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Old 12th December 2010, 09:33 PM   #1
TBKK is offline TBKK  Germany
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Default preamp

I have build this preamp, but the sound is to light.
does anyone knows what to change to get more mid

thanks see schematic
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Old 12th December 2010, 10:56 PM   #2
paulb is offline paulb  Canada
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Change the entire circuit to run on transistors.
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Old 12th December 2010, 11:01 PM   #3
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Quote:
Originally Posted by TBKK View Post
I have build this preamp, but the sound is to light.
does anyone knows what to change to get more mid

thanks see schematic
Increase the value of the coupling capacitors if possible.
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Old 12th December 2010, 11:20 PM   #4
tomchr is offline tomchr  United States
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Start by increasing the coupling caps. The one between the two 6N4's need to be 220 nF minimum (1/(220E-9 F * 1E6 Ohm) = 2.2 Hz). The one by the 12AU7's should be 100 nF min. This will give you less than 0.5 dB attenuation at 20 Hz.

Then I suggest moving the volume control to the input. I would change the 100 kOhm on the input to 1 MOhm. Connect the ends of the volume pot between the input connector and ground; connect the wiper to the input of the amp. Scrap the 50 kOhm resistor currently present on the input.
If this arrangement causes clipping in the input stage at high volume, you can add 10 kOhm from the input connector to the top of the volume pot.

If this doesn't improve the sound quality, I'd reduce the 9.09 kOhm on the output to something like 100 Ohm (or eliminate it completely).

~Tom
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Old 13th December 2010, 10:43 AM   #5
DF96 is offline DF96  England
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There is a lot of circuit there, just to give you a gain of about 6. It looks like it was designed by someone whose day job is in solid-state.

Changing capacitors as tomchr suggested will not make much difference, except possibly to introduce motorboating (LF oscillation), as there is heavy negative feedback via the 24.3K resistor.

There seems to be a mistake in the circuit. I assume the output stage is supposed to be a White cathode follower. If so, the 0.1uF capacitor joining the 12AU7 grids should go from the lower grid to the upper anode. Is it built correctly, or did you build the circuit as shown? Correct this, and see what happens.

As it is a line stage the volume control should be at the input, as tomchr says. The 9K at the output seems daft. You have lots of circuitry to generate a low output impedance then you throw it away with a big series resistor. I would suggest something between 100ohms and 1K here.

The 3.3uF in series with the 24.3K will give you a rising response below about 2Hz. Don't use this line stage after a phono preamp, unless the preamp has extremely good subsonic filtering or your records/turntable have no rumble/warps! The small coupling caps will help prevent this getting out of hand - maybe that is why they are small.
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Old 13th December 2010, 10:53 AM   #6
SY is offline SY  United States
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Quote:
Originally Posted by DF96 View Post
It looks like it was designed by someone whose day job is in solid-state.
I'd guess shoe sales- this is a terrifically incompetent circuit. Not quite as bad as the Lampizator I was just looking at, but close. Overload performance is guaranteed to be awful, linearity is... questionable, and the noise performance will be very sub-par. The "designer" has no idea of how to run heaters. The output impedance is totally destroyed by that 9k09 and the volume control.

Where does this stuff come from?
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Old 13th December 2010, 11:04 AM   #7
DF96 is offline DF96  England
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I thought Mr. Lampizator is regarded as a guru - although I must admit I struggled to understand his site. I was never clear whether he was struggling with English (quite understandable) or physics. There were probably a few gems there, but the effort of digging became too much for me.

I have just noticed the heater arrangements for the non-existent 12AX7. I think we can assume that the circuit diagram is not exactly what the OP built. Is this more Chinese junk? Maybe they need hard currency to fund all their African metal ore adventures?
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Old 13th December 2010, 11:08 AM   #8
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An a fixed input attenuator! For what? It may be best to re-use PS and casing and build something entirely different.
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Old 13th December 2010, 11:08 AM   #9
SY is offline SY  United States
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I'm guessing Chinese junk. Has all the hallmarks. If the circuit board positions the electrolytic capacitors right next to a hot tube, my suspicion is confirmed.
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Old 13th December 2010, 12:00 PM   #10
TBKK is offline TBKK  Germany
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i work fine, only some minor DC on the output.
it has drive but sound to light.

i have build this for less then 50 USD
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