phono stage

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shaun
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#1 phono stage

Post by shaun »

hi all

hopefully this is the right place for the less confident people like me :D

i'm thinking of trying a phono stage and would appreciate some input from you all. i find myself quite interested in the one shot eq idea even though it has it's own problems i fell quite keen to give it a go. i'm just at the thinking stage but the current idea revolve around using a 6c45p (donated by dave dove) as the input /gain valve into a directly coupled RIAA stage(in a Brian Epstien stylie) driving an aikido. my (nicks)current phono is quite a distance from the rest of my set up so the aikido might be nice. so the next thing is to do some maths and see where that goes :roll:

the big question is ''is it a bad idea?'' has anyone here tried this?. i'm still a bit new to all of this. i guess the best way to find out is to build and i'm happy to take my chances on how it will sound :twisted: but dont want to waste time on something doomed from the start.

anyone had any experience with such a thing?

from what i've read the 6c45p has a tendancy to oscillate but the MU of 50 and high GM look very interesting and well suited to the perpose.

any how i'll have a think and come up with something a bit more solid in terms of a design but if it's a crapp idea fell free to say so.

take care

shaun
shaun
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#2

Post by shaun »

ok to answer my own question

it's a crapp idea

i just did a block diagram and i can see why it wont work. deerrr

so i'm going to need to cap couple to the aikido. :evil: and it sounded like such a good idea. oh well i would still like some input on that if that's ok :?:
Andrew
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#3

Post by Andrew »

Hi Shaun

I haven't tried a 6C45pi as a phono front end, but I do have a design I did with James D for a 6C45 cascode, this has bags of drive so can deal with awful loads like say the 600R LCR phono, it be would instead of the E810F in something liks Nick's. I am now of the opinion that a pentode is a very good input valve for a phono and that cascode is perhaps even better, albeit at the cost of twice as many triodes.

So where does that leave you? I'd be tempted to try the 6C45 into a low-ish Z RIAA, as possible, obviously not as low as Nick's LCR.

Perhaps Simon might be kind enough to post his C3g -> Aikido phono to give you some ideas? You could, as a start, cap couple that to the front end of the 6C45 design, the choice of cap will effect bass response so might need to sized appropriately, 100nF might be a good starter.

cheers,

-- Andrew
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#4

Post by Andrew »

OK doing some quickie sums...

The Rp of the 6C45 is ~1100R, with a bypassed cathode, or when using battery bias. This will give a Zout of Rp || Rload or 1100 || 10000, about 990R so it you want you could use the 6C45 loaded with a 10K 300v B+ and Nicad battery, as per Jerermy's circuit. Now chop the C3g front end off the Aikido phono Simon and I did, putting the 6C45 in place of the C3g, now place 4k7 resistor before the first RIAA cap from the anode of the 6C45. The RIAA in Simon's amp has nice standard cap values and is quite low Z. You might need to tweak the 4k7 up or down by ear, it might be a tad too much bass, a lot depends upon the indidual spec of the 6C45; according to the data sheet Rp can vary betweeb 700R and 2k, worst cases. That's one problem with triodes Rp varies batch by batch and will also do so with age.

Anyway, if I have that right, that should give a matched Z between the 6C45 and the RIAA in Simon's phono.

cheers,

-- Andrew
simon
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#5

Post by simon »

My FTP program won't talk to the ISP for some reason so I can't upload the updated pdf tonight. An older one is here: http://www.atrj13.dsl.pipex.com/images/ ... _rev_B.pdf.

I've played around with the C3g cathode resistor to increase gain and went as low as 68R but getting the right sort of voltages on the anode and G2 wasn't possible, so I've got 82R there at the moment. Andrew has a cunning plan to try leds there to increase gain. The resistor from G2 to B+ is somewhere in the area of 25k if I remember correctly. I haven't had any time to play with the phono for a few weeks now so it's still a work in progress.
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#6

Post by Andrew »

Simon and I have mostly been mucky around with the C3g on his phono to get more gain. The rest of Simon's RIAA + Aikido will be a good enough starter for you if you want to go down this road. Simon, thanks for posting the link.

One thing Shaun, most folks seem to think that the ratio between Zout of the driver and the RIAA for a triode input phono, such as this one, should be a bit higher than my proposal, which is about 1:5 in this case. I only say this so you know what your getting into, there are both advantages and disadvantages to having a lower ratio.

If you want to have a go at this it will be good to hear what you find, but its up to you, sticking a cap after Jeremy's RIAA and adding the Aikido should also work.

cheers,

-- Andrew
shaun
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#7

Post by shaun »

thanks Andrew and Simon

i knew someone would rescue me :roll:

as you've guessed i know sweet fa about designing a one shot phono so it's nice to know help is on hand :) however i have designed a power amp so my first post made me cringe big time when i thought about it after. i always seem to forget the basics :evil:

i've just built an aikido 6np1-6np6 line stage with a series reg psu which is sounding pretty good so it made me think of doing a one shot into aikido to see what it sounds like. :?:

i'm gathering the parts for another reg psu for this project so it's going to take a while to get that sorted (a week or so). also a regulated supply for each fillament which is almost there (and hot).

simons phono looks interesting :idea:

i have some C3g with bases so i might try that one also if its ok with you guys.

thats pretty generous of you andrew i'm going to give those sugestions some thought.

thanks and take care

i'll keep you posted

shaun
Andrew
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#8

Post by Andrew »

We've been tweaking Simon's C3g input stage, but the RIAA seems pretty sorted. Simon would like more gain for his set up, so I have suggested using an LED for the C3g bias, thus eliminating the local feedback of the resistive bias and increasing gain accordingly.

Its up to you what you prefer, knowing Simon, I'm sure he'll furnish an updated schematic that has his build learning/updates in it and some notes on what he did and why/problems etc. Otherwise I have the original paper-only design, if you want it.

I also have some generic nowt special 470nF poly caps I could have a go at matching a pair if you like? No charge just cover the postage, whatever.

cheers,

-- Andrew
simon
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#9

Post by simon »

Here's the "up to date" circuit:

http://www.atrj13.dsl.pipex.com/images/ ... _rev_C.pdf

It's not a great deal different from the previous one. I've a feeling the 16.5k resistor isn't correct now - I've been playing with the loading on the anode and g2 - and I think it's a little higher so there's more voltage on the anode.

Hope to have some time to pick it up again soon.
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#10

Post by simon »

shaun wrote:i've just built an aikido 6np1-6np6 line stage with a series reg psu which is sounding pretty good
Glad you like it Shaun. Is it the version on John Broskie's website or have you amended it? I could do with a bit of drive in front of my WE91s, but I don't need much gain at all. Ideally I'd bodge my Aikido headphone amp to do both jobs but initial experiments weren't encouraging so I'm having another think.
shaun wrote:i have some C3g with bases so i might try that one also if its ok with you guys.
It's Andrew's phono really - I've just played at building it. We'd both be very interested to hear what you think of it if you build it for sure.
shaun
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#11

Post by shaun »

thanks simon

yes the aikido is pretty much JB's but with the 6n6p as the drive.

sounding better by the day. i'm pretty impressed.

andrew i'd still love to try the 6c45 as the first valve as an interesting experiment and will as you say use battery bias (i use that on my (nicks) WAD phono). b+ 300v into your suggested one shot (simons) riaa and then into aikido. it seems to me that the increased b+ leaves a bit more room for thought on the Aikido input valve. so i'm going to think on that one. i have some 6n3p which may be interesting to try out there but to be honest i've not got round to checking the data sheet on that yet so it could be a non starter.

andrew just to be clear about the placement of the 4k7 resistor. an i right in thinking that it replaces the 680r resistor on the first leg of the riaa?
sorry i'm still abit new to all this phono stuff. thanks for the offer of the caps but i have a matched pair stripped from and old brocken phono 480 and 481nf. the diode bias is an interesting idea :idea:

by the way as its an experiment i'm going to use trimmers for the resistors (ducks for cover). not the cheap and nasty jobs but nice vishay ones so i can change things without to much stress. when i have it right i'll put something nice in :P

thanks guys for the input i

t's nice

take care

shaun
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#12

Post by Andrew »

Hi Shaun,

Leave the RIAA as is, the 4k7 needs to be in series with the signal so take the signal off the 6C45 into the 4k7 the other end which goes to the start of the RIAA (see attached)

You might find 4k7 is a tad high, 4k6 might be better, (edit more cross checking start a 4k1 and work your way upwards) it really depends upon the 6C45s, how well matched and how close to spec. You're going to have to suck that and see. An inverse RIAA would be a help debug it, but basically, if the series R is too high you will get a tad too much bass.

It will be intersting to see if this works, if not I can do you a higher Z RIAA (based arounnd 220nF caps) which will mean more buffer resistance will be required to match the 6C45. The upside of this is it will help swamp out the changes in the 6C45 and any mismatches, and further changes as the valves age and the gm changes. The downside of a higher Z network is the whole signal path will be more than double the impedance.

If it were me I would build the same RIAA as Simon's and try both valves. If the 6C45 is a clear winner then you could try it in a cascode or with a higher Z RIAA, which may or may not be to you liking.

I'll look at the RIAA and see what 480nF will do, you might need to adjust some of the R's to compensate for more capacitance.

hope this helps,

-- Andrew
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shaun
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#13

Post by shaun »

thanks for that Andrew thats a big help. i think the idea of keeping the eq the same and swopping valves is a go one. thanks for the support.

i.m going to start getting the TX and a few bits ordered so i'll keep you posted on the progress :D .

take care

shaun
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#14

Post by Andrew »

Hi Shaun,

The 6C45 should give you about 12-13db after RIAA eq, so you need a second stage that gives about 27db for the magic 40db.

The C3g should give a gain of 15-16db, after RIAA eq, so you'd need about 24db; there really isn't much between the two gain-wise, about 3db.

Simon might want to chip in here given his experience with gain but I would I suggest looking at valves with a mu of over 40 for the Aikido first stage. 61NP is close for the the C3g, but might come up a tad short for 6C45, depends upon the sensitivity of your pre power combo. As you pointed out Aikido gain is half the mu so limits options for the choice of the first valve in a phono.

I must get some 6N1P to try for myself Simon and Dave Dove both really like them.

cheers,

-- Andrew
shaun
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#15

Post by shaun »

hi andrew

yes it does seem that getting enough gain from the aikido is going to be the tough thing. i think that finding a good sounding high mu valve is going to be fun :evil: sort of. but going for gain with this topoligy seems to cut down the number of suitable candidates so what :cry:

i'm not sure i can comment on the sound of the 6n1p as i've not had the chance to compare it with other valves in that circuit position. maybe when i start to swop things around it may be easier to comment.

what i can say is that the 6n1p and 6n6p in my line stage in my set up is very good indeed. the best i.ve tried by quite some margin. :lol:

so it's data sheet and load line time but of course any input on a suitable candidate for the aikido input would be most welcome..

again thanks for the input

take care

shaun

shaun
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