Small Clone Build- Slight drop in volume

[wademalorgan]

Hello all!
I just finished my small clone from the tonepad layout and it sounds great.  However, there is a slight loss of volume when the effect is turned on.  I just want to know if there is a cure for this.  I've read one post that stated that changing the feedback resistor after the op amp output stage might cure this.  I've searched through the forum and noticed that some get a small boost in volume and some get what I have.  Any ideas would be greatly appreciated.  Thanks!
wademalorgan

[Thomas P.]

http://generalguitargadgets.com/index.php?option=content&task=view&id=109&Itemid=139

[wademalorgan]

Do these small stone volume mods apply to the small clone?
wademalorgan

[Mark Hammer]

Appreciate Tomboy's quick response but the query was about a Small Clone, not Small Stone.  Here, I'll be embarrassed on your behalf so you don't have to be. 

Okay, with that slight detour out of the way,not that the Small Clone has an op-amp on the input and another on the output.  The input is noninverting and the output inverting.  Input has a feedback resistor of 33k and ground leg resistor of 6k8, providing a gain of 5.85.  The output has an input resistor of 10k and a feedback resistor of 33k, providing a gain of 3.3.  Bumping the feedback resistor of either stage up to the next standard value of 39k will provide new gains of 6.74 and 3.9, respectively.  The first represents a 6.74/5.85= 15% increase in gain, and the second represents a 3.9/3.3 = 18% increase in gain.  In theory, either should be suitable for what you describe as needing fixing.  My own preference would likely be for the second rather than the first, simply because  BBD chips are susceptible to some distortion if pushed hard, so gain recovery after the BBD seems to more helpful path, compared to gain boost at the front end.

Whether an 18% increase in gain is sufficient or appropriate for what you need is another thing, so perhaps you might want to consider either sticking a 10k variable resistance (pot/trimpot) in series with the 33k feedback resistor to achieve gains between 3.3 and 4.3 or making that output feedback resistor even higher in value (e.g., 47k) and sticking a 100k log pot in place of the 100k fixed resistor on the output.

Coming at it from a slightly different approach, note that the Small Clone, like a great many chorus pedals, uses a combination of pre-emphasis and complementary de-emphasis to achieve some noise reduction - a sort of dollar-store dolby.  In the SC, that pre/de network consists of the 6k8/.01uf pair on the input stage and 6k8/.01uf feedback pair on the output stage.  Those two provide some boost above 2.3khz on the input, which is counterbalanced by the lowpass filtering at the same frequency on the output.  Since the effect of the de-emphasis will depend on its relationship to the 33k feedback resistor, perhaps the smarter thing to do is to goose the gain by decreasing the 10k input resistor on the output stage, to 8k2 or perhaps 6k8.  That will increase gain the same way that increasing the 33k feedback resistor will, but without screwing up the symmetry of the pre/de networks.

[Thomas P.]

Appreciate Tomboy's quick response but the query was about a Small Clone, not Small Stone.  Here, I'll be embarrassed on your behalf so you don't have to be. 

hihi - one shot too fast

[puretube]

ye-ye: the old problems with the clones:
copying old designs from way back
straight like they were when they were as they were...

[wademalorgan]

Wow!  THank you very much for the informative response.  I will try the possible solutions and get back to yall. 
Much appreciated!
wademalorgan

[wademalorgan]

Hey,
I tried all of your suggestions but still did not get the volume boost I wanted to make it even with the bypass level.  I decided to probe through the circuit and see if I could spot where I was losing gain.  I noticed that the wet signal and dry signals before going through their respective mixing resistors (22k for the dry, 20k for the wet) were quite on par with the signal at the input of the circuit, as far as level and tone.  It's where the two signals meet that there is a volume drop and a small loss in high frequency.  I followed the signal further along and there is a gain increase after the output op amp stage.  However, this is not enough to bring it back to that same level heard before the mixing resistors.  Would anybody know what causes this.  Would trying smaller mixing resistors work?  I would really appreciate any sort of feedback.
Thank you!
wademalorgan

[puretube]

yes, I`d decrease those 22k & 20k;
that should leave the de-emphasis OK,
but increase the vol of dry & wet.

this can be done by parallelling them with larger Rs.
(experiment to taste)

or you replace these 2 with a pot (47k) to dial in (almost) any desired
balance between dry & wet (vib),
and parallell the pot`s branches with named larger Rs,
to adjust the 50/50 mix to the middle of the pot`s rotation,
and to get the overall vol right.

[wademalorgan]

Hey
I just added a 50k mix pot with 47k resistors in parallel with the pot branches.  It does the mixing fine.  On one side a get a nice LOUD dry signal and on the other a nice LOUD wet.  However, When I blend  them in the middle the volume drops.  I just can't get my head around this!
wademalorgan

[puretube]

try 39k, 33k, or even smaller in the 2 branches...

(with the 47k`s it`d only be ~ as loud as before, with the 22k/20k`s)

[wademalorgan]

I used 33k's and I'm still getting a volume drop. Seems like the mix between the two was a little louder- i'm not sure.  Maybe I'm not doing this right.  I have one resistor connected to lugs 3 and 2 and the other connected to 2 and 1.  Is that how I would put them in parallel with the pot?
thanks

[wademalorgan]

I tried 10k's and still a volume drop.

[puretube]

these suggestions were based on a guess - I never built that thingy.
(so I don`t know how large the volume-drop actually is).

next thing (let`s say you choose 33k as parallelling resistors),
would be slightly decreasing that 10k resistor that is following
in the circuit flow.
(e.g. parallelling the 10k with 100k, 82k, 68k, 47k... subsequently.

like Mark mentioned earlier, this however could shake-up the de-emphasis
a little...


Btw: the problem with the decrease of the 22k/20k not working immediately
as it was hoped to work
lies in the fact that they are not the "last" resistors in front of the inverting
input (= virtual ground), but only part of a total resistance,
which in turn is part of the lowpass around the opamp.

[wademalorgan]

Hey, thanks for the input!
I'm not quite sure I understand.  Are you saying that it's not just the resistance between the mixing resistors that control the mix but the resistance from the following op-amp section as well? So the following resistance in front of the inverting input also plays a part in how the signals are mixed?
I just do not get why there is no way to mix the two signals together without losing anything.  It's so strange
Thanks again!
wademalorgan

[wademalorgan]

Could jumpers be used instead of resistors to mix the wet and dry signals?

[puretube]

no