General Question Regarding OD build/Output stages!

[mrahc_jftd]

Hey hey!

With many thanks to the kind users on these forums, I've gotten an OD that I've been trying to put together really coming along now!

So my query is probably an easy one for you all but here goes -

I find that when the gain is all the way down and volume all the way up. It still doesn't boost the signal like it does on OD's that I've played.

I'm feeling that it's because the schematic I'm going off doesn't actually have an output buffer/stage haha. Just a volume pot that comes straight out after the Tone Pot.

Main two questions really are simply - is it true that my issue is just because there is no output stage included in the circuit & secondly are there any output stages you guys would recommend trying out?

Thanks everybody.

(See schematic attached)

[willienillie]

An output buffer won't add any gain or volume, what you seek is a second amplifying stage.  You can use the other half of the TL082 for that.

Also, C3 is backwards.

[mrahc_jftd]

Very cool! Thank you for the advice, I'm keen to have a tinker!

I did notice that actually when initially putting it together, thankfully I just thought I'd put it in the normal way just incase haha!

[idy]

Usually this kind of stage has a resistor in series with the gain pot so you are never completely shorting the output to the input. If you put a 1k (or more) there you will find minimum gain still gives some boost.

[antonis]

I thought we've dealt with that before.. 

https://www.diystompboxes.com/smfforum/index.php?topic=125668.msg1197912#msg1197912

[Steben]

Usually this kind of stage has a resistor in series with the gain pot so you are never completely shorting the output to the input. If you put a 1k (or more) there you will find minimum gain still gives some boost.

The diodes will start to conduct at +/- 0.6V, that is 5 times as high as a decent single coil chord.
This means the opamp stage can have a minimum gain of 5 without overdrive effects with single coils.
A Humbucker can go way beyond that of course... But a gain of 2 as a minimum wont give an audible effect.
sd-1's have 33k fixed feedback resistor, which means a min gain of 8, ts9's have 47k fixed giving min gain of 11.
On the other hand, you can always touch a little the gain knob as it pleases....

[Vivek]

I see Mark Hammer's STUPIDLY WONDERFUL TONE CONTROL again !!!

[iainpunk]

The diodes will start to conduct at +/- 0.6V, that is 5 times as high as a decent single coil chord.

i don't know, that sounds awfully low. i haven't tested it yet, but that sounds a bit underpowered. especially taking the "black ice" in to account, a passive Ge clipper (+/- 0,3V) that replaces a tone control capacitor, it supposedly also works on single coils, but i'm sceptic of both your and their claim... ill be testing that this afternoon.

I find that when the gain is all the way down and volume all the way up. It still doesn't boost the signal like it does on OD's that I've played.

I'm feeling that it's because the schematic I'm going off doesn't actually have an output buffer/stage haha. Just a volume pot that comes straight out after the Tone Pot.

Main two questions really are simply - is it true that my issue is just because there is no output stage included in the circuit & secondly are there any output stages you guys would recommend trying out?

if i were you, i'd put a big muff recovery gain stage on the end, after the volume control. this strengthens the output and boosts the signal significantly without adding clipping. it also provides you with a nice bit of transistor asymmetry and nonlinearity that sounds ever so sweet, something an op amp doesn't do.



cheers, Iain

[Vivek]

Is there any way to get linear gain from one Opamap along with the distortion ?

I mean,

If normally the peak output will be equal to 0.7V

Can be modify the same one Opamp to get peak output of 0.7V . x




Or in other words, suppose the transfer function is F(z), can we alter circuit to get transfer function of F(z) . u ?


or

Make one IC output same as (a clipper stage followed by a linear gain stage of gain u )  ??

[iainpunk]

more diodes in series
series resistance with the diode
higher minimum/overall gain.

>higher clipping threshold
>gain after the threshold is reached
>higher gain before threshold

cheers, Iain

[duck_arse]

C23 is about 100 times larger than it needs to be. 10nF would do fine.

[Mark Hammer]

1) Brian's drawing was intended to be an illustration for explanation purposes, not a complete "design".

2) If one wishes to turn it into a build, then a few additions are needed.  As shown, the Gain control is able to provide enough boost to drive most signals into clipping with the diode complement shown.  HOWEVER, when turned down to minimum, providing essentially a straight-wire feedback path from output to input, it becomes a unity-gain stage, and anything less than max volume setting will be less loud than bypass.
The remedy is to provide some minimum feedback resistance that is at least the same value, and preferably larger than 1k (R46), in series with the pot; which end (output or negative input) makes no difference to functioning.  I would suggest 4k7.  It will provide enough of a clean boost at minimum gain that you'll have a choice of volume settings to use with it.

3) As probably noted somewhere, there should also be a fixed resistor between C3 and the input lug of the Tone pot.  This will set the minimum treble cut.  When it is set to max resistance (10k), the Tone control and C29 starts the rolloff around 720hz.  If there is a 470R resistor between C3 and the Tone pot, the minimum treble cut (i.e., highest rolloff point) will be just over 15khz, and the most severe rolloff will now start around 690hz.  It IS a fairly shallow and gentle rolloff, so small adjustments will not seem to do much.

[Steben]

The diodes will start to conduct at +/- 0.6V, that is 5 times as high as a decent single coil chord.

i don't know, that sounds awfully low. i haven't tested it yet, but that sounds a bit underpowered. especially taking the "black ice" in to account, a passive Ge clipper (+/- 0,3V) that replaces a tone control capacitor, it supposedly also works on single coils, but i'm sceptic of both your and their claim... ill be testing that this afternoon.

http://tomsguitarprojects.blogspot.com/2014/12/electric-guitar-output-voltage-levels.html

Don't forget diodes follow a logarithmic pattern. There is no "straight" part of the I/V curve. What is flat is that part that gets squeezed so tigh in the scale it looks straight. If you look in the range of µAmps we get this:
http://qrp.gr/ultimatextalset/diodeiv.gif

[Vivek]

I find that when the gain is all the way down and volume all the way up. It still doesn't boost the signal like it does on OD's that I've played.

I'm feeling that it's because the schematic I'm going off doesn't actually have an output buffer/stage haha. Just a volume pot that comes straight out after the Tone Pot.

I made a separate thread on adding scaling resistors at output of Opamp, to make that one Opamp have greater output than the current configuration, without altering shape of the wave too much.

https://www.diystompboxes.com/smfforum/index.php?topic=125696.msg1198252

[Vivek]

An output buffer won't add any gain or volume, what you seek is a second amplifying stage. 

I made a post on how to get One Opamp to clip + multiply the clipped signal


You have the option to have no/very slight amount of clipping, yet the output levels will be high ie the circuit acts like a clean boost at very low gains

https://www.diystompboxes.com/smfforum/index.php?topic=125696.msg1198252

[Steben]

An output buffer won't add any gain or volume, what you seek is a second amplifying stage. 

I made a post on how to get One Opamp to clip + multiply the clipped signal


You have the option to have no/very slight amount of clipping, yet the output levels will be high ie the circuit acts like a clean boost at very low gains

https://www.diystompboxes.com/smfforum/index.php?topic=125696.msg1198252

Very elegant. If we swap the divider for a potentiometer we have a boost control.
Mind you, this is all with virtual ground or dual supply. Some might overlook.

[PRR]

...when the gain is all the way down and volume all the way up. It still doesn't boost the signal like it does on OD's that I've played.....

If that bothers you, don't do it! Why turn a knob all the way to zero if it isn't pleasing?