RCA Phono Stage

[The Shadow]

Hi folks

After having built the classic RCA Orthophonic RIAA phono stage into my preamp I have posted a write up on my website.

Has anyone else ever built this thing and what is the consensus of opinion these days on the sound of such an old design.

It is extremely quiet even with AC heating to the two 12AX7s gain stages.

I like it very much but then I would would I not? Fed by my JVC DD turntable and Ortofon Rondo Blue MC cartridge it sounds simply gorgeous; like valves and vinyl should.

I was inspired to build this after Andrew had expressed a certain disappointment in the sound of my JVC at the last EggFest and suggested that maybe my existing WAD phono was not up to the task anymore given the resolution of the rest of the system.

Anyway here it is; the full lowdown plus a bit about the phase plug mod to the Metronomes.

Steve

[Gerry]

Steve

Looks good. Nice layout !
So how does it compare to the Phono II?
Is it better, more revealing, quieter, or just different in its presentation?

Regards

[The Shadow]

Originally Posted by Gerry

Steve

Looks good. Nice layout !
So how does it compare to the Phono II?
Is it better, more revealing, quieter, or just different in its presentation?

It is quite easy to compare the RCA to the far more modern Phono II.

In terms of noise performance the RCA stage is far superior. In spite of the use of AC heaters to the 12AX7 gain stages, the silence of this Phono preamp is quite simply staggering.

The normal ticks and pops that inevitably accompany vinyl reproduction are very well supressed compared to the phono II. This results in a wonderful, almost CD like lack of noise behind the music, allowing all sorts of micro details through that the standard Phono II seems to gloss over.

The treble from the RCA stage is very sweet and clean and seems to be less etched than the Phono II. This could be a function of the power supply to the RCA stage being extremely beefy in comparison being a CRCRCRC filter fed by a big Hammond Transformer rather than a little toroid.

Similarly bass goes a lot deeper than the PhonoII with more detail. Fingering details and little string buzzes from upright basses give an immediacy to the lower end that the standard Phono II seems again to make less apparent.
Fast synthesised bass lines seem very slightly slower on the RCA stage but it is not really enough to detract from the overall musical experience.

The Phono II as standard is lighter in the midrange than the RCA and sounds a bit thin in comparison. I suppose one could say that the PhonoII is not as big-toned as the classic phono stage.

I think that the PhonoII was WAD's idea of a modern sounding stage in that it gives a fast etched sound that can be exciting in the right circumstances but can ultimately become tiring on some types of music.

The RCA stage is more of an all-rounder. It is a big-hearted, generous and colourful phono stage that seems to give even the grottiest recording a chance to give of its best.

Motown and Northern Soul in particular can sound hard and compressed but this phono stage gives a richness and drive to these old recordings that is hard to resist.

As to the mechanics of why this phono stage seems so right in my system I can only put it down to the massive power supply to the whole preamp.

Maybe the fact that a cathode follower is not necessary as the stage is only driving six inches of coax to the selector switch may also be a factor.

Maybe some of our phono experts can shed more light on this.

Steve

[NickG]

What circuit are you using Steve?

I must admit the last time I tried modeling a RCA phono, it was way down on its treble response.

--
Nick

[The Shadow]

Hi Nick

I'm using the circuit below.
The only difference is that I'm using an HT of 425V rather than RCAs 250V

This gives 250V at each 12AX7 and 2v on each cathode which is a better o/p for a 12AX7 (I think)
Although the HT is higher the ratio between the voltages at the anodes before and after the 39K resistor is the same.
The existing 100k plate resistors can't of course be changed without totally screwing up the RIAA
so apart from the higher HT the circuit is identical to the RCA original

It is feeding the input stage to the preamp directly.
RCA specifies a minimum load resistance of 220K.
The input stage impedance of my preamp is 1M so no problem there.

According to stuff I have read on various forums (Audio Asylum being one) if the RCA stage is used into a modern valve preamp circuit (100K input usually) then there will be an appreciable loss of treble and an increase in distortion. Also they reckon that a cathode follower stage on the output is no good either as the sound becomes flat' dull and boring.

Therefore it seems that the RCA stage unless built as an integral part of an old fashioned 1M input loaded preamp will be a bit of a disappointment in the sound quality stakes.

Given then that it is almost impossible to make anything useful out of it within today's context of standalone phono stages, not many people bother with it and it is not difficult to see why, given the above restrictions on its application in todays equipment.

But use it as it was intended in its proper context and it is a pretty nice sounding circuit.

Steve

[NickG]

Ok, I still think you are 3dB or so down at 20k, and that might match your description of the sound. But it obviously works well in your setup, so great :-).

[The Shadow]

Originally Posted by NickG

Ok, I still think you are 3dB or so down at 20k, and that might match your description of the sound. But it obviously works well in your setup, so great :-).

Nick
Do you reckon that this is because they have possibly rounded their component values in the eq circuit to standard ones as in the old days no one would have noticed anyway given the quality of the gear back then?

Steve

[Paul Barker]

I have built the rca and the true realism and the wad phono stage, I far prefer the true realism and the wad to the rca.

[NickG]

Do you reckon that this is because they have possibly rounded their component values in the eq circuit to standard ones

Maybe. I will try and have a play see if I can get the top up a tad.

I guess also that at that time, most sources gave up at 10k, as did most loudspeakers.

Very quick play, try removing the 180pf cap.

Whats it driving into, ok, its 1M , but what capacitance?

[The Shadow]

Originally Posted by NickG

Whats it driving into, ok, its 1M , but what capacitance?

Right here goes

The input capacitance of the 6J5 in the first stage of the preamp is 2.2 pF,
but the grid-plate capacitance of 4.0 pF is multiplied by the gain of 17,
so the Miller input capacitance is

2.2 + 4 x 17 = 70.2 pF.

(God did I just work that out? )

To this we add the cable capacitance to and from the selector switch. There is about a foot of coax there at 75pF per metre so that's about another 25pF
which makes

95.2pF altogether

Edit Oops! That's without the 180pF across the output. It's 270pF with that cap in. And what about strays? Either way there's a top end droop with that 180pF cap in by the looks of it

( Allright stop laughing at my maths Nick, Paul, James I'm only an amateur).

Steve