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Mic preamp without input transformer
Mic preamp without input transformer
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Old 4th February 2018, 05:50 AM   #11
MarcelvdG is offline MarcelvdG  Netherlands
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It could also be microphonic effect from the tubes. Some can ring on very high harmonics of the mechanical vibration that excites them.

If you have a computer with a sound card and an audio editing program, could you make a recording and post a waveform plot and an FFT? Maybe that will give someone a clue what the problem is.
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Old 4th February 2018, 11:24 AM   #12
turk 182 is offline turk 182  Canada
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would low pass filters on either the input, output or B+ supply lines reduce the possibility of a high frequency modulating audible audio? and i know the question remains would doing so result in an audible difference?
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Old 4th February 2018, 11:50 AM   #13
turk 182 is offline turk 182  Canada
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come to think of it wouldn't a transformer across the input exibit low pass?
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Old 4th February 2018, 01:22 PM   #14
MarcelvdG is offline MarcelvdG  Netherlands
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It would, but it would be a very expensive way to make a simple low-pass.

Filtering could definitely help if the problem is undesired demodulation of RF signals that get picked up somehow.
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Old 4th February 2018, 03:36 PM   #15
turk 182 is offline turk 182  Canada
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that's the other thing we don't know...just how well shielded this pre-amp is...are tube shields in place?
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Old 7th February 2018, 05:16 AM   #16
6V6dude is offline 6V6dude  Australia
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Quote:
Originally Posted by MarcelvdG View Post
If you have a computer with a sound card and an audio editing program, could you make a recording and post a waveform plot and an FFT? Maybe that will give someone a clue what the problem is.
That's a great idea. I'm not sure if anything can be seen by looking at a recorded waveform or what signal should be recorded. Perhaps I could do the test my self but don't know how. I did compare original sound file (song) with one I've recorded through the preamp in basic software frequency analyzers but could see no difference visually at all. ALl the peaks and frequencies looked same to me.

As for shielding, it is well shielded. Build in metal box and divided to three sections. I tried to that as best I could knowing I want to record with it. Tubes are at the top side. See pic.
(That solid state piece is there just to run headphones output and VUs).

Click the image to open in full size.

Last edited by 6V6dude; 7th February 2018 at 05:27 AM.
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Old 7th February 2018, 08:44 PM   #17
MarcelvdG is offline MarcelvdG  Netherlands
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Correct me if I'm wrong, but if I understood you correctly, the problem is that when you record silence, you hear some weird background sound you can't get rid of. You could try to record silence with a microphone connected and silence with the input shorted, select parts where the weird sound is clearly audible and post waveforms and spectra.
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Old 7th February 2018, 10:16 PM   #18
turk 182 is offline turk 182  Canada
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can we see the tube side?
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Old 7th February 2018, 11:33 PM   #19
6V6dude is offline 6V6dude  Australia
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No it doesn't have a strange sound on it's own. It doesn't even have a strange sound when I play music through it, sounds great. But it mounts up when I do number of recorded tracks.
It's kind of.....umm...annoying? My wife described it as it having an annoying pitch to it. To me it's like something in the upper midrange that sticks out.
Thinking about it, it is maybe reminiscent of EL84 tube sound which is very upfront and in your face. As oppose to 6V6 which is more subtle and sits back a bit. To my ears anyway. Could it be even possible that 12AX7 is not suitable for mic preamp the way I build it? I've noticed some mic preamps use both sides of the tube as single stage but apparently that's just for less noise. So I don't know....
The tube side here.

Click the image to open in full size.

Last edited by 6V6dude; 7th February 2018 at 11:39 PM.
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Old 8th February 2018, 12:20 AM   #20
majerjack is offline majerjack  United States
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I suspect the unpleasantness in the sound is related to the fact that you are building tracks with a Shure SM57 and are terminating it with an 1800 ohm impedance.


The SM57 has a frequency response that rolls off sharply at both bass and treble extremes, with a +6dB peak in the 5-7kHz range. Summing many tracks with that kind of upper midrange boost is going to make hash out of your mix. In addition, some people believe that terminating the SM57 (and some other dynamic microphones) with lower impedances improves the sound by damping the ringing in the response. You might try fixing a resistor in parallel with the microphone input to see if improves the sound. I suggest a parallel resistor of 1000 ohms to get an input impedance of close to 600 ohms. Your signal-to-noise ratio will take a hit, but perhaps that will not be a serious problem with this preamp.


Of course, you could always shop for a different microphone, one with a flatter, more extended frequency response.
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