BP-1 History (or, how we got to
here from there)
The Barge Concepts BP-1 is an enhanced interpretation of a single vintage Interfax HP-1 Harmonic Percolator® whose internal pictures appeared in various gear/music forums some time ago. The HP-1 is a much talked about effect pedal, but it is rarely seen in vintage form. Over the years a number of circuits have appeared on various forums and bulletin boards, each with its own story of accuracy and legitimacy. Modern era copies are available, with at least one of them claiming to be identical to a well-known original. The differences between all the published circuits and the modern copies are minor in most cases, with the basic structure of the circuit (the topology) pretty constant from version to version. The degree of change between all the units and schematics strongly suggests the normal ebb and flow of product evolution and revision: diode in, diode out, 2K resistor here, 10K resistor there, coupling capacitor up or down in value. All of it makes a kind of erratic sense because the general topology remains pretty stable.
We had decided to do an HP-1 implementation some time ago, and were at the edge of having a real product (based on some of the published schematics) when we remembered that series of photos of the vintage unit. The unit in the forum pictures had all the earmarks of a genuine vintage HP-1 – potted transistors, labeled controls on the front of the enclosure, and some other ‘less typical’ parts on the PCB. In addition, it had an overall appearance that just looked real and original, one that completely fit the HP-1 legend in all its oddities. We were confident that this one had to be a genuine vintage unit, so we decided to base our design on it to avoid the uncertainty of the differences in all the other sources.
After much research, we were sure we finally had an accurate schematic of a genuine vintage HP-1 rather than a years-later version, but something wasn't right. We had double- and triple-checked every trace and every component, but were still unable to (electronically) reconcile what we reverse-engineered from the photos with what we were able to get from all the other sources. After a considerable bit of head scratching and theorizing it finally made sense - The Answer!
What our reverse-engineered schematic showed was a circuit that actually wouldn't work, and all our attempts to figure out how it could work were met with the same answer – it won’t. What turned out to be wrong was our only electronic assumption about this unit - that the 2 transistors were of different types, one NPN and one PNP. All of the previously-published HP-1 schematics invariably included a pair of transistors, one NPN and one PNP. What we finally recognized was that both transistors of the unit pictured on the forum had to be NPN, no matter how many other units had been reverse-engineered to show an NPN/PNP combination in their circuits. For whatever reason, this one was different. If the transistor previously thought to be a PNP were actually an NPN, the circuit made complete electronic sense and would work. It was as if the final piece of a puzzle had dropped into place.
After digesting this new truth for a bit, another light went on and things suddenly made further sense in an entirely different way. In the original HP-1, the identity of both transistors is concealed by a combination of a metal cylinder and some potting compound attaching the cylinder to the body of the transistor. What's missing on these two potted and completely unidentified parts is any way of distinguishing between them. In any kind of electronics manufacturing, it's vitally important to be able to tell what part you're dealing with, and to conceal two different devices in an identical way makes it impossible to easily tell which is which. However, it makes sense to not mark them if you don’t have to - if they're both the same and there's no need to distinguish between them. As a piece of confirming evidence, the importance of this one cannot be overemphasized, even though it completely escaped us before the electronic truth materialized.
That left us with a fully defined circuit topology that had a few part types and values missing. Analysis of the circuit gave us a good starting point for the two unknown resistor values – values that ultimately wound up being very consistent with values from the other published NPN/PNP versions. The mystery component variously identified as a jumper, inductor, or capacitor, we concluded to be a normal cylindrical capacitor whose value could be set with some experimenting. That left the transistors, and the question of Silicon vs. Germanium had to be considered. There was no ‘for sure’ way to tell from the schematic or pictures which type was used. The vintage of the original unit was such that either type could reasonably have been used. Indeed, later HP-1 copies have used a mix of Ge and Si in their implementations.
It was right around this time when we decided to try running our NPN/NPN prototype unit with some gain up front, and the results were astonishing. The unit opened up and, with some experimenting, completely transformed itself from a nice enough but hardly extraordinary effect into one that put a BIG smile on our faces. As soon as we were able to regain our ability to speak, we started talking about adding an optional gain function to the unit we had planned. The result of that test session made the question of Si vs Ge a less pressing issue than figuring out why the thing sounded so much better with the extra gain. It turns out that the HP-1 generates its characteristic sound largely by a combination of soft, asymmetrical clipping and some duty cycle modification (the ratio of positive excursion time vs. negative excursion time). The additional gain in the front allows even more duty cycle change and an accompanying change in tonal quality, as well as an improved range of response to playing dynamics.
Later tests verified that the overall performance of the basic (un-gained and un-buffered) circuit changes only very slightly with changes in transistor type, even when leaving the transistor biasing unchanged. This tended to confirm the old electronics axiom that circuits mostly function the way they do as a result of topology, not due to device types. The change that results from the up-front gain, however, is SO much more fun than the “stock” unit.
The net result of all this work is a better understanding of the original HP-1, a newly-revised schematic that is available on our web site, and a new addition to the Barge Concepts product line, the BP-1. The BP-1 is more fully described on its own page on our web site, but we're proud to be able to tell you about its unusual origins. We hope you experience the same uncontrollable grin we have when the gain gets dialed up.
The Barge Concepts BP-1 is an enhanced interpretation of a single vintage Interfax HP-1 Harmonic Percolator® whose internal pictures appeared in various gear/music forums some time ago. The HP-1 is a much talked about effect pedal, but it is rarely seen in vintage form. Over the years a number of circuits have appeared on various forums and bulletin boards, each with its own story of accuracy and legitimacy. Modern era copies are available, with at least one of them claiming to be identical to a well-known original. The differences between all the published circuits and the modern copies are minor in most cases, with the basic structure of the circuit (the topology) pretty constant from version to version. The degree of change between all the units and schematics strongly suggests the normal ebb and flow of product evolution and revision: diode in, diode out, 2K resistor here, 10K resistor there, coupling capacitor up or down in value. All of it makes a kind of erratic sense because the general topology remains pretty stable.
We had decided to do an HP-1 implementation some time ago, and were at the edge of having a real product (based on some of the published schematics) when we remembered that series of photos of the vintage unit. The unit in the forum pictures had all the earmarks of a genuine vintage HP-1 – potted transistors, labeled controls on the front of the enclosure, and some other ‘less typical’ parts on the PCB. In addition, it had an overall appearance that just looked real and original, one that completely fit the HP-1 legend in all its oddities. We were confident that this one had to be a genuine vintage unit, so we decided to base our design on it to avoid the uncertainty of the differences in all the other sources.
After much research, we were sure we finally had an accurate schematic of a genuine vintage HP-1 rather than a years-later version, but something wasn't right. We had double- and triple-checked every trace and every component, but were still unable to (electronically) reconcile what we reverse-engineered from the photos with what we were able to get from all the other sources. After a considerable bit of head scratching and theorizing it finally made sense - The Answer!
What our reverse-engineered schematic showed was a circuit that actually wouldn't work, and all our attempts to figure out how it could work were met with the same answer – it won’t. What turned out to be wrong was our only electronic assumption about this unit - that the 2 transistors were of different types, one NPN and one PNP. All of the previously-published HP-1 schematics invariably included a pair of transistors, one NPN and one PNP. What we finally recognized was that both transistors of the unit pictured on the forum had to be NPN, no matter how many other units had been reverse-engineered to show an NPN/PNP combination in their circuits. For whatever reason, this one was different. If the transistor previously thought to be a PNP were actually an NPN, the circuit made complete electronic sense and would work. It was as if the final piece of a puzzle had dropped into place.
After digesting this new truth for a bit, another light went on and things suddenly made further sense in an entirely different way. In the original HP-1, the identity of both transistors is concealed by a combination of a metal cylinder and some potting compound attaching the cylinder to the body of the transistor. What's missing on these two potted and completely unidentified parts is any way of distinguishing between them. In any kind of electronics manufacturing, it's vitally important to be able to tell what part you're dealing with, and to conceal two different devices in an identical way makes it impossible to easily tell which is which. However, it makes sense to not mark them if you don’t have to - if they're both the same and there's no need to distinguish between them. As a piece of confirming evidence, the importance of this one cannot be overemphasized, even though it completely escaped us before the electronic truth materialized.
That left us with a fully defined circuit topology that had a few part types and values missing. Analysis of the circuit gave us a good starting point for the two unknown resistor values – values that ultimately wound up being very consistent with values from the other published NPN/PNP versions. The mystery component variously identified as a jumper, inductor, or capacitor, we concluded to be a normal cylindrical capacitor whose value could be set with some experimenting. That left the transistors, and the question of Silicon vs. Germanium had to be considered. There was no ‘for sure’ way to tell from the schematic or pictures which type was used. The vintage of the original unit was such that either type could reasonably have been used. Indeed, later HP-1 copies have used a mix of Ge and Si in their implementations.
It was right around this time when we decided to try running our NPN/NPN prototype unit with some gain up front, and the results were astonishing. The unit opened up and, with some experimenting, completely transformed itself from a nice enough but hardly extraordinary effect into one that put a BIG smile on our faces. As soon as we were able to regain our ability to speak, we started talking about adding an optional gain function to the unit we had planned. The result of that test session made the question of Si vs Ge a less pressing issue than figuring out why the thing sounded so much better with the extra gain. It turns out that the HP-1 generates its characteristic sound largely by a combination of soft, asymmetrical clipping and some duty cycle modification (the ratio of positive excursion time vs. negative excursion time). The additional gain in the front allows even more duty cycle change and an accompanying change in tonal quality, as well as an improved range of response to playing dynamics.
Later tests verified that the overall performance of the basic (un-gained and un-buffered) circuit changes only very slightly with changes in transistor type, even when leaving the transistor biasing unchanged. This tended to confirm the old electronics axiom that circuits mostly function the way they do as a result of topology, not due to device types. The change that results from the up-front gain, however, is SO much more fun than the “stock” unit.
The net result of all this work is a better understanding of the original HP-1, a newly-revised schematic that is available on our web site, and a new addition to the Barge Concepts product line, the BP-1. The BP-1 is more fully described on its own page on our web site, but we're proud to be able to tell you about its unusual origins. We hope you experience the same uncontrollable grin we have when the gain gets dialed up.
'Harmonic Percolator' is a Registered TradeMark ® of Chuck Collins.
