best solid state preamp design for a ribbon mic?

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blandman74

Well-known member
Joined
Aug 4, 2005
Messages
67
Location
Washington, DC
Howdy all, I am looking for a little advice on a preamp design. I searched the forum and couldn?t find what I?m looking for so if anyone has ideas, I?d love to hear ?em.

I just participated in a ribbon mic group buy (via www.analogindustries.com) and want to build a simple, transformer-input circuit with an unbalanced output. Talking with Tom (or was it Dave?) at cinemag it was suggested I use a 50-ohm impedance transformer that gets me 22 db of gain off the bat. I figure for the mic I need another 45 dB of gain after that. So I compulsively bought one. I have to stop that.

Anyhoo, I am mostly concerned with making sure I build a preamp that makes the most of my nice (hopefully ? it hasn?t arrived yet) new ribbon mic. Now that I?ve bought the CMMI-7C from cinemag, does anyone have any suggestions for a clean, solid-state preamp design for ribbon mics?

BTW, I plan on taking advantage of the alternate primary wiring (a 200-ohm primary) and just have a little switch to toggle between either primary. Will this toggle switch affect noise so much that it would impact the design?


PS I looked at this design:

www.groupdiy.com/index.php?topic=8626

which is a Walt Jung design, but I am afraid it does not afford enough gain (total of 50 dB, maybe a touch more with the cinemag) to be useful with my ribbon mic.

Any advice is appreciated!
 
[quote author="blandman74"]PS I looked at this design:

www.groupdiy.com/index.php?topic=8626

which is a Walt Jung design, but I am afraid it does not afford enough gain (total of 50 dB, maybe a touch more with the cinemag) to be useful with my ribbon mic.

Any advice is appreciated![/quote]

There's also the pre-pre-amp thread, you might have seen it already:
http://www.groupdiy.com/index.php?topic=5743&highlight=active+ribbon

That would solve your gain-concern, but I can image you'd rather have a complete approach than a 1+1.

Regards,

Peter
 
Yep, I saw that, but I didn't like it for two reasons, including the one you gave. The other is that I want to avoid coupling capacitors as much as possible, and I don't have phantom power anywhere (well, unless it is battery power for my goofy TapeOp mics) so I don't want to have to build something that requires it.

I forgot to mention that I already have a +/- 15 V power supply revved up and ready to go ....
 
What do you want to record with your ribbon mic?

I can't see which microphone model you are looking at, but let's assume it is fairly average in output.

I'd recommend at least 66dB gain for "medium" volume sources and upwards since you may want to pull the mic away a little to avoid bass tip-up since fig. 8 ribbons can get quite bass-heavy.

I'd say 70dB - 80dB gain would be safe for most sources (assuming the preamp was quiet.

Your Cinemag appears to be 1:7 ratio which suits many designs. You could try an A*PI 3*12 or a Baby Animal amongst many others...

I'd recommend using the 50 Ohm and 200 Ohm taps.

I'm currently in the middle of making a quad version of Samuel Groner's Shared Gain Preamp which I should really start a thread about as I have a few photos... You'd need to breadboard it, but it's simple. I think I'll be getting around 80dB from that once I've sorted it out... I'm building it specifically to be good with ribbons.

What preamps do you already have?

Roddy
 
seems you're looking at the colourless, discrete end of the scale... how about this:

http://www.borbelyaudio.com/eb103213.asp


Gain: 0-44dB (see bellow)
Open loop gain: 76dB
Freq. Response: 0dB gain: >1MHz
44dB gain: 650kHz
THD: 0dB gain, 1kHz, 3VRMS: 0.001%
0dB gain, 10kHz, 3VRMS: 0.0015%
40dB gain, 1kHz, 3VRMS: 0.0015%
40dB gain, 10kHz, 3VRMS: 0.0028%
Input capacitance: approx. 2.2pF
Input noise, 30kHz BW: 44dB gain, shorted input -137.8dBu
44dB gain, 50-Ohm source: -132.4 dBu
44dB gain, 200-Ohm source: -127.4dBu
Max. Output level: +24dBu
Minimum load: 100 Ohm

Normally the gain of the preamp is 44 dB, but can be increased to maximum 54dB. Used with a 1:2 input transformer the total gain is 50/60 dB, with a 1:10 transformer it is 64/74 dB

Balanced transformerless mic pre using two 213 amps.
Fig. 3 shows two 213 connected as a balanced, transformerless mic pre. Using the 54dB maximum gain option for the 213 will give 60dB gain in balanced mode. It is essential to use a very low-impedance gain-control to take advantage of the low inherent noise of the 213.

The circuit in fig. 3 can of course also be used with input transformer, which will increase the gain to 66 (1:2) or 80 (1:10) dB. This configuration is ideal for Ribbon mics.
 
I have built a JLM Dual 99v. This has enough gain for almost everything I need. It works well with my STC 4038 (30ohm) which has an obscenely low ouput. It sounds good, is a straight forward build and can be configured with a variety of opamps and transformers. Their customer support is also good, and can't be blamed for some of my metal work.

The board can also be built with a DC servo, which would get rid of the coupling caps you wish to avoid. I believe the 99v opamp is not the best choice for DC servo circuits though on account of it's high offset.

Depending on the opamp used, it can run off your bipolar 15v, and wont cry if you dont connect anything to the 48v rail.

see: www.jlmaudio.com
 
rodabod:

I have only a couple of homemade preamps based off of the INA217. They are very clean and suit my humble needs, but I don?t think they?ll have enough gain. After looking around a bit more, it seems that most preamps that are marketed as being appropriate for ribbon mics have gain up around 70 to 80 dB as you say. The Samuel Groner's Shared Gain Preamp sounds excellent. Does it use evil, evil coupling caps?

I have a little bit about the INA217 preamps on my dorked-out web site: doorstopelec.typepad.com

Mlewis:
I like that design, although it does have the coupling caps in it, which (I forgot to mention in my first post) I?m trying to avoid. Honestly, maybe my desire to avoid caps is just recycled juju, but John Hardy?s articles and interviews denouncing coupling caps have spooked me. (And the INA217 amps I built don?t have ?em, either.)

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.

Thanks everyone for your posts!
 
[quote author="blandman74"]The Samuel Groner's Shared Gain Preamp sounds excellent. Does it use evil, evil coupling caps?
[/quote]

It is excellent. It uses very few components if you don't need phantom power, but does use the dreaded de-coupling caps!

However, I don't worry about them. It's the sound that matters at the end of the day... Samuel suggested using bipolar caps to avoid distortion, and you obviously don't need one at mic level (ie. at the input stage) since it is transformer input (I get a bit sensitive about messing about with weak ribbon mic signals!)

You could (I think) get rid of the de-coupling caps by using opamps with a lower off-set and swap any remaining de-coupling caps with servos....
 
Sounds great. Do you have a schematic? Also, do you know where I could get a little instruction on how to craft a DC servo circuit? I can't find it in my books.
 
[quote author="blandman74"]Do you have a schematic? Also, do you know where I could get a little instruction on how to craft a DC servo circuit? [/quote]

The search function will answer 99% of your questions!

http://www.groupdiy.com/index.php?topic=8543&highlight=shared+preamp

Search for DC servos - there will be answers here. I havn't used one yet, although you appear to using them in your INA design, no?


By the way, I've not looked into this, but have you considered adding an output transformer to your curent preamps? You'd need to check the output impedance of them currently as it would be stepped-up by the turns ratio squared.

A 1:2 transformer would give you an additional 6dB gain (and balance the output) and a 1:3 would give about 10dB additional gain.

I need to learn more about opamp circuits and their output impedances - I know that at the opamp the outputs are usually ~ 0 Ohms, but I'm not sure how it works out in example circuits.

Roddy
 
there's a good example of a DC servo here

http://www.forsselltech.com/schematics/JE16%20Mike%20Preamp.PDF

great discrete opamps for sale and no coupling caps too. don't know about gain tho.
 
Thanks. I thought the shared preamp was it, but wasn't sure. I'll try that DC servo circuit (I wish I understood how it works, but I'll stare at it tonight and maybe it'll come to me). The one in the INA217 data sheet (which I did use) is even more mysterious.

I will patch this together when the transformer comes and report back!
 
the dc servo is just a Low pass amp with the pass freq set really low so that it only lets through DC to a few Hz of signal and then forces a feedback voltage to the input of the non-servo (gain) amp to correct the offset at it's output. i seem to remember there being inverting and non-inverting configurations. i'm far from being an expert on this though.
 
blandman74, if you are interested I'm happy to modify my "shared gain" preamp to your needs (e.g. with servos instead of coupling caps). Just tell me to go ahead and give me a few days.

Samuel
 
[quote author="blandman74"]

John Hardy?s articles and interviews denouncing coupling caps have spooked me. [/quote]

Victim of technical talk and babble, most polyprops are very transparent and they do no more to the sound than a transformer does. Most of what you read is other peoples opinions, grow your own so you can eventually mislead people like most "gurus" do.

analag
 
Samuel....well, I got carried away already:

groner_pre_adapted.gif


I am imagining a 1:7 step-up transformer producing a peak signal of about a maximum of 35 mV.

I'll leave the other coupling cap in for simplicity.

EDIT: The 25k FB resistor on the first op amp would be a log-tapered pot.
 
anyone actually use the Borbely preamp? i like the design but wonder about the sound..

anyone have the schematic for it with values?
 
As you can probably see, 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. So maybe it's best to skip the whole "I'm against coupling caps" theory for now, anyway.

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.
 

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