Reactive load - adding an XLR out and fan

[GainFreak]

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.

[Helmholtz]

Component values are adjusted for 8 Ohm.

Sorry, no.
The reactive load's impedance is greater than 16 Ohm.

[nickb]

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.

[GainFreak]

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.

[Chuck H]

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.

[nevetslab]

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.

[Helmholtz]

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

[nickb]

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.

[GainFreak]

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

[nickb]

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

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.

[GainFreak]

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.

[Helmholtz]

[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?

[nevetslab]

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.

[nickb]

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.

[GainFreak]

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.