Mic Pre Front End with Ribbon / Condenser Switch

[thermionic]

Hi,

I've had a Sowter 9045 for a while and have decided to make a single channel mic pre from a pair of DOAs. Please see attached drawing. The idea is to get around 35dB max. gain from the i/p stage. My questions are:

The 9045 can be switched between series / parallel primary for ribbon / condenser mode. Do you think I should use the spare pole on the switch to change the gain network, or will the feedback values / 470R pot give enough range?

Can you see any areas where the design could be more elegant?

Many thanks in advance.

Justin

 

[zebra50]

Hi Justin,

Not quite answering your question... is there a reason for putting the phase switch at the input, rather than down the line at the output? Common thinking is that fewer switches is better at the mic/preamp interface.

Stewart

 

[thermionic]

Good point, Stewart. I'll put it at the output t/former.

I've just realised one thing that's wrong: for approx 35dB the divider pot should be 470 R at its minimum, increasing resistance to lower gain. The schematic should therefore have a 470R wired permanently in series with the pot, with a pot value of say 5k, decreasing gain as it increases. I probably need to revise the gain scheme...

J

 

[PRR]

What are mike outputs at similar acoustic levels? I'm guessing 0.1mV for ribbon, 0.2mV for EV 635a, 0.6mV for smaller condensers, 1.2mV for larger condensers.

A 2:1 gain-shift is only partial compensation.

Old hot condensers sold into dynamic markets had 10dB pads.

Yes, your as-drawn gain control is wrong.

If you don't mind the risk of occasional/rare BIG pops, make "27K" your variable. Then it can be a common audio-taper pot and value. Your "RV1" must be less than secondary Z for low noise. Say under 10K. With 1:10 iron there is little excuse for more than 40dB gain in the amplifier. Then the feedback may be 1Meg.

I once had a board with 1:1 iron and 1k fixed 470K audio pot. Very sweet machine, though not the lowest hiss ever heard.

Your iron claims "Exceptionally high 20 Hz level", basically +24dBu or 12V at secondary. If you bought it for this reason, you really need a unity-gain extreme (even a loss) in your amplifier.

I have no idea where you have "a pair" of opamps.

Do not know why the cap and the 270K resistor.

Why the diodes on the gain-set cap?

I would not join the Phantom and the ratio-switch. When recording gnats you may need a (phantom) condenser plus a very high gain. When recording gunshots with 635a you will want low gain (though phantom won't hurt 635a). When recording childrens choir in large room with my Nakamichi mikes hacked for phantom, I need considerable gain. It's really two different issues, power and gain.

 

[thermionic]

>Your "RV1" must be less than secondary Z for low noise. Say under 10K

Having sketched the original attachment late last night, today I remembered that I did some experiments with the same op-amp being directly fed from a D-A chip (<50R o/p Z). I found that, for the same gain ratio between divider R and Rfb, the lower the divider R, the better distortion performance was – markedly. In the case of a phono preamp I’ve made with the op-amp, with 100R divider R, the THD is below my soundcard’s floor – an AP had to be used to measure it. I’ve modified the drawing, with the pot in the f/back loop – as per your suggestion. 

>I have no idea where you have "a pair" of opamps

The other will be used as the second gain stage, driving a t/former.

>Do not know why the cap and the 270K resistor.

I’ve removed the resistor. The cap (a 4u7 film cap?) is there to prevent DC offset from the op-amp getting onto the t/former secondary. If I short the DOA i/p to GND via an ammeter I get around 7uA flowing. Brian Sowter says this is not a problem as it’ll be far less across the coil. However…when I omitted the cap in another design with a different Sowter t/former, I felt it added a layer of fuzz. A client (who received the design blindly) also complained of additional fuzz… I was aware that core magnetisation loses headroom, but didn’t think 7uA would be enough to introduce fuzz…

The diode in parallel with the gain cap has been recommended by people more learned than I. Example 2 on the page offers an explanation: http://www.soton.ac.uk/~apm3/diyaudio/Signalpath.html Self goes to town on this one; he’s known to add a pair of series diodes in opposite directions across the cap.

>I would not join the Phantom and the ratio-switch.

Point absorbed into diagram.

The t/former was chosen because I wanted to try Sowter’s t/former with lowest THD and heaviest shielding – purely that (and the dual primaries accommodate what I have in mind).

As ever, I’m very grateful, PRR – many thanks.

Justin

 

[PRR]

> around 7uA

7 uA??? And THD with input resistance?

It's too soon to be looking for "elegance". More like you are hungry, and you find a spicy onion and a cut of beef.... but you have not told us the beef has its own strong flavor, and you have another one you have not shown.

7 uA input current and THD sensitivity suggests your unspecified opamp is intended for LOW source impedance, not the 5K-20K of this transformer.

It will "work", but elegance and <AP THD may not be on this path.

Also 7uA bias current in your proposed 270K is a 2V DC offset. I don't care to argue the merits of dioding the cap, but with 2V offset the diode will be slammed into conduction. Now the cap is shorted, DC gain is AC gain and up to 100.... you will have 200V DC offset at the output (or whatever the opamp can swing).

Also the 10K:100 gain-set, turned to minimum gain, throws the 100 ohms across the opamp output. Few opamps will drive this confortably. And you may well need unity gain with this high-ratio transformer in modern studios.

I'm suspecting this DOA is aimed at 500-1K source impedances, not 5K-20K.

 

[thermionic]

Thanks again, PRR. The opamp is happy driving 50R and lower without any real nasties showing up on the FFT, so I'm not worried there. However, your point about source impedance is most likely correct. I'm going to try the DOA, but if any nasties show up when fed with higher impedance sources I'll have to find another opamp.

Will keep you posted.

Justin

 

[thermionic]

Your "RV1" must be less than secondary Z for low noise.

Can anyone give me a rule for the relationship / ratio between the divider R (100R in the latter diagram, or "RV1" in initial sketch)) and input Z in a non-inverting situation to keep noise at bay? At what point will the noise start creeping up?

TIA.

Justin

 

[PRR]

> At what point will the noise start creeping up?

"Your "RV1" must be less than secondary Z for low noise."