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Old 1st September 2005, 07:23 AM   #1
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Default high end version of low end phono stage

I am wondering if anyone has any thoughts on using this basic phono stage design (http://www.geocities.com/rjm003.geo/.../diy_pho5.html) but using something like the Borbely discrete opamp. Pricewise, to do this would cost about $150 for a pair of opamps, + at least another $150 for caps worthy of the exercise and a pair of power supplies. Would this design be up to the task, or, for the same money, should I look at something like a bottlehead seduction, or Cary PH-1 kit? (those are both tube designs, I am not sure of other ss phono tages in this price range.)
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Old 1st September 2005, 08:53 AM   #2
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This single stage design is inherently limited in concept, and no substitution of parts will get past it. Better designs in terms of distributing the correction tasks over two gain blocks are available and provide lower noise, improved RIAA conformity and lower interaction between xover points. There are many posts on that here on the forum if you are willing to search for them. Walt Jung has also published an article on this issue complete with good arguments for the distributed topology.

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Old 1st September 2005, 02:55 PM   #3
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i'm not sure you will find agreement with all of those statements jan

noise and dynamic range are "best" with the single stage feedback eq - as long as you're looking at op amp style gain blocks

see thorsten's "el-cheapo" for a finely tuned implementation - even justiifes the high freq zero as being desirable

if you want to correct for the high freq zero then a extra buffer stage may be good - but i would not distribute the riaa constants over the 2 stages
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Old 1st September 2005, 04:41 PM   #4
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Default Borbely phono

You might consider ordering the 419 phono kit. Mine was about 300.00 USD delivered. It has the discrete gain stages, active hybrid or passive RIAA, and low noise discrete regulators.
The circuit board comes with fairly high grade parts to finish. You still need a raw 29 -31 volt dual rail supply, and chassis.
The gain stages are nice, use 2SK170 and 2SJ74 transistors. There are also servos to keep the dc drift down.
I have not built mine yet. Need to build a box to put it in first. From the layout and design I expect it to be excellent.

George
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Old 1st September 2005, 05:27 PM   #5
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Quote:
Originally posted by janneman
This single stage design is inherently limited in concept, and no substitution of parts will get past it. Better designs in terms of distributing the correction tasks over two gain blocks are available and provide lower noise, improved RIAA conformity and lower interaction between xover points. There are many posts on that here on the forum if you are willing to search for them. Walt Jung has also published an article on this issue complete with good arguments for the distributed topology.

I can't find an article referring to distributed topology in Walt's publication list. In page 28 of Op Amp Applications Ch. 6 he refers to passive EQ having better RIAA curve conformance but I see this as academic given the variability in cartridges and source materials.

I agree with jcx on active EQ (theoretical) superiority for noise and overload performance, though I believe in practice that component and layout choices are more important. As for RIAA conformance, I have verified +/- 0.1dB on a single stage active RIAA with 72dB Avol using single-stage inverse RIAA.

Of course xover points are interactive in single-stage topologies but this does not preclude close conformance if some experimental adjustment is possible, OTOH I don't see where distributed constants are detrimental except perhaps for overload performance when using a first stage with flat gain vs f.

I believe the main advantage of distributed topologies is in cases with low gain stage Avol and high output impedance (resistive loaded discrete), and in convenient calculation of RIAA network values for close conformance.
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Old 1st September 2005, 05:34 PM   #6
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Quote:
I believe the main advantage of distributed topologies is
Hi Nuvistor

Have you actually built one? It goes against the minimalist approach (unless you integrate the line stage) and is difficult to make fair comparisons but the split riaa sounds significantly better to my ears. Not to mention that it allows to use really high quality caps without having to resort to paralleling.
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Old 1st September 2005, 05:50 PM   #7
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Quote:
Originally posted by analog_sa


Hi Nuvistor

Have you actually built one? It goes against the minimalist approach (unless you integrate the line stage) and is difficult to make fair comparisons but the split riaa sounds significantly better to my ears. Not to mention that it allows to use really high quality caps without having to resort to paralleling.
Fair question. I was responding to janneman in a technical context only.

My only experience with distributed was a version of the Nat Semi AN-346 using 5534s with JFET first stage, built in the 80's, haven't used it in years, 3180/318us active followed by 75us passive. Recent experience has been only with single stage EQ, passive with op amps, active with CCS-loaded tubes, for MM cartridge, my personal preference is the tube stage. I have wanted to build a EAR-834 like topology (flat gain, 75us passive, 3180/318us active) for awhile but have not gotten around to it, I assume this is what you are thinking of as "split RIAA".
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Old 1st September 2005, 06:25 PM   #8
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Konnichiwa,

Quote:
Originally posted by janneman
This single stage design is inherently limited in concept, and no substitution of parts will get past it. Better designs in terms of distributing the correction tasks over two gain blocks are available and provide lower noise, improved RIAA conformity and lower interaction between xover points.
That is one view.

With my rather ancient mid 90's "Analogue Addicts" Phono I set out to demonstrate the opposite.

Okay, step one. Let us consider a MM Phono using "Op-Amp" structures. (we don't do MC here, a seperate stepup optimised for the job is a better choice).

Is it possible to attain better than 0.1db RIAA accuracy with off the shelf parts, reliably, in a single stage? Of course, easy peasy.

Is there any specific advante to the active single stage design?

Well, most Op-Amp's open loop gain rolls off as 1st order lowpass somewhere between 10 - 100Hz. The RIAA may be seen simplified as 50Hz lowpass (ignoring the midrange breakpoints), so the RIAA EQ around a single stage keeps actually the degree of negative feedback around the stage the same within a few db regardless of frequency.

Is there any specific disadvantage to 2-Stage topologies?

Of course, ignoring the extra parts etc., the 2 Stage topology invariably incurs a severe headroom penalty, a larger one if passive EQ is used.

Oh, it sounds much worse too.

I find that FOR OP-AMP structures ONLY, the single stage design as is the AA/ElCheapo and the more recent RJM is a better choice, objectively and subjectively.

I'm normally all down with Walt, but with solid state, limited rail voltages and all else a 46db gain single stage RIAA is the best choice. For MC only the split topology would seem to make more sense but suffers the same disadvantages, except the two stages are now essential ever which way, a good open loop MC Pre-Pre or a good MC Transformer usually outperforms other solutions though.

Sayonara
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Old 1st September 2005, 06:27 PM   #9
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Konnichiwa,

Quote:
Originally posted by nuvistor
I have wanted to build a EAR-834 like topology (flat gain, 75us passive, 3180/318us active) for awhile but have not gotten around to it
Just a note, the EAR is not like you describe, it has a "flat" gainstage outside the NFB loop and a fully active EQ (all three time constants) wrapped around a single ECC83 stage with cathode follower.

Sayonara
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Old 1st September 2005, 07:53 PM   #10
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KYW - Yes indeed, after all, you did post the EAR 834P schematic on this forum - thanks for the correction and for your observations.

So what are your opinions on the topology I incorrectly described as EAR 834P, specifically, flat gainstage with no NFB, followed by 75us passive, followed by 3180us/318us active? A solid-state example from this thread with this schematic.
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