andia

Mic-Preamp EAB 501 - Output-Level??
« on: December 04, 2019, 03:28:25 PM »
Hi all,

I have 2 of these EAB-Geiling E501 Micpreamps. they have 70dB of Gain +10dB with a Trimmer. with the GainSwitch you can also engage a 10dB Pad on the front-panel.
The problem is that the inputsignal gets padded by 40dB after the Input-transformer and you have to crank low level signals really hard, so that the signal gets often distorted internally.

I have checked every component in the circuit, changed caps and studied the schematic.
It seems to work as it should. At least I do not find anything.

Could it be, as this module was part of a mixing console, that the output, which is unbalanced by the way, of this module was trimmed this way for the next stages in the channel? Or do I overlook something (un)obvious.

If so, I thought I can put a 30dB Gainstage at the output to compensate for that??

thank you all!!


moamps

Re: Mic-Preamp EAB 501 - Output-Level??
« Reply #1 on: December 04, 2019, 03:41:49 PM »
The problem is that the inputsignal gets padded by 40dB after the Input-transformer and you have to crank low level signals really hard, so that the signal gets often distorted internally.
It's just part of the attenuator if your input signal is very high. At 70dB position, the overall gain should be 70dB. Do you have installed card KM-ET at the transformer's primary?

andia

Re: Mic-Preamp EAB 501 - Output-Level??
« Reply #2 on: December 04, 2019, 04:08:04 PM »
It's just part of the attenuator if your input signal is very high. At 70dB position, the overall gain should be 70dB. Do you have installed card KM-ET at the transformer's primary?

Yes, it containes a high-pass filter.

the overall gain is indeed 70dB from the lowest to the highest step of the gain switch, just 40dB lower than than it should be, if that makes any sense...

moamps

Re: Mic-Preamp EAB 501 - Output-Level??
« Reply #3 on: December 04, 2019, 04:42:52 PM »
the overall gain is indeed 70dB from the lowest to the highest step of the gain switch, just 40dB lower than than it should be, if that makes any sense...
Do you like to say that the preamp has regulating range of 70dB, but max. output level is 40dB below 0dBfs or +4dBu?

andia

Re: Mic-Preamp EAB 501 - Output-Level??
« Reply #4 on: December 04, 2019, 05:48:12 PM »
Do you like to say that the preamp has regulating range of 70dB, but max. output level is 40dB below 0dBfs or +4dBu?

lets say I feed a 775mV into the input, at the lowest gain position (labeled -10dB) I read something like 7,75mV (-40dB) at the output, at the next step (labeled 0dB) the level is -30db in relation the input voltage, next step -20dB, next step -10dB,  next step 0dB, next step +10dB, then +20dB, ...

radardoug

Re: Mic-Preamp EAB 501 - Output-Level??
« Reply #5 on: December 04, 2019, 07:00:33 PM »
Set it to the maximum gain. Feed in a signal that does not distort at the output. Measure the input level, and the output level.
Then determine the actual gain that you have. Sounds like you do not have 70 dB.

mjrippe

Re: Mic-Preamp EAB 501 - Output-Level??
« Reply #6 on: December 04, 2019, 09:47:42 PM »
I am going to venture that you do not have a 40dB pad after the transformer.  The resistors you have circled are part of the gain control switch, which is a three deck switch.  It looks to me that at the lowest settings (-10 through +30) the first gain stage is bypassed and above that the signal is routed through both stages.  Similar to how Neve did it.

To test the higher gain settings you should be sending a much smaller signal than .775 volts.  I typically test mic preamps with a -40dBm signal which is around .007 volts.  If you do this and still get -40dBm at the output with the gain switch set to 40 then there is likely a problem somewhere.  I doubt they would have a 40dB pad in the preamp circuit if there is a -10dB pad and a -10dB gain setting.

thekid777

Re: Mic-Preamp EAB 501 - Output-Level??
« Reply #7 on: December 05, 2019, 08:18:43 AM »
I had a pair of these and indeed they should work unbalanced without problems and they sound great!
Still, since I acquired same preamp but with complete signal path on it
I prefer the sound with the output amp and xformer
In my version they replaced the studer input xformer with a haufe rk880 (same specs)
and added haufe rk357-2 on the output (+amp)
You might be able to use output line drivers from other brands to complete signal path
once your problem is sorted out. (might be defective 414-416 transistors?)


 

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