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Baxandall circuit
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5199
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Author:  dimitris1811 [ 07 Jul 2015, 00:19 ]
Post subject:  Re: Baxandall circuit

Thanks Matt. I follow exactly the scematics not any difference at all. I think that problem might be the potentiometers types. I have to check them again since they might be linear type. I do not see something else.

Furthermore, bass & tremble are not working well and you can easily understand it by listening the sound. Actually it looks that there is no bass. I will revert with results

Author:  dimitris1811 [ 09 Jul 2015, 06:46 ]
Post subject:  Re: Baxandall circuit

Just a note. I hope that I have connected properly the potentiometers and are not reversed. Thanks for any comment.

Optional pad has already been installed

Author:  philip davies [ 20 Jul 2015, 18:11 ]
Post subject:  Re: Baxandall circuit

I have have recently run into problems with the circuit Baxandall circuit I used with my headphone amplifier. If I turn the bass potentiometer to full boost and rotate the amplifier volume to full the output of my headphone amplifier will oscillate quite loudly. Also if I set the volume to minimum the output of the amplifier will hum loudly. There will also be a loud pop if I turn the volume up to around half way. Its as if there is a D.C voltage present on the volume potentiometer.

Meassuring for any voltage accross the potentiometer revealed 0v.

I managed to eliminate the problem by installing a 100K resistor accross the output of the baxandall circuit. Right after the 0.47uF decoupling capacitor.

Can I leave the fix as is or should I be looking for a different fault that would cause the above symptoms?

Author:  Suncalc [ 20 Jul 2015, 19:16 ]
Post subject:  Re: Baxandall circuit

philip davies wrote:
If I turn the bass potentiometer to full boost and rotate the amplifier volume to full the output of my headphone amplifier will oscillate quite loudly.
Insufficient phase margin in the feedback loop. How is the circuit loaded?
philip davies wrote:
Also if I set the volume to minimum the output of the amplifier will hum loudly.
Bad ground on the potentiometer. Look for cold solder joints.
philip davies wrote:
There will also be a loud pop if I turn the volume up to around half way. Its as if there is a D.C voltage present on the volume potentiometer.
You'll need to post the entire schematic so that we can see how you've wired it in. Where is the volume control?

Author:  gofar99 [ 20 Jul 2015, 20:45 ]
Post subject:  Re: Baxandall circuit

Yes please post the whole schematic. I would add it sounds as if there might not be enough inter stage decoupling in the power supply.

Good listening
Bruce

Author:  philip davies [ 21 Jul 2015, 18:39 ]
Post subject:  Re: Baxandall circuit

The volume control has been placed between the output of the baxandall circuit and the headphone amplifier input. The pot has a value of 50K. I have also checked the ground on the potentiometer and gone over the solder joints.

I have attached a drawing of my amplifier and its psu. If I remove the baxandall circuit completely i.e disconnect it from the psu and remove it from the audio path then the amplifier works fine.

Author:  gofar99 [ 21 Jul 2015, 20:18 ]
Post subject:  Re: Baxandall circuit

Hi Check out Duncan's Web site for Tone Stack Calculator. It will do all the calculations for you and show graphically what happens. Nice thing is that it shows what happens if you alter the components. It does several popular types of tone controls not just the one you were using. It's a free program as well. I'll see if I can post the link.

http://www.duncanamps.com/tsc/download.html

I have used it and the values seem to match my expectations.

Good listening
Bruce

Author:  philip davies [ 22 Jul 2015, 18:30 ]
Post subject:  Re: Baxandall circuit

Hi Bruce,

Thanks for showing me the Tone Stack Calculator. It has shown me some interesting things when decreasing the value of the load resistor on the tone stack output. I am quite surprised how much of the low frequency is lost by just decreasing the value a little bit.

I did try my setup again today and I also removed the 100K resistor that I placed accross the the tone control output. Everything seemed to work fine but unfortunetly I couldn't get the amplifier to oscillate like it did before I added the resistor. It seems that there could be an intermittent fault somwhere either in my wiring or a bad tube socket or perhaps tube pins that are not clean.

I will try again tomorrow to see if I can get the amp to missbehave and find a proper fix to the fault.

Phil

Author:  philip davies [ 23 Jul 2015, 07:22 ]
Post subject:  Re: Baxandall circuit

Hi,

I have made some more testing today and the problem is back. Also the problem is more persistent so I have managed to use my multimeter to check voltages. Looking at the baxandall circuit that I am using I meassured for any dc offset voltage after the 0.47uf cap that's connected to the cathode of the cathode follower section. There appears to be some voltage present here so in other words a small dc voltage is going through the bass and treble pots. Also there is voltage after the 0.47uf cap on the output of the circuit. I am guessing there is voltage on the output due to the feedback going back to the bass and treble section. So if there is voltage going into the bass and treble section it will go straight to the output.

I am using brand new Mundorf caps so I'm hoping they haven't started to leak dc voltage.

Author:  dimitris1811 [ 05 Aug 2015, 00:38 ]
Post subject:  Re: Baxandall circuit

I finally made my pre-amp based on the attached drawing but I am not happy with tone control performance,it is not so good for my ears, so at the end I stay with the by-pass function with the optional pad. I used various types of potentiometers as advised with no significant improvements. Hence, I decided to stay with pure sound. The problem is that I have too much gain so, when I connect the pre-amp to my 6SN7-300B SE, using a CD player, I have to turn the potentiometer of the pre-amp almost at zero to 1, since I want to keep the amp's volume control high. Generally, I red that if the potentiometer is at low position then the ration signal/noise is decreased and noise effects increase. Any idea to reduce gain or to look somewhere in my circuit?

Is it better to have two stage pre-amp instead of a simple Universal 4S ? I tried the 4S and got good results, but I though that with two stage (a 12AU7 as follower) would get better results. Any opinion on that?

Thanks Dimitris

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