Another go at PP -> IT -> PP

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Max N
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#16 Re: Another go at PP -> IT -> PP

Post by Max N »

Here's the 45 driver stage. I had always been a bit worried about using DHTs to drive the bifilar ITs because of the risk of the high HT breaking down the enamel insulation between primary and secondary wires. So this was designed for fixed bias to drop the HT.
45 driver stage.jpg
Max N
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#17 Re: Another go at PP -> IT -> PP

Post by Max N »

Next I built up a 6b4g stage. I started building this as an alternative driver stage to compare against the 45. But at some point the plan changed, and I decided I would configure it as an output stage. I wanted to listen to the various experiments as well as taking measurements, and start learning and understanding how the measurements correlated with what I was hearing. I could have dusted off the 211 output stages, but I decided to keep things manageable and sane.
6b4g stage wip.jpg
My thoughts then crystallised into a plan to build a 'small' amplifier that I could use on my desk, fed from a DAC. This simplifies the requirements for the amp, it means that I don't need to worry about phase-splitters, and I don't need input switching, and I don't need have to worry about low-level sources like a phono stage. And it meant that I was working towards something that would actually get used!
Max N
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#18 Re: Another go at PP -> IT -> PP

Post by Max N »

Things went together quite nicely, using stuff I had accumulated over the years. I had the Lundahl LL1663s from when I built the Claus Byrith amp. The mains transformer was bought off Ray, the caps were originally upgrades for use in the WAD 300BPSE. Some of the chokes and the Black Gate cathode bypass caps were from the WAD 300B PP clone.
6b4g both channels cathode bias.jpg
Max N
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#19 Re: Another go at PP -> IT -> PP

Post by Max N »

I still needed a source of negative bias for the 45 stage, so I invested in the Tent/VdV module
bias module.jpg
Max N
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#20 Re: Another go at PP -> IT -> PP

Post by Max N »

I then got stuck into the measurements. I used a Focusrite 6i6 audio interface. I also have a Pete Millett sound card interface, but the Focusrite is so flexible that for most measurements I didn't need it. Also the Focusrite allows for balanced input and output.
measurements.jpg
RhythMick
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#21 Re: Another go at PP -> IT -> PP

Post by RhythMick »

Max N wrote: Tue Dec 22, 2020 6:16 pm Hi Mick
Which Lundahls do you have exactly?
As it happens I'm currently using a pair of LL1663 outputs for my experiments. I have thought that they may be a weak link, so may invest in something better one day. You've also reminded me that I have a pair of LL1671AM PP interstages somewhere which I should really measure and have a listen to. I think I originally bought them when I was thinking of 300B PP to drive 211 PP - I was going to try them in 2+2:1+1 step down. But I came to my senses so they are in a drawer somewhere.
I am roughly between Oxford & Reading - I think you are further north? Maybe we can meet up at an Owston one day, when the world is back to normal.
Sorry for the delay. Yes I look forward to the next Owston very much!

As interstages we used the LL1621PP. I think I have 8 of them sat doing nothing ! I also have spare LL input transformers but can't remember which ones ... 1540? I'll check later.
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#22 Re: Another go at PP -> IT -> PP

Post by Max N »

Apologies for the delay in updates. There was some really good input, thanks everyone. I've spent a few days mulling over what was said, it made me question some of what I had done and I went back to the bench and made a useful step forwards.
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#23 Re: Another go at PP -> IT -> PP

Post by Max N »

izzy wizzy wrote: Wed Dec 23, 2020 9:20 am I keep reading about pp cancelling 2nd as though it does it perfectly. Given the phase splitters aren't perfect and the gain devices aren't either the might be some cancellation but there must be some left. Been reading some Nelson Pass articles on distortion and apparently we like considerable 3rd harmonic as well. It's the higher odd harmonics that are the most objectionable.

Given we all have our own sonic preferences, I can say, for me, I've never had such good sounds at home. I find the involvement such that I lose all track of time and am not focused on what I should do next other than get them into smaller chassis. But it took a lot of work to get my amps to this point.
I agree with everything you say Stephen. We have both been on a long journey to get PP to work for us! Ref sonic preferences, over the course of various Owstons etc I have realised that my sonic preferences seem to be a bit of an outlier. On the plus side, I seem to be less sensitive to things like a flat frequency response. But I think I am perhaps more sensitive (overly sensitive?) to distortion.
A better way of stating my point about harmonic distortion in PP stages, is that some of the even harmonics will cancel. So to end up with low distortion in the whole amp, you have to work to keep the odd harmonics low in each stage.
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#24 Re: Another go at PP -> IT -> PP

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Nick wrote: Wed Dec 23, 2020 9:32 am All I can add is that I find it helpful to prevent losing touch is to try and work out just what you dislike in a design sonically and try and devise a plan to address what you hear. If you can correlate measurements with that all well and good, but work out what you are trying to fix first. I would agree that valve phase splitters are problematic, look at the work Radford did maybe with high frequency balance.

Eggs may have been sucked in this post by the way. Sorry.
And from Mark's epic and inspiring thread
Nick wrote: Wed Dec 23, 2020 2:56 pm I would agree with that Steve, I think the first part of the problem of
it appears to me to be quite a difficult thing to build really good hi fi
Is that it can take some time to find just what "really good hi fi" sounds like, or rather the number of things it can sound like. I am of the view that like convergent evolution, its possible to start in different places, with different plane, and by applying the picking the best and throwing the rest strategy you suggest, its possible to arrive and several entirety different solutions that in their own way produce a equally valid version of "really good hi fi"

I would also say that to do that you really have to apply the "throwing the rest" even though that can be inconvenient and expensive. Clinging to a solution for reasons of simplicity or as I often suggest dogma, can prevent the "really good" from ever being reached.

I would consider this thread as a shining example of that process in action and a striking example of the difficulty it can involve.
No eggs Nick. I was actually a bit stuck to be honest, and this helped get me moving forwards again.
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Nick
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#25 Re: Another go at PP -> IT -> PP

Post by Nick »

Glad to help. I guess I have to ask given your post about distortion (and I think I am with you on wanting it to be low) why are you using valves?
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Max N
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#26 Re: Another go at PP -> IT -> PP

Post by Max N »

Fair question Nick.
I think it was sometime in the 90s I went to a HiFi show (it may have been a Bristol show) and I heard some valve stuff. At that time I had some fairly decent solid state amps at home, but I was aware of listener fatigue or transistor ear (although i wasn't aware then what people called it).
I remember going into some of the small rooms, and finding that the valve amps didn't have 'that sound'. As I went along the corridor, I started guessing before going into the rooms whether what I heard from outside was valve or solid state. Maybe it was a fluke, but I was pretty successful - the SS stuff had a graininess, the tube stuff didn't. There were a couple of exceptions (Lavardin for instance sounded free of grain). I decided to buy some valve stuff, bought some WAD kits, and the rest is history - or not. Since then I have discovered that valves have their own problems, and do not necessarily provide long term satisfaction either. Plus kids and grand kids don't mix with valves.
So that's how I got started with valves, but why I persist with them is harder. I am certain that solid state has moved on in the last two decades, and I could find an SS amp now that would satisfy me. But the valve thing has become a hobby I enjoy, and felt like unfinished business. Plus I have memories of my Dad fixing old valve TVs, and I think that makes valves nostalgic for me.
And who knows, maybe I will succeed in making a very low distortion valve amp :D
Max N
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#27 Re: Another go at PP -> IT -> PP

Post by Max N »

Unfortunately, I haven't kept good enough notes and I can't remember everything I've tried, but this photo is from March 2019
output stage.jpg
This was using a Focusrite Forte to drive the 6B4G output stage into an old Kef centre channel (home theatre) speaker.
The level/gain was obviously too low to draw any real conclusions about the sound. But I remember thinking (not for the first time) that the Forte sounded good for what it was.
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#28 Re: Another go at PP -> IT -> PP

Post by Max N »

One of the limitations of the ITs I was working with was that the primary and secondary were centre-tapped, rather than being two separate windings. On my list of things to try was using them for SE -> PP, maybe in an iron concertina. So I needed to do a bit of surgery on them to separate the windings.
before surgery.jpg
Very fine wire....
after surgery.jpg
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izzy wizzy
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#29 Re: Another go at PP -> IT -> PP

Post by izzy wizzy »

This has been a timely thread for me as there are very few on PP amplifiers, especially transformer coupled ones. I have to correct or add some relevance to my previous post about input transformers. Mr Idler's post about measuring his input transfomer on the Newton 300B was also very relevant. I got a signal generator for Xmas and so could do some measurements and so revisiting many of my choices to see how they measure.

The post I mentioned about Lynn Olson measuring the Lundahls was found https://db.audioasylum.com/mhtml/m.html ... t%3DLL1676

I have the 1544a on the front of the line and measuring it wasn't quite as good as he claimed until I terminated it as the datasheet. The Sowter 3575 also looks good but I measured it first and haven't gone back to it but it needs loading too as the datasheets says. In fact they all do to damp the ringing. The 1544a I loaded and also used the RC combo as per datasheet. I liked the improvement in sound which I wasn't confident about before hand. Another prejudice gone. The 1676 is everything Lynn says too performance wise. What I'm trying to say is, I may not have auditioned these on a level playing field.

What I did find also was I hadn't wired up the 1676 as I should and it was not performing well at all. As Lynn noted, 1:1 performance is incredible, 1:1+1 as I had it, not so great. I have done it as 1:1 (1:0.5+0.5) now and it's a lot better but I lost too much gain so must now try it as 1:2+2 if that's possible. I can go back to 1:1 when my line stage has some gain in the future.

I think some distortions can be quite seductive but I was noticing after a time something going on that I couldn't work out until I did these measurements. Correcting it has shown what that distortion was.
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izzy wizzy
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#30 Re: Another go at PP -> IT -> PP

Post by izzy wizzy »

Nick wrote: Thu Dec 31, 2020 3:17 pm Glad to help. I guess I have to ask given your post about distortion (and I think I am with you on wanting it to be low) why are you using valves?
My understanding is that on a device level, the right valves can be super linear and so low distortion and have favourable distortion characteristics. It's all about what distortion you like anyway.
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