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Old 9th February 2011, 04:43 PM   #1
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Default My Wave Isn't Square.

The top trace is the input signal at about 3.8v P-P, 1K hz. The bottom trace is 10v P-P after the coupling cap.

-6 dB NFB has reduced the over shoot and most of the ringing. The issue now is the top and bottom of the wave are not flat. They both increase with time. I understand this represents a emphasized low hz response.

I presume the differance in the angle of the top and bottom is due to nonlinearity.

The output trace (not shown) is an inverted image with the highs over emphasized, decrease with time, and with a bit more angle. The angle increases as volume increases.

The amp is very quiet and clear but it's a bit bright.

How would I correct this type of frequency response?
Attached Images
File Type: jpg Sch.jpg (25.1 KB, 1372 views)
File Type: jpg SW 6SN7.JPG (107.9 KB, 1354 views)
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Old 9th February 2011, 04:48 PM   #2
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Out of curiosity: what does it look like if you disconnect the feedback?
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Old 9th February 2011, 06:04 PM   #3
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You are looking at high pass filter behavior, which is typical when looking at square waves. The lower you go in frequency, the more pronounced this will become. The higher in frequency, the less pronounced.

Schematic would help to identify the capacitor that is forming this HPF, but I would say you're fine if you are happy with the bass response of the amp.
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Old 9th February 2011, 06:06 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Troncones View Post
The top trace is the input signal at about 3.8v P-P, 1K hz. The bottom trace is 10v P-P after the coupling cap.

-6 dB NFB has reduced the over shoot and most of the ringing. The issue now is the top and bottom of the wave are not flat. They both increase with time. I understand this represents a emphasized low hz response.

I presume the differance in the angle of the top and bottom is due to nonlinearity.

The output trace (not shown) is an inverted image with the highs over emphasized, decrease with time, and with a bit more angle. The angle increases as volume increases.

The amp is very quiet and clear but it's a bit bright.

How would I correct this type of frequency response?
You haven't touched on the OPT you are using which is certainly responsible for most of what you are seeing. FWIW that is a fairly good looking SQ wave. No OPT is going to have sufficient HF response to pass a perfect square wave except at relatively low frequencies, (a couple of kHz or so) and that presupposes that the primary inductance is sufficient not to load the tube at the frequency of interest.
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Old 9th February 2011, 07:03 PM   #5
DF96 is offline DF96  England
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Why do you want to 'correct' the response? The negative feedback is doing, presumably, what you want it to do. It corrects the output (precisely, the NFB sampling point) by modifying the input. You are simply seeing the modification. You would see far 'worse' waveforms inside other amps.
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Old 9th February 2011, 07:13 PM   #6
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Originally Posted by DF96 View Post
Why do you want to 'correct' the response? The negative feedback is doing, presumably, what you want it to do. It corrects the output (precisely, the NFB sampling point) by modifying the input. You are simply seeing the modification. You would see far 'worse' waveforms inside other amps.
Indeed it is, and it would have been more interesting/useful to see the square wave at the output. I also agree having seen far worse waveforms in other amps as well.
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Old 9th February 2011, 07:16 PM   #7
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Instead of measuring the amplifier square wave performance you should do a simple frequency response measurement to determine whether there is anything beyond subjectivity in your comment about the amplifier sounding bright.

I suspect what you will find is the amplifier's power response is down several dB or more at 20Hz (or even higher) resulting in the subjectively bright sound.

Your source could be a bright/hard sounding CD player/dac and this amp is just telling it as it is.

Your speakers could have HF drivers that are inherently bright, analytical or....

It is also possible that you just don't like the sound of a pentode amp even with some global feedback applied. I know I don't in many cases..

You really need to give us a lot more information..
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Old 10th February 2011, 12:37 AM   #8
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How do the square waves look at the output?

I've seen the same thing myself. If you connect the probe to a Hi-Z point (as you did here) the apparent loss of high frequencies could be an artifact of cable and o'scope deflection amp capacitance.

I got some hideous-looking square waves, and what looked like an Fh of barely 20KHz by o'scoping a Hi-Z point. Connected to the more appropriate Lo-Z point (the output of the cathode follower grid drivers) the square waves were nice and flat, and the Fh measured to be some 117KHz.
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Old 10th February 2011, 03:56 AM   #9
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Default Thanks for your input

The first photo is input (1K hz, 4V P-P) and output (10V P-P) with -6db NFB. The second no NFB. I still have a little over shoot to work on.

The OPT's are 10w 5K-8 Edcor's (GXSE10-8-5K).

It's good to know I'm in the ball park. It's also good to know there is room for improvement. I will play around a bit more with the NFB. Mullard references NFB in the -20db range for pentodes in their Tube Circuits for Audio Amplifiers book. The sweep is a good idea.

I'm consistant with the listening test. Same source, speakers, tracks, room, volume. You might right, It may just be the new pentodes. I'm used to my old Mullards.
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File Type: jpg Output-No NFB.JPG (90.4 KB, 1151 views)
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Old 10th February 2011, 01:04 PM   #10
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Bear in mind that -6dB is far too little feedback. You either need none, or enough. With none, you just get the valve non-linearity (mainly low order). With enough (-20dB?) you get significant distortion reduction. At -6dB you get lots of high order products arising from the feedback, but little suppression of them. Worst of all worlds.
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