Author Topic: Tube phono preamps  (Read 8293 times)

Offline handsome

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Re: Tube phono preamps
« Reply #30 on: December 04, 2015, 02:53:13 PM »
riaa eq should be at least accurate to 0.2dB or so. I have an analogue scope at best it is probably only 5% accurate plus you have to infer the value off the screen - that is work out were the green line is sitting not exactly accurate by any means. a digital scope could be better but if you are measuring directly you still have to cope witha 40dB dynamic range and a digital scope will only be 8 bits (unless you are lucky enough to have one of them real funky machines) that is dynamic range of only 56dB..... with an inverse eq all you are doing is measuring (ostensibly) the same value: set your function generator's level to give you an output from the preamp of 1V then measure at different spot frequencies - you are just measuring the deviation from 1V. Makes things far easier allows for any wide bandwidth meter (most DVMs dont have the bandwidth) to be accurate on the measuring device side. the other problem with measuring riaa accuracy is because of the complexity of the filter, your response variations can and will wiggle all over the show - so you need to measure most of those frequencies and not just (say) 100Hz, 1kHz and 10kHz. with amplifiers on the other hand you pretty much can be sure the response is flat and you will be just looking for where the response starts to drop at the low and high end.

Offline handsome

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Re: Tube phono preamps
« Reply #31 on: December 04, 2015, 02:58:23 PM »
More importantly though: how does your d3a pre sound? and what front end are you using with it? i bought myself a packetful of d3as a while back meaning to build a similar pre but as usual have yet to get that project a-going yet... :whistler:

Offline tangmonster

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Re: Tube phono preamps
« Reply #32 on: December 04, 2015, 03:16:31 PM »
More importantly though: how does your d3a pre sound? and what front end are you using with it? i bought myself a packetful of d3as a while back meaning to build a similar pre but as usual have yet to get that project a-going yet... :whistler:

Blatant thread hogging from us now.

The 50ohm in parralel with 270uf cathode bias in that design I referenced is a problem. It had a definite effect even at lower frequencies. Making the design as it is a bit bass shy.
It was out by around 3dbv at 20hz with my measurements. 20khz was basically spot on though.

I have since changed to IR led and lowered the Cathode resistors to get around 11mA. Giving much increased bass.

Looking at a phono preamps like this:

http://www.diyaudio.com/forums/diyaudio-com-articles/163570-his-masters-noise-thoroughly-modern-tube-phono-preamp.html

Makes me want to experiment now with current source  on anode for d3a :)

Here is my setup (in the making) ( the hardboard pieces are there to hide my vinyl from small fingers and pets. not very good looking but it works):


Offline handsome

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Re: Tube phono preamps
« Reply #33 on: December 04, 2015, 03:33:39 PM »
Nice looking rig. Is that a lenco? nothing like an idler!! what cartridge and arm are you using? I did read sy's article it does look like a cool pre (in fact i have no doubt it is) and i usefully have a pair of those sowter 8055 xformers.  I recently found myself 20 PC88s which are basically ECC88s but with twice the gain, a smidgen more gm and only a single triode per envelope so was thinking of doing the next phonostage with them as they were pretty cheap - i might just keep my d3as to use on ebay - the combination of our rand and their scarcity could conspire to turn them into a decent investment hehehe. since i have more than one turntable i have been looking to design a stage that has low input capacitance so i can mix n match MM MC and their associated transformers without delirious effects. The transistor/valve cascode seems to be the best answer bar the PSRR issue or a common cathode type input if one can excuse the noise penalty....always a compromise somewhere  :-X

Offline tangmonster

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Re: Tube phono preamps
« Reply #34 on: December 04, 2015, 03:44:45 PM »
Turntable is a stock standard Dual 1218 with m91 cartridge. Incidentally is also a idler.

Bought it about 7 years ago over gomboom , frozen up and struggling to turn. Since then I have only lubricated in certain spots according to manual.

Replaced the stylus with eliptical n91.

And buy the odd vinyl every other month:

My humble collection:

http://www.discogs.com/user/tangmonster/collection

Offline Ampdog

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Re: Tube phono preamps
« Reply #35 on: December 06, 2015, 12:16:45 AM »
Gentlemans ....

Duly respecting members better endowed than this humble soul
        but what are we designing here?
                 an instrumentation amplifier??

I find a specification of within 0,2dB rather thick for something audio. It seems to be accepted that 1 dB is already pushing it as far as audible differences go. And then in direct comparison; not a signal today and the same 1dB better tomorrow. Then we come to valve production spreads: +/- 10% is - er - acceptable; see posts elsewhere of what the real picture out there is. Rough approximation: This will account for a spread in rp and thus gain of some 1 dB in this case.

Phono cartridge response vs. frequency? I can assure you that within +/- 1dB is pretty darn good; thats another possible 2dB of variation.

I found the circuit of post #18 giving the RIAA graph to within about 1dB with a bogey valve. Again, as always, you spend your money as you please. For myself I cannot see myself paying boutique money for 1% capacitors and such.

Handsome,
Input capacitance worries you? Use it as part of the RIAA h.f. cut. In fact, I have seen RIAA circuits for MM cartridges using an input capacitance only as the h.f. compensation, reportedly also giving superior h.f. performance of the cartridge. (Though I will place that 2nd best; one then forfeits the advantage of also attenuating fist stage hiss in the process.) 
 
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline handsome

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Re: Tube phono preamps
« Reply #36 on: December 06, 2015, 07:39:26 AM »
'tis the challenge sir, that be the joy! Actually it is far less impractical than you think. 1% capacitors are both cheap and commonplace nowadays - in both polystyrene and polypropylene. 0.1% resistors are also easily available and, whilst not so cheap (RS sells them in packets of 5)  not a huge bank breaker considering you only need 6 of the little buggers (i use 8 - see if you can identify that particular can of worms?) . And lets not forget that parting with upwards over R2000 for a speaker/interconnect/ 1x digital cable (with invariably RCA connectors that inherently enforce an impedance mismatch) cable is not considered a sign of early dementia, low blood sugar or a keeping-up-with-the-Joneses type variety of OCD these days. On a more realistic note though, almost every phono stage that i have ever seen tested in a magazine these past ten years seems to comfortable stay well within the 0.2dB limits. And you are correct it is doubtful anyone could hear such perfection but it is a engineering challenge and one that i for one do relish - even if it often just limited to a back-of-the-napkin exercise.

Obviously we cannot account for how cartridges are manufactured - but we have total control over our designs.....

Input capacitance as i said has to be considered in the network calculations. The problem is in the input valve - it could be ideal as the HF pole with certain cartridges but surely a decent phono stage should not restrict the user's choice of cartridge? However as you mention, would compromise the phono stages inherent noise performance. A 12AX7s input capacitance plus the probably 100pF of cable capacitance can easily exceed the correct amount of capacitance a given cartridge would require, hence my talk of cascodes an hybrids...

Valve variance and valve aging should also be considered in a design thus requiring a cunning choice of values within one's eq network to minimise the inherent frippery on the part of this Jurassic technology....

Offline Ampdog

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Re: Tube phono preamps
« Reply #37 on: December 06, 2015, 01:44:38 PM »
Enlightened then, good Sir!

I was not aware of the present state-of-market re precision components economically speaking; reveals me antiquity  :(

But fine, thanks for realising my point-of-departure.  (Not defending anything but I am surprised that tested phono stages achieve that degree of accuracy what with other variables than R-Cs - valve designs, of course! With op-amps the scene is rather different - but that you know.)

As said earlier, some deviation from Karel's original intentions. Thanks for tolerating us 'dwellers' in seasonal spirit! Now where were we .....

(Perhaps to repeat my stance expressed earlier: I still prefer an RIAA feedback system with the added ability to rapid cut-off below desired frequency. Also a challenge! ... but then perhaps even more demanding of component precision!  ;D )

Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline handsome

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Re: Tube phono preamps
« Reply #38 on: December 06, 2015, 02:43:34 PM »
Thank you sir. Come to think of it though, you are quite right to raise an eyebrow at the brilliant test results that i have consistently seen especially on the tube variety.....but perhaps those are feedback designs....... Feedback is but one way and the additional stability cannot be ignored - but at the end of the day you needs gobs of gain - not always so easy with our glass bottles. The fact that the amount of feedback will vary across the spectrum due to the RIAA characteristic and especially with our valves' vastly lower open loop gain, bothers me slightly. Thus feedback is notionally ideal for the solid state crowd......but as ever its that engineering challenge...and individual choice.....

Offline Ampdog

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Re: Tube phono preamps
« Reply #39 on: December 06, 2015, 05:26:05 PM »
As you say

The various ways of using feedback is an interesting field - lots of experiments by me, even before one had Spice available.

But unsure whether we are not a little off the OP's intentions.  Perhaps Karel would like contributions to get closer to his original intended circuit. As said I found a Spice analysis of that spot-on - with a bogey valve!  I have not gone over Tangmonster's analysis - the present temperature-of-day is not encouraging!

Perhaps a worst-case analysis involving valve parameter extremities later - which may not have other implications than that one has to live with that; one cannot expect to have tube measurements involved as well in practice.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline fredeb

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Re: Tube phono preamps
« Reply #40 on: December 06, 2015, 08:25:20 PM »
Fredep,

Very useful list that Millet one, BUT:

A cascade topology is mentioned - that is not the same as cascode. In cascode two valves are used in series (one on top of the other as it were).  The anode voltage on the bottom one (going to the cathode of the top valve) is somewhere in the 60 - 70s, maintained by setting the top triode G1 at some 60 - 70Vdc. The top valve anode operates at some 150 - 200V, in series with the usual load resistor to B+.

Cascode triode connections exhibit pentode gains and characteristics. The figures given in that list rather suggest one valve with a CCS as anode load. The last entry for a 6SN7 with gain of some 100 would indeed suggest a cascode. (Eg. ECC88 triodes in cascode give a gain of some 150+ if memory serves.)

Aaah ... thanks for the wisdom , I do need all the help I can get .


My humble collection:

http://www.discogs.com/user/tangmonster/collection

Nice collection !
ďAnd those who were seen dancing were thought to be insane by those who could not hear the music.Ē
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Offline peterc

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Re: Tube phono preamps
« Reply #41 on: December 31, 2015, 10:46:11 AM »
Any thoughts on this very simple design?

http://phonoclone.com/diy-pho3.html

Thx
Peter

Offline vinyljan

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Re: Tube phono preamps
« Reply #42 on: January 01, 2016, 07:30:32 AM »
Phonoclone is the Rjm Karel refers to in #18. Nice sound and I've got mine fairly quiet, but gain at 38 dB is a bit low. Works very well for me with MC and SUT.
Turneraudio.com had a design that reminds of Rjm, but with 12AX7's and additional buffer/cathode follower, gain is said to be 52 dB, but can be lowered if 12AT7's are used in the second gain stage.

In a maximum bank turn in a normal category airplane, like a typical Cessna, everything is fine until you turn across your own wake

Offline ghostinthemachine

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Re: Tube phono preamps
« Reply #43 on: January 01, 2016, 07:32:49 AM »
Any thoughts on this very simple design?

http://phonoclone.com/diy-pho3.html

Thx
Peter

That looks very simple and elegant enough to make a great build and to enjoy the fruit of your diy labour.
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Offline vinyljan

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Re: Tube phono preamps
« Reply #44 on: January 01, 2016, 07:36:10 AM »
"The Turner schematic canít be navigated to via the home page for some reason. Iíve kept it bookmarked: http://www.turneraudio.com.au/miscellaneous-schematics1.htm"
Thanks Matthew ..
« Last Edit: January 01, 2016, 07:46:27 AM by vinyljan »
In a maximum bank turn in a normal category airplane, like a typical Cessna, everything is fine until you turn across your own wake