Ampeg Scrambler drops in volume when blended

[bertmar]

Hello!
Just finished my first stompbox, the ampeg scrambler clone. My problem is that the volume goes low when I turn the blend knob. When on full fuzz or no fuzz its OK but in between it almost disappears. I will continue to see if I can find any obvious faults.. just wondering if anyone has any suggestions about this??

Thanks!

Martin

[Elektrojänis]

Volume on the clean side of the blend pot should be about the same as bypassed volume. On the full fuzz side volume is louder than bypassed (at least in the clones I have built). Through all the blend pot's rotation the volume is either the same as bypassed or louder (again, in the clones I have built).

A few questions.

1. What is the resistance value of the blend pot you are using?
2. What are you driving with it? Amp? Next effect? Input impedance of the next effect/amp?
3. Is the sound on the clean side of the blend pot really clean (or at least almost clean).

[bertmar]

1. 47k
2. Actually I havent tried it with anything else than my digital piano through the scrambler, to a couple of headphones. Will use it tonight with my marshall amp and see how that sounds. Dont know the input impendences of these, How do I find out?
3. The sound is pretty clean but a bit less bassy, but the volume is a bit lower than when bypassed.

I made it using only one of those boards with a lot of holes in it (ie not pre-made PCB-board, dont know what it is called) so a lot could have gone wrong, but Im almost there.

[Elektrojänis]

1. 47k

Should be OK. That's what I'm using.

2. Actually I havent tried it with anything else than my digital piano through the scrambler, to a couple of headphones. Will use it tonight with my marshall amp and see how that sounds. Dont know the input impendences of these, How do I find out?

So you go straight from the scrambler to headphones?

If so, this is probably the problem. Actually it's a small wonder it works somewhat usably at all.

Most fuzz boxes (and other pedals) are designed with the assumption that they will be driving a guitar amp (or at least line level input on something). Guitar amps generally have a high input impedance (1Mohm or so) so the output impedance of the effect does not have to be low. (Line level inputs can have an impdance from under 10kohms to at least 100kohms.) Usually as far as signal transfer is concerned, the signal should go from lower impedance output to higher impedance input. More about impedance can be found from geofex article: http://www.geofex.com/Article_Folders/impednc.htm

Headphones usually have an impedance that is somewhere between 8ohms and 600ohms (note no k for kilo or M for mega). On the ends of the blend pots rotation the output impedance can be quite low as one of the transistor stages is connected straight to the output. (How low? Well... I don't know... Im not that good in the theory/calculations. Or I'm lazy...) On the center it's another story as half of the pots resistance is between the transistor stages and the output and it's a good assumption that the scramblers output impedance is over 10kohms.

So... At the center of the blend pot rotation you are going from an output that has an impedance of over 10000 ohms to an input that is probably under 600 ohms... Likely 32 ohms or something. The signal voltage gets divided to 32/10032=0.00318979266 of what it was before loading it down (at least I think it goes like this). That something like almost -50dB (acording http://www.muzique.com/schem/decibel.htm if I used it correctly).

3. The sound is pretty clean but a bit less bassy, but the volume is a bit lower than when bypassed.

That's probably because the output capacitors are quite small for driving headphones. Impedance issue again.

Anyway that's how it goes acording to my knowledge... Which is not much anyway. If I made any mistakes I hope someone corrects those so I can learn more. My english skils are not too good either, but I hope my writings can be understood without too much trouble.

I think your unit is fine. You just need to feed a suitable input with it. Any guitar amp is probably fine. If you want to use headphones you need a headphone amp (or something that has one).

[Arn C.]

Yes, I would say you need to go through an amp first then to your headphones.   Even a small amp, anything to boost the signal

Peace!
Arn C.

[bertmar]

Thanks guys! Of course your right, I just didnt have the time to try it out with the real stuff.
It works great when I put it through my amp (what a wonderful sound!).

Martin