Silver Lady Guitar Preamp

[Kipper4]

I've been trying to develop a preamp with enough gain to direct record to my line in on the interface and still have that clean fender vibe.
So i borrowed the Fetzer Preamp Thanks ROG.
and followed it up with a buffer and gain stage.
I tried in vain to couple a baxandall tone stack on the end / inbetween the op amp stages but it always caused such an unrecoverable amount of gain i had to abandon the idea.
I did a quick and dirty test with my DAW Cubase SX3 and with the tone stack i was losing a full 20 + db of gain. so even when dimed the pre 's volume was too low to direct record.
I liked what the tone stack did a lot but i couldnt make it work inbetween anyway this is degenerating into a rant.
Heres my schematic.
Comments and help needed and welcome
thanks
Rich

https://www.dropbox.com/sh/7ma9n53c5jqdgsr/d-0TvwYXDL#f:Silver%20Lady.jpg

[Bill Mountain]

The buffer is excessive.  You could turn that buffer stage into an amp stage to boost before the tone stack and then your final stage will boost after the tone stack.  You will have to be careful with your staging to keep the opamps from clipping.  You could put some diode limiters in there.  It would be a hard limit that sounded better than opamp clipping and can help you in settings your gain stages for max clean gain.

[Kipper4]

i'm intrigued.
How do i do this please?

[Bill Mountain]

On my way home from work I was thinking about how I would build this and I would eliminate a whole gain stage.  I'll draw something up later tonight.

[Kipper4]

Thanks Bill i will look forward to that.
Mean while i'm spending the day looking after the chaos that is my 3 grandkids while mum and dad are at work.
Bring it on kids.

[Bill Mountain]

I had to rush this so some of the values may not be ideal.

I am by no means an expert but if I was building something like this I may build it like this:



That 50k trim for the opamp is just to set the maximum gain of the recovery stage.  The diodes are optional but I would use them to help me calculate my max output.  The diodes can be any combination of silicon, Ge, or LED diodes to give you a good max output voltage.  For example, if you use 2x regular red LED's you would have a max output voltage swing of about 3.8 volts.  I can't imagine needing any more than that for recording purposes.  To set the trim pot I would max all eq and volume knobs and adjust the trim pot until your hardest strummed chords barely started clipping the diodes.

The opamp stage has about a 40 db boost so it should easily fix any volume issues after your eq.

If you don't have a level control mixed in with the eq you could replace the 10k resistor at the end with a 1k pot.  You don't need 100k especially since what your plugging into may not be a high impedance input.  You could also make that trim pot for the opamp an exterior pot and use it like a sensitive pot for easily adjusting to different guitars.

[Kipper4]

Thanks so much Bill for the help.
I'll solder it up later and let you know how it goes.
Its very helpful having all the explanations that go with it.
I might even breadboard it first.

[Kipper4]

Yep i just breadboarded it and it works.
It has a very hot output even with the opamp 50k pot turned nearly all way down (or up) it will defoinitly need an output volume put.
How did you work out the gain (40db) for this please.
was it with the 470ohm against the potential 50k?

I might set the opamp trimmer up and then replace it with a static value resistor.
i still have a lot to learn thanks for all the help Bill Mountain and thanks the All at ROG for the Fetzer input stage.

[Bill Mountain]

Yep i just breadboarded it and it works.
It has a very hot output even with the opamp 50k pot turned nearly all way down (or up) it will defoinitly need an output volume put.
How did you work out the gain (40db) for this please.
was it with the 470ohm against the potential 50k?

I might set the opamp trimmer up and then replace it with a static value resistor.
i still have a lot to learn thanks for all the help Bill Mountain and thanks the All at ROG for the Fetzer input stage.

I'm glad it worked out for you.  The 40dB was based on the max gain of about 100x created by using the 470r and 50k pot.

There are a lot of things you could do with this design.  So keep learning what you need and I'm sure there's already a thread that'll show you how to do it.  That's how I learned.

[Kipper4]

i'm gonna perf board it tonight and tweek it from there. I'll post up the schematic later.
Thanks for everything.

[Gus]

Bill Mountain

You can remove a few parts if no EQ is going to be used
remove the 1/2 ref parts and the .1uf and 1meg.  You have have this set at for a low highpass frequency so direct coupled would be close to the same

direct connect the + input to the drain you might want to use a series resistor try 100 ohm to 1k ohm

The drain voltage will set the Vref

[Bill Mountain]

Bill Mountain

You can remove a few parts if no EQ is going to be used
remove the 1/2 ref parts and the .1uf and 1meg.  You have have this set at for a low highpass frequency so direct coupled would be close to the same

direct connect the + input to the drain you might want to use a series resistor try 100 ohm to 1k ohm

The drain voltage will set the Vref

Correct.  In fact , the OP's original design demonstrated some direct coupling of the opamp stages.  The OP had mentioned a desire to have an EQ and more volume so that's why added those parts.

[Kipper4]

Update; Well i made it on a board today and it does what it says on the tin.
I tried all sorts od diodes but my favorites are 3mm leds. I tried every kind i had that i thought might even be in the ball park of being usefull.
1n34,1n60,bat48,1n5187, 1n4148, red 5mm leds singles doubles all combinations.
depending on the diodes is how much clipping distortion i get and what kind.
Seeing as im going for a clean sound (silver face type thing) i liked the 3mm reds best.
It also depends on the biasing of the Fetzer front end but even then its not too nasty when its gritty.
I changed the 50k trimmer on the opamp for 100k for a tad more gain.
I easily get to -9db on the mixer in the daw with a hard strum.
Ideal seeing as i'm looking to get between -20 and -12.
it leaves me a little headroon to play with.
i have an issue with the treble pot on the tone stack that i need to sort tommorow night.
I'm now thinking of using the second half of the dual op amp to use as another stage so i can put an effects loop in between the two.
I havent had much luck with this from my experiments last week though so i'm not everso hopeful it would work.
I really appreciate all the input and i now know how to make a variable gain op amp stage.
I appreciate your input too Gus its taken on board and hopefully will help me out in future too.
Cheers guys.
I'll update you asap.
Rich

[Kipper4]

oh yer i forgot to say it has a switchable NPN silicon fuzz face on the front end too.

[Kipper4]

I tried a 100k and a 500k trimmer on the opamp feedback loop but it didnt really make enough differance in volume (gain) only a big differance in thermal noise. which was not needed.
I did however have some luck with the effects loop. I put a Beavis Audio Buff and blend on the output and it did a reasonable job.
Theres something about the way that it blends that doesnt sit well with me for some reason. I cant put my finger on it but it did what it said it was going to do so i guess i shouldnt moan.
Thanks again for all the help guys and i will rebuild this on another board at a later date because this trial boards looking a bit scrappy now and i'll box it up at some stage mean time heres a schematic on my dropbox.

https://www.dropbox.com/sh/7ma9n53c5jqdgsr/d-0TvwYXDL#f:Silver%20Lady%203.jpg

If anyones intrested i'll throw together an audio sample.