j-FET / Triode Phono front-end

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IslandPink
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#16

Post by IslandPink »

Paul Barker wrote:I would think the 417a's are doubleunobtainium by now.I never had any. Perhaps the 6c45pi is close enough.
417a is 5842
6C45pi is supposed to be equiv of 437a .
I'm not sure how much difference there is in those two, without checking .
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#17 Step-Ups

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Paul Barker wrote:Mark bought the last pair of S and B step ups AFAIK so he will be able to tell us his opinion in time.
I think bought the last pair sold at £180 Paul, wasn't it :wink:

After that they went up to £320 then became harder to get once they started selling them to Bent Audio .
In any case, I would say that Pieter's cobalt amorphous ones ( which I bought for E160 about 5 years ago ) are generally better, having more tone and clean detail in the mids and treble. I expect the S&B's might have it in the low bass , due to their extra size.
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#18

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IslandPink wrote:
izzy wizzy wrote:
Paul Barker wrote:The OZ-Loesch predated Thorsten's copy (no relation to Arthur).
And I suspect Allen Wright pre-dated that lot too.

Stephen
True, and Hewlett Packard and Tektronix pre-dated Allen !
Ah, but in our context we are only talking about the provenance of applying a well established technique to the Arthur Loesch phono stage. In that context my memory is of the OZ-Loesch the circuit for which was a very rough one back of a fag packet method, and it seems to have totally disappeared from the net.
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#19

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izzy wizzy wrote: I reckon this is a taste thing as the FET/valve cascode vs the stepup/valve route seems to polarise people. I just couldn't get on with the FET/valve cascode but then maybe I wasn't doing it right. It was a long time ago

Stephen
That's where I was 5 days ago . Tonight I discovered something, but it was with info gleaned from swapping valves at the weekend , plus reading of some of JC Morrison's articles on his microphone pre-amp, in the last two days .

The first point to make is, from my experience , however you set the thing up in detail , the bass tone and low bass / timing is better than any step-up I've used. Nick warned me about this :wink:
It's the tone/space 'naturalness' thing ( that is so important to me and I guess to most of you ) that is not initially right - and takes some work .
Oh, and the jFET settings shown in all the circuits above are not right , at least not right for my taste .

I should link to JC's site first to provide some extra reading :

http://www.labjc.com/

Thanks to James for telling me where he was hanging out .
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#20 Starting point

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Ok, yes as Paul says, maybe it was Mick Maloney who tried it first in a phono. He built a load of circuits about 15 years ago and wrote about all the results on the Joenet , I had the article once .

Ok, let me describe what I set up . I decided to use 2-off 2SK369 each side to get more current ( and hence Gm ), as the gain of the cascode is set roughly by the transconductance of the FETs multiplied by the anode resistor . I wanted to keep something around 12k there so I could at least drive a 100k RIAA network .
A 20k network , if the valve can drive it ( D3a-triode at 13ma does - zout 2.5k ) sounds better , but this is a cascode now, and the Zout is nearly the same as the anode resistor .

I kept in the D3a at first .
With 21R on the sources, I got 6ma in each FET , ie. 12mA through each D3a , and about 500 to 600X gain, as it was about the same as with my 1:10 step-ups into a D3a .

Now one other parameter available is the valve G1 voltage , set via a divider, which also roughly sets the D-S voltage across the FETs .
People have differed on this - Thorsten has said initally 3.5V , then later almost 0V !
Chris Brady says something like 3V . Nick said about 8V . I followed Nick, because his phono sounded great at Owston . So, I put a 220k + 6.8k divider from the (290v ) supply , with a 220u cap across the 6.8k . G1 connected to the middle .

OK, so this worked, and the bass was tremendous, great with rock, really really 'fast' , but a rather hard , aggressive sound, no charm . I got frustrated with it after a day or so .

Now the next thing was where it started to get interesting . I tried some valve swaps .....

Must take a break and eat some dinner .. back soon .
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#21

Post by Paul Barker »

If you can get the tone aswell then I would probably also build it.
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#22

Post by Laurence »

Also interested.
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#23

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OK, where was I ?
Well, I had D3a in there .
Now, I realised that E180F has the same pinout and similar filament current . I had Mullard E180F's and Russian E180F-equivalents in my stash . Now, Mick Maloney, these days , seems quite fond of 6688 ( ie. E180F ) above the FET :
http://supratekaudio.blogspot.com/searc ... results=50

So, I thought , I'll lob in some Russian E180F's, and see how it sounds . Oh, this is still in triode-mode, just to be clear .
Luckily , the current and gain are almost the same, as it's the FETs that are in charge here, as far as the voltages and current go .

Ok, the Russian E180F's sounded BETTER . Some more tone , less aggressive, more musical . Still not great , but better .

Ok, then I put in the Mullard E180F's . This was worse again, but not as hard and charmless as with D3a's .

So , in the last couple of days, I thought "what is the order of linearity in these three valves , in triode mode ? - surely it is : E180F Russian , E180F Mullard, D3a Siemens , going upwards" . This was in opposite order to the sound quality .
Also , Nick ( and Allen ) have used ECC88 , which is not known for pleasant distortion spectrum either - in the higher harmonics .

Now , OK, then I read a very useful snippet from JC's articles on the microphone pre-amp ( part 3 ) :

"the next change has to do with noise. in order to minimize distortion, i increased the voltage across the FETs by biasing the 12AY7s up, until i got the lowest 3rd harmonic. in this configuration, that turned out to be 18 volts. i would have guessed 7 or 9 would have done it. but the combination of these FETs with this tube worked out that way. this is NOT the lowest noise arrangement."

Now JC was working with a balanced input gain-stage here, so 3rd and 5th-harmonic would be very important to him .

Breather again ....
Last edited by IslandPink on Tue Jan 17, 2012 10:44 pm, edited 1 time in total.
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#24

Post by IslandPink »

izzy wizzy wrote:
I keep coming back to this and it makes me smile :D Brilliant!
cheers,

Stephen
Rest assured, Steve, I didn't make any effort to remove the circuitry from underneath, when I was using the router . I just took it out and put it on the Black & Decker workmate and got going . I did have some respect for the great Siemens GmbH , and removed the valves .
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#25

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Now then now then . From the above evidence, it would seems that the jFET has some distortion , probably including 3rd or higher-order distortion, even at these tiny signal levels - which needs correcting with a less-than-ideal valve on the top .

We are led to believe from the basic application of jFETS ( eg. http://www.tkhifi.com/div/Erno_Borbely_ ... icel_1.pdf ) that biasing the jFET above about 6V in this configuration should be plenty to ensure low distortion . He shows THD results as well, for various signal levels . However... there are often problems with using THD as the measure ; it is un-discriminating .

Today, I decided to add a preset pot to the voltage divider , adding extra R to the 6.8k in the voltage divider . I put in a 10k pot there, so I could try out voltages of 8V to 20V approx, across the FET .

My first effort this evening was with 14.5V across the FET .
Still with the Russian E180F's ....and ...

:love4:

Now we are cooking with Gaz . I have not felt the need to adjust anything all evening, and have been playing music .
I now have 90% of the bass snap, but now with tone, and soundstage depth , and emotion . I think I have exchanged most of the third-harmonic for a touch of 2nd-harmonic .

So, I think in this usage, the minimum for 2nd-harmonic or THD does not occur at the same D-S voltage as the 3rd-harmonic minimum .

I will now try out such things as using the Mullard E180F's and trying different D-S voltages with that one .

( This system will now play 'Diamond Dogs' by David Bowie, one of the sternest tests of any music system :!: )
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#26

Post by IslandPink »

Oh Paul, drums are just Fab Fab Fab . Got 'Time Further Out' on there now, cosmic .

By the way, this must be the all-time cheapo champion phono front-end : the Russian E180F's can be had for about £1 each .
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#27

Post by Paul Barker »

Great breakthrough by the sound of it. What about also trying the much loved 12AY7, what works for a mic. should work for a cart.
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#28

Post by JamesD »

Yippee!!!! :lol: :lol: :lol:

Fantastic stuff Mark - just fantastic!

and hats off to jc for sharing via his blog!
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#29

Post by Max N »

Seconded :!:
Congratulations Mark, and thanks for sharing.
I shall have a read of JCs blog
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#30

Post by izzy wizzy »

It's that whole charmless thing you mention that put me off when I tried it and although I tried a bunch of things, I didn't go as far as you Mark. Well done!

It is tempting to do this input stage for a whole bunch of reasons.

Although a cascode is similar to a pentode, can it be loaded down like a pentode? Fer instance, is there any chance of it driving a low Z EQ network of say about 20k? Maybe I ought to get LTSPICE out to have a fiddle.

JC's blog is ace!

cheers,

Stephen
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