Boost switch question, cap/resistor order (Vox UL/Hybrid 7-series preamp)

[seveneves]

Hi there,

I've been bored as of late so I decided to do some analysis on the (in?)famous Vox 7-series (you know, same as the Beatles used during the Revolver and Pepper sessions).

I've been studying actual amps with the schematic and notice something subtle with the boost switch wiring on the "normal" channel.

What I am finding is that there is a slight difference between what the schematic shows and the way (at least some of) the amps are wired (most if not all of the ones I am seeing, anyway).



For reference:



It seems that the way the amps are wired work as per the schematic (i.e., when the switch is in the "on" (down) position, R48 is shorted), we have essentially (as I understand it anyway) an HPF (i.e., LF is attenuated = "boosting" HF). So I get that (I think).

However, I am not sure what happens between the two when the switch is in the off position. I am wondering what the "off" position sounds like in each scenario?

Per the schematic, C28 comes after R48 in the off position. Is it rolling off less LF due to R48 before it?

Per an actual amp (if my analysis/studies are accurate), R48 comes after C28 in the off position. What is the purpose of R48 in this case? Is it doing anything? Reducing the signal? Does the boost even happen in this case because the cap affects the signal first? If so, it wouldn't be the first time an amp was wired differently from the schematic (e.g., 6G6-B Bassman, IIRC).

And no, I don't have an actual amp to hear the difference/experiment with the cap/resistor order...

[martin manning]

C28 is basically a bright cap on the volume control. It's a HP shelving filter. R48 acts to limit the treble bypass when it is not shorted, and the order of C28 and R48 does not matter.

[seveneves]

Thank you Martin for the education. Appreciate it!

[seveneves]

OK, I guess I am confused as I read it again:

R48 acts to limit the treble bypass when it is not shorted

In the scenario of the way an actual Vox 7(x)xx amp is wired up, with the switch off, it seems the limiting of R48 happens after the HP shelving of C28. IOW, in this case it seems that C28 has high-passed the signal already (?) and that R48 would have (little-to-)no function here?

But I guess that's why you followed up with:

and the order of C28 and R48 does not matter.

which I am still trying to understand...

[Reeltarded]

In series, electrical things are electrically identical, no matter what the order of identical things might be. There may be reasons you want something in a certain order, but the physics don't mind a bit. This is pretty much unrelated to your actual question abiut this circuit.

The second example you posted is the reason that is ubiquitous. They dumped the idea because the sound should not change when boosted. In this case, "boosted" is the way to hear it, but it isn't additive, It removes the resistor. It is like setting a clean drive before your amp but turning it down and then turning it off for solos. Switched or unswitched the bright remains.

The 2200n cap was placed in the circuit at first the same way me and Dave imagined the Liverpool he built for me would benefit from by talking about it and basing changes on conversations instead of what the amp sounds like.

ZAP! Better undone

[Reeltarded]

I changed that reply 5 times trying to be more parsimonious in my vernacular. I hope it worked.

[seveneves]

I changed that reply 5 times trying to be more parsimonious in my vernacular. I hope it worked.

Thank you. I appreciate it.

[Reeltarded]

Whoa, there. What's with the uppity attitude?

If this was the old railroad.. I swear. You kids..

[seveneves]

UPDATE: Since I posted last, I did a little proof-of-concept of sorts, built the preamp based on the schematic.

Everything seemingly works fine with the exception of the boost switch. I can't tell when it's engaged. Absolutely no difference.

Tried wiring it up both ways, per the schematic and per actual amps. No difference.

Any ideas?

[R.G.]

Perhaps some other problem in the build?

I have a Defiant amp, which has pretty much the same preamp as the UL series, and the boost switch does change tone, but not much.

[seveneves]

Perhaps some other problem in the build?

This is what I was wondering. I will triple check.

I'm wondering if I need to have the whole amp built up (and not just my preamp build going through another power amp) to realize the effect? Really don't think so but just throwing it out there...

I have a Defiant amp, which has pretty much the same preamp as the UL series, and the boost switch does change tone, but not much.

OK, good to know. Wasn't expecting a huge change, but, something.

I understand the effect at higher volumes will essentially be nil.

I know I'm a little older, but don't think I'm ready for hearing aids yet!

I thought with a bright cap of 2200pF, the effect would be pretty OBVIOUS...

[seveneves]

So, an update to the update.

I tacked a 0.022 across C28 and noticed a subtle, but discernable change in brightness.

I want to presume that I have it wired it up correctly but will double check anyway.

I was using a power amp/speaker setup possibly more voiced for E. bass so that could be the reason why it wasn't perceptible with the stock 0.0022 value?

[R.G.]

I was using a power amp/speaker setup possibly more voiced for E. bass so that could be the reason why it wasn't perceptible with the stock 0.0022 value?

Ah.

That could be it, all right. If the speakers have big treble losses (and bass speakers often do) then it's going to be hard to hear any treble boost.

Here's an ugly thought - try rigging it up through one channel of a stereo setup. These things are designed to be as flat as it economical to get, much flatter than guitar amps. If there's treble change, you'll hear it there. If you do this, start and stay at quite low volume settings, as guitar preamps can have large output levels, much more than the sub-1V levels of hifi stuff.