I want to build an amp selector (two amps a/b, one cab)

[Gtr0]

Hi Folks.

Several times I have built the same circuit amplifier with different components... and often I have found wanting to have a box that will let me switch between the two amps while sharing the same cabinet. Now I'd like to take this on.

In my mind I only need to create a switch that will change one amp's input (from it's speaker jack) to the output while switching the other to a... what? A 200 watt 8 ohm resistor?

I know there are lots more to consider, this is just my first thoughts on the matter... I assume we are talking moderately lower voltage but high current here. The amplifiers in question are 100 Watt amps that will go out to an 8ohm cab/load.

Until now I have recorded one and, powered off, switched heads, and recorded the other amp. A lot of impression is lost in that one minute and I often catch differences in the room that do not seem to come out on the recording with the same impression... for whatever reason (mic freq limitations etc).

If you have any experience in the area, please share! And thanks!

[Gtr0]

Perhaps one of these??
https://www.banzaimusic.com/dummy-lo...-ohm-200w.html

[glebert]

Hi Folks.

Several times I have built the same circuit amplifier with different components... and often I have found wanting to have a box that will let me switch between the two amps while sharing the same cabinet.

I am lazy. I would use a stereo 212 or 412 cabinet with one amp hooked to each side of the cab, then just A/B switch to the amps inputs. Use two mics (one on each side) and either record both or mix them into one channel.

Then again if you have several times built the same amp with different components then you probably aren't lazy.

[Gtr0]

I am lazy. I would use a stereo 212 or 412 cabinet with one amp hooked to each side of the cab, then just A/B switch to the amps inputs. Use two mics (one on each side) and either record both or mix them into one channel.

Then again if you have several times built the same amp with different components then you probably aren't lazy.

haha. true... however, very cool idea. I would have never thought of it. But as silly as it may sound to you, I want all variables taken out of the equasion, that includes different speakers (even same brand/model), different positions of the cab, different (again though same model) mic etc etc.

AND at home I have a single 1x12" 8 ohm cab ;-)

[Chuck H]

...

[glebert]

...

Asking because I don't know, but do you need to be concerned for a situation where you get a speaker cable wired backwards and would then be sending one amps hot output to ground through the other amps chassis? Last time I wired a stereo/mono cabinet I used TRS switchjacks to switch both the tip and the sleeve in and out of circuit, but don't know if that was really a concern or not.

[Helmholtz]

Asking because I don't know, but do you need to be concerned for a situation where you get a speaker cable wired backwards and would then be sending one amps hot output to ground through the other amps chassis?

You mean hot wire and ground wire interchanged at one end of the speaker cable? That would be a problem as it would short the output of the amp connected (via the switchbox and/or mains safety ground). So speaker cables should be tested for correct wiring.

[glebert]

You mean hot wire and ground wire interchanged at one end of the speaker cable? That would be a problem as it would short the output of the amp connected (via the switchbox and/or mains safety ground). So speaker cables should be tested for correct wiring.

Yes, that is what I was thinking. You could also use a 4PDT switch so that the "grounds" are also switched between the speaker and the dummy load.

[Pedro Vecino]

I would do it that way: a standard signal relay for assignment to each amplifier and a powerful relay for switching amps to the speaker. Both synchronized.
Muting the signal input in the inactive amplifier, absorption resistor is not necessary.
I imagine that this circuit would need the interruption of the grounding in one of the amplifiers to avoid ground loops. Using a small adapter in the plug.

[Helmholtz]

Muting the signal input in the inactive amplifier, absorption resistor is not necessary.

Are you sure that some amp might not start to self-oscillate without a load? Without the load loop gain strongly increases and tube load gets inductive and resonant at some frequency.

[Pedro Vecino]

But the output of the inactive amplifier is not open. Itīs also bridged through the relay. Nothing bad should happen.
At least I have never seen a problem with that kind of protection.

[Helmholtz]

But the output of the inactive amplifier is not open. Itīs also bridged through the relay.

You're right, if input and output of the amp are shorted/grounded, there shouldn't be risk.
The relays' wiring/action wasn't quite clear to me from your drawing without knowing the actual pinout, but now I understand and support the principle.

[Pedro Vecino]

I was referring to the common type of signal relay switching used in amps (Finder, Omron, etc.).
This is the scheme:

[Helmholtz]

I was referring to the common type of signal relay switching used in amps (Finder, Omron, etc.).
This is the scheme:

[ATTACH=CONFIG]55434[/ATTACH]

Clear now. Thanks.

Good solution!

[Gtr0]

I would do it that way: a standard signal relay for assignment to each amplifier and a powerful relay for switching amps to the speaker. Both synchronized.
Muting the signal input in the inactive amplifier, absorption resistor is not necessary.
I imagine that this circuit would need the interruption of the grounding in one of the amplifiers to avoid ground loops. Using a small adapter in the plug.

[ATTACH=CONFIG]55433[/ATTACH]

wow! so as long as the output of the inactive amplifier goes to ground, then it is okay? and a stompbox type box would be able to support the proper grounding for this (at the chassis)?