Post by Owen Y on Aug 15, 2018 20:08:00 GMT 12
A Line stage is virtually redundant these days. Back in the old days, when AUX sources like radio tuners & tape decks put out only 0.1 to 0.5 volts rms, consumer 'line level' standard was only around 0.3vrms. And so, you needed a line stage in your preamp or system to boost that up a bit in order drive the power amp stage. But these days, with digital sources putting out 2 volts or more, a line stage preamp can be regarded as redundant. You still need a volume control & usually source switching too. (Tape input/output facility also is now a thing of the past.) So, for digital-only systems, an all-integrated amp with source switching & volume control on-board can do the job. Unless you have a vinyl source, which will still be down at 1/4 (or less) of digital level, so a line stage preamp is still required - or, if you have a sensitive power amp, ie. with the equivalent of a line pre-amplification stage up front.
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Post by Owen Y on Aug 15, 2018 20:40:44 GMT 12
Back in around '98, I built this tube Line Stage, my first from-scratch amp build - called the WOT (With Output Transformer) Linestage. It was a published (in Sound Practices) design by Diego Nardi, who was at that time representing (I think) Audio Note UK in Italy. It had a gain of around 10x. The signal circuit was simply one tube stage (5687 triode) followed by Output Transformer coupling to drive cabling & the power amp (Although the PSU was more complex, with large capacitance, quasi-battery DC supply & sophisticated heater supply.) 
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Post by Owen Y on Aug 15, 2018 20:43:22 GMT 12
I was on a bit of a Kondo copper theme at the time & it also matched my soon-to-be-built Phi-42 phono stage) also designed by Diago Nardi. 
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Post by Owen Y on Aug 15, 2018 20:48:57 GMT 12
The WOT Line stage was nice, neutral, the OPT eliminated the usual coupling capacitor (which is always sonically a good thing) but.... it didn't have that certain 'magic'. Also, with super-sensitive horn loudspeakers coming my way, I couldn't get rid of some EM pick-up & noise...
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Post by michaelw on Aug 15, 2018 20:57:52 GMT 12
love the look of the preamp.
the combination of wood, copper and those vernier type volume controls is timeless.
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Post by Owen Y on Aug 16, 2018 10:02:18 GMT 12
Back then I knew even less about EM noise, transformer orientation, Grounding etc. than I do now. I didn't know the specifs of the OPTs (supplied by the designer in Italy). Despite the theoretical isolation provided by the OPT, the stepped attenuator I was using at the time made pops occasionally. I tried ripping out the PSU components & housing them outboard in a separate PSU box (below) - to no avail. 
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Post by Owen Y on Aug 17, 2018 13:31:57 GMT 12
Although the 5687-WOT + DIY Phi-42 phono smoked the ARC SP8 (MkI) that I previously used, I gave up on the noise problems of the WOT. I switched to a more conventional tube preamp design - another design published originally again in Sound Practices, Bruce Berman’s Homebrew Type 76 Preamplifier - I built the version by JE Labs (Jospeh Esmilla) labelled the JEL Line Stage. A simple 1-tube amplifier stage combined with a single (half) 6SN7GT (1941 designed dble triode) output cathode follower to provide low impedance output - fairly conventional topology. But the 76 is a nice tube, a single triode introduced in 1933, with old-school 'ST' (shouldered tube) shaped glass envelope & was a commonly used tube. Sadly, the old 5687-WOT Line stage was gutted, converted with a new, bigger outboard box to house the tube-rectified, double-choke smoothed HT & SS DC heater PSU. That was around 2010.... 
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Post by Owen Y on Aug 20, 2018 19:54:49 GMT 12
This line stage design was at one stage being built by Jeff Korneff & was given what amounted to a rave review by Peter Breuniger in the now-defunct Listener magazine in around 1999. Yes it had a bit of 'magic' to its sound. The first stage RCA 76 tube is a nice sounding older style tube with big curved internal plate elements, to which I at least partly attributed its big, spacious, clean sound. The 76 was direct-coupled to the 6SN7 - ie. no intervening coupling-capacitor. I used this preamp up until recently. However: - its output to the power amp still required a coupling capacitor (I was using nice AmpOhm alumin foil/paper in oil caps but I find that all caps impart a 'sound'). - the 76 tube was a little microphonic, which was part of the 'magic', it conveyed lots of low level detail, but also some tube microphony/noise.
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Post by Owen Y on Aug 20, 2018 20:41:37 GMT 12
 (A bit of a rat's nest test-bed but....) Film & oil cap + choke PSU decoupling, Mills wirewound & Shinkoh tantalum resistors - low parts count but quality & experimentation to get a natural tone & 'live' dynamics that I like. The transformer volume controls (Sowter, 2011) opened my ears to something I hadn't heard before - I like the way that 'coils' (transformers, chokes, etc) deliver 'power' better than resistor / resistance type volume controls. Especially down in the lower frequencies, where the power in music resides (weight, rhythm, power). Also I like the way that (well designed) audio transformers convey high frequencies, more naturally, less harshly than resistors. Music 'breathes', ebbs & flows with more dynamic freedom. This was one of the things that led me to re-visit the previously discarded WOT (With Output Transformer) line stage preamp.....
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Post by colinf on Aug 21, 2018 21:18:40 GMT 12
Nice looking pre. I’ve always wanted to give the Sowter TVC a go. Have you compared it to any other TVCs? What will your new WOT pre consist of? My friend tried the 76 for a long time in a simple but elaborate line stage. He could never quite get the magic out of it. Also a bit microphonic. Perhaps TVCs would help. An optimised 6sn7 circuit has slightly lower distortion than 5687. Even better would be 6j5, a single triode that is the same as half of a 6sn7 but because it’s in a bigger bottle seems to have a bit more fun factor to the sound.
AMR-iFi R&D
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Post by Owen Y on Aug 22, 2018 12:43:08 GMT 12
Thanks colinf, I had more time back then  (And an old pal to help with the woodwork.) Sowter: Brian Sowter was very accommodating when I wanted only a narrow range of 16 taps in the TVC (for lowest cost) & he suggested a design that was made. With hindsight, although I had wide dB steps in the seldom-used low & top end of the vol control, the close steps of 3dB have proven to be too coarse in a sensitive system. So, I've bought a pr of standard 9335 Sowter TVCs, 24 x 2dB steps - still to be installed. I haven't used other TVCs sorry. I have some Stevens & Billington SUTs, but I have to say that their sound has never totally won me over. Dave Slagle's Intact Audio Autoformer TVCs have a following in USA. New WOT: First up, I have taken out the 76s & wired some new Line OPTs to the 6SN7s on the chassis. (You'll see that I use 2 x 6SN7s, only using one half of ea dble triode - no reason other than Bruce Berman did this. However I've wired them mirrored, so that I can swap them occasionally to avoid any 'cathode poisoning' with no HT on one half of the tube. Also possibility some extra flexibility in matching tubes.) Line OPTs: I was lucky to acquire a pr of these locally, used/near new, holy grail for me.....  
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Post by colinf on Aug 22, 2018 18:53:58 GMT 12
Tangos, cool! The unused half of the 6sn7 will begin to self-poison pretty much straight away and after a year or so it’s emission will be noticeably down. Better to tie the cathode and grid together connected to 0V, and have a very small current, perhaps half a milliamp running on the anode. The anode voltage will be about 7v. The cathode will last longer that way.
AMR-iFi R&D
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Post by Owen Y on Aug 24, 2018 11:41:39 GMT 12
Thanks colinf - yes I should convert it to a single 6SN7 or I could rewire for 6J5s, of which I have only a couple I think. However these 6SN7s have been in use for 4+ yrs & emission is interestingly down on only one tube, one side. The transformer-coupled design I've now adopted is ultra simple, no coupling caps.... a-direct-heating-triode.blogspot.com/2016/04/I've re-calc'd my HT PSU a bit to tweak time-constants a little, in agreement with the philosophy espoused by this Euro chap.  A bit of a lash-up at the moment, on the old chassis, while I evaluate & finalise things.  What I will do next is to try the 76s through the Tangos, before I finalise this build. The 76s have higher plate resistance, which is why I didn't go immediately with them. The single tube stage inverts phase & I'm not sure how the Tango is wound, so I will put it on the scope & take a look, maybe trying reversed phase output connections. The sound is right up my alley - great 'power' & headroom, dynamic contrasts, strong LFs, music 'flows'. Gain is down a bit, maybe 6dB(?). Plenty of warmth with the Kiwame (just what I had on hand) 1k2 R load on the sec & Mills w/wound cathode R. (Circuit correction: C2 changed to 100uF.)
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Post by Owen Y on Aug 24, 2018 16:37:43 GMT 12
There's a pattern emerging.... LCR Phono - inductor coils TVC - Transformer Vol Control WOT Line stage - transformer coupling Time I think to convert my PP amps to IT-phase-split/coupled PP.
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Post by michaelw on Aug 24, 2018 18:00:54 GMT 12
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Post by colinf on Aug 25, 2018 11:26:02 GMT 12
Have you tried fiddling with the 1k2 output resistor value? As it affects both the frequency response and the harmonic distortion generated by the 6sn7. Lower resistance = more thd from the 6sn7 having a steeper gradient across its load line, and less frequency response. Although..... Tim de Paravicini always believed in inductive EQ and transformer coupling where necessary. I always thought they (transformers) would lead to phase shifts and frequency response aberrations, preferring flat frequency response. But the ear already has limited frequency response and the stereocilia (the hair cells in your cochlea that pick up vibrations from their surrounding fluid) act as mini spectrum analysers with nonlinear response. So the ear can deal with quite a bit of harmonic distortion of sine waves, and phase response anomalies at the frequency extremes and get away with it. That also has to do with the brain learning the characteristic sound of something and being able to filter it out to get to the fundamental changing signal. So low distortion amplifier circuits might not be the answer after all, unless transparent signal processing is needed. Circuits with a bit of musically consonant harmonic distortion might be modifying the way sound is reproduced in a psychoacoustically agreeable manner. In the studio industry there are sound apps that supply characteristic valve harmonic distortion for that reason. Perhaps that’s why some prefer a valve and transformers in their otherwise low distortion systems. I do!
AMR-iFi R&D
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Post by Owen Y on Aug 27, 2018 10:08:14 GMT 12
Hi colinf - no.... not yet tried changing sec loading on OPTs. I have set it up as per Tango datasheet for 20k:1.2k. An alternative is 10k:600, which according to their data, gives slightly flatter LF response - so of course I must try that (when I get around to digging for some more Rs or source some of the ones that I usually prefer. However 20k may suit the Rp of the tube better - the ears will tell us! (Graph below shows S-Ended source connection on the Prim) 
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Post by Owen Y on Aug 27, 2018 10:23:21 GMT 12
Tube operating point: colinf, I've adopted the set-up of the blog page link above, because it looked v good to me - ample voltage swing, a good low distortion op point (load-line below). It also worked out with the HT volts of my existing PTX/PSU. A much better op pt I think, than my previous 76-6SN7 line stage - with that cct, the HT needed to be high for the direct-coupled tubes, cascaded/stacked & my 76 was operating a little low on the curves (due to my PTX B+ being a touch low). 
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Post by Owen Y on Aug 29, 2018 9:54:20 GMT 12
The single tube stage inverts phase & I'm not sure how the Tango is wound, so I will put it on the scope & take a look, maybe trying reversed phase output connections. Indeed it was so, a quick look on the scope revealed exact 180 degree phase inverted output (pic below) (Top trace input, bott trace output.) 
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Post by Owen Y on Aug 29, 2018 9:57:49 GMT 12
I swapped the Tango NP-126 primary connections (rather than the secondary outputs) & correct phase output, matching the input signal, was restored. (Excuse the noise on the output trace, my rough & ready probe technique.) 
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Post by Owen Y on Aug 29, 2018 10:09:42 GMT 12
Importantly, auditioning reveals that the sound has changed slightly - more sharpness, 'precision' ('snap' & focus), HF brilliance, image width & 'expansiveness' most noticeable. Unsurprisingly I suppose, correct phase is important for best stereo reproduction - and also for mono I'd expect. Possibly easier to identify on valve/tube equipment, but any audio manuf. should specify it.
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Post by colinf on Aug 29, 2018 21:52:34 GMT 12
IME it does matter, especially for soundstage depth. The way the ear perceives harmonic distortion means that the depth perspective is changed. I can’t remember where I heard it but I recall that air has second harmonic distortion and contributes to the way the ear can determine how far something is away. If the absolute phase is reversed it would be modifying that in some way. If the equipment reverses phase both speakers' polarity can be reversed, or for phono stages, the cartridge polarity. Also, when the primary polarity is changed in your pre the inter-winding capacitive coupling changes very slightly as now the other end of the primary is effectively grounded. I agree that manufacturers should specify it. Generally SS equipment is phase correct but valve equipment could be either.
AMR-iFi R&D
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Post by Owen Y on Aug 30, 2018 20:45:28 GMT 12
Ah yes, as you say, the reason for swapping the primary connection (rather than the secondary), is to have +ve phase entering the 'start' of the coil winding (as opposed to the 'finish' end) - technically lower stray capacitance & audible too I believe. (Not dissimilar I think to connecting to inner/outer foil ends of wound film caps - the difference there is not small.)
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Post by colinf on Sept 2, 2018 18:36:13 GMT 12
Are you able to measure the frequency response? That way you can see if there's a difference with the primary connected normal phase or reverse. Otherwise the differences you’re hearing could be due to the interpretation of absolute phase.
AMR-iFi R&D
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Post by Owen Y on Sept 2, 2018 21:26:36 GMT 12
Hi colinf - yes I can do that in a rather laborious manual manner using an oscillator, scope & 20Log V1/V2 calcs - as i don't have any more sophisticated test gear At the end of the day I tend to trust the ears 
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Post by Owen Y on Sept 22, 2018 13:24:27 GMT 12
I managed to snag an analogue millivolt meter with dB scale, on Tr Me for 50 bucks. Now I can do those amp frequency sweeps much less laboriously. A China-made job (Tonghui), but it seems very good. 
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Post by Owen Y on Sept 22, 2018 13:37:37 GMT 12
Here's the sine freq response taken at 12 o'clock volume into 100k (with OPT sec loaded 1.2k): (Excel graphed.) 20-20kHz within 0.55dB - better than I thought, good bass, some HF rolloff, but overall very much like (slightly better than) the Tango datasheet above. I'll try 600 ohms sec loading when I get the Rs ordered from HiFi Collective in UK. Noise is not so flash (but not a big problem in-system), only ~-60dB residual (ref max. output), same at all vols - I'll have to look harder into that. 
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Post by colinf on Sept 22, 2018 18:52:31 GMT 12
Nice one. How did you generate the sine wave sweep and the graph? Can you measure phase as well?
AMR-iFi R&D
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Post by Owen Y on Sept 23, 2018 18:32:28 GMT 12
Hi colinf - actually, I took 'spot' freq readings at octave spacings midband & closer 1/3-oct readings at top & bott ends - so 'sweep' is a bit misleading sorry. Then, I put the data into Excel & line-graphed it. Still a bit laborious.
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Post by colinf on Sept 24, 2018 22:46:34 GMT 12
Still a nice looking graph :-)
AMR-iFi R&D
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