how to lower gain from this schematic

[milen007]

Hi Guys

i build this 6t4 preamp awhile ago. pairing it with 100wpc mosfet class AB amp gives too much gain for my systems. say the lowest volume is 7oclock. i reached my listening volume at 8 oclock.

my plan: using voltage devider of 1K and 24K resistors on the output of this pre. from calculation it gives 28db attenuation, Zin=25K and Zout=960ohm

while this schematic has roughly Zout = 800 ohm

do i have better option that this voltage devider to lower the gain?

Thanks in advance

erwin

here is the schematic

[assendor]

Hello Erwin,

If you only use Monica as a input source, you may lower her output level, by changing the I/V resistors to
a lower value.
In your case 1-1.2k maybe?

You may also reduce the gain of your preamp setup, using Negative feedback.
You may reduce the gain of your preamp here, by adding negative feedback.
NOW this something I HATE, but still you can  test it to see what you think.
It all depends on the equipment connected, how it will sound.
It may add odd harmonics to your preamp, but if you really need a LOT of feedback, it does not have to be so bad, or it can be good too- `?

Now the solution to reduce the output voltage of Monica is the one I would prefer first.

-Jonas

[assendor]

Hello Erwin,

Have you tried connecting your Monica DAC directly to this amplifier?
Just a Volume between them?

You do have the Buffer B1 Right?

-Jonas

[yeo]

hi erwin

i won't use voltage divider to lower the output voltage.

what is the amplification factor of this tube? this determines the gain. usually i just go for lower gain tubes.

yeo

[assendor]

Hello Erwin,

here is the datasheet for this tube, if it actually is a 6T4.
You have four tubes in your preamp then?

http://www.mif.pg.gda.pl/homepages/frank/sheets/108/6/6T4.pdf

-Jonas

[assendor]

Oh forgot something.

Amplification factor is lucky number 13.
This is why I wonder if you have tried Monica directly via a volume control.

BTW could not agree more with yeo here, tried the damping you suggest myself, and find it to kill speed of sound.

-Jonas

[milen007]

hi guys

thanks for the inputs, guys. this preamp is just too much gain for 100wpc amp.

but for the ta2024 of 10watt is very good. think will do wonder with charlize.

this is my 1st diy tube preamp. i know the gain is too much for my systems, but the simplistic urge me to try and the rave of this bettered the aikido and GG.

Hi Jonas

I never try just volume control from monica to this amp. as somehow inside me want to have tube sound to my systems. lol but maybe try this with my cathode follower.

IV resistor of 1K, this will lower the output of monica right? will this also decrease any details? or the other way around, lower the noise?

i have no knowledge on developing the NFB. any suggestion is very welcomed.

Hi Yeo

this preamp is claimed to has 8x gain. as i said i know this is too much gain for my systems. gain of 2-3 is good.

Erwin

[KenC]

hi,
few ways that I can think off,
1) as jonas suggested, use feed back
2) re-wire the to triode into a CF and the bottom triode as CCS, you can also boot strap.
3) use a step down input transformer?
4) remove the top tube (becomes a very simple GC stage), add another stage of RC PS filter to bring the B+ of your desire and place a anode sized to you required output signal amplitude.
4 a.) if you don't want to wast the 2nd tube, wire it as another CF.

5) Cheap and hassle free? solder in a voltage devider using 2 resistor as yeo suggested.

It would be most useful if you could tell us the signal amplitude you want.

CF is not all bad... just takes more tender care and effort to do them justice. Also, since you are talking about mosfet output stage, CF is not an luxury.

That should keep you busy for a while.

Cheers

KenC

[milen007]

Hi KenC

thanks for the suggestions. lol as you know noobs like me has to do lots and lots of research to try any of your suggestions.

which one do you recommend most? as i will dwell into the one suggest.

thanks alot

erwin

[KenC]

Erwin,

LOL.

Actually, the easiest fix (and my favorite) is to turn down your volume pot... That is all you need to do.... LOL

The next fix is 3) that is if learning is you main objective; Build a simple experimental circuit, keep things simple, experiment one thing at a time while you learn, increase the complexity bit by bit...AND Buy MJ's books (not expensive consider the mount of mistake you will avoid)....

Cheers

KenC

[milen007]

hi guys

k i wanna use NFB for this schem. but no idea how. is it kind of voltage divider? can some one point me the exact schematic but one with NFB and one without? with this kind of comparison, all will be very clear. try to find but to no avail.

this 6t4 schem has gain 6-7 (15-17db)

can someone help me to apply negative feedback to this circuit to get the gain of 2-3?

is it just adding another resistor from the input to the output of 2.2uF cap before the output ground resistor? if yes, whats the suggested value for that feedback resistor? whats the suggested value for the output ground resistor?

TIA

erwin

[wildbushman]

Hi Erwin,

I dont think you would like to put in NFB (sorry guys, NFB is for pussies).

You can add 100k resistor in between the input RCA and the volume pot. This effectively becomes voltage divider and reduces your gain by half. Also, the preamp would be happier to see the volume put turned up a bit more.

Hope this helps.

[Passtube]

Hi,

Never use voltage divider at output. It is very very inefficient! I can proof to you, with load current, voltage divider's efficiency can drop to less than 30%!!!

[milen007]

hi Passtube

how bout voltage devider at the input? before or after the pot?

tia

erwin

[Passtube]

The voltage divider at input is all right (in fact, your pot act as a voltage divider). If you wish to step down a lot of voltage, I suggest you to add an emitter follower (also known as current amplifier with voltage gain slightly smaller than one), then only go through your voltage divier. This is to ensure that the input impedence saw by your CD source is high and the output impedance to your voltage divider is low. With this, the power transmitted from your source to your voltage divier is high enough to ensure minimum losses.

But yet, I think what Yeo said is right, lower mu tube is more suitable in your case.

This is what you may like to do. Use low mu tube to build your buffer (cathod follower), then change your pot to 50k pot. This will 1st, reduce your voltage gain to about 0.85 after the buffer.  Note that the pot is placed after your buffer.