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  #1  
Old 2nd August 2019, 05:26 PM
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Phil Y Phil Y is offline
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Default Phono 2 CF

Hello all, a question for the technically knowledgeable.

Is there any reason that the cathode follower valve in the Phono could not be changed for a ECC82 to lower its output impedance?
Obviously the resistors in the cathode circuit would need changing to bias the different valve correctly, but beyond that is there any problem?

My reason for asking is that these days, very few things adhere to (the once standard?) 47K input impedance, and the Phono 2 needs at least 40K to be correctly loaded.

What started me on this was looking at headphone amps that mostly seem to be 27K input but some even lower. I also know of one (commercial) passive pre that has 10K input impedance!

I know I could build an active Pre but my CD and TV go through a modern DAC that will drive anything and my tuner also has transistor o/p, so going to the cost and time of building an active Pre for the benefit of 1 of my 4 sources would seem silly and wasteful.

Phil.
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Old 4th August 2019, 04:30 PM
bikerhifinut bikerhifinut is offline
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Default Re: Phono 2 CF

Where did you get the information that the output from an ECC83 CF needed a 40k load?
The ECC83 CF usually has an output Z of 600 ohms, give or take and with the cathode stopper resistor added in of usually 1k that's less than 2 k output Z.
Have you tried it into your chosen input, which is what? Is it a normal potentiometer volume control. bearing in mind a 50k pot has an input resistance of around 25k at the halfway position.

given the rule of thumb of a 10 to 1 ratio being adequate for output to input resistance to avoid signal losses then a 2k output resistance is fine into 20k and I certainly have no problems using a Wd or indeed other stages with an ECC83 CF into a solid state preamp.

the output capacitor at 2.2uF seems to be plenty large enough too in order to avoid losses in the LF end of the spectrum.


An ECC82/12AU7 type is a good choice for a standard bootstrapped CF as it offers good current drive for long cables to power amps, and its cheap, and if you want really low output resistance, an ECC88/6922 type will get output Z down to the order of just less than 100R. But for a phono stage usually only connected by about 1m of wire I wouldn't think there's an issue with cable capacitance issues.


Altering the Valve in the CF will have an effect on the RIAA correction as the input resistance and internal capacitance etc of the valve will have altered and this forms part of the final passive EQ filters.


Leave well alone is my advice and don't try to fix what isn't broken. Which may not be what you want to hear so apologies for that.

A.

Last edited by bikerhifinut; 4th August 2019 at 04:42 PM. Reason: bit extra info.
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Old 4th August 2019, 04:47 PM
bikerhifinut bikerhifinut is offline
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Default Re: Phono 2 CF

Quote:
Originally Posted by Phil Y View Post
I know I could build an active Pre but my CD and TV go through a modern DAC that will drive anything and my tuner also has transistor o/p, so going to the cost and time of building an active Pre for the benefit of 1 of my 4 sources would seem silly and wasteful.

Phil.
So do I deduce that you are using a simple passive attenuator between sources and power amps?
In which case unless its got a 5k pot, there shouldn't be any issues.
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Old 4th August 2019, 06:36 PM
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Default Re: Phono 2 CF

If that is the case then yes, nothing to worry about but I got the figure from here:
http://www.world-designs.co.uk/forum...ead.php?t=7681

I have no wish to start an argument but there does seem to be a difference of opinion!

Phil
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Old 5th August 2019, 10:22 PM
bikerhifinut bikerhifinut is offline
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Default Re: Phono 2 CF

Thanks for the link Phil, I worked on an online calculator and that will not have taken into account a few factors I think.

4k is still pretty good though and I do know the phono3 works very well into a 1980's Mitsubishi transistor preamp with no obvious frequency response losses that I can hear.

If you know the precise figure of the input resistance of your preamp then it should be possible to calculate the -3 dB point and take it from there.

Anyway, it should be possible to substitute another valve in the CF with a couple of component value adjustments but you may not get a significant improvement over the existing setup. I am sure that the Phono3 was designed with the ECC83 as the output CF for a reason, otherwise the designer would have used a different valve.


I've just checked the specs for my usual solid state preamp, a Rega Cursa3 and its input resistance is given as 10k. I Use Phono3 with this preamp quite happily and never noticed any problems with matching.


If it is a real matching problem then you have little choice than to put a buffer stage between the phono amp and preamp input. Theoretically I guess you could squeeze a CF of some sort inside the WD casework as theres a bit of room inside to cobble something in. I am also assuming that you don't wish to use a solid state buffer as a decent low noise op amp would do the job very well with the feedback resistors set for unity gain.

A.
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Old 6th August 2019, 08:50 PM
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Default Re: Phono 2 CF

If you have not experienced any problems with a load as low as 10k, perhaps a low load, though not ideal from an electrical point of view, in the real world is not all that significant.

My passive pre is 47k so fine. I was thinking of the situation of having a headphone amp with 27k input impedance in parallel with my 47k passive pre.
I have no pressing problem, just mulling things over really.

Phil.
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