Tube with Power IC Output Stage - JLTi

[halojoy]

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

[halojoy]

Here is a simplyfied schematic.

[Joe Rasmussen]

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
alike.

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).

http://members.ozemail.com.au/~joeras

Regards

Joe Rasmussen

[planet10]

Joe,

Welcome to diyAudio. It is always great to see those with a proven track record show up on the forum.

dave

(and halo -- good work hooking him in )

dave

[planet10]

Joe,

What is the purpose of the high-pass filter on the input of the chip?

dave

[Joe Rasmussen]

Originally posted by planet10
Joe,

What is the purpose of the high-pass filter on the input of the chip?

dave

Hi Dave

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.

Regards

Joe

[planet10]

Originally posted by Joe Rasmussen
Basically, if you look closely, there is in fact
a band-pass filter.... keeps the feed-back loop very short

Very clever, you VSE guys

Thanx for the explaination.

Is this beauty going to be available as a kit?

dave

[Joe Rasmussen]

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? -


Joe

[planet10]

Originally posted by Joe Rasmussen
Would Sony ever make kits? -

Probably not, but they aren't already making 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...

dave

[dimitri]

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.

[Joe Rasmussen]

Hi Dimitri

Tried the link you gave but it no longer seems
online?

I hear what you say, but to include the tube
stage is not practicable if you look closely at
the circuit. The buffer (unity gain) has no way
of easily accepting feedback.

I am loath to use a triode gain stage as there
is already too much feedback - I want to go LESS
not more. Also I've avoided 'warming up' the
sound - and adding a stage that increases 2nd
harmonic may well sweeten things up (and other
tricks). Actually lessening feedback did the
job for me and got the BALANCED result I was
seeking.

The circuit I've developed involved a lot of
pros and cons - and I'm happy where it stands.
It cannot NOW be easily changed. As for using
feedback to correct any anomaly I have to say
I'm not in favour of it. I suspect very much that
the basic goodness starts from the open loop
characteristics of the 'clone chip, and that
feedback basically - being an opamp - is a way
of setting gain.

As for further developments... for sure, but I
don't want to start from scratch but rather build
on the foundation I've got.

----------------------------------

Now for a different tack:

To both of you, Dimitri and Dave (and others).

The inverted 'clone circuit that Thorsten
supplied has served the DIY fraternity well. If
I may, and hopefully Thorsten won't mind too much,
I am going to post a very reconsidered version
- not tube - of the inverted clone. I will
prepare this thoughtfully over the next week or
two. It will include one interesting area that
can be tweaked, a kind of voicing mechanism.
They key is the way feedback is applied and also,
my main concern with Thorten's, the stablising of
the feedback loop as the level is changed.

It will incorporate some of the benefits of my now
commercial (not DIY) JLTi.

There will be full diagrams and component values
and suggestions on which can be tweaked. I am also
going to do some "Lynn Olson first 1W" type
harmonic distortion measurements (these are actual
dist/freq graphs).

I would like to see others try this new version
(can somebody think of a name to give it?) and
get feedback - we might be able to give this
whole topic a bit of a new boost.

It should be fun!

Stay posted.

Joe

PS: James Taylor (of Fire and Rain fame) is here
in Sydney at the moment. I heard him yesrerday
say, on a local radio station: "War is failure,
not a strategy."

[planet10]

Originally posted by Joe Rasmussen
Tried the link you gave but it no longer seems online?

Dimitri had mistyped the url... it is fixed now.

dave

[planet10]

Originally posted by Joe Rasmussen

I am going to post a very reconsidered version - not tube - of the inverted clone.

I look forward to that... my chips are still in there little plastic bags

"War is failure, not a strategy."

I heard that on the radio recently too... i'm not exactly positive who said it, by i think it was Jacques Chirac (sp?). (and i have to agree with the sentiment)

Nelson Mandala also had some not-nice things to say about W.

dave

[dimitri]

Hi, Joe
--------
... include the tube stage is not practicable if you look closely at the circuit. The buffer (unity gain) has no way of easily accepting feedback.
--------
You can reconnect from the follower to common cathode stage

-----------
I am loath to use a triode gain stage as there is already too much feedback - I want to go LESS
not more. Also I've avoided 'warming up' the sound - and adding a stage that increases 2nd
harmonic may well sweeten things up (and other tricks). Actually lessening feedback did the
job for me and got the BALANCED result I was seeking.
-------------
I could not get how you make LESS feedback, than in typical application from data sheet. What feedback makes wrong to you (and your gainclone)?
It 'warming up' the sound was not your aim why have you used the tube? For marketing purposes, just to add that nice word in your ad?

[Kuei Yang Wang]

Hi Joe,

(Or should that "Hey Joe"?)

> I am loath to use a triode gain stage as there
> is already too much feedback - I want to go LESS
> not more.

Actually, I feel the reverse is true. Around the Power Op-Amp you need a lot of negative feedback. Anyway, views vary.

The inverted 'clone circuit that Thorsten
supplied has served the DIY fraternity well. If
I may, and hopefully Thorsten won't mind too much,
I am going to post a very reconsidered version
- not tube - of the inverted clone.

Hell, the infamous "inverted gainclone" was simply the result of someone on the Gainclone board asking "how do I build an inverting Amp". I also kept a lot of considered "belt & braces" in there, which I personally would do without and designed in the volume control as fundamentally neccesary and stabilising element.

There are many things I'd optimise on that puppy myself. It was never meant as a suggestion of "crushing superiority - maximum tweaked out and freaked out eschaton of chip amp's". It was simply a quick slap-dash for people who wanted to try the difference between inverting and non inverting Amp's.

Tweak away to your hearts content.

Sayonara