a ceramic cartridge?
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Old 16th Jan 2010, 11:57 pm   #2
G8HQP Dave
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Default Re: RIAA equalisation for Ceramic cartridges?

Quote:
Does RIAA equalisation apply to ceramic phono cartridges?
Yes and no. Yes, it does apply because the record is cut with a need for playback equalisation. No, because most of the equalisation comes automatically for a ceramic cartridge, provided it has a high impedance load. A magnetic cartridge responds to the velocity of the groove, so is weaker at lower frequencies. A ceramic cartridge responds to the deflection of the groove. This means that the LF boost/HF cut is not needed.

However, a ceramic cartridge looks like a capacitor so if fed into a low impedance, such as a 47K MM input, this will cut bass so full RIAA equalisation will be needed as for MM or MC cartridges.

The lack of bass could be caused by poor cartridge/arm matching giving a high arm resonance. The strident treble could be caused by arm resonances. You can't expect an old record player to sound like modern hi-fi, and a ceramic cartridge put in a modern deck/arm could sound even worse as it will probably be a significant mechanical mismatch.
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Old 17th Jan 2010, 12:24 am   #3
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Default Re: RIAA equalisation for Ceramic cartridges?

The reason why we apply an RIAA equalisation is because of the way a magnetic or moving coil cartridge works.

The output of any electromagnetic pick up whether it be a tape head or a cartridge can be described as below

V out is proportional to the rate of change of magnetic flux.

If you double the frequency, you double the rate of change of magnetic flux, so you double the output.

This is where you get the expression 6dB per octave.

The RIAA curve was designed to compensate for this and to compliment the equalisation of the recorded signal before it was put onto the record.

I hope this helps you
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Old 17th Jan 2010, 11:24 am   #4
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Default Re: RIAA equalisation for Ceramic cartridges?

Thanks guys.

My interest came about due to recently getting and restoring to working order a Bush 31C and a horrible plastic modern Bush RPA1, both have ceramic carts. The "old" Bush sounds far superior to the plastic Bush. These are the only two working machines that I have with ceramic carts having lived with both MM and MC carts in my turntables for many years.

Did 1970's music centres with ceramic carts sound as bad as my Bush RPA1?
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Old 17th Jan 2010, 11:45 am   #5
Brian R Pateman
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Default Re: RIAA equalisation for Ceramic cartridges?

Most 1970s music centres sounded pretty good. Some of the higher end Hitachi and similar offerings sounded very good indeed and could hold their own against some of the mid range hi-fi systems of the time.

I suspect that the problem with the modern "Bush" is that it is, as you describe it, a horrible plastic thing. The problem doesn't lie with the cartridge or equalisation. If you were to connect it to a decent pair of speakers you might be surprised at the difference.

The proper Bush was provided with a good quality speaker and was housed in a good solid enclosure. Plastic is fine for cheap mass production but absolute rubbish from an acoustic point of view unless you are talking about engineering composites. These are very specialised and much too expensive for application to cheap mass produced equipment.
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Old 17th Jan 2010, 12:03 pm   #6
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Default Re: RIAA equalisation for Ceramic cartridges?

Hi Brian

It's more than just the inbuilt speakers as I have connected it via the headphone socket to a decent computer active sub/sat system and also listened using good quality headphones. The radio section of the Bush RPA1 is much better for audio quality.
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Old 17th Jan 2010, 12:34 pm   #7
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Default Re: RIAA equalisation for Ceramic cartridges?

Apologies in advance for getting mathematical, but the explanation of why it works is mathematical.

A magnetic cartridge is a velocity sensor. A ceramic or crystal cartridge is a position sensor. Now, since position is obtained by integrating velocity, it follows that a position sensor is electronically identical to a velocity sensor followed by an integrator. And the simplest integrator is a potential divider with a large resistor at the top and a capacitor at the bottom: v = 1 / C ( integral i dt), and the big resistor sort of vaguely approximates a constant current source.

Obviously the reactance of the capacitor will be less at high frequencies than at low frequencies, so this means we will get more output voltage at low frequencies than at high frequencies. With the addition of a simple tone control, the RIAA curve (which is created by two RC networks) can be approximated reasonably.
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Old 19th Jan 2010, 12:45 am   #8
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Default Re: RIAA equalisation for Ceramic cartridges?

To look at this question in another way, cartridges that produce an amplitude-responsive output – which group includes the crystal and ceramic types when appropriately loaded – do require equalization to compensate for the RIIA recording curve. The appropriate equalization curve can be visualized as being the standard RIAA replay curve rotated 90 degrees to the left (anticlockwise). Thus, the resultant curve is essentially flat up to a 500 Hz turnover point, whereafter it steps up just over 12 dB to a 2120 Hz turnover point, being flat thereafter. There should also be a 6dB/octave roll-off below 50 Hz.

This equalization could be done electronically within the amplifier, although such seems to be rare. At least from the late 1950s, when ceramic cartridges emerged for use in high quality audio applications, the “step” equalization seems to have been done mechanically within the cartridge. Thus such cartridges could be connected directly to a “flat” amplifier input of appropriate impedance. To some extent this facility seems to have led to the impression that amplitude-responsive cartridges do not require any equalization. The required 50 Hz turnover point on the equalization curve was probably approximated by the high-pass filter formed by cartridge self-capacitance and amplifier input resistance.

If the output from an amplitude-responsive cartridge is fed to a high-pass filter with turnover frequency above the audio range, say above 20 kHz, and 6dB/octave slope below turnover, then the equalization curve needs to be adjusted to have a 6dB/octave down-slope added to it, which is the same as rotating 90 degrees to the right, and so takes us back to the standard RIAA replay curve. Or, the high-pass filter converts the amplitude-responsive output into the velocity-responsive type, which may then be treated in the customary manner.

For crystal and ceramic cartridges, the high-pass filter is inherent in the interaction between their essentially capacitative internal impedance and the amplifier input resistance. Thus the turnover point may be varied by varying the amplifier input resistance. If the latter is high enough – typically 2M2 – the output will be amplitude-sensitive, whilst if it is low enough, say less than 100k, the output will be velocity-sensitive.

Back in the pre-RIAA days (early 1950s), when it was necessary to cater for multiple equalization curves, it seems that crystal cartridges intended for higher quality applications did not have self-equalization, and were typically loaded in a way that produced a velocity-sensitive output. Thus, with appropriate gain adjustment, crystal cartridge outputs could be fed through the same switched equalization/gain amplifier stage as were magnetic cartridges. As far as I know, it was done this way in the Quad QCII, although I haven’t done the rigorous analysis to be 100% sure.

As said, in the ceramic cartridge era, self-equalization appeared to be normal practice, perhaps because only the one RIAA curve needed to be considered. Obtaining suitably high input impedances, say 2M2, was no problem with valve amplifiers. And as ceramic cartridge output levels were approximately the same as those of radio tuners of the period, some valve economy in lower-cost amplifiers could be had by dispensing with the input gain and equalization stage typically required with magnetic cartridges.

With the arrival of solid state amplifiers, although high input impedances were possible, it was usually inconvenient to provide them, and there was a general reversion to “low” loading of ceramic cartridges to provide velocity-responsive outputs which could then be routed through the RIAA equalization stage with appropriate gain adjustment. The problem then was that the self-equalization got in the way. As a consequence the major ceramic cartridge makers, including Acos, Decca and Goldring developed matching pads (usually of the RC type) to go between their cartridges and amplifier inputs, essentially to cancel out self-equalization. Probably about 3 or 4 pads would have covered the majority of ceramic cartridges that would be used in the kinds of systems where good equalization was considered important. But I suspect that such pads were little used in practice, either as external ad hoc adds-on or by incorporation into amplifier circuits. By the end of the 1960s, lower-cost magnetic cartridges were becoming available and so displacing the ceramic type. In turn amplifier makers in general probably did not pay so much engineering attention to ceramic cartridge matching, in much the same way that many tuner makers (usually non-UK based) unnecessarily downgraded AM reception capability and quality.

Another approach, and one that took account of self-equalization, was to configure the input amplifier stage as a low-pass filter with turnover point matching that produced by the (high-pass) combination of cartridge self-capacitance with the amplifier input resistance. Thus a conventional RIAA stage could be used with relatively low input resistance, and with capacitative feedback replacing the RIAA feedback network. Quad did this with the 33 control unit, with the C1 position of the input board. As the feedback parameters are dependent upon actual cartridge self-capacitance, Quad evidently chose values that would suit a reasonable range of cartridges, and actually specified a cartridge capacitance range in its literature. I have a vague notion that Armstrong may have used a similar circuit in one of its amplifiers, but otherwise it does not seem to have been common practice. It is also apparent that the Quad 33 input matching arrangement would have lent itself to the incorporation of matching pads – one may envisage a “ceramic” input board with four different matching pads and RIAA equalization.

An interesting control unit design of the early solid-state era was the Linsley-Hood Modular, described in Wireless World of July, 1969. In this, the ceramic cartridge input stage is a FET/bipolar combination, providing 2M2 input resistance, and being unequalized. Alternatively, details are provided that enable use of the magnetic cartridge RIAA gain and equalization stage with ceramic cartridges. Here the feedback network could be arranged for flat frequency response, or it can be adjusted to provide for the full step mentioned above, or part of it, this provision said to be to counter inadequate self-equalization.

Although input resistances as high as 2M2 could be obtained using FETs, or bipolar circuits with bootstrapping, I suspect that in the late 1960s these were not much used by amplifier manufacturers because FETs were still relatively expensive, and two-transistor bootstrap circuits would probably increase the semiconductor count, either by being separate input modules or by replacing single transistor buffer stages. With ceramic cartridges assuming a secondary position in the hi-fi world, such complexity would have been hard to justify in most cases, although at the lower end of the market, Tripletone did offer a model using a FET/bipolar integrated circuit for good ceramic cartridge interfacing. The tape deck manufacturers faced a similar impedance matching problem, in that their early solid state models might need to work with existing valve amplifiers whose usually unbuffered tape outputs needed to look into a relatively high impedance, for example 500k minimum in the case of the Quad 22. Ferrograph used a FET source follower with 2M2 input resistance in its Series 7 models, and this idea was carried over to its F307 amplifier. Revox used a bootstrapped bipolar input amplifier in its A77; with 1M input impedance.

Some discussion of ceramic cartridge matching is provided in several of the Gordon King books of the late 1960s and early 1970s. I’ll provide the references later on.

Cheers,
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Old 20th Jan 2010, 10:26 am   #9
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Default Re: RIAA equalisation for Ceramic cartridges?

Thanks for the very comprehensive overview.

It would appear the reason for the appalling sound of the modern plastic bush is down to poor engineering both electronically and mechanically.
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Old 22nd Jan 2010, 7:54 am   #10
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Default Re: RIAA equalisation for Ceramic cartridges?

Well, insofar as the “problems” were capable of solution by application of appropriate mechanical and electronic engineering principles, one might well attribute poor performance to inadequate attention to detail. But as ceramic cartridges tended to be associated with lower cost equipment, deviation from the ideal might have been considered reasonable or even desirable in the interests of cost reduction. If those treading this pathway knew how to do it properly nevertheless, then any tradesoff could have been well-judged in a cost vs. performance sense. But sometimes one wonders if the “near enough is good enough” approach has not been used, with the tradesoff being more happenstance than calculated, and perhaps suboptimal in terms of what could have been achieved at the desired cost point.

That some of the cartridge makers were concerned about how their wares performed in situ is evidenced by their development of matching pads. But in any event their market was being eroded by low cost magnetic cartridges from Japan, and as a corollary the Japanese amplifier builders did not need to worry too much about input matching for ceramic cartridges.

Details of the books I referred to earlier, which have useful information on this subject, are as follows:

The Audio Handbook
Gordon J. King
Newnes-Butterworth, 1975
ISBN 0 408 00150 X


Pickups and Loudspeakers, How to Choose and Use
John Earl
Fountain Press, 1971
ISBN 0 852 4240 9


The Hi-Fi and Tape Recorder Handbook
Gordon J King
Newnes-Butterworth, 1969
No SBN or ISBN or LCC


I have also since recalled another approach to ceramic cartridge matching that I read about in one of the Australian magazines in the early 1970s. (Unfortunately I haven’t retained it, or if I have, I now cannot find it.) The article was about a design for what was therein described as something like a “charge-coupled amplifier", although it didn’t use any charge-coupled devices. My memory is now a bit vague, but I think it might have been an inverting, virtual-earth amplifier with capacitance alone in the shunt feedback and input pathways, the input capacitance being the cartridge itself. This would seem to give a broadband level frequency response for a self-equalized cartridge. But I can’t be 100% sure that it wasn’t a non-inverting design, more like the Quad 33 C1 input.

Cheers,
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Old 24th Jan 2010, 1:46 am   #11
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Default Re: RIAA equalisation for Ceramic cartridges?

Something else I remembered and was able to look up, as it appears in some literature for the Jason J 2-10 Mk III amplifier that I still have. Therein, under heading “Crystal Pickup”, it is stated: “This input has a sensitivity of 80 mV and an input impedance of 1.5 M ohm. In many designs of amplifier the crystal is shunted with a low value of resistance and the response then correct by the magnetic correction. Reputable authorities however claim that this increase the record wear considerably. In this design, therefore, a high input impedance is used, and with the crystal pick-up cartridge working into its correct load impedance, a flat response is achieved.”

It is an interesting proposition. Insofar as the cartridge is a generator, then the input power required to “drive” it will be proportional to the electrical output power taken from it. And as the load impedance seen by the cartridge drops, then the up to a point at least the electrical power output will increase. (Possibly the generator V vs. I curve in this case is of the convex, drooping type with the point of maximum slope being the peak power point.) So low impedance loading could increase the electrical power output, and therefore in turn the mechanical force required to move the stylus, perhaps to the point of increasing the record wear rate. But this assumes that the input power that is converted to electrical output power is a significant proportion of total input power. If that is not the case, and the majority of the input power is dissipated mechanically within the cartridge mechanism, then the effects of varying the electrical loading would be negligible.

Anyway, whether or not the reasoning is sound, the Jason cases illustrates that some amplifier builders did pay attention to the subject.

Cheers,
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