Lilliput Single Ended Amp

[Cressy Snr]

Here's a little project I'm on with at the moment



It's my version of the Lilliput single ended 6080 amp with a valve rectified PSU rather than the solid state one.

Only a couple of watts per channel but it's just a computer amp for the study.
It will feed a pair of KEF egg A/V speakers.

Here's the schematic for anyone interested




With all that Hammond iron on board it weighs a bloody ton! The PSU choke is inside the box.
Should make an interesting comparison with the push pull amp at a future fest.

Steve

[andrew Ivimey]

neat!

[Cressy Snr]

Thanks Andrew

Looking at the schematic makes me wonder if this circuit could be a candidate for direct coupling a la Loftin White 2A3.

Sticking 174V on the cathode of the 6080 should be no problem as the max cathode to heater voltage for this valve is 300V. I'd need a darn sight more HT though to maintain the right voltage relationships across the valve

Still - food for thought. I just wonder if it's worth doing just to get rid of one cap.

Steve

[andrew Ivimey]

You could do it but nb it ain't Loftin white just DC, but you know, 6AS7s can sound 'glassy' or hard, in the same way imho most Russian 'odd' valves do. ou'd prob. have to raise the voltage on the 6080 cathode and you'd lose the RC network of the cap and the grid resistor - worth doing just to see.

But by using a cap you can get some warm tone into the design, which could be more musical and you'd lose that by going DC.


p.s. (edit)
ha ha, I see your posting has gone 'long width like' and I now see you are aware of the voltage issues, to which I allude. Ah well, still worth a go. You know what you are doing. But musicality may need a cap.

[Nick]

Another way avoiding the big cathode resistor is to use stacked power supplies. I can't help thinking along the same lnes as Andrew, the extra cathode resistor and big bipass cap would do as much damage as one nice small coupling cap.

[Cressy Snr]

Yes...

Probably more trouble than it is worth really. But it was just a thought. A very short thought

Steve

[Paul Barker]

I am conscerened that there is insuficient voltage amplification. You would have to have your squeeze box turned up full for maximum power.

[Cressy Snr]

I am conscerened that there is insuficient voltage amplification. You would have to have your squeeze box turned up full for maximum power.

Yes that concerns me too Paul.

An ECC88 set up like this does not seem to me to be man enough to drive the outputs to anything approaching full power. He's got a big current swing going on with the "driver" but the corresponding voltage swing is not good enough IMO. To me it would be better to make the loadline less steep to give a wider voltage swing than he's got.

The work I did on the push-pull amp has proven to me that with this output valve it's voltage it wants. Current is far less of an issue than it is with most other output triodes. Others elsewhere are not of that opinion but it works for me.

A few people on DIYAudio like the sound of the amp so I'm reserving judgement until I've built the thing and listened to it.

I'm expecting a bit of a weedy sound to be honest but it'll be easy change if I don't like it.

Steve

[Paul Barker]

It's just one of the many circuits on the web which are ill conceived but popular with the unthinking masses.

[Cressy Snr]

Here's what I worked out. It's just about the most I think I can get out of this "driver"

This operating point and load line gives a 120V pk-pk voltage swing for the full output of a CD player. Linearity is very good and distortion looks low. BUT that's before losses associated with the cap coupling.

We'll see how it works out.

[Nick]

How about a CCS like I did with the 2a3, C4S at 5ma, 140v on the anode, should give you at least +-100v

[Andrew]

I was just about to suggest a CCS beat me to it Nick, it would maximize mu, and the C4S would work nice @ 5mA.

a CCS is not a cure all, by any means, horses for courses etc, but this could be one of those cases.....courses....whatever

cheers,

-- Andrew

[Cressy Snr]

Now that sounds more like it chaps

[Deco-Doctor]

removed

[Cressy Snr]

Hi Guys,

Thanks for the kind offer Martin but I'll pass on this occasion. Read on and you'll see why.

So!

I built up the Lilliput circuit as per the schematic off the Net and hmmm.. crystal clear but tending towards glassiness and hardness. Also as I had feared and Paul had warned, gain was pathetic, which robbed the sound of dynamics.

If you want an amp that is analytical and cold then build this one, Sure it digs up forensic detail but fails spectacularly at hanging things together so that a decent facsimile of a musical event seems to be taking place in the room. Hi Fi yes music...forget it.

As ever this is only in the context of my system and I wouldn't want to discourage anyone else from building it YMMV.

Building up the C4S current sources to feed the ECC88 resulted in a far better sense of drive and dynamics with again a superbly clear sound, but compared to the PP amp it was like listening in an igloo as an icy draught blew from the speakers. Bass however was nice and tight (for an SE amp ) What was desperately needed was an injection of single ended warmth and romance plus more drive.

So in went a third stage of amplification in the form of a Tung Sol 5687 driver. The CCS was removed and the ECC88 reconfigured to make volts rather than current. The 5687 was similarly configured, just using the Resistance Coupled Amplifier table in its data sheet, going for gain and plenty of voltage drive to the 6AS7.

The sound was transformed by the addition of a proper driver stage. warm, sweet, clean and clear. Bass is tuneful and goes very deep indeed though slightly looser in nature than the push pull amp, great for jazz.

I could put the CCS back in, which would probably tighten up the bass a tad but as it is being used as a study system with little KEF egg A/V speakers, it does not really warrant it.

Here's the thing though; in the main system downstairs it gives the push pull amp a good run for its money at a fraction of the cost. The PP amp is better but not by a huge margin. The PP holds together better at high levels due to its power advantage and has tauter bass.

The PP amp stays in the main system but it will be an interesting comparison to do at a Fest to see what others think. The amp does hum a little but it increases and decreases as the volume is raised and lowered so it's down to poor positioning of the inputs, whose cable passes right over the PSU and straight under the rectifier valve, once that is sorted we're cooking with gas. I'll move the input jacks to the front next to the input stage.

It's no longer a Lilliput now of course.

It seems that to get a good sound from a PP amp that compares favourably with single-ended, one has to go to a great deal of trouble; using triodes, eschewing global feedback and going right back to first principles. I still think it's worth doing. But the cheapo SE amp did give Big Jetson a bit of a fright.

Steve.