Will the correct Doug Self Wireless World RIAA please stand up?

[leadbelly]

I was trying to find a "best" RIAA eq for my preamp in this thread: http://www.diyaudio.com/forums/showthread.php?s=&threadid=73110 and the Doug Self circuit from 1996 is mentioned often. In Rod Elliott's Project 25 http://sound.westhost.com/project25.htm he does show a schematic and points out that 1 cap is likely the wrong value and if you change it you do get the 3180 us constant. However I found another project http://www.mhennessy.f9.co.uk/preamp/schematics.htm (click Phono stage schematic) where the designer also claims to use the Self eq but has other components with different values. Does anyone have the original Self article or know enough about the subject to verify what is correct?

 

[jackinnj]

you might want to read Nelson Pass' article on the Pearl (I believe that's the one) -- www.passdiy.com

 

[Cloth Ears]

I think 'cos it's still available from http://www.softcopy.co.uk/mag/default.asp?dir=ew&p=shopping&action=add&item=096070540&count=1 (watch out, this adds it to a 'shopping basket') for 4 pounds, it might not be readily available on the net.

As it's not on his own site (http://www.dself.dsl.pipex.com/ampins/ampins.htm) you might end up having to get a copy for yourself to verify what is correct.

 

[Paul Ranson]

I'm prepared to be wrong on this but I think the mhennesey value
for the big resistor isa conseqence of misreading the schematic in 'Self on Audio'.

I think 5220 Ohms in parallel with 14.32n and 63500 in parallel with 50n gives the right numbers.

Paul

 

[sreten]

Hi,

I have the book at home so I can check tonight.

However D.Self categorically states that the time constants interact
and you cannot get accurate EQ by using the time constants for each
section. Consequently for the proper values a simple R||C time constant
calculation should yield "incorrect" time constants.

🙂/sreten.

 

[BlackCatSound]

If I remember I'll check his book when I get home.

 

[davidsrsb]

I have one of Doug Selfs Wireless World design articles at work and can check tomorrow.

The best thing is to use a simulator like Simetrix and simulate against the ideal mathematical model. It's quite easy to get very close to ideal.

I have seen a few discrete preamps that are off at the 3180us point because they run out of gain

 

[rayfutrell]

I recall an article by Reg Williamson in Audio Amateur about 15 years ago on this subject. He had 4 different topologies for the RIAA network and the correct time constants for each topology to provide the proper RIAA break frequencies.

Finding the article may take some time.
Regards,
Ray

 

[jcx]

you might want to look at jung's Op Amp Applications book

section 6 has a dozen pages on riaa circuits

http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html

 

[davidsrsb]

Doug Selfs Wireless World February 1979 RIAA preamp
This was a single supply bipolar transistor design.
I have modeled it here with opamps for simplicity but the results are similar. Note that the gain is low from 50Hz to 1KHz, as much as 1.6dB so the 3180 us pole is wrong.

 

[EC8010]

That's unfortunate. Self spent all that time getting the HF bit right and allowing for the problem of the non-inverting amplifier just to get the LF wrong instead!

 

[davidsrsb]

I wonder if it was intentional, a small lift above 1 KHz but without excess HF peaking can sound good. Plenty of well regarded commercial phono stages are not flat.

 

[AndrewT]

Hi,
I converted the Williamson formulae into a spreadsheet at the time. BUT it was before Lotus 123. First I need to find it, if at all possible, then try to get it to run or extract the formulae. Don't wait around!

 

[richie00boy]

Well here is the Doug Self Precision Preamp '96 as printed in Electronics World/Wireless World:

An externally hosted image should be here but it was not working when we last tested it.

 

[davidsrsb]

Doug liked his rumble filters.

Living in a tropical climate I have a lot of trouble with fungus on vinyl, but at least my lps don't seem to warp anymore

 

[EC8010]

Living in a tropical climate I have a lot of trouble with fungus on vinyl...

If you have fungus, it must be living off something. Do you suppose it might be the mould release agent? (Pun unintentional.) If so, perhaps running the records through a record cleaning machine might help.

 

[mhennessy]

I had an email conversation with Doug Self about this when I was building it. My circuit is as close as intended to the original design, give or take 200 ohms. Allow me to quote from my site:

Also, when comparing my schematic to the published version, you'll note some resistor value changes in the RIAA feedback network - R22 and R23 are [or should be] 52K and 1K5 respectively. However, 52K is not an E24 preferred value - I queried this with Douglas Self, and he was surprised that no-one, including himself, had noticed this before. As the correct target value for this series combination is 53.5K, I got close to it with 51K and 2K7.

The circuit performs very well in my opinion, and has been shown (by simulation) to be extremely accurate RIAA correction - naturally, I recommend it.

If anyone wants more info, I'll have access to the old emails and simulation results when I'm back at work on Monday 😉

Mark

 

[SY]

If you have fungus, it must be living off something. Do you suppose it might be the mould release agent? (Pun unintentional.) If so, perhaps running the records through a record cleaning machine might help.

Much more likely to be the vinyl's plasticizer. They tend to be fatty esters (sebacates, adipates, or phthalates) at relatively high concentrations. Perfect food.

http://www.chemicalfabricsandfilm.com/pdfs_researchSection/techSupport/plasticizer.pdf

This also speaks to the issue of why one might be cautious about having vinyl contact solvent-sensitive materials like polystyrene.

There may also be viscosity modifiers added to the formulation which also provide excellent nutrition for growing molds and fungi.

 

[Bobken]

RIAA

Wilfred Harms wrote a very informative, and apparently authoritative, article on RIAA equalisation, which explained why most designs are wrong due to the interactions between the different stages needed to make up the required curve (IIRC).

Unfortunately I have not (yet!) recalled where I read this article, or I would post a reference to it.

I just did a Google search, and the reference showed "RIAA Network Limitations" which is bound to be the same article, and this apparently appears in the book "The LP is back" which I believe is an AudioXpress publication.

I don't have this book, but if any poster does, maybe he could glean some valuable information from this.

 

[leadbelly]

Also, when comparing my schematic to the published version, you'll note some resistor value changes in the RIAA feedback network - R22 and R23 are [or should be] 52K and 1K5 respectively. However, 52K is not an E24 preferred value - I queried this with Douglas Self, and he was surprised that no-one, including himself, had noticed this before. As the correct target value for this series combination is 53.5K, I got close to it with 51K and 2K7.

Well, I am still confused, since you refer to a 52K resistor, but in richie00boy's schematic we see a 68K resistor combined with the 1.5K.