I have a Hafler Pro 2400 and am the original owner since the 80's! It's been in storage for the last several years and I've recently decided to put it back in use. It does work but I'm wondering if there are any mods or upgrades I should be looking at doing. Also, for the cost of the upgrades, is it worth putting the money into it?
I just picked up an AVA Transcedence 8 preamp yesterday which is in somewhat rough condition. It was missing both tubes and had two holes cut in the top above where the tubes go. My best guess is the the PO upgraded to the 6CG7 tubes which are taller so they cut holes in the top to accommodate them. I'm curious to see if it works once I replace the tubes.
I just picked up an AVA Transcedence 8 preamp yesterday which is in somewhat rough condition. It was missing both tubes and had two holes cut in the top above where the tubes go. My best guess is the the PO upgraded to the 6CG7 tubes which are taller so they cut holes in the top to accommodate them. I'm curious to see if it works once I replace the tubes.
I was amazed that after 15+ years in the attic, it fired up and started pumping out sound! When you read many of the responses in the forums, you are led to believe that anything over 20 years old needs to be re-capped at least. Maybe that's true but I don't know how to tell if I would benefit from doing that.
I'm going to get the AVA preamp working, connect it to the 2400 and see how it sounds.
I'm going to get the AVA preamp working, connect it to the 2400 and see how it sounds.
So I got the AVA TR8 working. I actually called AVA and Frank Van Alstine picked up which is amazing! He recommended a tube replacement to what came in it originally and it works great. Now, I'm back to my question regarding the Hafler. Does this now become the weak link in my system and do I replace the capacitors or buy a used Adcom or Parasound amp?
I know this is an older thread, but here is the thing: fired up an old Hafler PRO 2400 today and was saddened to hear how bad it sounded... Congested and un-alive. So I looked at the schematic (attached) and did not like what I saw: 3 OP-AMPs in the input. Fine for converting a balanced signal to the single input. But unneeded for HI-FI.
So here is a quick fix: bypass the whole mess and drive the amp directly. Many ways to do that, but I found the easiest by far was to attech two female RCA inputs to the pin that goes to the wiper on the pots. Disconnect the pot wires (J5 and J6) and pick the middle wiper wire and the ground to solder onto the RCA inputs and viola! - now the amp sound faster and much clearer.
So here is a quick fix: bypass the whole mess and drive the amp directly. Many ways to do that, but I found the easiest by far was to attech two female RCA inputs to the pin that goes to the wiper on the pots. Disconnect the pot wires (J5 and J6) and pick the middle wiper wire and the ground to solder onto the RCA inputs and viola! - now the amp sound faster and much clearer.
Opamps are definitely compatible with high fidelity, especially for this sort of signal processing - but which opamp does matter. Here there are TL082's and NE5532's, shame that it wasn't 5532's throughout as that would reduce noise and distortion measurably.
If you can hear the input section it could well be oscillating as I don't see any decoupling capacitors mounted near U2, but the NE5532 absolutely requires them to perform correctly.
Try adding 100nF ceramic between pins 4 and 8 of U2, close to the chip (a few mm, not cm).
If you can hear the input section it could well be oscillating as I don't see any decoupling capacitors mounted near U2, but the NE5532 absolutely requires them to perform correctly.
Try adding 100nF ceramic between pins 4 and 8 of U2, close to the chip (a few mm, not cm).
Sure. OpAmps are fine for HiFi. I should have been a bit more precise here. Having built many tube, hybrid and MOSFET amps, my personla philosophy is the best part is no part. Or: the simplest circuit is the best. So If one does not need the balanced input of the PRO 2400, one can bypass the input OpAmps. And instead feed a single-ended input directly into C1 and C18. As it is easy to try, I just wanted to share. 
Well I totally don't agree - few parts usually means high distortion - you have to use the topology to correct for errors, and without enough parts you don't have the means or flexibility to correct error, extend operating ranges, cancel. The goal is the design, a good design places little demands on its parts, yet combines them to give high performance by ingenuity.
Look at the schematic of a typical opamp - often 15+ transistors are needed to get reasonable performance, so simpler is simply worse here. You need enough parts to compensate for the shortcomings, working together to bring better performance through careful design, be it cancellation, compensation, feedback, symmetry or whatever. The early opamps were too simple, had issues like phase-inversion, susceptible to damage on output-short-circuit, instability at unity gain... Now most modern designs work extremely well without quirks, the inner complexity makes for an external simplicity...
The classic 3-opamp differential->single ended circuit is a case in point - a single opamp circuit performs much worse than the three opamp circuit, compromising noise severely, or overloading the input and compromising distortion in an effort to keep resistor values small. Those two input buffers given enough freedom to the rest of the design to avoid compromize.
Look at the schematic of a typical opamp - often 15+ transistors are needed to get reasonable performance, so simpler is simply worse here. You need enough parts to compensate for the shortcomings, working together to bring better performance through careful design, be it cancellation, compensation, feedback, symmetry or whatever. The early opamps were too simple, had issues like phase-inversion, susceptible to damage on output-short-circuit, instability at unity gain... Now most modern designs work extremely well without quirks, the inner complexity makes for an external simplicity...
The classic 3-opamp differential->single ended circuit is a case in point - a single opamp circuit performs much worse than the three opamp circuit, compromising noise severely, or overloading the input and compromising distortion in an effort to keep resistor values small. Those two input buffers given enough freedom to the rest of the design to avoid compromize.