New LCR Phono : Pentode Aikido, Slagle 1.5k, C3g

[Andrew]

Thanks Mike, I knew we'd been here before...

[IslandPink]

As part of my slow and careful preparations for this project ... I have just installed a pair of the Mundorf ZN tinfoil/polyprop caps ( 0.68uF) on the output of my existing phono amp ; in place of some old Audyn KP-SN .
The Mundorfs are really just an update of the previous KP-SN . I've always liked the KP-SN which seem to have a fullness of mid and bass tone that metallised caps can't provide.

I'm lead to believe the ZN's should sound better in the treble due to the removal of the slight alloying of lead (Pb) in the tinfoil on the previous caps.

I'll report back in a few days , needs time to run-in and assess .

[IslandPink]

....several days later ....

Crikey these are great !!

[IslandPink]

My review of Mundorf ZN caps recently posted on DIY Audio :
"I can report the current Mundorf ZN's ( Tin/polyprop ) are proving to be excellent after about 15 -20 hrs running . This test is on the output of a phono amp at line-level signal.
They are significantly better than the old Audyn KP-SN to my ears . The tonal balance is better , sound a bit warmer than the slightly cold and bright character of the KP-SN. They have useful extra tone in the upper bass without losing anything elsewhere .
The top end is more refined without the slightly edgy character of the KP-SN .
They have excellent realism and 'immediacy' on well-recorded instruments and voices.
They seem to be overall a bit more classy and with more tonal colour in the mids, than the Audyn 'True copper' which I tested recently .
From my memory of the Mundorf silver/oils , I would say the ZN's would 'wipe the floor' with the silver/oils in bass and midrange tone, but the silver/oils might have just have a little more upper treble resolution."

So, the 0.68uF/630 are staying in the phono !

[IslandPink]

I've been messing about with LT Spice. Took Andrew's final LCR model , added in his Inverse RIAA source circuit, and ran various sims.
This one is with the 1.5K resistor on the end .
I spent a fair bit of time fiddling around with various values to get the very best results - well down in the 'noise' of uncertainty in real life, but I learned which components affect which parts of the curve, which will be v.useful for final tweaking .

One thing I noticed, and will post about later, is that it matters where you put the 1.5k load. There's about 0.6dB either way in the overall slope of the response for the two cases .

I'm also going to add in the miller capacitance of the next stage , I think this will be significant for the top-end .

[IslandPink]

This look about right (from here http://www.freewebs.com/valvewizard/pentode.html)

Gain: If both cathode and screen-grid are bypassed, the gain of the pentode is:
Av = gm * ra * Ra / (ra + Ra )
Since ra is very large, this can be approximated as:
Av = gm * Ra

If the cathode is not bypassed, the gain becomes:
Av(unbypassed) = gm *ra *Ra / [ ra + Ra + Rk (gm * ra + 1 )]
These formulae do not take into account the loading effect of the following stage, in which case Ra should be substituted for the value of Ra in parallel with Rl.

They are also very inaccurate unless very precise values for ra and gm are found.

Andrew -
This kept nagging at me - then I finally realised why I was confused .
On the power amps I've used , the pentode drivers always had a VR tube setting the G2 potential - and connected to the cathode . Therefore the cathode resistor takes the current from both the valve and the shunt through the VR tube . This means the current is doubled through it, and its value can be almost halved , for the same bias on the cathode . Hence the amount of local feedback reduces and the gain is closer to the fully by-passed case .

[Andrew]

Yes, spot on Mark, but a gas tube is also probbaly too noisy for the small signal at the front end of a phono, don't forget the cap you need on g2 is much bigger than you think it might be, I used > 10uF, I think.

Andrew

[IslandPink]

Yes I agree with you on both points there . I'll have to set the G2 ( in the phono ) on the input valve in a more conventional way, but as James explained to me , you have to size the cap ( to ground ) on the basis of the G2 impedance in parallel with the supply resistor .

[IslandPink]

No, don't get too excited !
Nothing being built yet, but have been talking to Nick about how he's done the 'V3' 7k3 LCR phono .
It's possible from Nick's info to get a good results with low enough noise from direct MC input to a D3a at the start . I'm just mulling over how much gain is possible with something like Nick's if I went that way.

I've just done a spice sim of my current phono ( conventional RIAA with D3a;D3a ) .
Circuit alone appears to deliver 220x gain ; with 1:10 step-up , that's 2200x . This should be 66.8dB .
I can trade-off a few dB once I'm using the full high-efficiency speaker set-up.

[rowuk]

If possible, try to get the gain close to your digital sources output.

My experience is that we do not always turn the playback down when switching sources and if one is appreciably louder, Murphy's Law states that you change it at midnight and piss someone very important off................

[IslandPink]

Actually I realised I made a mistake : no by-pass cap on the cathode of the first stage . My actual gain at the moment with step-ups is about 72dB