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Mic preamp without input transformer
Mic preamp without input transformer
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Old 31st January 2018, 05:57 AM   #1
6V6dude is offline 6V6dude  Australia
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Default Mic preamp without input transformer

Apart from Ohm matching, what else will input transformer do for a mic preamp?
Reason I'm asking is because I've build mic preamp some years ego, used it a lot but there was always something not right with the sound that I can't define. Almost like if some part of audio spectrum is missing but yet it sounds fine when compared to other preamps. I know, sounds strange. One interesting thing about this amp is that it is extremely quiet, almost no hiss. Probably only about 5% of hiss compared to transistor mic preamp. This is what leads me to think some frequency might be missing? I have no equipment to measure these things.
It'a s simple circuit, very much like a basic guitar preamp with two stage 12AX7 and 12AU7 cathode output. So maybe input transformer is necessary for use with mic?
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Old 31st January 2018, 06:32 AM   #2
6A3sUMMER is offline 6A3sUMMER  United States
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What kind of mic are you using?
Dynamic
Ceramic
Ribbon
Condenser
Electret

Is the mic balanced output?
Is the mic unbalanced output?
Does it require phantom power?

What is the model of the mike, i.e. Sure SM58, etc.?

Are you using the mic in a high RF or Electrical Field environment?

What is the schematic for your mic preamp?

What does the preamp drive, a PA amp and speaker, a recorder, etc.?

Sorry for all the questions, but this is a system, not just a single part.
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Old 31st January 2018, 06:34 PM   #3
PRR is offline PRR  United States
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The self-hiss of a dynamic microphone is near 0.2 microVolts.

The self-hiss of a vacuum tube tends to be over 1 microVolt.

A correctly picked and biased transistor can be in the 0.2 microVolt range.

Beyond that: your question is too vague and fact-free to even guess. You want to measure gain and frequency response. Once you can control gain, ear-tests of other mike amps *at the same gain* give a clue to hiss level.
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Old 2nd February 2018, 03:06 AM   #4
6V6dude is offline 6V6dude  Australia
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Sorry, should have posted the schema in the first place. I've all ready posted it one here, about a year ego when I was redesigning it from anode output to cathode follower, maybe some of you will remember.
Can you see anything in it that should make it sound "bad"? I've build hifi amps I'm using with similar design and they sound fine. The thing here is that the "effect" multiplies when I record multiple tracks. It's not a bad sound, just kind of midrangey and is very hard to mix because it seem there is no body to it or something.
I'm generally using SM57 mic and have it set to 1.8k on the input. The cathode follower should provide about 300 ohm so overloading the output is not a problem since the highest load I put on it is 10k.
The only thing I can think of that maybe the sound is caused by the my tube selection. I use one Mullard and one Miniwatt. When I tried Sovtek they were just way too noisy and harsh.
I think this circuit should sound good but maybe it's a different story for mic preamps, I'm not sure. (Hope there is no error in the schematic).
Click the image to open in full size.

Last edited by 6V6dude; 2nd February 2018 at 03:09 AM.
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Old 2nd February 2018, 02:51 PM   #5
turk 182 is online now turk 182  Canada
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what's the supply for this? i would look at regulation and insure there's no high frequency oscillation on the supply lines.
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Old 2nd February 2018, 03:50 PM   #6
MarcelvdG is online now MarcelvdG  Netherlands
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When the input potmeter is not at the very maximum, it will generate much more noise voltage than your microphone. The first tube and its cathode resistor do the same, but not quite as badly. With a step-up transformer you can improve the noise matching between microphone and amplifier.
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Old 2nd February 2018, 03:56 PM   #7
pieter t is offline pieter t  Netherlands
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Is the input capacitor really 0.1 mF (100 uF), an ugly electrolytic?
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Old 3rd February 2018, 02:18 AM   #8
6V6dude is offline 6V6dude  Australia
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No, these are polyester caps, the yellow tube type.
Yes I do get some more noise when the pot is not at maximum but still much, much lower than SS preamp so I am happy with that.
I don't have PS schematic but it is just a basic type. 300V PT going through SS rectifier and two stages, each having large filter cap and drop resistor. Pretty much same as found in most basic guitar amps. If that could cause some oscillation I don't know but I never figured out how to design anything better. These always work well in my hifi amps so I just went with the same. My main concern was hum which I don't have or at least I should say it's almost inaudible.
But if you see nothing wrong with the circuit, maybe it's the PS doing something I don't undersand? I have no means of measuring these things.
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Old 3rd February 2018, 05:59 AM   #9
MarcelvdG is online now MarcelvdG  Netherlands
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Quote:
Originally Posted by 6V6dude View Post
Yes I do get some more noise when the pot is not at maximum but still much, much lower than SS preamp so I am happy with that.
That makes me wonder what's wrong with your solid-state preamplifier. A well-designed solid-state microphone preamplifier should produce much less noise than this circuit, especially when the input pot is not at maximum.

Anyway, the two advantages of a step-up transformer are better noise matching and that you can use it to make the input balanced, which helps to suppress hum picked up by the microphone cable. If you have no problem with noise or hum, I don't see a reason to add a transformer.

Last edited by MarcelvdG; 3rd February 2018 at 06:05 AM.
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Old 4th February 2018, 02:31 AM   #10
6V6dude is offline 6V6dude  Australia
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I have several SS mic preamps. Commercial ones but nothing expensive, they all have much more hiss but also more body to the sound.
I am happy with the noise floor on my DIY preamp that's not a problem for me. Minor hum at 60hz but that gets EQed out anyway. But it's got some tone in it I can't even describe, maybe it's the oscillation. I think probably the only way to rectify this problem is by using an oscilloscope which don't have one so I might just shelf the preamp till maybe one day I'll be able to measure it.
But at least I know (since no one said so) there doesn't seem to be any major problem with the circuit design so that's a start.
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