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Author Topic: Simple Preamp please check my work  (Read 8639 times)

galaxiex

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Simple Preamp please check my work
« on: May 17, 2015, 02:09:31 PM »
Greetings all!  :) Yet another small project to work on.

Building a small practice amp.

I have a small power amp kit, this one >>> http://www.canakit.com/20w-bridged-audio-amplifier-kit-CK193-UK193.html?gclid=CM_jme-qycUCFZSCaQod-QwANg

Using a Eminence 820H 4 ohm driver. Not very efficient but it's what I have.

Power supply recommended for the above kit is a regulated 18V single ended supply.

I have a suitable supply, so now working on making a 2 input (High & Low) preamp with just a simple tone control.

The attached schematic is what I have come up with so far...

See any mistakes, obvious errors?
Any suggestions for improvements?
Maybe an additional buffer after the Vol control?
(perhaps some diode clipping for the High Input?)

TIA!
« Last Edit: May 17, 2015, 02:17:20 PM by galaxiex »
If it ain’t broke I’ll fix it until it is.

Roly

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Re: Simple Preamp please check my work
« Reply #1 on: May 17, 2015, 06:58:20 PM »
Well given that I have only just got up on this cold and frosty morning, nothing leaps out at me, all looks pretty reasonable.

I'll remark that I think you are going to find your "low" sensitivity input is pretty hot, and the gain channel even hotter - it's more normal to find, say, a 100mV High channel and a Low that's between 2:1 and 10:1 less sensitive, e.g. "Line" level, but it all looks basically sound and tweekable - suck it and see, and amend to taste.   :dbtu:

The upside is that if you want to drop one of the usual suspect tonestacks in at R11 with a typical 20dB insertion loss then you have gain to burn to compensate.  {but either lift your volume control to 1M or scale the tonestack impedances down by 10:1.}

Input sensitivity: 100mV / 47K Ohm

Quote from: galaxiex
Maybe an additional buffer after the Vol control?

Yes, with a 100k volume pot feeding a 47k input you are going to get a rather odd control characteristic, but failing redesign of everything after your op-amps for one-tenth the impedance, a simple emitter or source follower, or another op-amp should give you a preamp output impedance that is quite low enough.

Oh, and I'd move C5 to mid-rail to ground.  Where it is it will couple any noise on the supply to the mid-rail.

I'd personally stick a 0.1uF poly bypass right across the op-amp supply pins as insurance, but that may be gilding the lily.

Includes two gold-anodized heat sinks

Gold looks great, but black works better.   ;)

Keep us posted.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Simple Preamp please check my work
« Reply #2 on: May 17, 2015, 09:08:47 PM »
Awesome! Thanks Roly!  :)

Quote
cold and frosty


Winter down there already?

Here's a schem of the Cana Kit amp, in retrospect i should have included this before...

I added the component values, hope it shows up as readable.

Not sure how they arrive at 47k input impedance with a 20k volume pot to ground at the input?

Quote
move C5 to mid-rail to ground.


Done!

Quote
stick a 0.1uF poly bypass right across the op-amp supply

Yep, forgot to put it on the schematic.

Quote
Keep us posted.   :dbtu:

You betcha! Gonna mock this up and see how it works.
If it ain’t broke I’ll fix it until it is.

Roly

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Re: Simple Preamp please check my work
« Reply #3 on: May 18, 2015, 06:12:55 AM »
Quote from: galaxiex
Winter down there already?

You betcha.  Light frost this morning.  Snow down to 700m last weekend (430m here).  Often get the odd snowfall over winter in this area but it doesn't settle for long.  Sunny today though, and heading into town along the creek I almost fell over a big buck Eastern Grey Kangaroo sunning himself against my back fence - don't know who was more surprised.  :o  Bigger than me and he has been hanging around my garden lately, but thankfully pretty timid.

My phonepix are blurry, but this is how he was;


I rack up a lot of doona time this season.   ;)


Quote from: galaxiex
Not sure how they arrive at 47k input impedance with a 20k volume pot to ground at the input?

Well since the impedance can't be any higher than the resistance, that plumb evades me too.   :lmao:
If you say theory and practice don't agree you haven't applied enough theory.

g1

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Re: Simple Preamp please check my work
« Reply #4 on: May 18, 2015, 11:59:49 AM »
  Now you reminded me of a show we used to get up here when we were kids, "skippy the bush kangaroo"  :)
  We got a bit of a freak snowstorm here yesterday, just to remind us it's not quite spring yet.
  Cold and frosty you say?
https://www.youtube.com/watch?v=8BngJKGZe1k

Roly

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Re: Simple Preamp please check my work
« Reply #5 on: May 18, 2015, 01:06:40 PM »
Ha!  Yeah, I know; "You call that snow?  This is snow!"  Aussie doesn't have much snow worth talking about.  I lived in rural England for a while and getting snowed in was a winter reality.

You can tame a kangaroo but you can't make them act.  The claws-on-a-stick that were used so Skippy could fly helicopters, turn speedboats away from certain disaster, dial phone numbers, etc, are still a source of great hilarity today.  They are lovely critters, but they just ain't that smart.

The nine (or was it fifteen?) 'roos they used for each episode (they have one hell of a Union) were female Eastern Greys, but the northern morph which is only about 3-4 foot high.  My guy is southern morph and over 6 foot.

Thankfully they try to avoid trouble, but if cornered they sit back on their tail, making them even taller, and lash out with formidable claws on very formidable hind legs, and even if you are not opened up neck to knee you will go flying with broken bones.  Thankfully this is extremely rare, but when you almost fall over one unexpectedly it's something to keep in mind as you rapidly retreat.   ;)
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Simple Preamp please check my work
« Reply #6 on: May 24, 2015, 01:41:52 AM »
Ho-Kay... ;) Further to this pre-amp business, I now a have a (seems to be) working pre-amp. Not the one posted above.
I modified it quite a bit on the breadboard and have not yet (lazy) got around to re-drawing the schematic.
Will post that later....

The problem now seems to be the Cana Kit "power" amp.

They say this thing puts out 20W into a 4 ohm load but if I drive it with a 1V peak to peak signal it starts to clip, and even then is not very loud.

I realize that statement is subjective.... but I have heard other so-called 20W amps and they seemed waaaay louder than this one.
Even some 15W little practice amps with a crap 8" speaker can easily over-power this thing.
(Subjective after subjective, how subjective can we get...)  :lmao:

If I drive the amp with say a 1.5V P/P signal it is substantially louder but clips pretty hard and does not sound good at all...

I am using this speaker >>> https://www.amplifiedparts.com/products/P-A-820H-4
Granted, not terribly efficient but I have other speaker cabs, and no matter which one I use it jus' don't seem like 20 Watts.
It IS possible that all my speaker cabs are horribly in-efficient....

So I have been trying to understand the circuit but must confess to not understanding it at all...  :o

My thought was to see if I could calculate and increase the gain of the P/A somewhat, but have not a clue where to begin...
What controls the gain of this amp? R3 and R4? Why are they different values?
What parts should I be looking at for increased gain? Or at least to calculate what the gain actually is?

Actually, If you wouldn't mind...
A complete in-depth analysis with a layman's description of what every part does and why it's there would be good too...  :duh  ;)  :lmao:

Here's a redrawn schem of the Power amp. Hopefully it is easier to read.

Ya, I don't like the electros in the signal path but that's what the kit came with,
and I blindly assumed the guys that engineered this circuit knew what they were doing...  :duh

Oh ya, the power supply I have for this is somewhat overkill.
A regulated 18V 2A using a TO-3 7818 with a good heat sink.
At full volume (1V P/P in) and 20 minutes of playing the sink is barely warm and monitored voltage almost never dips below 17.5V
Big Transformer too. Torodial 100VA dual 18V secondaries in parallel, 4A bridge, 2X 10,000uf filter caps.
Approx 25.1V un-regulated.

Thanks!
If it ain’t broke I’ll fix it until it is.

phatt

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Re: Simple Preamp please check my work
« Reply #7 on: May 24, 2015, 04:46:49 AM »

FWIW, I'd stay well away from oddball circuits unless you know the outcome in advance.

The chip you are using is rated around 6 watts at 8 Ohms and even in bridged mode it's unlikely to ever compete with a simple lm1875 build or similar which is 20Watts unbridged and much easier to build 8|

IMO, Amplifiers under 20 watts are hardly worth it, heck you can likely pickup a complete 10watt combo at a jumble sale these days if you hunt around a bit.

Try this;
http://schematiccircuit.com/schematic-power-amplifier-with-lm1875/pcb-rangkaian-power-amplifier-lm1875-20-watt/

Or This;
http://sound.westhost.com/project72.htm

Also,,The preamp you posted earlier has no tone control and it's unlikely to deliver the flavor of a rock 'n roll guitar amplifier. The low input is just a buffer and gives no gain at all.
Can I suggest you breadboard something that is known to work then tweak to your taste.
Testing that preamp through a known power amplifier chip setup that is also known to deliver.

Just my 2 cents worth
Phil.
« Last Edit: May 24, 2015, 04:49:48 AM by phatt »

Roly

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Re: Simple Preamp please check my work
« Reply #8 on: May 24, 2015, 07:22:57 AM »
Quote from: galaxiex
My thought was to see if I could calculate and increase the gain of the P/A somewhat,

Increasing the gain isn't going to help.  You can already drive it into clipping and it isn't loud enough.  Increasing the gain will only lower the input level for a given output, it can't give you more output - that's limited by your supply voltage.

Quote from: galaxiex
Actually, If you wouldn't mind...
A complete in-depth analysis with a layman's description of what every part does and why it's there would be good too...

As it happens the TDA2003 datasheet has a table that explains what each component does and what effect changing its value has.


Taking your redraft, from the left-hand side;

This IC is basically a power op-amp, so the gain setting is the same;



Av = 1 + (R3 / R2)

1 + (240 / 15) = 17  i.e. a gain of x17

As with any op-amp the output of U1 will go to the value that makes the join of R2 and R3 identical to the input voltage on pin 1.


The drive for U2 is derived via R1 from the NFB attenuator on U1.  As noted above the signal at the join of R2 and R3 is identical to the input (but at much lower impedance).

U2 differs from U1 in that it is in an inverting configuration which means that the join of R1 and R4 are at virtual earth.  This in turn means that your R1 and R2 are effectively in parallel giving 15/2 = 7.5 ohms to ground, so the gain above must be modified accordingly;

1 + (240 / 7.5) = 33 or a gain of x33.


In the case of U2 the gain is;



Av = -(R4/R1)

-1 * (430/15) = -28.66666667 or an inverting gain of about x29.

This means that U1 will clip against the rails somewhat before U2, but that U2 will be driven with this clipped signal so it will appear to clip just before it gets to full rail-to-rail swing.


Since this is a "bridge" amp U1 and U2 drive opposite ends of the speaker load in opposite directions.  This allows twice the voltage swing of the supply, a considerable advantage in a vehicle where the supply is limited to 12V.  This means that overall we get twice that, ~24V peak-to-peak, across the load.

It also means that each amp "sees" half the total load impedance, i.e. 2 ohms with a 4 ohm speaker.


C8 and C3 are low frequency and high frequency bypass caps to tie the supply rail to ground for AC.  These should always be fitted as physically close to the amp supply pins with the shortest connections possible.  {you don't need to go to extremes, but they certainly shouldn't be back in the power supply or some other remote location.}

R5/C6 and R6/C7 are Zobel stability networks to prevent high frequency instability (and again should always be fitted physically close to their respective amplifiers).
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Simple Preamp please check my work
« Reply #9 on: May 24, 2015, 09:38:38 AM »
Thanks phatt, I had come to the conclusion that this "kit' is a lost cause. Gonna send it to the glue factory.  ;)

It's not worth mucking with any longer, tho I did learn something, so not a total loss.  :)

A quick glance at the 1875 chip looks more suitable for this project. I like the look of the ESP circuit...
and I'm pretty sure he knows what he's doing.  ;)

Quote
The preamp you posted earlier has no tone control and it's unlikely to deliver the flavor of a rock 'n roll guitar amplifier. The low input is just a buffer and gives no gain at all.

Ya, I have modified that pre-amp quite a bit as this thing evolved. I have a pre-amp now that I "think" will work ok.
I'll post the schem later.


Roly, Thanks much for that.
I was kidding about the detailed analysis, but do appreciate your time to explain all that. I will study and learn.   <3)

After I posted, I found that data sheet with the table etc,
my previous search for the data sheet turned up one that was less than helpful so I learned something there too...

NOT ALL DATA SHEETS FOR A GIVEN CHIP ARE THE SAME!!!   :o  :grr

Sorry about the yelling.

Next time I won't be so quick to just use the first sheet Google comes up with.  ::)

Ok, onward and upward with a careful re-think... As I've seen Roly post before.. "Long Think, Short Do"

I confess to not doing that with this project,
I just happened to have the Cana Kit amp on hand and dove into "making a little practice amp".  :-[

(Arnie voice) I'll be back.
If it ain’t broke I’ll fix it until it is.

Roly

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Re: Simple Preamp please check my work
« Reply #10 on: May 24, 2015, 12:04:50 PM »
Every company that makes an IC has a datasheet for it, but not all datasheets are created equal.  Bookmark that datasheet site.  I use it all the time and find it pretty good (compared to some others).

I'd still be interested to know what peak-to-peak voltage you are getting across your 4 ohm load at the onset of clipping (i.e. is the amp weak or the speaker deaf?).
If you say theory and practice don't agree you haven't applied enough theory.

galaxiex

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Re: Simple Preamp please check my work
« Reply #11 on: May 24, 2015, 02:14:50 PM »
Every company that makes an IC has a datasheet for it, but not all datasheets are created equal.  Bookmark that datasheet site.  I use it all the time and find it pretty good (compared to some others).

I'd still be interested to know what peak-to-peak voltage you are getting across your 4 ohm load at the onset of clipping (i.e. is the amp weak or the speaker deaf?).

Bookmarked!

I "think" it was ~ 10V P/P but that might be old guy memory stuff...

I started taking the mock-up apart, but not so-far that that I can go back and check what it actually was.

BRB...

PS, You recall that I have an old Serviscope S-51-A?

I recently discovered that the mains socket the CRO was plugged into has an open ground!!!  >:(

I have corrected this, and the CRO works MUCH better now.  <3)  8)
If it ain’t broke I’ll fix it until it is.

J M Fahey

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Re: Simple Preamp please check my work
« Reply #12 on: May 24, 2015, 02:46:02 PM »
FWIW that canakit amp is not bad at all, just not too loud.
Expect real 12W RMS or so.

FWIW I made for ages a very similar version of bridged TDA2003, at the beginning because there was nothing else (think 80's)  and in fact this was state of the art in car type (12V powered) bridge amps.

It's basically the datasheet example.
The right side amp has doubled value (R4 430r vs R3 240r) because the other NFB resistor, R1 15r is in series with the other amp one , R2 15r , so it's also effectively doubled.

Both halves end up having the same gain, 240/15=16X but out of phase, the way they are coupled is similar to common cathode coupling in tube long tail phase inverters.

Personally I couple them in a more traditional way, but that's just a minor detail.

That amp is fine as a portable busking one, fed from a 12.6V 4.5AH or 7AH lead acid battery, or to play alone or along acoustic instruments , but not enough to play by a drummer, at all.

Later I migrated to TDA2005, same thing inside a single chip, used, among others by Peavey Solo, Fender AmpCan, Pignose Hog 20, plus Crate and tons others, so it really does fill a niche.

Of course, you need a speaker as efficient as possible, and forget low frequencies (which the light speakers can't reproduce anyway).

In fact, I opened a dusty box and found a stash of about 25 or 30 old ones, the Hitachi or Toshiba version used in car radios, so I printed a few boards and am using them.

Since my board can be wired stereo or mono, I use them as killer PC speaker drivers, a couple 6" "ceiling PA" speakers driven by about 6W RMS in shoebox sized boxes are light years better than typical  2" plastic cone speakers in cigarpack sized plastic cases fed from +5V USB  :lmao:  and of course, as busking amps.

In fact, I have one model where 2 bridged TDA2003 or a TDA2005 drive a woofer, and another TDA2003 drives the tweeter: incredibly loud and clean for its size.

That said, if you can build a wall fed amplifier it will be better, batteries are very limited.

galaxiex

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Re: Simple Preamp please check my work
« Reply #13 on: May 24, 2015, 03:16:44 PM »
Thanks JM!

Ya the Cana Kit is not bad, but the speaker I am using is only 8" with a claimed 96.1 Sensitivity.

For this, the Cana Kit amp is not quite enough....

Surprisingly the speaker does have pretty good bass response, despite the tiny cabinet it's mounted in.


Roly, My memory is ok after all... :lmao:

It clips right at 10.1V P/P feeding it with a cheap Elenco Function Generator 1k Hz sine.

This F/G uses the XR 2206 chip and I'm not sure if something is wrong or not...

The sine has a tiny notch on the neg wave. The pos wave is nice smooth.

I seem to recall reading somewhere that neg wave notch is "normal" for that chip?
If it ain’t broke I’ll fix it until it is.

Roly

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Re: Simple Preamp please check my work
« Reply #14 on: May 24, 2015, 06:01:52 PM »
Quote from: J M Fahey
The right side amp has doubled value (R4 430r vs R3 240r) because the other NFB resistor, R1 15r is in series with the other amp one , R2 15r , so it's also effectively doubled.

Both halves end up having the same gain, 240/15=16X but out of phase

Sorry, but I beg to differ.

I calculated U1 Av = x33.
LTSpice says x32.98.

I calculated U2 Av = x28.6.
LTSpice says x28.6.

Pin 2 of U2 is a virtual earth point.
Pin 3 of U1 is an actual earth point.
Therefore R1 and R2 are effectively in parallel.

LTSpice result;
attach: TDA2003 Canakit bridge.jpg
If you say theory and practice don't agree you haven't applied enough theory.