I recently ordered one of Bob Cordell's distortion magnifier kits and I'm having a puzzling issue.
I'm attempting to apply the null to a 1v 1khz signal but I am getting roughly 15v on the output at the lowest gain pot setting regardless of whether the output of the amplifier is inverting, non inverting, or even plugged into the distortion magnifier unit at all.
The phase knobs have no effect on anything.
I checked and rechecked the board and everything seems to be in order.
I am beginning to wonder if I'm using it wrong or something.
I'm supplying the 1khz signal into the "source" input on the DM and to the amplifier input.
I'm feeding the output of the amplifier to the "amp" input on the DM.
I've connected my scope to the output of the DM.
Anyone have any troubleshooting advice for me?
I'm attempting to apply the null to a 1v 1khz signal but I am getting roughly 15v on the output at the lowest gain pot setting regardless of whether the output of the amplifier is inverting, non inverting, or even plugged into the distortion magnifier unit at all.
The phase knobs have no effect on anything.
I checked and rechecked the board and everything seems to be in order.
I am beginning to wonder if I'm using it wrong or something.
I'm supplying the 1khz signal into the "source" input on the DM and to the amplifier input.
I'm feeding the output of the amplifier to the "amp" input on the DM.
I've connected my scope to the output of the DM.
Anyone have any troubleshooting advice for me?
What happens if you short the input to the magnifier?
Are the right DC supply voltages on the op amps?
Too bad you can't post the schematic here.
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I don't have the schematic, just a block diagram.
Is the frequency of the AC output the same as your test signal?
Does it scale down when you reduce the input signal level?
CordellAudio.com - The Distortion Magnifier
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Yeah, same signal, just an increased amplitude. It does scale down with reduced input signal.
Doing a more exact calculation reveals the signal gain is about 18.5 on the output relative to the signal on the "source" input of the DM.
+/- 13v on the power pins.
Then set the input scaling to unity, and input the same test signal to both inputs.
You should be able to null now, if the circuit is working right.
Remember that there will be a significant output unless the two inputs
are identical and properly scaled.
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I would hope so, but maybe not. I don't see a switch on the block diagram.
There should be a switch to invert one of the paths, if needed.
Try rewiring the DUT to be noninverting and try again.
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Well, feeding the input signal to both the "source" and "amp" pins is equivalent to using a non inverting amplifier.
However, I now have a schematic and it seems that the output summing amplifier is designed to null when the "amp" signal is twice that of the "amp" signal.
But the signal can never null if the two inputs are out of phase and the "source" signal can never be below a gain of 1 because the source input stage does not support it.
So...it looks to me that the distortion magnifier requires a gain of at least 2 on the amplifier and it does not work with an inverting amplifier?
If this is true it is almost useless to me for most purposes.
However, I now have a schematic and it seems that the output summing amplifier is designed to null when the "amp" signal is twice that of the "amp" signal.
But the signal can never null if the two inputs are out of phase and the "source" signal can never be below a gain of 1 because the source input stage does not support it.
So...it looks to me that the distortion magnifier requires a gain of at least 2 on the amplifier and it does not work with an inverting amplifier?
If this is true it is almost useless to me for most purposes.
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The distortion magnifier (DM) as described in Cordell's article in Linear Audio is set up to accommodate device-under-test (DUT) gains from about 10 to about 35.
Other gains would require changes in the schematic. I have not tested this but I believe you'd need to adjust the gain of various stages of the DM by appropriate scaling input dividers and opamps' feedback networks.
DM has a differential input for DUT output, so inverting or non-inverting doesn't matter; you just swap hot and cold inputs.
Edited to add this: if you try to use the DM with a DUT with a phase- and amplitude response very different from a 1st order LPF or an integrator, you may be unable to null properly. The article has more details.
Other gains would require changes in the schematic. I have not tested this but I believe you'd need to adjust the gain of various stages of the DM by appropriate scaling input dividers and opamps' feedback networks.
DM has a differential input for DUT output, so inverting or non-inverting doesn't matter; you just swap hot and cold inputs.
Edited to add this: if you try to use the DM with a DUT with a phase- and amplitude response very different from a 1st order LPF or an integrator, you may be unable to null properly. The article has more details.
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I think both inputs must have the same phase to get cancellation, so if the DUT is inverting, your reference should also be inverted.
In fact, it says so in the very first sentence:
Features and issues
• Operates by subtracting input from scaled, phase-matched output of DUT
BUT! It also says:
The balanced inputs can be swapped for power amplifiers that are
inverting.
Note that the article details a test with a signal generator to see if the unit works properly before connecting a real amp. That may be useful.
All that from reading the user guide! Who would have thunk! ;-)
Jan
In fact, it says so in the very first sentence:
Features and issues
• Operates by subtracting input from scaled, phase-matched output of DUT
BUT! It also says:
The balanced inputs can be swapped for power amplifiers that are
inverting.
Note that the article details a test with a signal generator to see if the unit works properly before connecting a real amp. That may be useful.
All that from reading the user guide! Who would have thunk! ;-)
Jan
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I see now.
The differential amplifier input stage of the "amp" input section will invert the inverted signal internally as long as I make sure it is connected to the RCA jack that connects to the inverting input of the opamp in the differential stage.
Then I just need to ensure the amp has a gain of 10+.
Does that sound about right?
Edit: yeah it works now. Thanks guys
The differential amplifier input stage of the "amp" input section will invert the inverted signal internally as long as I make sure it is connected to the RCA jack that connects to the inverting input of the opamp in the differential stage.
Then I just need to ensure the amp has a gain of 10+.
Does that sound about right?
Edit: yeah it works now. Thanks guys
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There's an attenuation 10:1 in the leg for the power amp because it is supposed to handle gains between 10 and 35. So for unity gain you can set that to 1.
It's all in the article, I'll lower the price to just 0.50 (store won't accept 0).
Linear Audio | your tech audio resource
Jan
It's all in the article, I'll lower the price to just 0.50 (store won't accept 0).
Linear Audio | your tech audio resource
Jan
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Hi Folks
I had a problem when first using the DM and it turns out that the minimum DUT gain it can test is 15.6. The problem was that I was testing a JLH Class A amp that has a gain of just over 12, so it wouldn't null.
The solution is quite simple though, just pad down the 'Source' signal that goes to the DM. I used a 6dB attenuator made a from a pair of good quality resistors between the oscillator and the DM and the full level to the JLH amp. Works fine.
Cheers
Mike
I had a problem when first using the DM and it turns out that the minimum DUT gain it can test is 15.6. The problem was that I was testing a JLH Class A amp that has a gain of just over 12, so it wouldn't null.
The solution is quite simple though, just pad down the 'Source' signal that goes to the DM. I used a 6dB attenuator made a from a pair of good quality resistors between the oscillator and the DM and the full level to the JLH amp. Works fine.
Cheers
Mike
I had a similar problem with the distortion magnifier kit as supplied, but my problem was with phase not gain being out of range while trying to test the DM after I built it with the handy "Frankenstein" Pioneer SX-450 I keep around the shop.
Increasing the capacitor size in the Phase null circuit enabled the DM to null with this particular DUT. All in all a very useful piece of gear for those of us who can't afford AP gear, even used.
Increasing the capacitor size in the Phase null circuit enabled the DM to null with this particular DUT. All in all a very useful piece of gear for those of us who can't afford AP gear, even used.
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