New Guy with an interesting design idea

[Bajaguy]

Ok, I suppose I walked right into that one. No lemmings around here, but I might be able to scare up a oppossum or something. The last time I attached something to the wife I ended up with another mouth to feed, so hanging in the breeze works well enough for me :duh

As fdor the power feed, I understand what your getting at now, and I'll correct it and repost the schematic tomorrow. Thanks again for the help and the laughs :lmao:

Baja

[Bajaguy]

I finally had a few spare hours and got the preamp and amp tied together. I figured I'd throw the schematic up and see if anyone found holes in it before I took to breadboarding it.



The more I look at the preamp circuit, the happier I am with it  :tu:
Baja

[Roly]

Yeah, well I'm thrilled to bits with it as well , but the real points are A) does it actually WORK?, and B) what does it SOUND like?  It doesn't matter if it looks wonderful on paper if it fails either of these tests.

So what I'm interested in hearing is A & B.


{um ... shouldn't the south end of the Zobel networks be grounded?}

[phatt]

Re Pwr Amp?


Not sure about this but my understanding is that the + input of those black blobs (opamps) should have a ground ref or bias voltage ref. Head scratch?

Or maybe I've missed something?
Phil.

[Bajaguy]

um ... shouldn't the south end of the Zobel networks be grounded?

Yep, cleaned up one too many line on the schematic. Thanks for the catch.

[Bajaguy]

Re Pwr Amp?


Not sure about this but my understanding is that the + input of those black blobs (opamps) should have a ground ref or bias voltage ref. Head scratch?

Or maybe I've missed something?
Phil.

Man, I'm not saying I don't have it wrong, but I took the front end of that circuit right out of the spec sheet. I looked back through it and I can't find any of the circuits that aren't this way?

[J M Fahey]

Although drawn as Op Amps , TDA2003/2005/LM383 are "different" ones.
Meant for car radios and such they were designed by a Italian Genius , Bruno Murari  :dbtu: who invented the concept of a 5 leg Power Amp chip with minimum parts count.
He succeeded immensely of course.
Part of the trick(s) was to internally design it so the speaker output always stays at 1/2 the +V voltage.
With no need for resistive voltage dividers, neither external nor internal.
http://www.idea2ic.com/LM383/

[phatt]

Arrh,,, thankyou Jaun,   Now I see da tricky bit.
As you say,, Very clever. :tu:

Sorry to Bajaguy,, carry on all is well.
Phil. 

[Bajaguy]

Just got back from 14 days on the road stuck in a hotel without internet

Had some time to do some screwing around while sitting in the hotel, so I changed up my preamp a bit to try to give the whole 1 knob tone stack thing a try. I used the stack out of a Matchless Lightning and tried my best to fold it in with what I already had. I ordered up some MPF102 and some BC549C, but found out this morning that they won't be here for another week. I had a couple of questions about this one:

I left the gain from the first circuit alone and added the matchless stack in behind it, do I need that second FET behind the circuit to buffer the impedance for the input of the TDA2005, or will it work fine the way it is drawn? I really like the way this tone control works on my buddy's tube amp (lots of fat tone), but I'm not sure if it will sound the same with the 2005?

Here's the circuit:


Thanks ahead for the input, man am I glad to be back in touch with the web!

Baja

[J M Fahey]

14 days on the road stuck in a hotel without internet

Well, if you got stuck in this unconnected middle-of-nowhere crappy hotel.



in this crappy room



and with only your Company's *boring* Financial Manager to share it (we all know how dull Accountants are )



yes, I would also have felt depressed, very depressed  :lmao:

[Bajaguy]

Man, I want your job!

I was stuck in a hotel downwind of the dairy, with a train that came through every night at 2:30 am.

Your idea sounds way better.... :dbtu:

Baja

[J M Fahey]

I was stuck in a hotel downwind of the dairy,

Ouch!!

I live in Buenos Aires but I'm from an agricultural town in the middle of the Pampas.

*Many* times I was stuck driving for many miles in a narrow 2 lane highway behind a "cage" truck full of cows being carried to the slaughterhouse.

Man, did I have to "breath nature" !!!  xP  xP  xP

[Roly]

Man JM, where do I sign up?  I could use a coupla weeks months years of that.


{ahem} returning to more mundane matters...

Tone controls don't come much simpler than the ol' Top Cut, but all tone controls like to to be fed from a low impedance and work into a high impedance.  Is the input impedance of this chip amp high enough?  Dunno, would have to look at the data sheet; but given that the drive impedance is very low there is no reason to use such high values for the volume and tone pots (and is C10 really 1uF?  Seems very large to me).  As drawn the tone control will interact with the volume control having its greatest effect at mid volume and none at maximum.

I'd suggest if you want to stick with a single knob control you have a look at the Big Muff circuit;



...and place it ahead of the volume control so it is less dependent on the volume setting.

And again, the resistances can be scaled down and capacitances scaled up to get a lower impedance.

See Duncan's TSC.

[Bajaguy]

I like the big muff stack, but I read that it was lossy and needed a recovery stage before the power amp. I like the way it looks on the stack calc, but will I need another gain stage to compensate?

Baja

[Roly]

The simple top cut is about the only (partial) exception to the rule that passive networks can't boost anything, they can only cut less, so any passive control that has "boost" must have insertion loss as a starting feature, generally about 20dB, and it "boosts" by attenuating less; so normally some make up gain is required somewhere, generally following as in my original circuit.

However, if the following chip amp itself has a lot of gain then a make up stage might not be required.

One of the attractive things about the Big Muff over a simple top cut is that there is a lot more scope for tweeking the resistance and capacitance values to get more or less mid-band scoop or to move it up or down.