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Old 7th May 2013, 04:47 AM   #21
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Hi Ruffrecords,

thankyou for the walkthrough! This is starting to shed some proper light on my confusion. I have attached some diagrams of me laying it out to make sense of it. One question I have rolling around in my head was : Grid leak resistors. Does the 150k act as a load and a grid leak at the same time?

Also... What about the output stage of the preamp. That one diagram earlier had no output transformer... and no load resistor? What good would that drive?

For me, im interested in writing to my macbook which apparently presents a mic input impedance of 20kohms. What does this mean for the coupling part of this? Because of the high load... I could get away with a capacitor set up? or am I rather far off base here.
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Old 7th May 2013, 10:32 AM   #22
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Quote:
Originally Posted by woodrough View Post
Hi Ruffrecords,

thankyou for the walkthrough! This is starting to shed some proper light on my confusion. I have attached some diagrams of me laying it out to make sense of it. One question I have rolling around in my head was : Grid leak resistors. Does the 150k act as a load and a grid leak at the same time?

Also... What about the output stage of the preamp. That one diagram earlier had no output transformer... and no load resistor? What good would that drive?

For me, im interested in writing to my macbook which apparently presents a mic input impedance of 20kohms. What does this mean for the coupling part of this? Because of the high load... I could get away with a capacitor set up? or am I rather far off base here.
Your diagram indicates you have a good grasp of what is going on in the transformers.

The input impedance of a tube is essentially infinite but for biasing purposes the grid does need to be referenced to ground. As there is normally no grid current you can use very high values of resistance but the actual value does not matter and is usually chosen to set the desired input impedance. So, in this case the 150K does both jobs.

Depending on the sensitivity of your mac book mic input and the loudness of your acoustic source, you might be able to simply connect the output of the 1:37 transformer directly to it without any intervening amplification.

If you do need the extra gain then either of your proposed schemes will work. The 6AU6 wired as a triode was one of the very first tubes I tried as a mic pre. I used a 390 ohm cathode resistor and a 39K anode resistor which produced a gain of around 30dB. I would try the RC coupled scheme to start with. The resistor from the output capacitor to ground is there mainly to provide a dc charge path for the capacitor so its value is not critical - anything over 100K should be fine.. The output impedance is defined by the tube's plate resistance in parallel with the 39K plate resistor. I would guess this would be around 10K. This will drive a 20K load but it clearly does not meet the 10:1 impedance rule for no loss. This implies the transformer output might be a better option. One of the Edcor 15K:600 output transformers could work well here.

Cheers

Ian
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Old 8th May 2013, 04:50 PM   #23
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Hi Ruffrecords,

Thankyou so much for these tips! Im slowly getting a much clearer understanding of whats going on!

Its actually perhaps a pleasant surprise that I don't need too much effort to get from mic to laptop. But I am still curious in educating myself on how to properly assess and design a amplifier stage by stage (understanding impedance etc). I have just been slowly taking my time in absorbing this information.

I just have one question while I think about these things: What are the cons of a RC coupled method? It just looks so damn simple compared to the transformer alternative. cost, size, and potential room for distortion. I also feel that transformers for coupling act as giant magnetic pickups from nearby EMI etc. Or is the big pro for people the impedance control?
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Old 8th May 2013, 05:17 PM   #24
JZatopa is offline JZatopa  United States
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Here is a mic preamp I want to build myself that I think you should consider.

Aikido Mono Solo

It has quite a bit of gain, should be very low noise, has a PCB available and is a proven design in the hifi realm as a preamp.
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Old 8th May 2013, 05:35 PM   #25
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Quote:
Originally Posted by woodrough View Post
Hi Ruffrecords,

I just have one question while I think about these things: What are the cons of a RC coupled method? It just looks so damn simple compared to the transformer alternative. cost, size, and potential room for distortion. I also feel that transformers for coupling act as giant magnetic pickups from nearby EMI etc. Or is the big pro for people the impedance control?
It's no surprise that its a little of them all. The main one is impedance and drive capability. As I mentioned, the output impedance of the 6AU6 triode stage will probably be in the region of 10K. This is too high to drive normal pro inputs which an input impedance of around 10K. Using the old 10:1 rule you really want the output impedance of the 6AU6 stage to 'look like' 1K or less. This is the main drawback of the RC method and where a transformer comes in. We need to transform 10K into about 1K so a turns ratio equal to the square root of 10 will be about right. This will be a step down transformer and the output will be 10dB lower. So our overall mic pre gain becomes 20dB from the mic transformer plus 30dB from the 6AU6 minus 10dB from the transformer which equals 40dB. No too bad but a little on the low side.

Modern transformers produce very little distortion at normal signal levels and are not really prone to magnetic pick up unless you do something silly like stick a mains transformer right next to them.
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Old 8th May 2013, 05:40 PM   #26
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Quote:
Originally Posted by DJNUBZ View Post
Here is a mic preamp I want to build myself that I think you should consider.

Aikido Mono Solo

It has quite a bit of gain, should be very low noise, has a PCB available and is a proven design in the hifi realm as a preamp.
I have never been a fan of the Aikido. It's only novel property is its ability to reject HT hum and that would only be necessary if you had a really poor HT power supply design. Since a low current HT supply (for a mic pre) with virtually no noise is pretty easy to obtain there seems little point in using an Aikido. You would be just as well of coming straight out of the SRPP and save yourself a tube.

Cheers

Ian
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Old 8th May 2013, 10:46 PM   #27
Marik is offline Marik  United States
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The Edcor Pri DCR is 0.4 Ohm, so with ratio of 1:37 you get 0.4x37x37+87=634.6 Ohm (!!!) of pure noise, while typically the DCR (or impedance) of the ribbon in RE154 kit is around 0.15 Ohm. IOW, the noise of the transformer will be dominating. On top the 87 Ohm Sec copper resistance also works as a voltage divider (unless you run it unloaded), so you are losing some more signal there.

Typically, for a good noise performance you need the transformer Pri DCR AT LEAST a decade below that of a ribbon, esp. when you are trying to use it with tubes and need to squeeze every last bit of dB.

While a second 1:10 transformer probably is a necessity in what you are trying to do, on the other hand you are getting transformer losses twice. Single 1:100--1:200 ratio transformer will be much better idea, but those are not cheap.

You might want to try a FET->tube cascode, which will considerably cut the noise.

Generally speaking, I don't see a good enough reason to use tubes with ribbon mics.

Best, Mark Fouxman
Samar Audio & Microphone Design, LLC
www.samaraudiodesign.com

Last edited by Marik; 8th May 2013 at 11:07 PM.
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Old 9th May 2013, 11:24 PM   #28
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Check out this Lundahl ribbon mic transformer. Primary resistance 0.05 ohms:

http://www.lundahl.se/pdf/2912.pdf

Cheers
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Old 9th May 2013, 11:41 PM   #29
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Hi guys, these are all excellent points that are brought up. Again, trying to take it all in.

Although in my peripheral, I would like to design a preamp. More specifically, a 6s45p. I havent drawn up the values yet (and all the transformers that would come attatched to it) but I was curious how this special tube would perform at small signal. I only hear great reviews about it.

Click the image to open in full size.

And then for this noise deal, how exactly does resistance equate to dB of noise? the Lundahl you show here appears to have under half the quantity of ohms (264ohms) compared to the Edcor... which I purchased. I hear that people really dig the Edcor but what precisely am I combating here.
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Old 10th May 2013, 09:55 AM   #30
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You are combating thermal noise. In a 20KHz bandwidth at room temperature the noise voltage in a resistor R is just the square root of R/3096 microvolts. So 634.6 ohms of resistance produces about 0.45 microvolts of noise which is about -125dBu. On a quiet source, a ribbon mic might need 70dB of gain so this noise will be 70dB higher at about -55dBu.

For the Lundahl transformer the resistance is about 127 ohms which gives a noise voltage of 0.197 microvolts or about -132dBu i.e it is 7dB quieter and after 70dB of gain it is -62dBu.

Note, a good low noise preamp will have an equivalent input noise of around -130dBu at 70dB gain. With the Edcor transformer its noise is the dominant one. WIth the Lundahl, it and the preamp are nearly equal contributors.

Cheers

Ian
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