Touch sensitive circuits...

[KOULIS]

Hey all,is there a secret behind touch sensitive amp circuits?


Cheers

[klingo]

heavy gauge strings!

[Funkalicousgroove]

The secret is having the addl' gain stages after the tone stack, and a voltage divider after the recovery stage to put it very simply. Operating voltages and component selection also play a key role.

[David Root]

Funkaliciousgroove, (wish I'd thought of that handle myself!) if you put a cathode follower buffer between the first stage and the tonestack, and you have the voltage divider (or pot) after the recovery stage, how does that affect tone/touch sensitivity, in your experience/opinion? I know what I think, because I've done it in a Fender circuit, but I'm interested in your take on it.

[dehughes]

Oooh, me too...I'm interested in your opinions on that very subject.

[Funkalicousgroove]

It kind of depends on how you define "touch sensitivity" I define it as an amp that can go from clean chime to scream based on guitar volume and pick attack.

That being said, I think that the reason the gain stage/tone stack/recovery amp/voltage divider/ gain stage setup works for touch sensitivity is simply by virtue that the tone stack throws away MUCH more of your AC signal voltage than the 5F6-A/Plexi circuit with the cathode follower driving the tone stack. There is a greater voltage swing from voltage in to voltage out based on how hard you pick. I find that in my ODS amps they are FAR less touch sensitive with the PAB engaged, it's like full blown 3 mile island, whereas with the OD engaged, and the PAB off the amp will go from jangle to scream simply depending on how hard I hit the strings.

There are a million other factors such as cap type, DC filtering, ground scheme type/gauge/length of wire that can make an amp's slew rate faster or slower, but IMHO amps with cathode followers driving the tone stack are less responsive to touch, but have more "Evenness" (Is that even a word?) from note to note..........Of course if you don't have negative feedback it's a whole other ballgame!!

[Funkalicousgroove]

David and dehughes-

are you talking about a circuit that goes Input stage, Cathode follower, tonestack/volume, recovery amp??

I have no idea what that would do, although I suspect it would have a touch more headroom with the low impedance CF driving the tonestack......I really don't know, I'd have to try it!!

[David Root]

Yes, in a word!

Buffering the input to the tonestack after the first gain stage does give a cleaner and, in most cases, higher gain input to the tonestack (LC tonestack excepted, in my experience).

I'm interested in its effects as a result of changed interstage impedances i.e. overall gain architecture. This is something that is not obvious from the standard schematic, you have to run the numbers on Zo and Zin between stages too. It does seem to "smooth things out" as Funkalicious says, perhaps because the interstage dynamic range is reduced?

I think the power supply design and in particular the filter caps can make a huge difference in touch sensitivity too. I'm talking about energy transfer/resupply rates, not gross capacitance. Fast filter caps make an audible difference, even through a tube rectifier (5AR4 anyway). This is also true with tone/bypass caps, to a lesser extent. What think ye?

[Funkalicousgroove]

I definitely think Polyprop caps feel "faster" than say mylar caps, but I also find them less musical.

Ever tried Solens in your power supply? Talk about FAST!!

[David Root]

I have used Solens in a power supply, once. They are fast allright, but they're so damn big! I mounted them on top of the chassis like a can cap, using CA glue to hold them down, otherwise the chassis area you need is huge. Nice thing about them is of course they're nonpolar. But they do sound good as filter caps allright, and tone/bypass caps too in the smaller sizes. The tinfoil ones are even better.

Now I am using Black Gate electrolytics, the 47 + 47/500V WKZ on the power tubes and the little 22/350V VK for everything else. They are conventional size or smaller. The audible improvement is not subtle. The usual reaction, especially from pro players, is an immediate "Wow!" These things are so far ahead of even a good conventional cap like the Sprague Atom, it's amazing. BG claims they will last 5-10 times longer than wet electrolytics, because BGs are dry.

The 500V WKZs are awful expensive, but two 47 uF 630V Solens are about $35 these days and just take up too much space for me.

Now Rubycon has stopped making the BG WKZ and VK types. They still have the large nonpolar types, but like the Solens, they are physically huge! So I'm not sure how long I'll be able to get them. My source is the BG master distributor for the US and he has a good inventory I'm sure, but his main marketplace is the audiophile market, which is having conniptions about the loss of these caps.

Now I hear rumors that Vishay will soon discontinue Sprague Atoms, as being "obsolete"! That's not the point, of course, for us MI amp types, most of our technology has been technically obsolete for forty years!

[rhinson]

Yes, in a word!

Buffering the input to the tonestack after the first gain stage does give a cleaner and, in most cases, higher gain input to the tonestack (LC tonestack excepted, in my experience).

I'm interested in its effects as a result of changed interstage impedances i.e. overall gain architecture. This is something that is not obvious from the standard schematic, you have to run the numbers on Zo and Zin between stages too. It does seem to "smooth things out" as Funkalicious says, perhaps because the interstage dynamic range is reduced?

I think the power supply design and in particular the filter caps can make a huge difference in touch sensitivity too. I'm talking about energy transfer/resupply rates, not gross capacitance. Fast filter caps make an audible difference, even through a tube rectifier (5AR4 anyway). This is also true with tone/bypass caps, to a lesser extent. What think ye?

take a look at the input stage on the bass ch of the 6g6 series of blonde bassman heads. rh

[David Root]

That's it. Nothing new under the sun, right? Interestingly, when they designed the BF Bassman (AA864) and put the tonestack in one place, it's gone!

In my amp, which is really a modified brown Vibrolux with reverb replacing the normal channel, I just took the 6G11 bright channel, and inserted the cathode follower, then added a third gain stage after the TS recovery, which is not shared with the reverb recovery stage. I also put a Drive pot ahead of the third stage, and a Pre PI MV after it, so you can play around a bit with the gain architecture.