j-FET / Triode Phono front-end

[Paul Barker]

Thank you, that should be enough to excercise Mark's synapses.

In England the Wright and Weaire Ltd transformers fitted to their sister company Ferrograph's equipment and bundled with Reslo mic's make great sounding MC step ups. A hidden gem. But they are now achieving good prices beacause of the increased demand for Reslo mic's.

Reading what you say, which is in a field I am so far from in knowledge and experience as to make it too late to catch up. I am left at the mercy of what Mark Nick and others might bring to a meeting resulting from it.

I shall have to stick with my Wearite's and focus my attention on the better sounding valves, which is all I know about.

[Paul Barker]

Most LCR phonos use high gm valves for a low Z circuit. Thomas Mayer reckons it's the low Z nature of the thing that is the key more than the LCR bit. Also it might have something to do with how sensitive you are to some component type colourations.

It is the low Z which to me is the problem because from my small world of only understanding a little about valves a higher Z allows me to use valves and topologies which in pure valve terms I prefer. Take for example my Sowter TVC's. YUK! I hate them. But then if I weren't so stubborn about my nice valves and topologies, and fed them from a CF they may sound good. I would prefer people to make their stuff so that it matches my world please?

OTOH, maybe it's the sheer investment in it that means it is a final destination for many

I believe so. A very valid direction to take is the Jfet one on these grounds alone.

It seems a very odd direction for someone with arguably the best MC stepups (if Thorsten is to be beilieved, which in my small world has never been a given) to take. Except to validate his stepups by negative means.

I remember when studying the Marketing element of my Economics degree the stupidity of the human. They don't necessarily do a lot of research before buying but then there is a huge amount of attention paid to the whole area and all alternatives so they can validate what they have bought.

Not saying this is Mark's situation, because I know that Mark has a very technical job and approaches all these things with that kind of mind. I can only understand my kind of mind, which is much more prepared to form faith and belief in stuff as a direct response to listening discoveries and doesn't have the capability or need to explain it. suffice it to say "such and such sounds better than anything I did or heard before" is all I want from the situation.

I have noticed that some of the people who once ran well have gone off onto a tangent and don't know it following their techincal minds.

[Nick]

LCR still does something that RC doesn't, but the one I have is behind the curve now, one day I will have time to bring it up to date.

I knew I was going to be in trouble when I posted here about LCR being the end Don't know the topology. Most LCR phonos use high gm valves for a low Z circuit. Thomas Mayer reckons it's the low Z nature of the thing that is the key more than the LCR bit. Also it might have something to do with how sensitive you are to some component type colourations.

OTOH, maybe it's the sheer investment in it that means it is a final destination for many

cheers,

Stephen

Yes, I agree the low impedance RC networks seem to sound better than the high impedance ones, though as yet I am unsure if that is due to the network or the choice of valves used to drive them. However I (and others) think the important point about the LCR topology is that the network is a constant impedance type, so unlike the typical passive RC one, the driving valve sees a constant impedance with frequency.

[izzy wizzy]

Thanks for the great post jc. In going back to your blog, I noticed you don't have a blog archive mast thing so it makes it very hard to find stuff without wading through the entire thing which is getting pretty big now. Any chance of a blog archive can be retrofitted easily like those hifi heroin and vinylsavor guys have?

cheers,

Stephen

[Andrew]

LCR still does something that RC doesn't, but the one I have is behind the curve now, one day I will have time to bring it up to date.

I knew I was going to be in trouble when I posted here about LCR being the end Don't know the topology. Most LCR phonos use high gm valves for a low Z circuit. Thomas Mayer reckons it's the low Z nature of the thing that is the key more than the LCR bit. Also it might have something to do with how sensitive you are to some component type colourations.

cheers,

Stephen

Well it might be part of the issue, low Z, and certainly a 5k6 RC phono I built is very good but its not an LCR.

As I understand it, the real difference lies in the physics of the phase change (group delay?) the filter imposes upon the signal. An RC filter changes the phase relationship between the voltage and current since a cap in the circuit causes current to lead voltage (I think that's right way around). Using an L and a C the two effects, leading and lagging, are balanced so neither current nor voltage leads or lags, the upshot is a constant impedance for the driver, very helpful, and no change in the phasing versus frequency response and, if I have this correct, better power transfer - I guess it looks more like a transmission line.

In audio terms, it means the bass notes seem much more connected with the rest of the music.

That's my understanding to the best of my knowledge, a radio ham might come up with a better explanation, this is all bread and butter for them as at radio freq its all about power transfer since the current and voltage are rarely in phase hence their use of power meters rather than volt meters.

Andrew

[Paul Barker]

Yes, I agree the low impedance RC networks seem to sound better than the high impedance ones, though as yet I am unsure if that is due to the network or the choice of valves used to drive them. However I (and others) think the important point about the LCR topology is that the network is a constant impedance type, so unlike the typical passive RC one, the driving valve sees a constant impedance with frequency.

If that is so I will eventually have to relinquish one of my sacred cows, but I am not going down in the first venture from the trenches.

[izzy wizzy]

Yes, I agree the low impedance RC networks seem to sound better than the high impedance ones, though as yet I am unsure if that is due to the network or the choice of valves used to drive them. However I (and others) think the important point about the LCR topology is that the network is a constant impedance type, so unlike the typical passive RC one, the driving valve sees a constant impedance with frequency.

One of the reasons I have the circuit I do is so it could drive virtually any EQ network without alteration. Only the overall gain would change as the load on the driver dictates the gain. It is a pentode. So there was no change from a 100k network to a 20k network other than the network. I was sceptical to begin with. The lower Z one was far superior. Many would agree that the constant Z of the LCR is part of it's benefit. Most report a 'wholeness' from top to bottom.

cheers,

Stephen

[JamesD]

Wow! Many, many thanks, jc!!!

Brilliant post with so much info in it - it will take the weekend and more to process. I've been thinking about your Gm amp and how to use it best for a phono - must try it now you confirm again how nice it can sound...

James

[Paul Barker]

As I understand it, the real difference lies in the physics of the phase change (group delay?) the filter imposes upon the signal..... Using an L and a C the two effects, leading and lagging, are balanced so neither current nor voltage leads or lags, the upshot is a constant impedance for the driver, very helpful, and no change in the phasing versus frequency response................

In audio terms, it means the bass notes seem much more connected with the rest of the music.

Seems right explanation to me.

[Max N]

Thanks for the input JC, much appreciated. That was a lot of knowledge and experience crammed into one post!
Not wishing to put words into your mouth, just checking that I have understood correctly - are you saying your preferred phono front end is a SUT into a Gm amp?
I'm considering various options for phono stages, and any experience/opinions is very helpful.

[jc morrison]

well max, at the moment, yes. of course the onus falls on the transformer, and that isn't a trivial thing. that can be solved with money. but not everyone has that kind of money to part with. i haven't heard many "cheap" trannies that are good enough. there are some vintage things but the new stuff is better. permalloy is amazing. i would say the hashimoto step up, is entry level, these days, and it isn't at all cheap. even harder to argue with cobalt or nickel amorphous. silver litz is also a plus... but, i also believe that using a transformer, any transformer, means some sacrifice in the bass (and treble, that i can't hear anymore) and even very small details. trannies tend to slew things together a bit. less so than caps at small signal levels, absolutely. but, at least in my case, i don't mind giving that up. but i do believe you can achieve the same level of performance going transformerless. it is much more work, though.

that is why the fet/tube cascode is interesting. it is possible to use a pentode, such as a D3a or a 7788 without a transformer. in a Gm amp or with frank blöhbaums "super pentode" arrangement, it is possible to get to sub 100 ohm eq. noise resistance. noisier than a fet but maybe ok.

i think that noise masks problems with tube stuff and it is very interesting to see how far one can take the tube thing down into the muck. most of that work was done for radio and radar. 50 ohm and 75 ohm systems. very applicable to phono. there is a lot of really good work done on the subject... philips did a great deal of that work in the 60's. the last run of VHF tubes are the best of them all. low microphonics. high Gm. you need high slope tubes to get low noise. that means frame grids... that means getting the chemistry right.

anyway, when you really clean things up, it's a different thing. it isn't flashy, but it isn't dull. it took a while for me to really get an emotional handle on the subtleties. but now i think i get it... at least i think i do? dense music really shows it off. but it is still lovely for vocals. that is where i am at the moment. the phono/line pre i did for silbatone is the best attempt so far.

some of the russian VHF pentodes are amazing. and i agree that their E180F/6688 is lovely. great pentode, that. i believe the quietest tube blöhbaum has measured was a russian pentode: their version of the E810F. that does not make it the "best" sounding... sometimes one has to tackle one problem at a time. but, it could be all anyone wants or needs?

jc

[IslandPink]

Excellent, thanks for dropping in, JC !

Plenty to think about there for sure. I've printed off a lot of stuff from your blog and will be browsing through it over the weekend while I'm visiting my mother . Your experience & comments re the jFET characteristics at different set-up values is very helpful, as it seems I was coming to some of the same conclusions.

I think I will try to order a few of those P-Channel FETS this afternoon, as it is looking inevitable I will try this circuit at some stage . I can probably build it off-line on some veroboard and use a battery for the -12V , then wire it into the rest of the phono , I suppose .

I could just quickly add a couple of comments to some things Paul and Stephen have said :

Paul : the interest in trying this as opposed to the step-ups came from the great performance of Nick's phono in the Owston 'legendary Sunday' system . When I first put it together a couple of weeks ago, the obvious thing that struck me was how good the low bass and timing were . Clearly no contest against the very good step-ups I have . So the effort seems justified to try and get the rest 'right' . That may take a bit more effort than I anticipated ! Also, I have a suspicion the plain 103 might work better into the FET than it does into SUTs .

Stephen : The LCR thing , I'm certain of this, is that the bass and midrange are better integrated , tonally (harmonically) and timing-wise. There is a greater 'musicality' . It must phase and/or loading issues in the RIAA section . At ETF2005, I heard over 20 phono amps compared ; all of the 5 or 6 LCR or LR phonos had this special sound in the mids and upper bass .
Now I think the split (RC) RIAA goes some way towards getting this sound .

Attached a better version of my current circuit, without the entertaining errors . Please don't find any errors .

[IslandPink]

Yes, I agree the low impedance RC networks seem to sound better than the high impedance ones, though as yet I am unsure if that is due to the network or the choice of valves used to drive them.

I had 100k network and then 20k network in the D3a > D3a phono , mostly as shown above , and it was clear the 20k network was usefully more transparent, more dynamic and had a more tonal colour .
I think it's the lower series Z driving the coupling cap , or the caps to ground ; John Curl reckons it's the caps to ground . Either way, I think small signal circuits like this have problems getting the small stuff through the caps , and a big series resistance just makes it worse .

[Joe Roberts]

Most LCR phonos use high gm valves for a low Z circuit.

Yeah, why?

Obvious reason is that the circuit is a well-known entity, ripped from Pultec and propagated by Tango in their LCR module. I think it was published in Audio Cyclopedia.

The 600 ohm design is such in order to bring it in line with standard old school line impedance for pro gear in the US.

Most pro gear had 600 ohm transformers on the output. Amp--> 600ohm T network to stabilize impedance--> LCR filters. systems had excess gain so everything could be terminated and impedance pinned down with pads.

This is cool but hi-fi systems aren't envisioned and constructed in that way. We have to go out of our way to accommodate hard 600 ohm terminations.

jc's Silbatone design uses a higher Z network but drives it with a stage that could be jiggered to drive the 600 ohm job. One very valid practical reason for a different Z is that the design impedance can be selected to employ standard cap values. THAT is a plus.

Another is that the range of possibilities is way expanded if you can go up to a few kohms at least. What can drive 600 ohms? A 6080? White follower? Most sensible thing is to use a transformer, really.

The other question that needs to be addressed is the contributions to non idea behavior of real-world parts. Adherents of the 600 ohm approach say smaller inductors have less spurious C to cause unwanted resonances, lower DCR/higher Q, etc. H'mm...I'm not feeling it.

Personally I think the problem of driving 600 ohm load is a major problem, best avoided. You can drive the benefits of low impedance drive with higher Z networks that don't force you into that 600 ohm corner.

Moderate Z LCR network is a much more comfortable place to be.

That being said, hopefully I'll be listening to jc's new silbatone FET/tube phono stage in a few weeks.

If it beats his previous SUT/D3A gm amp design, I'll eat my hat.

morrison knows that I am a SUT guy but I'll hear him out on this experiment. The noise specs are totally nuts. Maybe a trans and a FET would be the ticket?

[Andrew]

Yes, good point 600R is a difficult load, I think in Mark's case he's hoping to drive a 1k5 load based upon the chokes Dave Slagle wound for me a couple of summers ago. I'd like to say I had true design input but, in truth, its just scaled up the Tango network played with in Spice until I was happy.

I did expend a good deal of effort measuring the resultant RIAA built from Dave's chokes at varying amplitudes and frequencies and they measured very well indeed.

Dave has got as far as 7k I believe.

Andrew