Phono Stage and Phase Plugs
The Job is Completed
The final addition to the 6J5 preamp was the phono stage. This has completed the system, which is now able to accept any source. The speakers have been modified with Planet 10 phase plugs from Dave Dlugos.
The Phono circuit I used was the classic RCA recommended 12AX7 phono stage from the back of the RC25 manual. It consists of one passive RIAA stage between two common cathode stages per channel and is very simple to build.
I moved the operating points of the valves a little further down the load line by running them with 420VHT rather than the 250V specified by RCA. This gives 250V at the valve anodes and 2V at the cathodes, which is more of a typical condition for a 12AX7.
Anode resistors remained unchanged at 100K to prevent the total screwing up of the RIAA. Only the linearity of the stage was changed by this decision (I hope) and it’s for the better I believe.
Note however the decrease in tidiness from my usual impeccable wiring standards:-)
I decided to hard-wire the RIAA part of circuit to the valve bases using only the component leads which produced the layout in the picture. I would have loved it to look a little better but given the very low signal levels involved I thought it prudent to use the shortest possible signal paths. Sometimes you just have to compromise your ethics for the greater good.
I have what I would now call a system that exhibits the classic valve sound. The classic valve sound is the sound I grew up with as a kid in the early to mid 1960s. It is the sound of a console hi-fi but with much greater resolution and beneath it all, the classic sound still remains intact.
Since I became interested in hi-fi as a 16 year old in the mid 1970s I have explored many sides of the hi-fi sound, from the dry punchy sound of a high end Linn/Naim system to the warm and wooly sound of a KEF Coda NAD 3020. But I have now come home to the sound that rocked my childhood. A strange journey indeed.
Here is what Scott Faller (June 2003, TNT Audio) has to say about the classic valve sound.
The classic tube sound is very distinctive. Imagine if you can, walking into a jazz bar where a trio is playing. The lead man is playing a hollow bodied guitar amplified through a vintage Fender tweed amp and has the treble turned down slightly. The Bass player is plucking away un-amplified on his upright and the drummer is using brushes so he doesn't run over the top of his bandmates on the stage. It's this laid back sound that best describes the classic tube sound for me (at least today anyway). The presentation of the music is very relaxed and highly non-audiophile sounding.
So as I sit here listening to “Motown - Big Hits and Hard to Find Classics” on vinyl there is this dark tonal balance and an unmistakable texture to the sound coming from the phono stage. It is warm and rich with nary a hint of getting in your face. There is more than enough detail to satisfy anyone but the desperate pedant and the sound-stage is big, tight and clean without a lot of smearing. By smearing I mean that vocalists appear about a foot wide in the mouth rather than pin points. The classic RCA phono stage paints its sound pictures with broader strokes but that is OK with me.
Using my JVC QL-Y3F direct drive turntable with Ortofon Rondo Blue MC cartridge into Sowter 9570 step-up transformers, music emerges from a black velvet silence. Surface noise is extremely well suppressed and completely non-intrusive, indicating a good overload margin.
This good noise performance is doubly impressive since there is AC heating to the two 12AX7s that make up the phono stage and I have no inclination to change just yet.
Sowter 9570 step ups are housed in this screened box
Here we see the two JJ 12AX7S small signal valves that make up the gain stages for the RCA phono circuit.
In the background are the four JAN Philips 6J5 single triodes that make up the preamp gain stage.
The two JJs are a nice, sweet- sounding tube with a delicate top end and a firm bottom.
After a bit of tube rolling with Mullards, EIs and Sovteks I settled on the JJs as having the best overall balance.
Here is the complete preamp in situ with the step up box next to it.
I found that trying to mount the Sowter step ups inside the preamp chassis was a waste of time as the level of hum was unacceptable and no amount of playing around with wire routing would shut it up.
Mounting the transformers in a screened box cured the hum completely.
Phase Plugging the Fostexes
The final icing on the cake was the Planet10 mod to the Fostex FE108EZ drivers I use in my Metronome speakers.
At the recent meeting at Steve Shiels’ place, Nick Gorham and I had noticed a definite shouty coloration coming from the drivers when they were being pushed hard in a large room.
As is always the case whenever we meet at Steve’s I end up going home with more questions than answers regarding my system’s performance. But that is fine as it always leads to an improvement in the sound of the system in the end.
Researching the problem I visited Dave Dlugos’ Planet10 hi-fi site and read about the improvement it was possible to make to the sound of some Fostex drivers by installing wooden phase plugs. Several WD board members had tried them with good results so I ordered a pair from Dave.
Fitting them involved irreversible surgery to the drivers by cutting out the dust caps with a sharp scalpel, therefore I was quite naturally a bit nervous about butchering these cherished drivers.
I needn’t have worried as the improvement made by fitting these plugs was startling.
The treble sweetened up, the shout disappeared and the general musicality of the system ratcheted itself up another few degrees.
Why Fostex don’t fit these plugs as standard in their drivers is a mystery.
So what of the sound of the system now?
I have at long last come home from the quest that started 32 years ago with my first proper
hi-fi system.
The classic sound of valves lives on in this system. The sound that came from Mom and Pop’s console hi-fi but now with a far higher degree of resolution is the key to it all.
I am in love with the sound of the 2A3 valve, single-ended triode amplification in general, a tube phono stage design from the 1950s and a pair of single driver home built loudspeakers that look like metronomes. All this and only 3.5W of power.
Gorgeous.
