Preamp Choise

[Shakey 1047]

I don't know if this is the right section but any way, I have several preamp kits, one of which I want to use as a booster for my guitar, just a small increase of maybe 3-5 dB. What I want to know is, having both inverted and non inverted preamps does it matter which I use, is one better than the other???
Cheers Dave.

[Jazz P Bass]

How do you invert a sine wave?
I realize you are refering to how the opamps inside the circuit are configured.
But I do not think it would matter, as the input is a sine wave & so is the output.

[Shakey 1047]

Thanks Jazz, much appreciated, basically what I want is a buffer but with a little extra gain so I'll use an NE5534 in the first board I pick up. Cheers Dave.

[J M Fahey]

Use a non inverting configuration.
It will:
1) keep the phase of your signal.
2) much more important: it will naturally have a higher input impedance.
Getting a classic 1M input impedance will only need hooking a 1M resistor from the + input to ground; the guitar signal is still going straight to such pin for low noise.
In an inverting configuration, that 1M resistor needs to be in series, i.e. between the pickup and the Op Amp "-" input, raising noise.
And for, say, 3X gain the feedback resistor also needs to be very high; 3M3 in this case.
In a nutshell: for what you need, non-inverting is better.

[Steve Conner]

The NE5534 maybe isn't the best choice for guitar use. It's designed for lower source impedances like microphones and pickup cartridges, and has a hefty bipolar input stage with quite a high input bias current, hence a high noise current too.

With a high impedance source like a guitar, the TL071 might well be quieter or sound better. The OPA134 or OPA604 would be an audiophile choice.

[pdf64]

I've had very good results with OP27 for this type of booster application; seemingly no noise and it doesn't mess with the tone.

[Shakey 1047]

Thank you all for your replies, now all I need is to know where to get OP27's

[Shakey 1047]

pdf64, I don't suppose you have a circuit for which you use the OP27's??

[pdf64]

I'll sketch it out and post it tomorrow.

[Jazz P Bass]

Use a non inverting configuration.
It will:
1) keep the phase of your signal.
2) much more important: it will naturally have a higher input impedance.
Getting a classic 1M input impedance will only need hooking a 1M resistor from the + input to ground; the guitar signal is still going straight to such pin for low noise.
In an inverting configuration, that 1M resistor needs to be in series, i.e. between the pickup and the Op Amp "-" input, raising noise.
And for, say, 3X gain the feedback resistor also needs to be very high; 3M3 in this case.
In a nutshell: for what you need, non-inverting is better.

Thanks for the excellent post, JH.
It never occurred to me just what the implications where.

[pdf64]

See scan for booster schematic.
None of the values are very critical, most values can be halved or doubled without significantly affecting operation.
The ground return of the battery can be connected to the 'middle' terminal of a stereo input jack socket; then when a mono jack plug is plugged in, the battery gets its connection to ground.
With an OP27, the circuit should be virtually noise free and clean, so can be left permanently in the signal path, say to raise the level of a strat up some.
Pete.

[Shakey 1047]

Many thanks for the diagram Pete, I shall look forward to building it. I have to go to Derby next weekend to get the parts from R F Potts as we have nothing in Leicester except Maplins, and only an idiot would pay 21p for resistors that cost 2p at Potts. Anyway thanks again.
Dave.

[oc disorder]

now all I need is to know where to get OP27's

The OP 27GP is available from Futurlec @AU$1.67


Components Op-Amps&main_menu=IC&sub_menu=ICOP-AMP

Datasheet there too.

GENERAL DESCRIPTION
The OP27 precision operational amplifier combines the low
offset and drift of the OP07 with both high speed and low noise.
Offsets down to 25 mV and maximum drift of 0.6 mV/∞C, makes
the OP27 ideal for precision instrumentation applications.
Exceptionally low noise, en = 3.5 nV/÷Hz, at 10 Hz, a low 1/f
noise corner frequency of 2.7 Hz, and high gain (1.8 million),
allow accurate high-gain amplification of low-level signals. A
gain-bandwidth product of 8 MHz and a 2.8 V/msec slew rate
provides excellent dynamic accuracy in high-speed, dataacquisition
systems.
A low input bias current of ± 10 nA is achieved by use of a
bias-current-cancellation circuit. Over the military temperature
range, this circuit typically holds IB and IOS to ±20 nA and 15 nA,
respectively.
The output stage has good load driving capability. A guaranteed
swing of ±10 V into 600 W and low output distortion make the
OP27 an excellent choice for professional audio applications.

[pdf64]

It's good to support local shops, if they've got what you need.
However, Farnell or RS are the big 2 parts suppliers in the UK. Neither have been making a delivery charge to me for a while, even for low value orders (~£15).
By the way, best to use film/foil or metalised film for all caps other than the large value >1uF (electrolytic or tant) or small value <500pF (ceramic) ones.

[Steve Conner]

While we're at it, how much does it cost to get from Leicester to Derby, and once the cost of travel is taken into account, wouldn't it be more cost effective to just get the parts from Maplin?

RS and Farnell are cheaper but I don't think they sell resistors singly.