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Author Topic: Any low sensitivity power amplifier?  (Read 3332 times)

sajy_ho

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Any low sensitivity power amplifier?
« on: December 21, 2015, 04:03:03 AM »
Hi guys, I'm planning on building a tube preamp/SS power amp style hybrid guitar amp.

The preamp section is basically from ax84 High-octane with tone stack:
http://www.ax84.com/hioctane/AX84_Hi-Octane_101004.pdf

After the preamp and tone stack I'm planning to use a fixed resistor divider in the place of MV to attenuate the signal level to around 3.5V p-p and feed it into an opamp buffer; something like this:
http://www.tremolo.pl/Przedwzmacniacze/JCM%20900%20Marshal.gif
I'm assuming the HV doesn't damage the opamp at start up, does it?

My main issue is the power amp; At first I was going to simply use a TDA20X0 chip, but it seems with a 12V+12V supply and 8 ohm speaker the input signal must be under 0.40V to avoid clipping.
So maybe I should build a discrete power amp section...
What's your opinion on this project?
Any help would be appreciated...

Thanks
Sajad
« Last Edit: December 21, 2015, 04:04:28 AM by sajy_ho »

Enzo

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Re: Any low sensitivity power amplifier?
« Reply #1 on: December 21, 2015, 05:20:57 AM »
Why not just attenuate the preamp signal to the level you need in the first place?

Roly

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Re: Any low sensitivity power amplifier?
« Reply #2 on: December 21, 2015, 06:51:51 AM »
...or, since you intend to build a valve preamp, go the whole hog, sidestep the problem entirely, and build a valve power amp (which is where most of the desired "valve tone" comes from anyway)?  e.g...

http://www.ozvalveamps.org/ava100/ava101lamington.htm
If you say theory and practice don't agree you haven't applied enough theory.

sajy_ho

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Re: Any low sensitivity power amplifier?
« Reply #3 on: December 21, 2015, 08:43:24 AM »
Thanks man, I already have a High octane tube amp, but I found myself playing it at around TV level and the master volume doesn't pass 9 oclock!
So practically the output section doesn't color the sound and I get all the tone from the preamp!

Besides I have a diy combo cabinet around with a Celestion 8" 15W inside, so it may be fun to build  a tube preamp to get some triode tone and then feed into a pure clean SS power amp to take care of speaker driving, and maybe add a switchable PT2399 delay line before the output section in the case of lead...

I also tought about attenuating the signal to appropriate level for the chip amp, but looking at the 12ax7 plate curves, the signal level coming out of the preamp will be around 180V p-p! So I need a serious attenuation here...

I choosed 3.5V p-p because it is the max input headroom of PT2399. The problem with Chip amps is that they all have preamp section for voltage amplification too, so they all will clip very soon.
I wonder if there is a schematic for any ONLY power amp section for around 6-12W into 8 ohms?

I found these schematics but since I'm a total noob in the SS realm, maybe someone here can take a look at these schematics or maybe come out with a better idea  :
http://circuitswiring.com/fet-based-audio-power-amplifier-with-12w-output/
http://circuitswiring.com/12w-amplifier-circuit-based-741-op-amp/

Thanks




sajy_ho

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Re: Any low sensitivity power amplifier?
« Reply #4 on: December 21, 2015, 08:59:26 AM »
Edit:
Besides here is the building diagram of what I have in mind: https://db.tt/G9a4Dfmm

J M Fahey

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Re: Any low sensitivity power amplifier?
« Reply #5 on: December 21, 2015, 09:49:27 AM »
Straight attenuate the 180Vpp Hi Octane raw output to 0.4V RMS (some 0.7Vp or 1.4Vpp) or even better attenuate it to around 1V RMS, add a Master volume control there, feed that into the PT and then into the chip amp.

The final volume adjustment will be made with the just added Master Volume.

Just to throw some numbers into it, you need around 100X attenuation, so a 10k master volume with a 1M resistor in series with it will be about fine ... and need no buffer, since that impedance is fine .

As of Chipamps vs. Discrete , they are basically the same, only the chipamp comes encapsulated.

What defines sensitivity is the negative feedback, which both use.

sajy_ho

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Re: Any low sensitivity power amplifier?
« Reply #6 on: December 21, 2015, 10:08:57 AM »
Thank you so much, that is all I needed to know.
There is just one question remaining for me; for the power supply should I use a single 24V supply or a dual 12+12 supply? Because I'm gonna use the same supply rail for feeding the tube heaters.

Thanks again...
Sajad

J M Fahey

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Re: Any low sensitivity power amplifier?
« Reply #7 on: December 21, 2015, 01:33:47 PM »
The chipamp datasheet *usually*  has first a split supply example (in this case might be 12+12V)  but for those who need it, often adds a single supply example near the end of the datasheet.

Use whichever you want, they are equivalent.

And if the datasheet example shows, say, +/- 15V or +/- 22V rails or whatever, usually somewhere there is a small graph showing performance (power and dissipation)  with other supply voltages.

As long as your chipamp is within the "permitted" voltage in that graph, you are fine.

To see it from another point of view: if you have 12+12V rails , it's the same whether you use TDA2030/2050 or LM1875 , they are all compatible there and will put out the same.

As of tube filaments, if fed DC all they care is voltage between filament ends and that they are somewhat related to Ground (i.e. not left fully floating) , so all options work.

sajy_ho

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Re: Any low sensitivity power amplifier?
« Reply #8 on: December 21, 2015, 02:43:37 PM »
Thanks for all your help J M Fahey and others, now with your help I'm going to build my amp.
I was wandering around the web and I found this thread interesting: http://www.diyaudio.com/forums/chip-amps/9852-gainclone-no-gain-yes-pain.html

Is this idea going to work?

J M Fahey

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Re: Any low sensitivity power amplifier?
« Reply #9 on: December 21, 2015, 04:47:55 PM »
Follow the datasheet example ... which says gain must be 20X or greater for stability.

DIY Audio is made out of a few real designers , say less than 100 and maybe that's too much, thousands of followers (not bad)  and quite a few *would be*  designers, who lacking real ideas try to do things "another way" , even if nonsense.

Think having soup with a fork and eating steak using 2 spoons "just to be original" .

phatt

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Re: Any low sensitivity power amplifier?
« Reply #10 on: December 21, 2015, 09:43:29 PM »


Regards your circuit idea;
http://www.tremolo.pl/Przedwzmacniacze/JCM%20900%20Marshal.gif

There is a real gotcha at the input of the opamp, that 47k R will render the tone control very in efficient. ALL those classic tone circuits depend on the output looking into a very high Z, you need at least a 1meg R value there.
Also some resistor noise can be a problem through the series 470k.
Others here will know more but,,Maybe back off the gain a bit at V3 will help keep the signal from swinging to much.
I have built a project based on the Mesa V twin and i'f the opamp after tone control distorted I never noticed it much.
Phil.

Enzo

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Re: Any low sensitivity power amplifier?
« Reply #11 on: December 21, 2015, 10:39:47 PM »
"Honey, could I have another slice of soup, please?"


Come to think of it, many years ago I lived on a commune, and on my cooking day, one thing I liked to make was bean soup.  I like my bean soup thick and hearty, not brothy.  But some of the others complained my soup wasn't soupy enough.  So I started calling it "bean surprise" instead of bean soup.  Problem solved.
« Last Edit: December 21, 2015, 10:42:15 PM by Enzo »

phatt

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Re: Any low sensitivity power amplifier?
« Reply #12 on: December 22, 2015, 01:19:34 AM »
Oh very funny,, yeah yeah Ok point made,,,, I know *Soup* was a bit harsh  :-[

I actually like chili beans as long as it's not insane Hot,,but my other half is not so keen on those kinds of foods Her tummy can't handle it.
Phil.

 

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