Telefunken v672 gain and impedance relationship

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David_H

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Hello everyone!
I'm in the process of racking a v672/a1, and while I've extensively used the search function there are still a few things that are not quite clear to me - most importantly the relationship between input impedance, gain and frequency curve of this preamp.
From what I understand and have researched here in the forum, the input impedance is set from resistors on the input. Many people have been experimenting with this, but for a gain range from 34 -65db, this manual http://audio.kubarth.com/rundfunk/g...YK96Y?5C8W,E]496-H;FEC86PM36%N=6%L [email protected]``

clearly states that this resistance should be 40 ohm, and that Rg for the negative feeback between 2,5k and 330k.
Now I know that a larger input impedance is typically desired, but isnt this the ONLY configuration that preserves maximum recommended gain while still being as close to linear in terms of frequency specs as possible?
So is the trade off for the higher gain settings the smaller input impdedance? It sure seems (if I understand the manual correctly) like this amp is really designed to work best at 34db gain with 2,5k input impedance and everything else is a trade off, is that correct?

The second headscratcher is the 15khz low pass. Many people have been investigating this issue, for example here:
or here

But is A.) the high freq rolloff at higher gains really an issue if one sticks to the original instructions as to how to use this amp as a mic pre (i.e. sticking with 40ohm input impedance)?
And B.) some manuals for the later models (like my v672/a1 or the ANT v672D) state that you can just bypass the low pass by bridging pins 11&12.

Final question: Is there a solution that keeps both input impedance high(ish) (around 1k or greater), gain in the 60db range and a linear frequency response?

Thanks in advance as always,
David
 

dmp

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Final question: Is there a solution that keeps both input impedance high(ish) (around 1k or greater), gain in the 60db range and a linear frequency response?

It's been a long time since I was working with these, but I think you can get ~45 dB. Say you use 1.5k for the input impedance (Rin=Re) and use a 300k R for max gain. Then v = (300+8.2)/(1.5)*1.2 = 806 = 47 dB
If you switched in a lower impedance, the gain will increase.
I didn't see in a english manual a 40 ohm recommendation for Rin (links to kubarth don't work, better to describe what file you are looking at so others can find it).
40 ohm seems low for a mic. If you did do that, I would put a few other options (including 1.5k) on a switch.
But going from 1.5k to 40 will increase the gain from 47 dB to 79 dB using the above formula.

I think I had a different version than you, so the things I found in those old posts for the frequency response might not apply. But once you get it up and running it is easy to sweep the frequency response and go from there.
 

David_H

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Sorry, you're right, I should have checked the link first... I am referring to the "Telefunken v672 technical manual" on kubi's site. I am not sure what the policy of attaching his files here is, so I just copied out the relevant chart from the manual, see below.
Your calculation, which is also in the manual, makes perfect sense when you add a 200ohm source impedance to the 40ohm resistors, which is apparently what one's supposed to be doing. Because then the calculation spits out 63db gain with 40 ohm resistors in place. This seems to be the way that all the "pro" companies (I am in no way qualified to judge this in any way) seem to offer their v672 products.
Now back to impedance. This may be a noob question, but isn't impedance the reflected property of the circuit that's behind the transformer? In other words, is the 40 ohm resistor in the front really the impedance?

Sorry if this is touching on subjecets asked and answered many times elsewhere, I guess I should just try out different settings and settle on the good ol' "sounds good, therefore it's right" principle. Thanks anyway, I might learn a thing or two.
 

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dmp

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isn't impedance the reflected property of the circuit that's behind the transformer? In other words, is the 40 ohm resistor in the front really the impedance?

These preamps have a 'zero' impedance input ( < 5 ohms). Typically there is an impedance that is reflected through the input transformer, but in this case the circuit is designed so even with a input transformer the preamp alone has an input impedance of near zero.
You can always raise the impedance into a preamp with resistors (but at the expense of gain)

Your calculation, which is also in the manual, makes perfect sense when you add a 200ohm source impedance to the 40ohm resistors, which is apparently what one's supposed to be doing. Because then the calculation spits out 63db gain with 40 ohm resistors in place.
This sounds right. I remember I had a switch with a few resistors to change the input impedance and it had a big effect on gain but not a major effect on tone. But a nice feature in racking these.
 

David_H

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Thanks for the explaination! I got around hooking the card up and wiring and choosing the resistors/pot has been a success - I have it up and running. Unfortunately, I have a follow-up question.
My module sounds fine with low signal flowing into it, but starts to distort in a nasty way once too much signal (i.e. louder speaking, so very low level still) is applied. Are there any quick guesses as to what could cause this reduced headroom? Somehow I am thinking of a wrongly balanced transistor, but there is no way to adjust the voltages internally with a trimmer, or is there? The PSU gives out a solid 24v.
 

David_H

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Good question, I am going to measure the output level as soon as I can, but the issue is seemingly unrelated to output level - I can get it to clip regardless of gain setting, but I can also get clean signal at all gain settings, as long as I dont raise the source volume (speaking louder) into the mic.
 

David_H

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Yeeees. Figured out what went wrong! Thank you radardoug for getting me on the right track! I found out that the pre doesn't clip at every gain setting - with a dynamic mic and the lowest (in my case 34db because of the wiring) setting I could get a clean signal. This led me to believe that the pre is working fine, but that I have a error in the wiring somewhere. I reread the manual carfully and found out that I had to strap together pins 22&23 on the output transformer (this is clearer in the old telefunken v672 manual, but because I have a newer model I stuck to the ANT manual, and there the strapping togehter of 22&23 is only mentioned very briefly in a subordinate clause, not in the wiring diagram!) Now I have a healthy signal without distortion :)). Much bassier too. I thought the signal was very bass shy before but having no experience with this amp I wasn't sure really. Nice!
 

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