Preamp Voicing

[mooreamps]

I drew this up for a friend of mine ; who asked the question. I thought perhaps others would be interested. I would not use this circuit myself since I do not use cathode resistors in the preamp... This would be a more of a noiseless approach of switching in the extra bypass voicing cap... I don't know if this has been captured before, anyone is welcome to use it...


-g

[guitarmike2107]

unless I am missing something special, I think you will find circuits like that are common as muck

[Gaz]

Well, let's not be rude when Mr. Moore actually wants to share!

[MWJB]

"You don't need C3. The 100K accross the SPST will do it by itself. R5 could even be a smaller value (27K?) but people generally have plenty of 100K lying around. I have seen as much as 1Meg used, you get a little thud on the first throw, subsequent throws are noiseless."


Oops, my bad, sorry Gary I now see you you are switching between the 2 caps, rather than switching a cap in/out. Apologies.

[guitarmike2107]

I wasn’t been rude.... well not deliberately anyway

Gary, Why are you using such small caps, at a glance the boost is above 10kHz.

[mooreamps]

It was a schematic I saw drawn on a white board over at King Amps. Someone wanted a switch to do some preamp voicing. But it was drawn ; on the white board ; with the switch just under the cathode, meaning it would create a loud pop when toggling between two different bypass caps.. I was only thinking this would be a more noiseless approach... The series resistor would keep the larger cap charged to the average level of the bias voltage ; but large enough to keep the effect of the larger cap out of circuit.. .. ... The values of the two bypass caps can be adjusted as desired... Like I said, I don't personally use this type of design, so the values of the caps would be calculated depending upon the desired effect....


-g

[Enzo]

This is a very old technique. Look up any thread on this forum about popping relays or channel switching or indeed even bypass cap switching, and you will find mention of the resistor across the switch to keep the caps charged. As long as there is a charging path, it doesn;t matter which side of the cap has the switch.

[Amp Kat]

Have also seen a large resistor (2M) from the switch to ground to shunt some of the pop to ground.

[mooreamps]

This is a very old technique. Look up any thread on this forum about popping relays or channel switching or indeed even bypass cap switching, and you will find mention of the resistor across the switch to keep the caps charged. As long as there is a charging path, it doesn;t matter which side of the cap has the switch.

That's fine. I probably figured as much after I posted it. . But at the time I was looking at the circuit ; as drawn ; I was just thinking there could probably be a better way to do, than what I was seeing on his white board.

-g

[redelephant]

Nothing wrong in trying to give good advice So many amps have popping noises when flipping switches, so someone might learn something new. But as mentioned by others I think this method is quite common. You never know though, so thanks anyways. (I've goofed many times myself, being sure I've figured out something new, only to find out the whole world knows)

[mooreamps]

Thank you...

-g

[TheTinMan]

As a minor contribution to our cesspool of knowledge, there are three different cathode switching arrangements in the two attached schematic fragments.

V1a in both examples just has a switch between bypass caps (although you really don't need the "pop" resistor under the smaller of the 2 caps since an effective 27uf is close enough to 25uf for our purposes).

In #1, V1b either has a partially bypassed cathode using a large cap or a fully bypassed cathode using a smaller cap. This worked out well in a build where not-Bright was actually for harmonica and I wanted less feedback and therefore less gain.

In #2, V1b's cathode is bypassed either way but the value of the cathode resistor changes in addition to the bypass cathode. 3K||25uf or 1.5K||6.8uf

And no, I don't think I invented any of these.

Cheers,

Chip

[mooreamps]

Looks all good to me.... well for preamps using self bias resistors.....

Thank you Chip for the contribution...

-g

[jrfrond]

If you eliminate the switch and change R5 to a pot connected as a variable resistor, and use a large C3 and small C2 (I use 22uF and .68uF respectively), you can change the voicing gradually using the pot. Now follow me here: if your voicing pot was a tandem control with the 1MA volume pot (after C1), and wired reverse (so that the low-frequency response trailed off as the volume pot was increased), the amp will be tighter at higher volumes, as low-frequencies are automatically limited through the stage as the volume control is turned up.

This is a design I came up with a long time ago and even built for awhile. Have never seen it anywhere else. The only drawback was that it needs a two different pot values (I used 1MA and 10KL) to accomplish this "trick" correctly.

[guitarmike2107]

I have thought about doing this on a current build, but no longer have the need. I was going to use a dual 1M pot and just use a resistor to get the capacitor "cut off" resistance where I wanted it.