Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on November 04, 2021, 06:04:30 am

Title: Crosstalk when using LND150 FET
Post by: Leevi on November 04, 2021, 06:04:30 am

I have been wondering the crosstalk that happens when using LND150 Fets and tubes in the same circuit.
That is an isssue for instance in two channel amps or in buffered effect loops where LND 150s are used.
The crosstalk happens even if the channels are separated  from each other with a switch.Is the reason the same voltage source or some ground current?
/Leevi

Title: Re: Crosstalk when using LND150 FET
Post by: pdf64 on November 04, 2021, 09:20:29 am

You need to be a lot more specific, provide detail.

Title: Re: Crosstalk when using LND150 FET
Post by: Willabe on November 04, 2021, 09:24:10 am

Quote from: pdf64 on November 04, 2021, 09:20:29 am

You need to be a lot more specific, provide detail.

A schematic would help a lot.

Title: Re: Crosstalk when using LND150 FET
Post by: Leevi on November 04, 2021, 12:11:16 pm

This is the current case.  If I select "Power amp In" and plug guitar to normal input the guitar signal comes out from speaker very weakly.
Also there is a switch before the master volume. It selects the signal either from the power amp in or preamp.Reverb has been added and implemented with LND150s (please see the pdf).Hopefully the pictures make analyzing easier.
/Leevi

Title: Re: Crosstalk when using LND150 FET
Post by: acheld on November 04, 2021, 03:39:49 pm

I'm confused.   

The way the circuit is drawn, there is a "normal" input which allows signal to pass through your reverb circuit via the voltage divider.  Just before the Master volume pot, there is a switch that allows you to select which signal that is then passed to the LTP and the output tubes. 

The switch allows either the "normal" channel to pass, or ?  -- what appears to be a jack with a single gain stage, and no tone stack.   Is that correct? 

Title: Re: Crosstalk when using LND150 FET
Post by: Leevi on November 05, 2021, 12:01:55 am

Quote

The switch allows either the "normal" channel to pass, or ?  -- what appears to be a jack with a single gain stage, and no tone stack.   Is that correct?

Yes that is correct.


And this is not the only case with LND150 where I have had the same issue. A part of the signal passes through via the LND150s even if the signal paths are totally isolated from each others.


/Leevi

Title: Re: Crosstalk when using LND150 FET
Post by: acheld on November 05, 2021, 10:14:01 am

Well, I'm stumped.

Assuming it is wired as in your schematic, and there is no connection except through the switch, I can't see how the guitar signal would arrive in your power section. 

That said, I have little experience with transistors.   There are plenty of smart dudes here who do, however.

Title: Re: Crosstalk when using LND150 FET
Post by: shooter on November 05, 2021, 10:33:52 am

Got a scope?


WAG; you are feeding through the PS.  Try and use the same B+ tap for both FETS

Title: Re: Crosstalk when using LND150 FET
Post by: Leevi on November 05, 2021, 11:50:55 am

I have scope, have to investigate further.
Wiring is correct, checked with multimeter.
I think it leaks through the PS.
/Leevi