Siemens U274 input gain attenuator

[la-2a]

Hi folks , I was wondering how to make the Siemens U274 input less sensitive . Just lowering the input -15, - 30 dB with an toggle switch or a pot would do the job for me.
Is  a simple input resistor (H attenuator)  a good choice . ??  I don't need 70dB amplification so starting at -20 up to 50dB  would do the job  very well for me . Which components  in the circuit should I change to get less amplification. My electronic skills are unfortunately not so high.
For your help and suggestions I would be very grateful.

Best regards

 

[helterbelter]

I simply used a 40dB U bridge attenuator with a 2P2T switch in front of the thing.  I chose 40dB so the unit can also be used as linelevel diode bridge  limiter.

 

[etheory]

Which components  in the circuit should I change to get less amplification.

The first-stage gain is determined by the initial npn transistor sitting before the diode bridge.
It's labelled as Ts1/BCY66 on most of the schematics.

Note how the gain is decreasing as the feedback resistors running from it's collector to base, via that 5uF cap? So you can see that going lower than the lowest 24K shown will decrease gain.

The second stage output amp, has gain determined by the feedback resistor R18, which is generally listed as 51K on the schematic. Lowering this value decreases the gain of this stage. If you decrease this value, be careful that you also adjust the output bias control R14 to ensure half the supply voltage (about 12V referred to GND) appears at the emitter-collector junction of the output transistors (before the output cap), and you should be good.

Attenuation on the input is a lot easier however....

 

[Rob Flinn]

I simply used a 40dB U bridge attenuator with a 2P2T switch in front of the thing.  I chose 40dB so the unit can also be used as linelevel diode bridge  limiter.

I think I did the same sort of thing.

 

[woodsound]

Hello,
i know a lot of time passed from the start of this topic but, i'm trying to understand how this module works,
do you think it will be possible to change attack, release and threshold working point of the diode bridge?

here the schematics thankx to Mr. Kubarth:
http://audio.kubarth.com/rundfunk/getfile.cgi?f=%3D%2C34W.%238W%2CSDQ.5%5DU%2BW-I96UE%3BG-%3F%3D3%28W-%22YG%3A68%60%0A

The switch for the gain setting is dual, i can have gain setting just using a 250k log pot instead of the R1-R5 on the switch S1-I, but what's the S1-II is for?  (anyway, is it correct to do so in this circuit?)
When the pot is at 0 ohm i don't have silence, but some loud amplification, maybe about 20 or 30 db (not measured), slowly rotate it will bring volume to be very low but very distorted, then volume is increasing as expected, it's a very strange behaviour for a negative feedback preamp (i have some experience with Telefunkens V672 first and newer version, and other modules).

Anyway, when default and unmodded, my U274 appears to have a variable threshold point: the more the gain, the lower the threshold, i had this feeling because with silence on the input i got different noise for the higher gain settings from diode bridge on or off (S2 I II III), not a linear increase.

Hope someone will have some useful info, in the meantime i will try something to have less gain on the first stage, on the input maybe

thnkx!

 

[moamps]

You shouldn't set the pot to 0 ohms, place in series a resistor of 27k or so. The equivalent resistance in emitter path should be changed simultaneously also.  The gain/threshold can be lowered by changing relation between R18 and R14.  It's a simple limiter, you can't expect too much from it.

 

[Rob Flinn]

It's a simple limiter, you can't expect too much from it.

For me it's a bit like a level loc but better.

 

[PRR]

The LevelLoc has better control of timing. This one is crude.

 

[Rob Flinn]

The LevelLoc has better control of timing. This one is crude.

I have both & prefer the sound of the Siemens.

 

[woodsound]

Thank you guys for reply

You shouldn't set the pot to 0 ohms, place in series a resistor of 27k or so. The equivalent resistance in emitter path should be changed simultaneously also

I'd like to understand why 0 ohm should not be used, i understand 0 ohm will transfer the negative feedback without attenuation, so almost no signal amplified...
(i'm not an expert but i'd like to dig into these old German designs, i just started working on modules i collected over the past 3 years, i have many different);
A resistor of about 24k in series with the pot will give me the first 44db of the stepped witch, it would be good to have less than those 44 db as a starting point, will it work to use the switch and replace the resistors to different values? 
like 7.5k - 15k - 30k - 60k - 120k - 200k ?

Reading the papers i understand it has 400 ohm input impedance, it's a low value as far as i know, i used a switch to have default/10k input (2x 5.1k) and it lower the output a lot, and lowers the noise too, and the color is darker (as far as it's far from a linear preamplifier, and even more color from the limiter, it's a "colorbox" module, i thought a change in freq. response is not a problem as far as it's musical and not annoying)
Any suggestion about input resistor to set a different input impedance? differences in sound from a U pad?

What's the equivalent resistance in emitter path - S1 II - is for? (sorry for this maybe stupid question)

The gain/threshold can be lowered by changing relation between R18 and R14.  It's a simple limiter, you can't expect too much from it.

Yes it's a simple limiter i know, the possibility to change the threshold is interesting, can you explain how it works?
no way to change attack and release playing with capacitors?
10 ms attack is slow for a limiter, a switch to set 1/10 ms will be killer, and the release is very slow, will be nice to have it faster.

thnkx

 

[moamps]

I'd like to understand why 0 ohm should not be used, i understand 0 ohm will transfer the negative feedback without attenuation, so almost no signal amplified...

This way you are shorting input to output of an amplifier. It never sounds good.
More about this you can find here:
https://wiki.analog.com/university/courses/electronics/text/chapter-9

A resistor of about 24k in series with the pot will give me the first 44db of the stepped witch, it would be good to have less than those 44 db as a starting point, will it work to use the switch and replace the resistors to different values? 
like 7.5k - 15k - 30k - 60k - 120k - 200k ?

44dB is a gain of the whole chain: input transformer-input stage-output stage-output transformer, it's not gain of the first transistor alone.
You can test your idea using 7k5 in front of your pot. 
Also, you can try to use another tap for sensing gain on the output transformer and get different threshold.

Reading the papers i understand it has 400 ohm input impedance, .....
Any suggestion about input resistor to set a different input impedance? differences in sound from a U pad?

It's designed for connecting a microphone. You can try to lower overall gain and increase input impedance by swapping primary and secondary of the input transformer.

What's the equivalent resistance in emitter path - S1 II - is for? (sorry for this maybe stupid question)

The gain of the common emitter amplifier is about  -Rc/Re where the Re is remaining resistor chain from the capacitor bypass.
It is additionally reduced with NFB from C to B of the transistor. 

Yes it's a simple limiter i know, the possibility to change the threshold is interesting, can you explain how it works?
no way to change attack and release playing with capacitors?..

Maybe:
https://groupdiy.com/index.php?topic=51773.20

 

[woodsound]

Thank you Moamps, very useful info ad resources

i'm trying to mod it, if it will work and worth the effort there will be a lot of controls, so a lot of drills, and not enought space in the metal front panel of the module... i need a "workaround"

Test-wiring "on the fly" - Frankenstein is coming!