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Reactive load - adding an XLR out and fan

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  • GainFreak
    replied
    Interesting. I see you can very quickly spend a lot of $$$ on impulse "packs".
    There are free packs which are quite good There are millions of IR's out there but in the end you settle on couple of them and that's it. Also you can make your own impulses.

    Generally there are two types of IRs: 1/captured with an SS amp and 2/ with a tube amp. The first type is a "flat"type reproducing the actual speaker/cabinet response. With tube amp IRs you get the "smile" impedance curve and the tube amp response. So if you have a reactive load (which as close to a speaker load as you can get) you should use the first type otherwise you'll get two impedance curves - the one from your reactive load and the other one captured in the impulse. If you're running a preamp only and want the tube amp response and the speaker/cabinet impedance curve than you should use the second type IR's.
    Using resistive load with a tube amp is good only for measurements. Soundwise it's just mediocrel. That's why most resistive attenuators usually suck big time.
    Note that a tube amp will start distorting at impedance peaks which will generate harmonics (mainly in the low mid region) which you can not add in any way after that if you use a resistive load.

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  • nickb
    replied
    Interesting. I see you can very quickly spend a lot of $$$ on impulse "packs".

    But, there is a problem here. Take the Mooer Radar, it expects a flat frequency response on the both the input and the output. Using a reactive load and taking the amplifier output voltage to the DI is not quite right to the end of a flat response. You actually need a resistive load. That said the error is quite small at around 1 dB but there is significant extra cost in the hardware and all it did for you was introduce an error.

    OTOH it's a bit silly talking about precision in the land of guitar amps

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  • GainFreak
    replied
    What is missing is the electro-acoustical filtering by a real guitar speaker. I've seen elaborate active filter circuits to emulate a speaker's response, but Nick's solution is much simpler and for free.
    I'll be feeding the signal to an impulse response player so this problem is solved. It can be a PC based one or an external pedal like Mooer Radar for example or similar. IR's are the most accurate speaker/cabinet simulations to date. The reactive load signal can be fed also to a small SS amplifier and this way you practically get an attenuator.
    If the amp feeding the reactive load is a tube one then the second amp must be SS otherwise you'll get the impedance curve at the speaker one more time.

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  • Helmholtz
    replied
    I have to say that Nick's proposal is an excellent addition to a plain reactive load. It's simple, passive and cheap (though I don't exactly understand why it works).
    It is often overlooked that while good a reactive speaker load can make the amp behave as if it was connected to a real speaker, the signal at the amp's output is hardly suitable for direct processing, recording or feeding a monitor.
    What is missing is the electro-acoustical filtering by a real guitar speaker. I've seen elaborate active filter circuits to emulate a speaker's response, but Nick's solution is much simpler and for free.

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  • GainFreak
    replied
    Unless you have the input jack sleeve isolated from the chassis. Do you?
    The input jack is isolated from chassis. It has a separate chassis sleeve which contacts the chassis and further connects to the PCB chassis plane (fan circuit, input jack and ground switch are on this PCB). Input jack ground is separate from the PCB chassis plane. The ground lift switch connects both (or not).
    The line out circuit is mounted on another PCB where XLR combo socket ground/chassis pin and pin 1 connect to chassis. This means the chassis is always grounded either at the input or output. If both ends are grounded a ground loop is expected

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  • Helmholtz
    replied
    Originally posted by Chuck H View Post


    I think it's strange that the ground lift on your box is correcting a ground loop. At least as it is shown in the schematic. Because the actual ground for it is achieved via speaker cable contact with the jack sleeve to the amp. And that remains regardless of what position the switch shown in the schematic is in. Unless you have the input jack sleeve isolated from the chassis. Do you?
    A ground lift is typically used to interrupt an undesirable low impedance ground current path between different mains connected equipment.
    In this sense the switch as shown in the schematic should work as it separates the grounds of the guitar amp and the equipment connected to the Line Out.


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  • Chuck H
    replied
    Originally posted by GainFreak View Post

    I don't think it's worth the effort. What is important the circuit is correct and working properly.
    I don't agree with discarding analysis on the basis of the problem no longer being present. There's no "understanding" in that. But then, since there was never any clues as to what the actual problem was, and what seemed to fix it doesn't analyze as causing it, it's probable that any model created won't be duplicating the actual circumstances.?. So in that light light I'm less inclined to take the time. But on the subject of analysis...

    I think it's strange that the ground lift on your box is correcting a ground loop. At least as it is shown in the schematic. Because the actual ground for it is achieved via speaker cable contact with the jack sleeve to the amp. And that remains regardless of what position the switch shown in the schematic is in. Unless you have the input jack sleeve isolated from the chassis. Do you?

    EDIT: It occurs to me that you probably don't have the input jack sleeve isolated. Because if you did you would lose the XLR sleeve when the ground lift was engaged, causing different noise problems.?. At least as shown in the schematic you provided for analysis. An exception being if the terminal signal end of the XLR is also sleeve grounded. Since you indicated plugging this rig into PC based digital media that seems unlikely, but I don't know.
    Last edited by Chuck H; 07-14-2020, 02:21 PM.

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  • GainFreak
    replied
    I suppose modeling that circuit, and going thru selectively shorting out portions of the bridge would finally yield the problem you had.
    I don't think it's worth the effort. What is important the circuit is correct and working properly.

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  • nevetslab
    replied
    Say no more. Mostly it was a question to solve the what seemed a simple problem yet perplexing all of us. I suppose modeling that circuit, and going thru selectively shorting out portions of the bridge would finally yield the problem you had.

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  • GainFreak
    replied
    What did you find in the bridge you removed, with regards to what the bridge turned into?
    Honestly I was so pissed at this issue so as soon as the new bridge worked I immediately threw the old one in the garbage. I pulled that one from an old box with left over parts and maybe it was bad in the first place. I bought a new one (same type) from the local retailer and it worked both ways.

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  • nickb
    replied
    Originally posted by Helmholtz View Post

    What was your literature?

    Did you compare your sim result with a published SPL of the speaker?

    I guess single star "*" means multiplication and double star "**" means power/exponent like "^" in Excel?
    It was a paper by Neils Elkjaer Iverson "Introduction to Loudspeaker Modelling and Design". It's a gem.

    Here is the SPL overlaid with my model output light green trace. I'd reckon that is pretty good. The TS models don't even try to get the cone effects at higher frequencies corrent so that difference is to be expected.
    Attached Files
    Last edited by nickb; 07-13-2020, 07:16 PM.

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  • nevetslab
    replied
    I still have no explanation for the GND Lift Switch changing anything. What did you find in the bridge you removed, with regards to what the bridge turned into? Which shorted or open cells of the bridge. Circuit still confuses me, but glad replacing the bridge cured it.
    Last edited by nevetslab; 07-13-2020, 06:21 PM.

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  • Helmholtz
    replied
    [QUOTE=nickb;n909129
    If what I read is true, the the following should give an audio output that is closer to the audio output ...
    [/QUOTE]

    What was your literature?

    Did you compare your sim result with a published SPL of the speaker?

    I guess single star "*" means multiplication and double star "**" means power/exponent like "^" in Excel?

    Leave a comment:


  • GainFreak
    replied
    Glad to inform you the problem was finally solved. The credits go nevetslab who suggested a bridge recto problem. After replacing it with a new one now the fan is working in both modes.
    Ground lift also works as expected. When I connected the XLR (unbalanced via mono jack) to PC I got loud hum indicating ground loop. Lifting the ground eliminated it.
    Still pissed about this dumb problem but sometimes it's right in front of your eyes and you still can't see it.
    Thanks to all for your suggestions and comments.

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  • nickb
    replied
    Since Aiken is the source for the first two link and what you used let's focus on that. He states that his first model ( the one you used) is 16 ohms and that the voice coil resistance is wrong and then goes on the give better models. For all three at no point is it claimed that the DI signal out is representative of the SPL but it is claimed that it looks like a speaker, which it does, up to a point. You wanted 8 ohms so I put in the effort and calculated the values for you. The formulae are there if you want to plug in the TS parameters for a different speaker and you will see the same formulae in the 3rd link you gave. I've also shown an improvement that gives a better DI output signal for a PC or a PA and that uses less components. What's not to love about it?

    I had a thought on the the fan stopping with the lift switch open. A bridge rectifier designed for 120Hz operation is not going to be so good at the oscillation frequency of say 50KHz where capacitance and stored charge are going to become significant. That said, I really don't think these effects are big enough to explain it but I can't find good datashets to confirm it so I'll just throw it in as a remote possibility.

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