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Technik => Tech-Talk TT-Projekte => Thema gestartet von: Leadman am 8.09.2020 22:52

Titel: TT LX18 without master volume
Beitrag von: Leadman am 8.09.2020 22:52
Hello,
What components would be deleted for build the TT LX18 without master volume? Maybe C6, c7, P4 and r28?

Thank you in advanced.
Titel: Re: TT LX18 without master volume
Beitrag von: tuxele am 10.09.2020 15:39
Well, just delete C7 and P4 and connect the free pin of C8 with C6. R28 is needed.
Titel: Re: TT LX18 without master volume
Beitrag von: Striker52 am 10.09.2020 23:21
Hi, I think it would be better to keep all components at their place and only replace the volume pot with a voltage divider (like R10 and R11) to avoid a too strong signal at the input of the phase invertier.
Titel: Re: TT LX18 without master volume
Beitrag von: Leadman am 11.09.2020 12:27
Thanks for the replies,

Why r28 is needed? What is the funtion of r28 resistor without another resistor for a voltage divider? I thought R28 is in parallel with the master volumen pot for softening the volumen control curve.

Best Regards!
Titel: Re: TT LX18 without master volume
Beitrag von: Striker52 am 11.09.2020 13:03
R28 together with C6 build a frequency filter (high pass). R28 is not in parallel with P4; the both are seperated by C7. As I wrote before you can use a voltage divider (2 Rs in series) to replace P4. Use P4 first to determine the signal level you want to have at the input of th PI, then measure the resistances between input/taper and taper/ground of P4, and replace P4 with the according Rs.
Good luck!
Titel: Re: TT LX18 without master volume
Beitrag von: Leadman am 11.09.2020 14:12
I got it now!
Thank you so much for the help


Best Regards!
Titel: Re: TT LX18 without master volume
Beitrag von: tuxele am 11.09.2020 16:50
R28 together with C6 build a frequency filter (high pass). R28 is not in parallel with P4; the both are seperated by C7.

But surely, of course, R28 is in parallel with P4 in AC terms! I guess that C7 is even needless due to its big value.
Heaven knows why Dirk implemented this additional capacitor ...
Titel: Re: TT LX18 without master volume
Beitrag von: Dirk am 11.09.2020 17:35
I guess that C7 is even needless due to its big value.
Heaven knows why Dirk implemented this additional capacitor ...

Give it a try and you will see (hear) why C7 is needed  ;)

Best regads, Dirk
Titel: Re: TT LX18 without master volume
Beitrag von: tuxele am 11.09.2020 18:33
Give it a try and you will see (hear) why C7 is needed  ;)

Best regads, Dirk

Unfortunately, I haven't built this amp, so there is no chance for me to try it out.
I'd like to learn more about designing guitar amps. So what does this mystery cap do? Please give us a hint.

Titel: Re: TT LX18 without master volume
Beitrag von: Dirk am 11.09.2020 20:06
oh, my mistake, I did not read the full thread. You want to remove the Master. In this case you do not need the C7, C8 and R28 but you must replace P4 with a voltage divider as Striker52 described earlier. I highly recommend to simulate the circuit with SPICE so you can test and simulate different modifications to get an idea if they will work or not and when you have a good working circuit then you can build the amp.
Titel: Re: TT LX18 without master volume
Beitrag von: tuxele am 12.09.2020 12:05
In my opinion, the coupling cap C8 should be left in place when a voltage divider instead of pot P4 is going to be used in order to block the DC component (grid voltage of about 45 Volts) of the phase inverter. i agree with Striker52 that C6 and R28 are used to do a highpass job and should be also left in.  As far as the "mystery cap" C7 is concerned - I cannot explain the use of it ...
Titel: Re: TT LX18 without master volume
Beitrag von: Striker52 am 13.09.2020 11:42
I fully agree that c28 has to be kept in place. The PI Grid gets its ground reference from the bottom of R16.
Titel: Re: TT LX18 without master volume
Beitrag von: Leadman am 19.09.2021 00:39
Iīm still thinking about C7 cap funtion.
What is the purpose of the c6 r28 c7 node before the master volume?
What will happend if I change de c7 cap value. Could I put a smaller cap like 100nF?

Thank you in advance for the help.
Titel: Re: TT LX18 without master volume
Beitrag von: Dirk am 19.09.2021 19:14
Why do you want to remove the master if you do not know how it sounds or how it works ? This makes no sense for me.

My feeling is that some people think that they must make some changes just to make some changes without any idea of what they are doing or what the result will sound like. I have see this some many times now...  and when they run into problems, the frustration is going up because they do not know how to solve the problem and at the end they have an amp with some cool mods but also without any function.

The LX18 (or any other of our kits) are well tested and known to work fine if you build them correctly. I highly recommend that you build it first and make sure that the amp works staple and after this you can think about some changes.

You don't have to remove the master. Just turn dial it up and keep it there so you do not have to change anything and it works like a none-master.

Best regards, Dirk
Titel: Re: TT LX18 without master volume
Beitrag von: Leadman am 19.09.2021 20:45
Hello, the amp has been built like the real one, it goes fine and sound superb.
But I want to know more a more about electronics, and I donīt find nothing alike that Cap, I only wanted to know how the master (and that cap) works. Itīs only about curiosity ....

Best Regards.

Titel: Re: TT LX18 without master volume
Beitrag von: Dirk am 19.09.2021 23:37
The master is a simple adjustable voltage divider. R28 is a kind of attenuate which helps to reduce the signal level a little bit (ca. 15 - 20%) and C7 is needed to decouple the potentiometer from the R28 otherwise the R28 will be parallel to the potentiometer and then the master will not work as it should. Changing the value of C7 will affect the bass/treble. We found 680 pF okay even with maximum overdrive.

Best regards, Dirk
Titel: Re: TT LX18 without master volume
Beitrag von: Leadman am 25.09.2021 19:33
Thank you for your help!

Best Regards!