microphone circuit

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Gus

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microphoneschematic.PNG


Instead of changing the circuit in microphone that use a circuit like this try a
simple adjustment.

I have changed one of my MXLv67s to this circuit.
stock transformer, grill, transistors and capsule
2n5087 is because the sim program had it, use the stock PNP
something a little different
This is one of a few adjustments/sims/builds to the circuit
Something to try using most of the stock parts
Screenshot of an edited sim
comments?
 

3nity

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whats the improvement results on??
Isnt the 2n5087 used on the dorsey mod?
Looks like i should try it on my v67.
 

Gus

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Stock PNP is fine.  The sim program did not have the stock transistor anyway it is an EF.
EQed for male voice and the stock parts I left some of the tantalum caps in circuit.

What do you see that is different and not what you find in other microphone circuit adjustments? 
I dislike the term modification for simple circuits

I simmed and adjusted and then changed some parts.  I paid attention to gain, biasing, EQ, current draw and the idea of keeping most of the stock parts

About 10VDC at the drain is for people without a signal generator and scope or THD meters to bias the first stage you could try to do it by ear.

It is a compromise for noise, gain, overloading but it is a low cost change of the circuit to learn from.
 

dai h.

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hi Gus.  :)  Thanks for this.

Gus said:
What do you see that is different and not what you find in other microphone circuit adjustments? 

do you mean not in the other V67 circuit type "mods"? The feedback point?
 

Gus

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I posted this as a something to get people thinking about adjusting the two transistor JFET PNP circuit like this you can find in a few microphones.

Maybe the people who might see what I was doing and why are just not posting.  I can understand that.

I set limits for myself as the smallest amount of parts to change and add keeping the capsule and transformer.

Why did I use a 47K in the JFET stage?
Why the bias change to the EF section and its input resistance?
Why the cap and resistor value adjustments?
Why...

 

dai h.

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Right, so the object not to be a simple spoon-feeding exercise.

smallest amount of parts to change and add keeping the capsule and transformer.

Makes sense as a general principle (i.e. to try to make best use of what is already there as opposed to knee jerking into capsule changes or whatever the trend is). That being said, I only have a vague idea of what is going on.

Why did I use a 47K in the JFET stage?
Why the bias change to the EF section and its input resistance?
Why the cap and resistor value adjustments?
Why...

If the 47k was a plate R (on a tube) I guess that would increase gain (pass more of the output onward).

The bias change is to load the FET output less (and P48)?

The cap values are smaller than stock.

What I recall reading was that (in the mics sharing this basic cct), the FET and EF are not biased correctly (for least distortion--I suppose understandable in a low cost mic) and the added EF loads down the power supply. Also gathered that a transformer like the original (in the u87) probably isn't going to happen since it (one w/high ratio and good performance) would be too expensive for a budget priced mic.
 

gyraf

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Do we really want R13-R14? They contribute quite some to the mic's noise impedance - wouldn't it be better to shovel in some 47uH's in their place (as for the HF rejection)

Jakob E.
 

Gus

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Jakob
Good point about noise I am not posting this is the best adjustment just a set of compromises in the limits I set for myself
I left R13 and R14 for two reasons
One, stock parts
Two, to help with transformer input preamps

dai h.
EF bias change is for two reasons
One, more stable/predicable than stock bias
Two, sets the input resistance to a set value and works with the output of the first stage and C6 for the high pass at the section.

Anyone note the C8 value?
 

mitsos

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Trying to follow along here, not much to add..

Is the original schematic available?
 

tommypiper

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Gus, I really appreciate the thread.  However, you seem to like to pose questions and riddles to provoke people into thinking (and learning).  Fine.  But the problem is many of us don't have enough understanding or foundation to even begin to follow your riddles. 

Instead, would you consider explaining some of the foundation in the circuit so we could be brought up to speed to the point where we'd understand what you're talking about?  When things are explained, I get it, and I learn.  And I feel deeply appreciative and it's why many of us are here.  But when they are posed as cryptic riddles in a circuit we don't know how to read it's just a waste of time. 

Not all of us are designers or techs.  We're musicians, we're busy with music and production and gigs and scheduling and licensing and mixing and troubleshooting, and we're here because we have the intellectual curiosity to understand as much as we can about our gear, how to maintain it, how to service it, how to use it best, sometimes how to build it...  please take these considerations into account.  Many are not even amateur techs or designers... 

Many of us are beyond the point in our lives and careers where an EE class is an option, and we don't have time to search old threads and hunt for clues without knowing even where to begin, or even guess at what questions to ask, without some context and foundation.  We need to be efficient with our time or we sink. 

It seems it would be much more generous and friendly to simply explain as you go, otherwise you are losing some of your audience. 

I have several stock V67s but really have no clue what you are talking about in this thread so far and it's frustrating.  Some background information would be most helpful.  Don't assume anything.  Please. 

If you don't mind that some of us will not get what you're talking about and cannot participate, well fine.  I'm merely suggesting the effectiveness of your posts could increase dramatically and find a larger appreciative audience. 

I know you hate to give out "free" information, from having followed your posts for years, here.  You want people to discover things on their own.  However, this riddle style just leaves me totally in the dark.

This is all meant as constructive feedback, please don't attack me with sarcasm or dismissals, ,etc.  I'm glad you are posting to begin with, just expressing some points which may, or may not, be helpful.  :) 

PS: I'll give you some riddles in return:  What is EF biasing?  Where do you control it?  What does it do?  How do you "sim?"  What is the purpose of that discussion?  Which cap and resistor adjustments are you referring to?  Why and how would I begin to know what they do?  Etc.  I hope you get the picture.  :) 

If you refer me to off-thread resources, I won't have time unfortunately, though it may be helpful in a larger context.  Instead, would it be possible for you to explain a bit what's going on in this specific circuit you have posted and therefore make your riddles specific and definable?
 

tchgtr

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Gus said:
Anyone note the C8 value?

C8 is smaller than stock, I'm guessing to cut the lows a bit more. Perhaps the low-budget transformer just can't deal with the low-end info, and cutting back a bit keeps it from overloading...
 

grantlack

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Gus said:
EF bias change is for two reasons
One, more stable/predicable than stock bias
Two, sets the input resistance to a set value and works with the output of the first stage and C6 for the high pass at the section.

Anyone note the C8 value?

let me see if i have this right wrt to biasing: in the original configuration, the bias point may be shifty due to non-linear behavior of C6.  Adding R100 solidly fixes the bias point by creating a voltage divider between V+ and ground.
this would let you bias 'hotter' since there's less uncertainty as to whether your bias point will drift up and lead to clipping.
gold star?

i'll tack my name on the 'c8 change is anti-saturation insurance' line of thought.
 

gemini86

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You did say this was tuned for the male voice, so I'm guessing that rolling off some of the low lows will both help with proximity effect and keep the average male singer voice from being muddy. Could also help with plosive booms and thumps, which are exaggerated in cheap mics.

Gus said:
Jakob
Good point about noise I am not posting this is the best adjustment just a set of compromises in the limits I set for myself
I left R13 and R14 for two reasons
One, stock parts
Two, to help with transformer input preamps

dai h.
EF bias change is for two reasons
One, more stable/predicable than stock bias
Two, sets the input resistance to a set value and works with the output of the first stage and C6 for the high pass at the section.

Anyone note the C8 value?
 

Marik

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Gus said:
Screenshot of an edited sim
comments?

Hi Gus,

Do you have the sim data for the 2SK170? I am wondering if you had chance to compare that circuit with the 3819 and how those FETs gain differences affect EQ curves. If you need data for the output transformer let me know--I have a few of those and can give you measurements for different conditions.

Best, M
 

zebra50

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The whole capsule biasing arrangement is not what we are used to seeing.

Feedback from the output - R1 R2 R3 C1 C3 C5.

I haven't got into the whole sim thing, so can't really figure out the effect on the frequency response.

Plus the bias thing that Grant has pointed out.
 

MagnetoSound

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I am sure that what Tommypiper wrote rings true for a lot of people reading this thread.

For my own part, I am not familiar with the original V67 circuit, so perhaps posting that schematic together with your amended version would be helpful for comparison.

 

abechap024

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Maybe Gus designed this circuit so that the original PCB in the mic could be used without much modification? It is only a guess, but it is a though stimulating question...
 

Gus

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This is a adjusted schematic of one of my MXLV67m microphones.  In the screen shot I have notes for what parts were changed.
The MXL 2001 and v67 schematic is on the web somewhere I think Flatpicker posted it.  Flatpicker and I compared notes years ago with traces of the microphones IIRC it was posted here many years ago.

The EQ is basically the same as an U87 network
330K capsule voltage source resistance
.47uf to the network

the .47uf coupling cap was not about saturation of the transformer more the circuit response.

The sim shows for the circuit (not the capsule and circuit)
About -3dB at 110Hz and -3dB at 7Khz
About  -15.5dB at 40Hz and -11.5dB at 16Khz

EDIT found the link I was looking for look at the chart for the U67 and M269c  db at 40Hz and 16KHz
http://recforums.prosoundweb.com/index.php/m/38519/0/

The 2SK170 model I used does not seem to be correct it gives a way off source resistor value
The sim a was more for the frequency response.

I was thinking male singer in a rock mix for the EQ and stock capsule/grill.  Also noise might not be as much as an issue

The screenshot is edited I removed the source resistor value and transformer inductance

I did not design this circuit I adjusted values of a few parts and added one resistor

look at this thread
http://www.groupdiy.com/index.php?topic=46109.0
 

tommypiper

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Gus, is this meant to be an exercise, or a sincere attempt to improve the microphone?

I understand the benefit of an exercise, we learn about the circuit, etc.

If it's a sincere attempt to improve the microphone, then we also hope to actually make some changes to the mic that may result from the discussion...

The former may be frustrating if not disclosed or understood.  I'm having trouble understanding the point of your thread.

 
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