TDA2822 doesn't play nice with op-amp buffer

[antonis]

Sorry but you lost me..
We're mixing many different topics and I can't follow you..

From closed-loop input impedance viewpoint, there is no practical difference between JFET and bipolar op-amps..
What counts is input bias current, hence bias configuration resistance for Vbias offset..
(ignoring capacitances, MosFets can be biased with virtually infinite value resistors..)

For BJT inputs, you have 2 distinct (and numerous between them) choices:
1. Use the lower, for affordable drop, value bias resistor and bootstrap it..
2. Use high value bias resistor and offset Vbias accordingly to voltage drop across bias resistor..

P.S.
Of course, No 2 has much more uncertainty than No 1..

[fryingpan]

What I meant is, if you need more than so much input impedance, with a bipolar input you will probably have more noise or more offset, or a combination of the two. So effective "useful" impedance is lower. In fact I've always seen bipolar input opamps used in line level/low impedance applications, and FET-input opamps for the rest. (In audio).

[PRR]

> using the TL072 because the NE5532 sounded noisier.

Just for fun, try shorted input. (Or 150ohm dynamic microphone.) The 5332 should now be the quieter (less hissy highs) choice.

Yes, the TL072 is often best (or best cheap) input for guitar.

Then why use bootstrapping in the first circuit?

Because that is an input for PIEZO transducers. Whole other thing from wound guitar pickups. With weeny piezos any loading drops the bass out, gutless, no body.

[fryingpan]

> using the TL072 because the NE5532 sounded noisier.

Just for fun, try shorted input. (Or 150ohm dynamic microphone.) The 5332 should now be the quieter (less hissy highs) choice.

Yes, the TL072 is often best (or best cheap) input for guitar.

Then why use bootstrapping in the first circuit?

Because that is an input for PIEZO transducers. Whole other thing from wound guitar pickups. With weeny piezos any loading drops the bass out, gutless, no body.

Yes, I know, but I mean, you can use a FET-input and whatever large resistor (be it 47Mohms, even), with basically no bias current you can use one and you don't need to go through the trouble of bootstrapping.

Or, even better, use a FET-input opamp for a charge amplifier.

[antonis]

use a FET-input opamp for a charge amplifier.

Do you want to drive any piezoelectric accelerometer or what..??

P.S.
YES, you are free to use an FET-input op-amp..!!
Nobody will stand in your way..!!
    

[fryingpan]

use a FET-input opamp for a charge amplifier.

Do you want to drive any piezoelectric accelerometer or what..??

P.S.
YES, you are free to use an FET-input op-amp..!!
Nobody will stand in your way..!!
    

No, I actually see the advantages of using a bipolar-input opamp. Source impedance is low, noise is low, the insertion of a large value resistor in the circuit is probably inconsequential and you get more part choice. Still, as Rod notes, with a charge amplifier (which might seem overkill) you get even lower noise.

[Ksander]

Anyway, here is the 'Moonstone':  two watts of power packed into a 12x12x8cm enclosure, blasted out by a 4" speaker. It features a bell filter (thank you Rob Strand!), and it sounds great. Thank you all for helping me make this 

[ElectricDruid]

Then why use bootstrapping in the first circuit?

https://sound-au.com/project202.htm

According to the formula, assuming even a gain of ten (20dB), and assuming that an audio opamp can easily have 100dB open loop gain (the 4580 states 90dB minimum, 110dB typical):

Zin = 300000 * (100000 + 10) / 10 =
approx. 3Gohms. Even the lowest capacitance piezo pickup may require, say, 100Mohms load impedance for full bandwidth.

(If you use the above formula with 90dB open loop gain, approx. 30000x, then it still works out at 900Mohms).

This is why we generally just *ignore* the impedance of the op-amp itself when we're working things out. It's just regarded as "very large" and then forgotten about. The *other* resistances in the circuit will all be far more significant in the vast majority of cases. The op-amp input is taken as "infinite" ("mostly", "generally", "for practical purposes", etc etc).