Can Someone Check This Mic Pre Schematic Please?

Hi guys,

After trying to convert my K+H preamp module into a decent mic preamp, I have realised that it would be best to make a new preamp using the Beyer transformer from the original module.

Samuel Groner very kindly modified a mic pre design which he thought would be suitable for my needs.

Would anyone with better experience than myself be able to look over the schematic to confirm that there are no major errors? It looks ok to me, but I want to make sure before I order all of the components.

I think the Beyer transformer has the following specifications;

Ratio: 1:7
Primary DC resistance: 50 Ohms
Secondary DC resistance: 3.45 KOhms
Transformed impedance: 10 KOhms
Primary open-circuit inductance: 6H @ 50Hz
Primary open-circuit impedance: 1.885 KOhm @ 50Hz
Rg: 200 Ohms

Here is the schematic:



Thanks,

Roddy

Looks fine to me.

You might want to include some local bypassing caps on the power supply lines.

Nice not boring circuit.

But some synthetic load for trafo ?

xvlk

[quote author="xvlk"]But some synthetic load for trafo ?
[/quote]
That way (I have no time to compute resistor,
it is simple but for overall gain -10 for 1M feedback resistor.

Some Zobel network for added OP amp inp. possibly needed ,
because usual way to taming( 150p paralel to input is nonsense (trafo capacitance load is bad))
will see you in circuit sample testing.

Here is schemo, you must cut and paste link to other
browser if you want to see someth.

http://mujweb.cz/www/xvlkxvlk/mod.gif

Have a succes,

xvlk

I'd start by hanging a 75k resistor on the transformer's secondary for loading, then build it and see if the frequency response is reasonable. As was remarked someplace, the electronic stages reverse polarity, so make sure the trafo's input is similarly reversed. Oh, and if I remember these transformers correctly, they saturate at kind of low levels, so you may want to be a little cautious about what you put into them.

Peace,
Paul

Thanks for your advice, guys.

[quote author="pstamler"]I'd start by hanging a 75k resistor on the transformer's secondary for loading[/quote]

I am not familiar with adding resistors for loading the transformer.

Is this resistor placed in parallel with the transformer secondary?

What is the reason for this? Is it to get a suitable reflected impedance across the primary?

Thanks again,

Roddy

Is this resistor placed in parallel with the transformer secondary?

Click to expand...

Yes.

What is the reason for this? Is it to get a suitable reflected impedance across the primary?

Click to expand...

Partly that, but mainly to kill top-end resonances ("bumps" in top-end frequency response) stemming from transformer capacitance//leakage inductance that gets significant at high load impedances. Also it is common to load the transformer with a "snubber" - a resistor/capacitor series connection. I think I read a PDF from Lundahl about this once - they may have it on their site..

Jakob E.

Looks kind like the same basic layout of the SSL 571 mic pre, (which uses Jensen TX's btw) in so far as the single-gang pot cleverly controls the gain of both stages, by reducing the forward-fed (source) impedance to stage 1 as it increases the NFB impedance to stage 1, so both stages increase gain; whether or not it's perfectly matched between the stages isn't too much of an issue: as long as both stages shift gain, the overall gain is a "shared task" if you get my drift.

Keith

I'd like to follow along with this. The OPA604 performance sheet is here.
http://focus.ti.com/lit/ds/symlink/opa604.pdf
Unless there is another revision you are looking at. Voltage supply is +/-25V Maximum but best performance is at 15V +/-.
Looks like a great breadboard project.

I'll be quiet now.
:grin:
Cheers

StrayCat said:

Voltage supply is +/-25V Maximum but best performance is at 15V +/-.

Click to expand...

Where did you get the +/- 15V value from? Was it because that was the test voltage for typical characteristics? I thought higher supply voltages might be better to allow a better output voltage swing?

Do you know what would be a suitable and simple power supply to build for this?

Thanks,

Roddy

A mic probably won't drive that to the point where you need ten thousand volts of headroom.

And don't worry about the termination resistor yet. The ringing on a 1:7 will be way out of even a wired hair snozzer's hearing range. Is snozzer a word?

Check out the PDF. All of the testing was done at +/- 15V. All the best results also sit at around the +/- 15 V area. This circuit mind you might operate better at 18v or 24 V but the testing from the company which would more than likely try and show the chips best performance was at 15V.

Just my observation on the spec sheet.

Cheers

[quote author="StrayCat"]Check out the PDF. All of the testing was done at +/- 15V. All the best results also sit at around the +/- 15 V area.
Cheers[/quote]

I have been operating OPA604 opamps on +-22 volts with excellent results. In the application I use them they sound better than running them at 15 volt rails. Specification sheets are poor conveyors of sound quality.

Looks kind like the same basic layout of the SSL 571 mic pre, (which uses Jensen TX's btw) in so far as the single-gang pot cleverly controls the gain of both stages, by reducing the forward-fed (source) impedance to stage 1 as it increases the NFB impedance to stage 1, so both stages increase gain; whether or not it's perfectly matched between the stages isn't too much of an issue: as long as both stages shift gain, the overall gain is a "shared task" if you get my drift.

Click to expand...

Nice construction indeed. The Oakley-stuff uses it (pre-amp of the VCF-1) & I thought I had seen something like it in some Peavey schematics as well (not totally sure about the latter, but it sure was an 'unusually connected' gain-pot).
Just curious, is that SSL 571 mic pre schematic anywhere online ?

Regards,

Peter

I don't think so...

I got a pair of 571s racked up in a metal box with inbuilt power supply. (In fact it was the veroboard prototype of the SSLtech power ±18V + 48V phantom supply which people have taken to referring to as the 9k PSU)

I use it as my standard, "go-to" mic preamp for location stereo work where interference is an issue: Sometimes only a really good transformer will do!

The 571 is basically a JT-115 tx folowed by almost exactly the circuit above, but with electronically-latched things like pad and phantom, and total recall on the switching and gain settings. It also has an LED peak meter for each channel, which I regard as an absolute requisite for all location work where the preamp is out of easy reach of the operator's position... No sense in running there and back twenty times while you find out if you're overloadoing or under-driving! :wink:

If you can live without total recall and electronic latching, the above circuit and a Jensen is basically the SSL 571, all you need to do is add a simple LED ladder driver comparator chain or the LM3915 data sheet schematic and you've got the same thing. The resistor values might be different, but that's basically it.

Keith

Keith

Thanks for the additional info,
Cheers,

Peter