| Author |
 Topic  |
|
|
Laurie
Double Platinum Member
Canada
4854 Posts |
 Posted - 03/02/2009 : 15:13:26
|
G'day! This is probably common knowledge for folks who have been building pedals for a while, but it has been a learning experience for me... 
As some members know I've built a few FA-1 clones - more as a technical exercise than anything else. Playing with the FA-1 convinced me to design an equivalent high impedance booster - the Hi-Z Driver, or "HZD" (again initially more as a technical exercise).
I've built a few of the HZD pedals and have been evolving the design for the last couple of months and through that process I have learned a LOT about pedal input impedance and the interaction of input impedance with guitar tone.
The average Boss pedal input impedance is "a couple of hundred k-ohms" (e.g. the DS-1 is 220k). This is close to the average single coil guitar pickup impedance, which makes things "interesting" from an impedance matching viewpoint (see http://www.bossarea.com/forum/topic.asp?TOPIC_ID=5778).
Additionally "a low impedance load will reduce the high-frequency response of the pickup because of the filtering effect of the inductance" (Wikipedia).
As I've been playing with the design of the HZD pedal, I have had the opportunity to observe the behaviour of a Hi-Z pedal directly with test instruments. I can report my experience as follows...
With some careful engineering, my input design for the HZD has a genuine AC small-signal input impedance of over 1 Meg Ohm. Comparing the bypass signal to the signal that has been Hi-Z buffered by the HZD, the two things that are apparent are:
1) An increase in high frequency transparency - frequencies above about 5kHz are slightly rolled off in bypass.
2) A large increase in dynamic range - the dynamic range of the buffered input is about 6dB greater than in bypass.
To the ear, what this means is that the "chime" is preserved (the high frequencies), and the touch-sensitivity is amazing. With the pickup driving a Hi-Z input, it is possible to go from light clean picking to room-filling overdriven chords, just by varying the picking strength - this is simply not possible in bypass. I am working on making some sound samples.
The only negative thing I've found is that with the increased dynamic range it is possible to get a drive signal of several volts out of the HZD which is great if it is plugged directly into a tube amp, but it can push the input of following pedals into clipping - I have a small pedal chain for testing: HZD -> DD-2 -> CE-5 and the CE-5 input can be driven overload (nasty clipping) if the HZD is set too high. I'm thinking the ultimate chain is have the initial HZD set to maybe 3dB gain to drive the pedal chain, an then a second HZD at the end of the chain so it can drive the amp directly at whatever gain you want.
I would not have thought it possible for there to be so great a difference between Hi-Z buffered and un-buffered tone. Boss knew what they were doing when they built the old FA-1.
From now on my pedalboards will always have a Hi-Z pedal like this as the first one in the chain.
|
Edited by - Laurie on 03/07/2009 05:24:45 |
|
|
verivorax
Platinum Member
Canada
1185 Posts |
Posted - 03/02/2009 : 17:28:08
|
Very interesting.. I definitely want to hear some of these.
Is the next step a switchable/blendable FA-1 + hiZbooster? Is such a thing possible?
 GASsing. |
 |
|
|
Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 03/02/2009 : 17:41:58
|
Hmmm... not sure what you are describing with "switchable/blendable FA-1 + hiZbooster"? The FA-1 and my HZD are both Hi-Z boosters - the HID was designed to be a "modern equivalent" to the FA-1... you only need one of them 
Oh. Wait. Nope, you need two of each - one as the first pedal to give you a Hi-Z load for the guitar and drive the pedalboard, and another one right at the end of the chain to drive the amp (I've designed the HZD so that it can be powered by up to 24V to give massive drive into a tube amp).
Of course, the "tone" is different between the HID and the FA-1. The FA-1 uses the old HA1457W opamp, and the HZD uses the 4558D so the breakup is different when the opamp is overdriven into clipping. I prefer the 4558D because it is "gentle" to your tone and gives clipping that sounds somehow smoother.
|
Edited by - Laurie on 03/03/2009 02:03:29 |
|
|
|
Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 03/03/2009 : 07:11:08
|
hi guys
Laurie has asked me to move this thread from:
"Boss Products/Modifications and Technical Issues",
to
"Other Products / Non Boss Gear Discussions"
regards dr. bob - moderator |
Edited by - Dr. Bob on 03/03/2009 07:11:51 |
|
|
|
Right Foot Boss
Gold Member
USA
881 Posts |
Posted - 03/07/2009 : 03:55:17
|
| I'll testify to that 'digital clipping' My AC-3 does it hard with my FA-1 clone. Just turn the level down and it's still smooth as butter. |
Edited by - Right Foot Boss on 03/07/2009 04:07:34 |
|
|
| |
Topic |
|
Pedal input impedance and touch sensitivity
