Phono3 3.18uS correction

[Richard]

Originally Posted by bob orbell

To add on from Richards post, I use both CD and vinyl, my issue reared its head after changing my WAD Phono II for an all III set up, CD was still good but, vinyl produced a lot of very annoying sibilance, it became that bad I changed my cartridge for a new one, (same Ortofon) but after fitting and listening the sibilance was still there, I tried all of Richards suggestions but to no avail, after much reading on the subject of RIAA, it was decided to remove the 3.18 added resistors and the rest is now history, I also know others who posted similar issues, but I think a lot just gave up and moved on, pity, because this Phono is very good, save for the 3.18 time constant that I feel is implemented incorrectly. I wonder if Andy Groove could be contacted ? after all he designed the original, I think. Bob

Well it sounds like you've done the right thing then Bob and just reverted to Andy's original Phono II design without 3.18. Building it on the later 3S pcb with the extra decoupling caps etc gives you the best of all the versions. Sometimes we all go round in circles only to find that the original set up or parts were better after all. Me, I can only say I didn't have the sibilance issue in this instance, but I have spent many pounds on other mods and components in the past only to revert to the originals.

As a matter if interest the first write up for Phono II included cathode bypass caps on the ECC83 so designers change their mind too.

Look closely at the Operating Instructions for Phono 3S and you'll see R21/22 is shown as 2k2 instead of 14k3. Maybe a typo (why would the circuit be re-typed?) or maybe to keep the earlier corner correct and still allow a measure of adjustment at the extreme top? I don't recall it being mentioned.

[Greg.]

Thank you, gentlemen. All very informative. I have personally never owned a Phono 3 or 3s, although I’ve built some. I had (still have it) one of the first Phono 2 kits produced which did not include a 3.18uS time constant correction. It sounded fine to me ears. My recollection is that it was Paul Barker who first raised the matter of a need to include the correction in the circuit. At the time I did this tweak and although the sound was marginally different, there was nothing ‘night and day’ about it. I recall it offered a little more air around high frequency sounds. I have never suffered from sibilance since introducing this modification when used with a variety of cartridges. At the time, Neal Gibbons also tried the mod, liked it and then wrote it up for the FAQ.

As Richard suggests, only way forward here is to suck and see for ones self. Depending on how we hear as individuals will determine whether to keep the correction in circuit, modify it or remove it altogether. Certainly if sibilance is experienced, trying the mod or removing from the circuit could be a solution.

Must say, this is a great thread to read and think about. I’ve really enjoyed it.

[bob orbell]

Richard, I too have that circuit diagram on paper in the build instructions supplied with the kit from Peter C, it too has 2K2 for R21-22, but in the kit R21-22 were 14K3, I assumed it was a typo error because of the FAQ tweak from Neal G for the Phono II, now this is interesting since you have bought it up, just which value is correct, I am quite happy to leave mine as is with no 3.18 correction, but could anyone on here do the maths to find the exact value and placing of the 3.18, I have the equation, but it would require a good mathematician to work it out and it would be very interesting to see the results, any takers. Bob

[A.N. Beal]

Dear All,

As previously discussed, it appears that the theory behind the '3.18us correction' does not stand up: producers may have applied various ultrasonic filters when recording but there was no standard Neumann ultrasonic filter. Therefore the merit of this 'correction' depends on how it interacts with individual systems (do other components have ultrasonic roll-off and can amplifiers tolerate an ultrasonic level boost without distorting?) and recordings (what ultrasonic filter, if any, was applied during recording and is the ultrasonic information recorded on the disc good musical harmonics or junk and distortion?).

It is also worth remembering that many moving coil cartridges and some moving magnets already have a rising top end response, with an ultrasonic peak of 3-5dB. If the cartridge has a +5dB response peak at 50kHz and the '3.18 us correction' applies a further +3dB (or maybe +5dB if the 'correction' is not quite right) the result could be a total ultrasonic boost of up to 10dB at 50kHz. This is probably inaudible in itself but could well be troublesome for amplifier stages downstream, with sibilance and scratches pushing them into distortion.

For my money the arguments strongly favour building the Phono 3 with 'standard RIAA equalisation' as standard and make the '3.18us correction' (correctly done with a 165kohm resistor) an optional possible 'tweaker's option'.

While we are at, I understand the usefulness of the 65pF variable capacitor in parallel with 180pF in the Phono 3 for those who intend to try substituting various different valves such as 5751s for the standard ECC83s. However for those of us who are happy to stick with ECC83s a fixed capacitor of the appropriate value would be more convenient and, if it is better quality, might sound better. I found that when tuning the variable capacitor by ear I ended up at exactly half way, as recommended in the instructions, which equates to 180 + 36 = 216pF. I decided to replace the standard capacitors with Evox Rifa 220pF capacitors, which are excellent quality but not expensive (60p each from HiFi Collective). The result is (to my ears) a definite improvement in treble quality and strongly recommended.

Regards,

Alasdair

[NealG]

Greg is correct, it was PB who read about the 3.18us then suggested it and I just wrote up the FAQ. It’s a tweak, a subtle one as I wrote back then. It made no difference to sibilance but added a touch more air at the top end. I would suggest if anybody is experiencing that much sibilance they should go and look for the source of it elsewhere, it doesn’t sound correct that removing the mod would make such a dramatic difference. I continue to use my Phono II albeit heavily modified with a variety of turntables and cartridges and don’t suffer from sibilance.

[colin.hepburn]

Hi All
I Am Aware this subject has been talked about a lot here but I think it is just adding to the confusion over part numbers and values and looking at schematics with typos in them isn't helping

Ok so bear with me on this if you can thanks
I will be ordering some parts to build up my Phono 3S Board shortly and would like to clarify the parts values for R21/22 plus C9/10

I don’t plan on doing the 3.18uS correction and I don’t have the variable caps C9aC10/b just using C1/2 as 0.1uf Valves are the 5751/ECC83-S
I NOTE in the old phono 2 this is R180k and C 220pf but I have two schematics for the phono 3 one has the same R180k and C 220pf and has a led hanging off the cathode of V4a and R39/39k 1w to the led

The other phono 3 schematic has R21/22 as 14.3k and C9/10 as 180pf and C9a/10/b as 65pf note this is for phono 3 version the phono 3A Has R21/22 as 2.2k and C9/10 as 180pf C9a10/b as 50pf

Sorry, this is getting more confusing now.
So ok for my phono 3A board without variable cap C9a/10/b I can use 220pf +10pf to get the 230pf if C9a/10/b is 50pf or 220pf+25pf if C9a/10/b is 65pf

And I still need the correct value for R21/22 is it 2.2k or 14.3k
Thanks for the help and your patents reading this
Colin

[bob orbell]

Hi Colin, to some of your questions, the first phono 3 CD said R21/22 2K2, later it said 14K3, this is the 3.18uS correction resistor, if as you say you do not want to have this, then fit wire links in place of the resistors, this disables the 3.18uS, I do not have it in mine and sibilance is now not a problem. Capacitor C9/10 should be 180pF, if you do not want to fit the trimers C9A/10A, then 220pF is needed in C9/10 and leave C9A/10A empty. Bob

[colin.hepburn]

Hi BOB
Thanks for the follow up to my post that’s cleared up the confusion now
Thanks again Bob for your input
Colin