TDA2030A success! Dumb Question - how to wire a headphone jack... - diyAudio
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Old 17th January 2004, 07:40 PM   #1
Morse is offline Morse  United States
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Default TDA2030A success! Dumb Question - how to wire a headphone jack...

Hi everyone and thanks up front for any help or advice you can offer!

The title says it all - as of just after 3AM local time, my TDA2030A went online with both channels - and no oscillation. The "fix" was just to add the 100uF PS bypass caps directly to the pins of the chips and to substitute 26 ga wire (made it easier to wire it up outside the chassis and "drop" it in). As of now, I'm giving it a shakedown cruise with "One Hit Wonder's of the '60's" and my RS40-1354/RS40-1270D speakers (played it first with a pair of $2 car speakers just to find out it worked okay...). DC offset measures 2mV in one channel and 3mV in another. Is this okay for a cheap chipamp? Hum measures less than 1mV (off the scale at the bottom end!).

Now for the question. Originally I was going to wire in a OPA2134 based chipamp to drive headphones in the same chassis. I've already got the headphone jack mounted. Now, in keeping with the theme of "cheap 'n sleazy" that this <$45USD amp (about $25 of that on chassis and fittings...) was designed around, it strikes me that I could just use a voltage divider to the headphone out and wire the speaker out's to the switching (NC) jack so that when I put in the headphone plug the (reduced voltage) output will be diverted to feed the 'phones.

Is there a problem with that, and if not, what sorts of values would you recommend for the voltage divider? Should I wire 'bounce' caps between the contacts of the "speaker switch" so that it doesn't pop? Could that affect stability of a chipamp?

Sorry for the dumb questions, but normally I would just a. use a different OPT from a valve amp or b. use a discrete chip (a'la the original plan). This is an exercise in being REALLY CHEAP.

Thanks for any help or advice in advance!
All the best,
Morse
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Old 17th January 2004, 08:15 PM   #2
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For 8 Ohm headphones I would try a 270 to 330 ohm resistor in series with each headphone - Amp -> 330 -> 8 -> Gnd.
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Old 24th January 2004, 11:08 PM   #3
johnnyx is offline johnnyx  United Kingdom
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Default headphone jack

Iwouldn't use a switching jack socket to switch the speakers. Use a proper switch for the speakers, you can leave the resistors for the headphones permanently connected. Switched sockets are used in many low-cost commercial amps, and often the sound cuts out altogether. Peace of mind for the price of a switch
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Old 25th January 2004, 12:34 AM   #4
Morse is offline Morse  United States
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Default 'Fraid it's a fait accompli by now....

Hi JohnnyX;

Thanks for the tip; as it turns out I've had the headphone jack installed and running for a little while now. Yep, it's a switching jack (both NC and NO switches).

I ended up using a 150r/10r voltage divider and installed another copy of the output zoebel on the 10r leg of it (just like on the 8r speaker output). So far so good - it's a really nice looking jack that I picked up in a small shop in Hiroshima over the holidays. Wish I would have bought a few more of 'em....

If this were a "serious amp" I'd follow your advice (and if it starts causing problems I will take your advice and rewire it with a switch!), but for now I'll leave it as is and get on with my next project.

That's an LM1875 noninverting amp. Still haven't decided if I'll use another of my 'cheapskate special' enclosures or if I'll bite the bullet and build something "special" for the occasion (thick aluminium and real wood....); then there's the question of a ps trafo - toroid or EI core - and how many VA for the best performance. Decisions, decisions....

Ideas anyone?

All the best and thanks again!
Ja mata,
Morse
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Old 26th January 2004, 10:18 PM   #5
johnnyx is offline johnnyx  United Kingdom
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Default LM1875 PSU

I used an 80VA toroidal transformer, with 0-18v,0-18v secondaries, a 25A, 200v bridge, and 2 off 4700uF reservoir caps for the power supply. It worked well, powering 4 LM1875 boards. Maplin used to sell pcbs for them, but they have severely cut back on the types of components and kits. I used them to try out different active crossovers. I ripped the guts out of an old commercial amp - I forget which - and built it in that. It was "experimental" but it was stolen in a burglary a few years ago,
I admire the work other members have have done, totally amazing design & finish, but it aint me. I do "experimental"
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Old 27th January 2004, 02:43 AM   #6
Morse is offline Morse  United States
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Hi JohhnyX;

>>>...I used an 80VA toroidal transformer, with 0-18v,0-18v secondaries, a 25A, 200v bridge, and 2 off 4700uF reservoir caps for the power supply. It worked well, powering 4 LM1875 boards...<<<

Thanks for the tip! I've been looking at an Avel Lindberg with 18V-0-18V secondaries that's rated to 160VA - I guess it'd be overkill for the LM1875! Maybe if I bridged 'em to make a 2 channel amp, or perhaps a 4 channel amp set up for biamping....hmmm....

FWIW, I'm currently using a 25.2VCT 50VA rated EI core on the TDA2030A. The high end extension is really quite a lot better than when first built - I 'oopsed' a calculation and was knocking down the high end rather harder than necessary with the RC feedback loop that's used to attenuate RF oscillations via NFB. That's all fixed now, though.

>>>...Maplin used to sell pcbs for them, but they have severely cut back on the types of components and kits...<<<

It's the same story here with Radio Shack. It looks like DIY'ers are going to be stuck special ordering our parts rather than being able to buy 'em locally. On the bright side, I've been stocking up my parts bins with all the "junk" Rat Shack is discontinuing. Tomorrow after work I'm going to check out one a few miles from here and see if they still have anything worth buying before it disappears forever. If not, I'll just use it as an excuse to restock my 1/4 watt resistors and ceramic disc caps bins with some combo packs.

>>>...It was "experimental" but it was stolen in a burglary a few years ago...<<<

Sorry to read that!

Thinking about it, for an extra $4USD I can go up to a 50VCT, 250VA toroidal PS trafo - think I ought to consider using LM3886's instead?

Maybe for the LM1875, I could just use a 25.2VCT EI along with a 12.6VCT EI; I'd just parallel the primaries and series connect the secondaries (after ensuring correct polarity!) - then make sure that the average current draw stays under the 2A of the 25.2VCT trafo. That'd give me 37.8V @ 2A (with a fullwave bridge and the centre taps on the trafos left floating!) for a pair of LM1875's.

What do you think?

Thanks again for the tips!
All the best,
Morse
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