Is it ok to pass a DC current trough a ribbon mic element? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 23, 2022, 11:57:48 pm
content: Will it mess up the response of the mic if I pass like 1mA trough the ribbon?

author: bob91343
author_rank: Super Contributor
timestamp: July 24, 2022, 03:22:27 am
content: It depends. If the element moves out of the uniform field, yes. Dynamic range may be reduced.

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2022, 09:30:00 am
content: It doesn't take much DC to push a very light ribbon right out of the magnetic gap, stretching out its corrugations at the same time. Why do you want to put DC through it? A ribbon mic normally has a built in step-up transformer.

author: Brumby
author_rank: Unknown
timestamp: July 24, 2022, 09:43:35 am
content: My instinctive response was an immediate NO, but the actual impact will depend on how much current you are talking about. It won't take much to affect performance and I would be concerned about permanent damage.If we are running through a transformer, then the steady state will not cause deflection - BUT - the initial surge would be a risk - and the collapsing field upon disconnection a bigger risk.The question does beg, however, why?

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2022, 12:28:56 pm
content: Quote from: ELS122 on July 25, 2022, 07:46:43 amHow about connecting it via a transformer, or AC coupling capacitor? If you opt for the transformer, you'll either need one which can handle 1mA of DC bias, or apply DC times the turns ratio, of the opposite polarity through a different winding.

author: CaptDon
author_rank: Super Contributor
timestamp: July 25, 2022, 12:43:30 pm
content: I doubt the grounded grid topology would have any value over a transformer coupled grid driven tetrode or pentode. Seems like a negative benefit vs. all of the downsides. Because the signal amplitude input and output will be relatively small you can surely find a very very linear part of the plate curves of a pentode or tetrode where the non-linear artifacts would be lesser that probably the distotion component of having D.C. current in the ribbon. Ribbon mic amplifiers have been done and done well with low-noise tubes since the 1920's. No need to re-invent a square wheel, they never did work very well.

author: jonpaul
author_rank: Super Contributor
timestamp: July 25, 2022, 01:01:14 pm
content: no, use standard ribbon mic Z matching transformers.Any DC bias will cause distorted audio.j

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2022, 08:32:36 pm
content: Without a transformer the signal level is extremely low and one would need an extremely low noise amplifier - so more like a high current BJT based one to get in the range well below 1nV/sqrt(Hz).I would expect a tube based amplifier to be way to noisy. There is a reason for using a transformer with band mics, as the source impedance is extremely low.The transformer does not have to be large - just large enough to get reasonable efficiency and not too much extra loss / noise. My guess would be a small torroid - today ideally some nanoperm or permalloy.p.s. So the transformer could be smaller than most tubes. So if size matters use transistors instead of tubes.

author: TimFox
author_rank: Super Contributor
timestamp: July 25, 2022, 10:22:27 pm
content: A common error when considering low-noise amplification from a low-impedance source (e.g., ribbon microphone) is to connect the source to a grounded-grid, grounded-base, or grounded-gate amplifier, whose input impedance (at the cathode) is comparable to the source impedance.However, this is not good for SNR. A given active device has an optimum source impedance for lowest noise figure.With tubes, to first approximation, the higher the source impedance the better, since the input noise current is very low.With BJTs and FETs, the input noise voltage and noise current are both important. At a given (audio) frequency, the optimal source impedance is given byRs=en/in,the ratio of the input noise voltage density and noise current density (assumed to be uncorrelated).For example, a 2N4416 JFET at a few kHz center frequency has an optimal source impedance of 150,000 ohms (see p 106 of the reference below).For a full discussion of this relevant to audio, see C D Motchenbacher and F C FitchenLow-Noise Electronic Design, Wiley 1973.At higher frequencies, there is some correlation between the noise densities for real devices, and the optimal impedance becomes complex.The non-obvious result (discussed in chapter 12 of the reference) is thatthis optimal source impedance is essentially the samefor grounded-grid and grounded-cathode amplifiers (etc.) at the same operating point (except for minor changes due to the auxiliary components in the circuit).Therefore, connecting a grounded-grid amplifier's cathode to your microphone will decrease the signal level, but the "noise match" is still very bad from the low-impedance microphone.This is why, for almost 100 years, low-impedance microphones are always connected to the grid of a vacuum tube through a step-up transformer, rather than to the cathode.The step-up increases the signal voltage linearly with the turns ratio while the noise voltage of the tube remains constant, so the SNR improves until the noise voltage from the noise current through the higher impedance seen by the tube becomes important.[edited for better explanation in the last sentence]

author: jonpaul
author_rank: Super Contributor
timestamp: July 26, 2022, 01:44:22 am
content: Matching a.01 Ohm ribbon to any amplifier reasonable Z required a high ratio transformer.As a magnetics designer decades ago, I can testify that low distortion audio band matching transformers arewell documented and available for both ribbon microphone and ribbon tweeters.These use thin tapewound high perm cut cores or tapewound toroids, with mirror lapped faces, and special winding technique.Lundstrom in Sweden and Jensen Transformers in California have written papers and manufacture.Just the ramblings of an old retired EEJonFinally RAAL ribbon tweeters in Belgrade have perfected their own design.