Vox Top Boost preamp... Mud problem

[JimSoprano]

Hi guys!

Fairly new here. I'm working on a cool project.
I wanted a Top Boost channel to use as rhythm channel and a more overdriven channel for solos. Please see below pictures for schematic and photo of my test preamp.

The preamp and switching works fine, no hum or noise, but I can't get a good brightness out of the top boost part. The overdrive channel works better and is easier to adjust.

Does anyone has an idea to brighten it up in a good way? Could it be possible due to the long leads?

I've tried smaller bypass caps, but won't do much. The only thing worked best was to bypass the 220k resistor after the tone stack with 4,7n and 22n caps. But that sounded like the treble worked in oposite way.

Thnaks!

[xtian]

Remember that any length of hookup wire adds capacitance. It may be that you've got so much wire running around that you've dulled the top end.

You can increase the value of the 120p bright caps.

[Stevem]

Do you have a scope to detect if you have a oscillating problem that would be making for phase canceling?

[JimSoprano]

Remember that any length of hookup wire adds capacitance. It may be that you've got so much wire running around that you've dulled the top end.

You can increase the value of the 120p bright caps.

I’ve tried this, but the effect is to little. This circuit should be ver bright on it’s own, right?

[JimSoprano]

Do you have a scope to detect if you have a oscillating problem that would be making for phase canceling?

I don’t have a scope, but do you suggest phase canceling by wires overlap? I can poke arround with the wires. Funny thing is, it’s dead silence with all this wire…

[pdf64]

So the ‘out’ is feeding a remote power amp?
If so, how long is the cable between them?

[JimSoprano]

So the ‘out’ is feeding a remote power amp?
If so, how long is the cable between them?

Correct, I run it throug a Marshall 9005 or 18 watt EL84 poweamp. Cable is 3 mtr.

I've ran it side by side through my vox AC30CC2X as well, the amp's preamp had way more brightness, presence and punch.

Could it be the filtering? Should I add a 47uf after the Voltage pump?

[pdf64]

The source impedance from your ‘out’ is extremely high, >220k. When combined with cable capacitance, a low pass filter will be formed.
With a 3m cable, it only takes 120pF/metre for a 2kHz LPF.
The effect of that may be what you are hearing.

From Aiken q&a http://aikenamps.com/index.php/technical-q-a

Q: A lot of amps have effects loops. Which ones are buffered and what does that mean?
A: "Buffering" means the fx loop circuit has an amplifier with a low-impedance output to drive cables and effects inputs without signal loss. There is also usually an accompanying buffer on the return side to receive the signal from the effects unit. Essentially, the "buffer" isolates the effect from the amplifier circuitry.

A typical rack-mount line-level type effect may have an input impedance of around 10K ohms, so if you drive it with the typically high-impedances found in guitar amplifier stages, you'll lose signal level, and may get unwanted clipping/distortion.

In addition, a high impedance is far more susceptible to high-frequency rolloff due to cable capacitance, so a low-impedance drive is best to preserve frequency response.

How about if you get rid of the 220k resistors on the channel outputs?

[JimSoprano]

The long guitars cables were a big part of the problem. Thanks! Now I'm using shorter cables and ended up with 0,68uf cap on the first cathode. Little less bottom. 22/25uf will be on a switch in final version for sure.

I now have one 220k into two 1M master volumes. I will try removing the 220k as well. But I thought it's often used to make the MV work better. I'm also thinking of a output buffer with a LND150 or IRF820 I've laying arround. Any ideas?

It's now actually a very cool two channel preamp now. Need to test it with my ac30 poweramp next week.

[bepone]

I now have one 220k into two 1M master volumes. I will try removing the 220k as well. But I thought it's often used to make the MV work better. I'm also thinking of a output buffer with a LND150 or IRF820 I've laying arround. Any ideas?

for sure with the output buffer you will remove cable problem, try it you have place, it is easy to do

[TUBEDUDE]

I'd still tame that birds nest in the chassis. Shorter wires lower capacitance, crosstalk and potential oscillation.

[JimSoprano]

I'd still tame that birds nest in the chassis. Shorter wires lower capacitance, crosstalk and potential oscillation.

Yes for sure, but this is only a test setup. If i’m happy with the sound I want to build it in a pedal. I didn’t know the long lead, etc. Would cut so much treble and punch.

The idea for this preamp is that’s almost imposible to get a nice lead biost in gain and volume with some gain on the top boost channel…

[TUBEDUDE]

You need another tube so you can have a cathode follower output.

[ChopSauce]

That or a MOSFET follower, which has the side bonus to drain the caps.

[JimSoprano]

That or a MOSFET follower, which has the side bonus to drain the caps.

This is what I looking for, do you have a solid design for this application? I have a irf820 or lnd150 which I can use.