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HF-85 needs upgrades to coupling caps & power supply caps...

I have posted a few times concerning this item. The archives searches should prove worthwhile for mhardy and all. However, since we are "in the neighborhood" a short description is in order. This preamp uses AC filaments and a wimpy amount of power supply capacitance. A few weeks ago, maybe a month, there was a long thread about this preamp which started with the OP pinging my moniker. That thread had the OP's revised scheme of what his HF-85 would have for RIAA EQ, plus someone else posted the ST-84 scheme below that. The ST-84 was a revised HF-85 with a slightly better power supply arrangement, which can easily be added to any HF-85. Check that thread out...

My ideology is to always restore the keeper vintage items first; then upgrade. I think the HF-85 has a great sounding line stage. There is an extra coupling cap that can be removed or rearranged in Y fashion; or left stock. But, the original coupling caps that are ceramic or waxy paper are detrimental to the sonics and deserve better upgraded types. If you want that classic sound, polyester/mylar/paper/paper in oil will do the trick. If you want a bit more accurate and seemingly faster sounding, some polystyrene or polypropylene are in order. The power supply needs lots more capacitance for each stage after the rectifier output cap.

The HF-85 phono stage is just alright. It can be made to better standard. Plus, and it is a big plus, the Mic position is an easy place to add another phono EQ circuit. The Mic input has a ceramic input cap wired at the rotary switch. This can be bypassed with wire and a 47K resistor added at the mic input jacks to ground, for the proper RIAA phono loading. Somewhere between those "bypassed" ceramic caps and the switch area common to the tape head and phono EQ feedback circuits, you can add your favorite phono EQ circuit so the Mic position becomes Phono 2. The Tape Head wiring can also be rearranged for yet another phono stage. The 3 3/4 & 7 1/2 toggle switch can be rewired to accommodate various EQ values, which is important to many vinylphiles. I would probably choose two different HF cut networks at that toggle switch.

So, concerning the phono stages, consider that the HF-85 uses a unique feedback arrangement where the first stage is simply high gain and the second stage uses feedback EQ fed back to the second stage grid. Normal/Standard operating procedure for RIAA active EQ usually has the feedback EQ from the second stage plate fed back into the first stage's cathode. This can easily be accommodated, once the wiring is sorted out. The cathode bypass caps at the phono tubes would be removed. Then again, it is going to be your preamp. Thus, study all the circuits you would want to try. The HF-85 can actually be modded for different types of RIAA EQ for the different positions, once you sort out the wiring & parts arrangements. Likewise, the HF-81 integrated, with similar preamp stages, can wholly benefit from this idea.

That sorting out of the wiring is the biggest trick of resto mods. Draw out the stock wiring so you know what you "were" looking at, before you change the wiring. That way, you can always go back to stock. Do not depend on published threads and revised schemes from others, but get inspired from them. Learn the circuitry and learn what differing circuits will sound like. It is very rewarding and enlightening to then actually realize what changes made the sound right or wrong. Now, concerning actual phono EQ network values, again, rely on yourself after studying other published circuits. While the math is important to many, even the parts types actually make sonic impact. So, if your RIAA network EQ poles start out as 850 pf across 91K or 100K(HF cutoff), in series with 2700 pf across 2M(bass boost), depending on your parts type choices, including tubes, you might tweak the circuitry to 750 pf across 95K, in series with 2500 pf across 3 or 4Meg.

For the disbelievers of using your ears, go ahead and use the math. Check out all the vintage active RIAA EQ circuits with all the different tubes, gain setups and topologies. Then, you will surmise that many active/feedback EQ circuits are not that different despite differing overall topologies. I am influenced by the previous work of some genius engineers; plus, I can still trust my ears. I choose to tweak from my experience; when needed. The parts available now were not available when many of these circuits were devised. It is clearly audible that a mica EQ cap will sound different than a polystyrene or ceramic EQ cap of the same value. For absolute assurance, you might desire to build or buy an inverse RIAA EQ circuit and measure frequency response as flat through your EQ circuit passing through the inverse circuit. Then again, how do you know your inverse circuit that you built or bought is a better reference ? Maybe, using well known discs and vinyl systems is just as good, or even a better reference. Best of Luck with your resto efforts...

The benefit of bringing these vintage artifacts back to life, sounding better than they ever did before, is both worthwhile for your brain and your ears...ENJOY !



Edits: 07/01/12

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  • HF-85 needs upgrades to coupling caps & power supply caps... - Interstage Tranny 16:14:31 07/01/12 (0)

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