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Old 30th March 2013, 01:24 PM   #3071
Magz is offline Magz  United States
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Quote:
Originally Posted by merlin el mago View Post
No, without OPT as Iko did.
I don't know about you guys, but I can clearly hear transformer ringing. It gives a harshness to the highs.

Here's an example of ringing on the input transformer of my 26 preamp (pic #1), which was cleaned up with an RC network on the secondary (pic #2) and series resistance on the primary (pic #3). The sound went from very harsh, to a little harsh, to smooth and clean going from picture 1 to picture 3. Ended up with 2nF and 850 ohms across secondary, and 800 ohms on primary.

Note that using a resistor across the secondary will drastically lower your gain, while using an RC will not.

I had a similar experience with the output transformer of my 2C22 preamp.


Note: these pictures are 10kHz square waves.

Last edited by Magz; 30th March 2013 at 01:26 PM. Reason: Added note.
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Old 30th March 2013, 01:26 PM   #3072
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schematic
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File Type: png 26 VR150+CCS+output cap.png (54.1 KB, 207 views)
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Old 30th March 2013, 01:39 PM   #3073
stajo is offline stajo  Sweden
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Interesting Magz!

Merlin. You need an anode load. Resistor, choke, OPT or gyrator.
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Old 30th March 2013, 01:41 PM   #3074
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Most simple is resistor, wich value? or better how can calculate?
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Old 30th March 2013, 01:56 PM   #3075
stajo is offline stajo  Sweden
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Im in the mall. Look at suggested value for Ra for 150 v in the datasheet
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Old 30th March 2013, 02:15 PM   #3076
iko is offline iko  Canada
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Are you giving up on your output transformers? If you want to keep your output transformers then you cannot use an output cap.
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Old 30th March 2013, 02:33 PM   #3077
stajo is offline stajo  Sweden
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Originally Posted by stajo View Post
Im in the mall. Look at suggested value for Ra for 150 v in the datasheet
I didnt find any suggested value in the sheets. Try 50 k. You might need to up the B+ to compensate for the fall over the anode resistor.
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Old 30th March 2013, 02:49 PM   #3078
stajo is offline stajo  Sweden
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Originally Posted by Magz View Post
RC network on the secondary (pic #2)
Could you draw or explain this. Is it a classic RC or a zobel?

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Staffan
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Old 30th March 2013, 02:58 PM   #3079
Magz is offline Magz  United States
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Originally Posted by stajo View Post
Could you draw or explain this. Is it a classic RC or a zobel?

Best regards
Staffan
Simple. R+C across transformer secondary. The best way to figure out the values (which are different for every transformer and circuit) is to use a small cap such as 1nF and a 5K or so pot in series across the secondary. Adjust the pot to get the best looking square wave. Then go up in capacitance to 2nF and do the same, then go to 3nF, etc. You want to choose the combination that has an acceptable square wave using the lowest C and the highest R. Once you have those values, substitute a fixed resistor for the pot. I like to use TX2575 resistors from Texas components for this, since they will custom make the resistors to whatever exact value you need; a little pricey but really, really nice low noise resistors.

I usually split the resistor value in two and use an RCR network, just so the resistors isolate the transformer from the cap somewhat.

In the case above, I found that the input impedance to the transformer was crucial to achieving the best performace, so I put a pot in series with the primary and adjusted that to get the best square wave after I optimized the secondary, then replaced the pot with a fixed resistor.
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Old 30th March 2013, 03:05 PM   #3080
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Quote:
Originally Posted by iko View Post
Are you giving up on your output transformers? If you want to keep your output transformers then you cannot use an output cap.
I want to try the both before deciding which to use
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