Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: raza on January 18, 2021, 04:17:50 am

Title: POP between two presence circuits.
Post by: raza on January 18, 2021, 04:17:50 am

Hello
I'm just finishing designing a three channel amp.
I want to apply a presence circuit to the first channel and another circuit to the other two higher gain channels.

It turns out that I have tried RELAY and have a POP noise when I switch between the two presence circuits.

Any suggestion?
I leave handmade outline.


One possibility would be to use LDRs, would it have pops?
(https://i.ibb.co/crcQc4C/1610964847271.jpg) (https://ibb.co/pLvdvYR)
<a target='_blank' href='https://es.imgbb.com/'>free png images[/url]


THANKS

Title: Re: POP between two presence circuits.
Post by: sluckey on January 18, 2021, 04:43:10 am

Sure it's gonna pop. All the cathode current for the LTP PI is flowing through your presence circuits. I would try putting a 100K resistor from point A to ground. Might help. No guarantees.

Title: Re: POP between two presence circuits.
Post by: d95err on January 18, 2021, 06:54:15 am

I would do something like this (see attached image).

Keep the 4k7 in the circuit all the time and switch in a parallell resistor (approx 420R) to get the lower resistance.

Also, I like to keep switching on the ground side of things when possible.

Title: Re: POP between two presence circuits.
Post by: raza on January 18, 2021, 07:38:19 am

I tried R100k to ground fixed and POP!!!

this afternoon Ill try this solution.

What is better for do this swith opto or rele?

Title: Re: POP between two presence circuits.
Post by: SnickSound on January 18, 2021, 08:56:31 am

Quote from: raza on January 18, 2021, 07:38:19 am

I tried R100k to ground fixed and POP!!!

this afternoon Ill try this solution.

What is better for do this swith opto or rele?

The problem is you're switching the resistor to ground, there is DC on that and so changing to a different value changes the operation of the whole phase inverter.

You need to keep the 4k7 in at all time.

I'm also curious about the 390R value on one of the circuits. Sounds like it would be pretty much like having no NFB at all if you're using the typical 27k to 100k resistor in the NFB path. If on the other hand you're using a less than 10k resistor then the 4k7 would be way too much.

Title: Re: POP between two presence circuits.
Post by: raza on January 19, 2021, 02:15:02 am

Hi

Nice!!! no POPS!
PERFECT!!!

What is better to do the switch? Rele or OPTOS? All amp runs with optos, but I dont know if in this part of circuit is better Rele or opto.
(https://i.ibb.co/0mZd8HV/Whats-App-Image-2021-01-18-at-21-07-39.jpg) (https://ibb.co/sbQMk8J)

THANKS!

Title: Re: POP between two presence circuits.
Post by: MrsColumbo on January 19, 2021, 03:27:39 am

How do ya plan to switch it? Remotely aka floorboard? Or just a toggle on the front/back? I'm talking about "memory" - do you want the amp to "remember" the last switch positions after powering it off?

Title: Re: POP between two presence circuits.
Post by: raza on January 19, 2021, 03:35:09 am

This isn't a problem. I designed a board with a arduino to program a rele or opto.

Each channel has optos assigned for switching, so I would only have to update the arduino software so that the clean channel has one presence and the PLEX / LEAD another.

So my doubt whether to use an opto or relay.
(https://i.ibb.co/19kNXdG/Whats-App-Image-2021-01-19-at-10-30-15.jpg) (https://ibb.co/wKfqQwh)

Title: Re: POP between two presence circuits.
Post by: raza on January 22, 2021, 06:24:54 am

Well I tried this:
(https://i.ibb.co/MNKSvVz/Whats-App-Image-2021-01-22-at-13-21-35.jpg) (https://ibb.co/bdYm0B4)


POP! I don't know why with a manual switch I don't have pops and with OPTO apears the pops.


what do you think?

Title: Re: POP between two presence circuits.
Post by: pdf64 on January 22, 2021, 09:08:47 am

If there's a Vdc step change in a stage that's in the signal path, then a pop or thump is pretty much a given. I've no idea why it isn't happening with the manual switch  :dontknow:

Title: Re: POP between two presence circuits.
Post by: shooter on January 22, 2021, 09:42:32 am

possibly a make before break switch  :dontknow:

Title: Re: POP between two presence circuits.
Post by: sluckey on January 22, 2021, 09:59:47 am

The pop probably comes from the control side of the optocoupler.

Title: Re: POP between two presence circuits.
Post by: PRR on January 22, 2021, 01:25:53 pm

The top of the 4.7k must bop from 5V to 0.5V and back when the 470 is switched out and in. While the bottom of a perfect long-tail has some CMRR, the Fender is not optimum for CMRR.

You need to block-off and leak-off the DC.