audio-talk

SteveTheShadow · Posted: Mon Apr 13, 2009 8:22 pm Post subject: Another Loftin White

This one uses triode strapped 46s driven by paralleled 6N1Ps.

I need a backup amp to play with so this one should prove interesting.

As usual here's a schematic to ponder.

Steve

andrew Ivimey · Posted: Mon Apr 13, 2009 8:47 pm Post subject:

I'm pondering!

I'm trying to do something with 46 parafeded - this is interesting.

Have you got a 10k output transformer? I'd have thought that 5k would be enough.

SteveTheShadow · Posted: Mon Apr 13, 2009 8:54 pm Post subject:

SteveTheShadow · Posted: Sun Apr 19, 2009 5:06 pm Post subject:

A bit of progress on the 46 Amp today.

The case has been built and all the hardware mounted.

I'm not doing any more on it tonight but it's ready for wiring.

I've still not quite finalised the design details yet but it should work OK.

What it'll sound like is another matter Wink

Here's a couple of views of what it will look like when finished.

Between the small pillars flanking the gate we pass between the output
transformers and walk up the procession way to worship at the great altar of Rectifius GZ37 beyond Very Happy

A general view.

Steve

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Forgive me Steve, but aren't those power TX in the 'least optimal' orientation' or am I hum paranoid after having built one too many phono's.... Rolling Eyes

-- Andrew

SteveTheShadow · Posted: Sun Apr 19, 2009 5:14 pm Post subject:

Ah well....you see

The mains tranny is positioned so that its coils are at right angles to the coils of the left hand output transformer in the box, now I didn't want the choke interfering with the right hand output transformer so that too is at right angles to the right hand output tranny.

Unfortunately this puts the choke and the mains transformer in line with each other so i've put them as far apart as possible, with the rectifier in the middle. So a compromise. I can soon swing the choke round if there are any problems.

Well that's my story and i'm sticking to it Very Happy

Steve

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

But doesn't the hum field run worst along the line of the lams? Or have I got that the wrong way around?

-- Andrew

SteveTheShadow · Posted: Sun Apr 19, 2009 5:57 pm Post subject:

Yep your right Andrew

I'll rephrase the previous. The trannys are positioned so that the lams are mutually at right angles between the mains TX and the output TXs.

You can't see the OPTXs as they are in the boxes but their lams run right to left whereas the visible TX and choke run front to back.

Mind you the potting boxes have lids underneath so totally enclose the OPTXs, so a bit of earthing should completely screen the trannys within them from any interference, meaning I could safely spin the choke through 90 degrees anyway. Smile

Steve

SteveTheShadow · Posted: Sun Apr 19, 2009 6:41 pm Post subject:

There ya go! Smile

Steve

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

OK, but on your photo, doesn't the left hand side's (as I'm looking at it) hum field run straight through the OPT?

Course it might make very little difference I an continuously amazed about what you can get away with on power amps when compared to hummin phono's.

I'll get me coat.

Embarassed

-- Andrew

SteveTheShadow · Posted: Sun Apr 19, 2009 7:15 pm Post subject:

Good this isn't it! Smile

So if I turn the power TX thro 90 degrees it'll be feeding an unrectified 50Hz sinewave into the choke and if I reorientate the choke back to where we started it'll be feeding 100Hz ripple into the other OPTX. Looks like I can't win either way with this layout. Sad

Good job the OPTXS will be totally enclosed in earthed metal boxes I say.

Steve

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Well, I agree, Steve, you have the measure of it, certainly. Dammed if you do and dammed if you don't, as they say....

If it were me, and I know its not, I would rather opt for running the choke and PSU TX left to right rather than front to back or even at right angles.

Why?

Doesn't the step down of the OPT give you a large PSRR?

I would have thought what you don't want to do is induce that 50/100Hz into the secondaries of the the OPTs where there is buggar all in the way of PSRR/step down on your side.

Whereas a few mV of hum on the primaries, caused by B+ ripple, is nowt when you are likely swinging well over 100v!

-- Andrew.

Nick · Posted: Sun Apr 19, 2009 8:41 pm Post subject:

I would have thought the 50hz field from the power transformer would be swamped by the 100hz current being fed into the choke.

SteveTheShadow · Posted: Sun Apr 19, 2009 8:47 pm Post subject:

Well,

So intrigued was I with this whole problem that I took a look at MJ's Building Valve Amplifiers.

He seems to favour pointing the Mains transformer lams straight at the driver stage with the OPT lams going left to right across the chassis,
which is how I've got it at the moment.

He also argues that if you are using a CLC supply then the choke won't induce much into the OPTX as ther will be little AC current across it.
A choke input supply with a large AC current would need aligning properly though, he says

Steve

cressy · Posted: Sun Apr 19, 2009 9:01 pm Post subject:

this problem was the reason i went for a 2 box layout on the 45/2a3 amp, i couldnt get my head around it all. if the psu txs are 3 feet away then problem solved!