Hello all,
Ok, I took in what most of you written and I have been to the norman koren site before. I do like the idea that there is an option for modified tone controls, but the whole point to redoing the tone controls would be to get something that can easily be replaced down the road. At first glance this looked to be a cool circuit, but then I noticed that the bass control needs to have a second gang on it. I don't think I would have an easy time finding a dual gang 500k control. A dual gang 50k, 100k, 200k, yes...but a 500k??
Yes, I also saw the guy is selling boards but $109 for just the pcb's? That is a bit expensive.
I will say though that this circuit IS a step in the right direction, but I was wondering if there was something 'simpler'.
I did notice the circuit on the end with 'no feedback' and that is something that appeals to me. But how (if possible) would one add tone controls to a circuit like that.
CpuZapper wrote:For me the biggest improvment was the removal of the tone controls.
As I mentioned above, I don't really want to consider that an option. I WILL consider switchable filtered stages. As Joe pointed out to me with the site he mentioned, I DID see a selector switch tone control variant and that seems interesting, but it also seemed like that is the most complicated circuit.
I am very much a firm believer in "KISS" (Keep It Simple Stupid). But I already built a simple pre-amp and I would like to have one with tone controls.
RIAA FIX
The stock PAS phono stage compromises some performance in exchange for simplicity. Plus, its RIAA compensation departs from ideal by 4db. The output impedance is quite high (30K) so low impedance loads (less than 100K) will cause LF rolloff. While it is a major undertaking to lower the output impedance, it is relatively easy to address the RIAA compensation and the LF rolloff issues.
RIAA Fix:
1. Remove the 750pf caps (both channels) from the selector switch.
2. Remove the 27K resistors (both channels) from the selector switch.
3. Replace the RIAA 68pf cap on the PC-6 pc board with a 820pf (polysty or silver mica).
4. Parallel the 100K RIAA resistor with a 2M (metal film).
5. Replace the 2750pf RIAA cap with a parallel combination of 2200pf and 470pf.
6. Replace the 4.7M RIAA resistor with a 2M (metal film).
7. FYI - DO NOT remove the 47K feedback resistors between the cathodes of each tube.
LF Rolloff:
1. Jumper the 0.1uF output cap connected from the plate of the second stage(s) with a solid wire (both chanels).
2. Change the value of the 0.1uF output coupling caps, connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel) to 0.47uF / 400VDC.
After you complete these mods, you can use any of the 3 low level inputs (Phono, Tape, Special) for your turntable - all 3 will be RIAA compensated. RIAA error will have improved from +/- 2db to +/- 0.2db. Output impedance will not be changed so the original caution of low impedance loads (attached to the tape output connections) apply.
Good Luck with the updates.
I am taking notes here for the RIAA section and pretty much here I WILL do what it takes to get it right. I would like to use this pre-amp to 'warm up' the sound of records, not only for listening to, but also for CD creation via my computer. So I would like this stage to sound as good as possible.
mesherm wrote:Do you have PAS-2, PAS-3 or PAS-3x units?
They are PAS-2 units, but one might be a PAS-3. I am sure they are not the 3X variants with the special tone controls. I have heard of these and that the center detent position effectively bypasses the tone tone controls.
I DO think it is a cool feature, but I am sure that my pre-amps do not have it.
My suggestions:
1. Replace the stock selector switch and rewire all inputs with shielded and twisted cable.
2. Replace the PCBs with new ones using modern quality components.
3. Make the Curcio mods to the RIAA board and hard wire one set of RCAs in the back just for the RIAA input.
4. Replace the can capacitor and the voltage doubler components.
5. Clean all switchs and pots with DeOxit.
6. Disable all the 2 wire line plugs.
7. Use NOS 5751 tubes in the line amp and Sovtek 12AX7LPS tubes in the RIAA section.
I will say that overall I was going to replace the PCB's and update the power supply. But I am wondering if they might make 'hand wiring' PCB's. I am thinking more along the lines of going with a different circuit.
Believe it or not I was even thinking about going with something octal based, such as a 6SN7 tube. I have LOADS of these tubes.
I don't mind using an SRPP circuit for the line stage. In fact I do prefer it over the cathode follower. It is just that I do not know how to add tone controls to an SRPP.
As with my interest in SRPP, I do prefer a no feedback line stage design as well.
As I said above, I don't mind pulling the stock PCB's out and putting new ones in that can be hand wired from scratch. I would even use the Rosenblit SRPP pre-amp design again, BUT I just would need to know how to add a tone control to it.
This was my biggest complaint about my original Bruce Rosenblit pre-amp design. It had NO tone control. What I did do is buy myself a special volume control that had a 40% loudness tap and I just copied a popular loudness control contour into the design. So I had a bit of fun with that. The cramped quarters of my tiny 1 rack space chassis also forbade me to to install a standard selector, and relays would have been complex and I didn't have room for them. So I ended up only having the capability for 2 inputs. Not much for a pre-amp. But then again, I originally built that pre-amp because I wanted a good tube pre-amp to warm up my tracks when I had my recording studio. For that purpose I really didn't want the tone controls on the pre-amp. The loudness contour was effectively out of the circuit when turned off. So it was great for recording purposes.
But now that I want to build my own amplifier and settled on a design, I would like a nice pre-amp to go with it and since I have two PAS's already. May as well modify one.
But again, I am looking for a simple circuit, preferably NFB. Would be nice if I could use octals (6SL7, 6SN7). But some kind of tone control is mandatory. It could be something switched though...I am open to that idea.
Thanx for the input.
Geo