Valve amplifier phono input

[PJL]

I have now got the SP21 design within 0.32dB 20-20KHz by further reducing the load from the next stage and adjusting the CR network. Given it is both noisy and at the very limit of the gain I need I am thinking I will build one of yours.

The 2-stage triode design is brilliant as it achieves a huge gain and delivers high input impedance. Do you think the design has any impact on distortion? I have done the same manipulation with yours and trimmed the values to remove the roll-off. If the simulation is correct it now tracks the RIAA extremely closely (<+.3dB) with just a slight roll-off of -0.8dB at 20Hz.

[kalee20]

If you build it, I'll be really interested to read your comments. I built two, in small diecast boxes, as a stereo pair. But |I'm not using a really high-end magnetic cartridge.

Distortion? Well, the first stage has to cope with everything the cartridge throws at it, so yes could overload with record clicks or anything impulsive, as it is handling raw signal. But it should behave gracefully up to half a volt peak input. The second stage rolls-off at high frequencies so will be comparatively immune - it sees its largest amplitudes at low frequencies.

As before, the input 0.01µF and 1MΩ could be omitted if you don't want the (non-RIAA) LF roll-off.

[GrimJosef]

Originally Posted by kalee20

... As before, the input 0.01µF and 1MΩ could be omitted if you don't want the (non-RIAA) LF roll-off.

And strictly speaking the input 100pF will be partly made up of the capacitance of the input cable I imagine.

Cheers,

GJ

[PJL]

It's over 2 years later and I am looking at this again! Since this post I managed to obtain a Beam Echo STEP 11 converter (a little rusty) with the intention of using it to house a triode phono input amplifier based on Kalee20's design as it already has the correct PSU lead. I had thought it was a variant of the STEP 21 ECC83 tape pre-amplifier but it turns out to be a phono version almost identical to the Kalee20 circuit. LTspice confirmed it to be within 1dB of the RIAA curve which is significantly better than the SP21 phono input.

I have now replaced all the out-of spec parts and removed the decoupling capacitor from the first triode as I won't need full gain. I am also changing the input design from the current switchable attenuation giving 67K/10K input impedance to 47K/270pF to match the V15 III bought when I was a student in the 70's (spec. says 47K and 400-500pF).

It will be interesting to see if reality matches theory and noise is lower and bass increased.

[kalee20]

Can't wait to read the report!

Originally Posted by PJL

...I managed to obtain a Beam Echo STEP 11 converter (a little rusty) with the intention of using it to house a triode phono input amplifier based on Kalee20's design as it already has the correct PSU lead. I had thought it was a variant of the STEP 21 ECC83 tape pre-amplifier but it turns out to be a phono version almost identical to the Kalee20 circuit.

That would be unintentional, my circuit was not based on anything 'out there,' I just wanted to use the workhorse high-µ ECC83 triode.

[PJL]

Very difficult to invent something entirely new! The circuit is below but I have missed out some HT smoothing and the input attenuation switch.

Interestingly, there is a 120 ohm resistor in series with the -ve supply and earth. I assume this is to reduce current flow along the signal cable screening caused by multiple earthing routes.

[PJL]

The noise is definitely lower and it sounds remarkably good. Gain is still below ideal but I could alter the feedback on the SP21 EF86's to fix it. It's a bit difficult to do a comparison so, in the tradition of audiophoolery when you spend time & money on audio gear, I conclude that the bass is now full and dynamic bringing out subtleties in the recording that weren't evident with the SP21 phono stage.

I have had various hifi amplifiers in my time, most of them dissapointing, but this 55 year old amplifier is as good as the best and far better than most. Anyone who says the essence of valve amps is their smoothness has probably been listening to a DAC90 as a good valve amp is remarkably dynamic. OK it may not be entirely 'accurate' and doesn't have a 0.0001 ohm output impedance to suck the life out of the speakers but it does have charm and a secondary use as a room heater.

[Top Cap]

I don't like the active RIAA's myself and have built many of the old WAD passive filter units with great success. I use direct as possible connections as shown in the link.
http://goo.gl/zRYoCI

[PJL]

I have gone for authenticity rather than the best possible technology. Other than a few very minor changes to the STEP11 design and stacks of new caps and resistors, it is a complete original Beam Echo setup.

If I get some time I might build a passive pre-amp and revised phono amplifier but lots of other things to do at the moment.

The hiss from the triode phono amplifier is very noticeably less than the pentode and from what I remember better than transistor versions I have had before,