AVC measurements?

discussion of magnetic volume control design for both line and speaker levels.

dirkwright
Posts: 56
Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

I think you might find this pleasing. Zero distortion @ -18 dB input level @ -18 dB attenuation. 0dB is 2Vrms in this case. Noise is in the basement. Very nice!
Attachments
Slagle AVC FFT -18dB.JPG
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dirkwright
Posts: 56
Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

I checked the input/output linearity @ -18dB attenuation and it looks perfect to me. So, I'm excited about using these in my headphone amps.
dirkwright
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Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

Frequency Response with full inductance lam stack. Much better bass response. Only down 0.3dB @ 20Hz! Rock on!
Attachments
Slagle AVC FR full lam stack.JPG
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dirkwright
Posts: 56
Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

And here is the FFT for the full lam stack, this time only -6dB attenuation but the input signal is the same as before @ -18dBFS (-3dB is 2Vrms). Distortion is zero. That little pip @ 2kHz was not a harmonic since it was just part of the noise floor. I just happened to hit "print screen" when it was up. This is really outstanding performance! I'm very pleased!
Attachments
Slagle AVC FFT -6dB.JPG
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dirkwright
Posts: 56
Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

10kHz square response. Input set to 2Vrms, attenuation is -6dB. Really nice! Inductance is 218H @ 100Hz.
Attachments
PA200205 - sm.JPG
PA200205 - sm.JPG (122.77 KiB) Viewed 10532 times
docali
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Joined: Thu Oct 16, 2008 7:16 am

Post by docali »

Hello Dirk!

Very nice measurements!

But as such results much depends on the source impedance would you please tell us what was the source impedance of you function generator and what was the schematic for measurement? Was this device loaded?

Thanks!
Docali
dirkwright
Posts: 56
Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

docali wrote:Hello Dirk!

Very nice measurements!

But as such results much depends on the source impedance would you please tell us what was the source impedance of you function generator and what was the schematic for measurement? Was this device loaded?

Thanks!
Docali

the circuit is the AVC as an input, with an opamp set to unity gain directly connected to the output of the AVC. power was by 9V batteries. so no, the AVC is unloaded on the output since the unity gain opamp presents an infinite load (effectively) to the AVC. This is basically the way I intend to use it.

Source impedance for the FFT's is unknown, but low. I used a Tascam UH-7000 for those. For the square wave results, I used an analog oscillator that has a 600 ohm source impedance. It's a Kenwood oscillator.
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Post by docali »

Hello Dirk!

You state that the source impedance of the oscillator is about 600 Ohms. In this case it is no wonder that there is less ringing. The 600 Ohms damps the TVC quite good. But what when you drive it with abot 200 Ohms or even 50 Ohms? Then you see the truth :wink:

You can do this while driving the oscillator into an resistor divider 560R-56R and drive the TVC around the 56R resistor. The ringing should be much more pronounced. These TVCs should be driven from fairly low source impedances. I doubt that 600 Ohms will do the best job but Dave can maybe say more about what should be the source impedance of the driving stage.

Thanks!
Bernd
dirkwright
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Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

I'm not concerned with ringing, so I'm not going to do the test you describe, thanks. I checked it with a digital generator that has very low source impedance and it performed very well as I recall. It's not an issue for me.

What is an issue is DC balance for the following gain stage after the AVC. Since I use op amps and feedback, I need to make the feedback network have equivalent DC resistance as the AVC. This is not possible with resistors, so I have to use an inductive feedback network in order to achieve low enough DC resistance there. I am in the process of developing such a network myself, however, the cost for the entire product will be much higher because of that. I do not make, nor am I interested in, passive preamps.
dirkwright
Posts: 56
Joined: Sat Jul 25, 2015 9:50 pm

Post by dirkwright »

docali wrote:Hello Dirk!

You state that the source impedance of the oscillator is about 600 Ohms. In this case it is no wonder that there is less ringing. The 600 Ohms damps the TVC quite good. But what when you drive it with abot 200 Ohms or even 50 Ohms? Then you see the truth :wink:

You can do this while driving the oscillator into an resistor divider 560R-56R and drive the TVC around the 56R resistor. The ringing should be much more pronounced. These TVCs should be driven from fairly low source impedances. I doubt that 600 Ohms will do the best job but Dave can maybe say more about what should be the source impedance of the driving stage.

Thanks!
Bernd
Still no ringing. I used a nice digital oscillator with very low output Z and there was no ringing. They were perfect square waves.
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