Sorry not with you. I was not suggesting a 'blanket just load it with 150K'. I said that was what some manufacturers recommend. Here's a link to a Jensen app note:
http://www.jensentransformers.com/as/as020.pdf
Cheers
Ian
For the noise calculation it is important to take into account the ribbon, as well.
What we have here is:
Edcor (0.15+0.4)x37^2+89=841.95 Ohm=0.52uV noise=-125.6dBv
Lundahl (0.15+0.05)x37^2+59=332.8 Ohm=0.32uV noise=-129.6dBv
For comparison, our own toroidal ribbon transformers have Pri DCR=0.002 Ohm and Sec DRC=2.7 Ohm (for 1:36 ratio):
Samar RT36 (0.15+0.002)x36^2+2.7=199.69 Ohm=0.25uV=-131.9dBv
Also, we will have signal loss due to the secondary copper resistance. Say, we have a load of 1.5K (very common value for modern preamps), then the losses are:
Ecor 0.49dB
Lundahl 0.33dB
Samar 0.017dB
Best, M
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Yes, indeed they do. The preamp designer has to be aware that this isn't the load that the secondary will actually see and add a network to make up for that.
Marik, your transformers are impressive!
Is it really equivalent:
A) thicker ribbon and more ratio tran or
B) thinner ribbon and regular ratio tran ?
And, lets go back to preamp design. What is best one for a ribbon mic?
No. I said, "The preamp designer has to be aware that this isn't the load that the secondary will actually see and add a network to make up for that." For example, the circuit in Post 39 will load the secondary with 100k in parallel with (roughly) 140pF- not just 100k. The transformer datasheets can't possibly show data for every combination of load resistance and parallel capacitance, so the designer needs to determine the proper RC network experimentally.
They will as well- the graphs shown on the datasheets show one source impedance only (in the case of the Jensens, I think it's 150R). For my ribbon mikes and output transformer, the source impedance is roughly 180 ohms, so I used 120R in series with the 50R output of my square wave generator to set the values of the RC network (yes, that's a 10 ohm error, but I was lazy).
Yes, it's the advantage of not having a "real" studio and being able to have separate preamps for each mike! A commercial pre could in theory have different plug ins or switch settings for specific popular mikes. Hmmmm, business opportunity...?
There is no such thing as "regular ratio transformer". Indeed, most of the transformers on the market are "one size fits all", which by no means is an optimized value for a chosen ribbon.
Besides the good noise performance we also want to achieve a certain output impedance and a certain damping on a ribbon. The impedance of the ribbon depends on ribbon dimensions--the thicker/shorter/wider ribbon-the less impedance, on the same manner the thinner/longer/narrow ribbon--the higher impedance (of course, we assume minimal losses at the ribbon clamps). What the transformer does is just basically raises that impedance in a square transformer ratio relationship. What you want is to see what the ribbon impedance is, and then chose a suitable ratio transformer for optimum noise/impedance/damping value.
As I mentioned, the only design I’d consider for a passive ribbon is a FET/tube cascode as a first stage:
More Phono Preamp Circuits
Followed by another gain stage. I had a very good experience with that topology.
Best, M
Fet/tube cascode huh? What are the advantages for doing that in the opening stage?
Also check out this thing. Its probably for condenser mics, but am curious on pros and cons between different topologies.
Im a sucker for as least parts as possible, but I feel like I cant apply that knowledge to the super pre amp world.
Yes, I agree with impedance points.
What I meant was rather voltage. My understanding is - thicker ribbon produces less voltage, so it should require xfrmr with more more ratio.
Am I right? If so. Will be following setups sonically equivalent?:
- thick ribbon - high ratio xfrmr
- thin ribbon - lower ratio xfrmr
(both produce ~same output impedance on secondary)
Or thin ribbon should sound better?
Why I am asking - working with and using thicker foil is more practical.
If it is possible to compensate drawbacks of thicker foil (lower sensitivity) just with using a transformer with higher ratio, would it be a solution?
If so, I could take out 1:37 Lundahl and put your Samar 1:100 instead of refoiling with 3 times thinner foil. Is this valid?
Thanks, Marik, I'll consider this.
Currently I reading about Mu-followers.
It looks (from articles) like this topology is superior from other ones in respect of distortions and performance. Your FET-Triod reminds Mu-follower (but inversed). Did you have an experience with Triod-Triod and Triod-Penthod Mu-followers?
What others can comment on Mu-follower as an ribbon preamp?
Should it be better than the topology posted in beginning of this thread?
Should it be better than RCA OP-6?
my god, I forgot to attach a link...
Battery Powered Vacuum Tube Microphone Pre-Amplifier for the Purpose of ?bootlegging? a Concert
What do you guys think/critique about this. Pros and Cons etc? (Portability is not a concern)
Battery Powered Vacuum Tube Microphone Pre-Amplifier for the Purpose of ?bootlegging? a Concert
What do you guys think/critique about this. Pros and Cons etc? (Portability is not a concern)
1. I think that 50dB gain is not enough for average ribbon mic.
But it, of course depends, how you build it, how strong magnet flux in ribbon area will be, how light (thin) is your ribbon and what will be a distance from front of ribbon to back around baffles.
2. This topology is regular common cathode with all its "character". You may like this character or not, it depends. But what the point of building very clear, neutral and fair sounding mic, if you then will colorate it in preamp?
I am currently looking into mu-follower topologies. These are quite simple, have good gain, low distortions, and capable to suppress PSU noises.
I just cannot find a schematic with extremely high gain 80dB. Looks like I'll have to design it.
80dB is a lot of gain to have at one frequency in one box. Stability may be an issue. Anything much above 60dB can be interesting!
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