On Jul 16, 3:07*pm, flipper wrote:
On Fri, 15 Jul 2011 02:03:07 -0700 (PDT), Patrick Turner
wrote:
snip
A drawback - a despised electrolytic as a shaping component.
Btw, there's another means of achieving a modest 'LF shelf', using a
concertina, and that's to boostrap the gain stage load off the
concertina cathode. Gain drops at LF as the boostrap rolls off.
This has been done in some Dynaco schematics which have the concertina
cathode driving the top of a RLdc via an electro cap, therefore
supplying positive FB to the input pentode by means of increasing the
RL the pentode anode "sees" so that pentode gain goes much higher
therefore making a given amount of GNFB much more effective. Basically
while peter robs paul and you light a fire under both, they tend to
become very active indeed.
Of course it's been done before.
If the input tube is a lowly triode who's gain is determined by its
lower µ the PFB effect is minimal, and whether there is a bootstrap or
not won't change VLF gain very much, so the shelving networks seen
throughout my website are perhaps the better way with triodes.
Depends, but 6 dB isn't a bad rough cut rule of thumb.
Sometimes 'just a little more' is all it takes and, at least with the
ones I've done, the capacitor values are usually, or can be, small
enough to use film and avoid electrolytics.
My "Stealth AX" amp does it sort of 'in reverse'. The gain triode Rl
is under the concertina splitter, for the 'infinite impedance'
bootstrap, and the signal is cap coupled to the concertina grid.
It has the added advantage of boosting stage gain and theoretically
lowering distortion from the (idealized) 'infinite impedance' of the
bootstrap
The bootstrap from concertina cathode could be taken from an
additional triode cathode follower which is driven directly off the
cathode so concertina R values remain unchanged, ie, anode RL =
cathode RL. The CF can then be used to drive a two resistor R divider
to V1 anode. One has to be careful that cut off distortion with the AC
coupling does not happen.
It "could be" but it seems an unnecessary waste of a tube.
The concertina resistors need adjusting to re-balance the load, and
that causes an offset toward B+, but that can be an advantage as well
since direct coupled concertinas on modest B+ rails can end up with
rather low anode voltage on the gain stage and the offset bumps that
up.
Yes, indeed. I would say I prefer the LTP driver with cathode CCS with
say 6SN7/6CG7/12BH7/12AU7 or EL84. Then the LTP acts as a balanced
pair with very low THD.
It isn't as low as a 100% NFB cathode follower, like the concertina,
and it's more tubes.
It depends. Have a look at
http://www.turneraudio.com.au/300w-1...tput-jan06.htm
Here I have an LTP which produces less than 0.5% THD at two phases of
85Vrms from each driver triode.
To do a similar thing with concertina, you'd have a triode making
170Vrms Va-k, and one might do that using an EL84 in triode, and the
drive voltage to the Concer grid would be 95Vrms. The previous stage
has to make a shirt&trouser load sized signal, maybe with 5% THD. Now
the concer stage OLG THD might be 10%, and with the CLG gain reduction
to about 2, the 10% is reduced to 1.2%, but basically you end up with
more 2H than I get with LTP which has natural cancelling of 2H. Now in
a typical Williamson, the balanced amp needs to make only 2 phases of
about say 32Vrms to power a couple of 6550 in triode. So the concer
makes only two phases of 2V and input tube makes only 2.2V at
clipping. Now with the Willy, the input concer stages usually have
such LOW signals at normal listening levels, say less than 0.2Vrms,
the THD is down at 0.05% and is reduced by the GNFB to utterly
negligible levels. The same goes for the use of the LTP in my amps.
So the discussion of "what is best" becomes of academic interest only.
In small amps with UL output stages with EL84, the SE input triode
with SE concer stage is all one needs and sounds/measured fine. If you
use an extra triode and make an LTP, you might find the amp can be
made more sensitive, and the THD will be almost indentical to the
concer stage but with less 2H present. Both ideas work fine where
signals are low, but in my high powered amps the concer drive stage is
not so hot. McIntosh amps have balanced driver stages.
Most makers use LTP or balanced with larger tubes.
*Whatever the input tube is, pentode or
triode, it has only to produce a low signal in SE mode so THD remains
lower than if you have an input tube needing to make slightly more Va
than is applied to each output tube grid.
Distortion is also increased by the extra tubes.
I happen to like the LTP too but that doesn't mean everything else is
crap and, like all design tradeoffs, "it depends" (on everything
else).
There's more than one way to do things.
A concertina is enough to drive a pair of 6BQ5s all by itself without
interposing another set of tubes and while that may not be your cup of
tea it makes for a perfectly fine little amp.
I agree entirely.
An LTP driven single ended has only half the gain of Williamson's
concertina double driven short tail and while I know you think
converting that to an LTP 'reduces distortion' it's at a place in the
amp likely to be of little consequence since the vast majority of
distortion is in the output stage. So far that's a "why not?" but the
LPT does cause overload interaction on positive grid drive, as does a
concertina *if* it's the thing doing the driving (but it's buffered in
the Williamson). It might not seem like such a big deal but I've seen
that cause HF instability on both the concertina and LPT and is the
reason some add a series grid resistor between the concertina and
output tubes when driving them directly.
Usually HF instability is due to the OLG phase shift characteristic
and once you have the correct gain shelving and zobels the HF becomes
entirely stable. And of course the grid stoppers are always a good
idea.
There's plenny of headroom for the function of the shelving network.
The LTP does not need to be directly
driven from input gain tube anode, so the LTP can have much more
headroom than found in most samples of amps like Leak and Manley Labs
et all.
What 'headroom'? The shelving network is zero loss in band.
Or do you mean for your shelf's 'peaking', which a bootstrap roll off
doesn't cause?
All I said is there was "another means," and I don't know that I'd do
it for 'just that', but it's an interesting aspect to consider if one
also wants the added gain.
Observation of the error signal at V1 anode output when feeding the
amp a level input signal between 1Hz to 100Hz will show that the LF
shelving network is OK. The reference 1kHz signal should produce half
maximum l Vo. Then if you drop the input F and plot the response at
all electrodes, you'll understand. And what will help matters is that
you have passive input CR input filter with pole at 8 Hz. You just
don't need to try to have the amp vainly try to make a big effort with
a 3Hz signal.
Patrick Turner.