Help with circuit: Acoustic guitar preamp based on Fender

[Mikheil]

I would like to build acoustic guitar preamp based on Fender Acoustasonic circuit.
I got 15V power supply and some TL072, I need one-channel preamp with line-level output of 600-700mV.
Could you help me to understand, following the input and EQ (U1) which part (op-amps) of the circuit to use and how ?

[J M Fahey]

Hi Mikheil.
Not recommended .

This is a quite complex Acoustic Guitar preamp, lots of features which are not big deal if assembled in some robotic factory but deal killers in a homemade one.
To boot it has two very different channels, yet you don´t indicate *which* one do you want.
What is your intended purpose? to drive an external power amp or powered cabinet or send line out to a mixer?
You intend it as a floor unit or built as, say, some kind of rack preamp?
Will you drive it straight from a raw Piezo pickup or passive Dynamic capsule or you already have some kind of preamp or active pickup built into your guitar?

To boot, you´ll have to *design*, print, etch and drill your own PCB, not an easy task and which takes some time to learn.
I suggest you build the *excellent* sounding yet simple preamp found in the Peavey Solo acoustic guitar amplifier.

The reason why I am trying to avoid the straight Fender design is that although it´s excellent, is way too complex but much worse: uses very hard to get (not only in Georgia but most anywhere else, including most of Europe) C/reverse Log taper potentiometers which are a staple in Fender designs but which, of course, they custom order by the thousands; not easily available on standard Electronic parts vendors.

[Mikheil]

Log taper pots: what they do such an exclusive to the circuit?

I would like to build a circuit with just two TL 072 op-amps and get a line-level output of 600-700mV. The Acoustasonic schematic is much larger and complex and little bit hard to realize which 'spot to cut' after input and EQ sections.

Here is the question:
Could you please share your thoughts on the issue and indicate those particular op-amps with R-C periphery that would suit the application with standard 10kOhm input impedance amplifier?

Attached is the option...

[eschertron]

If the volume pot is a 30% log reverse taper, I'd source a linear taper to replace it.

[Mikheil]

Concern about the EQ section: I have never seen such a small EQ circuit of just 2 caps, does anyone have? should it be switched with other TL072 of the circuit?
Thank you!

[MarkusBass]

Concern about the EQ section: I have never seen such a small EQ circuit of just 2 caps, does anyone have? should it be switched with other TL072 of the circuit?
Thank you!

I've seen it thousand times. This is typical cost-saving design. I thought that you already decided to build preamp based on this schematic. And now you suggest that the schematic is incorrect . I can assure you that the schematic is correct (they build many amps using this schematic). Such a design (three tone pots and one opamp) is very popular on the internet.
If you want other schematic, use other schematic.

EDIT: more info at: http://www.diyaudio.com/forums/instr...ed-fender.html

[Mikheil]

... And now you suggest that the schematic is incorrect .......

Hi!
I do not say the circuit is incorrect. As you see, I have cut U2 op-amp from the next section and switched it to U1 myself, so, the question was ob this issue on whether I did it right way ???
Sorry for confusing ...... and thanks for comments!

[J M Fahey]

You missed the:

C/reverse Log taper potentiometers which are a staple in Fender designs

part

That said, you *may* replace it with, what else? a Linear pot but be warned that you will have a lot of gain variation on a small part of the pot range.
Say, not much effect from 0>7 and a big jump from 7>10
I suggest you use a larger diameter Volume knob which will make handling easier.

The block you cropped is fine, I thought you needed other features included in that amp.

The phase inversion section and switch is not needed, since you are building a single channel preamp, but won´t hurt either.

To get 600 mV out with everything else being the same, increase R10 to 33k and you are done.

Those Op Amps require +/- 15V on pins 8 and 4 respectively; IF it´s the front end for an amplifier, no big deal; if it´s intended as a belt pack (or "pedal" case) preamp it complicates things a little, you might need a +9V to +/-15V converter.

If power amp input impedance is 10k, then replace R23 with a 100 ohm resistor.

Good luck.

[Mikheil]

[QUOTE=J
To get 600 mV out with everything else being the same, increase R10 to 33k and you are done.

Those Op Amps require +/- 15V on pins 8 and 4 respectively; IF it´s the front end for an amplifier, no big deal; if it´s intended as a belt pack (or "pedal" case) preamp it complicates things a little, you might need a +9V to +/-15V converter.

If power amp input impedance is 10k, then replace R23 with a 100 ohm resistor.

Good luck.[/QUOTE]


Thanks!
Yes, the negative feedback resistors do regulate the output volume of op-amps but the other rule is that the final section op-amps are used as filters requiring both input and feedback resistors to be of the same value, isn't it ?