WKSmith
Power User
Edit 3/20/24:
The 1200+ following posts hacking the FR series preamps turned into a plug and play replacement preamp for the FR-10/12 that solves the hiss problem.. and has several other major design improvements... named the Archangel V2.
If you are just getting here and trying to get on the waitlist, click on my forum name and send me a PM with your email address (important!)
Here are some bookmarks to help navigate this 66 page thread.
The V2 online owners manual.
V2 Features
V2 Boards
V2 Clip Light
V2 Pricing and IC upgrade options, and more waitlist details
V2 Review #1 and Review #2 from my awesome beta testers
V2 Crossover tuning and charts
V2 EQ spice Modeling vs stock eq graphs.
V2 balanced differential input tuning vs stock with Graphs
Gutshots of the original Stock FR preamp and F-corp's "factory hiss fix" secret IC Swap
V2 vs FR-10 vs FR-12 freq Charts
FR dis-assembly instructions from another forum member
Instructions to Bypass the stock EQ with one piece of wire.
V2 tuning vs stock tuning comparison graph with all eq knobs centered, cut knobs on 0
V2 Flat(ish) eq Settings
The Hiss that started all of this.
And Here is where this rabbit hole started:
I hacked the 1st 2 amp stages chasing the hiss. The hiss is still apparent and is probably in the eq. More on that later. I have a much cleaner eq design for this thing.
The FR12 has a true balanced differential front end andcan can almost take +4dB line levels. The first 2 stages are using a nasty 4558 (tube screamer) op amp. The first stage is the differential input and averager. Into the volume pot then into the second amp half for gain. The stock gain is set at 1+ 10k/ 1.5k so 1+ 6.66 for 7.66X gain.
Which is too high for even rail-to-rail op amps, and way too high for the 4558.
Edit: gain is still too high, even with first stage signal losses of -12dB from bad design in the stock preamp.
I replaced the poly caps with metal film box caps, replaced 10k feedback resistor with a metal one, Swapped out the 1.5k gain resistor for a 10k trimmer, swapped the electro cap for a metal film 1uf, and socketed a Burr Brown OPA2134.
The hiss is only moderately better. The tone though ..is very nice. It was nice before, now it sounds a lot more refined in the top end , much faster slew rate for the bottom. It's a very musical sounding op amp.
First pic is stock, 2nd pic is modded,
The 1200+ following posts hacking the FR series preamps turned into a plug and play replacement preamp for the FR-10/12 that solves the hiss problem.. and has several other major design improvements... named the Archangel V2.
If you are just getting here and trying to get on the waitlist, click on my forum name and send me a PM with your email address (important!)
Here are some bookmarks to help navigate this 66 page thread.
The V2 online owners manual.
V2 Features
V2 Boards
V2 Clip Light
V2 Pricing and IC upgrade options, and more waitlist details
V2 Review #1 and Review #2 from my awesome beta testers
V2 Crossover tuning and charts
V2 EQ spice Modeling vs stock eq graphs.
V2 balanced differential input tuning vs stock with Graphs
Gutshots of the original Stock FR preamp and F-corp's "factory hiss fix" secret IC Swap
V2 vs FR-10 vs FR-12 freq Charts
FR dis-assembly instructions from another forum member
Instructions to Bypass the stock EQ with one piece of wire.
V2 tuning vs stock tuning comparison graph with all eq knobs centered, cut knobs on 0
V2 Flat(ish) eq Settings
The Hiss that started all of this.
And Here is where this rabbit hole started:
I hacked the 1st 2 amp stages chasing the hiss. The hiss is still apparent and is probably in the eq. More on that later. I have a much cleaner eq design for this thing.
The FR12 has a true balanced differential front end and
Which is too high for even rail-to-rail op amps, and way too high for the 4558.
Edit: gain is still too high, even with first stage signal losses of -12dB from bad design in the stock preamp.
I replaced the poly caps with metal film box caps, replaced 10k feedback resistor with a metal one, Swapped out the 1.5k gain resistor for a 10k trimmer, swapped the electro cap for a metal film 1uf, and socketed a Burr Brown OPA2134.
The hiss is only moderately better. The tone though ..is very nice. It was nice before, now it sounds a lot more refined in the top end , much faster slew rate for the bottom. It's a very musical sounding op amp.
First pic is stock, 2nd pic is modded,
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