Oktava 319 Distortion

[craigmorris74]

I have an Oktava 319 that sounds great on kick drum.  However, if I place the mic any closer than approximately 3ft. away from the drum, it distorts.  I have confirmed that the microphone is the source of distortion.  Has anyone else had this problem with a 319?  Can the mic be tweaked to have higher headroom?  I'd love to use it in a similar way to a U47FET.

Thanks,
Craig

 

[RuudNL]

It seems you are exceeding the maximum SPL.
Maybe a readjustment of the FET bias can give you more headroom.

 

[craigmorris74]

That's what I was thinking.  I can't imagine these things being fine tuned before they left the factory.

 

[craigmorris74]

The FET in my microphone is labled 75 FN.  This isn't totally clear, it could also be 715 FN, or 7B FN. I can't find anything on the internet about that number.  Anyone know what FET is used in a 315?

Thanks,
Craig

 

[mjrippe]

Not sure about the stock FET, but a nice upgrade is the Toshiba 2SK170BL.

 

[RuudNL]

When the bias is fixed (resistors, no trimpot), you can assume that the FET bias is incorrect.
I have re-biased several microphones and they all improved after the re-bias.
Less distortion (even at lower levels) and a higher maximum SPL.
The problem is that there is a lot of tolerance between FETs, even from the same batch.
You simply can't use fixed resistors and just put a FET in, the bias should be adjusted for that specific FET!
(Also after changing a FET, even the if you put in the same type, the bias should be readjusted.)

 

[Gus]

The 219 319 Oktavas have two selected resistors.  You can sometimes find numbers written on the transformer case.

The bias and circuit is "better" in some ways compared to the KM84 like circuits.  Study the schematics you can find.

The overload is due to the gain of the amp.  The fet47 is a different type of circuit.

Simple one gain device circuit design is a set of compromises.  You can't have it all with simple gain stages.  You design it for what you want from it.

Why is a 170 a good upgrade?  Something you read on the web?

 

[craigmorris74]

Thanks for all of your replies.  I'll probably keep the stock transistor because I really like the sound of the mic, just would like to increase the headroom a bit.

Gus, I noticed on another thread that you stated that the  stock FET on the 219/319s were usually biased correctly.  Have you noticed this is true on several mics that you've tested?  Excuse my ignorance, but when you say that overload is due to the gain of the amp, do you mean that the amp is overloading some component that follows?

If I wanted to check that the mic was biased correctly, it appears the R7 and R8 bias the FET.  Could I substitute a trimpot for these resistors to make sure the bias is correct?

I really appreciate all the help.

Craig

 

[RuudNL]

It seems Oktava selects the fixed resistor values during production.
If this is correctly done, there is nothing against it of course.
But I have seen many microphones that all had the same(!) resistor values while the FETs were all incorrectly biased.
In fact I would prefer (correct!) fixed resistor values instead of a trimpot for better long term stability.
I would leave the original FET in. In general a manufacturer does not make choices without a reason...

 

[craigmorris74]

There was some resistor selection going on at Oktava.  The resistor values are written on the transformer, and I've seen many different values written on other people mics.  If this selection is done correctly, then I guess this mic "is what it is". 

 

[mjrippe]

Why is a 170 a good upgrade?  Something you read on the web?

Because I installed one as part of an upgrade kit for a client and the mic was night and day different from a stock one when completed.  Of course the other components helped 

 

[Gus]

Did you change one part at a time? If not you don't know what changed the sound.

The Oktava  is a mixed fixed and cathode bias with the source resistors selected for the overall gain and bias.

Look for the Vishay jfet app notes.

 

[pasarski]

The bias and circuit is "better" in some ways compared to the KM84 like circuits.  Study the schematics you can find.

It's the combo constant-current/self bias found in Siliconix AN102. I understand it's advantage is that it works better with wide range of FET's if fixed bias resistor values are used,  but I don't yet understand why they use it in mk219 whith selected bias resistors. Because there's no zener like in km84? Better tolerance to P48 variatons?