Gain and grounding: have I got it right?

[ohnoezitasploded]

Hi Everyone,

I've spent a lot of time reading this forum, and I think I understand how to apply the ideas here to my scenario. But if you would read over my approach and critique, I'd really appreciate it:

My sister has an electric cello, the internals of which consist of a pickup and 1/4" jack-- it doesn't contain any active amplification. I am building her a combination practice amplifier and pre-amplifier for when she has gigs. I'm using the TDA7052A chip IC which is simple and sounds nice.

I don't want to screw around with mains voltage at all, so I am using a 9V wall wart with a jack onto it which will plug into the enclosure. Then I'm stepping the voltage down to the chips' ideal 6V using a 7806 regulator. I'm building it into a conducting enclosure (metal box).

So for the grounding, I should star-ground everything internally, but not ground the chassis to the signal ground, right? There's no mains voltage inside the chassis so there's no safety concern, and the signal ground isn't connected to earth ground, so it wouldn't help shield the internals from RF, is my reasoning.

Also, as far as gain, it's variable with a pot up to 40x. That's about the right range to take a cello pickup up to line level, right? The chip will drive a small speaker from a tape head, so I'm thinking it would be about right for this as well.

Lastly, anyone have a clever method of heatsinking a dip-8 package? This little guy gets hot at the full 1W output.

Suggestions, critiques, rude gestures all appreciated!

[pacificblue]

Originally Posted by ohnoezitasploded

I don't want to screw around with mains voltage at all, so I am using a 9V wall wart with a jack onto it which will plug into the enclosure. Then I'm stepping the voltage down to the chips' ideal 6V using a 7806 regulator.

Why bother? The TDA can work from as much as 18 V.

Originally Posted by ohnoezitasploded

So for the grounding, I should star-ground everything internally, but not ground the chassis to the signal ground, right?

Rephrase that to, you don't need to ground the chassis, but it helps. Such a connection adds virtual mass to the ground and makes it more stable.

Originally Posted by ohnoezitasploded

Also, as far as gain, it's variable with a pot up to 40x. That's about the right range to take a cello pickup up to line level, right?

That is a question you'd better put in the Musical Instruments section of the Forum.

Originally Posted by ohnoezitasploded

Lastly, anyone have a clever method of heatsinking a dip-8 package?

How about self-adhesive heatsinks? They are widely available for computer RAMS, but also for DIL packages.http://www.conrad.de/ce/ProductDetai...FB.EPS_400.jpg

[madtecchy]

1 Watt to amplifiy a cello at a gig????? Unless you use a very sensitive speaker im guessing the unamplified sound from the cello will be louder than your 1 watt with the tiny speaker you propose.
Regards Mad Mark

[ohnoezitasploded]

Originally Posted by madtecchy

1 Watt to amplifiy a cello at a gig????? Unless you use a very sensitive speaker im guessing the unamplified sound from the cello will be louder than your 1 watt with the tiny speaker you propose.

Hi Mark,
It's an electric cello. There's a pickup inside like an electric guitar, but there's no sounding board, so the cello doesn't make any noise without an electric amplifier.

Originally Posted by pacificblue

Why bother? The TDA can work from as much as 18 V.

Hi PacificBlue,
That was my initial thought since that's what the datasheet says, but I've found in practice that it really wants 6V-- anything above that and it gets really, really hot. Using the 6V regulator has the additional advantage of removing ripple current from my cheap power supply, so there's that.

Originally Posted by pacificblue

You don't need to ground the chassis, but it helps. Such a connection adds virtual mass to the ground and makes it more stable.

It seems like since it's not connected to earth ground, any voltage the chassis came in contact with would affect the signal, rather than being drained to earth, or am I wrong?

After thinking about the gain issue, the maximum gain of the chip is set to 40x, so if I end up needing more than that I'm sunk anyway. But, if I put a 10K pot on the incoming signal, she can trim the incoming signal to a lower range if she needs it.

Thanks much for your informative reply!

[pacificblue]

Originally Posted by ohnoezitasploded

It seems like since it's not connected to earth ground, any voltage the chassis came in contact with would affect the signal, rather than being drained to earth, or am I wrong?

It is actually the other way round. Since it has no contact to earth, there should be no interaction. Pretty much like birds that can sit on high-tension power lines without being affected, because they do not come into contact with earth potential at the same time.

[ohnoezitasploded]

Originally Posted by pacificblue

It is actually the other way round. Since it has no contact to earth, there should be no interaction. Pretty much like birds that can sit on high-tension power lines without being affected, because they do not come into contact with earth potential at the same time.

That makes total sense, thanks.

[madtecchy]

HI
I assumed it was an electro acoustic.
I would suggest an electric guitar preamp/ buffer as pickups can be affected by the wrong input impedance. Then connect output of preamp/ buffer to small amplifier. This will allow you a much wider choice of more suitable amplifier chips.
Mark

[jaycee]

40x gain might be out of range for that chip, and causing oscillation...hence why the chip is getting so hot. You might also find that that 9v wallwart, if it's unregulated, is producing a much higher voltage and thats why the chip is hot, although I wouldnt expect it to produce more than 14v even off load.

[ohnoezitasploded]

Originally Posted by madtecchy

HI
I assumed it was an electro acoustic.
I would suggest an electric guitar preamp/ buffer as pickups can be affected by the wrong input impedance. Then connect output of preamp/ buffer to small amplifier. This will allow you a much wider choice of more suitable amplifier chips.
Mark

Hi Mark,
I've been reading up on this, and it's sounding like I might want something more like a traditional guitar preamp circuit. Which makes sense, actually. I'm going for something that sounds decent and is pretty rugged-- that's what attracted me to the TDA7052A.

So based on that criteria, do you really think I would benefit from a separate let's say FET based pre-amp? Or should I say good enough with what I've got and start building the circuit?

Thanks again for the help!

[ohnoezitasploded]

Okay, having bought some parts and read a lot more about this, the signal path I'm looking at is:

1. a low pass filter, with the corner frequency set at about the limit of human hearing.
2. a discrete FET buffer amp, with a 3Mohm impedance input, to handle the piezo pickups.
3. The output jack, so connected so that plugging a cable into the jack disconnects the speaker. Otherwise, it's in practice amp mode, so the signal is passed to:
4. The TDA7052A amp and 8ohm speaker.

Attached is a schematic of the above as I'm planning to do it. I tried to breadboard it but with all the wires everywhere it's more of a radio antenna than anything right now. I'm going to try making the pcb and wiring one up, let you know how it goes.