lower the gain

[Jempie]

Hi,

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

mow the gain is mutch to high

 

[DF96]

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.

 

[Frank Berry]

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.

 

[Jempie]

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

 

[Jempie]

i use 6z4 tube as regulator

 

[Frank Berry]

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.

 

[DF96]

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.

 

[Jempie]

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

 

[Jempie]

the voltage is 265 volt and goes to the 6z4 tube
on the tubes 12.6 V

 

[Jempie]

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

 

[DF96]

The preamp voltage gain is about 18dB, so -10dB attenuation is not enough.

 

[Jempie]

The preamp voltage gain is about 18dB, so -10dB attenuation is not enough.

i have remove the resistor of 9,09K and connect the 47K and lower the 100K
to 55 K does this work?

 

[ballpencil]

You can try skipping the second gain stage? Bypass the grid to the plate using, say, 1k resistor. It should be an easy trial to do.

 

[Jempie]

You can try skipping the second gain stage? Bypass the grid to the plate using, say, 1k resistor. It should be an easy trial to do.

what do you mean, could you draw this on the schematic

 

[ballpencil]

salam olahraga

first of all, what's your source? a cd player should output more than enough voltage to be fed directly to your amplifier (which means that you don't need any preamp, sell it in kaskus or something).

your amplifier needs only 1.7 Vpp for full output power. a typical CD player will output 2 Vrms, which is about 2.84 Vpp, so it's more than enough to feed directly to your amplifier.

here's my suggestion drawn on your schematic:

An externally hosted image should be here but it was not working when we last tested it.

EDIT: don't follow this suggestion!

 

[SY]

That will not be healthy for the tube- you've put a high DC potential on the grid.

 

[ballpencil]

That will not be healthy for the tube- you've put a high DC potential on the grid.

oh my! you're right. it's an easy fix by unplugging the tube? unplugging only works depending on how he shares the 6N4 double triode on the stereo preamp.

jempie, you have been warned by SY. don't follow my suggestion.

i will now shut up.

 

[Merlinb]

Also, your White cathode follower has a drawing error: The feed for the bottom valve should come from the anode of the top valve, not the grid of the top valve.

 

[SY]

You can lower the 24k3 feedback resistor (and increase the capacitor proportionately) to reduce the gain. You'll want to confirm stability, easily done with square waves, and add a compensation cap internally (e.g., across the 249k plate resistor) or across the feedback resistor if you see oscillation or ringing.

 

[kouiky]

I see there is a 20k potentiometer on the output. There are several peculiarities within the scope of this design. My advice would be to replace the 50k and 100k resistors on the input with a 60dB 100k potentiometer as a starting point.