Tillman: the squeal doctor ??!

[demym]

Hi,

i'm a newbie, and i've built five or six circuits till now, and especially: the Tillman preamp, a Thor, a BSIAB2 and a Carvin Legacy JFET emulation.

All builds, apart from Thor and Tillman, have squeal problems: big squeal with the BSIAB2 at almost any gain setting  and a little squeal with the Legacy when maxing Treble or Gain. The Thor has no squeal problems.

But, a strange thing happens: whenever i put the Tillman (which i have in a plastic box) in front of the 'squealers' (BSIAB2 and Legacy), the squeals disappear, and i can hear those pedals in a human way... Tillman also adds some spank to the following circuits, and that is good... but, can someone explain me why this happens ? The sound of the to pedals, apart from squealing at some settings, is the right one though.

I love the Tillman addon to my sound, but i would preferr to have pedals that work correctly in every aspect by themselves. Thx anyway to the Tillman for making me hear how the pedals i build would sound without the squeals.. :-)

In a recent post of mine, it was told me that the Tillman inverts the phase of the signal; but as i'm at the beginning of this wonderful learning experience, what does this matter on the signal flow ?

The Legacy has 4 gain stages, and the BSIAB2 (as you surely know) has two mu-amps followed by a single JFET stage (i write this as i've read that having even or odd gain stages makes a difference..)..

Can someone explain me why Tillman is doing the doctor ? :-) The guitars i use when testing pedals are Ibanez RG with DiMarzio Evolution bridge and neck pickups.

Thx in advance, have a nice time

[R.G.]

The Tillman preamp lowers the output impedance of the guitar. Many circuits are sensitive to the source impedance of that which they amplify, and will squeal if it's too high. You have apparently run into that case.

[demym]

Thanks R.G. !

Isn't there a way to lower the output impedance of my guitar without having to to use a Tillman, doing it right on the circuits ? And will these eventual circuit addons conflict whith the use of the actual tillman pedal in front ?

Thanks very much

Demy

[R.G.]

There isn't much in the way of simple, cheap, practical ways to lower your guitar's output impedance. Otherwise, it would be standard in all guitars by now.

You can use a transformer: expensive if good, not good if cheap, and lowers signal level as it lowers impedance. Passive means (i.e., no battery) all lower signal level and probably increase noise. Active means are just about the same as using Don's preamp.

Sorry.

[tempus]

The Tillman is really a buffer, although it has a small amount of gain. The 'spank' you're hearing is probably a combination of these 2 things; the extra gain is self-explanatory, but you'd probably get a little extra brightness from the lower impedance of the Tillman as well.

From that, I would think that you should be able to get rid of the squeal problems by adding a simple buffer to the pedals that are giving you trouble.

[demym]

Thanks, this is a very good news !

And, can you indicate me a schematic of the right buffer for this purpose ? (i've seen that there are many, inverting and non-inverting, just want to be sure to build the right buffer for this case).

Many many thanks in advance !

[Ben N]

Required reading: http://www.muzique.com/lab/buffers.htm
If all you need is to lower the impedance feeding your effects, the BJT input buffer from a Tubescreamer will do you fine. If you also want the "brightening" effect of a higher input impedance facing your guitar, the jfet (or mosfet) may be the ticket.

Look at this one, too: http://www.muzique.com/lab/tsbuff.htm

[demym]

Thanks Ben. I'm reading the Basic Buffers article.. I will breadboard some of the buffers (BJT and JFET) and then decide which to use.

Just one more, that is not clear to me: why doesn't high gain circuits (as BSIAB2 or others) have incorporated these kind of buffers ? I mean, it's only a problem with my pickups ?

Excuse my ignorance :-)

Thx anyway !

[Ben N]

Since I'm here, I'll try to answer to the best of my ability, although if RG comes back, I'm sure he can explain it a lot better. I don't know much about your pickups, or to tell the truth pickups generally, but I am guessing that your are "hot" pickups with a lot of windings, which is why they have a high impedance. Those circuits may work fine with lower impedance pickups; plus, many users will have buffers or boosters ahead of their high-gain pedals as a matter of course, whether by way of a dedicated input buffer for their pedalboard or a commercial buffered pedal, such a tuner, overdrive or whatever, ahead of it in the signal chain. Just for the heck of it, try your BSIAB behind a Boss, DOD or Ibanez (or similar) pedal with good quality buffered bypass, turned off, and see if they don't work as well as the Tillman to cure your squealies. Finally, such buffers are pretty much plain vanilla circuit elements that any DIY-er can choose to put in any pedal as needed; as such, they are not part of the basic design, but of the "wrapper" (to borrow RG's term).

[demym]

Ben, i think you've been very very clear. That makes sense to me. I have very high output pickups (DiMarzio Evolution).

Will try some of the buffers tonight, but i also already like the extra punch that Tillman is giving me. I thought it was not permitted by 'law' to use it before those distortion pedals.

But now it makes sense.

Thanks a lot !

Have a nice time