Tube with Power IC Output Stage - JLTi
Tube with Power IC Output Stage - JLTi Tube Hybrid Amplifier
I found this nicelooking and interesting Hybrid.
It is on the homepage of our newest member - Joe Rasmussen
and he is from Sydney Australia. The Golden Land of hybrid thinking .... Seems like it ;)
JLTi Tube Hybrid Amplifier - Photos
I wonder what "monolitic chip" you are using, Joe?
/halojoy - the curious man
Here is a simplyfied schematic.
Wow, that was QUICK!!!
I have only just posted the web site and before
you know it... I am getting reactions to it
from all over the place, New Zealand, Holland
etc and Thorsten emailed me last night asking me
re the thread he had started on Toobz and
Gainclones. I wasn't, so I had a look of course.
It's good that I am NOT the only one who has
thought of this marriage - great minds think
Seriously, this is only early days. I have
made four out of the first batch of ten, and
three are out on loan to gauge reactions and the
fourth is getting a phono stage (still in
development stage) fitted and has been presold
and going to Denmark. That will be the first
export. The others are likely to stay in Australia
and possibly one going to New Zealand - but there
are more to come.
The use of the tube front-end only really makes
sense with inverted 'clones. I did NOT want
to use a pot here as it means the feedback is
not held stable and not Lo-Z. A buffer means you
can get a better control on things and I am also
of the opinion to use LESS feedback than
Thorsten's Lin pot version which drops to +17dB
gain when the pot is in mid position (that's why
you get away with using a LINEAR pot - usually
better channel balance - but f/b varies betweeen
17 and 27dB. It shapes the pot to behave more like
a log. It is a clever idea provided the f/b
variations have no adverse effect on sound... I
believe it does.
SO - I wanted to use LESS feedback - and that
means mine, with buffer, sits at a constant +32dB.
Depending on the original Lin pot position, I am
using between 5 to 15dB LESS feedback than
Thorsten's original non-inverted 'clone.
At the end of February I will be able to export
these - but likely to those who may have (?????)
pre-ordered theirs. But that remains to be seen.
You are welcome to look at the site. There is
more detail and an interesting essay to read (not
quite completed yet).
Welcome to diyAudio. It is always great to see those with a proven track record show up on the forum.
(and halo -- good work hooking him in :) )
What is the purpose of the high-pass filter on the input of the chip?
Basically, if you look closely, there is in fact
a band-pass filter. The initial cap in series is
needed for two reasons 1) isolate the DC off-set
from the Tube Buffer (about 1V) and 2) also to
make sure the DC off-set into loudspeaker is kept
down to a few mV.
The low-pass is inserted as a part of the feed-
back and is much more difficult to explain. It
too has two functions, one to shunt the feed-back
to signal ground (that's the cap you can see
actually go to ground) at frequencies well above
audio. This keeps the feed-back loop very short,
down to centimeters, where it is most desirable
(high up). We can do this because it's above the
audio. The resistor BEFORE this cap, in series
with the signal, forms a low pass filter (and
hence completes the band-pass mentioned earlier)
- 1st order and very benign - to reduce potential
slew rate problem in the 3875. I was actually
able to induce slew rate distortion just above
300KHz, so we need to be down by a good margin
at that point. Without this it sounded very clear
but also a certain brightness was eliminated. This
was confirmed by a number of members of my
unofficial listening panel. :)
Hope that explain things.
Thanx for the explaination.
Is this beauty going to be available as a kit?
Sorry, afraid not. Just simply not designed with
that in mind... but who knows, later? Wouldn't
count on it though.
Would Sony ever make kits? -
With year over year growth in the DIY marker at something like 20% (guess based on increase of unit sales of raw drivers from manufacturer to distributors) that end of the market is growing -- but so to, i would guess, is the market for stuff that is reasonably priced and really sounds good.
Perhaps when you catch up a bit...
Joe, may I make a suggestion?
As I got, you try to mix the best from the two worlds.
On the page http://www.dself.demon.co.uk/thermald.htm there is a very nice distortion plot of TDA 1552. The thermal distortion really exists in power IC (gainclones). May I propose to reconfigure the feedback so the tube stage will appear the input stage of the whole amplifier. In this case it should be common cathode stage with AC overall feedback in cathode, now your amp will be free from thermal distortion completely.
My next suggestion is to make output resistance adjustable. You will need only several passive components for that.
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