Another go at PP -> IT -> PP

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

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Nick
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#32 Re: Another go at PP -> IT -> PP

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Max N wrote: Thu Dec 31, 2020 4:01 pm 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
I am glad you didn't think it was a factitious question, because it wasn't.its one I asked myself some time ago. I went through a very similar experience, and like you the one exception I heard at the time was the Lavardin. There is a lot to be said though in trying to find the best of both words. My current thinking is that valves are at their best in the small signal world, and for me trying to make a high power valve amp is trying to solve the wrong problem. Hence my CV19 limited wandering into Lowthers and Supravox's.

But its true I believe that SS amps have also moved on a hell of a lot, and maybe its worth sometime catching up if only to calibrate your goal’s.

I am not convinced that "low distortion" means the same thing in the valve and solid state world, I had to go to what would be zero distortion (in terms of what I could measure) to start to make a SS amp I liked, far more distortion is fine with the 211, but it would be horrible at the same levels with the SS amp.

I am overrun with amps at the moment (couple in for repair), I have the 211 Set, a pair of Transformer coupled 300b's, the Solid State P6100's, a Transcendent Sound OTL, a Leak Stereo 20 and a class D thing somewhere. To be honest I am not sure I could pick one from that to just live with. I can narrow the list down a bit, but cant say which would be the winner.
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Max N
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#33 Re: Another go at PP -> IT -> PP

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izzy wizzy wrote: Thu Dec 31, 2020 4:21 pm 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.
Very true, I feel like we are treading some of the same path! The Jensens I mentioned earlier need to be terminated with 10K, then they are 'perfect'. If you can't do 10K, they need a zobel, but they are designed for 10K and then they are very good. I have been spoiled by the bifilar ITs that are the subject of the measurement thread - they are the only ITs I have measured that don't need a specific termination, they are just flat, this is the magic of well-designed bifilars. I also have experience with some of the amorphous Lundahl line-level ITs, and in at least one case the data sheet hook-up was not the best arrangement. I'll see if I can find my notes on that.
A long time ago, someone told me that transformer ringing could be ignored, but like you i have found this not to be the case. Over time, there is something about the sound that bites every now and then.
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#34 Re: Another go at PP -> IT -> PP

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izzy wizzy wrote: Thu Dec 31, 2020 4:24 pm
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.
Its easy to say that (not having a go at you, I hope you understand), but in reality that "super linear" thing always seems to be more marketing than engineering. Yep, a DHT will have low distortion if you squint at it, but the moment you actually try to use it to amplify, or even worst do work in a power amp, that super linear characteristic, becomes at worst lowish distortion, or just not that much distortion.

I would argue, to be awkward, that on a "device" level a LM3886 is far lower distortion that any valve you may name.
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#35 Re: Another go at PP -> IT -> PP

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When I read a post like Nick's above; that's exposure to a wide range of amplifiers and to come to that conclusion is in itself something amazing. No judgement in any way but much food for thought. Having only owned 3 amplifiers in 40 years, not sure I would welcome the choice.
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#36 Re: Another go at PP -> IT -> PP

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Nick wrote: Thu Dec 31, 2020 4:51 pm Its easy to say that (not having a go at you, I hope you understand), but in reality that "super linear" thing always seems to be more marketing than engineering. Yep, a DHT will have low distortion if you squint at it, but the moment you actually try to use it to amplify, or even worst do work in a power amp, that super linear characteristic, becomes at worst lowish distortion, or just not that much distortion.
I would argue, to be awkward, that on a "device" level a LM3886 is far lower distortion that any valve you may name.
None taken. I get where you're coming from and welcome hearing some level headed thought about the various takes on this. If this was simple, we would all have the "perfect" amplifier.
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#37 Re: Another go at PP -> IT -> PP

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Max N wrote: Thu Dec 31, 2020 4:48 pm A long time ago, someone told me that transformer ringing could be ignored, but like you i have found this not to be the case. Over time, there is something about the sound that bites every now and then.
We must have read the same thing and it's a prejudice I think that has gone in the last few days. Spitty consonants like P, T, S etc have been spreading the sound yet in real life they localise sound to a place. Terminating the 1544a now localises those sounds. Just by way of an example. Your bite analogy is another. I suspect I have small rising responses above 10k in some other of the transformers too which may require termination. All my gain stages apart from the first one in the phono are basically the same.
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#38 Re: Another go at PP -> IT -> PP

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I also use the 1544a as input, in my case to convert balanced to single ended. I've only ever had it terminated as per datasheet. It does sound very good.
Max N
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#39 Re: Another go at PP -> IT -> PP

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Going through photos and notes again, the transformer surgery was October 2019. There was then quite a long gap before any real progress.
Actually, all the pieces were in place, it just took a while for an idea to percolate to the surface.
  • I wanted an amp that could live on or under my desk, so it had to be 'small' (although I suspect that when I put it into a box it will be bigger than I would like)
  • The only source would be a balanced DAC.
  • The speakers would be close, so not a lot of power needed.
The idea that finally came was this:
DACamp.png
A few notes:
Unless you have very sensitive speakers, the DAC needs to be capable of pro-level output. There are plenty to choose from, the Forte is a good cheap option, still available secondhand, and that's where I started. It's good for +16dbU, or about 13V peak-to-peak. I did quite a lot of research, so in a bit I'll list some other candidates, and where I have ended up.
The ITs need to have separate secondary windings - fortunately, I had separated the primary windings on the bifilars and could just use these as secondaries.
I already had the Tent -ve bias supply/servo module, which has turned out to be ideal. It starts off with the valves turned off, and then gently turns them on and controls each tube separately to the target current.
I'm going to claim that there are no caps in the signal path. Fixed bias means that there are no cathode by-pass caps. The power supply uses a beefed-up version of the statistical regulator, using 5W zeners, so the PSU caps are isolated by the CCS.
I'll be honest, the above arrangement still doesn't have enough gain to drive the intended speakers (Dynaudio Audience 50s at 86dB) even as near-field desk 'monitors'.
But I borrowed some Monitor Audio Bronze 2s from my son's setup, 90dB, and this got me close enough to start listening.
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#40 Re: Another go at PP -> IT -> PP

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All my experiments with ITs had reinforced the value of a low source impedance - pro DACs generally have a couple of hundred ohms. And the trend for difficult to drive headphones means that the often have quite beefy output stages. So we are getting the best out of the ITs. This is where I was about a month ago.
morites.jpg
There still wasn't enough gain, depending on the source material. A lot of modern recordings were OK (compression wars...), but some of my older CDs (actually FLAC rips of my older CDs) are good quality with a bit of dynamic range and headroom, and these were too quiet. With hindsight, some digital gain could have been applied to those tracks....

At these relatively low signal levels, the un-shielded ITs are quite susceptible to pick up. Everything from hum from the EI transformers feeding the VCCS filament modules (hence the switch to toroids) to data coming over the power line network adapters.

So back in August I ordered a pair of these:
https://www.sowter.co.uk/specs/9063.php

12 week lead time from Sowter at the moment, so I only got them about a month ago.
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#41 Re: Another go at PP -> IT -> PP

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I said I would list a few more candidate pro DACS:
The Focusrite Forte sounds better than the Scarlett range. Apparently the Clarett range uses the same circuitry, and is good for +18dBu, or 17.4V peak-to-peak.
Mytek Brooklyn DAC+ looks great, includes MQA if that interests you, also a phono input(!), and is good for +22 dBu or about 27 volts peak-to-peak
The daddy is the Chord Hugo, about the same output level as the Mytek, but hugely powerful for a DAC. Actually it can drive speakers directly, so would be ideal for my use case and no need for valves!
RME ADI-2 DAC fs is good for +19dBu but also has some digital gain so is as loud as the previous 2 when playing a track with some digital head room. It also has loads of other features including a 5-band parametric EQ. I ended buying one of those as well.

With hindsight, I was definitely guilty at this stage of persisting with an idea for reasons of simplicity and dogma. I had what I thought was such a good idea that it just had to work. It wasn't sounding as good as I wanted it too, and I was probably guilty of trying to throw money at it. I could justify the Sowters because I needed the gain and the shielding, but the RME was really just desperation. So Nick's posts really hit home. Fortunately, they prompted me to go back to measuring things to find the problem - and luckily for me I think I did.
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#42 Re: Another go at PP -> IT -> PP

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Why not just add a gain stage in front of the interstage. Solid State maybe balanced in balanced out, or just a simple 5687 LTP. Thats all you are doing by using a DAC with higher level outputs.
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#43 Re: Another go at PP -> IT -> PP

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Because I have tried and tried to make that (5687 PP) sound good, and to me it never has. I tried that for ages to drive the 211 PP, and I tried it again to drive 6B4G PP. Andy Grove's original suggestion to drive the bifilar ITs was 5687 PP, so that's where I started. There was always something I didn't like about the sound. That gap between October 2019 and August 2020 was mainly me trying everything to get a gain stage to sound good driving the ITs. Parallel 5687, E55L, tried everything I could think of. I know you made it work in your DAC, but I couldn't make it work when trying to get a 10 or 15 or more volt swing out of it. Admittedly I didn't try solid state, but the RME DAC's distortion when swinging full output is as good as at 4V or 2V. And it turns out that some of it's other features are really useful.....
Also with the 5687 you are then driving the ITs with about 2000 ohms instead of 200 ohms, and I know (think) that lower is better for the ITs.
Similarly the gain from the Sowter step-up ITs seems to be with vanishingly low distortion.
I need to go and join the family for our wild New Year celebration in front of the TV....all three of us. Will continue the story tomorrow :D
Happy new year everyone - good riddance to 2020!
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Nick
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#44 Re: Another go at PP -> IT -> PP

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Yep I did suggest solid state first. Did you try the 5687 with CCS loads and the primary driven parafeed. Like the raven preamp? Avoids current in the tx. Anyway. Yep have a good 21.
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#45 Re: Another go at PP -> IT -> PP

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Four of the amps mentioned in the corner of the room.

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