G & L Cavalier circuit

[JebemMajke]

Recently I've been searching for a perfect preamp for my neck pickup, in the bridge i have EMG Hz 3 with that DIY emg 81 circuit ( revolutniodeux blog spot ). It sounds wicked.

Now I've found this Cavalier circuit and i like it's sound, but it's gain is insane. My guess is that's due to the fact that in the original guitar there are treble, bass and vol pots in front of the circuit. How can i reduce it's gain?

Here is the schematic

[samhay]

^How can i reduce it's gain?
Reduce the value of the 220k resistor in the feedback loop of the op-amp. Gain is current 23x ( 1 + 220k/10k ).

Edit - changed gain equation to more convention form to prevent any more confusion.

[Kipper4]

I'd either change the 220k in FBI loop 2 to a lower value or
Put a bigger limiting resistor on the output(1k in the schemo) or put an output pot before the switch however you might sacrifice some tone.
That's where I'd start. Probably not the best solutions but I will wait and learn what others have to say.

Optionally you could put a 250k pot in feed back loop 2 I guess

[JebemMajke]

I'll try 100 k in there ( instead of 200 k)

Thanks

[Kipper4]

Question?
Why isn't the gain 24x ?

[JebemMajke]

Ok

2k resistor equals no hiss ... but low volume. So 4,7k is next on the test tomorrow.

I really liked it with 20k in there with humbuckers, but with single coil it was a disaster ....

[MaxPower]

Question?
Why isn't the gain 24x ?

samhay posted the equation (Rf+R1)/R1.
Written differently:  Rf/R1+1.

Where R1 is the 10k resistor and Rf is the 220k resistor.

[samhay]

Ok

2k resistor equals no hiss ... but low volume. So 4,7k is next on the test tomorrow.

I really liked it with 20k in there with humbuckers, but with single coil it was a disaster ....

Is that the feedback resistor (originally 220k)?
If so, perhaps you could make it switchable.

[Kipper4]

Question?
Why isn't the gain 24x ?

samhay posted the equation (Rf+R1)/R1.
Written differently:  Rf/R1+1.

Where R1 is the 10k resistor and Rf is the 220k resistor.

Thanks I've not seen it written thus before.

[JebemMajke]

Thing is , the single coil bastard has a lot of noise with 20k ( yep feedback resistor ), and works ok with 4,7k, but the amount of volume/gain is pretty much the same as when the circuit is bypassed. Maybe a bit more treble is added with the circuit on. So I guess I'll place 18k in that place and leave it with my humbucker in the neck position, on the other guitar. And the sing coil will stay passive.

I guess that with a regular single coil it would work fine with 10-15k feedback resistor, but i have bill lawrence black label s1, which on it's own is great, but it lack's that oomph when i switch from active humbucker to passive single coil.

[JebemMajke]

Right now I'm going through this emg 81 like circuit that's been mentioned on revolution deux blog

Schematic


For some time now I've been using this thing and it did sound great with everything that i threw at it.

I would like to make it a bit smaller than it is now. And to use tantal cap in the place of that 1uf non polarized. So my question is, how to orient it? My geuss is that the negative side would be oriented towards the exit.

[Gus]

The first circuit looks like too much gain at 9VDC.
Does the 200pf really have no connection at the one side?

When I have built an opamp based boost at 9VDC with a circuit like that I sometimes set it at a low gain
so for the first one you could make the 10K a 220k and the 10uf tant a .47uf cap (from pin 2 to ground RC)
or  100K and 1uf
The opamp used has a current programming input so it might not be a good idea to lower the 220k feedback resistor  I would increase the 10K value

Also you need to sometimes change a passive network with different pickups
there are reasons you will find 250K, 500K 1meg etc. guitar volume and tone controls in different guitars with single coils and humbuckers
And when you change the controls values you often need to change the cap values

Not knowing what the G&L pickup inductances are this make it hard to model what the passive part of the G&L circuit was designed to do.

However you could LT spice etc model the pickups you are using and try different passive tone  volume circuits before the active circuit.
The input resistance of the active part is 1.1meg kind of like an amp but without the added cable cap and 68K etc parts

what was the issue(s) with single coils?

[JebemMajke]

"what was the issue(s) with single coils?"

Well, either it had too much noise, or no boost at all.

With humbuckers it sounds really nice.

Especially in the neck position with strings 13-65, mahogany body, A> drop G tuning. Brings out the clarity. And since I've lowered the gain it works well with GE Fuzz Face, which naturally wouldn't

[samhay]

^The opamp used has a current programming input so it might not be a good idea to lower the 220k feedback resistor  I would increase the 10K value

Good point Gus. OP - are you using a LM4250 op-amp?

[JebemMajke]

Yep.

I've ordered 2 and 10 of tl072's last week, and the guys from the shop sent me 10 4250's and 2 tl072's. And since 4250 is 1 euro and 20 cents in my country and tl072 25 cents that was a good thing for me, financially that is

[JebemMajke]

Edit

replacing 10 k with 220k worked fantastic.

Thanks

[PRR]

> gain 24x ?

NO idea how you get 24 from 10K and 220K.

> looks like too much gain

+1. Way too much gain.

I'd put the "220K" at 22K". Gain of 3 (3.2) is reasonable for an amp taking guitar and eating 9V. Gain of 22 (or 23 (or 24)) seems much-too-much.

What does 22K do for you?

> The opamp used has a current programming input

Good point; but the LM4250 data-sheet shows 8uA ((9V-0.5V)/1Meg) gives HIGH current performance, ample output drive.

22K+10K will be significantly lower resistor hiss than 220K+220K; however the LM4250 is such a hissy little beast that it may not make much difference.

[JebemMajke]

Thing is 220k+220k =less his than 10k+22k. I am not sure how it should work on paper, but in practice 220k+220k work better for me.

Sincerely

[samhay]

220k / 220k, gain = 2
22k / 10k, gain = 3

more gain = more signal noise

[JebemMajke]

Well it does lack gain compared to the bridge pickup, but I like it this way, because it's loud enough, but i can use germanium fuzz face without it sounding like a bucket of noise and crap.