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Old 27th November 2010, 08:22 PM   #1
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Default Gyrator Frequency Response?

I built a test bed to evaluate different types of anode loads for my next version of the 12b4a preamp.

My current 12b4a preamp is using a 10m45s CCS as the load. It was a quicky to clean up the wiring mess I had initially, but some simple measurements of the frequency response showed it to be flat out past 30kHz.

I am now playing with Wavebourn's Gyrator. Attached is the schematic, pcb used and the response curve. I don't think a gyrator has a response like this indicating I did something wrong. The response curve is shown with a calibration run so it is the circuit response that drops off starting at 10kHz. I have switched tubes, played with bias, and moved the anode voltage up and down. The curve is taken at 140V anode, 20mA bias with 200V b+ into the gyrator.
Your help would be greatly appreciated.
Attached Images
File Type: png 12b4 Gyrator v1 brd.png (141.3 KB, 476 views)
File Type: png 12b4 Gyrator schem v1.png (67.6 KB, 484 views)
File Type: jpg version 1 gyrator freq response.jpg (127.8 KB, 466 views)
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Last edited by SGregory; 27th November 2010 at 08:24 PM.
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Old 27th November 2010, 08:44 PM   #2
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What is output & transfer capacitances summed of KSA1156Y?

What is input capacitance of your test equipment?
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Old 27th November 2010, 08:48 PM   #3
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Daft question, but I will ask anyway: are the grid stoppers actually 150? If you accidentally used 150K instead then you could get the HF rolloff due to Miller effect.
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Old 27th November 2010, 09:10 PM   #4
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The input capacitance of the test equipment should be quite low. The cal curve was taken with the same set-up and cables. It is a 1626m soundcard.

In regard to the KSA1156Y I can't find the output or transfer capacitance in the fairchild spec sheet. I do have some MJE350STU on hand but I find the capacitances for those either.

DF96 good question will check.
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Old 27th November 2010, 09:13 PM   #5
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Grid stoppers are right on 150R
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Old 27th November 2010, 09:14 PM   #6
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Hello SGregory,
I do not have an answer. I do have a question.
How is the E-Mu and software set up? Setup in terms bits and frequency (16, 48), (24, 96) (?,?).
Could be the brick wall filter your plots are showing.
Does the same thing happen when you FFT your Salas JFET Riaa?
DT
All just for fun!

Last edited by DualTriode; 27th November 2010 at 09:18 PM.
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Old 27th November 2010, 09:24 PM   #7
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48kHz, 32bit. Filter shoould be about 22kHz as is shown on the red plot. I took that using direct feed through just before I measured the 12b4 (blue line). The output of the card is set to 1Vrms. Then attenuate it with a ladder attenuator to so that the output of the circuit is 1Vrms.

The Salas JFET Riaa was measured with the same set-up
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Old 27th November 2010, 09:46 PM   #8
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Here is some measurements
B+ 278V
V anode = 142V

With respect to B+:
The KSA1156 has 12.1V at its base, 136.2 at the collector, and 11.7V at it emitter.
The BC559C has 6.18V at its base, 11.7 at the collector and 5.53 at its emitter.

The zener diode is flowing 1.3mA
R5 is passing 0.28 mA

The circuit appears to be working, I suspect that the current may be too low through the zener for the ksa1156

Current was fine. Doubled it and still same rolloff.
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Last edited by SGregory; 27th November 2010 at 10:02 PM.
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Old 27th November 2010, 11:06 PM   #9
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Hmmm... How big is inductance of C2?
What if to shunt it out by something else?
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Old 27th November 2010, 11:12 PM   #10
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C2 is an MKT. I will try a lower value.

After searching for a while, I have come to the conclusion the the capacitance of the KSA1156 has to be quite large. There is no documentation for it so it wasn't an issue in the design of the part. Found quite a few replacements that specify low capacitance but either they don't have a high enough voltage rating or can't dissapate enough heat. I know they are out there just havn't looked in the right spot yet.
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