best solid state preamp design for a ribbon mic?

[quote author="blandman74"]Another thought is that, from what I understand, ribbon mics have such a heavy bottom end, especially when close-micing, that a little roll-off caused by a coupling cap might be a good thing.[/quote]

The coupling caps won't roll off any noticeable bass - we spec them so that the RC filter rolls off lower than the audio band. They will however, cause some distortion which can be avoided partly by choosing better caps - eg. bipolars.

Have a look at my shared-gain preamp thread in the lab - I will update it occassionally.

By the way, can anyone enlighten me as to how we specify the output impedance of a preamp? Is it just the path between the output pins and ground?

Roddy

EDIT: The 25k FB resistor on the first op amp would be a log-tapered pot.

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No, no, no! Carefully recheck my original schematic: Shared_Gain_PreAmp_r1.gif

The cool thing about this design is that you can use one linear pot which adjusts the gain of both stages--a log pot in the feedback network is nonsense.

I put it through the on-line circuit simulator with a dc servo and noticed a slight phase shift in the output, which makes me think that having a dc servo circuit may have an audible impact just like (or more than) a capacitor would.

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It is clear that any analogue highpass filter (and a DC servo is just that as is a coupling cap) produces phase shift--but with a DC servo you can move it way down in frequency without using huge caps. The phase shift you simulated is likely dominated by the two 470 uF caps and not the servo circuit.

I'd use an inverting servo for the second stage anyway.

Samuel

BTW, I simulated the overall gain response and the response of the two stages a few weeks back: sharedgainpre.pdf

For the simulation I used resistor values which are slightly different from the schematic, but the result will be similar.

Samuel

Yeah, the linear pot thing is cool.

Just remember it is a shared-gain preamp.

[quote author="Samuel Groner"]BTW, I simulated the overall gain response and the response of the two stages a few weeks back: sharedgainpre.pdf

For the simulation I used resistor values which are slightly different from the schematic, but the result will be similar.
[/quote]

Cool! Did you do this simulation with any input transformer, or just the circuit alone?

I'll be using 1:7 trannies which gives 20*log 7 = ~19dB gain, but was also thinking of perhaps adding a 1:2 for balanced outs...

Did you do this simulation with any input transformer, or just the circuit alone?

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Just the active electronics alone--add your 19 dB to the overall response.

I was also thinking of perhaps adding a 1:2 for balanced outs...

Click to expand...

I'd be careful with this; a 1:2 presents a considerable load to the driving amp (i.e. 150 ohm when driving 600 ohm). 150 ohm is indeed to much for about any IC.

Samuel

There's a problem with the servo circuit as drawn. The capacitor from the output terminal needs to have its other end connected to the inverting input of the servo amp, not to ground.

Meanwhile, I wouldn't be concerned about the servo causing problems. If I understand the theory correctly, a first-order servo with a cutoff frequency of X Hz causes the exact same phase shift as a regular coupling capacitor+resistor circuit with the same cutoff frequency. The advantage of the servo is that it lets you get the same cutoff with much smaller capacitors, which can then be film devices rather than 'lytics.

Peace,
Paul

[quote author="Samuel Groner"]

I was also thinking of perhaps adding a 1:2 for balanced outs...

Click to expand...

I'd be careful with this; a 1:2 presents a considerable load to the driving amp (i.e. 150 ohm when driving 600 ohm). 150 ohm is indeed to much for about any IC.[/quote]

Ahh! I forgot about the impedance reflected back from the load to the final opamp stage! I was only thinking about the output impedance being stepped-up, and forgot that the load which the final opamp would be stepped-down.

That's ok though - 80dB gain is fine for me. 86dB gain is probably taking things too far :razz:

This might be (probably is) a really stupid question but...

Why would you need coupling caps on a ribbon mic preamp anyway? When I look at the INA217 datasheets the only reason they use coupling caps is to isolate the inputs from the phantom voltage. I see no reason you need those if you leave out the phantom supply.

Can't you just use an ina217 (or even one of those THAT chips) as a frontend without the coupling caps followed with a switchable second stage (for example a ne5534) to give some 20dB extra gain. and one of those burr brown line drivers as an output?

Mendelt, I agree with you, and your INA217 + NE5534 design sounds ok, but if we are using a transformer input then we don't need a de-coupling cap at the input anyway.

Where we do need caps is to block offset voltages between stages and perhaps as protection at the output in case someone puts phantom power in the wrong way.

I have heard people complain that the INA217 was too sterile and not particularly musical... I still need to hear one though.

What I use for both Royer R121 and ShynnyBox ribbons most of time is a micpre I did using a JT16B input, Forssell 992 opamp and cap output.
Sound just perfect!!

:guinness:
Fabio

[quote author="rodabod"]Mendelt, I agree with you, and your INA217 + NE5534 design sounds ok, but if we are using a transformer input then we don't need a de-coupling cap at the input anyway.[/quote]
Ok, so we're using an input transformer for flavor then. :grin:


[quote author="rodabod"]
Where we do need caps is to block offset voltages between stages and perhaps as protection at the output in case someone puts phantom power in the wrong way.[/quote]
But we could be using servo's and an output transformer here.

[quote author="rodabod"]
I have heard people complain that the INA217 was too sterile and not particularly musical... I still need to hear one though.[/quote]
I havn't tried them out yet either. As far as i know the are ok but there are probably better ways to build a first stage for a mic preamp. Maybe one of the discrete opamps...

[quote author="blandman74"]
tall_phil, I don?t know why I skipped the JLM stuff. They say they sell just the opamp, from which I could adapt a design pretty easily. Do you think I should go with the 99v or use John Hardy?s 990C? One is more like an API and the other is the Deane Jensen design .... I don't know the difference.
[/quote]

I have never built anything with the JH990C amps, so I can't give you any oppinions. Everything I know about it you can find easily with a little help from google.

But I am inclined to believe either would be better than adequate. It is an easy circuit to adapt to use either for personal comparason.

I don't think you should fret about it for the time being. If you choose to build it, use one of them (or any other option available). If you subsequently wish to tinker and test, do this. I think you should hold a similar attitude about the DC servo.