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22nd September 2016, 09:38 AM  #1 
diyAudio Member
Join Date: Mar 2003
Location: Zürich

An amplifier to test chokes
Last week I put together this simple Single Ended amplifier to test Power supply chokes.
The choke (DUT) is connected in series with the drain, and parallel to the choke I connected a resistor, with value based on expected inductance at about 100Hz (which is the frequency at which the PS choke will be working). I tested a 159M from Hammond (15H at 100mA), so took a 12k resistor I had on hand. The big resistor in the source is to further increase the output impedance of the drain. With the potentiometer one can dial in the desired DC current. I used Steps (Arta) to generate a bode plot (sorry, ommited phase in the graph) to measure the frequency response of this circuit, obtaining the plot below. I fed about 50mV RMS to the gate, output was about 5V RMS (voltage across the choke). The way I interpret it: the frequency response is relatively flat between 1k and 10k.This is the region where the inductive reactance of the choke is so high that the load to the amp is dominated by the 12k resistor. We are interested in the 3dB points at both extremes: I would say that at about 150Hz we have a 3dB relative to the 1k to 10k band. So at 150Hz the inductive reactance is = load resistor = 12k. Calculating backwards (12000/(2*pi*150)) = L = 12.73H (close to the claimed 15H). At the top end the 3dB is at about 50kHz, one can calculate capacitance from that. Is this approach more or less sane? And I was quite impressed that the choke goes up to 20kHz with less than 1dB loss. Erik
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22nd September 2016, 04:31 PM  #2 
diyAudio Member
Join Date: May 2015
Location: Parana

Very clever!
Knowing the attenuation points is an interesting tactic, because it will lead to know the real inductance for the application with the custom signal applied. So I will not find strange to find some different values provided by the manufacturer due to differences between methods. But if I make one (and will !), I prefer to use a bigger source resistor, only to be at safe side about the drain resistance. Gary Pimm used one of their modified CCS for measuring a DC power supply (one also based from GPCCS's), injecting variable DC+"AC" (variable DC) current to check load response/output Z etc , so this is a tool for several uses. I've seen a some time ago a guy using a pentode for this approach. Without degeneration, the rp is somewhat low and variable, unless one use a signal pentode like 6AU6 with >1M for ra, but with some heavy cathode resistance the behaviour will be the same (good for tube maniacs like me!). If one needs also to know the low signal response, is possible to use the FR1 or FR2 from ARTA. It uses a pink noise to test, and save some time for measuring.
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22nd September 2016, 08:16 PM  #3 
diyAudio Member
Join Date: Aug 2010

I agree, very clever.

22nd September 2016, 10:12 PM  #4 
diyAudio Member
Join Date: Feb 2009
Location: Melbourne, Oz

A common assessment is to keep the voltage across the DUT choke a constant, so as to avoid changing the choke's inductance.
Did you measure the shunt choke capacitance in the more typical way, and compare results ? Did you measure the nonDC bias inductance using your circuit, and compare that to the typical measurement using just mains frequency applied voltage and a current meter? 
22nd September 2016, 11:13 PM  #5 
diyAudio Member
Join Date: Apr 2008
Location: Carlisle, England

I have designed LLC SMPS and needed to know the leakage inductance of the transformer.
I shorted the outputs together. I then applied a sine wave through a known capacitor. I moved the frequency up and down until I got minimum signal across the series LC and this is the resonant frequency. I know the C so its just a matter of rearranging the formula to work out the L. f=1/(2 pi sqroot(LC) )
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23rd September 2016, 11:50 AM  #6  
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Join Date: May 2015
Location: Parana

Quote:
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As always, lots of ideas, enough components, little time available, and several errors ... 

23rd September 2016, 12:01 PM  #7 
diyAudio Member
Join Date: Mar 2003
Location: Zürich

Thanks for all comments!
I have not done any other tests, yet  so thanks for your input and suggestions! Cheers, Erik
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