Question about one switchable boost stage "cancelling out" another one

[rm77]

I breadboarded what I hope will soon be my first "from scratch" build. A switch at the input selects either a transistor boost or an opamp boost, into a gain control pot, into a bazz fuss, then a tone stack and gain recovery stage. A problem I ran into was that, while when the switch was set to run the input through the opamp boost into the gain pot/bazz fuss it worked fine, when I switched to run the input through the transistor boost into the gain pot/bazz fuss instead, there was no sound. It seemed as though the opamp was interfering with the transistor boost in this configuration, but not vice versa.

I tried using an SPDT switch instead of an SPST. It still switched the input path between one boost or another. But now the opamp output, instead of going straight to the gain pot, was wired to the switch so that it would be grounded when the transistor boost mode was turned on. Success! However, I don't understand why I needed to ground the opamp output in order for the input signal to properly pass from the transistor boost into the gain pot and bazz fuss, yet I do not need to ground the transistor output in order to have the opamp be the booster. That transistor output still goes right into the gain pot, and doesn't seem to affect the opamp boost operation or tone at all. Can anybody help explain that?

[GibsonGM]

Can you draw it up (schematic)?   Clearly there's some path there that is robbing your signal....maybe the opamp feedback loop?

Might be easy to see if you can present it graphically (at least for me).

Sounds like a good build

[Digital Larry]

It's hard to tell without a schematic, but:

#1 Don't ground an Op-amp OUTPUT.  That's a good way to make it unhappy.

#2 If you have two outputs connected together, the one which is lowest impedance will dominate.  The output impedance of a transistor boost is equal to to the collector resistor.  The output impedance of an op-amp is maybe 10 ohms open loop and goes down by the voltage gain factor of the stage (I think).  So if the two output stages are joined together somehow, then yes the op-amp is not going to let the transistor signal wiggle.

[rm77]

Thanks for the replies, here's a link to a very crude schematic of the version that DIDN'T work...I'm still learning about schematics and not sure how to easily illustrate the change I made

http://s189.photobucket.com/user/Bwv1048/media/workinprogress.jpg.html?filters[user]=48788638&filters[recent]=1&sort=1&o=0

This shows how I used the SPST switch at the input, to direct the input either to the transistor boost or the opamp boost, which resulted in the opamp boost working fine but nothing from the transistor boost. What I wasn't sure how to illustrate is how I replaced the SPST with an SPDT. Three of the SPDT lugs are being used just like in the picture, to take the input and then send it to one of the two boosts or the other. On the other three lugs, I have the opamp output to an outer one, the middle lug connects to the gain control pot right before the bazz fuss, and the final lug is unwired. This lets me switch back and forth between the two with no problem (at least I thought).

Digital Larry's explanation seems to cover why the opamp boost was interfering with the transistor boost, yet the opamp boost seemed immune to the same effect from the transistor boost. Thanks! But what is the issue with grounding the opamp's output the way I do? I haven't noticed any negative effects so far, but am I in danger of damaging the opamp like this?

EDIT: I have been referring to the switches as SPST and SPDT, but I meant that I started with a SPDT to route the input and then used a DPDT as part of my fix.

[Johan]

Thel link don't work.  But. .A single transistor voltage amp( assuming  that's what you have) is typically inverting . most opamp  voltage amps are non inverting.  So yes, there is a possibility they will cancel each other. . (Not having seen the schematic)
J

[JFace]

I can't see the schematic either, but you will need to switch your opamp output on one lug, your transitor output on another lug, and your volume control on the common lug. If the transistor output is permanently fixed to the op amp output, the transistor is seeing a very small resistance to ground. This is effectively like a 10 ohm resistor to ground from the transistor output. They need to be isolated from one another in some way.