Phono3 3.18uS correction

[bob orbell]

Well Alasdair, I cant' say I am that good, but ever since I had a Phono III with the 3.18uS correction I found the sound too bright, this is with high end MC cartridges, I then read about RIAA correction, Wiky ( see my post RIAA correction ) and it appears that it is not needed and few manufactures fit it as RIAA only specifies 3 time constants, this 3.18 time constant should not make the HF sound brighter, but in the case of the WD Phono, it does, and I think it is incorrectly implemented, I do wish I had the mathematical ability to prove so, but anyway, I removed it from my Phono and all sounds very good, I am not alone with my thoughts. More comments on this subject please. Bob

[A.N. Beal]

Dear Bob,

From reading various posts on this site plus other Internet material such as the Keith Howard Stereophile article I agree with you that this aspect of the Phono 3 design is worth revisiting.

To recap, the starting point is the original network in the Phono, which had standard RIAA equalisation, achieved by a 180 kOhm resistor at what is now R19/R20, a direct link at what is now R21/R22 and a 220pF fixed capacitor where now there is C9/C10 (180pF fixed) in parallel with C9a/C10a (65pF variable). At the time the Phono 3 was designed, 'enhanced RIAA' equalisation, which boosts extreme treble, was fashionable and this was achieved by inserting 14.3 kOhm R21/R22 in series with C9.

According to John Caswell the variable capacitor was introduced to assist tweaking with different valves; the modifications gave correct 'enhanced RIAA' equalisation with 5751 valves but for ECC83 valves R19/R20 should have been reduced to 165 kOhm. Since the Phono 3 was designed articles such as those by Keith Howard and Stanley Lipshitz have poured cold water on the theory behind the 'enhanced RIAA' treble boost.

I agree with your views about 'enhanced RIAA' equalisation: in my system and with my ears the current Phono 3 circuit introduces a sharpness to extreme treble which is initially impressive (it would 'go well in the showroom') but not right. In my system reverting to 'standard RIAA' equalisation sounds correct. However even if 'enhanced RIAA' is wrong in principle I can imagine its slight extreme treble boost suiting some people's taste and also nicely balancing up systems where e.g. the cartridge or power amplifier has a rolled-off extreme treble or where the combined input capacitance of Phono 3 and tonearm lead is higher than optimum for the magnetic cartridge used. (In this context it would be nice to have confirmation of whether I have calculated the Phono 3 input capacitance correctly.)

Based on this I was thinking of suggesting to Matthew Snell that for future Phono 3 kits, in addition to the current parts list he should also supply a pair of 165kOhm resistors. Then he could add a note saying that for standard RIAA equalisation R19/R20 should be 180 kOhm and a wire link should be inserted at R21/R22; for 'enhanced RIAA' equalisation R19/R20 should be 165 kOhm and R21/R22 should be 14.3 kOhm.

What do you think?

Regards, Alasdair

[bob orbell]

I will let you know tomorrow Alastair when Scotland's finest has past through my system as Zebedee said, time for bed. Bob

[bob orbell]

Sorry for the late reply Alasdair, I do agree with what you say in the most, I did try changing to 165k, but the sound was still not correct, I also tried 5751 valves, this was even further in the wrong direction, I remember my original WAD phono was very good but chose to move on to the 3S but omitting the low filters, and from that time my record collection took on sibilance that almost made me give up with vinyl . Reading papers on RIAA equalization and Stanley Lipshitz and Walt Jung who state that the 3.18 time constant is unnecessary, I removed and fitted jumpers, shortly after that I added a fully regulated PSU and now playing vinyl is excellent with no sibilance. I cant' comment on the input capacitance as I only use MC cartridges. I have spoken to Matthew about the 3.18 correction and also about fully regulated HT and LT, he may do something. Bob

[bob orbell]

The official RIAA standard defines three time-constants with pre-emphasis rising indefinitely above 75 μs, but in practice this is not possible. When the RIAA equalization standard was written the inherent bandwidth limitations of the recording equipment and cutting amplifier imposed their own ultimate upper limit on the pre-emphasis characteristic, so no official upper limit was included in the RIAA definition.

Modern systems have far wider potential bandwidth. An essential feature of all cutting amplifiers—including the Neumann cutting amplifiers—is a forcibly imposed high frequency roll-off above the audio band (>20 kHz). This implies two or more additional time constants to those defined by the RIAA curve. This is not standardized anywhere, but set by the maker of the cutting amplifier and associated electronics.

The so-called "Enhanced RIAA" curve or "eRIAA" curve attempts to provide complementary correction for these unofficial time constants upon playback.

Background
In 1995, a nonacademic source erroneously suggested that Neumann cutting amplifiers applied a single high-frequency pole at 3.18 μs (about 50 kHz) and that a complementary zero should therefore be included upon playback.[10] However, no such pole exists.[11]

For example, the RIAA pre-emphasis in the popular Neumann SAB 74B equalizer reaches a maximum at 100 kHz, and in addition to this, the circuit also applies a second-order roll off at 49.9 kHz, implemented by a Butterworth (maximally flat) active filter, plus an additional pole at 482 kHz.[2] This cannot be compensated for by a simple zero even if it were necessary, and in any case, other amplifiers will differ. Correction upon playback is not, in fact, required, as it is taken into account at the cutting stage when manual equalization is applied while monitoring initial cuts on a standard RIAA playback system. Nevertheless, the use of the erroneous zero remains a subject of some debate among amateur enthusiasts.

Many common phono preamplifier designs using negative feedback equalization include an unintentional zero at high frequencies, similar to that proposed by Wright. This was illustrated, for example, in the seminal 1980 work on RIAA playback equalization by Lipshitz/Jung,[12] though it was noted as unwanted. Some phono preamplifiers include additional circuitry to correct this and ensure that the output follows the RIAA curve accurately.[11] In most, however, this is omitted. From Wiky. Bob

[Greg.]

Thank you, Bob. So to be clear, what components need to be removed to eliminate the 3.18us elements from the Phono 3 PCB and do any wire links need to be introduced?