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

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

    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!

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    Quote Originally Posted by Gtr0 View Post
    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.

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    Quote Originally Posted by glebert View Post
    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 ;-)

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    Bent Member Chuck H's Avatar
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    ...
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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Quote Originally Posted by Chuck H View Post
    ...
    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.

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    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.

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    Quote Originally Posted by Helmholtz View Post
    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.

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    Senior Member Pedro Vecino's Avatar
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    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.

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    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.

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    Senior Member Pedro Vecino's Avatar
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    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.

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    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.

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    Last edited by Helmholtz; 10-05-2019 at 11:27 PM.
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    I was referring to the common type of signal relay switching used in amps (Finder, Omron, etc.).
    This is the scheme:

    Click image for larger version. 

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    Quote Originally Posted by Pedro Vecino View Post
    I was referring to the common type of signal relay switching used in amps (Finder, Omron, etc.).
    This is the scheme:

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    Clear now. Thanks.

    Good solution!

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    Last edited by Helmholtz; 10-05-2019 at 11:45 PM.
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    Quote Originally Posted by Pedro Vecino View Post
    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.

    Click image for larger version. 

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    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)?

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    On second thought, it would probably be better to make it a box that sits on top of the amp with a footswitch, as drawn. At first I was thinking all in one floor item, but then there's lots of cable run that way.

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    Senior Member Pedro Vecino's Avatar
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    The idea is a unit located above the amplifiers (like any attenuator) and make the change remotely with a footswitch.
    Grounding of the output on the inactive amplifier works as long as the amp input is also muted (indispensable condition)
    .
    The most delicate part in my opinion is the choice of the power relay. Once the appropriate model was selected, another one connected in parallel could be used for security purposes.

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    Quote Originally Posted by Pedro Vecino View Post
    The idea is a unit located above the amplifiers (like any attenuator) and make the change remotely with a footswitch.
    Grounding of the output on the inactive amplifier works as long as the amp input is also muted (indispensable condition)
    .
    The most delicate part in my opinion is the choice of the power relay. Once the appropriate model was selected, another one connected in parallel could be used for security purposes.
    I had Finder brand relays in mind, like this --> https://www.tube-town.net/ttstore/fi...5v-2a-12v.html

    I assume a pair of those would suffice?

    Thanks!

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    Senior Member Pedro Vecino's Avatar
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    For switching input signals, no problem. They are the ones that Engl uses, for example.

    But for speaker switching you need something stronger since itīs a critical function. As a reference, Mesa uses in the Roadking (this amp has automated switching to different speaker outputs) a set of Nec MR301-12HSL. They are relays that can handle 10A. You would need two to complement the switching in that part. At least with that exact model.

    This is the datasheet (Page 8):

    https://www.digchip.com/datasheets/p...-12HSL-pdf.php

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    OK, this is just my opinion, but this seems like a lot of work to hear the difference between two almost identical amps, which you have said doesn't seem to show any difference when recorded.

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    Quote Originally Posted by glebert View Post
    OK, this is just my opinion, but this seems like a lot of work to hear the difference between two almost identical amps, which you have said doesn't seem to show any difference when recorded.
    Ditto - although as an engineering exercise for switching heavy power/currents, the problem/solution has some value. I can't argue with the motivation either, but I wonder if given the part tolerances of 5% and 10% - or even 20% on pots and such, never mind transformers - how realistic is it that any tone difference could be attributed to anything specific in the implementation or circuit.

    Personally, I've chased after some particular tone goal and found cases where relatively minor changes in part values can make significant differences. In other cases, it's hard to hear any change with part variations of +/-50%. How would you know what's really causing the tone difference, if/when you do indeed hear one?

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    Quote Originally Posted by Pedro Vecino View Post
    For switching input signals, no problem. They are the ones that Engl uses, for example.

    But for speaker switching you need something stronger since itīs a critical function. As a reference, Mesa uses in the Roadking (this amp has automated switching to different speaker outputs) a set of Nec MR301-12HSL. They are relays that can handle 10A. You would need two to complement the switching in that part. At least with that exact model.

    This is the datasheet (Page 8):

    https://www.digchip.com/datasheets/p...-12HSL-pdf.php
    Noted, and thanks! I figure later that the finder didn't have enough to take the current. I will look into your recommendation.

    On a side note - What happens in the even of a catastrophic error where one of the set of inputs/outputs were reversed? So that the amp that is supposed to be going to ground on it's output is the one also receiving signal? I have to ask :-)


    Quote Originally Posted by glebert View Post
    OK, this is just my opinion, but this seems like a lot of work to hear the difference between two almost identical amps, which you have said doesn't seem to show any difference when recorded.
    Quote Originally Posted by uneumann View Post
    Ditto - although as an engineering exercise for switching heavy power/currents, the problem/solution has some value. I can't argue with the motivation either, but I wonder if given the part tolerances of 5% and 10% - or even 20% on pots and such, never mind transformers - how realistic is it that any tone difference could be attributed to anything specific in the implementation or circuit.

    Personally, I've chased after some particular tone goal and found cases where relatively minor changes in part values can make significant differences. In other cases, it's hard to hear any change with part variations of +/-50%. How would you know what's really causing the tone difference, if/when you do indeed hear one?
    First, I want to do it because I can and it's another experience with hashing out and building a useful item.

    But, my usage in this type of device would be for the differences that I do hear in the room, not really what is recorded. This item seems to be unnecessary by listening to playback of two amps recorded... my point was that there are lots of things *I think* I hear in the room that do not come across in recording.... or at least seem more significant in the room than on ... disk (I almost said "tap"e - shows my age ;-) )

    If I start with two amps that are in the ballpark (perhaps even using the same brand of everything)... then I will see the profound difference of swapping Xicon polypropylene caps with orange drop 716P (as an example) - and listen to it in real time in the room... not just on a recording.

    As an added bonus, occasionally I may want to replicate an amp, say an old Marshall. I can catch subtleties this way rather than relying on a recording that are minutes apart. Again, in the room.

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    Bent Member Chuck H's Avatar
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    Noble efforts in my opinion. But don't complicate peripheral tools to the detriment of achieving the goal. Do you want to make the most ideal switcher, or just hear the difference between designs so you can then make the most ideal amp!?! I think Pedro has one element correct. The switching of the input is critical if only because of the additional load on the guitar when plugged into two amps. So...

    Use the very simple amp switching design I provided above, which is a safe and the quickest way to the goal, and then use an A/B pedal (readily available) for the amp inputs. Yes, it's two whole buttons to press!!! Big deal. I don't think the amount of time it takes to press two buttons instead of one is a deal breaker for this. At least it shouldn't be. Like I said, keep the real goal in mind. You want to get to the best amp design. NOT the ideal switcher. Putting time into developing the ideal switcher, which is eminently more than is needed for the project only takes time away from getting to the design of the ideal amp. JM2C on that.

    And yes, to use the above schematic you'll need to make sure your speaker cables are wired correctly. While your at it you will also want to make sure your AC plugs are wired correctly, your speakers are in correct polarity and your amps bias is adjusted, etc. So really, what the hell sort of a nit pick is that? Commercially wired, and therefor 99.9999% likely to be correct speaker cables can be purchased. If you're wiring your own then you have only yourself to be accountable to.

    Find the quickest way to your end goal and don't muddle in the process steps where it isn't necessary. Unless the process, rather than results that achieve is the original goal is the point. Which I guess it may be for some. It's all choices.

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    Quote Originally Posted by Chuck H View Post
    Noble efforts in my opinion. But don't complicate peripheral tools to the detriment of achieving the goal. Do you want to make the most ideal switcher, or just hear the difference between designs so you can then make the most ideal amp!?! I think Pedro has one element correct. The switching of the input is critical if only because of the additional load on the guitar when plugged into two amps. So...

    Use the very simple amp switching design I provided above, which is a safe and the quickest way to the goal, and then use an A/B pedal (readily available) for the amp inputs. Yes, it's two whole buttons to press!!! Big deal. I don't think the amount of time it takes to press two buttons instead of one is a deal breaker for this. At least it shouldn't be. Like I said, keep the real goal in mind. You want to get to the best amp design. NOT the ideal switcher. Putting time into developing the ideal switcher, which is eminently more than is needed for the project only takes time away from getting to the design of the ideal amp. JM2C on that.

    And yes, to use the above schematic you'll need to make sure your speaker cables are wired correctly. While your at it you will also want to make sure your AC plugs are wired correctly, your speakers are in correct polarity and your amps bias is adjusted, etc. So really, what the hell sort of a nit pick is that? Commercially wired, and therefor 99.9999% likely to be correct speaker cables can be purchased. If you're wiring your own then you have only yourself to be accountable to.

    Find the quickest way to your end goal and don't muddle in the process steps where it isn't necessary. Unless the process, rather than results that achieve is the original goal is the point. Which I guess it may be for some. It's all choices.
    Well said.

    Yes, you'd be correct - I am, at this moment, more interested in the best amp design. Two switches is not an issue. However I wouldn't mind having the more "involved" plot for later use for when I do have some free time, which occasionally does happen.

    My first go at finding the appropriate relay was not fruitful... though I haven't spent long so far. My concern is that I haven't seen a DPDT with these parameters. More intense search later this afternoon.

    Thanks!!

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    DPDT relays are not hard to find.. Mouser has EXTENSIVE options. I looked at Jameco, because they are often a handy source for the hobbyist. Here is a selector guide of their line:
    https://www.jameco.com/Jameco/catalogs/c181/P72.pdf

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    On a side note - What happens in the even of a catastrophic error where one of the set of inputs/outputs were reversed? So that the amp that is supposed to be going to ground on it's output is the one also receiving signal? I have to ask :-)
    Feeding the amp which has its output shorted must be avoided. It would cause redplating and very short tube life.
    In case of doubt it would be better to switch the outputs to dummy resistors instead of grounding them.

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    Supporting Member loudthud's Avatar
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    My reasons for doing something like this are slightly different. I want to set up an experiment to see if people can tell the difference between two amps, one tube and one solid state. The problem I see is making adjustments to each amp without cluing the test subject as to which is which.

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    How would you know what's really causing the tone difference, if/when you do indeed hear one?
    I think the procedure will be changing components/circuits based on educated guess or simply trial and error until the tone difference between the amp under test and the reference amp is minimized.
    (Care should be taken to not compare amp tones of different sound levels.)

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    The main thing to be careful about when switching speakers is to avoid intermittent load, which can cause voltage spikes and destroy the output transformer.

    Even if the unused amp is switched to a dummy load, the relays will bounce a bit when switching. If there is no other protection, this would be very risky.

    A simple protection is to put a resistor across the amp speaker out signal (e.g. 270 ohms/5W) that is always in the circuit. This is enough load to protect the amp, without affecting the tone.

    The safety resistor can be added to the amp (I use it on all my builds), or part of the switch box (on the speaker signal input from each amp, before the relay).

    I would also recommend adding a circuit to mute the input of the unused amp.

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    ^^^Good point!

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    Senior Member Old Tele man's Avatar
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    Absolutely! "Opening" the OT output while signal is applied is akin to "automotive breaker points 'opening' on older ignition systems," i.e.: reflecting a VERY high voltage...back into the OT rather than to a spark plug.

    A 10-times-Z, non-inductive, "swamping" resistor in parallel with each amp's OTs winding input to switching circuit (not the speaker output) should mitigate this problem.

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    Well it all sounds dangerous for the amps. I'm not sure if I have enough faith in components to feel confident that this would work in a long term.

    I know George Metropoulos has/used to have one, which triggered my initial thoughts to have one... this was a while ago, I will go see if I can find a video. He doesn't show the switch but he seems to have full faith in the item he is using. Assuming that he is using the same cab. I will go seek that out in the meantime.

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    Found it... He's demoing A/B'ing one of his 12,000 series heads

    https://www.youtube.com/watch?v=Fcabc00kZ7w

    He seems to have faith in the switcher, especially switching with an original 1968. His switcher is built into a little metro style head.

    I assume he is using the two switch amp input/output to ground.

    Okay, faith renewed and reading over the last several posts with great care.

    Thanks everyone!

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  34. #34
    Member mhuss's Avatar
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    I built a two-relay box similar to the one shown above, but used full-time 200 ohm 5w resistors across the amp jacks (to keep a minimum load). 200 ohms because... that's what I had laying around! I used it for a bit with no problems, but haven't used it in a while. (I was using it for two different amps/sounds, not for A/B testing).

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  35. #35
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by mhuss View Post
    I built a two-relay box similar to the one shown above, but used full-time 200 ohm 5w resistors across the amp jacks (to keep a minimum load). 200 ohms because... that's what I had laying around! I used it for a bit with no problems, but haven't used it in a while. (I was using it for two different amps/sounds, not for A/B testing).
    Any details about pop noise? How you mitigated it? etc...

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    Last edited by Chuck H; 10-13-2019 at 02:40 AM.
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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