Should 3.18uS time constant correction be included in Phono amp circuits?

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Greg
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#1 Should 3.18uS time constant correction be included in Phono amp circuits?

Post by Greg »

Prompted by a thread on this topic in another place, I thought I would start a thread here to explore views. Condensing what has been said elsewhere, it seems there are differing views on whether this correction should or should not be included in Phono circuits.

Further reading here. https://en.wikipedia.org/wiki/RIAA_equalization

What does the panel think?
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#2 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by Nick »

Well, what I can add is that having measured many commercial phono stages, that their RIAA eq is so far from the undisputed part of the standard than the additional;pole is irrelevant.
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#3 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by Greg »

Nick wrote: Wed Mar 24, 2021 12:39 pm Well, what I can add is that having measured many commercial phono stages, that their RIAA eq is so far from the undisputed part of the standard than the additional;pole is irrelevant.
Interesting. What do you apply in your own phono stages?
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#4 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by Nick »

The MCj3 has some upper roll of over 50k, but not exactly full. I used the same approach that MH did to calculate the parameters in the eq then apply that as a filter to the R&S analyser. Then use that filter to text the stages against. Same for the PH1. In both cases I tweak the response to give a measured flat response. The Ref Phono has the extra pole in the network.
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#5 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by izzy wizzy »

The 3.18us time constant as I understand it is a boost around 50kHz. I've lived with it but don't use it now. I can see why some would like it.

It's hard to find solid evidence that it was used universally. From sims, it's not that easy to switch in and out without altering more than one thing. The other unknown is when does the boost to counter the cut in the recording end?

Another wrinkle is a boost in this area is playing around where MC carts start to get a bit weird; an area where many recordings aren't exactly dull either.

Recordings I like the most are from late 50s to early 80s classical and a bit of jazz. My guess is most of this gear wouldn't have been flat that far out so why correct for a cut to the RIAA boost that probably happened naturally. Did the tape of the time go that high up? Say to 100 kHz?

It's for all these reasons, I don't bother. The worst that might happen is it might sound a bit dull but overall that isn't my experience.

I should have read Greg's link first before blabbing. I'll go do that now.
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#6 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by rowuk »

Hmm,
knowing the resonant behaviour of vintage microphones (really nasty between 25 and 35k), I certainly would not want more output over 20K for direct to disc recordings. The head gap on analog tape machines is a brick wall filter that is simply not capable of playing back frequencies smaller than the gap. An additional thought is that LPs made for stereo FM also had issues with extended response - aliasing due to the 19kHz pilot tone.

I want my phono stage to have as extended response as possible but I am convinced that LPs with extension >20k or so will be few and far between.
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#7 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by Nick »

The extra pole has an effect down at 20k
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#8 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by IslandPink »

I was just thinking about that, and what's happening with many cartridges, which are heading towards a tip resonance that nay be at 20 or 25kHz. That also causes a phase rotation at lower Hz. There will be some interaction between the two phase rotations ( in the 10 to 15kHz region ) that may or may not make the 3.18uS correction successful for your individual case.
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#9 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by Nick »

Yep, and I also think the 3.18us is more important if its a phono that uses the network in a feedback loop to set the RIAA. With a passive eq, the gain is limited to the eq having a gain of one, cant get any higher, In a negative feedback loop, the max gain is the open loop gain of the circuit, and that may be much much higher, and also the level of feedback is related to the distortion characteristic, so you are going to get more of a frequency dependent distortion profile.
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#10 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by rowuk »

So the answer to the question "Should 3.18uS time constant correction be included in Phono amp circuits?" is maybe, but at least switchable?

I find it interesting that these specifications were created during a time when >16kHz was not common - even in many expensive systems.
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#11 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by izzy wizzy »

rowuk wrote: Sun Mar 28, 2021 9:55 pm So the answer to the question "Should 3.18uS time constant correction be included in Phono amp circuits?" is maybe, but at least switchable?
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#12 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by JamesD »

Hi Chaps,

Got some time this morning to respond to this.

The 3.18us pole was attributed to Allen Wright and he introduced this after discovering that a Neumann cutter head amp had a 50KHz pole - actually two pole filter at 50KHz rolling off the response of the cutter head. He saw this as a way by Neumann to minimise the effects of cutter head resonance and as it, like Nick stated, affects the response at the top end of the audio band Allen attempted to correct for this effect by reversing the roll off...

All good so far... but I only know of one Neumann model cutter head amp that does this at 50KHz most of them do this at 80KHz and some at 100KHz... the reason for its existence is the h.f stability of the cutter head system under a potentially infinite rising RIAA response - clearly not sustainable and closing the h.f. bandwidth in a controlled manner is just good engineering - Note: not necessarily to control the cutter head resonance itself but to control the entire cutter head system h.f. response to prevent instability and any other effects caused by spurious signals above the audio bandwidth. This applies to all good cutter head amp systems - they all have an engineered h.f. roll off above the audio bandwidth but there is no standard and they vary quite a bit by manufacturer and by model number.

So as ROWUK stated - if included make it switchable....

My view - this opens up a can of worms - All vinyl cutting shops had and have a vinyl mastering engineer who job is to setup the vinyl cut - this always includes equalisation within the audio pass band for engineering reasons of the specific cutter systems and for the 'taste' of the vinyl cutting shop and the individual engineer and this is far more drastic in its affect on the sound of the vinyl than a 50KHz double pole filter is... so to some degree this is a moot point so include if you want - it really doesn't matter in an absolute sense so whatever floats your boat :-)

All that by way of saying I don't bother now-a-days cos I can't hear much above 15KHz except for 'air' in the recording...

As my favourite youtube amateur astronomer says - "its all pointless and we're all going to die" :-)

edit: forgot to add if you want to read up on cutter head systems and modern electronics for the same try https://www.lathetrolls.com/

ciao

James
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#13 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by rowuk »

Which raises the question why we do not have a couple of bands of parametric equalization on a phono corrector. There are typical "issues" with preemphasis and 3 or 4 channels of frequency/Q/gain could work wonders.
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#14 Re: Should 3.18uS time constant correction be included in Phono amp circuits?

Post by JamesD »

Agreed - there is a case to be made for a multiband parametric equaliser in the audio chain - after all why restrict it to the phono section only - although one can argue that its most use there but I have a lot of early CDs that are top heavy to my ears ... so with a para eq one can be, to some extent, ones own transfer mastering engineer :lol:

Years ago I had a five band para eq in my system and used it lots for the first six months and then less and less until after 18 months I took it out of my system... although there are some records and some CDs that I no longer play as I don't like the sound of them that the para did improve - of course with different settings for each item and so too much faffing about :D
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