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lower the gain
lower the gain
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Old 14th January 2014, 02:34 PM   #1
Jempie is offline Jempie  Indonesia
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Default lower the gain

Hi,

i have build this preamp, is there a way to lower the gain
see schematic.

mow the gain is mutch to high

pre.GIF
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Old 14th January 2014, 02:42 PM   #2
DF96 is offline DF96  England
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Reduce the 100k resistor at the input. This preamp seems to operate by having lots of gain which is then reduced first by negative feedback to around 12 and then by adding an attenuator at the beginning to reduce it to around 8. This is still far too much gain for most systems, which generally need no extra gain at all.

You could reduce gain by adding more feedback but this could bring stability problems. Better to go with the flow and just add more attenuation.

Alternatively, just omit the preamp from your system.
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Old 14th January 2014, 03:40 PM   #3
Frank Berry is offline Frank Berry  United States
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You have not indicated the B+ voltage. If the preamp has enough headroom on the output, you might add a little attenuation to the output. As DF96 indicated, with further attenuation you will be getting quite close to unity gain. Your best solution might be to eliminate the preamp completely.
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Old 14th January 2014, 03:52 PM   #4
Jempie is offline Jempie  Indonesia
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if i play at 9. th sound is to load

hereby the specification of the amp

Power Output: 200 watts @ 8 ohms, 20 Hz - 20 kHz, <0.05% THD; 300 watts @ 4 ohms, 20 Hz - 20 kHz, <0.05%
THD Signal to Noise: 112 dB "A" weighted
Input Impedance: >10K Ohms
(RCA=16K Ohms, XLR=30K Ohms)
Gain: +27dB (1.7V sensitivity) for full output

if i lower the 100K resistor to 47K that mean?

I will check the voltage
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Old 14th January 2014, 03:54 PM   #5
Jempie is offline Jempie  Indonesia
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i use 6z4 tube as regulator
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Old 14th January 2014, 03:58 PM   #6
Frank Berry is offline Frank Berry  United States
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Does the preamp output potentiometer have an audio taper? That might explain why the volume is too loud with the control at a low position.
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Old 14th January 2014, 05:39 PM   #7
DF96 is offline DF96  England
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Assuming CD with the standard 2V output, and a modern disc with most of the music near the limit you could be getting 500mV as a typical output.

Starting from the other end, most people run their system at surprisingly (to them!) low power outputs - unless they have a huge room or very inefficient speakers. Let's guess 20dB down, so that there is some room for peaks (not that the CD will deliver peaks). That is 170mV, so all you need is a volume control.

Alternatively, just note that 2V is greater than 1.7V so no more gain is needed. So either way of looking at it tells us that all the gain provided by the preamp must be thrown away in attenuation; if not in the volume control then elsewhere.
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Old 15th January 2014, 12:05 AM   #8
Jempie is offline Jempie  Indonesia
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Quote:
Originally Posted by Frank Berry View Post
Does the preamp output potentiometer have an audio taper? That might explain why the volume is too loud with the control at a low position.
yes
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Old 15th January 2014, 12:09 AM   #9
Jempie is offline Jempie  Indonesia
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the voltage is 265 volt and goes to the 6z4 tube
on the tubes 12.6 V
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Old 15th January 2014, 03:33 PM   #10
Jempie is offline Jempie  Indonesia
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Quote:
Originally Posted by DF96 View Post
Assuming CD with the standard 2V output, and a modern disc with most of the music near the limit you could be getting 500mV as a typical output.

Starting from the other end, most people run their system at surprisingly (to them!) low power outputs - unless they have a huge room or very inefficient speakers. Let's guess 20dB down, so that there is some room for peaks (not that the CD will deliver peaks). That is 170mV, so all you need is a volume control.

Alternatively, just note that 2V is greater than 1.7V so no more gain is needed. So either way of looking at it tells us that all the gain provided by the preamp must be thrown away in attenuation; if not in the volume control then elsewhere.
If i use Rothwell Attenuators of -10Db at in input the gain is much better
but still to much for my amps
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