Simple solid state circuit for K67?
« on: February 25, 2019, 11:46:57 AM »
I have a few cheap chinese but not-too-awful-sounding K67 on my desk, but I have yet to work with any circuit with de-emphasis.

Are there any simple solid state circuits out there that would fit the K67, preferably electronically balanced (But not a must), maybe even some readily available PCBs?

Thanks

GB



Re: Simple solid state circuit for K67?
« Reply #1 on: February 25, 2019, 12:51:40 PM »
Built this circuit recently, and it fited sm57 type of body. Sounds great, and you don't really need a pcb.

http://www.audioimprov.com/AudioImprov/Mics/Entries/2017/5/7_Schoctava%2C_a_simple_mic_circuit.html

rogs

Re: Simple solid state circuit for K67?
« Reply #2 on: February 25, 2019, 04:00:26 PM »
I tried out that unbalanced circuit from Henry linked to above, but had some very strange noise problems with using one 'side' of the phantom power just for capsule bias?

In the end - and for only a couple of pounds extra cost, component wise - I stuck with my own simple balanced  'Schoeps' type circuit....copy attached ( just link out R4 and R5 - and don't fit C7 - for lowest noise 'flat' response) .
I personally think the few extra components needed to balance the output are worth the money - and the external  capsule  bias voltage multiplier can be a nice cheap way of providing some extra 'noise free' gain!

(No PCB, but some extra construction notes here:  http://www.jp137.com/lts/LDCX2.notes.pdf  )

« Last Edit: February 27, 2019, 05:36:14 AM by rogs »

Re: Simple solid state circuit for K67?
« Reply #3 on: March 20, 2019, 04:18:21 AM »
I tried out that unbalanced circuit from Henry linked to above, but had some very strange noise problems with using one 'side' of the phantom power just for capsule bias?

What preamp had the noise problems with this circuit?

Was it by any chance, an old transformer i/p preamp with P48 fed to a centre tap?
« Last Edit: March 20, 2019, 04:23:12 AM by ricardo »

OneRoomStudios

Re: Simple solid state circuit for K67?
« Reply #4 on: March 20, 2019, 03:58:05 PM »
The de-emphasis in the 67 was due to feedback applied to the backplate. You could get a transformer made with a tertiary winding and do the same thing, but that would be expensive and probably take some tweaking. I've had good luck using the cheater method from the E LAM 251 to tame high frequencies - they hang a ~100pF cap to ground from the plate that rolls off a little top-end. You could just add a passive filter cap before the OT (or after the last gain stage if going transformerless).

Khron

Re: Simple solid state circuit for K67?
« Reply #5 on: March 20, 2019, 05:21:57 PM »
Is there anything wrong with the U87 deemphasis / feedback arrangement?

Sure, it's not identical, but it (still) works ;D

The de-emphasis in the 67 was due to feedback applied to the backplate. You could get a transformer made with a tertiary winding and do the same thing, but that would be expensive and probably take some tweaking. I've had good luck using the cheater method from the E LAM 251 to tame high frequencies - they hang a ~100pF cap to ground from the plate that rolls off a little top-end. You could just add a passive filter cap before the OT (or after the last gain stage if going transformerless).
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

RuudNL

Re: Simple solid state circuit for K67?
« Reply #6 on: March 21, 2019, 04:08:22 AM »
Keep in mind that there is a difference between low pass filtering (plate capacitor) and HF feedback.
A bigger plate to ground capacitor will increase HF distortion.
There is a solution for every problem!

http://www.vansteenisaudio.nl

rogs

Re: Simple solid state circuit for K67?
« Reply #7 on: March 21, 2019, 03:59:20 PM »
What preamp had the noise problems with this circuit?

Was it by any chance, an old transformer i/p preamp with P48 fed to a centre tap?

No, it was  a Sound Devices USBPre 1.5 preamp

I seem to recall that the noise problem presented as  some form of handling noise, which seem to be related to the double mesh in the head basket ?.....
But didn't pursue the investigation --  keeping things balanced didn't cost much more , so I just spent a few pennies more and 'went Schoeps' as it were....and the noise problem disappeared.

Khron

Re: Simple solid state circuit for K67?
« Reply #8 on: March 21, 2019, 05:41:21 PM »
Back to the initial topic - has anyone tried adding an RC across the drain resistor in a Schoeps circuit?

I couldn't have been the first to get an inkling of this, but i remember raising this point over on the Micbuilders Yahoo group, and someone did indeed suggest that there.

Hint: LTspice is your friend ;D
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

cyrano

Re: Simple solid state circuit for K67?
« Reply #9 on: March 21, 2019, 07:17:30 PM »
No, it was  a Sound Devices USBPre 1.5 preamp

I seem to recall that the noise problem presented as  some form of handling noise, which seem to be related to the double mesh in the head basket ?.....
But didn't pursue the investigation --  keeping things balanced didn't cost much more , so I just spent a few pennies more and 'went Schoeps' as it were....and the noise problem disappeared.

I used to have a Sound Devices USB preamp 1 (never heard about a 1.5) and the phantom power is a bit weird. My old AKG451EB's needed up to a minute to stabilise and while doing that produced weird noises. One of my MKH's sometimes started making other strange noises.

Never happened with other interfaces. I put it down to the SD USB preamp being powered from the USB bus. Since I never found the schematics I passed it on for a RME FF400. FW bus power has more power, so no more problems.

I loved that thing, wanted to provide another power source, but without schematics I was afraid of ruining it. It's an Altera FPGA inside and I know nothing about those.
Why is it people love to believe and hate to know?


Re: Simple solid state circuit for K67?
« Reply #10 on: March 21, 2019, 09:17:04 PM »
Quote
    What preamp had the noise problems with this circuit?

    Was it by any chance, an old transformer i/p preamp with P48 fed to a centre tap?
No, it was  a Sound Devices USBPre 1.5 preamp

I seem to recall that the noise problem presented as  some form of handling noise, which seem to be related to the double mesh in the head basket ?.....
In theory, taking P48 power only from one side like Henry' Schoctava will upset certain old Transformer i/p preamps.  I know this is the case with some early 70's N*** sh*t.

But I thought Sound Devices used the Lundahl, Sowter, Jensen system with 2 x 6k8 resistors.  If you've still got your USBPre 1.5 and are happy to open it, could you post the model no. of the Lundahl transformer in it.

I'm not sure if this would appear as handling noise.

Quote
Back to the initial topic - has anyone tried adding an RC across the drain resistor in a Schoeps circuit?

I couldn't have been the first to get an inkling of this, but i remember raising this point over on the Micbuilders Yahoo group, and someone did indeed suggest that there.
That was me.  Detailed analysis under Files/ricardo/ChinaMod+U87hybrid

Still not sure what true U87 EQ is like.  I was using my own Circuit Analysis programme.  Now that I'm a pretend LTspice guru, perhaps I should re-visit this.

Some other suggested EQ schemes will give more noise & THD than my approach.  But I have to remember, not everyone is interested in the very best noise & THD  8)

Khron

Re: Simple solid state circuit for K67?
« Reply #11 on: March 22, 2019, 07:16:11 AM »
For what it's worth... (although do notice the capacitor values used there)

http://www.audioimprov.com/AudioImprov/Mics/Entries/2014/2/7_Mic_Electronic_Eq_Pt.1.html
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

rogs

Re: Simple solid state circuit for K67?
« Reply #12 on: March 22, 2019, 08:35:27 AM »

But I thought Sound Devices used the Lundahl, Sowter, Jensen system with 2 x 6k8 resistors.  If you've still got your USBPre 1.5 and are happy to open it, could you post the model no. of the Lundahl transformer in it.

No transformers in the USBPre 1.5... I think SD adopt a number of different configurations in their various ranges of kit...

Khron

Re: Simple solid state circuit for K67?
« Reply #13 on: March 22, 2019, 09:16:58 AM »
I seriously don't mean to be rude, but wouldn't this rather belong in its own thread? :)

No transformers in the USBPre 1.5... I think SD adopt a number of different configurations in their various ranges of kit...
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Re: Simple solid state circuit for K67?
« Reply #14 on: March 22, 2019, 11:39:42 AM »
I have the measurement of two black vintage u87s NFB which i made using REW.

However i find it to be useless unles one is using original capsule. For any other capsule/grille the curve needs to be adjusted to match original FR.

And usually the problem is the circuit needs some additional EQ for taming low end, as the original capsule has less low end that any other k67 capsule i have ever tried.

Taming only high end to match u87 high end response leaves you with a boomy mic, which has about 6db too much at about 50hz.

So instead of mid focused mic which u87 is, you end up with a dull sounding mic.

Re: Simple solid state circuit for K67?
« Reply #15 on: March 23, 2019, 03:06:39 AM »
I have the measurement of two black vintage u87s NFB which i made using REW.
Please post your electrical (or acoustic) measurements.  Are you feeding into the 'calibration' point for the electrical measurements?

Quote
And usually the problem is the circuit needs some additional EQ for taming low end, as the original capsule has less low end that any other k67 capsule i have ever tried.

Taming only high end to match u87 high end response leaves you with a boomy mic, which has about 6db too much at about 50hz.

So instead of mid focused mic which u87 is, you end up with a dull sounding mic.
If you are using my Schoeps based circuit, reduce C2 & 8 to 27n or 12n5

Re: Simple solid state circuit for K67?
« Reply #16 on: March 23, 2019, 09:14:38 AM »
I injected the signal directly into the fet . I remember trying to inject into the test points but something was off. If i remember well in that case LF response was flat, so i chose to feed the fet directly.

rogs

Re: Simple solid state circuit for K67?
« Reply #17 on: March 23, 2019, 12:13:24 PM »
I seriously don't mean to be rude, but wouldn't this rather belong in its own thread? :)
My apologies if this 'diversion' is causing problems....
I thought it might be relevant, as the circuit you linked to at the top of the thread doesn't very work well under some circumstances - and an SD USBPre is one of those cases.

Simply creating a balanced 'Schoeps' type output for a few extra pence solves the noise  problem, with no other changes needed.

I was grateful that someone had tried to help discover why this particular preamp might have a problem with this unbalanced circuit

Sorry for any inconvenience caused....

Re: Simple solid state circuit for K67?
« Reply #18 on: March 23, 2019, 06:12:10 PM »
I tested again that mic with Henry's circuit and i have to say its not the quietest one i have. I never noticed that as i use it as live vocal mic, so close proximity and no compression = no obvious noise problems. However i wouldn't use it in studio.

Blue Jinn

Re: Simple solid state circuit for K67?
« Reply #19 on: March 25, 2019, 02:20:25 AM »
There's this one:

it has an HF filter for use with "K67" style capsules. Haven't tried it out, I'm putting one together with a cheap C12 Chinese clone.

https://hlpimpedalice.wordpress.com/
not my thing. I am here just to unbalance the equation --pucho812

 http://www.bluejinn.org


 

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