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Reactive load - adding an XLR out and fan

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  • GainFreak
    replied
    If the OT secondary of the host amp IS grounded either independently or via the output jack then the ground lift switch on your load box shouldn't make any difference at all to the circuit operation. So if this is the case there should be no change that would cause your problem when lifting the ground as shown in the schematic above.
    OT secondary IS grounded. I meant to mention that but somehow missed it in the relies above.

    Why should lifting the ground from the chassis make any difference to the fan supply?
    This is the question

    What is the current rating of the bridge diodes? Is it a molded bridge, or discrete diodes making the bridge. .
    It's a molded one - KBP210 (1kV/2A).

    Seems like one or more of the negative sides of the bridge are shorted
    I'll check that.


    Basic reactive load read:

    https://www.aikenamps.com/index.php/...-load-emulator
    https://www.thegearpage.net/board/in...-load.1072793/
    https://circuitdigest.com/electronic...nt-rlc-circuit









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  • nickb
    replied
    Doing a bit of research I've learned that it's the current through the (100uF in your case) capacitor that is representative of the audio output from the speaker. Also the model used represents an infinite baffle which is unlike most guitar amps. The value of 16.5 ohms for the voice coil DC resistance shown is way off for an 8 ohm equivalent, 5.5 ohms would be nearer the mark.

    I found the Eminence Governor 12in 8 ohm speaker has the following TS parameters and used them to calculate the electrical equivalent. C1 and C2 below are combined into one in the circuit you are using. I put the values you have in parenthesis. You can see that Rc and C1+C2 are rather different,

    TS Params:
    Revc 6.81
    Levc 0.00041
    BL 15.3
    Qms 11.41
    Mms 0.032
    Fs 101
    Cms 0.08
    Sd 0.05195


    Rc 6.8 ohms (16.5)
    Lc 0.4 mH (0.5mH)
    C1 28.7 uF
    C2 136.7 uF (C1+C2 100uF)
    L2 18.7 mH (20mH)
    R2 131.6 ohms (75)

    If what I read is true, the the following should give an audio output that is closer to the audio output (for an infinite baffle). The 0.33 ohms resistor gives about 1V rms out for 20V rms ( 25 watts) in.

    Click image for larger version

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  • Helmholtz
    replied
    Why should lifting the ground from the chassis make any difference to the fan supply?

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  • nevetslab
    replied
    What is the current rating of the bridge diodes? Is it a molded bridge, or discrete diodes making the bridge. Seems like one or more of the negative sides of the bridge are shorted. With 20VAC source, as you go from around 2.4VAC across the 100 ohm resistor feeding one side of the bridge when the bottom side of the bridge is grounded, opening that switch, you now have around 18.3VAC across that 100 ohm resistor. If both (or either) negative side bridge diodes are shorted, you have forward-biased diodes in series with 100 ohms across the 20VAC source.

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  • Chuck H
    replied
    Originally posted by GainFreak View Post
    Power amp chassis is grounded via 3 prong cable.It's the same as a Slave Out found in many amps only instead of connecting a speaker you connect the reactive load...
    This really isn't clarifying anything for me. It was questioned whether or not the OT secondary on the host amp is grounded in the amp itself. What you have told us is that the chassis of that amp is grounded and knowing that doesn't really help the discussion. The consideration being you would drive a simulated speaker load with the amplifier output. A "slave out" is typically a line level output. Often as not exclusive of the OT in the amp. The simulated speaker load you have there is designed as a reactive load intended to "act like" a speaker when driven by watts so that your amplifier reacts similarly to when it's driving a speaker. It will not "sound like" a speaker when signal is processed through it. That would require a "speaker emulator" circuit.

    I don't think the above is the case. I just think we all need to be on the same page with circuit monikers to avoid confusion. If you were actually driving that load box with a "slave out" I don't expect you would get much activity from the fan. That said...

    If the OT secondary of the host amp IS grounded either independently or via the output jack then the ground lift switch on your load box shouldn't make any difference at all to the circuit operation. So if this is the case there should be no change that would cause your problem when lifting the ground as shown in the schematic above. That can only mean an inaccuracy either in the wiring or the reporting. So I hope you'll be patient and try to answer any questions amicably and accurately and perform requested tests.

    JM2C

    EDIT: In light of what Nick said it seems that the ground lift for a circuit like that is pretty much a vestigial appendage and won't make any difference whether the host amp OT secondary is grounded or not.?.

    Rather than drawing circuits onto an existing diagram I think it would be best if you arrange a proper schematic of the actual circuit. So often a poster will excuse their own errors while becoming frustrated with responses because their actual circuit is different from what they've presented for analysis. Which seems really obtuse, but it happens all the time.
    Last edited by Chuck H; 07-12-2020, 07:13 PM.

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  • GainFreak
    replied
    MY actual reactive load we're discussing has component values adjusted for 8 Ohms. The schematic attached is for illustration purposes only - to show the concept, sound path and connections.

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  • nickb
    replied
    Originally posted by GainFreak View Post

    The whole thing is built into its own separate from the amplifier enclosure made from 1.2mm steel.
    OT secondary ground is grounded.



    The reactive load circuit (speaker equivalent circuit) is a classic from Aiken from a very long time and was built by thousands of DIY-ers. There's ton of info about it all over the place.

    No oscillation is present at speaker's out (unit input) when ground is lifted and fan not working.

    1/ Voltage across 100 Ohm resistor at onset of fan rotation ground NOT lifted 0.95VAC. Voltage at load 4.8VAC
    Voltage across 100 Ohm resistor (at at same load voltage of 4.8VAC) ground lifted 4.6VAC.

    2/Voltage across 100 Ohm resistor ground NOT lifted 2.4VAC. Voltage at load 20VAC
    Voltage across 100 Ohm resistor (at same load voltage of 20VAC) ground lifted 18.7VAC.

    Fan specs are 24VDC/0.68W (low noise).
    Just because it's everywhere doesn't mean it's not flawed. The circuit simulates a speaker load approximately but the signal you pass out the XLR will not sound quite correct. As has been demonstrated, neither the voltage (which this uses) or current reflect the SPL. It remains an open question as to whether it's near enough.

    So it's a separate box. In that case all closing the lift switch does is connect the steel box to the output i.e. there is no substantive change in electrical connection. Makes no sense. Clearly something is still missing from the picture.

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  • Helmholtz
    replied
    Originally posted by GainFreak View Post

    Component values are adjusted for 8 Ohm.
    Sorry, no.
    The reactive load's impedance is greater than 16 Ohm.

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  • GainFreak
    replied
    With the component values shown, amp output impedance should be set to 16 Ohm. Otherwise power tube screens are at risk.
    Component values are adjusted for 8 Ohms.

    How did you measure AC voltages? Oscillation frequencies are typically above 100kHz.
    With a PC scope.

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  • Helmholtz
    replied
    Originally posted by GainFreak View Post

    The whole thing is built into its own separate from the amplifier enclosure made from 1.2mm steel.
    OT secondary ground is grounded.



    The reactive load circuit (speaker equivalent circuit) is a classic from Aiken from a very long time and was built by thousands of DIY-ers. There's ton of info about it all over the place.

    No oscillation is present at speaker's out (unit input) when ground is lifted and fan not working.

    1/ Voltage across 100 Ohm resistor at onset of fan rotation ground NOT lifted 0.95VAC. Voltage at load 4.8VAC
    Voltage across 100 Ohm resistor (at at same load voltage of 4.8VAC) ground lifted 4.6VAC.

    2/Voltage across 100 Ohm resistor ground NOT lifted 2.4VAC. Voltage at load 20VAC
    Voltage across 100 Ohm resistor (at same load voltage of 20VAC) ground lifted 18.7VAC.

    Fan specs are 24VDC/0.68W (low noise).
    How did you measure AC voltages? Oscillation frequencies are often above 100kHz. What is the power rating of the 100R resistor?

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by GainFreak View Post
    Power amp 8 Ohm output goes to the reactive load as per schematic.
    With the component values shown, amp output impedance should be set to 16 Ohm. Otherwise power tube screens are at risk.



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  • GainFreak
    replied
    I'd conclude that this is built in to the amplifier and that the speaker output is not grounded.
    The whole thing is built into its own separate from the amplifier enclosure made from 1.2mm steel.
    OT secondary ground is grounded.

    Therefore, the speaker simulator design seems fundamentally flawed.
    The reactive load circuit (speaker equivalent circuit) is a classic from Aiken from a very long time and was built by thousands of DIY-ers. There's ton of info about it all over the place.

    No oscillation is present at speaker's out (unit input) when ground is lifted and fan not working.

    1/ Voltage across 100 Ohm resistor at onset of fan rotation ground NOT lifted 0.95VAC. Voltage at load 4.8VAC
    Voltage across 100 Ohm resistor (at at same load voltage of 4.8VAC) ground lifted 4.6VAC.

    2/Voltage across 100 Ohm resistor ground NOT lifted 2.4VAC. Voltage at load 20VAC
    Voltage across 100 Ohm resistor (at same load voltage of 20VAC) ground lifted 18.7VAC.

    Fan specs are 24VDC/0.68W (low noise).

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by nickb View Post
    Therefore, the speaker simulator design seems fundamentally flawed.
    Well it simulates a speaker load to the guitar amp. But requires the signal to be finally reproduced (at lower volumes) by a real guitar speaker. Or some filter circuit that emulates a speaker's response.

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  • nickb
    replied
    Originally posted by GainFreak View Post

    There isn't anything special about it. Power amp 8 Ohm output goes to the reactive load as per schematic. Power amp chassis is grounded via 3 prong cable.It's the same as a Slave Out found in many amps only instead of connecting a speaker you connect the reactive load so the Jensen circuit is connected correctly to the load.
    The XLR out is supposed to connect to PC to be used with IRs. The problem occurs either way when the Jensen circuit is disconnected from PC or from the reactive load. I'll check with my PC scope if any oscillation is present when ground is lifted.
    I know it's obvious to you how it's constructed but we simply don't know. Is this built into it's own little metal box or internal to the amplifier? What about the secondary from the OPT; does it connect to any kind of ground or the chassis?

    So, not having the XLR connected to anything has no effect on the problem. If this is in it's own box, as I had envisaged it, then that makes little sense. I'd conclude that this is built in to the amplifier and that the speaker output is not grounded. That would mean that the speaker output wires could have some common mode signal, how much depends on things like layout, transformer design and frequency. This means that when there is no OPT ground the OPT secondary wires could couple back to more sensitive section(s) of the amplifier.

    The oscillation seems to be ultrasonic so your PC scope will need to have sufficient bandwidth, say >50Khz. In other words a scope using the audio card might not help.

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  • nickb
    replied
    Originally posted by Helmholtz View Post

    Depends on how the line-out is going to be used. I assumed it would feed a (small) linear power amplifier connected to a guitar speaker like with the Ultimate Attenuator. In this case taking the signal from the input ( = main amp output) makes sense.

    I agree that some of a speaker's SPL will show in its current - at least at mid frequencies. But e.g. the bass resonance will show as a dip or notch as opposed to a peak in real SPL.
    If speaker current would directly correlate with its SPL, the SPL would be linear when the speaker is driven from a CCS.
    I was looking as it being used as a DI, as it seems it is. So, SPL isn't the current at LF but nor is it the voltage at HF. Therefore, the speaker simulator design seems fundamentally flawed.

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