AVC measurements?
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AVC measurements?
Hello, I can't find complete measurements for these devices. I saw the thread about impedance and frequency response, but I need distortion and signal to noise measurements also. I need to know the total DC resistance of the AVC as well. I would like to use these in my product. I sent an email but have not heard back in several days. Anyone have any of these measurements? Thanks.
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Well, simulations in SPICE are showing the typical high distortion at low frequencies, which is common with magnetic devices like transformers, chokes, etc. There is a way to mitigate that, but I haven't figured out how to apply it to an AVC yet. The THD @ 20Hz is something like 100 times that @ 1kHz.... I'm not sure I can sell a product with that much difference. Humans are not very sensitive (actually virtually deaf) to low frequency distortion, so I doubt this would have much sonic impact, but I'm afraid of reviewers laughing at my product for the large amount of bass distortion if I were to use an AVC in it. I need to think about this some more...
BTW, how much DC can these handle with the standard gap?
BTW, how much DC can these handle with the standard gap?
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I measure less than .1% distortion midband and with 1V input @ 20hz I get .2%. Upping the voltage to 7V @ 20hz takes the distortion to 1%. (the 1% number comes from loading a 12B4 so there may be a small bit of tube distortion. I'll post a FFT later.
No DC offset is permissable.
dave
No DC offset is permissable.
dave
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OK, great, thank you. My simulations show lower THD, but I don't have a good AVC model.
These numbers are with your normal air gap? What was the source impedance and the load impedance? (sounds like the load impedance was the grid of a vacuum tube?)
If the source impedance were near zero, and the load impedance near infinity, and the lams were stacked to make maximum inductance, I think that we'd see much lower THD. Do you agree?
These numbers are with your normal air gap? What was the source impedance and the load impedance? (sounds like the load impedance was the grid of a vacuum tube?)
If the source impedance were near zero, and the load impedance near infinity, and the lams were stacked to make maximum inductance, I think that we'd see much lower THD. Do you agree?
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50R source and the load was the grid of a tube. Inductance was in the 100hy range. Tube was a 12B4 loaded with 60hy of inductance.
I think the bulk of the distortion was from the AC levels on the autoformer. They are intended for line level use so 2V or so input. 4V @ 20hz gives about 1% distortion at 20hz. The only info at 20hz in music is subharmonics and I have yet to see full signal output at that frequency.
dave
I think the bulk of the distortion was from the AC levels on the autoformer. They are intended for line level use so 2V or so input. 4V @ 20hz gives about 1% distortion at 20hz. The only info at 20hz in music is subharmonics and I have yet to see full signal output at that frequency.
dave
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FYI, from Sowter in the UK:
"9335 uses a 100% Mumetal core and the distortion over the critical range of 1kHz to 10 kHz will be less than 0.002%.
Above 10 kHz to 100 kHz and below 1kHz to 100 Hz it will rise to 0.01%.
Below 100 Hz the distortion depends very much on the applied level rising to 0.1% at +10 dBu and in a worst case scenario of +20 dBu at 20 Hz it will rise to 1%."
I wonder why a transformer has lower THD than an autotransformer?
"9335 uses a 100% Mumetal core and the distortion over the critical range of 1kHz to 10 kHz will be less than 0.002%.
Above 10 kHz to 100 kHz and below 1kHz to 100 Hz it will rise to 0.01%.
Below 100 Hz the distortion depends very much on the applied level rising to 0.1% at +10 dBu and in a worst case scenario of +20 dBu at 20 Hz it will rise to 1%."
I wonder why a transformer has lower THD than an autotransformer?
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measuring 0.02% distortion is no easy task since coming up with a source that has an order of magnitude less distortion is very difficult. I was simply measuring the "worst case" situation at 20hz when I got 1% and my numbers were not far off from sowthers. (+20dBu is about 7.5V) The cause of this dostortion is simply a function of the intensity of the magnetic field in relation to saturation. More turns will lower the B field and the distortion at the cost of high frequency behavior.
dave
dave
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for reference here are some pics of my loop of my soundcard at 25hz and 1Khz. I went with 25hz to avoid the 60hz line frequency and it looks like the source harmonics are below -90dB. The 0dBfs reference is 2Vrms.
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- 1Khz loop.bmp (2.33 MiB) Viewed 73641 times
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- loop.bmp (2.33 MiB) Viewed 73641 times
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These all have a 2Vrms input signal and the output is taken from a -10dB tap on an autoformer and it looks like the third is -75dB at 25hz... not sure where the 2nd is coming from since an unbiased transformer should only create odd order distortions.
I'm also unclear why in the 1Khz case the loopback thrid is at -95dB and the autoformer is -105dB from the fundamental.
dave
I'm also unclear why in the 1Khz case the loopback thrid is at -95dB and the autoformer is -105dB from the fundamental.
dave
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- -10db.bmp (2.33 MiB) Viewed 73637 times
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- -10db 100hz.bmp (2.33 MiB) Viewed 73637 times
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- -10db 1K.bmp (2.33 MiB) Viewed 73637 times
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Hello Dave, sorry for the delay but I have been very busy lately. Here is one of several screen shots of FFT's of your AVC with a unity gain op amp buffer on the output. The performance is very good. I am impressed enough that I think I want to use these in one of my products.
Input was set at 2Vrms for 0dB. The oscillator output was then set to -10dB. I think the tap I'm using is -6dB.
Performance may improve with max inductance lam stack.
Input was set at 2Vrms for 0dB. The oscillator output was then set to -10dB. I think the tap I'm using is -6dB.
Performance may improve with max inductance lam stack.
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- Slagle AVC FFT 20Hz.JPG (95.31 KiB) Viewed 73470 times
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- Slagle AVC FFT 200Hz.JPG (91.96 KiB) Viewed 73470 times
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- Slagle AVC FFT 2000Hz.JPG (93.66 KiB) Viewed 73470 times
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OK, here is the FFT for -3dB input level (0dB is 2Vrms, so -3dB should be 1Vrms), with attenuation set to mid level, -18dB. Performance is still excellent. Noise is extremely low, as expected. I like this a lot.
What seems to happen is that as the level goes up, the harmonics multiply, but that could be the op amp, since it's only operating off of +/-9VDC batteries.
What seems to happen is that as the level goes up, the harmonics multiply, but that could be the op amp, since it's only operating off of +/-9VDC batteries.
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- Slagle AVC FFT -3dB input.JPG (94.27 KiB) Viewed 73455 times
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I did a loop back on the L (red) channel in this screen shot. As you can see, the oscillator adds 2nd and 3rd harmonics, so we subtract that from the blue trace and see that the 2nd and 3rd added by the AVC is actually lower than shown. I can't figure out yet how to subtract the left from the right channel in the software.
You can also see that the setup of AVC + op amp adds less than 10dB to the noise floor.
You can also see that the setup of AVC + op amp adds less than 10dB to the noise floor.
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- Slagle AVC FFT -3dB L+R.JPG (97.21 KiB) Viewed 73454 times
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In this case I don't think the TCR really matters since the DCR appears as a small series resistance and when you compare the 60 or so ohms of DCR to the impedances involved a 10% change isn't going to matter.
Now when you talk about copper resistors, the TCR is a fair bit more of a concern but for some reason they still sound good (and I hate resistors). Would I use them in a situation where precise resistive value is critical like an RIAA filter.... no. (at least not yet) However there are lots of places where 10% here or there isn't an issue and when you run DC through them they tend to heat up and that often swamps any minor ambient changes.
dave
Now when you talk about copper resistors, the TCR is a fair bit more of a concern but for some reason they still sound good (and I hate resistors). Would I use them in a situation where precise resistive value is critical like an RIAA filter.... no. (at least not yet) However there are lots of places where 10% here or there isn't an issue and when you run DC through them they tend to heat up and that often swamps any minor ambient changes.
dave
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Yeah, I don't think the TCR of the copper windings is critical to an AVC. I would be curious about the VCR though, which is the change in resistance with changes in applied voltage. A typical metal film resistor is 5ppm, for example, which is very low. So, for an AVC, the relevant value would be a change in attenuation with changes in voltage. Since we can't apply DC to an AVC, then I guess what I can do is change the AC voltage input and plot it against attenuation (voltage output). It should be a straight line. If not, then there's a problem.