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Old 9th June 2003, 08:37 PM   #71
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Hi,

Quote:
Finally, I remember reading something from the good ol' days (RDH?) that specifically stated that grounded grid amps were not suitable for audio work. Anyone else see that?
Yes, I do recall that and the same objection holds true for cascode stages.

Re: the GG, I remember other people saying they sound spectacular at first but too clinical to their taste in the long run.

Not surprisingly these circuits present a harmonic distortion spectrum that contains mainly odd harmonics.
I don't know about the GG circuit but the cascode looks like a penthode if you'd care to plot it.

This may not be the only explanation but it would make me think twice about using such a circuit.

Using PIO caps may camouflage this stridency but I don't like to resort to such brush and carpet techniques.

Cheers,
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Old 9th June 2003, 08:39 PM   #72
EC8010 is offline EC8010  United Kingdom
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Hello Joel,

RDH could well have said that grounded grid wasn't useful for audio work, but I'm not searching through 1498 pages of fine print to find it! More to the point, times have changed. Although we may bemoan the price of an NOS KT88, the fact is that the real price of valves is lower than it used to be. We can now contemplate using any topology we like, and if it means using more valves, we only worry about the cost of the heater supplies. There's no reason whatsoever why we shouldn't use grounded grid amplifiers if the situation demands it. Possible uses might be an MC input stage, or following a current output DAC. 1950s audio simply didn't have the environment or the money to use grounded grid, so we should be careful to consider the context in which RDH made recommendations.

Dave S,

I'd say it could easily be the 12AU7 that is to blame. They produce far more distortion than 6SN7. The other possibility is that your pre-amplifier is oscillating at RF. Grounded grid stages have rather greater bandwidth than grounded cathode, so RF instability is a possibility. Have a look at the anode with an oscilloscope. Judicious grid-leak resistors or local HT bypassing will cure the problem if it exists.
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Old 9th June 2003, 09:32 PM   #73
dhaen is offline dhaen  Europe
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Hi,

Common grid stages shouldn't (and in my tests don't) sound better or worse than common cathode, so long as the source impedance is low.
With a high or varying source impedance, anything can (and does) happen.
I second EC8010's suggestion of them being useful for impedance transformation. And as Frank said, they're used every day in cascode stages.

Joel, I hold RDH as an authority too. But as I've mentiond before, it was a book written for radio designers, who had to save every penny. A common grid stage does not make economic sense for audio, but most of us in DIY are not penny pinchers.

And what's more, there's no miller C

Cheers,
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Old 9th June 2003, 11:16 PM   #74
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Hi,

IME, limited as it may be GG are O.K. for DACs and MCs and that's about it.

They show a tendency towards edgy, strident sound probably because of osscillation.
Adding grid stoppers usually brings that down but you better make sure to use high quality neutral sounding ones or you just listening to those again.
Which in turn ups the cost considerably...

Nothing's ever easy,
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Old 10th June 2003, 11:07 AM   #75
Dave S is offline Dave S  United Kingdom
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A friend of mine whose opinion I value highly has been measuring and listening to the GG ( his system is Cambridge CD2, Grant Valve Power amp and Gale 401s - all old but remarkably good!). He thought the GG had great mid and top but subjectively increased bass (remarkable in that it measures very flat) here are his comments :

"Thought you might be interested in the attached results -- test bench data
for your Rozenblit preamp! Nothing of any remark in the sense that
performance is pretty much "blameless". Slight anomaly between THD for
left and right channels is intriguingly resistant to swopping valves
around.... I'd have expected this to follow the valves in some shape or form
but it seems not to do so. That said the actual level of distortion is
small, is mostly 2nd harmonic and falls gracefully away into the noise as
the signal level is reduced (e.g. 0.03% at -10 dBu output, or about 250 mV
rms) so at normal signal levels it'll be totally negligible (since the pot
is at the preamp INPUT).

Bandwidth is pretty exemplary -- so much so it's pointless plotting the
result. The small gain difference between channels DID follow the valves and
was cleared by swopping the shared valve with one of the ones devoted to
left or right alone.

But nothing to explain the gross difference in tonal balance we both heard
in different ways! Hmm. Maybe it's that the response is actually FLATTER
than I'm used to (i.e. has a lower LF cutoff than my usual preamp) but this
wouldn't explain the difference I heard between my (10 k) passive pot and
the GG preamp."


I built the GG using P-P with chassis mount sockets mounted on a a pcb ground plane. Grid stoppers are right on the sockets. Decoupling is 22uF elco in // with 1uF stacked film polyester (Epcos naked type) in all 3 locations where this is required.

Tubes are Mullard - second hand but fully tested (and FOC).

Output cap is 1uF 400V Epcos naked stacked film polyester - I believe polyprops often have a "zing" to their sound which I don't like. Maybe this part is to blame but past experience indicates these parts are neutral to bland rather than bright.
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Old 20th June 2003, 10:21 AM   #76
Dave S is offline Dave S  United Kingdom
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It's that old devil called system matchng again!

I have persevered with the GG and changed other parts of my system, including the speakers and am now getting very good results with the GG.

It is definitely better than my best SS preamp - it just sounds more vibrant and realistic.

So maybe the 12AU7 is a "tizzy little fart bottle" as someone described, but it's now producing some quite appealing farts in my system!



Quote:
Good design is good design. Throwing together a circuit that happens to have tubes in it is not a guaranty of anything. You also make another curious statement that "hi-fi" is tiresome for you to listen to. Isn't high fidelity the point of this whole hobby?
According to many the GG is a good design. My scratch buit version is not thrown together but carefully hardwired using a ground plane, star earthing, close decoupling etc. I've been building stuff long enough to know that correct implementation is vital to good sound. Hi-fi is tiresome when it's not working properly - a good example is at hi-fi shows where about 75% of the systems are "tiresome".
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