G9 Frequenzgang, muss der so wenig Bass haben?

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Beauvais
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Registriert: Do Jan 29, 2009 11:31 am
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Beitrag von Beauvais »

OK, but then I don't understand what causes the bass loss if this is the ideal way of doing it. there are other amps with +-30dB/10k pads that don't exhibit this behavior.

is that by design and part of the compromise of using the same input transformers for both modes or is it due to the OEP's and using Lundahls for example would be better in this regard?

ioaudio
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Beitrag von ioaudio »

1. the ideal solution is one transformer for mic and another one for line (neve 1073 etc)
2. yes, it's a compromise (the pad attenuates the signal and the mic transformer raises the level again)
3. the pad was designed for the "standard" input impedance of ~10k (4,7 + 4,7) so the 470R termination is resulting from that.
the lundahl has a higher primary inductance so it will work better with a 470R source.

BUT: every transformer extends it's bass response when driven from a lower source impedance. i think the OEP should do nicely with the 1k+1k / 100R

Beauvais
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Registriert: Do Jan 29, 2009 11:31 am
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Beitrag von Beauvais »

OK, thanks for clearing the first two points. But I must still be missing something. Because I still don't understand why a h-pad would not be beneficial in this case. One could choose values that both represent a 10K input impedance and a 200ohm (or even lower) source for the input transformer. No?

Harpo
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Beitrag von Harpo »

Beauvais hat geschrieben:OK, thanks for clearing the first two points. But I must still be missing something. Because I still don't understand why a h-pad would not be beneficial in this case. One could choose values that both represent a 10K input impedance and a 200ohm (or even lower) source for the input transformer. No?
No. You don't want impedance matching here. Whatever value series resistors connecting between shunt and transformer would be seen by the transformers primary in series to the shunt, so resistance could only increase, but here you want just the opposite. (in real world, this already is a balanced H-pad, but fortunately trace resistance between shunt and transformer will be negliable above zero ohms).
As drawn by Jakob, the external line source sees (4K7+470R+4K7)=9K87. The source most probably will drive this ~10K load without any problem. This balanced L-pad drops amplitude by log(1/(4K7+470R+4K7)*470)*20=-26,4dB. The transformers primary sees at least the 470R shunt, which might decrease to 1/(1/470+1/(4K7+0+4K7))=447R6 with external source connected, being in parallel with the shunt. With your 1:6,45 ratio stepup transformer, the reflected transformer secondary impedance will be about 447,6*6,45^2=18K6.
Decreasing the resistor values in the pad from 4K7/470R/4K7 to 1K/100R/1K or 2K2/220R/2K2, as suggested by Max will both give the same -26,4dB drop in amplitude, but the external source now would see a 2K1 or 4K62 load instead. Even a TL07x should be able to drive this (for usual capacity/ft of cable is a more limiting factor) and this lower resistive load might only rise the HPF at output from the external source by a fraction of Hz. The transformers primary now will see 95R2 or 209R5 with source connected. Reflected secondary impedance will decrease to 4K or 8K7.
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Beauvais
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Registriert: Do Jan 29, 2009 11:31 am
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Beitrag von Beauvais »

Oh wow, thanks for all that information. But I was not looking to match impedances, but rather to get an even lower impedance than the standard u-pad by using an h-pad.

I've toyed around with some of those online pad-calculators and came up with a h-pad that has 10k input and 50 output impedance with the following values: 4988ohm in series, 50ohm for the shunt and 0ohm for the following series resistors, which equals a u-pad. I lose about 6db (the pad is 29dB) compared to the 'original' but get a 50ohm impedance for the transformer.

The reason I'm looking to get such a low impedance is the fact that the OEP apparently is VERY sensitive to even slightly higher impedance.

I don't know how I've missed it before, but I've just stumbled across this thread (http://www.prodigy-pro.com/diy/index.php?topic=9813.0) and apparently what I'm experiencing with my G9 is 'normal' behavior.

And there are apparently two frequent causes for bass roll of in the G9. One is when using the Lundahl LL5402 output transformer where the standard ECC82 are having a hard time driving the output tranny due to inductance problems. However this is not the case when using the OEP output, that's why I have not experienced this.

The other cause for bass roll off is that the OEP A262-A3E is VERY picky about its driving impedance, causing the bass to roll of with anything higher than as little as 50ohm. This is obviously what I'm experiencing and apparently does not happen when using the Lundahl LL1528 input tranny.

So to sum up all the information from that rather long thread: The best frequency response without doing further mods is to use Lundahl LL1528 for the input and OEP A262-A2E for the output. There are of course many other options but for noobs like me sticking to the PCB has its advantages.

Now my first thought was of course to exchange the OEP input with its Lundahl brother. But of course the easiest solution would be to just replace R5 with a 50ohm resistor.

So my question would be: Will I run into any troubles with that mod (ringing, high frequency ramp etc.) or will I simply loose 6dB (more like 10dB according to said calculator) of gain (which I would happily trade for a linear frequency response)?

Beauvais
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Registriert: Do Jan 29, 2009 11:31 am
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Beitrag von Beauvais »

OK, to bring this topic to an end, here's some more things I did.

First thing was to add a 56R resistor in parallel to R5/R105, effectively changing the impedance of the pad to 50R (please correct me if that's wrong). This did flatten the frequency response somewhat but caused problems in the extreme high frequency (ringing?), so I abandoned that idea. See for yourself, left channel is 'stock', the right channel is with the paralleled resistor.

Bild

So I went ahead and bought some Lundahl transformers to see if this would improve things. This improved the FR significantly, it's almost within +-1dB between 40Hz and 20KHz.

Bild

But I didn't really like the sound of the G9 with the Lundahls, it loses quite a bit of the edge I really like about a G9 with OEPs. So I've reverted to the OEPs and just accept the fact that the G9 is basically unusable as a line-amp. But it still is a great mic pre which I absolutely love!!

gyraf
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Beitrag von gyraf »

And there are apparently two frequent causes for bass roll of in the G9. One is when using the Lundahl LL5402 output transformer where the standard ECC82 are having a hard time driving the output tranny due to inductance problems. However this is not the case when using the OEP output, that's why I have not experienced this.
Yes, I'm aware that there is a user on the international forum that continue to claim the LL4502 to be a problem when driven from a ECC82 - but in several tests I've made, it keeps up nice (down to some 35Hz or so). Note that this is with a 4u7 output capacitor driving the 5402 - use a larger capacitor, and you won't get lower like you may think, because lowest octave is kept up by trafo/cap resonant area. Yes, it's cheating, but sounds nice :-)

For input transformer, The LL1530 wired 1:7 is also a nice option.

Jakob E.

Beauvais
Beiträge: 79
Registriert: Do Jan 29, 2009 11:31 am
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Beitrag von Beauvais »

gyraf hat geschrieben:
And there are apparently two frequent causes for bass roll of in the G9. One is when using the Lundahl LL5402 output transformer where the standard ECC82 are having a hard time driving the output tranny due to inductance problems. However this is not the case when using the OEP output, that's why I have not experienced this.
Yes, I'm aware that there is a user on the international forum that continue to claim the LL4502 to be a problem when driven from a ECC82 - but in several tests I've made, it keeps up nice (down to some 35Hz or so). Note that this is with a 4u7 output capacitor driving the 5402 - use a larger capacitor, and you won't get lower like you may think, because lowest octave is kept up by trafo/cap resonant area. Yes, it's cheating, but sounds nice :-)

For input transformer, The LL1530 wired 1:7 is also a nice option.

Jakob E.
I'm sorry, I didn't mean to pass on false or at least subjective information. I never had Lundahl output transformers installed, so I shouldn't comment on that. All I can say from personal experience is that there is a significant bass roll off when the line input is used with OEP's. It's better with Lundahls, but still not perfect, I guess for that an independent line input transformer would have to be used.

So from my subjective viewpoint, the G9 is a fantastic mic preamp (I prefer OEP input transformers over Lundahl) with the option to use it as a line amp, one just has to be aware of the drawbacks. All in all, I love it! :D

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