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First Hybrid

Started by baene123, October 08, 2009, 01:17:15 PM

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joecool85

Quote from: Brymus on October 09, 2009, 08:11:03 PM
Here is a better way of expressing what I wanted to say before.
If your guitar has a signal of 1V and your pre-amp amplifies it by a factor of 10
Then your last stage- a voltage follower needs to have a gain of .1 to bring the signal back down to 1volt to input your chip amp  :tu:

Wait a minute...1v into a chipamp?  I ran my guitar straight into my LM3886 and didn't get full volume, my pickups put out 2+ volts.  Then I hooked it up to my LM386 and ran that into it and got full volume, not sure how many volts it was putting out, but had to have been 5-6v.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

5thumbs

Quote from: joecool85 on November 03, 2009, 02:50:00 PM
Wait a minute...1v into a chipamp?  I ran my guitar straight into my LM3886 and didn't get full volume, my pickups put out 2+ volts.  Then I hooked it up to my LM386 and ran that into it and got full volume, not sure how many volts it was putting out, but had to have been 5-6v.

I'm working on projects that are based upon the Tripath TA2020 and the LM1875 chip amps.  The TA2020 has a max input voltage of 6V.  (Yes, that's an absolute max rating...the recommended RMS input voltage wasn't specified on the datasheet, so it could be 1V or perhaps higher...unknown to me at this time.)  So you can feed 6V to a TA2020 chip amp and not fry it, but that doesn't mean it will not distort like crazy when fed that level.  (I don't know what voltage levels I can feed my TA2020 chips for clean operation, but I'll get the voltages "in the ballpark" in the design phase, then tweak to final values during the "prototype tweak" phase.)

The LM1875 is max rated for input voltage from -Vee to Vcc, which in my application is 48V (-24V -Vee to +24V Vcc.)  I own a couple of commercial hybrid amplifiers which use the LM1875.  The last valve section feeds +80V into in an inverting 4558 which effectively halves the signal before presenting it to the LM1875 chip amp.  So in this implementation, they have played exactly "by the book" and are halving the voltage to keep it within the spec'd input voltage range.  (In this particular amp, they use a single supply of +40V on the LM1875, but they are still within spec for that chip amp.)

So all of that is more or less reinforcement for what others have said about making sure your preamp output is within spec for the input voltage rating of your power amp.  After reading this thread and thinking it over, I'm going to add an inverting op amp with variable gain reduction just before the power amp outputs.  It's a trivial task to do at the design stage that will make tuning the first prototype that much easier.

Thanks to all on this thread for giving me some good food for thought as I work through my own design issues.  :)
Pet Peeve:
Those who claim that discrete transistor amplification is the only way to get true audiophile sound...but they then reject any pedal/device that does not use mechanical "true bypass" because the discrete transistors in the switches/buffers of the pedal "color the sound."

phatt

Quote from: J M Fahey on October 18, 2009, 08:51:56 PM
*If* you have an Alembic preamp, or any tubed type "pulled" from a Classic Fender, you´ll have to do the following, to keep the good original sound; not screeching trebly, chip killing level, noisy , etc.
Let´s do some math:
To begin with, a tube fed with 300 Vdc can swing some 40 volts less, say around 260 Vpp , or around 80 Vrms.
To safely lower it to chipamp levels, you´ll need a *resistive* attenuator (flat response) , and not a capacitive one (bass killer).
I suggest the following: take your signal from the triode plate through a .047x400 cap, as originally specified, and connect it to the hot pin of a 10K pot, *through a 470K resistor*.
That way, the triode will be happy seeing a load higher than it´s original 100K plate resistor, overloading gracefully as intended, and you´ll get somewhat more than 1V rms at the chipamp input.
You will need no cathode/voltage follower, and the output impedance will be acceptably low: 10K (the pot´s value)
I think it´s clear enough, but if not, I´ll post a drawing, now I´m on a borrowed computer away from home.

Sorry JMF,, I missed this one,,,, :-[
Once again you nailed it ,,,Yes of course that idea would be the better way to go.
Good on you,,, :tu:  your a wealth of info.  Phil.

Brymus

I just about finished putting a 4 gain stage Marshall type tube pre amp(a modified Dockery TMB in cascade mode with selectable boost and an NFB circuit) into a small SE amp (Valve Jr).
I added an FX loop right after the MV breaking directly before the input to the power tube
an EL84.
I can drive anything with it !!!!!!!!!!!
Seriously with the MV down I can drive the input of any of my SS gear.
And it sounds sweet.
And it works super going into another tube amp as well.
Still needs a boost in front to get the gain I am after  8| But wow with some drop tuning
it is the $h1T.
I see no reason why a Hybrid tube pre/SS PA shouldn't be as simple.
FWIW the Dockery pre uses a cathode follower into the PA and it made the tonestack sound better to me.So the last stage is a CF followed by the TMB tonestack and then into the MV into the FX loop into the EL84(PA)

Yeah Joe I found the same thing with most chip amps they need more than 1V input.
I thought the question was in regards to limiting the input to 1 V for a certain chip.

J M Fahey

 :P :P :P(Drooling) :P :P :P
Hey Brymus, post something else. Pictures, schematics, MP3s, whatever.
What you write is enticing. :tu:
Congratulations.
Juan Manuel Fahey

Brymus

Hey JM sorry its been awhile,been busy.
Here is a schematic,This is the original I think I changed a couple of small things since in the FX loop and clipping section nothing major.
No MP3s yet,

The "guts"

More pics
http://s419.photobucket.com/albums/pp276/Bry928/High%20Gain%20EVJ/
I should probably start another thread for this. Later when i have more time.

phatt

Nice work Brymus ,,
but my concern is the NF path ,,, efx loop in between?? Head scratch?? Like what's gonna happen if the returning signal is in phase??

Surely that would then be a positive FB situation?
Hey if it works then no matter,, an I maybe missing something. ;)

Phil.

J M Fahey

Hi brymus. Two things:
1) I *loved* thios preamp, i'm sure it's a killer.
2) agreeing with Phatt, you can't bridge so far-away points in the amp with a NFB loop; even less if there are "modifiers" included in iot: Volume, tone stack and, horror of horrors !! an effects loop.
Just forget it, and, besides, the amp sounds better without it.
3) URGENT URGENT ¡¡¡¡¡NOW!!!! solder a 1M resistor across the effects return jack or even better, from EL84's grid pin to ground !!!!!!!!!!!!
The SECOND your output tube loses ground reference (such as a faulty jack or an output DC  isolated effect) your poor power tube will go to Tube Heaven, to play harp and fly from cloud to cloud.
4) If you make Phatt and I scratch our heads too much, we´ll start to get bald early in our youth !! (now that's a good excuse, hehe)
5) Liked the layout too.

Brymus

Ah yes it did run away w/o the ground reference,caused me quite a headache .
That has been fixed.
The NFB works great when the MV is at full volume.
Tames it just enough,but if there is a better way I am all for trying.
The FX loop was for trying the pre-amp with other amps more than adding FX but works both ways,still not very user friendly unless you know how its wired.
This should really be a two channel amp instead of having those switches to take the pre from clean to over the top but I was working with in the confines of the EVJ chassis and PT.
Thanx for the feedback any revisions are more than welcome.