Hello all-
I recently picked up this Audionics of Oregon stack for cheap - a BT-2 preamplifier with a PZ3 Series II amplifier.
They were sold as-is, and upon testing, I found the preamp to sound very good, but it exhibits the classic snap-crackle-pop-fizz of failing silicon.
The folks at Audionics decided to pot the circuit boards, presumably to protect their IP. Unfortunately, this made them absolutely impossible to service. They were violating right to repair way before it was cool!
I tried replacing the two small signal transistors that weren't potted, but they didn't have any effect. Based off of the 8-pin packages visible from the bottom, this looks like a simple op-amp based line amplifier to me. Even if I could service the board, finding these ancient op-amps would be a tall order in itself.
I love the simplicity and weight of the chassis, and the switches/pots are of very high quality, so I would like to revive this preamp by installing modern boards.
What are some favorite DIY preamp boards of the forum? I think I would like to keep it op-amp based to keep in the spirit of the original circuitry, but am open to discrete ideas as well. I want to buy boards in kit or pre-assembled form, not looking to break out the proto board this time. I don't mind buying PCBs and parts separately though.
I would like to replace both the line and phono boards, so suggestions for both are welcome.
Thanks!
I recently picked up this Audionics of Oregon stack for cheap - a BT-2 preamplifier with a PZ3 Series II amplifier.
They were sold as-is, and upon testing, I found the preamp to sound very good, but it exhibits the classic snap-crackle-pop-fizz of failing silicon.
The folks at Audionics decided to pot the circuit boards, presumably to protect their IP. Unfortunately, this made them absolutely impossible to service. They were violating right to repair way before it was cool!
I tried replacing the two small signal transistors that weren't potted, but they didn't have any effect. Based off of the 8-pin packages visible from the bottom, this looks like a simple op-amp based line amplifier to me. Even if I could service the board, finding these ancient op-amps would be a tall order in itself.
I love the simplicity and weight of the chassis, and the switches/pots are of very high quality, so I would like to revive this preamp by installing modern boards.
What are some favorite DIY preamp boards of the forum? I think I would like to keep it op-amp based to keep in the spirit of the original circuitry, but am open to discrete ideas as well. I want to buy boards in kit or pre-assembled form, not looking to break out the proto board this time. I don't mind buying PCBs and parts separately though.
I would like to replace both the line and phono boards, so suggestions for both are welcome.
Thanks!
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Best wishes with the preamp, but from me a note on the power amp. You might want to sweep the 'net for notes on that one.
It was famous for blowing up and I don't recall if it oscillated or what. I think it changed a lot over the production run - efforts to make them not come back to the factory under warranty.
(edit) here's some helpful PZ3 stuff http://www.nutshellhifi.com/library/bobsickler.html
It was famous for blowing up and I don't recall if it oscillated or what. I think it changed a lot over the production run - efforts to make them not come back to the factory under warranty.
(edit) here's some helpful PZ3 stuff http://www.nutshellhifi.com/library/bobsickler.html
I read the article, it sounds like a real charmer of an amp lol. I know this one powers on...I haven't plugged it into anything yet though. I can see the triple driver transistor bodge he talked about sitting in there, so mine must be a little later in production. At the very least I have a super-duty power supply and a nice chassis. I've always wanted to try Class A....
The existing power supply gives ±15v.
I had a BT-2 purchased new in the day and used if for a long while. My daughter uses it now. Audionics replaced one of the potted modules under warrenty. It always sounded good though it falls well short of modern designs. Always pleasant though. I would not attempt to clone it. The casework connectors and knobs are all very nice, so I would look toward basing a new preamp on something like the Pass 2022 Front End board. Should be easy and work brilliantly. You could probably do the new Pearl3 as well, though that would be a bigger challenge.
A friend gave me a dead Audionics CC-2, one of two he had. It had blown and been repaired twice, and he wasn't going to have it done again. I gutted it, saving the superb power transformer, and used it with LM3875 chipamp cards for a long while then converted it to an external power supply box for Neurochrome Mod 86 amps in a separate chassis.
Hope this provides a bet of context.
A friend gave me a dead Audionics CC-2, one of two he had. It had blown and been repaired twice, and he wasn't going to have it done again. I gutted it, saving the superb power transformer, and used it with LM3875 chipamp cards for a long while then converted it to an external power supply box for Neurochrome Mod 86 amps in a separate chassis.
Hope this provides a bet of context.
When I was an undergraduate, there was a file cabinet containing all sorts of service manuals and piecemeal trace-outs for expensive stereo gear. Have a look at this little example. You can see why they potted the circuits in opaque black epoxy; either they were deeply embarrassed, or else they were achingly proud, of the performance they were able to get from surprising component choices.
_
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Perhaps a healthy amount of both...
Thanks for the schematics! That cabinet sounds like a goldmine; documentation is hard to come by for these more obscure pieces.
You don't have anything for the PZ-3 Series II do you? I would like to at least give the poor thing a quick rebias.
The Pass 2022 Front End sounds like a decent contender for a replacement line amp. Looks like diyAudio is fresh out of PCBs right now though, might have to keep looking for now.
Yeah that massive transformer and 30,000uf of filtering is looking mighty tempting for use in other projects after reading multiple horror stories of the Series 1s. The Series 2s seem a bit more respected though. I'll see if I can give it a quick service and go from there.
It looks like your power supply board is more primitive than the 'Series II' drawings that got posted?
Anyway the modularity of the active stages should make it a piece of cake to retrofit different modules for phono & line. I don't know what store-bought kits exist for that & will let others comment. I'd just perfboard up a couple drop-ins, but you've ruled that out. Should be a fun project.
Anyway the modularity of the active stages should make it a piece of cake to retrofit different modules for phono & line. I don't know what store-bought kits exist for that & will let others comment. I'd just perfboard up a couple drop-ins, but you've ruled that out. Should be a fun project.
The phonostage + RIAA equalizer from the Advent 300 receiver {designed by Tomlinson Holman (link) } is a much-loved, freqiently-reviewed, all discrete circuit containing only seven transistors. And two of those seven are capacitance multipliers for supply rail noise elimination -- many years sooner than anybody else copied/used them.
Like other preamps from the early and middle 1970s, the Advent 300's phonostage was designed for moving magnet cartridges exclusively. No MC for you.
https://www.audioservicemanuals.com/a/advent-300/377773-advent-300-rec-sm
edit added later- or you could consider James Bongiorno's "Distortionless Preamplifier" from Popular Electronics magazine, June 1972. Three transistors is all he needed.
Like other preamps from the early and middle 1970s, the Advent 300's phonostage was designed for moving magnet cartridges exclusively. No MC for you.
https://www.audioservicemanuals.com/a/advent-300/377773-advent-300-rec-sm
edit added later- or you could consider James Bongiorno's "Distortionless Preamplifier" from Popular Electronics magazine, June 1972. Three transistors is all he needed.
Those of the NAD3020 have also been retooled and i believe at least Gerbers exist.
dave
Thanks for the links, interesting read. Audionics, my first “high end system”
I now use the chassis and power supply of the BT-2 preamp for a DIY dac. Still have the guts and front plate if anyone is interested. Also have two CC-2 amps and their T-52 speakers. Not in regular use but still working fine.
I now use the chassis and power supply of the BT-2 preamp for a DIY dac. Still have the guts and front plate if anyone is interested. Also have two CC-2 amps and their T-52 speakers. Not in regular use but still working fine.
A great "thank you" to Mark Johnson for BT2 schematic.
I have been looking for it since 2020.
Schematic of both (phono and line) sections of this preamp are non-conventional under many aspects, and it's really weird that a so old op-amp had been used.
My attention goes to that diode at output of IC that forces a positive voltage toward it while being in conduction leaves ac signal going to the output transistor.
This topology, I guess, was the secret that Barry Thornton wanted to keep hide.
This configures as a particular implementation of 709 opamp.
Only little frequency compensations and resistors to fix at 20 db the voltage gain and stable running of 709 and current source for output transistor.
Under that black epoxy there are the goodies.
F
I have been looking for it since 2020.
Schematic of both (phono and line) sections of this preamp are non-conventional under many aspects, and it's really weird that a so old op-amp had been used.
My attention goes to that diode at output of IC that forces a positive voltage toward it while being in conduction leaves ac signal going to the output transistor.
This topology, I guess, was the secret that Barry Thornton wanted to keep hide.
This configures as a particular implementation of 709 opamp.
Only little frequency compensations and resistors to fix at 20 db the voltage gain and stable running of 709 and current source for output transistor.
Under that black epoxy there are the goodies.
F
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