Old Buffers.

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andrew Ivimey
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#1 Old Buffers.

Post by andrew Ivimey »

IMHO that impedance is a very little understood thing but that it is actually crucial to getting things very RIGHT.

A buffer stage is a unity gain stage that allows for high impedance input, so as not to affect the output of the source, to a low impedance output, so as not to affect what comes next.

I am of the growing opinion that TVCs really do need a buffer stage before them in most applications. TVCs really are strange and quite unlike resistor ladders or basic pots; very strange indeed!

now, for instance isn't the WAD2pre just this albeit with up to X5 gain? Aren't most preamps (though not all) low impedance outs.

Anyone got any good pointers to ultra simple buffer stages? I need one!
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Nick
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#2

Post by Nick »

Thats exactly what I intend to do with the Slagle AVC (which is waiting for me to pay the inport duty at the moemnt, so should be heare tomorrow). I intend to use that d3a anode follower I had at Witham to drive the autoformer, and see how that goes.

The AF I built with d3a has just less than unity gain, but you can adjust for whatever gain you need by the ratio of two resistors. as the gain increases, output impedance rises though. As it stangs the op Z is about 150R, bipassing the cathode with a cap would make that about 80R
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#3

Post by Clive »

It's the high inductance of TVC's that stresses valve phono stages most of which are like the Phono II & III, ie they are designed to feed an active preamp or at least a buffer.

This may be heresy but I'm currently using a SS buffer from www.BursonAudio.com it'd so discrete that you can't tell it's SS, it has 6db of gain. Presumably the reason that TVCs sound better with valve buffers before them vs downstream is that the TVC steps down the buffer output impedance.

Thorsten once tried to get me to build a 2A3 stage parafeeding my S&B. Could be interesting, if expensive.
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#4

Post by Clive »

This one too:
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andrew Ivimey
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#5

Post by andrew Ivimey »

isn't the cathode follower, in principle, a low impedance output?
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#6

Post by Andrew »

andrew Ivimey wrote:isn't the cathode follower, in principle, a low impedance output?
Yes, (and no).

As I understand it, the CF may have a low output Z but that only translates to useful 'drive' if it can deliver the power, so you need the correct IV regimen. In other words, an ECC83/ECC82 running at 1mA or so, may have a low-ish Zout but won't be able to deliver the goods.

I reckon, 6N30P, 6N6P, 6C45, C3g triode, D3A triode, 5687 and so on with 10mA minimum behind them should do the biz.

cheers,

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#7

Post by Nick »

Yep, thats another anode follower Clive has posted.

Yes, Andrew, the CF is low output impedance. So thats another choice for you.

One of the advantages of the AF though, is the valve cathode can be near 0v as normal, so you don't need an additional cap on the input as the input sums to a virtual earth point.

And you can use the same circuit to give greater or less than unity gain.
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#8

Post by JamesD »

Driving a TVC in parafeed needs a little care as the parafeed low end response is determined by the inductance load of the TVC and the parafeed cap so if the inductive laod of the TVC changes with volume then so will the lf response... This is not to say it shouldn't be done but its one more factor to take into account and one more reason why the sound of the TVC will change with the system it is used in...

ciao

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#9

Post by Nick »

Driving a TVC in parafeed needs a little care as the parafeed low end response is determined by the inductance load of the TVC and the parafeed cap
Yep, see my comment about tha AVC.

I am trying to work out if the feedback loop on the anode follower helps here, but I suspect it starts to require a big cap.
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#10

Post by andrew Ivimey »

So, although I can't think why I can't use a pair of e.g. WAD pre2 output transformers as a step down 5:1 (10K to 600 ohms) as a buffer, I thought let's be reasonably conventional first.

I bought a PCB all soldered up form Hongkers - £26. 'Tube buffer Pre Amplifier PCB with full parts Circuit design ref. Musical Fidelity buffer preamplifier'

It is probably an exact copy - and why not!

Hennyway, I soldered leads, put it in a ally box and switched on.

It adds body - in a rather nice way - nothing else - and the TVC seems to like it. I can see why some folk recommend one of these devices in between CD player and the outside world.

Good idea.
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#11

Post by pre65 »

Hi-after Andrews last post i looked on e-bay to find the buffer

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... :IT&ih=015

I see they do the PCB as a separate item as well.
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#12

Post by andrew Ivimey »

It is a furiously busy PCB Philip.

It was so much easier just to get the work done for me.
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#13

Post by Clive »

I hope it sounds better than my X10D's did.....
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#14

Post by andrew Ivimey »

X10ds is the musical fidelity jobbie is it?

Oh it does then, it does! :wink:
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#15

Post by Clive »

Is it the same as the circuit in Philip's link? Seems like the X10D circuit but maybe your components are better than the MF ones. I still have an X10D somewhere. I must sell sell it!
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